The influence of short strong hydrogen bonding on the structure and the physicochemical properties of alkyl-N-iminodiacetic acids in solid state and aqueous systems

Article abstract of DOI:10.1021/ja021012i

Alkyl-N-iminodiacetic acids with varying alkyl chain lengths have been prepared and characterized with respect to structure, acidic properties, and ability to form aggregates in water. The alkyl-N-iminodiacetic acids are the group of ligands with the lowest molecule weight which can be characterized as chelating surfactants, compounds with surface chemical properties which at the same time have a high ability to bind metal ions. The solid alkyl-N-iminodiacetic acids have a unique structure with neutral zwitterionic units linked together to polymer chains through a short strong hydrogen bond, d(O(-H)...O) ≈ 2.46 A, and where the ν(O-H) stretching vibration at ca, 720 cm^-1 supports the presence of such a hydrogen bond. The polymer chains are cross-linked together to bilayers through relatively strong hydrogen bonds between ammonium and carboxylate groups, and where the parallel alkyl groups are interdigitating each other; the bilayer surface consists of hydrophilic iminodiacetic acid groups. The acidic properties of monomeric alkyl-N-iminodiacetic acids in water are in the expected ranges with pK_a values of about 1.7, 2.3, and 10.3. n-Octadecyl-N-iminodiacetic acid, present as aggregates in water, displays very acidic properties of the first proton, and a substantially weakened acidity of the second proton, pK_a2 = 5.5-7.5, depending on ionic strength, and pK_a3 = 9.5-10.5. This pattern of the acidic constants strongly indicates that the polymer structure with short strong hydrogen bonds is maintained in the aggregates and that such bonds can exist in aqueous systems if they are supported by a strong and rigid backbone structure, as the bilayers of well-organized long interdigitating alkyl chains in the studied systems. Hydrogenbis(methyl-N-iminodiacetic acid) perchlorate precipitates from perchloric acidic solutions of methyl-N-iminodiacetic acid. The structure is built up of dimers of zwitterionic methyl-N-iminodiacetic acid units linked together by an extra proton in a short strong hydrogen bond, d(O(-H)...O) ≈ 2.456(6) A, and ν(O-H) = 789 cm^-1.

Full text of DOI:10.1021/ja021012i

Published on Web 02/28/2003
The Influence of Short Strong Hydrogen Bonding on the
Structure and the Physicochemical Properties of
Alkyl-N-iminodiacetic Acids in Solid State and Aqueous
Systems
†
†
‡
†
¨
¨
Leif Haggman, Cecilia Lindblad, Hans Oskarsson, Ann-Sofi Ullstrom, and
Ingmar Persson*,†
Contribution from the Department of Chemistry, Swedish UniVersity of Agricultural Sciences,
Box 7015, SE-750 07 Uppsala, Sweden, and Akzo Nobel Surface Chemistry AB,
SE-444 85 Stenungsund, Sweden
Received July 23, 2002; Revised Manuscript Received November 5, 2002; E-mail: Ingmar.Persson@kemi.slu.se
Abstract: Alkyl-N-iminodiacetic acids with varying alkyl chain lengths have been prepared and characterized
with respect to structure, acidic properties, and ability to form aggregates in water. The alkyl-N-iminodiacetic
acids are the group of ligands with the lowest molecule weight which can be characterized as chelating
surfactants, compounds with surface chemical properties which at the same time have a high ability to
bind metal ions. The solid alkyl-N-iminodiacetic acids have a unique structure with neutral zwitterionic units
linked together to polymer chains through a short strong hydrogen bond, d(O(-H)‚‚‚O) ≈ 2.46 Å, and
where the ν(O-H) stretching vibration at ca. 720 cm-1 supports the presence of such a hydrogen bond.
The polymer chains are cross-linked together to bilayers through relatively strong hydrogen bonds between
ammonium and carboxylate groups, and where the parallel alkyl groups are interdigitating each other; the
bilayer surface consists of hydrophilic iminodiacetic acid groups. The acidic properties of monomeric alkyl-
N-iminodiacetic acids in water are in the expected ranges with pKa values of about 1.7, 2.3, and 10.3.
n-Octadecyl-N-iminodiacetic acid, present as aggregates in water, displays very acidic properties of the
first proton, and a substantially weakened acidity of the second proton, pKa2 ) 5.5-7.5, depending on
ionic strength, and pKa3 ) 9.5-10.5. This pattern of the acidic constants strongly indicates that the polymer
structure with short strong hydrogen bonds is maintained in the aggregates and that such bonds can exist
in aqueous systems if they are supported by a strong and rigid backbone structure, as the bilayers of
well-organized long interdigitating alkyl chains in the studied systems. Hydrogenbis(methyl-N-iminodiacetic
acid) perchlorate precipitates from perchloric acidic solutions of methyl-N-iminodiacetic acid. The structure
is built up of dimers of zwitterionic methyl-N-iminodiacetic acid units linked together by an extra proton in
a short strong hydrogen bond, d(O(-H)‚‚‚O) ≈ 2.456(6) Å, and ν(O-H) ) 789 cm-1
.
Introduction
The aim of this paper is to understand the crucial chemical
requirements for a chelating surfactant system with predictable
properties and to set the basis for the following papers in this
project describing the structural and physical-chemical properties
of the alkyl-N-iminodiacetic acids in the presence of different
metal ions in aqueous systems. This paper presents the prepara-
tion of three alkyl-N-iminodiacetic acids, R ) n-hexyl, n-
dodecyl, and n-octadecyl, the determination of the solid-state
structures of three alkyl-N-iminodiacetic acids, R ) methyl,
n-hexyl, and n-dodecyl, and of one singly protonated dimer,
hydrogenbis(methyl-N-iminodiacetic acid) perchlorate, the acidic
properties of the methyl-, n-hexyl-, n-dodecyl-, and n-octadecyl-
N-iminodiacetic acids including variation of total concentration
and ionic strength, the aggregation properties of the n-hexyl-
and n-dodecyl-N-iminodiacetic acids as a function of pH and
total concentration as determined with steady-state fluorescence
measurements, and infrared, mass, and 1H NMR spectroscopic
data and elemental analyses of the alkyl-N-iminodiacetic acids
for characterization purposes.
The interest for surface-active compounds, which at the same
time have the ability to bind metal ions strongly and/or
specifically, so-called chelating surfactants, is markedly increas-
ing as many technical applications are in sight, such as, for
example, soil remediation. The number of physical-chemical
studies on chelating surfactants is scarce, as it in many instances
is difficult to obtain reliable information due to the difficulty
to prepare pure and well-defined compounds. In this study, we
have chosen to study the group of ligands with the lowest
molecule weight which can be defined as chelating surfactants,
the alkyl-N-iminodiacetic acids, R-N(CH2-COOH)2. The
alkyl-N-iminodiacetic acids will serve as a model for larger
molecules with longer and/or more alkyl chains and with EDTA-
like multidentate entities for efficient and/or specific complex
formation of metal ions.
† Swedish University of Agricultural Sciences.
‡ Akzo Nobel Surface Chemistry AB.
9
10.1021/ja021012i CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 3631-3641
3631

A R T I C L E S
Ha¨ggman et al.
Syntheses of Alkyl-N-iminodiacetic Acids. Chemicals. 2-Propanol
(IPA) (analytical grade, Merck), ethanol (Primalco, 99.7%), n-hexyl-
amine (Fluka, 99%), n-dodecylamine (Fluka, >98%), n-octadecylamine
(Aldrich, 99%), and sodium monochloroacetate (SMCA) (Skoghall,
>97%) were used as purchased.
This study shows clearly that the hydrogen bonding plays a
very important role in the structure and the physical-chemical
behavior of alkyl-N-iminodiacetic acids, as it also does in many
other chemical systems. Hydrogen bonding is the key factor in
explaining many chemical and biological structures and pro-
cesses such as, for example, the unique properties of water,1
enzyme conformation,2 the behavior of lipid membranes,3
catalysis,4 and the double-helical structure of DNA.2 It is
therefore not surprising that the hydrogen bond energy can vary
over a very wide range, ca. 5-150 kJ mol-1.5 In recent years,
much attention has been put on the upper energy range, often
referred as short strong hydrogen bonds (SSHBs) or low-barrier
hydrogen bonds (LBHBs). The basic demands for being an
SSHB/LBHB include (a) a short distance between hydrogen
bond acceptor and donor, in the range 2.36-2.65 Å in oxygen-
containing systems,6 and (b) that the energy of the hydrogen
bond is at least 40 kJ mol-1.7 It has also been proposed that
high polarity and entropic disorder of the surroundings weaken
a hydrogen bond,8 while equal pKa values of the hydrogen bond
acceptor and donor should strengthen it.9 There has been strong
controversy over the existence of strong hydrogen bonds in
aqueous solution. Cleland and Kreevoy have stated “formation
of a short (less than 2.5 Å), very strong low barrier hydrogen
bond in the transition state or in an enzyme-intermediate
complex can be an important contribution to enzymatic cataly-
sis.”10 Guthrie has argued against this view and pointed out that
so far no short strong hydrogen bond has been proven to exist
in aqueous solution.7 Theoretical calculations support Guthries
view that short and strong hydrogen bonds do not exist in media
with permittivities larger than 10, such as, for example, in
water.11 This paper will discuss the possibility of forming SSHBs
in surface-active systems in aqueous systems using results from
acidic constant determinations of alkyl-N-iminodiacetic acid
systems in water.
n-Hexyl-N-iminodiacetic Acid. n-Hexylamine (102.14 g, 1.009 mol)
was dissolved in 2-propanol (250 cm3) and charged to a double jacket
1000 cm3 glass-reactor with a cooler, a Teflon-covered mechanical
stirrer (RW 20 DZM, Janke&Kunkel, IKA Labotechnique), a pH-stat
(Titrino 719S, Metrohm), and a digital thermometer (Ebro TFX 392).
Next 270.66 g (2.30 mol) of SMCA, dissolved in 300 cm3 of deionized
water, was added to the reaction mixture at 350-355 K during a period
of 1.5 h at a constant pH of 8.5, adjusted by addition of 30% sodium
hydroxide from a Dosimat (Titrino 719 from Metrohm Instruments).
The postreaction continued for 7 h at 350 K and pH 8.5. Thereafter,
the product was purified and concentrated. Excess SMCA was
hydrolyzed at 350 K and pH 12 for 2.5 h. The product was poured in
an E-flask and concentrated, and thereafter 4 mol dm-3 of hydrochloric
acid was added until pH 1.80 was reached. The solvent was partly
evaporated off, which caused precipitation of sodium chloride and the
product; the filter-cake contained 14.5% sodium chloride. The product/
salt filter-cake was dried and dissolved in ethanol (99.7%). In hot
ethanol, sodium chloride precipitated and was filtered off. The filtrate
was allowed to cool slowly overnight. By the following day, the product
had precipitated to a hard solid. The solid was divided into small pieces,
which were dried with diethyl ether. The compound was recrystallized
from aqueous solution at pH 1.9 to get single crystals suitable for X-ray
diffraction studies.
Analyses. Elemental analysis of n-hexyl-N-iminodiacetic acid: C,
calc. 55.28%, exp. 54.0, 54.3%; H, calc. 8.81%, exp. 8.6, 8.7%; N,
calc. 6.44%, exp. 6.3, 6.3%.
Mass spectroscopy: Calc. 217.265 g mol-1; exp. 217.131 g mol-1
.
1H NMR data in Me2SO-d6: δ 0.86 (3H), δ 1.22 (6H), δ 1.34 (2H),
δ 2.60 (2H), δ 3.38 (4H); see Table S1 for details.
n-Dodecyl-N-iminodiacetic Acid. First 100.88 g (0.544 mol) of
n-dodecylamine was dissolved in 250 cm3 of IPA and charged to the
reactor described above. Next 129.94 g (1.116 mol) of SMCA, dissolved
in 195 cm3 of deionized water, was added during a period of 1.5 h at
350-355 K and at a constant pH of 8.5, adjusted by the addition of
30% sodium hydroxide from a Dosimat. The length of the postreaction,
the purification and concentration of the product, and the hydrolysis
of excess SMCA were the same as those described for n-hexyl-N-
iminodiacetic acid above. The product was poured in an E-flask, and
ca. 150 mL of 4 mol dm-3 hydrochloric acid was added to the reaction
mixture until a pH value of 4.5 was obtained. The solvent was
evaporated off, which was complicated by formation of a lot of foam
during the evaporation. The resulting product was dissolved and refluxed
in ethanol (99.7%). The sodium chloride salt precipitated out from the
warm ethanol and was filtered off. The filtrate was slowly cooled, and
crystals were formed. The crystals were filtered off, and a second crop
of crystals was collected. The third crop of crystals was dissolved in
water/ethanol (75/25), and the pH was adjusted from 6.1 to 1.82. It
was a clear solution after 1 h of reflux. The solution was slowly cooled.
No crystals had been formed after 2 h, and the solution was left
overnight. By the next day, nice flake-shaped crystals had formed. The
crystals were filtered off and washed with cold ethanol under nitrogen
atmosphere. The crystals were dried, first under vacuum and then in a
desiccator.
Experimental Section
Chemicals. Methyl-N-iminodiacetic acid (Aldrich, 99%), perchloric
acid 70-72% (Merck, p.a.), sodium hydroxide solution (Merck,
Titrisol), pyrene (Aldrich, twice recrystallized from ethanol), and ethanol
(Kemetyl, 99.5%, spectroscopic grade) were used as purchased, and
all water used in this study has been deionized and Milli-Q-filtered.
Analyses of Methyl-N-iminodiacetic Acid. Elemental analysis of
methyl-N-iminodiacetic acid: C, calc. 40.82%, exp. 40.5, 40.6%; H,
calc. 6.15%, exp. 6.1, 6.2%; N, calc. 9.52%, exp. 9.4, 9.4%. The
elemental analyses were performed by Mikrokemi AB, Uppsala. The
methods used are described at http://www.mikrokemi.se.
Mass spectroscopy: Calc. 147.131 g mol-1; exp. 147.051 g mol-1
.
1H NMR data in Me2SO-d6: δ 2.38 (3H), δ 3.34 (4H); see Table
S1 for details.
(1) Silverstein, K. A. T.; Haymet, A. D. J.; Dill, K. A. J. Am. Chem. Soc.
2000, 122, 8037.
(2) Bra¨nde´n, C.-I.; Tooze, J. Introduction to Protein Structure; Garland
Publishing: New York and London, 1991.
(3) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; Wiley-VCH: New
York, 1999.
(4) Shan, S.; Loh, S.; Herschlag, D. Science 1996, 272, 97.
(5) Kumar, G. A.; McAllister, M. A. J. Am. Chem. Soc. 1998, 120, 3159.
(6) Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. Chem.-Eur. J. 1996, 2, 925
and references therein.
Analyses. Elemental analysis of n-dodecyl-N-iminodiacetic acid: C,
calc. 63.76%, exp. 63.1, 63.2%; H, calc. 10.37%, exp. 10.3, 10.3%; N,
calc. 4.64%, exp. 4.6, 4.6%.
(7) Guthrie, J. P. Chem. Biol. 1993, 3, 163.
(8) Pan, Y.; McAllister, M. A. J. Am. Chem. Soc. 1998, 120, 166. Perrin, C.
L. Science 1994, 266, 1665. Perrin, C. L.; Thoburn, J. D. J. Am. Chem.
Soc. 1992, 114, 8559.
Mass spectroscopy: Calc. 301.426 g mol-1; exp. 301.222 g mol-1
.
1H NMR data in Me2SO-d6: δ 0.85 (3H), δ 1.23 (18H), δ 1.36 (2H),
δ 2.60 (2H), δ 3.39 (4H); see Table S1 for details.
(9) Gerlt, J. A.; Gassman, P. G. Biochemistry 1993, 32, 11943; J. Am. Chem.
Soc. 1993, 115, 11532.
(10) Cleland, W. W.; Kreevoy, M. M. Science 1994, 264, 1887.
(11) Chen, J.; McAllister, M. A.; Lee, J. K.; Houk, K. N. J. Org. Chem. 1998,
63, 4611.
n-Octadecyl-N-iminodiacetic Acid. First 99.54 g (0.369 mol) of
n-octadecylamine was dissolved in 295 cm3 of IPA and charged to the
9
3632 J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003

Alkyl-N-iminodiacetic Acids
A R T I C L E S
Table 1. Summary of the Acidic Constants of Methyl-, n-Hexyl-,
and n-Octadecyl-N-iminodiacetic Acid (CnH2n+1-IDA, n Is Number
of Carbon Atoms in the Aliphatic Chain) in Aqueous System
(Temperature Was 295 ( 1 K)
hydroxide and supporting electrolyte. The alkaline solutions of methyl-,
n-hexyl-, and n-dodecyl-N-iminodiacetate were prepared from degassed
water, and the solutions were kept under nitrogen atmosphere. A 0.4
mmol dm-3 suspension of n-octadecyl-N-iminodiacetic acid was
prepared by ultrasonication during 1 h. The solution is turbid in the
investigated pH range 2-13, and it seems obvious that the n-octadecyl-
N-iminodiacetic acid is present as aggregates. These turbid solutions
were titrated with both sodium hydroxide and perchloric acid solutions.
The titrations showed that the stoichiometry of weighed product is
correct and that all iminodiacetic acid groups in the aggregates are in
contact with water, and in equilibrium with acids, bases, and other
particles they may react with. Heavy stirring was necessary to maintain
an aqueous phase of evenly dispersed aggregates; otherwise a slow
accumulation of an amorphous phase took place at the bottom of the
vessel.
Fluorescence Measurements. A stock solution of 5.0 × 10-7 mol
dm-3 of aqueous pyrene was prepared; this is below the solubility of
pyrene in water. n-Hexyl-N-iminodiacetic acid was dissolved in the
pyrene stock solution by means of ultrasonication close to isoelectric
pH. Aqueous solutions of n-dodecyl-N-iminodiacetate at neutral pH,
6.1-7.9, were prepared by dissolving monosodium n-dodecyl-N-
iminodiacetate in the aqueous pyrene stock solution. Alkaline solutions
of n-dodecyl-N-iminodiacetate, pH ) 10.8-13, were prepared by
dissolving n-dodecyl-N-iminodiacetic acid in a 4.5 × 10-7 mol dm-3
of pyrene and 0.10 mol dm-3 of sodium hydroxide stock solution by
ultrasonication. Lower concentrations of n-hexyl-iminodiacetic acid,
1.5-150 mmol dm-3, and neutral and alkaline n-dodecyl-N-imino-
diacetic acid, 0.1-200 and 0.01-200 mmol dm-3, respectively, were
prepared by dilution of the pyrene-containing stock solutions. The
samples where kept in darkness to prevent photodegradation.
Potentiometry. In the acid-base titrations of the alkyl-N-imino-
diacetic acid systems in aqueous solution, the pH was recorded by an
Orion Research EA 940 potentiometer with a Mettler Toledo InLab422
electrode or an Orion 720A potentiometer with an Orion 9102AP pH
electrode. Calibrations were performed with Orion standards at pH 4.01,
7.00, and 10.01. All measurements were performed at ambient room
temperature, 295 ( 1 K. The titrator solution was manually added from
a digital buret (Brand Bu¨rette Digital II or Metrohm 665 Dosimat).
Titrations of the aqueous systems of n-octadecyl-N-iminodiacetic acid
showed that the time to reach a stable pH reading was ca. 15 min. up
to the second half-equivalence point and above the second equivalence
point, while ca. 2 h was required for the range in between. The time to
reach stable pH values was significantly faster for the other alkyl-N-
iminodiacetic acid systems, but with the observation that the slowest
reading up to 45 min was in the region between the second half-
equivalence and equivalence points in n-dodecyl-N-iminodiacetic acid.
The acidic constants were calculated from pH titration data by means
of the program MAINPH.12
pK
a1
pK
a2
pK
a3
ionic medium
MIDA
1.82(5)
1.55(4)
2.22(2)
2.37(2)
7.61(9)
6.97(8)
6.04(4)
5.72(4)
5.64(10)
9.59(1)
10.412(7)
9.45(12)
9.4(2)
10.65(14)
9.92(8)
0.1 M NaClO4
0.1 M NaClO4
no supp. electrolyte
0.01 M NaClO4
0.1 M NaClO4
0.5 M NaClO4
1.5 M NaClO4
C6-IDA
C18-IDAa
9.48(16)
a Present as aggregates.
reactor described above. Next 88.19 g (0.757 mol) of SMCA, dissolved
in 100 cm3 of deionized water, was added during a period of 1.5 h at
a constant pH of 8.5 adjusted by the addition of 30% sodium hydroxide
from a Dosimat. The length of the postreaction, the purification and
concentration of the product, and the hydrolysis of excess SMCA were
the same as those described for n-hexyl-N-iminodiacetic acid above.
The product precipitated together with sodium chloride, and the bottom
phase of sodium chloride and product was separated. The product/salt
mixture was acidified in ethanol/water by adding 5 mol dm-3 of
hydrochloric acid. The addition of acid was stopped at pH 3 as the
product precipitated. The solvent was removed, and the product was
recrystallized in dichloromethane and 2-propanol.
The product was finally dissolved in ethanol (99.7%). At 333-343
K, the product appeared as a cloudy upper phase, and sodium chloride
precipitated in a bottom phase. The product was filtered warm, the
sodium chloride was filtered off, and the solvent was finally evaporated
off from the product.
Analyses. Elemental analysis of n-octadecyl-N-iminodiacetic acid:
C, calc. 68.53%, exp. 67.6, 68.0, 68.3%; H, calc. 11.24%, exp. 11.2,
11.2, 11.3%; N, calc. 3.63%, exp. 3.6, 3.6, 3.7%.
Mass spectroscopy: Calc. 385.5877 g mol-1; exp. 385.3204 g mol-1
1H NMR data in Me2SO-d6: δ 0.86 (3H), δ 1.22 (30H), δ 1.35 (2H),
δ 2.60 (2H), δ 3.41; see Table S1 for details.
Synthesis of Hydrogenbis(methyl-N-iminodiacetic Acid) Perchlo-
rate, [(CH3-N(CH2-COOH)2H]ClO4. Methyl-N-iminodiacetic acid
was dissolved in 1.0 mol dm-3 of perchloric acid in the stoichiometric
ratio 1:1 giving a pH ≈ 0.50. Water was gently evaporated off, and
crystals were formed.
Elemental analysis of hydrogenbis(methyl-N-iminodiacetic acid)
perchlorate: C, calc. 30.43%, exp. 30.4, 30.5%; H, calc. 4.85%, exp.
4.8, 4.9%; N, calc. 7.10%, exp. 7.0, 7.1%.
The mass spectroscopy spectrum of hydrogenbis(methyl-N-imino-
diacetic acid) perchlorate shows the exact same fragmentation pattern
as methyl-N-iminodiacetic acid (Figure S7a). It therefore was not
possible to determine the molecule weight of hydrogenbis(methyl-N-
iminodiacetic acid) perchlorate by mass spectroscopy.
Preparation of Solutions. pH Titrations. All solutions prepared
had the ionic strength 0.100 mol dm-3 using sodium perchlorate as
supporting electrolyte, except for the n-octadecyl-N-iminodiacetic acid
aqueous systems where the ionic strength was varied (Table 1).
Solutions of neutral methyl- and n-hexyl-N-iminodiacetic acids were
titrated with both 0.100 mol dm-3 of sodium hydroxide and 0.100 mol
dm-3 of perchloric acid, and alkaline solutions of methyl- and n-hexyl-
N-iminodiacetate were titrated with 0.100 mol dm-3 of perchloric acid.
The solutions of methyl- and n-hexyl-N-iminodiacetic acid contained
20.0 mmol dm-3 of neutral iminodiacetic acid and 80.0 mmol dm-3 of
supporting electrolyte, and the alkaline solutions contained 20 mmol
dm-3 of iminodiacetic acid and 80 mmol dm-3 of sodium hydroxide.
The acidic n-dodecyl-N-iminodiacetic acid aqueous systems contained
1.0 and 5.0 mmol dm-3 of iminodiacetic acid, 2 or 3 equiv of perchloric
acid and supporting electrolyte, and the alkaline solutions contained
20, 5, 1, and 0.5 mmol dm-3 of iminodiacetic acid, 4 equiv of sodium
.
Single-Crystal X-ray Diffraction. Data were collected on a Bruker
Smart Platform equipped with a 1k CCD area detector13 and a graphite
monochromator using a fine-focus sealed X-ray tube generating Mo
KR (λ ) 0.71073 Å) radiation. The crystal to detector distance was 50
mm. The temperature was 295 ( 1 K. A hemisphere of data (1271
frames) was collected for each structure using the omega scan method
(0.3° frame width). The first 50 frames were remeasured at the end of
each data collection to monitor crystal and instrument stability. Intensity
decay was negligible for all crystals in this study. The structures were
solved by the direct methods in Bruker SHELXTL14 and refined using
full-matrix least squares on F 2. Non-hydrogen atoms were treated
anisotropically. The ammonium and acetic acid hydrogen atoms in
methyl-N-iminodiacetic acid were located in the difference map, while
(12) Sandell, A., personal communication; Physical Chemistry 1, Chemical
Center, Lund University, Sweden. The program is available from the
corresponding author of this paper.
(13) Bruker SMART and SAINT, area detector control and integration software,
Bruker Analytical X-ray Systems, Madison, WI, 1995.
(14) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
9
J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003 3633

A R T I C L E S
Ha¨ggman et al.
Table 2. Crystallographic Data of the Alkyl-N-iminodiacetic Acids Determined in This Study
chemical formula
formula wt (g mol-1
space group
a (Å)
b (Å)
c (Å)
C5H9NO4
147.13
C10H19NO4
217.26
C16H31NO4
301.42
C22H43NO4
385.57
Pbca (No. 61)
8.60(1)
10.07(2)
56.95(9)
90
90
90
4932(23)
8
0.71073
295
C10H19ClN2O12
394.720
)
Pbca (No. 61)
9.291(6)
10.469(7)
13.489(8)
90
Pbca (No. 61)
8.412(2)
9.589(2)
28.063(7)
90
Pbca (No. 61)
8.3774(17)
9.7376(19)
41.525(8)
90
P1h (No. 2)
5.5253(15)
7.169(2)
10.823(3)
81.321(5)
88.518(5)
76.042(5)
411.28(19)
1
0.71073
295
1.594
3.00
R (deg)
â (deg)
90
90
90
90
90
90
γ (deg)
vol (Å3)
1213.0(14)
8
0.71073
295
1.490
1.30
0.1947
0.0614, 0.1286
0.0996, 0.1421
2263.5(10)
8
0.71073
295
1.275
0.98
0.1325
0.0519, 0.1019
0.1045, 0.1161
3387.4(12)
8
0.71073
295
1.182
0.83
0.1493
0.0601, 0.1130
0.1307, 0.1299
formula units Z
λ (Å)
T (K)
F
calc (g cm-3
)
µ (cm-1
)
Rint
0.0244
0.0666, 0.1764
0.1079, 0.1907
R indices R1, wR2 [I > 2σ(I)]a
R indices R1, wR2 [all data]b
2
a Definition of R: SHELXTL, R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑(w|Fo|)2]1/2; SHELXL, R1 ) ∑||Fo| - |Fc||/∑|Fo. b wR2 ) (∑[w(Fo
2
4
- Fc )2]/∑[[wFo ])1/2
.
in the other structures the acetic acid protons were calculated in
estimated idealized positions on the basis of the relationship between
O-H bond distances and ν(O-H) stretching vibrations.6 The hydrogen
atoms in the alkyl chains were calculated in ideal positions riding on
their respective carbon atom. Crystal and experimental data are
summarized in Table 2.
Steady-State Fluorescence Measurements. The steady-state fluo-
rescence measurements were performed on a SPEX Fluorolog 1680
combined with a SPEX Spectroscopy Laboratory Coordinator DM1B.
The excitation wavelength was set to 337 nm, and the emission was
collected at front face, typically in the range 360-560 nm. The
background emission due to solvent and alkyl-N-iminodiacetic acid was
subtracted by pyrene-free solution spectra.
using a direct probe with evaporation of the samples into the ion source
kept at ca. 475 K and an electron beam energy of 70 eV. High-resolution
(HREI) measurements were made at a resolution of 5000 and using
perfluorokerosene as internal standard.
NMR Spectroscopy. 1H NMR spectra were recorded on a DRX400
Bruker Avance spectrometer operating at 400.13 MHz and 303 K. A
pulse angle of 30° was used in all experiments, the pulse repetition
time was 5.0 s, and the number of scans was 32. Deuterated dimethyl
sulfoxide (Me2SO-d6, 99.9%), ethanol (C2D5OD-d6, 99.9%), and water
(D2O-d2, 99.9%) were used as solvents.
Results
Vibration Spectroscopy. The IR spectra of the solid alkyl-
N-iminodiacetic acids in the neutral zwitterionic form are given
in Figures 1 and S1. The C-H stretching vibrations in the
aliphatic chain fall in the region 3000-2850 cm-1, and
absorption bands in the region 3700-3100 cm-1 are ordinarily
due to various O-H and N-H stretching vibrations.20,21
However, strong hydrogen bonding tends to broaden the peaks
and move them toward lower wavenumbers.6 The crystal-
lographic characterization of the alkyl-N-iminodiacetic acids
shows the presence of a short strong intermolecular hydrogen
bond, see below.
The aggregational behavior of the n-hexyl- and n-dodecyl-N-
iminodiacetic acids was examined using steady-state fluorescence
spectroscopy of the emission from pyrene in aqueous solutions of alkyl-
N-iminodiacetic acids. The dissolution of pyrene in micelles is well
established, as well as its different spectrophotometric behavior in
hydrophobic and hydrophilic environments.15,16 From the changes in
the total emission intensity in the region 360-560 nm,17 and the pyrene
monomer bands, first and third vibrational peaks, at ca. 383 and 372
nm, respectively, conclusions can be drawn of the critical micelle
concentration.18 The excited pyrene might also be quenched by a
nonemissive deactivation process by another quenching molecule.19 The
Stern-Volmer equation for collisional quenching is given as Io/I ) 1
+ KD[Q], where Io and I are the fluorescence intensities of the
unquenched and quenched fluorophore, respectively, KD is the Stern-
Volmer quenching constant, and [Q] is the quencher concentration.
Vibration Spectroscopy. The infrared spectra of the solid samples
were recorded at room temperature for characterization. A Perkin-Elmer
1720 X spectrometer with DTGS detector and Mylar beam splitter was
used to record the mid-IR spectra in the range 400-4000 cm-1. The
solid samples were measured as KBr pastilles containing 5-10 wt %
sample. In each spectrum, 200 scans were collected and averaged at 4
cm-1 resolution.
It has been shown that there is a semiquantitative relationship
between the stretching frequency ν(O-H) and the O-H and
O(-H)‚‚‚O distances in a hydrogen bond.6 According to this
relationship, the observed oxygen-oxygen distances around 2.46
Å in the alkyl-N-iminodiacetic acids should be in conformity
with a ν(O-H) stretching frequency down to 1200-750 cm-1
The spectra of n-hexyl-, n-dodecyl-, and n-octadecyl-N-imino-
diacetic acid show a broad unsymmetric peak at ca. 730 cm-1
.
,
which consists of a fairly sharp band at ca. 730 cm-1 and broader
band at ca. 720 cm-1, as seen from the second derivatives of
the spectra (Figure S2). The band at 730 cm-1 is assigned to
unbranched alkyl chains longer than four methylene groups,21
and the 720 cm-1 band is assigned to the ν(O-H) stretching
vibration (Figures 1, S1, and S2). There is also a very broad
Mass Spectroscopy. Mass spectra were obtained using MS-1 of a
JEOL JMS-SX/SX102A (JEOL, Japan) tandem mass spectrometer
operating at an accelerating voltage of 10 kV and equipped with JEOL’s
XMS software. Electron ionization (EI) mass spectra were recorded
(15) Almgren, M.; Griezer, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101,
279.
(20) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic Chemistry,
3rd ed.; McGraw-Hill Book Company (U.K.) Limited: London, 1980;
Chapter 2. Criddle, W. J.; Ellis, G. P. Spectral and Chemical Characteriza-
tion of Organic Compounds, 2nd ed.; John Wiley & Sons: Chichester,
1980; Chapter 2.
(21) Skoog, D. A. Principles of Instrumental Analysis, 3rd ed.; CBS College
Publishing: Philadelphia, 1985; p 349.
(16) Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984, 88, 1098.
(17) (a) Zhao, C.-L.; Winnik, M. A.; Riess, G.; Croucher, M. D. Langmuir 1990,
6, 514. (b) Wang, Y.; Winnik, M. A. Langmuir 1990, 6, 1437.
(18) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
(19) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press:
New York, 1983.
9
3634 J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003

Alkyl-N-iminodiacetic Acids
A R T I C L E S
Figure 2. Structure of zwitterionic n-dodecyl-N-iminodiacetic acid, polymer
form.
Table 3. Hydrogen Bond Distances (Å) and Angles (deg) of
Alkyl-N-iminodiacetic Acids (CnH2n+1-IDA, n Is Number of Carbon
Atoms in the Aliphatic Chain) Obtained from the Crystallographic
Study
compound
D−H‚‚‚A
d(D−H) d(H‚‚‚A) d(D‚‚‚A)
(DHA)
Figure 1. Infrared spectra, 2000-500 cm-1, of solid methyl-, hexyl-,
dodecyl-, and octadecyl-N-iminodiacetic acids and solid hydrogenbis(methyl-
N-iminodiacetic acid) perchlorate.
CH3-IDA
N(1)-H(6)‚‚‚O(2)#1 0.98(4) 2.03(4) 2.857(4) 141(3)
O(4)-H(44)‚‚‚O(2)#2 1.08(6) 1.45(6) 2.480(4) 157(4)
C6H13-IDA N(1)-H(16)‚‚‚O(1)#2 1.10(5) 1.72(5) 2.782(5) 160(4)
O(4)-H(44)‚‚‚O(2)#1 1.08(-)a 1.42(-) 2.464(4) 160
band just below 1200 cm-1 which may be assigned as the
ν(O-H) stretching vibration. In the case of methyl-N-imino-
diacetic acid, the ν(O-H) stretching band seems to be shifted
toward an even lower wavenumber, 698 cm-1, the lowest value
reported so far. This may indicate that the hydrogen bonding
in methyl-N-iminodiacetic acid is somewhat stronger than that
in the alkyl-N-iminodiacetic acids with longer alkyl chains.
Somewhat higher ν(O-H) stretching wavenumbers are found
for hydrogenbis(methyl-N-iminodiacetic acid) perchlorate, 789
cm-1, and n-octadecyl-N-iminodiacetic acid hydrochloride, 794
cm-1, as well as for ethylenediaminetetraacetic acid, 802 cm-1
(Figure S2).6
Crystal Structures. Description of the Crystal Structures
of Alkyl-N-iminodiacetic Acids. The crystal structures of the
methyl-, n-hexyl, and n-dodecyl-N-iminodiacetic acids have
been determined crystallographically, while it has only been
possible to accurately determine the unit cell parameters of
n-octadecyl-N-iminodiacetic acid; the crystallographic data are
summarized in Table 2. All compounds form colorless crystals,
with those of methyl-N-iminodiacetic acid as blocks, and those
of n-hexyl-N-iminodiacetic acid as prisms or octahedrons, while
the crystals of n-dodecyl- and n-octadecyl-N-iminodiacetic acid
are very thin and fragile flakes. The single-crystal X-ray
diffraction study has shown that the alkyl-N-iminodiacetic acids
crystallize in their zwitterionic form with one carboxylic acid
group deprotonated and the nitrogen protonated into an am-
monium ion. The molecules are oriented with the iminodiacetic
acid groups in the a-b plane and the alkyl chain parallel with
the c-axis. This causes the length of the c-axis to increase
linearly with increasing alkyl chain length according to c(Å) ≈
13.3 + 2.4n, where n is the number of carbons in the saturated
linear alkyl chain.
C
12H25-IDA N(1)-H(16)‚‚‚O(1)#1 1.08(6) 1.75(6) 2.792(5) 162(4)
O(4)-H(44)‚‚‚O(2)#2 1.08(-)a 1.41(-) 2.455(6) 160
(CH3-IDA)2- N(1)-H(6)‚‚‚O(7)#2 0.95(5) 2.16(5) 2.930(8) 137(4)
HClO4
O(2)-H(22)‚‚‚O(2)#3 1.228(3)b 1.228(3) 2.456(6) 180.000(1)
O(4)-H(44)‚‚‚O(1) 1.00(2) 1.57(2) 2.558(5) 173(5)
a The position of the proton is estimated from the ν(O-H) stretching
vibration frequency, and the relationship between the O(-H)‚‚‚O distance,
O-H bond distance, and ν(O-H) stretching vibration frequency in ref 6.
b The position of the proton is forced to be in the center of symmetry, while
in reality the proton is equally distributed ca. 1.1 Å from one of the
carboxylic oxygens according to the ν(O-H) stretching vibration frequency.
The growth of the crystals into thin and brittle flakes directly
reflects the layered lamellar buildup of the structure. The basic
building block is a polymer chain of zwitterionic units linked
together with an exceptionally short and strong intermolecular
hydrogen bond between two carboxylic groups, one protonated
and one unprotonated, d(O(-H)‚‚‚O) ) 2.455-2.480 Å (Figure
2 and Table 3). The proton in this strong hydrogen bond was
localized in methyl-N-iminodiacetic acid giving an O-H bond
distance of 1.08(6) Å, and an O-H‚‚‚O angle of 157(4)° (Table
3). The very strong hydrogen bond in the methyl-N-imino-
diacetic acids is further supported by the very low wavenumber
of the ν(O-H) stretching frequency, 698 cm-1 (Figure 1). The
results from methyl-N-iminodiacetic acid are perfectly in line,
as an extreme point, with the correlation between O(-H)‚‚‚O
distances, O-H bond distances, and ν(O-H) stretching fre-
quencies given by Bertolasi et al.6 It has, however, not been
possible to localize the hydrogen position in the strong hydrogen
bond in n-hexyl- and n-dodecyl-N-iminodiacetic acid. The
O(-H)‚‚‚O distances close to 2.46 Å and the very low
wavenumbers of the ν(O-H) stretching frequency, ca. 720
cm-1, show that the O-H bond distance in these compounds
9
J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003 3635

A R T I C L E S
Ha¨ggman et al.
Figure 3. Structure of n-hexyl-N-iminodiacetic acid, packing model seen
along the c-axis. The hydrogen bonds are marked with dashed lines.
is very close to that observed in methyl-N-iminodiacetic acid,
and the carboxylic acid hydrogen position has been placed
accordingly.
The hydrogen bond in the solid alkyl-N-iminodiacetic acids
is among the shortest and strongest ever observed and is of the
(-)CAHB (negative charge assisted hydrogen bond) type
according to the classification made by Bertolasi et al.6 The
bond order in the two parts of the hydrogen bond can be
estimated from the O-H bond distance and/or the ν(O-H)
stretching vibration using a formula given by Bertolasi et al.6
These bond orders have been calculated to ca. 0.65 and 0.35
for the O-H and H‚‚‚O bonds, respectively, which is in total
agreement with previous observations and calculations.6
The polymer chains of zwitterionic iminodiacetic acid units
are cross-linked via other relatively strong hydrogen bonds
between an ammonium group and an oxygen in a carboxylate
group in a neighboring polymer chain of alkyl-N-iminodiacetic
acid to form a two-dimensional network stabilizing the lamellar
sheet structure, d(N(-H)‚‚‚O) ) 2.782-2.857 Å (Figure 3,
Table 3). It is possible to distinguish the carboxyl hydroxo and
carbonyl oxygen atoms in the carboxylic acid groups from the
C-O bond distances, because they are significantly shorter in
the carbonyl groups than in the carboxyl hydroxo groups (Tables
S3, S5, and S7).
Figure 4. Structure of n-hexyl-N-iminodiacetic acid seen along the b-axis.
The alkyl chains are packed in such a way that two adjacent
alkyl chains in a polymer chain are closer together in a “pair”
to give space for another interdigitating “alkyl chain pair”
coming from the opposite direction in the double sheet (Figure
4). The “paired” alkyl chains have a dumbbell shape, clearly
seen in dodecyl-N-iminodiacetic acid (Figure S3), to optimize
the van der Waals forces, as the distance between two nitrogen
anchors is longer than the ideal van der Waals distance between
two alkyl chains. A mismatch between the imino-N-diacetic acid
and the alkyl chain areas causes a tilted structure (Figures 5
and S4).3 By viewing the alkyl chains along the a-axis,
perpendicular to the polymer chain, we find that the alkyl chains
are arranged with every other chain coming from above and
below, respectively (Figures 5 and S4).
The very strong hydrogen bonds along the alkyl-N-imino-
diacetic acid polymer chain, the hydrogen bonds cross-linking
these polymer chains, and the very efficient packing of the alkyl
chains at van der Waals distance make the structure strong and
rigid in the a-b plane (Figures 2 and 4). The forces parallel
with the c-axis between two neighboring bilayers are due to
weak van der Waals interactions (Figure 4). The crystal growth
Figure 5. Structure of n-hexyl-N-iminodiacetic acid seen along the a-axis.
directions are certainly in the a-b plane. In the alkyl-N-
iminodiacetic acids with a short alkyl chain, the stabilization
of the layers is weaker, and no tendency to form flake-shaped
crystals is observed. It shall, however, be stressed that the
principal buildup of the crystals of alkyl-N-iminodiacetic acids
is maintained also for analogues with short alkyl chains. The
atomic parameters and selected bond distances and angles are
summarized in Tables S2-S7.
Hydrogenbis(methyl-N-iminodiacetic Acid) Perchlorate.
The hydrogenbis(methyl-N-iminodiacetic acid) perchlorate crys-
tallizes as rhombohedrally shaped colorless crystals in the
triclinic space group P1h (No. 2). The principal buildup of the
structure is similar to the methyl-N-iminodiacetic acid but with
9
3636 J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003

Alkyl-N-iminodiacetic Acids
A R T I C L E S
Figure 6. Structure of hydrogenbis(methyl-N-iminodiacetic acid) perchlo-
rate, dimeric form.
Figure 8. pH titrations of alkyl-N-iminodiacetic acids in aqueous solutions.
The arrows show the titration direction. (a) 0.4 mmol dm-3 of n-octadecyl-
N-iminodiacetic acid (no supporting electrolyte), (b) 0.4 mmol dm-3 of
n-octadecyl-N-iminodiacetic acid (I ) 0.1), (c) 1.0 mmol dm-3 of dodecyl-
N-iminodiacetic acid (no supporting electrolyte), (d) 1.0 mmol dm-3 of
dodecyl-N-iminodiacetic acid (I ) 0.1), (e) 20 mmol dm-3 of hexyl-N-
iminodiacetic acid (I ) 0.1), (f) 10 mmol dm-3 of methyl-N-iminodiacetic
acid (I ) 0.1).
experimentally obtained pH titration functions did follow theory
using the ion product of water at the applied ionic strength and
medium, 0.1 mol dm-3 of sodium perchlorate, pKw ) 13.79.22
However, some data points immediately before the second
equivalence point had to be deleted for a good fit (Figure 8).
The solutions of alkyl-N-iminodiacetic acids with short alkyl
chain were clear before, during, and after the titrations, the
n-dodecyl-N-iminodiacetic acid solutions were turbid up to ca.
pH 7, but above the second equivalence point they were clear,
while the n-octadecyl-N-iminodiacetic acid solutions were turbid
throughout the titrations. The correct stoichiometry in the
titrations with alkyl-N-iminodiacetic acids in aggregate form
shows that the aggregates most probably consist of individual
hydrated bilayers and that the mean size of the aggregates seems
to increase with increasing alkyl chain length, thus increasing
the mechanical stability of the bilayers.
Figure 7. Structure of hydrogenbis(methyl-N-iminodiacetic acid) perchlo-
rate, packing model seen along the c-axis. The hydrogen bonds are marked
with dashed lines.
formation of dimers held together with a very strong hydrogen
bond between the carboxylate groups in two zwitterionic units
sharing an extra proton, d(O(-H)‚‚‚O) ) 2.456(6) Å (Figure 6
and Table 3). The position of the proton is forced to be at the
center of symmetry, while in reality the proton is equally
distributed ca. 1.08 Å from one of the carboxylic oxygens
according to the ν(O-H) stretching vibration frequency and the
correlation given by Bertolasi et al.6 The hydrogen bond linking
the two zwitterionic methyl-N-iminodiacetic acid units is indeed
very strong as seen by the very small wavenumber of the
ν(O-H) stretching vibration, 789 cm-1 (Figures 1 and S2b).
The hydrogen bond distance O(4)-H(44)‚‚‚O(1) linking the
dimers is fairly short and strong, 2.558(5) Å (Table 3).
Furthermore, there is a relatively weak hydrogen bond between
the protonated nitrogen and a perchlorate oxygen, d(N(-H)‚‚‚O)
) 2.930(8) Å, holding the layers of hydrogenbis(methyl-N-
iminodiacetic acid) dimers and the interlaying perchlorate ions
together in the three-dimensional lattice (Figure 7). All hydrogen
bond distances are summarized in Table 3, and the atomic
parameters and selected bond distances and angles are sum-
marized in Tables S8 and S9.
The pH titrations of methyl-N-iminodiacetic acid from low
pH with sodium hydroxide and from high pH with perchloric
acid are in complete coherence. Also, the titrations of the
n-hexyl-N-iminodiacetic acid system are also in coherence with
the exception of a somewhat smeared-out region below the
second equivalence point around pH 6. The acidic constants
of the methyl- and hexyl-N-iminodiacetic acids are as those
expected for an iminodiacetic acid,23 pKa1 ) 1.7 and pKa2
)
Potentiometry. The pH titrations of the methyl-, n-hexyl-,
n-dodecyl, and n-octadecyl-N-iminodiacetic acid systems were
straightforward, and the calculations of the acidic constants from
(22) Maeda, M.; Hisada, O.; Kinjo, Y.; Ito, K. Bull. Chem. Soc. Jpn. 1987, 60,
3233.
(23) IUPAC Stability Constants Database; Academic Software: Sourby Old
Farm, Timble, Otley, Yorks, LS21 2PW, United Kingdom.
9
J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003 3637

A R T I C L E S
Ha¨ggman et al.
2.3, and with a pKa3 value for ammonium group of 9.6 and
10.4 for methyl- and n-hexyl-N-iminodiacetic acid, respectively
(Table 1).
The titrations with n-octadecyl-N-iminodiacetic acid with
sodium hydroxide were performed at different ionic strength
to study how an aggregated system is affected by the presence
of salt. Titrations with acid have only been performed at the
standard ionic medium in this study, 0.1 mol dm-3 of sodium
perchlorate. The time to reach equilibrium is very long, ca. 2
h, and the reproducibility of the octadecyl-N-iminodiacetic acid
system is lower than that for the other systems studied. The
results show that the first acidic constant, Ka1, is very acidic
and any value could not be determined. The pH value of the
prepared solutions with the composition n-octadecyl-N-imino-
diacetic acid hydrochloride, [C18H37-N(CH2COOH)2]‚HCl, is
equal to the logarithm of the molar concentration of the salt;
thus, it behaves as a strong acid. The second acidic constant is
substantially less acidic, pKa2 ) 5.5-7.5 depending on the ionic
strength, than that in the analogue monomer. The acidity of the
ammonium group, Ka3, is in the same order of magnitude as
that in the analogue monomer, and it is less sensitive to ionic
strength than Ka2 (Table 1). The slight ionic strength dependence
on Ka3 may indicate the same kind of intermolecular hydrogen
bonding of the ammonium proton in n-octadecyl-N-imino-
diacetic acid in aqueous suspensions in a similar way as found
in the solid states of the alkyl-N-iminodiacetic acids (Table 1
and Figure 3).
Figure 9. Relative total intensities, I/Io (a-c), and III/I intensity ratios
(d-f) from the pyrene fluorescence spectrum versus concentration of alkyl-
N-iminodiacetic acid. (a) and (d) n-hexyl-N-iminodiacetic acid at isoelectric
pH; (b) and (e) n-dodecyl-N-iminodiacetate in neutral pH solutions; (c) and
(f) n-dodecyl-N-iminodiacetate in alkaline pH solutions. The III/I ratio of
aqueous pyrene is inserted for comparison (dashed line). The arrows show
corresponding y-axis.
enough to dissolve pyrene seen from the small but significant
difference in the III/I ratio. In the alkaline n-dodecyl-N-
iminodiacetate, the poor pyrene emission is not typical of that
found in a well-defined hydrophobic moiety, for example, in
micelles.
Changes in Total Emission Intensity. In the n-hexyl-N-
iminodiacetic acid system, the fluorescence emission is decreas-
ing with increasing concentration up to 100 mmol dm-3, and
thereafter it increases up to the solubility limit at ca. 150 mmol
dm-3 at pH 2.0. In terms of the Stern-Volmer equation, there
is linearity in Io/I up to 80 mmol dm-3 of n-hexyl-N-
iminodiacetic acid, suggesting a collisional quenching by the
monomeric form. The intensity is slightly increasing above 100
mmol dm-3, and the fine structure in the pyrene monomer
spectrum is well developed, which may indicate that aggregates
are formed. The negative slope from the Stern-Volmer equation
has not a physical interpretation but indicates that the local
environment is different and shielding the probe from quenching
and enhancing probe emission.
When examining the intensity variations with concentration
of n-dodecyl-N-iminodiacetate, we find that the intensity
decreases up to 10 mmol dm-3 in both neutral and alkaline
solutions and follows the Stern-Volmer equation for collisional
quenching in this concentration range. The emission above 10
mmol dm-3 of n-dodecyl-N-iminodiacetate differs completely
for the neutral and alkaline solutions. In neutral n-dodecyl-N-
iminodiacetate, there is an abrupt jump to intensities higher than
that for the unquenched aqueous pyrene, and a steadily
increasing gain in emission intensity with concentration is seen.
In alkaline n-dodecyl-N-iminodiacetate, the pyrene emission is
almost completely quenched. Thus, at neutral pH and concentra-
tions above 10 mmol dm-3, n-dodecyl-N-iminodiacetate is
mainly present in aggregate form, while it is impossible to
determine the state of aggregation in alkaline dodecyl-N-
iminodiacetate. The results from the steady-state fluorescence
experiments are displayed in Figure 9.
As there is some kind of equilibrium/distribution between
monomers and aggregates in the n-dodecyl-N-iminodiacetic acid
system in water, it is not possible to determine any acidic
constants as the concentrations of the monomeric and aggregate
forms are not known, and it can be seen from the other systems
that the acidic constants are strongly dependent on the state of
aggregation. The titration function of n-dodecyl-N-iminodiacetic
acid seems to be a mix of the titration functions of n-hexyl-
and octadecyl-N-iminodiacetic acid (Figure 8). The obvious
discontinuity in pH (Figure 8d) depends on partial precipitation
of the zwitterionic n-dodecyl-N-iminodiacetic acid, seen as a
significant turbidity. The precipitation in the titrations takes place
at decreasing pH with decreasing concentration of alkyl-N-
iminodiacetic acid.
Steady-State Fluorescence. III/I Ratios. When examining
the ratio of the first and third vibrational bands of the pyrene
monomer, III/I, in n-hexyl-N-iminodiacetic acid at pHie ≈ 2.0,
we find that the ratio is constant at 0.594(5) in the concentration
range 1.5-150 mmol dm-3 (Figure 9), revealing that pyrene
does not sense a polar environment different from that of water.
It does not exclude the possibility of aggregate formation, but
these then do not have sufficiently large hydrophobic dimensions
to dissolve pyrene.
The III/I ratio of pyrene in neutral and alkaline solutions of
n-dodecyl-N-iminodiacetate was examined. Up to ca. 1 mmol
dm-3 of n-dodecyl-N-iminodiacetate, the III/I ratio is constant
at 0.598(8) and thereafter is increasing up to 10 mmol dm-3
.
Above 10 mmol dm-3 in the neutral solutions of n-dodecyl-N-
iminodiacetate, the III/I ratio is constant at ca. 0.85, but in the
alkaline n-dodecyl-N-iminodiacetate solutions, the pyrene emis-
sion is almost completely quenched. Thus, at neutral pH and
higher concentrations than 10 mmol dm-3, n-dodecyl-N-
iminodiacetate seems to form hydrophobic aggregates large
Amounts of Alkyl-N-iminodiacetic Acid Possible To Dis-
solve in Water. The amounts of alkyl-N-iminodiacetic acid
possible to dissolve in water at different pH values have been
determined. It is possible to dissolve methyl-N-iminodiacetic
acid in molar concentrations, in water at the isoelectric pH (2.0).
On the other hand, only small amounts of methyl-N-imino-
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3638 J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003

Alkyl-N-iminodiacetic Acids
A R T I C L E S
Table 4. Fragmentation Pattern of Alkyl-N-iminodiacetic Acids
(CnH2n+1-IDA) Obtained in MS; n Is Number of Carbon Atoms in
the Aliphatic Chaina
solubility of n-dodecyl- and n-octadecyl-N-iminodiacetic acids,
13C NMR data are too noisy and weak to gain aggregational
1
information. The H NMR data are summarized in Table S1.
fragment
CH −IDA
3
C6H −IDA C12H −IDA C18H −IDA
13 25 37
-‚COOH
102(100) 172(100) 256(100) 340(100)
Discussion
-‚COOH-CO
-‚COOH-CO-CH2O
-‚COOH-CO-‚OH
-‚COOH-COO
-Cn-1H2n-1
74(45)
44(62)
57(54)
58(14)
146(0)
129(7)
144(2)
114(2)
127(8)
128(48)
146(62)
199(6)
228(1)
198(5)
211(2)
212(5)
146(68)
283(1)
312(1)
282(6)
295(20)
296(13)
146(55)
367(5)
The Crystal Structures. The short strong hydrogen bond,
causing a polymerization of the alkyl-N-iminodiacetic acids in
their zwitterionic form, has an amazing impact on self-
aggregation, crystallization, and physical-chemical behavior as
observed by several techniques. This hydrogen bond is of
negative charge assisted hydrogen bonding ((-)CAHB) char-
acter6 linking two molecules through a strong hydrogen bond
resulting in a very short O-(H)‚‚‚O distance, including a long
O-H distance as found from the crystallographic study of alkyl-
N-iminodiacetic acids, as well as from the extremely low
-H2O
a Molar weight and height (in parentheses) are given.
diacetic acid, low mmol dm-3 range, are possible to dissolve
in ethanol, thus giving a factor of around 1000 in difference
between water and ethanol.
The solubility of n-hexyl-N-iminodiacetic acid at pH 2.04,
close to the isoelectric point, is 0.15 mol dm-3, which at elevated
temperatures approaches the molar range, reflecting the entropy
effect. Addition of a half equivalent of sodium hydroxide (pH
2.59) increases the solubility to 0.23 mol dm-3. A highly viscous
gel phase is formed at concentrations exceeding 2.5 mol dm-3
and at room temperature. This gel phase is maintained above
pH 7 at similar concentrations.
The solubility of the neutral zwitterionic form of n-dodecyl-
N-iminodiacetic acid is in the submillimolar range. As for
n-hexyl-N-iminodiacetic acid, the amount of n-dodecyl-N-
iminodiacetic acid possible to dissolve increases dramatically
in solutions with neutral pH, ca. 1.4 mol dm-3, giving a stiff
gellike phase.
The amounts of n-octadecyl-N-iminodiacetic acid possible to
dissolve is in the submillimolar range in the entire pH range,
reflecting the dominating hydrophobic effect present due to the
long hydrocarbon chain. This is supported by the fact that the
amounts possible to dissolve in ethanol are higher than those
in water, while the opposite trend is observed for methyl-N-
iminodiacetic acid as mentioned above.
wavenumbers of the ν(O-H) stretching vibration, ca. 720 cm-1
,
as observed in the IR spectra (Figure 1). The alkyl chains in
the alkyl-N-iminodiacetic acids are highly ordered, being parallel
to each other in a polymer chain. The van der Waals forces
between the alkyl chains are fairly strong as the alkyl chains
are densely packed, giving them a dumbbell shape as the
distance between the nitrogen anchors is longer than the ideal
van der Waals distance between two alkyl chains (Figures 4
and S2).
It is evident that the primary ordering of polymer chains is
made into two-dimensional layers by cross-linking hydrogen
bonds. From the crystal structures, it is shown that the two-
dimensional structure is bilayers where the alkyl chains from
the layers are interdigitating each other and the zwitterionic
headgroups make up the surfaces of a very well-ordered lamellar
structure. The headgroups have the same structural form and
size independent of the length of the alkyl chain. The thickness
of the bilayer and the length of the c-axis are a direct function
of the length of the alkyl chain (vide infra).
pH Dependence on Aggregation Studied by Steady-State
Fluorescence. Aggregated structures of n-hexyl-N-iminodiacetic
acid at isoelectric pH are probably formed in water above 100
mmol dm-3 when interpreting the increase in the total emission
as shielding from quenching. However, the aggregates are not
thick enough to dissolve pyrene in a hydrophobic environment
because the III/I ratio does not shift to higher values. It is
concluded that the n-dodecyl-N-iminodiacetic acid system is
mainly monomeric up to ca. 1 mmol dm-3 where aggregation
takes place in the neutral solution as found from the high
emission intensity and the shifted III/I ratio at concentrations
above 10 mmol dm-3, which is not observed for the alkaline
n-dodecyl-N-iminodiacetic acid. In n-dodecyl-N-iminodiacetate
in alkaline solution, there is an extremely quenched pyrene
signal above 10 mmol dm-3, most possibly due to quenching
by the completely deprotonated dodecyl-N-iminodiacetate.
However, it is impossible to draw any definite conclusions as
quenching might occur in both unaggregated and aggregated
systems.
The low solubility of the neutral zwitterionic form at pHie
was used at the purification of the synthesized material. The
resulting products are stunningly pure as found from NMR and
mass spectroscopy, and elemental analyses.
Mass Spectroscopy. The obtained fragmentation patterns are
presented in Table 4 and Figure S7. In all mass spectra, the
largest peak is obtained when one carboxylate radical is split
off from the alkyl-N-iminodiacetic acid, and this peak is
normalized and set to a value of 100. The fragmentation pattern
of methyl-N-iminodiacetic acid is different in comparison to
the other alkyl-N-iminodiacetic acids, probably due to the shorter
hydrocarbon chain.
NMR Spectroscopy. 1H NMR measurements were performed
in the deuterated solvents dimethyl sulfoxide (Me2SO-d6),
ethanol (C2D5OD-d6), and/or water (D2O-d2). The spectrum of
n-octadecyl-N-iminodiacetic acid is lacking several expected
bands in water and in dimethyl sulfoxide as observed in the
other alkyl-N-iminodiacetic acid systems, probably due to low
solubility. Therefore, NMR data of n-octadecyl-N-iminodiacetic
acid are only presented in the ethanol, the solvent with
sufficiently high solubility of n-octadecyl-N-iminodiacetic acid.
The lowest chemical shifts are observed in the aprotic solvent
dimethyl sulfoxide, and the highest values are observed in the
protic ethanol. These results do not follow the expected trend
related to the polarity of solvents. Because of the very low
The fluorescence measurements show no synergy effects of
an oligomerization leading to lowered critical aggregation
concentrations. The observed critical aggregation concentrations,
ca. 100 mmol dm-3 for n-hexyl-N-iminodiacetic acid at iso-
electric pH and ca. 10 mmol dm-3 for neutral n-dodecyl-N-
iminodiacetic acid, are reasonable relative to the critical micelle
9
J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003 3639

A R T I C L E S
Ha¨ggman et al.
concentration of sodium n-hexyl sulfate (0.42 mol dm-3) and
sodium n-dodecyl sulfate (8.2 mmol dm-3).24
ultrasonication is required to get manageable concentrations,
then in aggregate form, for titration experiments. As the proton
stoichiometry in the aggregates is in accordance with the
weighed amounts of iminodiacetic acid, this strongly indicates
that the aggregates are present as individual bilayers where the
iminodiacetic acid/iminodiacetate surfaces are strongly hydrated.
For the n-dodecyl-N-iminodiacetic acid system, the steady-state
fluorescence studies show that the fraction monomer form
increases with pH and increasing charge of the headgroup. This
shows that there is a fine balance between hydration and
formation of monomers and aggregate formation with its
hydrogen bonds in the surface and the van der Waals forces
between the densely packed alkyl chains. The van der Waals
forces in the n-octadecyl-N-iminodiacetic acid system are
sufficiently strong to maintain the aggregate form, while for
n-hexyl-N-iminodiacetic acid the hydration is sufficiently strong
to break up the aggregates into monomers. As short strong
hydrogen bonds seem to exist in aqueous systems if they are
supported by, for example, a rigid structure as in the present
systems, it is reasonable to believe that they can also be present
in other strong and rigid structures as, for example, biomol-
ecules, if the structural backbone support is strong enough to
resist structural breakup due to hydration effects.
Protolytic Properties of Alkyl-N-iminodiacetic Acids in
Water. The determination of the acidic constants of the alkyl-
N-iminodiacetic acids in aqueous solution clearly shows two
principal cases. First, the short-chained alkyl-N-iminodiacetic
acids (methyl- and n-hexyl-N-iminodiacetic acid) are present
predominantly as monomers in aqueous solution. The other clear
principal case is the long-chained alkyl-N-iminodiacetic acids
(n-octadecyl-N-iminodiacetic acid, and to some extent also
n-dodecyl-N-iminodiacetic acid), which are present mainly as
aggregates, and the monomer concentration is negligible. The
alkyl-N-iminodiacetic acids behave in both cases as ordinary
three-protonic acids in titrations with strong acid or base but
with the exception that the kinetics to reach stable pH values
in solutions containing aggregates are very slow. However, the
results in the two cases are significantly different and will be
discussed separately: (a) The acidic constants of the methyl-
and n-hexyl-N-iminodiacetic acids (Table 1) are in coherence
with literature data, respectively,23 and the acidic constants are
those expected for compounds with ammonium and acetic acid
groups.23 (b) The first acidic constant, Ka1, of n-octadecyl-N-
iminodiacetic acid in aggregate form is more acidic than that
in monomeric forms of alkyl-N-iminodiacetic acids and acts as
a strong acid, while the second carboxylic acid proton,
participating in the short strong hydrogen bond, acts as a much
weaker acid than that in the analogue monomers (Table 1). This
strongly indicates that the short strong hydrogen bonds linking
the iminodiacetic acid units into polymer chains in the solid
state are maintained in aggregates in aqueous systems. It is very
difficult to protonate the carboxylate group participating in the
short strong hydrogen bond, and the acidity of this group, Ka1,
becomes very strong. The proton participating in the short strong
hydrogen bond, Ka2, is very much stabilized, and its acidity
properties decrease therefore substantially, ca. 105 times, in
comparison to the corresponding proton in alkyl-N-iminodiacetic
acids present as monomers (Table 1). Furthermore, the Ka2 value
is unusually dependent on ionic strength with the acidity
increasing with increasing ionic strength. It is well known that
the stability of colloids decreases with increasing ionic strength,25
and the increasing acidity of the strongly hydrogen bound proton
with increasing ionic strength is in line with this fact. The acidic
properties of the ammonium group, Ka3, seem to be fairly
independent of the state of aggregation of the alkyl-N-imino-
diacetic acids (Table 1).
Conclusions
The alkyl-N-iminodiacetic acids form exceptionally strong
intermolecular hydrogen bonds between neutral zwitterionic
units forming polymer chains in the solid state; the O(-H)‚‚‚O
distance is in the range 2.45-2.48 Å, the O-H bond distance
is ca. 1.08 Å, and the ν(O-H) stretching vibration is ca. 720
cm-1. A protonated dimer, hydrogenbis(methyl-N-iminodiacetic
acid), as perchlorate salt, has been structurally characterized in
the solid state. This structure has also a short strong hydrogen
bond as the neutral zwitterions of the alkyl-N-iminodiacetic
acids. The acidic properties of monomeric alkyl-N-iminodiacetic
acids (methyl and n-hexyl) follow those expected with pKa
values of about 1.7, 2.3, and 9.6/10.4. However, when present
as larger aggregates in aqueous solution (n-octadecyl- and partly
n-dodecyl-N-iminodiacetic acids), the first acidic constant is very
strong, behaves as a strong acid, and no acidic constant could
be determined, while the second acidic constant decreases
substantially, ca. 5 orders of magnitude. The acidity property
of the ammonium group, Ka3, is independent of the state of
aggregation. This strongly indicates that the lamellar structure
observed in the solid state most certainly remains in aggregates
in aqueous systems and that the proton in the extremely strong
hydrogen bond has weak acidic properties. This also shows that
short strong hydrogen bonds exist in aqueous systems if the
hydrogen bond is supported by a strong backbone structure, as
the long interdigitating alkyl chains are in bilayers of alkyl-N-
iminodiacetic acid systems.
These results show that short strong hydrogen bonds can be
present in aqueous systems if this hydrogen bond is supported
by a very strong backbone structure, in this case in the form of
long interdigitating alkyl chains and hydrogen bonds within the
surfaces of the aggregates consisting of iminodiacetic acid
groups. The shorter the alkyl chain is, the less stable the
interdigitating bilayer becomes. There are weak indications of
aggregate formation in n-hexyl-N-iminodiacetic acid, while it
is obvious in the n-dodecyl- and n-octadecyl-N-iminodiacetic
acid systems. The solubility of n-dodecyl- and n-octadecyl-N-
iminodiacetic acid in water is very low, and/or very slow, and
Acknowledgment. Akzo Nobel Surface Chemistry AB,
Stenungsund, Sweden, is gratefully acknowledged for financial
support. Sincere thanks go to Dr. Anders Cassel for his
continued interest and many important discussions, and to Mr.
Suresh Gohil for recording and analyzing the mass spectra and
Mr. Rolf Andersson for collecting NMR data. The Department
of Physical Chemistry at Uppsala University is kindly acknowl-
edged for putting their fluorescence spectrometers at our
disposal.
(24) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann,
I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80,
905.
(25) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press: Oxford,
1998; Chapter 20.9.
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3640 J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003

Alkyl-N-iminodiacetic Acids
A R T I C L E S
Supporting Information Available: Crystallographic data in
CIF format. Tables S1-S9 give the NMR data of the synthe-
sized alkyl-N-iminodiacetic acids, the fractional atomic coor-
dinates, and interatomic bond distances and angles of n-dodecyl-,
n-hexyl-, and methyl-N-iminodiacetic acid and hydrogenbis-
(methyl-N-iminodiacetic acid). Figure S1 shows the mid-infrared
spectra of n-dodecyl-, n-hexyl-, and methyl-N-iminodiacetic acid
and hydrogenbis(methyl-N-iminodiacetic acid) and n-octadecyl-
iminodiacetic acid hydrochloride, and with the second deriva-
tives in Figure S2. Figures S3-S6 show the structures of the
n-dodecyl-, n-hexyl-, and methyl-N-iminodiacetic acids along
a direction different from those in Figures 2-7. The fragmenta-
tion pattern in mass spectroscopy of solid methyl-, n-hexyl-,
n-dodecyl-, and n-octadecyl-N-iminodiacetic acid is given in
Figure S7 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA021012I
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J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003 3641