Synthesis, characterization and conductivity of quaternary nitrogen surfactants modified by the addition of a hydroxymethyl substructure on the head group

Article abstract of DOI:10.1007/s11743-012-1360-1

Two novel series of hydroxymethyl group-appended quaternary nitrogen surfactants (QNSs) based on the aliphatic N-alkyl-trimethylammonium and aromatic N-alkylpyridinium head groups were synthesized from the appropriate nitrogen head group precursor and 1-bromoalkane. The QNSs were characterized using ¹H and ¹³C nuclear magnetic resonance and infrared spectroscopy, and their purity confirmed using elemental analysis. The solution behavior of the QNSs was investigated by conductivity, assessing both the aggregation concentration as well as the amount of counter-ion dissociation. The results showed a general decrease in the aggregation concentration for the compounds with the hydroxymethyl addition, where the pyridinium compounds were more affected than the ammonium QNSs. In contrast, the extent of counter-ion dissociation (α) from the aggregate was slightly increased for the ammonium compounds but that of the pyridinium compounds was not generally affected by the structural modification.

Full text of DOI:10.1007/s11743-012-1360-1

J Surfact Deterg
DOI 10.1007/s11743-012-1360-1
ORIGINAL ARTICLE
Synthesis, Characterization and Conductivity of Quaternary
Nitrogen Surfactants Modified by the Addition
of a Hydroxymethyl Substructure on the Head Group
•
Deborah Jordan Eng Tan Dylan Hegh
•
Received: 15 November 2011 / Accepted: 18 April 2012
Ó AOCS 2012
Abstract Two novel series of hydroxymethyl group-
appended quaternary nitrogen surfactants (QNSs) based
on the aliphatic N-alkyl-trimethylammonium and aromatic
N-alkylpyridinium head groups were synthesized from the
appropriate nitrogen head group precursor and 1-bromoal-
kane. The QNSs were characterized using 1H and 13C nuclear
magnetic resonance and infrared spectroscopy, and their
purity confirmed using elemental analysis. The solution
behavior of the QNSs was investigated by conductivity,
assessing both the aggregation concentration as well as the
amount of counter-ion dissociation. The results showed a
general decrease in the aggregation concentration for the
compounds with the hydroxymethyl addition, where the
pyridinium compounds were more affected than the ammo-
nium QNSs. In contrast, the extent of counter-ion dissociation
(a) from the aggregate was slightly increased for the ammo-
nium compounds but that of the pyridinium compounds was
not generally affected by the structural modification.
pharmaceuticals for decades. An investigation was under-
taken to synthesize and observe the effect of adding a
hydroxymethyl group (CH2OH) to the head group of qua-
ternary nitrogen surfactants (QNSs). Small alterations to
affect the hydrophilicity of the head group have previously
been shown to modify a QNS’s solution behavior [1–3]. In
this work, we report the synthesis of two novel series of
aliphatic and aromatic QNSs containing the hydroxymethyl
addition (6–10 and 16–20, Fig. 1). For comparison, the
corresponding trimethylammonium and pyridinium QNSs
were also studied (1–5 and 11–15). To provide molecules
with a range of hydrophobicities, a progressive series of
alkyl tail lengths was investigated. Saturated straight even-
numbered chains from 8 to 16 carbons were chosen for a
predictable increase in hydrophobicity to the limit of water
solubility at room temperature. The syntheses of the QNSs
were designed to be high yielding, using either pyridine or
ammonia based head group compounds to effect nucleo-
philic substitution on the appropriate 1-bromoalkane. The
critical micelle concentrations (CMC) were measured
using conductivity to investigate differences in the QNSs
solution behavior to gauge any effect the hydroxymethyl
group may have on both the aggregation of the QNS
molecules and the extent of counter-ion binding to the
aggregate.
Keywords Synthesis ꢀ Cationic surfactants ꢀ Head groups ꢀ
Hydrophobicity ꢀ Aggregation
Introduction
Quaternary nitrogen based cationic surfactants have been
employed as detergents, antimicrobial agents and in
Experimental
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-012-1360-1) contains supplementary
material, which is available to authorized users.
Materials and Instruments
Head group compounds and 1-bromoalkane for QNS synthe-
ses, N-dodecyl-N,N,N-trimethylammonium bromide (13),
N-tetradecyl-N,N,N-trimethylammonium bromide (14),
N-hexadecyl-N,N,N-trimethylammonium bromide (15), solvents
D. Jordan (&) ꢀ E. Tan ꢀ D. Hegh
University of Otago, Chemistry 700 Cumberland Street,
Dunedin 9016, New Zealand
e-mail: djordan@chemistry.otago.ac.nz
123

J Surfact Deterg
Fig. 1 The structures of the
QNSs investigated highlighting
the hydroxymethyl addition to
the head group of the modified
QNSs. The QNSs will be
referred to in the main text as
the number in bold
Br
Br
R N
R
N
CH2OH
n = 6 (6), 8 (7), 10 (8), 12 (9), 14 (10)
CH2OH
n = 6 (1), 8 (2), 10 (3), 12 (4), 14 (5)
Br
Br
corresponding to each
compound
R
N
R N
n = 6 (11), 8 (12), 10 (13), 12 (14), 14 (15) n = 6 (16), 8 (17), 10 (18), 12 (19), 14 (20)
CH2
CH2
CH3
where R =
n
and general chemicals were sourced from Sigma-Aldrich
(Australia) and used without further purification. Elemental
analyses were performed at the Campbell Microanalytical
Laboratory, University of Otago, New Zealand. Infrared
spectra were recorded on a Perkin-Elmer 1600 FTIR spec-
R
N
120 o
C
N
+
+
R
R
Br
Br
(1) - (5)
R
N
1
trometer. H- and 13C-NMR spectra were recorded on a
N
80 o
C
Br
Acetonitrile
500 MHz Varian VXRS300 spectrometer at 25 °C. The
NMR spectra were recorded as solutions in deuterochloro-
form using tetramethylsilane (TMS) and deuterochloroform
as internal standards for 1H and 13C NMR respectively.
Br
OH
OH
(6) - (10)
90 o
Acetonitrile
C
Synthesis
+
N
R
R
Br
N R
Br
The compounds (1–12 and 16–20) were synthesized by
Menshutkin reactions, reacting the appropriate nitrogen
containing head group compound and 1-bromoalkane. The
reactions were generally carried out in polar, aprotic sol-
vents at varying temperatures and times and were purified
by washing with non-polar solvents (Fig. 2). The full
experimental protocols and characterisation of each QNS,
including 13C-NMR and IR spectra can be found in the
supplementary information.
(11) - (12)
80 o
C
+
Br
N
N R
Br
HO
HO
CH3OH, CHCl3
(16) - (20)
R = C8H17, C10H21, C12H25, C14H29, C16H33
1
2
3
8
4
9
5
6
7
10
11
16
12
17
18
19
20
Measurement of Conductivity
Fig. 2 Synthetic routes for the QNSs investigated
Conductivity measurements were carried out in triplicate
using a Suntex SC-170 conductivity meter referenced to
0.100 mol L-1 KCl in water-jacketed beakers for temper-
ature control within 0.5 °C. Solutions of the QNSs were
made up to appropriate concentrations for the length of the
alkyl chain.
the concentration vs conductivity curve was reached. At
least five data points above and below the CMC were
recorded to minimize error in the determination of the
CMC value.
Generally, the hexadecyl molecules were made up to
2 9 10-3 mol L-1, tetradecyl to 6 9 10-3 mol L-1
,
Results and Discussion
dodecyl to 2 9 10-2 mol L-1, decyl derivatives to
9 9 10-2 mol L-1 and the octyl derivatives to 0.5 mol L-1
in Milli-QÒ water. The conductivity was established and
the solution diluted with Milli QÒ water. At each dilution
the solution was allowed to reach equilibrium while being
stirred, before the conductivity was recorded. This was
continued until a noticeable difference in the gradient of
Synthesis
The compounds were made by nucleophilic substitution of
the nitrogen head group onto the appropriate 1-bromoal-
kane. The reactions were carried out with an excess of the
1-bromoalkane to increase the rate of reaction, and the
123

J Surfact Deterg
8 0.86 (x-CH3, 3H, t, J = 7.0 Hz), 1.30 ((x-CH2)9, 18H,
m), 1.99 (b-CH2, 2H, qt, J = 7.5 Hz), 4.76 (a-CH2, 2H,
t, J = 7.5 Hz), 4.92 (c-CH2, 2H, s), 8.09 (b-py, 2H, d,
J = 7.0 Hz), 9.03 (a-py, 2H, d, J = 7.0 Hz).
unreacted alkyl bromides were removed by washing with a
non-polar solvent. The different head groups and tail
lengths required slightly different reaction conditions. The
synthesis of the 4-hydroxymethylpyridinium compounds
(6–10) was performed in acetonitrile as solvent in less than
three hours. The reactions to prepare N-alkyl-N,N-dime-
thyl-N-ethanolammonium derivatives (16–20) took longer
to reach completion than for the pyridinium compounds
and required a mixed solvent system to adequately dissolve
the two starting materials. In all cases, the compounds with
longer alkyl tails were easier to purify as they were solids.
The crude materials were dissolved in a suitable polar
solvent then precipitated via dropwise addition of the
solution into a non-polar solvent to afford the solid prod-
ucts, which were then recovered by filtration. These solids
were re-crystallized to obtain the highest purity. The
compounds with shorter alkyl tails were purified by
washing their aqueous solutions with an organic solvent.
9 0.86 (x-CH3, 3H, t, J = 7.0 Hz), 1.23 ((x-CH2)11,
22H, m), 1.99 (b-CH2, qt, J = 7.5 Hz), 4.77 (a-CH2, 2H,
t, J = 7.5 Hz), 4.92 (c-CH2, 2H, s), 8.09 (b-py, 2H, d,
J = 7.0 Hz), 9.05 (a-py, 2H, d, J = 7.0 Hz).
10 0.86 (x-CH3, 3H, t, J = 7.0 Hz), 1.28 ((x-CH2)13,
26H, m), 1.98 (b-CH2, 2H, qt, J = 7.5 Hz), 4.76 (a-CH2,
2H, t, J = 7.5 Hz), 4.92 (c-CH2, 2H, s), 8.09 (b-py, 2H,
d, J = 7.0 Hz), 9.03 (a-py, 2H, d, J = 7.0 Hz).
16 0.86 (x-CH3, 3H, t, J = 7.0 Hz), 1.27 ((x-CH2)5,
10H, m), 1.74 (b-CH2, 2H, qt, J = 7.0 Hz), 3.35 (a-CH3,
6H, s), 3.53 (a-CH2, 2H, m), 3.74 (b-CH2, 2H, m), 4.12
(c-CH2, 2H, m), 5.01 (–OH, 1H, s).
17 0.87 (x-CH3, 3H, t, J = 7.0 Hz), 1.27 ((x-CH2)7,
14H, m), 1.75 (b-CH2, 2H, qt, J = 7.5 Hz), 3.37 (a-CH3,
6H, s), 3.53 (a-CH2, 2H, m), 3.75 (b-CH2, 2H, m), 4.14
(c-CH2, 2H, m), 5.01 (–OH, 1H, s).
Characterization
18 0.88 (x-CH3, 3H, t, J = 7.0 Hz), 1.29 ((x-CH2)9,
18H, m), 1.75 (b-CH2, 2H, qt, J = 7.5 Hz), 3.37 (a-CH3,
6H, s), 3.53 (a-CH2, 2H, m), 3.76 (b-CH2, 2H, m), 4.15
(c-CH2, 2H, m), 5.00 (–OH, 1H, s).
An absorption peak at *1,600 cm-1 appeared in the FTIR
of all the QNSs which confirmed the presence of the cat-
ionic nitrogen centre [4, 5]. The compounds with the added
hydroxymethyl groups had an extra absorption peak at
3,250–3,450 cm-1, indicative of the O–H stretching
19 0.87 (x-CH3, 3H, t, J = 7.0 Hz), 1.27 ((x-CH2)11,
22H, m), 1.74 (b-CH2, 2H, qt, J = 7.5 Hz), 3.36 (a-CH3,
6H, s), 3.53 (a-CH2, 2H, m), 3.75 (b-CH2, 2H, m), 4.13
(c-CH2, 2H, m), 5.00 (–OH, 1H, s).
1
vibration. H-NMR spectra clearly indicated the formation
of a positive charge by the downfield shift of the N?–CH2
signal on the tail. The proton of the hydroxyl group of the
novel hydroxymethylpyridinium QNSs was not detected in
20 0.88 (x-CH3, 3H, t, J = 7.0 Hz), 1.31 ((x-CH2)13,
26H, m), 1.76 (b-CH2, 2H, qt, J = 7.5 Hz), 3.37 (a-CH3,
6H, s), 3.53 (a-CH2, 2H, m), 3.75 (b-CH2, 2H, m), 4.17
(c-CH2, 2H, m), 5.01 (–OH, 1H, s).
1
the H-NMR spectra. The assignments are as follows with
the key shown in Fig. 3 with chemical shifts in ppm from
TMS.
Elemental analyses were performed to confirm the
purity of the prepared QNSs and the experimental values
were within acceptable error of the calculated values. The
results are shown in Table 1.
6 0.82 (x-CH3, 3H, t, J = 7.0 Hz), 1.27 ((x-CH2)5, 10H,
m), 1.98 (b-CH2, 2H, qt, J = 7.5 Hz), 4.77 (a-CH2, 2H,
t, J = 7.5 Hz), 4.91 (c-CH2, 2H, s), 8.08, (b-py, 2H, d,
J = 7.0 Hz), 9.07 (a-py, 2H, d, J = 7.0 Hz).
7 0.86 (x-CH3, 3H, t, J = 7.0 Hz), 1.30 ((x-CH2)7, 14H,
m), 1.99 (b-CH2, 2H, qt, J = 7.5 Hz), 4.76 (a-CH2, 2H,
t, J = 7.5 Hz), 4.93 (c-CH2, 2H, s), 8.09 (b-py, 2H, d,
J = 7.0 Hz), 9.03 (a-py, 2H, d, J = 7.0 Hz).
Table 1 Elemental analysis of the hydroxymethyl containing QNSs
Surfactant
C %
H %
N %
Found
Calc.
Found
Calc.
Found
Calc.
6
55.5
58.2
60.0
62.4
63.8
50.8
54.2
56.6
59.1
60.2
55.6
58.2
60.3
62.2
63.8
51.1
54.2
56.8
59.0
60.9
7.9
8.5
8.0
8.5
4.6
4.3
4.1
3.6
3.4
4.9
4.5
4.1
3.9
3.6
4.6
4.2
3.9
3.6
3.4
5.0
4.5
4.1
3.8
3.6
7
c
8
8.8
9.0
HO
β
Compounds
ω
ω
9
9.6
9.4
N
b
6 - 10
Br
10
16
17
18
19
20
10.0
9.8
9.7
x
x
α
a
10.0
10.4
10.7
11.0
11.2
b
HO
a
Compounds
16 - 20
β
10.2
10.5
10.9
11.3
c
N
Br
α
a
Fig. 3 Labelling of the protons of the hydroxymethyl containing
QNSs
123

J Surfact Deterg
Conductivity
compounds [6–8]. These values were presumably not sig-
nificantly altered by the hydroxymethyl addition to the
QNSs in this study. The critical micelle concentrations
(CMC) were determined by the intercept of the two linear
conductivity-concentration plots [9]. The ratio of the gra-
dients of these plots before (S1) and after (S2) the CMC was
used to measure the degree of dissociation (a, Eq. 1) [10].
The measurable CMC and a values for each QNS are in
Table 2.
The conductivity of the synthesized surfactants was mea-
sured over a range of concentrations at 25 and 37 °C
(Fig. 4). The Krafft temperature for similar surfactants has
been reported to be primarily influenced by the length of
the alkyl tail, and measured to be approximately 0 °C for
dodecyl, 14 °C for tetradecyl and 25 °C for hexadecyl
a ¼ S2=S1
ð1Þ
Table 2 shows the CMC and a values for each of the
QNSs. The values given are consistent with those previ-
ously reported in the literature [11–13]. The missing values
for the shorter alkyl tail lengths were due to no observed
change in the conductivity-concentration gradient up to
concentrations where solubility became an issue. The
longer chain pyridinium compounds (5 and 10) do not have
recorded CMC at 25 °C as they were insoluble, however
they were soluble at 37 °C.
A Stauff-Klevens plot allowed for the relative effects of
the head group and tail length on the CMC of each QNS
family to be assessed (Eq. 2, Table 3, Fig. 5) [9].
Fig. 4 Typical conductivity versus concentration plot used to deter-
mine the CMC of the QNS compounds
ꢁLogðCMCÞ ¼ aðR ꢁ lengthÞ þ b
ð2Þ
Table 2 CMC and extent of counter-ion dissociation from conductivity data for the four surfactant families
CMC
(mmol L-1) 25 °C
CMC
(mmol L-1) 37 °C
a
a
25 °C
37 °C
1
2
250 30
–
0.42 0.03
0.37 0.01
0.307 0.008
0.303 0.006
–
–
Br
54
3
53
5
0.46 0.02
0.35 0.02
0.347 0.009
0.41 0.06
–
3
12.1 0.6
12.8 0.9
3.3 0.3
0.80 0.03
–
R N
4
2.9 0.2
5
–
–
6
–
Br
7
41
3
47
11
4
1
0.42 0.02
0.39 0.09
0.308 0.009
–
0.41 0.02
0.39 0.02
0.36 0.02
0.436 0.009
–
OH
R
N
8
10.3 0.7
9
2.4 0.2
2.7 0.3
0.70 0.04
–
10
11
12
13
14
15
16
17
18
19
20
–
–
–
Br
49
3
62
3
0.32 0.01
0.28 0.02
0.248 0.009
0.257 0.009
–
0.380 0.008
0.30 0.02
0.34 0.02
0.322 0.008
–
R
N
N
15.8 0.9
3.8 0.2
1.02 0.07
–
16.4 0.9
3.9 0.3
1.04 0.04
–
OH
Br
73
5
82
8
0.39 0.02
0.33 0.02
0.303 0.009
0.346 0.008
0.43 0.03
0.34 0.03
0.36 0.02
0.362 0.009
R
14.5 0.9
3.7 0.3
16.9 2.1
3.8 0.3
0.91 0.04
0.87 0.06
123

J Surfact Deterg
The data displayed an increased CMC for the QNSs at
the higher temperature, consistent with previous studies
[13]. The CMC data also showed that QNSs with longer
alkyl tails have lower CMC values as expected [9]. The
change in CMC with alkyl tail length and temperature is
comparable between each of the head groups. However,
modifying the head group by incorporating the hydroxy-
methyl group also had an effect on the CMC values.
The addition of the hydroxymethyl group to the pyrid-
inium QNSs generally caused the CMC to decrease by
more than 10%. In contrast, the hydroxymethyl addition to
the ammonium compounds generally showed a small
decrease in the CMC which was within the uncertainty of
the determined value. However, in one case, comparing
compounds 12 and 17, the CMC dramatically increased for
the compound with the hydroxymethyl addition (17). A
difference in dissociation behavior between the QNSs with
pyridinium and ammonium head groups upon addition of
the hydroxymethyl group was also observed, as indicated
by the a values. There is a negligible to small increase in
the amount of dissociation for the hydroxymethylpyridi-
nium compounds while a significant increase was observed
for the trimethylammonium compounds.
Table 3 The coefficients a and b of the Stauff-Klevens equation for
each of the four surfactant families
a
b
1–5
0.32 0.01
-1.90 0.01
Br
R
N
N
6–10
0.31 0.01
-1.69 0.03
Br
OH
R
11–15
16–20
0.28 0.01
0.32 0.01
-1.55 0.02
-1.99 0.05
Br
R
N
OH
Br
R
N
The effect on the CMC by the addition of a small
hydroxyalkyl group to the headgroup of QNSs has been
previously studied in similar ammonium systems, generally
as chloride salts. The CMC of compound 18 as the chloride
salt has previously been determined by surface tension
measurements to be two orders of magnitude lower than
the chloride salt of 13 [1]. In a different study, the CMC of
compounds 15 and 20, as the corresponding chloride salt
were determined by conductivity [3]. The data showed a
slight decrease in the CMC of the hydroxy derivative
consistent with what is observed here. The addition of a
hydroxymethyl group to the pyridinium compounds has
been much less studied. The only reported study showed
that the decyl derivative (comparing 7 to 2) showed a
decrease in the CMC on a scale similar to the change
observed in this research [2]. However, as the ionic
strength was much higher in the previously reported
experiment, the CMC values could not be directly
compared.
Fig. 5 Stauff-Klevens plots for the N-alkyl-4-hydroxymethylpyridi-
nium family (6–10) at 25 °C
The results showed the a values were all very similar
and positive demonstrating that as the alkyl tail got longer,
the molecule became more hydrophobic regardless of head
group as expected [14] The values for b were negative
indicating the head groups are hydrophilic. The values for
b calculated in this study show a different trend upon the
addition of the hydroxymethyl group to the ammonium
compounds (11–15 and 16–20) than has previously been
reported [14] In this study, the b values become more
negative for compounds with the hydroxymethyl addition
where previously they had been shown to become less
negative. The pyridinium compounds had trends that
agreed with the previously reported ammonium com-
pounds, where the hydroxymethyl addition causes less
negative b values which has been attributed to a decreased
repulsion between the head groups by the formation of
hydrogen bonds due to the hydroxyl addition [14].
Conclusion
The data that has been previously presented for QNSs with
a hydroxymethyl addition to the head group has been
reflected in this work across a range of alkyl tail lengths.
The change in the CMC is generally consistent with
changes to the alkyl tail length. However, there is one case
for the decyl-ammonium compounds (comparing 12–17)
where the CMC value increases for the hydroxymethyl
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J Surfact Deterg
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lengths where the expected increase of the hydrophilicity
of the head group would have a more dominant effect. In
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compounds with the hydroxymethyl addition [3], however
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the same addition. The Stauff-Klevens plot showed that the
addition of the hydroxymethyl group makes the ammonium
compounds more hydrophilic; however it had the opposite
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Author Biographies
Deborah Jordan is
a research investigator and tutor in the
Department of Chemistry at the University of Otago. She received
her Ph.D. in Chemistry in 2012. Her research interests are in the
biological activity of surfactants and their mechanism of action.
Eng Tan is a senior lecturer in the Department of Chemistry at the
University of Otago. He received his Ph.D. from the University of
Adelaide in Australia in 1991. His research interests are with regard to
molecular interactions in biological chemistry with current projects in
enzyme reactions and surfactants.
6. Perche T, Auvray X, Petipas C, Anthore R, Perez E, Rico Lattes
I, Lattes A (1996) Micellization of N-alkylpyridinium halides in
formamide tensiometric and small angle neutron scattering study.
Langmuir 12:863–871
Dylan Hegh is a Ph.D. student in the Department of Chemistry at the
University of Otago. He received his M.Sc. in Chemistry at the
University of Otago in 2011. His research thesis is in the area of novel
switchable surfactants and their applications.
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