Physico-chemical properties of N,N-di(2-hydroxyethyl)alkylamines

Article abstract of DOI:10.1134/S1070363214030104

In view of potential application of N-alkyl derivatives of diethanolamine RN(CH₂CH₂OH)₂ (R = C₈H₁₇, C₁₀H₂₁, C₁₂H₂₅, and C ₁₄H₂₉) as su

Full text of DOI:10.1134/S1070363214030104

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 3, pp. 474–478. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © T.Yu. Nasrtdinova, D.V. Koltashev, A.V. Radushev, L.G. Chekanova, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84,
No. 3, pp. 416–420.
Physico-Chemical Properties
of N,N-Di(2-hydroxyethyl)alkylamines
T. Yu. Nasrtdinova, D. V. Koltashev, A. V. Radushev, and L. G. Chekanova
Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences,
ul. Akademika Koroleva 3, Perm, 614013 Russia
е-mail: tina.nasa@mail.ru
Received May 14, 2013
Abstract―In view of potential application of N-alkyl derivatives of diethanolamine RN(CH2CH2OH)2 (R =
C8Н17, C10H21, С12H25, and C14H29) as surfactants, we have studied their physico-chemical properties,
constructed the surface tension isotherms at the liquid–gas interphase boundary in the cases of water and
aqueous hydrochloric acid, and estimated critical micelle concentrations as well as surface activity of the
studied compounds.
DOI: 10.1134/S1070363214030104
Etoxylated amines are known as nonionic sur-
factants; however, in acidic medium they are
protonated and thus can act as cationic surfactants [1].
Potential applications of N-alkyl derivatives of
diethanolamine [N,N-di(2-hydroxyethyl)alkylamines I]
include cationic surfactants as component of latex
paints [2]; synergists of biocides in diesel oil, masout,
jet fuel [3], paints, and adhesive coatings [4];
dispersants enhancing stain removing action of fabric
bleaching agents [5]; antistatic agents [6]; pH
regulators, resin solubilizers [4]; corrosion inhibitors
[4, 7]; hydrolysis inhibitors for phosphites and
phosphonites [8]; slurry traps [9]; and others. Certain
physico-chemical properties of compounds I have been
reported [10–12]; however, their systematic studies
have been lacking to the very best of our knowledge.
Studies of changes of physico-chemical properties in
the series of surfactant homologues would aid in
development of their practical applications. Recently,
we have carried out similar investigation of N-alkyl
derivatives of 2-aminoethanol [13].
1464–1460 (С–Н), 1369–1368 (О–Н), and 1045–1041
(С–О). The observed spectral features coincided well
with the reference data [14, 15].
Table 1 shows the data on compounds I solubility
as well: they were found to be poorly soluble in water
and moderately soluble in aqueous HCl. Solubility in
the both media decreased with increasing the alkyl
chain length; however, the solubility decrease was less
steep in the case of aqueous HCl. As the studied
compounds could form micelles in the solutions,
accurate determination of the actual solubility was
complicated due to absence of macrophase separation.
Nevertheless, the obtained solubility values provided
reasonable estimate for the concentration range
suitable for further surface tension studies.
According to thermal analysis results, compounds I
were stable up to 200°С. At 200–325°С all the studied
samples were decomposed, the process being
accompanied with significant mass loss.
Compounds I were more basic than the parent
diethanolamine (рΚa 8.78 ± 0.03) [16]. The compounds
basicity was slightly decreasing with increasing the
alkyl chain length; the determined рΚa values are given
in Table 1.
N-Alkyldiethanolamines RN(CH2CH2OH)2 (R =
C8Н17, C10H21, or С12H25) are transparent yellowish
viscous fluids, compound I with R = C14H29 is white
crystalline solid (Table 1). 1H NMR data of the studied
compounds are collected in Table 2. IR spectra of
compounds I contained the following absorption bands
(cm–1): 3364–3348 (О–Н), 2924 (С–Н), 2854 (С–Н),
Chemical stability of compounds I was reflected in
their resistance towards hydrolysis, most likely giving
the corresponding primary amine and ethylene glycol.
474

PHYSICO-CHEMICAL PROPERTIES OF N,N-DI(2-HYDROXYETHYL)ALKYLAMINES
Table 1. Physico-chemical parameters of RN(CH2CH2OH)2
475
Solubility, mol/L
рΚa
R
bp, °C
nD
in 0.1 mol/L
(n = 6–8, Р 0.95)
in water
HCl
C8H17
C10H21
168–170 (5 mm Hg) {128–133 (0.5 mm Hg) [11]}
183–185 (5 mm Hg) {203–204 (12 mm Hg) [10]}
190–192 (4 mm Hg) {193–195 (3 mm Hg) [12]}
27–28а (hexane–ethyl acetate)
1.4620
1.4635
1.4655
–
9.2 × 10–3
4.8 × 10–3
1.8 × 10–3
6.6 × 10–4
1.6 × 10–1
1.4 × 10–1
1.2 × 10–1
9.4 × 10–2
8.49 ± 0.05
8.48 ± 0.05
8.45 ± 0.05
8.33 ± 0.05
C12H25
C14H29
a Melting point.
Table 2. 1Н NMR spectroscopy data (δ, ppm) of RN(СН2CН2ОН)2
R
ОН (s)
СН2ОН (t)
СН2СН2ОН (t)
RСН2N (t)
RСН2СН2N (m)
ССН2С (m)
1.24–1.31
СН3 (t)
C6Н13
С8Н17
С10Н21
С12Н25
С14Н29
С16Н33
3.79
3.17
3.80
4.09
2.80
3.18
3.59
3.60
3.59
3.59
3.61
3.62
2.63
2.64
2.63
2.62
2.66
2.76
2.50
2.51
2.50
2.50
2.53
2.60
1.40–1.47
1.42–1.48
1.41–1.47
1.42–1.49
1.43–1.48
1.44–1.49
0.88
0.87
0.87
0.87
0.87
0.87
1.24–1.30
1.23–1.30
1.22–1.28
1.21–1.27
1.22–1.30
studied compounds, σ was sharply decreasing with the
concentration; in more concentrated solutions, surface
tension decrease was less steep. The curves shape was
due to decreasing of number of surfactant-free
adsorption centers. At far higher concentration of the
surfactant, micelle formation was evident [17].
RN(CH2CH2OH)2·HCl + 2HOH
RNH2·HCl + 2HOCH2CH2OH.
[H+]
The compounds chemical stability was estimated
by TLC using N-dodecyldiethanolamine as repre-
sentative example. The most suitable solvent was 1 : 2
mixture of chloroform and methanol, Rf values of the
studied compound and the hydrolysis product (do-
decylammonium hydrochloride) being of 0.87 and
0.18, respectively. At 20°С, N-dodecyldiethanolamine
was stable over at least 1 day; at 60°С it was hyd-
rolyzed after 3 h. The compound resistance towards
hydrolysis was sufficiently high in view of their
potential flotation agent and surfactant applications.
The above-described results revealed that the
studied compounds were surfactants of moderate sur-
face activity: the lowest σ values were of 27–48 mJ/m2
(water) and 36–41 mJ/m2 (0.1 mol/L HCl) (Table 3).
The critical micelle concentration CMC is known
as certain measure of surfactant molecular solubility:
further addition of the surfactant portions will not
change its concentration at the interphase boundary,
influencing only the molecules micellar organization in
the solution.
One of the key surfactant properties is its ability to
decrease the interfacial surface tension; the surface
activity of compounds I was determined in neutral and
acidic (0.1 mol/L HCl) media.
In the series of compounds I, minimal surface
tension values decreased and micelle formation ability
was enhanced with increasing the alkyl chain length
(Table 3). In the case of R = C14H29, σmin could not be
reached due to limited solubility.
Figure 1a shows isotherms of surface tension of
compounds I (R = C12H25 and C14H29) in water and in
aqueous HCl; the plots in the cases of other
compounds I were similar. At low concentration of the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 3 2014

476
NASRTDINOVA et al.
(a)
(b)
σ, mJ/m2
σ, mJ/m2
с, mmol/L
с, mmol/L
Fig. 1. Isotherms of surface tension (σ) of RN(CH2CH2OH)2 (a) in water and (b) in 0.1 mol/L HCl. R = (1) C12H25 and (2) C14H29.
Surface activity G is another important surfactant
That was due to protonation of the compounds amino
group: CMC of cationic surfactants is generally higher
than that of the nonionic surfactants [19]. Another
reason for the described trend could be the broader
range of spherical micelles existence in acidic medium
as compared to neutral aqueous solutions [18].
parameter. It is known that in the homologous series of
surfactants G increases with the alkyl chain length
[18]. The G values calculated for the studied com-
pounds I are given in Table 3. G increased with the
alkyl chain length, the exception being R = C14H29;
that was likely due to insufficient solubility of the
compound in water (Table 1) not allowing for satura-
tion of adsorption layer.
The experimental surface tension isotherms were
recalculated into the adsorption isotherms following
the Gibbs equation. Contrary to the expectations, the
calculated isotherms did not follow the classical shape
of Langmuir isotherm [20], and maxima were observed
in the plots [Fig. 2 displays the curves in the case of
C8H17N(CH2CH2OH)2, the representative example].
Similar adsorption behavior of surfactants was
discussed in [21].
Generally, the surfactant activity is enhanced in the
presence of strong electrolyte [18]. In the case of
studied compounds I, decrease (in any) of the minimal
surface tension was not significant; however, the
critical micelle concentration was increased and the
surface activity was decreased in acidic medium (Table 3).
Table 3. Medium effect on surface activity of RN(СН2CН2ОН)2 at room temperature
Minimal surface tension σmin, mJ/m2
Critical micelle concentration CMC, mmol/L Surface activity G, mJ m mol–1
(с, mmol/L)
R
in water
in 0.1 mol/L HCl
in water
in 0.1 mol/L HCl
in water
32
in 0.1 mol/L HCl
2
C8H17
48
(0.9)
34
41
(20.0)
38
0.40
0.80
0.30
0.68
0.06
0.46
0.18
–
1.00
5.00
2.5
C10H21
C12H25
C14H29
132
338
108
4
19
(0.6)
30
(18.0)
36
18
0.86
4.00
0.28
–
(0.5)
52
(10.0)
36
127
(0.6)
(0.6)
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PHYSICO-CHEMICAL PROPERTIES OF N,N-DI(2-HYDROXYETHYL)ALKYLAMINES
477
G × 106, mol/m2
chloride electrodes. In particular, the 0.1000 g
specimen was dissolved in 4 mL of KСl solution
(1 mol/L) and titrated with 0.1 mol/L aqueous HC1.
Content of N-alkyldiethanolamines and their рΚа
values were calculated as described in [13].
G × 106, mol/m2
Compounds solubility was determined by
gravimetry [23]. Their thermal stability was estimated
using the Q−1500D Paulig–Paulig–Erdei derivato-
graph (in air, at 20–600°С and 10 deg/min).
2
1
The compounds resistance towards acid hydrolysis
was studied by incubation of the 5 × 10–2 mol
specimen during 1 day at 20 and 60°С. The reaction
mixture was regularly sampled and analyzed by TLC
(Silufol plates, 1 : 2 chloroform–methanol, develop-
ment with 3% chloranil in toluene at 70°С, visual
inspection with the reference sample).
с, mmol/L
с, mmol/L
Fig. 2. Isotherm of C8H17N(CH2CH2OH)2 adsorption at the
water–air interphase boundary (1) and at 0.1 mol/L HCl–air
interphase boundary (2).
Surface tension at the interphase boundary between
the compounds solution (either in water or in 0.1 mol/L
HCl) and air was measured with stalagmometer [20].
Critical micelle concentration was determined
graphically as the kink of surface tension plot as func-
tion of concentration.
At lower concentrations, the surfactant adsorption
at the liquid surface occurred due to hydrogen
bonding. With increasing concentration, interaction of
the alkyl tails of the surface-adsorbed surfactant mole-
cules with micelles in solution was possible. Likely,
that was the reason for appearance of the maxima in
adsorption isotherms. Adsorption reached the limit at
sufficiently high surfactant concentration, when no
further growth of micelles in the solution was possible.
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