PHYSICAL AND CHEMICAL PROPERTIES OF N-(2-HYDROXYETHYL)ALKYLAMINES
1477
reduced (except reagent II in 0.1 M HCl, where there is
a maximum solubility). The choice of these solvents was
portion of the curve reflects a gradual filling of the surface
layer by the surfactant molecules thereby the adsorption
made from practical considerations: application of HEA attains the limiting value, the further input of the surfac-
to the flotation of metals.
tant leads to the micelle formation in a bulk of the solution
and does not effect on the surface tension of the solution.
The further growth in the surfactant concentration causes
a transition from spherical micelles formed in the solu-
tions at the small surfactant concentrations to asymmetric
ones, aspheric, that affects the state of the surface layer
[7]. These changes result in the shape of the following
portion of the isotherm where a further gradual decrease
in the surface tension occurs.
Based on data of the thermal analysis we established
that up to 140°С the compounds are stable. In the tem-
perature range 140–275°С there is thermal decomposi-
tion of all the samples accompanied by a considerable
loss of weight.
Like the aliphatic amines, the compounds under study
have the basic properties due to an capability of lone pair
of electrons of the nitrogen atom to attach a proton. Ba-
sicity of HEA can be characterized by the values of acid
dissociation constants Ka of conjugated acids:
The results are evidence that HEA series belongs to
a class of the strong surfactant since minimal values of
the surface tension σmin of compounds with R = C8H17,
C10H21, C12H25 are 25–45 mJ m–2 in water, of compounds
with R = C8H17, C10H21, C12H25, C14H29 in 0.1 M HCl,
30–35 mJ m–2 (Table 3). The slight decrease in the surface
[R–NH2–CH2CH2OH]+
Ka
→ R–NH–CH2CH2OH + H+.
(4)
tension by compounds with R = C14H29 in water (σmin
=
63 mJ m–2) and with R=C16H33 in water and in 0.1 M HCl
Results of the computations of рKa of the conjugated
acids RNHСН2CН2ОН series, calculated according to
data of the potentiometric, show that the constant values
are practically independent of the length of the HEA
radical; in comparison with monoethanolamine [6] the
basicity of the compounds under study changes slightly.
(a)
Chemical stability of HEA is determined by their
resistance to hydrolysis, which will most likely be oc-
cur with the formation of the primary amine (A) and
ethylene glycol. The chemical stability was assessed
by thin layer chromatography (TLC) on the example of
N-(2-hydroxyethyl) dodecylamine. The most suitable was
chloroform–methanol solvent system (1 : 2), for which
values of the relative speed of movement Rf of the reagent
under study and a hydrolysis product (dodecylamine)
were 0.45 and 0.60, respectively. Found that the reagent is
chemically stable to within a 1 day in 0.1 M HCl solution
at 20°C. At 60°C after 45 min in the chromatogram were
revealed two spots corresponding to the initial reagents
and dodecylamine; after 1 h incubation reagent was
completely hydrolyzed: only one spot of dodecylamine
was revealed.
(b)
Figure 1 shows he isotherms of the surface tension
of the solutions of HEA with R=C8H17 (curve 1), C12H25
(curve 2), and C14H29 (curve 3). The isotherms of the
reagents with R = C10H21, C16H33 were analogous. In
the region of the HEA small concentrations a value of σ
decreases sharply and then a decrease in the surface ten-
sion slows upon the concentration growth. A descending
Fig. 1. The isotherms of the surface tension σ (mJ m–2) of
compounds RNHCH2CH2OH (a) in water and (b) in 0.1 M
HCl. (с) Concentration, M. R: (1) C8H17, (2) C12H25, (3) C14H29.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 8 2010