Associative interactions of the various species found in the premicellar concentration region of aqueous
fatty acid solutions have been investigated using acid−base titration. In previous studies, aqueous films
of fatty acid salts were investigated at various bulk solution pH values. It was found that there exists a
pH where minimum evaporation of water, maximum foamability, maximum foam stability, minimum
contact angle on PMMA surface, maximum single-bubble stability, and maximum surface viscosity are
observed. It was also found that this optimum pH value is near the measured pK
a of the fatty acid, which
in turn depends on the length of the fatty acid chains. Titration of the homologous series of C8−C12 fatty
acids to determine the solution pK
a has shown an increase in apparent pK
a with concentration. The increase
in pK
a is maintained at concentrations well below the critical micelle concentration (cmc). Thus, similar
to micelle formation, the submicellar aggregates must be responsible for the increase in pK
a as compared
to that of soap monomers. Mixing of soap molecules of unequal chain length decreases the pK
a of the
solution as compared to that of the two individual components because of disorder produced by the unequal
chain length. Results indicate that premicellar surfactant aggregation and molecular association well
below the cmc of the soap considerably affects ionization of the polar group. This molecular association
results in an increase in the measured pK
a of soap solutions.
The orientation of surface-active molecules at interfaces is extremely important in determining the dynamic and equilibrium properties of such systems. Films of fatty acid salts were studied at various pH values of the solutions. It was found that there exists a pH where minimum evaporation of water, maximum foamability, maximum foam stability, minimum contact angle on PMMA surface, maximum single-bubble stability, and maximum surface viscosity are observed. It was also found that this optimum pH value is near the pKa of the fatty acid salts. The experimental results are explained in terms of area per molecule, intermolecular spacing, and cooperativity among soap molecules at the interface. It was further shown that the chain length of the soap molecules can modulate the area per molecule, and hence the intermolecular distance in the film, and thereby influence the ionization behavior of the fatty acid carboxyl group. The pKa increases from 6.5 to about 9.0 as the chain length of the fatty acid salt increases from C8 to C16. Cooperativity among surfactant molecules at the interface is controlled by the area per molecule and intermolecular spacing in the adsorbed film. A small change in intermolecular distance of 0.03 Å can significantly influence various technological processes such as foaming, emulsification, wetting, and retardation of evaporation.
We have deposited Langmuir-Blodgett multilayers of dipalmitoylphosphatidylcholine onto a mica substrate from the surface of an aqueous solution of uranyl acetate. No transfer could be achieved when the subphase was pure water. X-ray diffraction confirms the formation of multilayers; the distance between bilayers is 57 ± 1 Á.
The critical micelle concentrations (CMC) of nine commercial nonionic surfactants (Tween 20, 22, 40, 60, and 80; Brij 35, 58, and 78) and two pure nonionics [C 12 (EO) 5 and C 12 (EO) 8 ] were determined by surface tension and dye micellization methods. Commercially available nonionic surfactants (technical grade) usually contain impurities and have a broad molecular weight distribution owing to the degree of ethoxylation. It was shown that the surface tension method (Wilhelmy plate) is very sensitive to the presence of impurities. Much lower CMC values were obtained with the surface tension method than with the dye micellization method (up to 6.5 times for Tween 22). In the presence of highly surfaceactive impurities, the air/liquid interface is already saturated at concentrations well below the true CMC, leading to a wrong interpretation of the break in the curve of surface tension (γ) vs. concentration of nonionic surfactant (log C). The actual onset of micellization happens at higher concentrations, as measured by the dye micellization method. Furthermore, it was shown that when a commercial surfactant sample (Tween 20) is subjected to foam fractionation, thereby removing species with higher surface activity, the sample yields almost the same CMC values as measured by surface tension and dye micellization methods. It was found that for monodisperse pure nonionic surfactants, both CMC determination methods yield the same results. Therefore, this study indicates that precaution should be taken when determining the CMC of commercial nonionic surfactants by the surface tension method, as it indicates the surface concentration of all surface-active species at the surface only, whereas the dye method indicates the presence of micelles in the bulk solution. FIG. 1. Ultraviolet-visible absorbance spectrum of eosin Y in aqueous surfactant solution. The wavelength maximum (λ max ) shifts from 518 nm in the absence of surfactant to 538 nm as the surfactant concentration increases. The rise is most significant at 542 nm. 54 A. PATIST ET AL. FIG. 2. Critical micelle concentration (CMC) determination of Tween 20 (CMC = 0.042 mM) using the dye micellization method (absorbance at 542 nm). Eosin Y concentration: 0.019 mM. Horizontal dashed line represents dye absorbance in water in the absence of surfactant.
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