Behavior of cationic surfactants and short chain alcohols in mixed surface layers at water-air and polymer-water interfaces with regard to polymer wettability. I. Adsorption at water-air interface

Abstract

Measurements of the surface tension of aqueous solutions were carried out at 293K for mixtures of cetyltrimethylammonium bromide (CTAB) with short chain alcohols such as methanol and ethanol, as well as for 1-hexadecylpyridinium bromide (CPyB) with the same alcohols. The concentration of CTAB and CPyB in aqueous solutions was in the range from 10(-5) to 10(-3) M, and methanol and ethanol was in the range from 0 to 21.1M and from 0 to 11.97M, respectively. Moreover, the surface tension of aqueous solution mixtures of cationic surfactants with propanol in the concentration range from 0 to 6.67M was also taken into consideration. The obtained isotherms of the surface tension were compared to those calculated from the Szyszkowski and Connors equations. The constants in these equations were determined by the least squares method. It appeared that they depended on the type of surfactant and alcohol. From comparison of the experimental and theoretical isotherms of the surface tension it is possible, at first approximation, to describe the relationship between the surface tension of aqueous solutions of cationic surfactants with short chain alcohol mixtures as a function of alcohol molar fraction in the bulk phase by the Szyszkowski and Connors equations. Furthermore, changes of the surface tension of aqueous solutions of CTAB and CPyB with alcohol mixtures at each constant concentration of cationic surfactant can be predicted by the Fainerman and Miller equation, if it is possible to determine the molar area of cationic surfactant and alcohol in the mixed monolayer. Based on the surface tension isotherms the Gibbs surface excess concentration of cationic surfactants and alcohols at water-air interface was determined, and in the case of alcohol, this concentration excess was recalculated for that of Guggenheim-Adam. The Guggenheim-Adam surface excess concentration was applied for determination of the real concentration of alcohol in the mixed surface monolayer. The real concentration of cationic surfactant was assumed equal to Gibbs surface excess concentration. For determination of the composition of the surface layer, its proper thickness was assumed. The composition of the surface mixed monolayer was discussed with regard to the standard free energy of cationic surfactant and alcohol adsorption at water-air interface determined in different ways. The standard free energy of adsorption of "pure" cationic surfactants determined from the Langmuir and Aronson and Rosen equations was compared to that deduced on the basis of Cp(20), and the surface tension of the cationic surfactant tail and tail-water interface tension.