Thermodynamics of Micellization of Sodium Alkyl Sulfates in Water at High Temperature and Pressure

Abstract

Apparent molar volumes V Φ ,S were determined for sodium octyl, decyl, and dodecyl sulfates in water at 2 and 19 MPa from 25 to 130 °C. The shapes of V Φ ,S vs the surfactant concentration curves depend on the surfactant alkyl chain, temperature and pressure. The standard partial molar volumes were calculated from data in the premicellar region whereas the partial molar volumes of the surfactant in the micellar phase were obtained from data in the postmicellar region. The partial molar expansibility and compressibility were evaluated from the dependence of the partial molar volume on temperature and pressure, respectively. Attention was focused to the expansibility and its pressure coefficient since studying the pressure effect on the expansibility is equivalent to studying the temperature effect on the compressibility. The hydrophilic and the hydrophobic (methylene group) contributions to the expansibility were evaluated. The comparison between the present and the alkyltrimethylammonium bromides data evidenced that, contrarily to the expectation, the pressure effect on the expansibility (or temperature effect on compressibility) is the same for the two polar heads in the micelles and independent of the nature solvent (micelles or water) for −SO4Na. The pressure effect on the methylene group expansibility does not depend on the polar head in the micelles whereas it does in water. At a given temperature and pressure, the volume of micellization ΔV m was calculated by assuming the pseudo-phase transition model. ΔV m decreases with temperature according to the negative expansibility of micellization. The temperature at which ΔV m assumes a null value depends on pressure and on the nature of the surfactant. In particular, at a given pressure, the inversion of the ΔV m sign occurs at lower temperature the longer the alkyl chain is. Moreover, for each surfactant, ΔV m shows a sign inversion at lower temperature by increasing pressure.

A comparative study of the physicochemical properties of perfluorinated and hydrogenated amphiphiles

“…This fact has been previously reported in the literature as deviation from linearity for surfactants with shorter chain length [75]. This magnitude also decreases with temperature, de Lisi et al [76] finding negative slopes for V φ of sodium octyl, decyl, and dodecyl sulfates in water against temperature in the range 25 • to 130 • C at different pressures, attributing this negative slope value to the expansibility of the surfactant in the micellar state being smaller than that in the aqueous phase. Attwood et al [77] found this decrease with temperature for chlorpromazine, an effect which has been attributed to the dehydration of the ionic head group.…”

Volumes of aqueous block copolymers based on poly(propylene oxides) and poly(ethylene oxides) in a large temperature range: A quantitative description

“…Moreover, it illustrates the dependence on temperature of both the contributions for the aggregates formation and the shift of the micellization equilibrium (equation (6)). The aggregation process involves a decrease of E U in agreement with the hydrophobic desolvation as it occurs for conventional surfactants [46]. Therefore, the constant E U in the low and the high temperature domains reflect the E m and E M values, respectively.…”

Effect of terbium(III) chloride on the micellization properties of sodium decyl- and dodecyl-sulfate solutions

“…Considering the electrostatic interactions between Tb(III) and alkyl-sulfate chains, the interactions with water must also be taken into account. In support of this, SDeS shows a higher solubility in water than SDS, and the stronger interactions with this surfactant can also be seen through various thermodynamic properties [27,28].…”