Thermodynamics of micellization of hexadecyltrimethylammonium bromide in propylene glycol-water mixture: A conductivity study

Abstract

Micellization of hexadecyltrimethylammonium bromide (syn. cetyltrimethylammonium bromide, CTAB) in propylene glycol-water (30% v/v) binary mixture, as well as the thermodynamic properties of the resulting micelles, were investigated by electrical conductivity measurements. The conductivity data were used to determine both the critical micellar concentration (CMC) and the micellar ionization degree (∞) of CTAB in the temperature range 298.2-310.2 K. The equilibrium model of micelle formation was applied in order to obtain the thermodynamic parameters (the standard molar Gibbs free energy, ∆Gmº, enthalpy, ∆Hm0 and entropy, ∆Smº) of the micellization process. The values of ∆Gmº and ∆Hmº were found to be negative at all investigated temperatures, while the values of ∆Smº were positive and became more positive as temperature increased. A linear dependence between ∆Smº and ∆Hmº, i.e. an enthalpy-entropy compensation effect, was observed.

Konduktometrijski je ispitivana micelizacija heksadeciltrimetilamonijum-bromida (sinonim cetiltrimetilamonijum-bromid, CTAB) u binarnoj smeši propilen-glikol-voda (30%, v/v), kao i termodinamičke osobine nastalih micela. Merenjem specifične provodljivosti određeni su kritična micelarna koncentracija (KMK) i stepen jonizacije micele (∞) CTAB u opsegu temperatura 298,2-310,2 K određeni su. Primenom ravnotežnog modela za proces micelizacije izračunati su termodinamički parametri: promena standardne molarne Džibsove slobodne energije, (∆Gmº), entalpije (∆Hmº) i entropije (∆Smº) micelizacije. Vrednosti ∆Gmº i ∆Hmº su bile negativne na svakoj od ispitivanih temperatura, dok su vrednosti ∆Smº bile pozitivne i povećavale su se sa porastom temperature. Na osnovu linearne zavisnosti između ∆Hmº i ∆Smº utvrđen je tzv. entalpijsko-entropijski kompenzacioni efekat.