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 (a) 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, DG_m⁰, enthalpy, DH_m⁰ and entropy, DS_m⁰) of the micellization process. The values of DG_m⁰ and DH_m⁰ were found to be negative at all investigated temperatures, while the values of DS_m⁰ were positive and became more positive as temperature increased. A linear dependence between DS_m⁰ and DH_m⁰, i.e. an enthalpy-entropy compensation effect, was observed.

TERMODINAMIKA MICELIZACIJE HEKSADECILTRIMETILAMONIJUM-BROMIDA U SMEŠI PROPILEN-GLIKOL-VODA: KONDUKTOMETRIJSKO ISPITIVANJE

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 (a) 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, (DG_m⁰), entalpije (DH_m⁰) i entropije (DS_m⁰) micelizacije. Vrednosti DG_m⁰ i DH_m⁰ su bile negativne na svakoj od ispitivanih temperatura, dok su vrednosti DS_m⁰ bile pozitivne i povećavale su se sa porastom temperature. Na osnovu linearne zavisnosti između DH_m⁰ i DS_m⁰ utvrđen je tzv. entalpijsko-entropijski kompenzacioni efekat.