Thermodynamics of micellization of hexadecyltrimethylammonium bromide in propylen glycol-water binary mixture

Abstract

The effect of cosolvent (propylen glycol, PG) on micellization of an cationic surfactant, hexadecyltrimethylammonium bromide (hereinafter cetylecyltrimethylammonium bromide, CTAB) in aqueous solution was studied. The thermodynamic of micellization and the aggregation behaviour of CTAB in PG-water mixture at different temperatures (from 298.2 K to 310.2 K) have been studied conductometrically. The conductivity data were used to determine the critical micellar concentration (CMC) and degree of micellar ionization (α) of CTAB in PG-water (20% v/v) binary mixtures at different temperatures (298.2 K, 306.2 K and 310.2 K). The equilibrium model of micelle formation was applied to obtain the thermodynamics parameters (the standard molar Gibs free energy (ΔGm 0), enthalpy (ΔHm 0) and entropy (ΔSm 0)) of micellization. Those parameters were determined using temperature dependance of CMC. Value of DGm 0 was found to be negative at all temperatures and becomes less negative as the temperature increases. Values of ΔHm 0 and ΔSm 0 were found to be negative in all cases, and increase dramaticaly with temperature in 20 % (v/v) PG. An entalpy-entropy compenzation effect was observed. Moreover, based on experimental data, it can be concluded that the micellization process in the examined system is substantially similar to the CTAB micellization in water, and the presence of PG has a negligible effect on the CTAB-water interactions.

U radu je ispitan uticaj korastvarača (propilenglikola, PG) na micelizaciju katjonskog surfaktanta, heksadeciltrimetilamonijum-bromida (cetiltrimetilamonijum-bromida, CTAB) u vodenom rastvoru. Termodinamika micelizacije i agregaciono ponašanje CTAB u smeši PG-voda na različitim temperaturama (od 298,2 K do 310,2 K) ispitivani su konduktometrijski. Kritična micelarna koncentracija (KMK) i stepen jonizacije micele (α) CTAB u smeši PG-voda (20% (v/v)), na različitim temperaturama (298,2 K, 306,2 K i 310,2 K) određeni su merenjem specifične provodljivosti. Primenom ravnotežnog modela za proces formiranja micela i iz temperaturne zavisnosti KMK, izračunati su termodinamički parametri (promena standardne molarne Gibsove slobodne energije, (Δm 0), entalpije (ΔHm 0) i entropije (ΔSm 0)) micelizacije. Vrednost DGm 0 je negativna na svakoj od ispitivanih temperatura; ove vrednosti se smanjuju po apsolutnoj vrednosti sa povećanjem temperature. Vrednosti ΔHm 0 i ΔSm 0 su negativne na svakoj od ispitivanih temperatura i drastično se povećavaju po apsolutnoj vrednosti sa povećanjem temperature u smeši PG-voda. Za proces micelizacije CTAB u smeši PG-voda (20% (v/v)), dobijen je tzv. entalpijsko-entropijski kompenzacioni efekat. Štaviše, na osnovu eksperimentalno dobijenih podataka može se zaključiti da je proces micelizacije u ispitivanom sistemu suštinski sličan micelizaciji CTAB u vodi, i da prisustvo PG ima neznatan uticaj na interakcije između CTAB i vode.