Micellization behavior and thermodynamic properties of cetyltrimethylammonium bromide in lithium chloride, potassium chloride, magnesium chloride and calcium chloride solutions

Abstract

The micellization behavior and thermodynamic properties of cetyltrimethylammonium bromide (CTAB) in single lithium chloride (LiCl), potassium chloride (KCl), magnesium chloride (MgCl₂) and calcium chloride (CaCl₂) solutions were investigated at 288.15−318.15 K. Result showed that the critical micelle concentration (CMC) values of CTAB in all solutions decreased to a minimum value around 298.15 K and then increased with further increasing the temperature. In all cases, the CMC values decreased with increasing salt concentration at each temperature. Additionally, the introduction of any single salt resulted in a reduction of CMC values for CTAB, attributed to the combined effects of counterions and entropy-driven interactions. The observed trend for CMC values was as follows: ${\text{CMC}}_{H_{2}O}$ > ${\text{CMC}}_{\text{KCl}}$ > ${\text{CMC}}_{\text{LiCl}}$ > ${\text{CMC}}_{{\text{CaCl}}_2}>{\text{CMC}}_{{\text{MgCl}}_2}$. Furthermore, standard thermodynamic parameters, including standard free energy of micellization ($\Delta G_m^0$), standard enthalpy of micellization ($\Delta H_m^0$) and standard entropy of micellization ($\Delta S_m^0$), were calculated based on the obtained CMC values. The negative values of $\Delta G_m^0$ indicated that the formation of CTAB micelles was a spontaneous behavior. The variations in $\Delta H_m^0$ and $\Delta S_m^0$ suggested that micellization was primarily entropy-driven at temperatures between 288.15 and 298.15 K, while it was influenced by both entropy and enthalpy from 298.15 to 318.15 K. Fourier transform infrared spectroscopy (FTIR) and transmission electron microscope (TEM) were employed to further explore the effects of salts on the micellization behavior of CTAB.