Solubilization of n-alkylbenzenes into decanoyl-N-methylglucamide (Mega-10) solution; temperature dependence

Solubilization of benzene, toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, n-pentylbenzene, and n-hexylbenzene into micelles of decanoyl-N-methylglucamide (Mega-10) was studied at 303.2, 308.2, 313.2, and 318.2 K, where equilibrium concentrations of the above solubilizates were determined spectrophotometrically. The concentration of the above solubilizates remained constant below the critical micelle concentration (cmc) and increased linearly with an increase in Mega-10 concentration above the cmc at each temperature above. The Gibbs free energy change of the solubilizates from aqueous bulk to their liquid solubilizate phase was evaluated from dependence of their aqueous solubility on alkyl chain length of the solubilizates, which leads to the ΔGCH20 values (−3.60 to −3.38 kJ mol−1), the energy change per CH2 group of the alkyl chain with no strong temperature dependence. The first stepwise solubilization constant (K1¯) was evaluated from the slope for the change of solubilizate concentration vs. Mega-10 concentration. The Gibbs free energy change (ΔG0,s) for the solubilization decreased linearly with the carbon number of alkyl chain of the solubilizates, and the ΔGCH20,s values (−2.71 to −2.54 kJ mol−1) obtained from the linearity showed a slight increase with temperature. The ΔGCH20 values are less than the ΔGCH20,s values, where the latter values clearly indicate that the location of alkyl chain is a hydrophobic micellar core. The fact is also supported by the absorption spectrum of the solubilized molecules. Temperature dependence of ΔG0,s indicated that the solubilization is entropy-driven for the solubilizates with shorter alkyl chains, while it becomes enthalpy-driven for those with longer alkyl chains.