Thermodynamics of Micellization of Hexadecylpyridinium Bromide in Aqueous and Alcoholic (C1-C4) Solutions

Abstract

In this work, we studied effects of temperature, alkyl chain, branching and increasing number of hydroxy groups of aditives on the critical micelle concentration (CMC) and thermodynamics of micellization of 1-hexadecylpyridinium bromide (CPBr) using conductometric techniques in a series of solvents: water and C1-C4 alcohols (methyl (MeOH), ethyl (EtOH), n-propyl (n-PrOH), isopropyl (i-PrOH), n-butyl (n-BuOH), isobutyl (i-BuOH), tert-butyl alcohol (t-BuOH), 1,2-ethanediol (EtdOH), 1,2-propanediol (PrdOH) and 1,2,3-propanetriol (PrtOH)). From the conductivity curves measured in water and alcohols in the temperature interval 20-50 °C, the CMC and concentrations of CPBr corresponding to a change of the micelle shape were calculated. For alcohols, the plots of the CMC vs temperature display U-shaped curves with minima at 30 °C, whereas in water, the CMC increases nonlinearly over the temperature interval studied. According to their effect on the CMC, the solvents can be arranged in the order H2O < n-BuOH < i-BuOH < t-BuOH < PrtOH < n-PrOH < i-PrOH < PrdOH < EtOH < EtdOH < MeOH. Degrees of counterion binding (β), as calculated from slopes of the conductivity curves, were found to decrease as the temperature increases. Thermodynamic functions of micellization - standard molar Gibbs energy, enthalpy and entropy - were analyzed in terms of the model of phase separation as well as the model based on the mass-action law. While the drop in the ∆mG0 values over the given temperature interval is only 1-2 kJ mol-1, the lowering of ∆mH0 and ∆mS0 is much more pronounced. Enthalpy-entropy compensation plots were used to calculated the compensation temperatures which characterise the solvent-solute interaction and lie in the range 289-313 K. The reference temeratures for ∆mS0 converging to zero range from 318 to 344 K. The values of molar heat capacity ∆mCP were also calculated. In the mixed (water-alcohol-tenside) solutions with the volume alcohol-water ratios φ = 0-1.0, the CMC were determined at 25 °C.