Microcalorimetric Study on the Critical Micelle Concentration and Thermodynamic Functions of Di(2-ethyl-hexyl) Phosphate Salts in Organic Solvent +sec-Octyl Alcohol Systems at 298.15 K

Abstract

The power−time curves of the micelle formation process were determined for three kinds of quasi-surfactants [lithium di(2-ethyl-hexyl) phosphate, sodium di(2-ethyl-hexyl) phosphate, and potassium di(2-ethyl-hexyl) phosphate] and five kinds of alkanes (octane, decane, dodecane, tetradecane, and hexadecane) with sec-octyl alcohol by titration microcalorimetry. The critical micelle concentration (CMC), aggregation number (n), formation constant of micelle (K), and thermodynamic functions (ΔrHmθ, ΔrGmθ, and ΔrSmθ) were obtained. For different quasi-surfactants and homologue of the alkanes, the relationships between the carbon number (N) of the alkanes with the critical micelle concentration (CMC) and the thermodynamic properties of the quasi-surfactants are discussed. It can be concluded as follows: (1) The microcalorimetric method is a good method to study the process of micelle formation. (2) The CMC decreased with an increase of the carbon number (N) of the alkanes, and the relative order of the CMC of different quasi-surfactants was D2EHPA(K) > D2EHPA(Na) > D2EHPA(Li) at the same carbon number. (3) The aggregation numbers (n) and formation constant of micelle (K) increased with an increase of the carbon number (N) of the alkanes. The relative order of different quasi-surfactants was D2EHPA(K) > D2EHPA(Na) > D2EHPA(Li) at the same carbon number. (4) The ΔrHmθ and ΔrSmθ increased, while ΔrGmθ decreased with an increase of the carbon number (N) of the alkanes. The relative order of the different quasi-surfactants was D2EHPA(Li) > D2EHPA(Na) > D2EHPA(K) at the same carbon number.