Effects of temperature on the partitioning and adsorption of C12E5 in heptane–water mixtures

Abstract

The temperature dependence of the partition coefficient of the monomer of the surfactant dodecyl pentaethylene glycol ether (C₁₂E₅) between heptane and water has been measured. The monomer partitions strongly in favour of the oil throughout the temperature range in which aggregates form (at high C₁₂E₅ concentrations) in either the water, oil or a third phase. The partition coefficient P(=[C₁₂E₅]_water/[C₁₂E₅]_oil) decreases with increasing temperature and the standard molar enthalpy of transfer from water to heptane is 46 ± 10 kJ mol^–1. From interfacial tensions the critical surfactant concentrations required for aggregate (microemulsion droplet) formation (cµc) in both the oil and water phases have been measured. The temperature dependence of the cµc in heptane is the inverse of that of the partition coefficient, whereas the aqueous phase cµc is virtually temperature independent. The area occupied per C₁₂E₅ molecule in close-packed monolayers at the planar oil/water interface is only weakly dependent on temperature and the values are intermediate between the values of the head- and tail-group areas for the surfactant present in the curved monolayers stabilising microemulsion droplets.