Effects of temperature on the partitioning and adsorption of C12E5in heptane–water mixtures

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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.

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