Hydration pressure of a homologous series of nonionic alkyl hydroxyoligo(ethylene oxide) surfactants

Abstract

The hydration pressure of a homologous series of nonionic surfactants of the type CH₃(CH₂)_n−1(OCH₂CH₂)_mOH (C_nE_m) was determined by using sorption gravimetry. The hydration pressure shows a non-exponential decay on hydration and the curves are best fitted with a rational function. The non-exponential character is explained by contributions arising from the bending energy of non-lamellar liquid crystalline phases. The parameters obtained show that hydration of pure nonionic surfactants is correlated with the number of the oxyethylene groups in the headgroup. However, they are almost independent of the length of the alkyl chain. Exceptions are surfactants in the solid crystalline state that almost prevent hydration. Furthermore, the respective mesophase structures do not dominate the hydration behaviour. The Gibbs free energy (free enthalpy) of hydration is −1.1 kJ mol⁻¹ per oxyethylene group.