Calcium surfactants: a review

This article presents an overview over research on binary and ternary anionic surfactant systems with calcium counterions over the past 30 years, with a focus on recent studies. General phase behavior of surfactants and the underlying electrostatics are given a synoptical treatment. With N-alkyl amide sulfates, a new class of surfactants is described in which the Krafft point of the calcium surfactant is lower than that of its sodium counterpart. The exemplary ternary systems treated in depth are calcium dodecyl poly(oxyethylene) sulfate/C14 betain/water, calcium dodecyl sulfate/medium-chain alcohol/water, and α-sulfonated methyl myristate, Ca2+ salt/2-ethylhexylmonoglyceride/water. It is shown that ternary systems of this kind frequently show elements of the behavior of non-ionic or double-chain surfactants, e.g. a tendency toward micellar growth and the formation of vesicular and lamellar structures, the existence of highly swollen, iridescent lamellar phases and also the formation of L3 (sponge) phases. This is explained by the high degree of membrane association of the doubly-charged calcium ions and the resulting low apparent surface charge density of the surfactant/water interface. The methods discussed in this article include small-angle neutron scattering (SANS), UV-Vis photometry, differential scanning calorimetry, oscillating rheometry, conductivity and electric birefringence measurements and electron microscopy.