Krafft Points of Anionic Surfactants and Their Mixtures with Special Attention to Their Applicability in Hard Water

Article abstract of DOI:10.1021/j100455a015

The Krafft points of the sodium and calcium salts of typical anionic surfactants and their mixtures have been measured to examine their applicability in hard water.The pure model compounds of the linear alkylbenzene sulfonates, α-olefin sulfonates, and alkylpoly(oxyethylene) sulfates were synthesized and used for Krafft-point measurements.Among the above three types of surfactant, the alkylpoly(oxyethylene) sulfates are shown to be the best surfactant for their practical uses in hard water, since their sodium and calcium salts as well as their mixtures are readily soluble at room temperature.The Krafft point vs. composition curves observed in binary surfactant mixtures have been classified into two groups.In group I, there exists a minimum in the Krafft point at a certain composition, whereas the Krafft point varies monotonously with the composition change in group II.It is found from the composition analysis of the solid phase that both components are immiscible in group I but are completely miscible even in the solid phase in group II.The thermodynamic theory for freezing-point depression has been favorably applied to the Kraff point vs. composition curves in group I.Theoretical calculations for the Krafft point vs. composition curves (liquidus curve) and the corresponding solidus curves in group II have also been made, assuming the ideal solutions in both liquid (micellar) and solid phases.The calculated curves are in poor agreement with the observed ones probably because of the nonideality of the solution especially in the solid phase.