Anionic Surfactants with Divalent Gegenions of Diffuse or Separate Electric Charge: Solubility and Micelle Formation

Abstract

In order to clarify the relationships among solubility, micelle formation, and Krafft point, anionic surfactants with three types of divalent gegenions were prepared and their micelle formation and solubility examined. The first gegenion type was characterised by localized charge(divalent metal ion), the second by diffuse charged,(1,1′-dimethyl-[4,4′] bipyridinium ion or methylviologen ion), and the last by separate charged,(l,1′-alkanediyl-bis-pyridinium ion). The effects of the unlocalized gegenions were compared with those of the localized gegenions, and the following conclusions could be drawn from the experimental results: (i) the crystalline state with gegenion of diffuse or separate charge is less stable energetically and has higher solubility, and (ii) the difference in cmc value among the three types of surfactants is relatively small when the charge separation is small, but a large charge separation gives rise to marked decrease in cmc value suggesting that the alkylchain of separate gegenion folds and penetrates the inner hydrophobic part of the micelle. Also we observed that surfactants with more water of crystallization are easier to dissolve with a smaller enthalpy change of dissolution.