Effects of Calcium Ions on the Solubility and Rheological Behavior of a C22-Tailed Hydroxyl Sulfobetaine Surfactant in Aqueous Solution.

Effects of calcium ions (Ca2+) on the solubility, aggregate structure, and rheological behavior of a C22-tailed zwitterionic surfactant, erucyl dimethyl amidopropyl hydroxyl sulfobetaine (EHSB), have been investigated in aqueous solution. In comparison with sodium ions (Na+), Ca2+ ions exhibit a much higher efficiency in decreasing the Krafft temperature (TK) of EHSB. Specifically, contrary to Na+ ions which have no obvious effect on the rheological properties of the EHSB solution, Ca2+ ions increase the viscosity of the EHSB solution at lower EHSB concentration, and enhance its elasticity at higher EHSB concentration. Moreover, Ca2+ ions raise the temperature needed for the elastic-to-viscous transition of the EHSB solution at higher concentration. At lower EHSB concentration, the hydrophobic interaction between the ultralong hydrocarbon chains induces a tighter packing of the hydrophobic chains by forming a more stretched configuration, while at higher EHSB concentration, the electrostatic attraction between Ca2+ ions and the sulfonate groups of EHSB induces a tighter packing of the headgroups by forming Ca2+-mediated bridges among the EHSB headgroups. Besides, the above interactions may strengthen the hydrogen bonding of OH groups and/or of C═O amide groups, which in turn facilitates the compact packing of the surfactant molecules in aggregates and promotes the growth and entanglement of wormlike micelles. Thus, the EHSB solution shows Ca2+-dependent rheological behaviors. The solubility and rheological properties of the ultralong chain surfactant solution can be simultaneously improved with the addition of divalent Ca2+ ions.