Interactions between dodecylammonium chloride and carrageenans in the semidilute regime

Abstract

Interactions between dodecylammonium chloride (DDACl) and κ-, ι- and λ-carrageenans (abbreviated as KC, IC and LC) in the semidilute regime have been studied using dynamic light scattering, zeta potential and rheological measurements. Addition of oppositely charged surfactant monomers to carrageenan solutions leads to the formation of various soluble or insoluble complexes (PSC). An increase in the carrageenan linear charge density shifts the appearance of mixture turbidity and phase separation toward higher DDACl concentration. A decrease in PSC size and charge as well as in solution viscosity with increasing DDACl concentration indicates collapsing of carrageenan chains. At higher surfactant concentrations, light scattering shows the existence of larger PSC that remain negatively charged close to phase separation. Steady-state rheological measurements showed that all samples exhibit shear thinning behavior, well fitted by the power law equation, indicating that they mainly maintain carrageenan rheological properties. The change of flow behavior parameters with increasing DDACl concentration indicates progressive transition from shear thinning to nearly Newtonian behavior. The effect of DDACl concentration on flow behavior parameters increases from KC to LC, suggesting that PSC structural properties are mainly determined by the conformation and charge of carrageenan chains. Small-amplitude oscillatory shear measurements reveals viscoelastic behavior of all single carrageenan solutions. Upon the addition of DDACl the viscous contribution to viscoelastic deformation increases with increasing DDACl concentration indicating that the elasticity of carrageenan solutions was slightly reduced for all KC mixtures and for IC and LC mixtures at lower DDACl concentrations. The loss of carrageenan solution viscoelastic properties strongly depends on the carrageenan type and DDACl concentration.