Effect of cations on surfactant induced membrane wetting during membrane distillation

Abstract

Membrane wetting is regarded as challenges to the saline surfactant wastewater membrane distillation process, and the reason for wetting discrepancies under the assist of salt remains ambiguous. Herein, a comparative investigation was performed to reveal the mechanism of salt on sodium dodecyl sulfate (SDS) induced membrane wetting via the direct contact membrane distillation (DCMD) process. The results showed that different cations exhibited contrasting influences on membrane wetting. Compared to Na+, Mg2+ and Al3+, Ca2+ and K+ triggered significant flux increase. However, Ca2+ alleviated membrane wetting once feed temperature decreased. Feed solution characteristics indicated that Ca2+ and K+ significantly enhanced the Krafft point and hydrophobicity of micelles. Molecular dynamics simulation revealed the microcosmic interaction of Ca2+ and SDS, and verified that Ca2+ compressed the headgroup volume of SDS. By combining the critical packing parameter (CPP) of surfactants, dehydration of cations and micelle filtration experiments, it was concluded that the cations with weak hydration function in compressing the hydration layer of headgroups and facilitating the formation of more compact micelles that carry more SDS monomers, thus creating the transient adsorption of SDS in membrane pores and rapid membrane wetting. This study provides a new understanding of surfactant induced membrane wetting.