Effects of Charged Surfactants on Interfacial Water Structure and Macroscopic Properties of the Air-Water Interface.

Abstract

Surfactants are used to control the macroscopic properties of air-water interfaces. However, the link between the molecular structure of surfactant adsorption and macroscopic properties is poorly understood. Using sum-frequency generation spectroscopy and molecular dynamics simulation, we show that two ionic surfactants (dodecyl trimethylammonium bromide and sodium dodecyl sulphate) with the same carbon chain lengths and head group charge magnitude (but different signs) can interact and reorient interfacial water molecules differently. DTAB forms a thicker but sparser interfacial layer than SDS. It is due to the deep penetration into the adsorption zone of Br- counterions compared to smaller Na+ ones, and also due to the flip-flop orientation of water molecules. SDS alters two distinctive interfacial water layers into a layer where H+ points to the air, forming strong hydrogen bonding with the sulphate headgroup. In contrast, only weaker dipole-dipole interactions with the DTAB headgroup are formed as they reorient water molecules with H+ point down to water. Hence, with more molecules adsorbed at the interface, SDS builds up a higher interfacial pressure than DTAB, producing lower surface tension and higher foam stability at a similar bulk concentration. Our findings offer improved knowledge for understanding various processes in the industry and nature.