Enhancement of water transport through the liquid-vapor interface by surfactants

Interactions between surfactants and water were monitored from the perspective of liquid-vapor interfaces. The surfactants studied were chosen to be the paradigms of the three major types of amphiphilic molecules: the anionic surfactant (e.g. sodium dodecyl sulfate), the cationic surfactant (e.g. dodecyltrimethylammonium chloride), and the nonionic surfactant (e.g., n-dodecyl-beta-D-maltoside). Droplets were deposited on octadecyltrichlorosilane (OTS) covered silicon surfaces and their contact angle and contact base diameter were observed as a function of time to obtain information on the drying dynamics. Using a new method to evaluate the solubility parameters of liquids via evaporating sessile droplets, information on the assembling of surfactants as well as their interfacial behavior at the water-vapor interface is extracted. It is concluded that surfactants accelerate drying in pre-micellar solutions. Formation of micelles retards this acceleration. Surfactants, depending on the type and concentration, interfere with the intermolecular force among liquid, with the most significant influence observed around the critical micelle concentration. Monomers with hydrogen bond-forming head groups and longer tails influence the liquid more than the other types of surfactants. Small electrolyte molecules affect the drying dynamics of ionic surfactant solutions more than nonionic surfactant solutions.