The effect of alkane tail length of CiE8 surfactants on transport to the silicone oil–water interface

Detailed surfactant transport studies have typically been restricted to the air–water interface. This is mainly due to the lack of experimental devices and techniques available to study liquid–liquid interfaces. As a result, there is a lack of relevant data and understanding of surfactant behavior in microfluidic studies and emulsion applications. Using a novel shape fitting algorithm for a pendant drop capable of handling fluids of similar densities, i.e. low Bond numbers, we measure the dynamic surface tension as a function of bulk surfactant concentration at the silicone oil–water interface for a homologous series of CiE8 nonionic surfactants. We show that the isotherms governing equilibrium at the oil–water and air–water interfaces are very different. Using a scaling analysis comparing two governing mass transport timescales, we demonstrate that there exists a transition from diffusion-limited to kinetic-limited dynamics at the silicone oil–water interface. Adsorption rate constants are determined from a one parameter nonlinear fit to dynamic surface tension data. These results demonstrate that the dynamics of interfacial transport are highly dependent on the immiscible fluids that form the interface.

Graphical abstractA scaling analysis is used to determine whether dynamic surface tension experiments exhibit diffusion-limited or kinetic-limited transport. If diffusion-limited, the equilibration timescale follows scaling expected for diffusion in a spherical geometry (solid line). If kinetic-limited, the timescale deviates from the diffusion scaling. Identifying when kinetics are relevant aids in correctly analyzing dynamic surface tension data to determine kinetic rate constants.Figure optionsDownload full-size imageDownload high-quality image (35 K)Download as PowerPoint slideResearch highlights► Scaling arguments determine relevant transport mechanisms for surface tension evolution. ► Diffusion-limited dynamics aid in fitting isotherm parameters. ► Adsorption rate constant decreases with increasing alkane tail length.