Effect of the Surfactant Charge and Concentration on the Change in the Forces between Two Charged Surfaces in...

A combined atomic force microscope (AFM)-peristaltic pump system was used to determine the effect of a flow on the forces between two negatively charged surfaces (silica particle and silicon wafer) in aqueous solutions containing surfactants. The effect of the surfactant charge on the forces was determined by using an anionic surfactant (sodium dodecyl sulfate, SDS) and a cationic surfactant (dodecyltrimethylammonium bromide, DTAB) of the same chain length. The surfactant concentration effect was determined by using concentrations up to the critical micelle concentration. In the case of SDS, a flow reduced the range and magnitude of the repulsive forces. The force range reduction was explained by a shrinking of the diffuse layers, due to the deformation of the diffuse layer by the flow. The force magnitude reduction was explained by (1) the increased electrostatic screening due to the thinner diffuse layers and (2) an increased adsorption of specific ions, such as Na⁺, to the silica surfaces. In the case of DTAB, a concentration (8.0 mM) that gave an attractive force in the absence of a flow gave a repulsive force in the presence of a flow. Comparison of AFM images of a silicon wafer in DTAB measured in the absence and presence of a liquid flow showed that the number of DTAB patches adsorbed to the silicon wafer increased with a liquid flow. The change in the forces with a flow was therefore explained by this change in the DTAB adsorption to the negatively charged surfaces. As a liquid flow can change the charge of a surface, it may be possible to control the aggregation/dispersion of charged particles via the flow rate, if the appropriate surfactant type and concentration are used.