Electrical double layer interactions in a non-polar liquid measured with a modified surface force apparatus

Surface force measurements have been made between mica surfaces immersed in a non-polar liquid, decane, in the presence of an amphiphilic electrolyte, sodium di-2-ethylhexyl sulphosuccinate (AOT), using a modified Surface Force Apparatus (SFA). The modifications, notably the employment of a pair of sensitive force measuring springs and the implementation of a magnetic drive mechanism, have appreciably improved the sensitivity of SFA, which enables the detection of a weak long-range repulsion in the non-polar system. The origin of the repulsion is ascribed to electrical double layer interactions, and it may be satisfactorily described by a counterion-only double layer theory. This comprises the first report of electrical double layer interactions in a non-polar liquid.