Interaction forces between hydrophilic surfaces in electrolyte dimethyl sulfoxide solutions measured by atomic force microscopy

Interaction forces between tip-mica and silica-mica surfaces in lithium chloride dimethyl sulfoxide (DMSO) solutions were measured using atomic force microscopy (AFM) to investigate the detailed structure of adsorbed layers on surfaces. It is found that there exists a firm primary adsorbed layer which varies from a single layer of DMSO molecules to that of solvated Li+ ions with increasing electrolyte concentration. In this study, the dynamic procedure of AFM was also introduced to evaluate the fragile structure of the adsorbed layer. We show that the fragile secondary layer does exist outside the firm primary layer whose thickness increases with the electrolyte concentration. The existence of the fragile secondary layer is also confirmed by experiments on the restructuring time of the adsorbed layer. All the results are consistent with our hypothesis proposed in our previous report, where aqueous solutions were employed so that the existence of the secondary layer was not as clear as the present one.