The results of surface force measurements between water-plasma-modified mica surfaces in solution as a function of non-ionic surfactant (pentaoxyethylene dodecyl ether) concentration are presented. In the absence of surfactant the forces are purely repulsive and characteristic of silica in that there is an electrostatic repulsion and a repulsive deviation from DLVO theory at short separations. Only a slight decrease in the range of the extra repulsion was observed when the surfactant concentration was brought to one-third of the CMC. There was no significant effect on the surface force until the concentration was slightly higher than the CMC at which point a small amount of material became trapped between the surfaces and a small adhesion was observed. After a long equilibration period a long-range, hysteretic but reproducible force was observed. It is speculated that this force might be due to a local surface-induced phase transition. The forces measured in this system are compared with those measured between real silica surfaces.