The mechanism by which macroscopic measured contact angles on quartz surfaces change is disucssed. Measurements made with decane-water-quartz systems indicate an increase followed by a decreae in the contact angle as the concentration of CTAB is increased. The angle does not change substantially when SDS is used. The predicted contact angles using on the measured DLVO components of disjoining pressure do not indicate the same trend. The decrease in contact angles at above the c.m.c. can be attributed to the formation of bilayers at the quartz surface. Any attempt made at predicting changes in contact angles must include the structural components in the disjoining pressure. This component dominates all the other interactions in the calculation of the apparent contact angle.