The hydrophobicity of a titania surface was increased by adsorption of octadecyltrihydrosilane (OTHS). The modified titania surface was characterised using a number of techniques: tapping mode AFM, streaming potential, X-Ray photoelectron spectroscopy and contact angle measurements. With a partial surface coverage of OTHS, a bell-shaped dependence of contact angle with respect to pH was observed, with the maximum advancing contact angle occurring at the point of zero charge of the titanium dioxide substrate. The contact angle change was also influenced by the surface coverage and salt concentration. At extreme pH values there was no further decrease in the contact angle, i.e. saturation occurred. A thermodynamic model was used to describe the influence of surface charge on the contact angle. The results were interpreted as a reduction in the solid-liquid interfacial free energy. The model described the results well until the point where saturation occurred.