Correlation of oil-water and air-water contact angles of diverse silanized surfaces and relationship to fluid interfacial tensions.

The use of air-water, θ(wa), or air-liquid contact angles is customary in surface science, while oil-water contact angles, θ(ow), are of paramount importance in subsurface multiphase flow phenomena including petroleum recovery, nonaqueous phase liquid fate and transport, and geological carbon sequestration. In this paper we determine both the air-water and oil-water contact angles of silica surfaces modified with a diverse selection of silanes, using hexadecane as the oil. The silanes included alkylsilanes, alkylarylsilanes, and silanes with alkyl or aryl groups that are functionalized with heteroatoms such as N, O, and S. These silanes yielded surfaces with wettabilities from water wet to oil wet, including specific silanized surfaces functionalized with heteroatoms that yield intermediate wet surfaces. The oil-water contact angles for clean and silanized surfaces, excluding one partially fluorinated surface, correlate linearly with air-water contact angles with a slope of 1.41 (R = 0.981, n = 13). These data were used to examine a previously untested theoretical treatment relating air-water and oil-water contact angles in terms of fluid interfacial energies. Plotting the cosines of these contact angles against one another, we obtain the relationship cos θ(wa) = 0.667 cos θ(ow) + 0.384 (R = 0.981, n = 13), intercepting cos θ(ow) = -1 at -0.284, which is in excellent agreement with the linear assumption of the theory. The theoretical slope, based on the fluid interfacial tensions σ(wa), σ(ow), and σ(oa), is 0.67. We also demonstrate how silanes can be used to alter the wettability of the interior of a pore network micromodel device constructed in silicon/silica with a glass cover plate. Such micromodels are used to study multiphase flow phenomena. The contact angle of the resulting interior was determined in situ. An intermediate wet micromodel gave a contact angle in excellent agreement with that obtained on an open planar silica surface using the same silane.