Contact angles and surface properties of nanoporous materials

The validity of thermodynamics at the nanoscale has been questioned, but we demonstrate that it can be applied to determine surface properties of a nonporous and of a mesoporous silica from the measured adsorption isotherms of three hydrocarbon vapors. These measurements give the total amount of vapor adsorbed and liquid formed in the pores as a function of pressure. We compare these measurements with the thermodynamic predictions when the pressure dependence of the contact angle inside the pore is taken into account and determine the values of the surface properties by requiring the thermodynamically predicted isotherms to have the same pressure dependence as the measurements. We assess the procedure by considering the consistency of the property values obtained with each of the three vapors, and find the properties differ by only a few percent. This consistency indicates that thermodynamics is valid at least in pores down to a radius of 1.3 nm.