Effect of surface modification on the pore condensation of fluids: experimental results and density functional theory

The physisorption and pore condensation of a polar fluid (CHF₃) in a series of MCM-41 type mesoporous silica materials with native and chemically modified pore walls has been studied over the temperature range 168–293 K, corresponding to reduced temperatures T/T_c in the range 0.56-0.98, where T_c is the critical temperature of the fluid. Chemical modification of the pore walls by attachment of Si(CH₃)₃ groups causes a shift in pore condensation to higher relative pressures p/p0. This effect is most pronounced for materials with narrow pores (2.9 nm) at low temperatures. In the theoretical part of the work density functional theory based on a simple cubic lattice model of the confined fluid has been used to analyse the combined effect of a reduced pore width and weaker fluid-wall interaction caused by the surface coating. For realistic values of the model parameters it is found that the effect of the lower pore width is outweighed by the opposing effect of the lower fluid-wall interactions. The weaker temperature dependence of the pore existence curve observed experimentally for the surface modified materials can be traced back to a crossover from a two-step to a single-step process of pore filling predicted by the model.