Filled Nanoporous Surfaces: Controlled Formation and Wettability

Abstract

The controlled filling of hydrophobic nanoporous surfaces with hydrophilic molecules and their wetting properties are described and demonstrated by using thiocholesterol (TC) self-assembled monolayers (SAMs) on gold and mercaptoundecanoic acid (MUA) as the filling agent. A novel procedure was developed for filling the nanopores in the TC SAMs by immersing them into a "cocktail" solution of TC and MUA, with TC in huge excess. This procedure results in an increasing coverage of MUA with increasing immersion time up to an area fraction of approximately 23%, while the amount of TC remains almost constant. Our findings strongly support earlier observations where linear omega-substituted alkanethiols selectively fill defects (nanopores) in the TC SAM (Yang et al. Langmuir 1997, 12, 1704-1707). They also support the formation of a homogeneously mixed SAM, given by the distribution of TC on the gold surface, rather than of a phase-segregated overlayer structure with domains of varying size, shape, and composition. The wetting properties of the filled SAMs were investigated by measuring the most stable contact angle as well as contact angle hysteresis. It is shown that the most stable contact angle is very well described by the Cassie equation, since the drops are much larger than the scale of chemical heterogeneity of the SAM surfaces. In addition, it is demonstrated that contact angle hysteresis is sensitive to the chemical heterogeneity of the surface, even at the nanometric scale.