Hydrophobic silicas with well-defines chemistry and geometry: Synthesis, adsorption, and interfacial properties

The synthesis of mesoporous silicas with a highly symmetric ordered network of pores is an area of extensive research with potential applications in catalysis, adsorption, and materials chemistry. Particularly attractive is the possibility to enhance these materials by covalently binding organosilanes to the surface, thereby opening a path toward structured materials with an extensive range of surface properties. As it is well understood that the underlying substrate can have much influence on functionalization, an extensive study into the effects that silica surface curvature has on the ability to covalently bind alkylsilanes of increasing chain length (C1 to C30) will be presented and discussed in Chapter 1. Three types of substrate geometries will be the focus of this chapter: convex surfaces of fumed silicas, concave surfaces of ordered mesoporous templated silicas, and the relatively flat surfaces of large pore silica gels. Comparisons will be made in regards to packing density and molecular ordering, with special attention being placed on the surface functionalization of templated silicas.;Functionalization of templated silicas can be accomplished through one of two separate and distinct techniques: (1) the covalent binding of organosilane groups to the surface in a process known as post-synthesis grafting or (2) through the direct incorporation of organosilane groups into the silica structure in a process known as co-condensation. Over the past several years, co-condensation has gained much attention as it enables one to seemingly synthesize a well-ordered functionalized material in just a single step. While a significant number of publications have been written in regards to this technique, very few have compared material synthesized through this method to material synthesized through traditional post-synthesis grafting. The work presented in Chapter 2 will systematically compare these two methods, focusing primarily on the characteristics of the prepared materials when functionalized with alkylsilane chains of increasing length. Homogeneity of the surfaces, long-range ordering in the pores, and apparent hydrophobicity will be discussed.;Lipid bilayers on the surface of silica will be the focus of Chapter 3. Here, lipid bilayers consisting largely of phosphatidylcholine will be deposited on the surface of mesoporous silica with pore sizes ranging from 6 nm to 250 run in an effort to determine their intrinsic flexibility and ability to conform to the contours of the underlying surface. Lipids supported on solid surfaces are of great interest due to their ability to serve as artificial model membranes, thus playing an important role in gaining insight into the physical and chemical characteristics associated with these biological barriers.;While the adsorption of lipid bilayers provides insight into the cell membrane, the adsorption of dissolved gas onto hydrophobic silicas provides insight into a well known phenomenon within the chromatographic community. Commonly referred to as "phase collapse", the sudden loss of analyte retention in reverse phase HPLC due to the use of an eluent with a high aqueous percentage is observed quite frequently when attempting to separate polar species that do not interact strongly with the stationary phase. This phenomenon has primarily been attributed to either a physical collapse of the alkyl chains or a de-wetting of the modified hydrophobic surface. Unfortunately, these current proposed mechanisms do not completely explain the experimental data and are not supported by direct investigation of the water/hydrophobic interface. Here, an alternative explanation is presented which involves the adsorption of dissolved gas to form a vapor barrier between the hydrophobic stationary phase and the surrounding water. This will be discussed in Chapter 4.;Although still focused on the chemistry of silica, the final chapter departs from the previous four chapters in that silicon dioxide is no longer the principle substrate. Here, a manufacturer of printing plates sought guidance on a dilemma regarding the quality of their product. For customers located in areas of high heat and high humidity, an unusually high number of printing plates were producing printed media of substandard results due to a separation of the top silicone layer from the underlying titanium layer. The final chapter (Chapter 5) will focus on why these plates were failing, providing a clear assessment of what is actually occurring at the titanium/silicone interface in addition to detailing what can be done to alleviate this issue.