Preparation of porous silica materials via sol-gel nanocasting of nonionicsurfactants: A mechanistic study on the self-aggregation of amphiphiles for the precise prediction of the mesopore size

Sol-gel nanocasting is used to imprint the soft-matter structures of lyotro pic phases of nonionic n-alkylpoly(ethylene oxide) amphiphiles ("CxEy") int o solid porous silica. Small angle X-ray scattering (SAXS), nitrogen sorpti on, and transmission electron microscopy (TEM) are used to investigate the dependence of the porosity on the block lengths or the block volumes, respe ctively. It is found that the size of the mesopores is a function of the le ngths/volumes of both the alkyl chain (N-A) and the PEO block (N-B). Moreov er, the materials contain a substantial degree of additional microporosity. A quantitative model is developed that relates the amphiphile organization during the nanocasting to the size of the mesopores and the microporosity. In particular, it turns out that depending on the number of EO units a fra ction of the PEO chains contributes to the mesoporosity, while a significan t portion leads to additional micropores. This model provides a quantitativ e description of the distribution of the hydrophobic and hydrophilic blocks within the lyotropic phase itself. Our findings indicate that the interfac e areas b(2) of single surfactant chains are a function of the block length s, which can be described by a scaling law b(2) proportional to (NANB0.4)-N -0.16. Mixtures of chemically equivalent amphiphiles with different block r atios are studied in further detail. It is seen that every pore size betwee n the size originating from the "parent" templates can be adjusted simply b y mixing various amounts of two surfactants, proving that true mixed phases act as a template for the silica pores.