Preparation of Porous Silica Materials via Sol-Gel Nanocasting of Nonionic Surfactants : A Mechanistic Study on the Self-Aggregation of Amphiphiles for the Precise Prediction of the Mesopore Size

Abstract

Sol-gel nanocasting is used to imprint the soft-matter structures of lyotropic phases of nonionic n-alkylpoly(ethylene oxide) amphiphiles ( CxEy ) into solid porous silica. Small angle X-ray scattering (SAXS), nitrogen sorption, and transmission electron microscopy (TEM) are used to investigate the dependence of the porosity on the block lengths or the block volumes, respectively. It is found that the size of the mesopores is a function of the lengths/volumes of both the alkyl chain (NA) and the PEO block (NB). Moreover, 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 fraction of the PEO chains contributes to the mesoporosity, while a significant portion leads to additional micropores. This model provides a quantitative description of the distribution of the hydrophobic and hydrophilic blocks within the lyotropic phase itself. Our findings indicate that the interface areas b2 of single surfactant chains are a function of the block lengths, which can be described by a scaling law b2 ∞ NA 0.16 NB 0.4. Mixtures of chemically equivalent amphiphiles with different block ratios are studied in further detail. It is seen that every pore size between the size originating from the parent templates can be adjusted simply by mixing various amounts of two surfactants, proving that true mixed phases act as a template for the silica pores.