Phase separation of mixed micelles and synthesis of hierarchical porous materials.

Abstract

The mixed micelle template approach is one of the most promising synthesis methods for hierarchical porous materials. Although considerable research efforts have been made to explore the formation mechanism, explicit theoretical guidance for appropriately choosing templates is still not available. We found that the phase separation occurring in the mixed micelles would be the key point for the synthesis of hierarchical porous materials. Herein, the pseudophase separation theory for the critical micelle concentration (cmc) combined with the Flory-Huggins theory for the chain molecular mixture were employed to investigate the properties of mixed surfactant aqueous solutions. The cmc values of mixed surfactant solutions were experimentally determined to calculate the Flory-Huggins interaction parameter between two surfactants, χ. When χ is larger than the critical value, χc, the phase separation would occur within the micellar phase, resulting in two types of mixed micelles with different surfactant compositions, and hence different sizes, which could be used as the dual-template to induce bimodal pores with different pore sizes. Therefore, the Flory-Huggins theory could be a theoretical basis to judge whether the mixed surfactants were the suitable templates for inducing hierarchical porous materials. We chose cetyltrimethylammonium bromide (CTAB) and n-octylamine (OA) as a testing system. The phase separation behavior of the mixed solutions as well as the successful synthesis of hierarchical porous materials by this dual-template indicated the feasibility of preparing hierarchical porous materials based on the concept of phase separation of the mixed micelles.