Pore expanding effect of hydrophobic agent on 100 nm-sized mesoporous silica particles estimated based on Hansen solubility parameters

Mesoporous silica particles (MSPs) are synthesized in aqueous medium by a sol-gel reaction, wherein a surfactant cetyltrimethylammonium bromide (CTAB) promotes silica growth and controls the mesoscopic structure of MSP. To estimate an essential role of hydrophobic agent, herein we synthesized the 100 nm-sized spherical MSPs in the presence of hydrophobic agents, and investigated their pore size distribution. The coexistence of cyclohexane, hexane, or 1,3,5- trimethylbenzene with CTAB micelles resulted in bimodal pore size distributions, with larger pore sizes up to 10.5 nm. In contrast, the coexistence of benzene or toluene resulted in a single peak of pore size distribution ranging from 2.8 to 3.3 nm, which was similar to control (2.3 nm, without hydrophobic agents). Because the hydrophobic core region of CTAB micelle can be considered as an assembly of hexadecane molecules, Hansen solubility parameter distance (Ra value) was calculated for hexadecane and each hydrophobic agent. A lower Ra value indicates higher affinity of CTAB micelle and hydrophobic agent, and the volume fraction of large pores increased with decreasing Ra values. No pore expansion effect of benzene could be consistent with its highest Ra value among the hydrophobic agents tested here. It is concluded that the effect of hydrophobic agent on pore expansion in 100 nm size MSPs can be estimated based on the Ra value as indicator for micelle-hydrophobic agent affinity.