Influence of Surface Chemistry on the Surfactant Organization and Interfacial Structure of Mesoporous Silica and Organosilica

Abstract

Mesoporous silica (SiO2) and periodic mesoporous organosilica (PMO) with divinylbenzene (DVB)- and divinylaniline (DVA)-bridge groups are studied by solid-state NMR and molecular dynamics simulations to examine the influence of the surface chemistry on the interfacial interactions and the assembly structures of the surfactants. The NMR signals of the surfactants inside the mesopores appear at different chemical shift values depending on the organic bridging groups. Two-dimensional NMR spectroscopy shows that the surfactants assemble as micellar structures, where the polar head groups directly interact with the pore walls, and the interaction strength of the surfactants with pore wall is in the order of SiO2 >DVA-PMO > DVB-PMO. To elucidate the functional dependence of the interfacial structure of SiO2 and PMOs, the radial distribution function is calculated from molecular dynamics simulations, which provides individual atomic correlations quantitatively. For SiO2, the surfactant head is preferentially adsorbed to the inorganic layer with extended aliphatic tail group packing, whereas the adsorption of the surfactant head group to the pore wall is weaker for PMOs. We find that the hydrophobic interaction between DVB and the surfactant tail, which is stronger than that in the case of polar DVA, weakens the interaction between the surfactant head and organic layer for DVB-PMO more than for DVA-PMO. From the conformational analysis, we directly observe a more abundant gauche defect of the surfactant in DVA-PMO than in DVB-PMO. The upfield NMR shift in DVB-PMO is revealed by more planar aromatic ring orientation, which is caused by the steric effect between the organic groups. Our study suggests that the hydrophobicity and bulkiness of the organic bridge can be exploited in the design of PMOs for molecular machines, absorbents, and enzyme immobilization materials.