Raman Investigation of Surface OH-Species in Porous Silica

Surface in porous media has a key role for both theoretical and technological aspects. A comparative analysis of the surface vibrational properties of sol−gel synthesized porous silica monoliths with different porosity is presented. Raman spectroscopy investigation of the fundamental O−H stretching range (3000−3800 cm-1) reveals a dependence of the surface reactivity on pore dimensions. The pore surface curvature plays an important role affecting the hydrogen-bonding interaction between surface hydroxyls and modifying the distribution of the OH species at the surface; the relative contribution of interacting hydroxyls and adsorbed water with respect to isolated species is larger in smaller pores. Water and silanol vibrations were singled out as a function of porosity.