Detection of Local Density Distribution of Isolated Silanol Groups on Planar Silica Surfaces Using Nonlinear Optical Molecular Probes

Crystal Violet has been used as a nonlinear optical molecular probe for the detection of local density distribution of isolated silanol groups on planar silica surfaces. Because of its large size (∼120 Å 2 ) and nearly flat adsorption geometry, Crystal Violet has successfully separated the truly isolated silanol groups (i.e., the silanol groups that cannot form a hydrogen bond with their neighbors by any means, ∼9.3 ×10 13  cm -2 ) into two major classes. The first class includes those isolated silanol groups; each is surrounded by a large empty surface area (≥120 Å 2 ) in which a Crystal Violet cation can be placed. The surface density of this type of silanol groups is ∼1.1 ×10 13  cm -2 . The second class includes the rest of the isolated silanol groups with a surface density of 8.2 × 10 13  cm -2 .