Hydrogen-Bond-Donating Acidity and Dipolarity/Polarizability of Surfaces within Silica Gels and Mesoporous MCM-41 Materials

Linear solvation energy (LSE) relationships are employed to characterize the internal polarities in the channels of a siliceous MCM-41 material and of a silica gel synthesized by a sol−gel process from tetramethoxysilane (TMOS). The polarity of the surfaces inside the silica gel and the mesopores of the MCM-41 can be quantitatively described by three independent terms: the dipolarity/polarizability (π*), the HBD (hydrogen-bond-donating) acidity (α), and the HBA (hydrogen-bond-accepting) ability (β). These terms are defined by means of the Kamlet−Taft solvent parameters α, β, and π* as a reference system. The following indicators have been encapsulated by the sol−gel process within the silica matrix or adsorbed to MCM-41: 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate ( 1 ), dicyanobis(1,10-phenanthrolin)iron(II) ( 2 ), and Michler's ketone ( 3 ). The solvatochromic UV/vis spectroscopic band shifts of these indicators correlate with the Kamlet−Taft solvent parameters α and π*. The hydrogen-bond-donating acidity, α, within MCM-41 increases significantly with decreasing temperature, whereas the dipolarity/polarizability term, π*, is temperature-independent. With 2 as indicator, an average hydrogen-bond acidity (α) is measured, whereas 3 is suitable to distinguish between differently acidic silanols within MCM-41 as shown by surface titration of the adsorbed indicator 3 with a sterically hindered base.