Hydrogen-Bond Donating and Dipolarity/Polarizability Properties of Chemically Functionalized Silica Particles

Linear solvation energy (LSE) relationships are employed to characterize the surface polarity of organically functionalized silicas. The polarity of the silica surfaces should be quantitatively described by three independent terms: the dipolarity/polarizability (π*), the hydrogen-bond donating ability (α), and the hydrogen-bond accepting ability (β). These terms can be defined by using the Kamlet−Taft solvent parameters α, β, and π* as a reference system. Kamlet−Taft α and π* values and Reichardt's E T (30) values are presented for 30 differently functionalized silica particles and bare silicas. The surface polarity parameters α and π* were determined by means of correlation analyses of the energy of the UV−vis absorption maxima (ν max ) of selected solvatochromic probe dyes which are adsorbed to the particle surfaces. The following surface polarity indicators have been used: 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate and its penta- tert -butyl substituted derivative, cis -dicyanobis(1,10-phenanthroline)iron(II), and bis-4,4‘-( N, N -dimethylamino)benzophenone. The α values of organically modified LiChrosphers, which were synthesized with monofunctional silanes, linearly decrease with the amount of surface coverage of the functional group μmol m - 2 . Using trifunctional silanes as reagents, the α value of modified silicas is influenced in a more complex manner because the functionalization process occurs not uniform. Correlations of the values of surface polarity parameters with each other and with literature data demonstrate that the reported values are relevant empirical constants. The E T (30) parameter of chemically modified silicas can be expressed by the specific LSE equation: E T (30)(measured) = 14.84α + 5.33π* + 36.1; n = 30, r 2 = 0.9334. This E T (30) LSE relationship, derived for functionalized silicas, is compared to the equation derived for pure solvents as reported by Marcus (Marcus, Y. Chem. Soc. Rev. 1993, 409).