ET(30) surface polarity parameters of alkyl- and aryl-group-functionalized silica particles:: Differentiating the surface environments by means of the application of differently substituted Reichardt's dyes

E-T(30) surface polarity parameters are presented for alkyl-functionalized silica particles. Sixteen different chemically modified Aerosil and LiChrospher samples were used as silicas with various degrees of grafting and different chain lengths. The surface polarities of the silica samples were examined by measuring the UV/vis absorption maxima of differently substituted 4-(2,4,6-triphenyl-N-pyridinio)phenolate betaine dyes adsorbed to functionalized silica samples suspended in 1,2-dichloroethane or cyclohexane. The value of the E-T(30) surface polarity parameter of the silica particle decreases with increasing degree of functionalization of the silica surface with alkyl groups. The lengths of the alkyl groups have no observable influence upon the value of the E-T(30) parameter. The measured value of the E-T(30) surface polarity parameter of a partially alkyl-group-modified silica sample is lowered more by using dyes with sterically demanding alkyl substituents in the 2 and 6 position of the phenolate ring of the dye than by using the standard dye. Contributions of hydrogen bonds and dipolarity/polarizability effects on the specific E-T(30) values measured of individual silica particles with various residual silanol and alkyl groups, respectively, are discussed.