Kamlet−Taft's α (hydrogen-bond acidity) and π* (dipolarity/polarizability) parameters of three different
silica samples and a siliceous MCM-41 material determined in 17 various solvents are presented. Fe(phen)2(CN)2 [cis-dicyano-bis(1,10)phenanthrolineiron(II), (1)] and Michler's ketone [4,4‘-bis(N,N-dimethylamino)benzophenone, (2)] were used as solvatochromic surface polarity indicators. The UV/vis transmission
spectra of the two surface polarity indicators 1 and 2 adsorbed on the silicas from the solvents were
measured, and the absorption maxima were used to calculate the α and π* values of the silica surface.
α of the silanols on the freely accessible surfaces decreases with increasing HBA (hydrogen-bond accepting)
capacity of the solvent whereas the π* term of the solvent only marginally modifies the polarity of the
external surface silanol groups. In the mesopores of MCM-41, the UV/vis shift of the solvatochromic dyes
shows a different behavior as a function of solvent polarity than that on freely accessible silica surfaces.
The dipolarity/polarizability of solvent molecules inside the pores affects significantly the solvatochromic
shift of the dye in the pores. The specific polarity properties of the solvent/silica interface at freely accessible
surfaces and inside the MCM-41 pores are discussed.
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