Surface polarity of dimethylsiloxane-grafted silica particles

The surface polarity of dimethylsiloxane-grafted, silica particles (DMS/silica) has been studied as a function of surface coverage by measuring the UV/Vis spectra- of coadsorbed solvatochromic probes. Fe(phen)(2)(CN)(2) [cis-dicyano-bis-(1,10)-phenanthroline-iron(u), (1)] Michler's ketone [4,4'-bis(N,N-dimethylamino)-benzophenone, (2)], coumarine 153 (3) and 3-(4-amino-3-methylphenyl)-7phenylbenzo[1,2b:4,5b']difuran-2,6-dione (4) were used as solvatochromic surface polarity indicators. Kamlet-Taft's a (hydrogen bond acidity), beta (hydrogen bond basicity), and pi* (dipolarity/polarizability) parameters of the DMS/solid acid interfaces were calculated using different LSE relationships of v(max) data sets of the probes 1 and 2 or 3 (for alpha and pi*) as well as 1 and 4 (for beta). The values for a decrease significantly with increasing conversion of silanol groups with grafted DMS chains. Then, alpha asymptotically approachs a constant value when more than about 75% of the residual silanols on the surface are,grafted with DMS when measured in 1,2-dichloroethane slurry. The pi* values of DMS/silica solvent interfaces seems unaffected by the degree of surface functionalization. The beta values of DMS and grafted DMS/ silica are clearly lower than the values for a and pi*.