Electrokinetic and solvatochromic studies of functionalized silica particles

Zeta potential plots, Kamlet-Taft's alpha (hydrogen bond-donating, HBD), be ta (hydrogen bond-accepting, HBA), and pi* (dipolarity/polarizability) para meters as well as Gutmann's DN (donor numbers) are presented for 12 differe ntly functionalized (i.e. alkyl, cyanoalkyl, aminoalkyl, and imidazolidine) silica particles. The pH-dependent zeta potential plots were measured in a queous solution, whereas the solvatochromic studies were performed in 1,2-d ichloroethane and cyclohexane as solvents using a special UV/visible spectr oscopic technique. For the determination of the solvatochromic polarity par ameters, a set of carefully characterized polarity indicators was employed: Cu(tmen)(acac)(+) B(C6H5)(4)(-) for beta and DN; dicyano-bis(1,10-phenanth roline)iron(II) for alpha; 4,4'-bis-(N,N-dimethylamino)benzophenone for pi* ; and an aminobenzofuranone derivative {3-(4-amino-3-methylphenyl)-7-phenyl -benzo[1,2b : 4,5b']difuran-2,6-dione} (ABF) for alpha, beta, and pi*. The calculation of the polarity parameters was carried out via linear solvation energy (LSE) correlation equations using the Kamlet-Taft solvent parameter s as a reference system. A linear correlation between the electrokineticall y determined value of the isoelectric point and the beta parameter of the f unctionalized silicas is demonstrated.