Probing the Surface Polarity of Various Silicas and Other Moderately Strong Solid Acids by Means of Different Genuine Solvatochromic Dyes

Abstract

Reichardt's E T (30) as well as Kamlet-Taft's R (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and aluminafunctionalized silica particles, are presented. The E T (30) values of the solid acids were directly measured by UV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorinesubstituted derivative of 1a (1b) as surface polarity indicators. Kamlet-Taft's R and π* parameters for the various solid acids are analyzed by means of Fe(phen) 2 (CN) 2 [cis-dicyano-bis(1,10-phenanthroline)iron(II)] (2) and Michler's ketone [4,4′-bis(N,N-dimethylamino)benzophenone] (3) as solvatochromic surface polarity indicators. The chemical interpretation of the R and π* parameters and the nature of the surface sites which they reflect are discussed. The correspondence of the UV/vis spectroscopic results to those of the IR-sensitive probe benzophenone and the fluorescence probe pyrene (literature data) is also discussed. It is evident that the solid surface environments observed by the various indicators are moderately strong dipolar/polarizable (π* ) 0.38 to 1.04) and are fairly strong hydrogen-bond donors (R ) 1.00 to 1.99). Theoretical E T (30) values of solid acids are calculated by applying linear solvation energy (LSE) relationships using the independently measured R and π* values of the solid acids according to E T ( 30) ) [E T (30)] o + aR + sπ*. The respective dependence of the R and π* terms, expressed by the coefficients a and s, upon the measured E T (30) value for solid acids is discussed in comparison to related multiple LSE correlations known for wellbehaved regular solvents and functionalized silicas. The results show in general that values of polarity parameters of strong HBD and dipolar surfaces are strongly influenced by the experimental conditions applied.

Empirical polarity parameters for various macromolecular and related materials

“…The following correlation equations are used in this paper for the calculation of the a and p* values by simultaneous use of the indicators 2 and 3 50) . Eq.…”

“…We especially recommend the iron complex 3 as a very suitable surface polarity indicator for moderately acidic surfaces because it can also be used for stronger acidic environments 50) . Fig.…”

Employing perichromism for probing the properties of carboxymethyl cellulose films: an expedient, accurate method for the determination of the degree of substitution of the biopolymer derivative

“…Probes have been employed in order to study the properties of inorganic solid surfaces, e.g., silica, alumina, and titanium dioxide Fidale et al 2010;Khristenko et al 2006;Spange et al 2000Spange et al , 2005Zimmermann et al 2002). They have also been employed in order to probe the surface properties of: Untreated and treated celluloses and other carbohydrates (microcrystalline cellulose; bacterial cellulose; cotton linters; regenerated cellulose; sulfite kraft pulp; starch; chitin, chitosane) (Fidale et al 2010;Fischer and Spange 2000;Fischer et al 2002;Spange et al 2003 Fig.…”

Probing the dependence of the properties of cellulose acetates and their films on the degree of biopolymer substitution: use of solvatochromic indicators and thermal analysis

“…1, vide infra. Probes have been employed in order to study the properties of inorganic solid surfaces, e.g., silica, alumina, and titanium dioxide (Khristenko et al 2006;Spange et al 2000Spange et al , 2005Zimmermann et al 2002). Fig.…”