Adsorption Of Monocarboxylic Acids
ADSORPTION OF MONOCARBOXYLIC ACIDS
ON THE SURFACE OF ORGANOSILICAS
L. A. Belyakova, N. N. Vlasova, A. M. Varvarin,
L. P. Golovkova, D. Y. Lyashenko
Institute of Surface Chemistry of National Academy of Sciences, Kiev, Ukraine
To conduct adsorption of monocarboxylic acids the chemical modeling of active centers on the surface of amorphous highly dispersed silica has been performed:
– chemical grafting of aminopropyl groups on the surface of amorphous highly dispersed silica;
– chemical grafting of imidazolyl groups on the surface of amorphous highly dispersed silica;
– preparation of trimethylsilyl hydrophobic “pockets” with grafted aminopropyl groups on the silica surface;
– preparation of hydrophobic silica surface by the substitution of surface silanol groups by trimethylsilyl radicals;
– preparation of hydrophobic silica surface by application of water insoluble polymer;
– preparation of silicas with irreversible adsorbed albumin.
The adsorption of some monocarboxylic acids (benzoic, salicylic, a-picolinic and bile acids desoxycholic, glycocholic and taurocholic) from water solution at different pH on the surface of synthesized organosilicas and initial hydroxylated highly dispersed silica has been studied. The carboxylic acids, which were selected for the investigation, have various constants of acidic dissociation.
It was established that salicylic and bile acids adsorbed on the surface of highly dispersed silica. All these monocarboxylic acids are poor soluble in water because of formation of intermolecular hydrogen bond for salicylic acid and presence of large hydrocarbon radical in bile acids. The parameter, which characterizes the molecule’s hydrophobicity, is the coefficient of distribution between organic phase (for example, n-octanol) and water. The values of distribution coefficients for the bile acids are significantly higher than the same
values for the benzoic and picolinic acids. It allowed us to consider that the adsorption of bile and salicylic acids on the surface of highly dispersed silica is determined by “hydrophobic ejection” of molecules from water. The adsorption on the solid surface is more preferable than energetically disadvantageous solvation. The shape of curve, which represents the dependency of adsorption value against pH, indicates that acids adsorbs, in general, in molecular form. The adsorption of acids significantly decreases, if pH increases. The formation of acid anions in solution and negatively charged ºSi-O- groups on the silica surface takes place in the solution at pH>pK. The repulsion of likely charged particles prevents the adsorption of carboxylic acids.
All the studied carboxylic acids are adsorbed on the surface of organosilicas, which contain donor aminopropyl and imidazolyl groups. Comparing to the adsorption of bile acids on initial silica, the pH range extends when adsorption of these acids takes place on the surface of organosilicas. The comparison of distribution diagrams for the acid-base forms of adsorptives in the solution and distribution of protonated and neutral forms on the surface of organosilicas (the calculation were performed by GRFIT program) has shown that the studied carboxylic acids are adsorbed on the modified silicas in the forms of molecules and anions. The equilibrium constants for the formation of corresponding adsorption complexes are calculated. The stability of these complexes correlates with protonization constants of surface functional groups and dissociation constants of carboxylic acids.
It was established that the increase of adsorption ability of silica towards the monocarboxylic acids, including bile acids, is observed as a result of its modification by hydrophobic groups of different chemical composition and structure. The adsorption of carboxylic acids on the surface of silicas modified by albumin is observed. It was shown that the increase of albumin content on the silica surface (up to the concentration which corresponds to the monolayer) results in the increase of adsorption of salicylic and desoxycholic acid. The dependency of adsorption versus pH has its maximum, which corresponds to the isoelectric protein point (pH»5).
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