JOURNAL OF COLLOID AND INTERFACE SCIENCE 192, 166–178 (1997) ARTICLE NO. CS974985 Aqueous Suspensions of Fumed Silica and Adsorption of Proteins V. M. Gun’ko,* ,1 V. V. Turov, * V. I. Zarko, * V. V. Dudnik, * V. A. Tischenko, * O. A. Kazakova, * E. F. Voronin, * S. S. Siltchenko,² V. N. Barvinchenko, * and A. A. Chuiko * * Institute of Surface Chemistry, 31 Prospect Nauki, 252022 Kiev, Ukraine; and ² University of Kansas, 204 Lippincott, Lawrence, Kansas 66045 Received February 17, 1997; accepted May 9, 1997 tion–desorption isotherm hysteresis (8). A study of the in- Thermally stimulated depolarization spectra of frozen aqueous teraction between proteins and highly disperse silicas is also suspensions of fumed silica show a dependence of relaxation pro- of interest because these oxides are used as sorbents and cesses on the concentration of silica ( C SiO 2 ) due to a change in the supports for drugs immobilized for therapy purposes ( 9, 10 ) . particle–particle interaction (their electrical double layers) and Proteins can also be considered as models of some toxins, the concentration of bulk water (ice) with increasing C SiO 2 .A which can be strongly adsorbed by fumed silica as an entero- significant fraction of agglomerates of primary particle aggregates sorbent. Further, mineral particles containing such silicas as are not decomposed in aqueous suspensions of fumed silica. The asbestos aerosols can take part in the oxidation of proteins, thickness of an interfacial water layer perturbed by the silica sur- DNA, etc., and the active sites of the silica phase play an face or protein molecules was estimated from the dependence of 1 H NMR signal intensity of unfrozen water on temperature below important role in these reactions (9b). 273 K, separating the signals of water molecules weakly and The primary particles of fumed silica synthesized at high strongly bound to the surface. Protein molecules adsorbed on temperature (diameter of ca. 10 nm) form aggregates (ca. fumed silica aggregates form hydrogen bonds with GSiOH or 200 nm) by siloxane bonds via coalescence of these hot GSiO 0 groups by amino groups, e.g., H / NR from the zwitterion particles; thereupon agglomerates ( above 2 mm) of hydro- fragments. Aqueous suspensions of mechanochemically activated gen-bonded aggregates cooled to ambient temperature are fumed silica do not lose protein adsorption ability during a long formed in air (11, 12). In aqueous suspensions of fumed period in which the particles remain as microscaled agglomerates. silica, these agglomerates can be partially or completely de- According to AM1 – SM1 calculations, complexes such as composed due to interactions between water molecules and GSiO(H)rrrH / NRCOO 0 are stabilized in aqueous suspensions hydrophilic silica surfaces (11–15). Fumed silica in aque- of silica and the protein desorption rate decreases with increasing molecular weight of proteins.  1997 Academic Press ous suspensions possesses a high adsorption ability for pro- Key Words: fumed silica; aqueous suspension; TSD; protein ad- teins ( 10 ) and their interaction can change both the structure sorption; 1 H NMR; AM1–SM1 calculations. of the electrical double layer (EDL) of the silica particles or biopolymer molecules and the suspension stability ( 16, 17). The active sites on hydrophilic oxide surfaces form INTRODUCTION strong hydrogen bonds with water molecules; therefore, the structure and properties of interfacial water depend critically The mechanisms of protein interaction with solid surfaces on the nature of the surface and an active site distribution are of interest for biotechnology, medicine, ecology, and the for solids and biopolymers. With increasing temperature the food industry (1–3). According to studies of the kinetics hydrogen bond network in water is partially broken and the of protein interaction with silicas, clays, and polymers (4– interfacial layer thickness decreases (18). The temperature 7), a few stages of their adsorption were found to be (a) of water freezing near a hydrophilic surface is lower than diffusion of macromolecules toward the surface, (b) forma- that in the bulk due to the influence of the electrostatic field tion of adsorption complexes, and ( c ) transformation of mo- of the solid particles, leading to partial decomposition and lecular conformations due to formation of new bonds in disorder of the hydrogen bond network in the interfacial multicentered adsorption complexes. The last stage is the water layer. The molecular mobility (e.g., rotation) of some slowest and its rate is influenced by the nature of the proteins molecules in this layer increases as part of the degrees of and their concentration in solution. The formation of multi- freedom of the molecules is not linked. The unfrozen water centered adsorption complexes at this stage leads to adsorp- layer thickness was estimated from 1 H NMR data, measure- ment of viscosity, etc. (19–21). Additionally, disordering 1 To whom correspondence should be addressed. Fax: 380 44 264 0446. of the hydrogen bond network in interfacial water in suspen- 166 0021-9797/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.