Impact Of Some Organics On Structural
Impact of Some Organics on Structural
and Adsorptive Characteristics
of Fumed Silica in Different Media
V.M. Gun\’ko1, V.I. Zarko1, E.F. Voronin1, V.V. Turov1,
I.I. Gerashchenko1, E.V. Goncharuk1, E.M. Pakhlov1, N.V. Guzenko1,
R. Leboda2, J. Skubiszewska-Zięba2, W. Janusz2, S. Chibowski2
1Institute of Surface Chemistry, Ukrainian National Academy of Sciences, General Naumov st. 17, 03164 Kiev, Ukraine,
2Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowskasq. 3, 20-031 Lublin, Poland
Fumed silica consisting of spherical primary particles forming a variety of secondary structures (aggregates of primary particles, agglomerates of aggregates and visible flocks) can markedly change the structural characteristics on heating, suspending-drying-suspending, adsorption of dissolved surfactants and polymers, interaction with polar or nonpolar solvents, etc. Therefore any pretreatment of fumed silica (structurally non-rigid in respect to its secondary particles, whose stability typically decreases with their size) can alter many of the properties important for different applications, as this material does not exist in the form of individual primary particles. The aim of this work was to study the impact of polymers (bovine serum albumin, egg albumin, gelatin, poly(vinyl pyrrolidone) PVP), an ionogenic surfactant (Aethonium, 1,2-ethylene-bis(N-dimethyl carbodecyloxymethyl) ammonium dichloride) and a polar solvent (ethanol) on the structural and adsorptive characteristics of fumed silica in the aqueous suspensions and dried powders prepared from the solid residual of centrifuged polymer/silica suspensions.
Pristine fumed silica and powders prepared by drying of solid residual of centrifuged aqueous suspensions of fumed silica, silica/PVP or silica/protein (egg albumin, gelatin) were studied using nitrogen adsorption and IR spectroscopy methods. Aqueous suspensions of silica, silica/polymer, silica/surfactant or silica suspended in a water/ethanol mixture were investigated using photon correlation spectroscopy, 1H NMR and protein or PVP adsorption methods. Proteins (albumin and gelatin) have a weaker
influence on the silica structuring on suspending-drying in comparison with PVP due to the difference in their adsorption in the form of globules (as proteins are characterized by strong intramolecular bonds) and unfolded PVP molecules (weak intramolecular forces) stronger rearranging primary particle swarms. Suspending-drying of silica and silica/polymers leads to reduction in the specific surface area but the pore volume increases in comparison with the pristine fumed silica, while polymers covering the surfaces of primary particles and their aggregates provide additional diminution in S but relative reduction in the pore volume in comparison with that for dried pure silica suspension is smaller (addition of 5 wt. % PVP even enhances the pore volume). Covering of silica by polymers blocking silanols changes the adsorption potential distributions (for water hydrogen-bonded to ºSiOH groups with great contribution of electrostatic interactions and nitrogen adsorbed through mainly dispersion interaction with any sites) and nitrogen adsorption energy distributions, as the adsorption potential is reduced at low coverage (narrow pore contribution decreases) but enhanced at great coverage corresponding to secondary filling of large mesopores.
Fig. 1. Pore size distributions in respect to dVp/dRp for pristine fumed silica A-300 and powders prepared by drying aqueous suspensions of (a) silica, egg albumin/silica, and gelatin/silica and (b) silica and PVP/silica.
Suspending-centrifuging-drying of polymer/silica changes the pore structure differently depending on a polymer kind and its concentration (Fig. 1). The availability of egg albumin or gelatin at different concentrations causes a weak effect (related mainly to filling of pores by protein molecules) in contrast to PVP (markedly rearranging secondary silica particles and pores in them) due to features of adsorption of these polymers. Albumin and gelatin can be adsorbed on silica surfaces in the globular form due to strong intramolecular forces and hydrophobic interaction between nonpolar side groups of amino acids. Typically, albumin or gelatin adsorbs on fumed silica in amounts of 300-600 mg/g. Strong adsorption of PVP corresponds to lower amounts approximately 100-150 mg per gram of silica. PVP in the amounts above 10 wt. % (g = CPVP/CSiO2 = 0.1) provides disturbing of a major portion of accessible ºSiOH groups, as the band at 3750 cm-1 practically disappears at CPVP = 17.5 wt.%. In the case of BSA/silica samples, this band is else observed at significantly larger BSA concentrations, e.g., CBSA = 30 wt.%, than CPVP. Consequently, one can assume that PVP molecules are adsorbed in the unfolded form (as intramolecular bonds are weak) in contrast to proteins keeping the globular structure in the liquid media or adsorption state. Additionally, strong interaction between unfolded PVP molecules and primary particles of fumed silica leads to decomposition of a significant portion of particle swarms, as nearly linear PVP molecules can penetrate into the aggregate channels and decompose the hydrogen bonds between primary silica particles.
Ionogenic surfactant Aethonium (low concentrations
Acknowledgements
This research was supported by NATO (grant No. EST.CLG.976890), the Polish State Committee for Scientific Research and Ministry of High Education and Science of Ukraine (grant No. 2М/303-99).
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