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Adsorption of short-chain tetraalkylammonium bromide on silica

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  • Chemistry


Langmuir 1993,9, 3553-3557 Adsorption of Short-Chain Tetraalkylammonium Bromide on Silica J. C. J. van der Donck, G. E. J. Vaessen, and H. N. Stein' Laboratory of Colloid Chemistry and Thermodynamics, Department of Chemical Technology, Eindhoven University of Technology, P.O. Box 513,5600 MB Eindhoven, The Netherlands Received March 18, 1993. I n Final Form: July 2 , 1 9 9 9 The adsorption of tetraalkylammonium (TAA) ions on silica shifts the isoelectric point to higher pH values. The IEP shift increases with increasing alkyl chain length. Also, tetraethylammonium (TEA) and tetrapropylammonium (TPA) ions can be adsorbed in larger amounts than tetramethylammonium (TMA) ions. The chemisorption of TAA ions can be explained by assuming an attraction between hydrophobically hydrated regions around surface siloxane bridges and around the TAA ions. The adsorption data of TMA ions are compatible with the stimulated adsorption model. Introduction Solutions containing both silicate ions and tetraalky- lammonium (TAA) ions are of interest in chemical technology because they are used in zeolite syntheses;14 in such solutions precursors of zeolite crystals are expected to be formed. This refers among others to TAA ions with rather short alkyl chains (up to butyl). In these solutions pronounced quantities of oligomeric silicate ions (cyclic trimer, cubic octamer) have indeed been found.7-9 From a theoretical point of view the interaction between the TAA and the silicate ions, which is responsible for such a precursor formation, is interesting. This interaction is expected in first instance to be an attraction between the TAA and the silicate ions due to their opposite electrical charges. However, the viscosity of tetraalkylammonium silicate solutions indicates that in addition a repulsive force is present.1° This conclusion is supported by the observation that, in solutions con- taining a sufficient amount of TAA and silicate ions, plus other ions (e.g., sod

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