Hydrophobicity of Siloxane Surfaces in Smectites as Revealed by Aromatic Hydrocarbon Adsorption from Water

Abstract

Abstract--The nature of the siloxane surface in smectites was investigated by measuring the adsorption of aromatic hydrocarbons from water by organo-clays. The organo-clays were prepared by replacing the hydrophilic, inorganic exchange cations of a series of smectites with the small, hydrophobic organic cation, trimethylphenylammonium (TMPA). Smectites with a range in charge densities were used that resulted in different TMPA contents in the organo-clays. Adsorption isotherms of benzene, alkylbenzenes, and naphthalene from water by the TMPA-smectites indicated that sorption was inversely related to TMPA content. The Langmuir form of the isotherms suggests that the aromatic compounds adsorb to the clay surface. Possible adsorptive sites in TMPA-smectites are limited to the TMPA cations and the siloxane oxygen surfaces. Because sorption increased as layer charge and TMPA content decreased, the organic compounds must adsorb to the siloxane surfaces.Calculations based on an adsorbed compound monolayer, which was estimated by fitting adsorption data to the Langmuir equation, and the N2 specific surface area of each TMPA-clay, indicate that the surface area occupied by each adsorbed molecule increases as the planar area of the molecule increases. This strongly indicates that the planar surfaces of the compounds adsorb directly to the clay surface. Apparently, the TMPA cations function to keep the smectite interlayers open. Interactions between the phenyl groups of TMPA cations on opposing interlayer clay surfaces may act to increase the size of the adsorptive regions. These results show that the siloxane surfaces of smectites can effectively adsorb aromatic hydrocarbons from water if the hydrophilic, inorganic exchange cations are replaced with small, hydrophobic organic cations. The strong adsorption of hydrophobic organic molecules from water demonstrates the hydrophobicity of the siloxane surfaces in smectites.

Kinetic Study of Denatonium Sorption to Smectite Clay Minerals

“…However, when sorption activation energies are 40-800 kJ/mol, chemisorption is posited. In our study, an activation energy of + 25.9 kJ/ mol was calculated from which, we hypothesized that sorption proceeded via physisorption in agreement with others' work (Dogan et al, 2006;Gü rses et al, 2006;Rodríguez et al, 2010), with denatonium sorbing to the clay surface in a manner similar to other hydrophobic quaternary cations ( Jaynes and Boyd, 1991). Experimental support for a physisorptive or cation exchange mechanism was provided via triplicate single-point sorption experiments as a function of background electrolyte ionic strength.…”

Development and characterisation of integrated microfluidics on waveguide-based photonic platforms fabricated from hybrid materials

“…It has previously been demonstrated that polycondensation reactions of the silanol groups into irreversible siloxane groups are catalyzed by the inclusion of ZPO (Oubaha et al 2003). The result is a significant increase in siloxane bonds which are well-known to be hydrophobic (Jaynes and Boyd 1991). The underlying mechanism that causes increased hydrophobicity for 100% compared to 50% hydrolysed materials is similar to that explained above for increased ZPO content.…”

Crystal architectures of a layered silicate on monodisperse spherical silica particles cause the topochemical expansion of the core-shell particles

“…[16,23] The cation exchange capacity (CEC), which directly correlates with the negative-layer charge density, is an important determiner of the distance between adjacent interlayer cations (the spatial density). Achieving an appropriate distance can allow the material to act as a molecular sieve for nonionic organic compounds [24][25][26][27][28][29] or to improve the photofunctions of photoactive molecules. [30][31][32][33][34] It has therefore been recognised that one of the advantages smectites offer (in addition to the many other important properties of smectites and their intercalation compounds) is that the CEC can be varied in order to control the spatial distribution of organic moieties.…”