The Effect of Surface Modification by an Organosilane on the Electrochemical Properties of Kaolinite

Abstract

Abstract--The electrochemical properties of kaolinite before and after modification with chlorodimethyloctadecylsilane have been studied by electrophoretic mobility, surface charge titration, and extrapolated yield stress measurements as a function of pH and ionic strength. A heteropolar model of kaolinite, which views the particles as having a pH-independent permanent negative charge on the basal planes and a pHdependent charge on the edges, has been used to model the data. The zeta potential and surface charge titration experimental data have been used simultaneously to calculate acid and ion complexation equilibrium constants using a surface complex model of the oxide-solution interface. The experimental data were modeled following subtraction of the basal plane constant negative charge, describing only the edge electrical double layer properties. Extrapolated yield stress measurements along with the electrochemical data were used to determine the edge isoelectric points for both the unmodified and modified kaolinite and were found to occur at pH values of 5.25 and 6.75, respectively. Acidity and ion complexation constants were calculated for both sets of data before and after surface modification. The acidity constants, pKat = 5.0 and pKa2 = 6.0, calculated for unmodified kaolinite, correlate closely with acidity constants determined by oxide studies for acidic sites on alumina and silica, respectively, and were, therefore, assigned to pH-dependent specific chemical surface hydroxyl groups on the edges of kaolinite. The parameters calculated for the modified kaolinite indicate that the silane has reacted with these pHdependent hydroxyl groups causing both a change in their acidity and a concomitant decrease in their ionization capacity. Infrared data show that the long chain hydrocarbon silane is held by strong bonding to the kaolinite surface as it remains attached after washing with cyclohexane, heating, and dispersion in an aqueous environment.

Surface Charge Properties Of Kaolinite

“…-~ 4.8 at the lowest ionic strength, but the disagreement between the model predictions and experimental aH data was significant. As with Braggs et al (1994), a short equilibration time in the experiments of Schindler et al (1987) is a possible explanation of a lower p.z.n.p.c. Zhou and Gunter (1992) did not report an equilibration time but their titration data indicate p.z.n.p.c.…”

“…The corresponding ranges for qo were -0.2 to +5.2 and 2.7 to 8.1 mmolc kg -~ (data not shown), decreasing with increasing -log[H+]. In general, qLi and qcl both decreased with increasing equilibration time for a given -log[H+], although this trend was more marked for qLi ' Braggs et al (1994) found p.z.n.p.c. --~ 3 for a Georgia kaolinite, far below the values reported in this study.…”

HEATS OF K/Ca AND K/Pb EXCHANGE IN TWO TROPICAL SOILS AS MEASURED BY FLOW CALORIMETRY 1

“…Thus, the exchange sites were energeti cally homogeneous in the Oxisol. The main com ponents capable of cation exchange at the pH of the experiment (pH ϭ 4.7) were organic matter and kaolinite (ZPCs Ͻ 3 and 3 to 5, respectively; Braggs et al, 1994;Ferris and Jepson 1975;Sparks, 1995). Both materials have external cation ex APPEL, RHUE, MA, AND REVE SOIL SCIENCE change sites, which may exhibit constant affinities for K as exchangeable K content varies.…”

Characterization of Montmorillonite Surfaces After Modification by Organosilane

“…The objective of the acid pre-treatment is to remove octahedral cations, each of which leaves behind two additional Si-OH groups (RausellColom and Serratosa, 1987). Kaolinite, with edge-OH groups estimated at --10%, was examined also by direct reaction with silane in solvent (Braggs et aL, 1994). Sepiolite, with a comparatively larger -OH content, was examined for direct grafting either with ~'Present address: Novellus Systems, Inc., 4000 N. First Street, M/S 81A, San Jose, California 95134, USA.…”