Influence of the structure of boundary layers and the nature of counterions on the position of the isoelectric point of silica surfaces |
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Authors: | L E Ermakova M P Sidorova N F Bogdanova |
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Institution: | (1) Department of Chemistry, St. Petersburg State University, Universitetskii pr. 26, Petrodvorets, St. Petersburg, 198504, Russia |
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Abstract: | Dependences of electrokinetic potentials of different silica materials (nano-and ultraporous glasses, a quartz glass plane-parallel capillary, and monodisperse spherical particles of silicon oxide) on the pH of solutions containing single-, double-, and triple-charged cations have been compared. It has been shown that the degree of hydration of a single-charged cation and the structure of an interface substantially affect the position of the isoelectric point (IEP). The most hydrated Na+ ions have virtually no effect on the position of the IEP up to their concentration of 0.1 M irrespective of the thickness of an ion-permeable layer at the surface of a solid phase. A reduction in the radius of a hydrated cation (K+, Cs+) enables its penetration into an ion-permeable layer and, as a consequence, causes the IEP to shift toward larger pH values depending on the parameters of this layer. Two IEPs are observed in LaCl3 solutions: one at a pH value close to pHIEP in NaCl solutions and another at a higher pH value corresponding to the charge reversal of the Stern layer. |
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