Simple model of surface-induced electrolytic dissociation of weak acids in organic solvents
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Simple model of surface-induced electrolytic dissociation of weak acids in organic solvents Marek Kosmulski
Received: 7 April 2010 / Accepted: 26 May 2010 / Published online: 25 June 2010 © The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract The electric conductivity of solutions of oxalic and phosphoric acid (up to 0.025 M) in ethanol and methanol has been studied in the presence of TiO2 (1–10% by mass). TiO2 enhanced the conductivity of solutions of oxalic and phosphoric acid in the both alcohols. The experimentally observed behavior was successfully modeled using a model with two types of surface sites. Sites of the first type bind the acids in molecular form. Sites of the second type bind the acids in form of hydrogen oxalate and dihydrogen phosphate anions, respectively, and protons are released to the solution, and contribute to enhanced conductivity. The adsorption model properly reflects the electrokinetic potential of titania particles in alcoholic solutions of oxalic and phosphoric acid. Keywords Organic solvents · Non-aqueous solvents · Degree of dissociation · Electrolytic dissociation · Adsorption · Surface heterogeneity · Zeta potential
1 Introduction Kosmulski et al. (2009, 2010) reported enhancement in the electric conductivity of oxalic and phosphoric acid solutions in lower n-alcohols in the presence of titania. This result is rather surprising, and it differs from usual behavior observed M. Kosmulski () Center of Excellence for Functional Materials and Graduate School of Materials Research at Laboratory of Physical Chemistry, Åbo Akademi University, Åbo, Finland e-mail: [email protected] M. Kosmulski Department of Electrochemistry, Lublin University of Technology, Nadbystrzycka 38 A, 20-618 Lublin, Poland
for aqueous solutions, in which the presence of adsorbents (including titania) depresses the conductivity of electrolyte solutions. The difference between aqueous and alcoholic solutions was qualitatively interpreted in terms of different degrees of dissociation of acids in different solvents. In water, the degree of dissociation is relatively high, and titania adsorbs existing ions. In alcohols, the degree of dissociation and the concentration of pre-existing ions are low, and titania adsorbs acids chiefly in molecular form. The interaction between electroneutral acid molecules and titania surface produces titania-phosphate (or titania-oxalate) surface complexes and protons in solution. This phenomenon, termed “surface-induced electrolytic dissociation” enhances the concentration of ionic species in solution. The surfaceinduced electrolytic dissociation is probably responsible for high electrokinetic charges in nonaqueous media. Namely, high zeta potentials have been reported in solvents, in which the concentrations of pre-existing ions are low (van der Hoeven and Lyklema 1992; Morrison 1993). The very existence of electric double layer requires presence of sufficient concentration of ions. The concentrations of ions calculated from solution chemistry in
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