engleski

Thermodynamics of diamond/aqueous electrolyte solution interface

Diamond is an example of a chemically inert hydrophobic material which does not bear surface functional groups and therefore should neither chemically react with water molecules nor with ions from an aqueous electrolyte solution. However, due to the hydrophobicity water dipoles are repelled from the surfaces and orientation of water molecules is more ordered than in the bulk of the solution. Both the distribution and ordering of water molecules and ions in the vicinity of the surface cause the formation of an electrical interfacial layer (EIL) [1]. It was found that, for inert hydrophobic materials, the electrophoretic mobility, and thus the surface charge and surface potential, are pH dependent [2]. The value of the isoelectric point was found to be in the acidic region (2 < pH < 4). The origin of electrical charge at an inert hydrophobic material/aqueous electrolyte solution interface and pH dependency have been a subject of numerous debates in recent years and is still not solved [3]. In this research, we analysed process within the interfacial layer of the diamond/aqueous electrolyte solution. The distribution of H+ and OH− ions between bulk of solution and interface is described as accumulation of H+ and OH− ions at the interface. Total reaction is described as an exchange of H+ and OH− ions between the bulk of solution and the interface [ ] (Figure 1). The thermodynamic parameters of the above mentioned processes are obtained indirectly from the temperature dependency of equilibrium parameters. By determining temperature dependence of electroneutrality points (isoelectric point or point of zero charge) enables the evaluation of the difference in standard distribution enthalpies of H+ and OH−. Electroneutrality points of diamond were obtained in the temperature range from 10 °C to 50 °C by using three separate methods, namely streaming potential measurements, electrophoretic mobility and potentiometric mass titration. Additionally, molecular dynamics simulations of aqueous electrolyte solution on diamond surface where performed to attain atomistic level insight into the molecular structure of diamond/aqueous electrolyte solution interfaces.

diamond, ion distribution, electrical interfacial layer, point of zero charge, isoelectric point

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