Electrostatic Potential of Specific Mineral Faces

Abstract

Reaction rates of environmental processes occurring at hydrated mineral surfaces are in part controlled by the electrostatic potential that develops at the interface. This potential depends on the structure of exposed crystal faces as well as the pH and the type of ions and their interactions with these faces. Despite its importance, experimental methods for determining fundamental electrostatic properties of specific crystal faces such as the point of zero charge are few. Here we show that this information may be obtained from simple, cyclic potentiometric titration using a well-characterized single-crystal electrode exposing the face of interest. The method exploits the presence of a hysteresis loop in the titration measurements that allows the extraction of key electrostatic descriptors using the Maxwell construction. The approach is demonstrated for hematite (α-Fe(2)O(3)) (001), and thermodynamic proof is provided for the resulting estimate of its point of zero charge. Insight gained from this method will aid in predicting the fate of migrating contaminants, mineral growth/dissolution processes, and mineral-microbiological interactions and in testing surface complexation theories.

Effect of surface site interactions on potentiometric titration of hematite (α-Fe2O3) crystal faces

“…Specific interaction of aqueous ions with transition metal oxide mineral surfaces such as hematite (a-Fe 2 O 3 ) underpins interfacial reactions such as dissolution, growth, electron transfer, and phase transformation, processes that impact waste remediation, solar energy conversion, microbiological/mineral interactions, and soil evolution [1,2]. One of the most fundamental specific interactions is protonation/deprotonation.…”

“…PZP is used here when relating to our measured values, where a negligible contribution from counter ions is possible. Using this approach, titration results on single faces of metal oxide crystals have shown both linear [30,31] and non-linear pH-potential relations, including asymmetries [1,29,32,[35][36][37]. Some of these deviations from expectations appear to arise from slow proton adsorption and desorption kinetics [38].…”

“…Our recently proposed thermodynamic model [1,25] was applied to measured acidimetric and alkalimetric titration hysteresis loops, collected from highly organized (001), (012), and (113) crystal face terminations using pH equilibration times ranging from 15 to 30 min. Hysteresis loop areas indicate that (001) faces equilibrate faster than the (012) and (113) faces, consistent with the different expected ensembles of singly-, doubly-, and triply-coordinated surface sites on each face.…”

The Effects on the Response of Metal Oxide and Fluorite Single Crystal Electrodes and the Equilibration Process in the Interfacial Region

“…[57,58] In the case of specific adsorption or unsymmetrical association of counterions, the point of zero potential could be obtained by several proposed experimental methods. As one example the appearance of hysteresis of electrode potentials during acid base titration, [10] the method of a common intersection point of measured electrode potential functions at different ionic strengths, [11] a saddlelike surface potential function Ψ0(pH). [53] During the potential measurements with the metal oxide SCrE the electrolyte solution is occasionally stirred, liquid flowing along a solid crystal plane could affects the distribution of ions and arrangements of water molecules within the EIL.…”

“…Cyclic titration from acidic to basic pH (alkalimetric), and the reverse (acidimetric), often exhibits hysteresis, [9] which was exploited for determination of the point of zero potential. [10] In the absence of hysteresis and counterion association another method was proposed which is based on a common-intersection point of measured electrode potential functions at different ionic strengths. [11] Last decades numerous attempts ( Figure 2) were made to measure the inner surface potential of metal oxides, an important quantity that affects the state of charged surface sites through their interfacial activity coefficients and distribution of ions within electrical interfacial layer.…”