Electrochemical Double Layer Processes, PDF

Dissociation of surface functional groups is usually preceded by the adsorption of water molecules to produce a hydroxylated surface. For an oxide for example: MO (surf) + H2O = MO.H2O (surf) = MOH.OH (surf) This is followed by the protonation or deprotonation of the surface hydroxyls to produce a c...

Dissociation of surface functional groups is usually preceded by the adsorption of water molecules to produce a hydroxylated surface. For an oxide for example: MO (surf) + H2O = MO.H2O (surf) = MOH.OH (surf) This is followed by the protonation or deprotonation of the surface hydroxyls to produce a charged surface: MOH (surf) + H+ = MOH2+(surf) MOH (surf) = MO- (surf) + H+ For sulphides: MS (surf) + H2O = MS.H2O (surf) = MOH.SH (surf) The MOH undergoes protonation or deprotonation as above and the thiol group (SH) may also undergo protonation or deprotonation: SH (surf) +H+ = SH2+ (surf) or SH (surf) = S- (surf) + H+ The hydroxyl groups present originally on carbon surfaces or produced as a result of interaction of the oxygen-containing surface functional groups of the carbon surface with water may behave in a similar manner: CO (surf) + H2O = COH.OH (surf) which then protonates or deprotonates as in the case of oxides or sulphides to give charges to the carbon surface. Selective desorption or dissociation of the constituent surface atom can occur in the following manner: MA (surf) = M+(surf) + A- 3.2 or MA (surf) = A- (surf) + M+ Electric Double Layer As indicated above, for all particles in aqueous solutions, which develop surface charge whether by specific chemical interaction, preferential dissolution of surface ions or lattice substitution, the surface acquires a potential with respect to the solution. For electroneutrality the surface charge must be compensated by an equal charge distribution in the aqueous phase. The surface charge together with the charge distribution in the solution is referred to as the electrical double layer. The FIG. B3 below is a schematic representation of the electric double layer and the potential drop across it. 35 Figure B3: A schematic representation of the double layer 3.2.1 Development of the Electric Double Layer Model The electric double layer model was first conceived by Helmholtz (1853) and has undergone series of modifications first by Guoy and Chapman, followed by Stern (1924) and then Grahame (1947). The general assumptions used in developing the electrical double layer model are: 1. the surface is flat, extends infinitely and is uniformly charged 2. the ions in the diffuse part of the double layer are point charges and obey the Boltzmann's point charge distribution as follows; n+ = no exp[ -zey ] RT n- = no exp[ +zey ] RT where n+, n-- are the number of positive and negative ions per unit volume at points where the potential is ψ (ie. zeψ is the electric potential energy), n○ is the bulk concentration of each ion species, z is the charge number and e is electronic charge. The net volume charge density, ρ, is given by, ρ = ze(n+ - n--) 3. the solvent's dielectric constant is the same throughout the diffuse part and influences only this part of the double layer. 4. the presence of a single symmetrical electrolyte of charge number z such as NaCl or KCl is assumed. The 'thickness' of the diffuse layer is usually estimated by 1/κ where 1/κ = 3.04 x 10-10(Cz2)-½ (m) at 25°C and C is the concentration of the electrolyte in mol.dm-3 is often referred to as the Debye-Hückel thickness parameter. Electrolytes, in fact, tend to compress the diffuse layer according to the above expression. 3.2.2 Potential determining ions These are ions that establish the surface charge. These ions may include ions of which the solid is made of; hydrogen and hydroxyl ions; or in flotation, collector ions that form insoluble metal salts with ions making up the surface of the mineral or solid as found in flotation; and ions that can form complex ions with surface species. 3.2.3 Counter ions These ions possess no special affinity for the surface and therefore are adsorbed by electrostatic attraction. Examples of counter ions are for example in flotation, collector anions at low concentration and chloride ions adsorbed on a positively charge surface. 3.2.4 Co-ions These (which in our schematic representation are negatively charged) are ions that possess the same charge as the solid or particle surface and hence are repelled from the surface into the diffuse layer. It should be noted that for electroneutrality, the diffuse layer charge density, σ d, must equal the surface charge density, σs, ie. σs = -σd 36 3.2.5 Total double layer potential The total double layer potential, Ψo, is defined as the potential difference between the surface and the bulk of the solution. Some important parameters in discussing the double layer potential include: o The Stern Plane, which is a hypothetical plane next to the surface is defined as the closest distance of approach of hydrated counter ions to the surface. o The potential difference between the Stern plane and the bulk solution is Ψδ and it is generally called the Zeta Potential, ζ. o For a particle suspended in an aqueous solution, its surface charge greatly depends upon the pH of the solution (ie. hydrogen ion activity of the bulk solution). At a certain pH the surface may be uncharged and that pH or H+ activity is referred to as the point-of-zero-charge, PZC, of the particle. For non-conductive solids it is not possible to measure Ψo. However, with the knowledge of the PZC it can be estimated according to the following relationship; yo = RT a+ RT aln o = ln nF a+ nF a-o where R is the gas constant (8.3143 J mol-1 K-1 ), T is absolute temperature (in K), n is valence and F is the Faraday's constant (96500 coulombs ). a +, a- are the activities of potential determining ions in solution and a+o , a-o are the activities at the PZC. o Another important phenomenon concerning charged surfaces is that of Isoelectric Point, IEP, which is defined as the point at which the zeta potential is zero. There should be no confusion regarding IEP and PZC; the two are considered to be the same only when the zeta potential is measured in the presence of uni-univalent acids or bases. Zeta potential is normally used to characterize the electrokinetic behaviour of a particle since unlike the surface potential, it is measurable. It is usually measured using two common techniques – electrophoresis, streaming potential or electroacoustic method. 3.2.6 Measurement of the Zeta Potential 3.2.6.1 Electrophoresis In this technique, the electrophoretic mobility of charged particles are measured by observing their velocities under a potential gradient and in the absence of convective flow. The zeta potential is then calculated using the relation; z = 4p m V × ×9 x10 4 Volts where μ is the solution viscosity in poise (10-1 N.s.m-2 in SI units), DC Dc E is dielectric constant, V is particle velocity in cm/sec, and E is the applied potential gradient in volt/cm. 3.2.6.2 Streaming Potential This technique involves forcing a solution through a packed bed of particles at whose opposite ends are placed electrodes. The streaming potential developed at these electrodes is then used to estimate the zeta potential according to the relation; 37