Surface potentials of solutions

Surface potentials of solutions. If a dissolved substance is adsorbed at the surface of a solution it alters the contact potential between the solution and air, or between the solution and any other phase with which it may be in contact. The change in potential due to the solute, called as usual the surface potential of the solute, may be due to electrical double layers set up by the orientation of the molecules adsorbed, re-orientation of the solvent molecules, redistribution of ions, or to all these causes. 

The method first used for measuring surface potentials of solutions is often known as the method of flowing junctions It was used by Bichat and Blondlot.2 Kenrick,2 and more recently by Frumkin.4 

The solvent, and the solution, are made to flow, one in a column down the centre of a vertical tube, the other down the walls, so that there are constantly renewed surfaces of solvent and solution, of large area, in fairly close proximity. The difference in the air-liquid potential between the two liquids (which is the surface potential of the solute) causes a difference in potential between the liquids, and since the liquids are constantly renewed, current must be supplied to one liquid, and taken from the other, in order to maintain this difference. This current is large enough to be measurable by an electrometer. A circuit is therefore constructed with reversible electrodes in contact with the insulated reservoirs containing a supply of each of the liquids, an electrometer to detect the flow of current, and a potentiometer to impose any desired potentials on the liquids. The potentiometer is adjusted until the electrometer shows no flow of current then the applied potential is equal to the difference in 

The method would probably give erroneous results for solutions of substances adsorbed too slowly for the surface to reach equilibrium very quickly with the interior, but for solutions of salts and organic substances of moderate molecular weight it is probably trustworthy. Frumkin and Donde1 have used a thin stream of falling mercury instead of the inner column of aqueous solution. 

Sawai2 applied the technique of the air electrode covered with a radioactive substance, described for insoluble films in Chapter II, to stationary solutions contained in a funnel filled to the brim the funnel was first filled with the solvent, which was quickly replaced with solution. The difference between the potentials when solvent and solution are in the funnel is the surface potential. The method works quite well and is to be preferred to a flowing junction for solutes which diffuse slowly to the surface. 

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Surface Potentials of Solutions of Organic Substances Frumkinf... 

The other method used for liquid surfaces is the flow method of Kenrick (14) in which a jet of one solution is passed down the center of a tube whose walls carry a flowing layer of a second solution. The potentials between the flowing liquids are monitored with a quadrant or other electrometer. This method has been used with good results by Randles (15) and Parsons (16). Case and Parsons (17) compared the Kenrick and radioactive electrode methods for methanol-water mixtures. They found good agreement except at elevated methanol concentrations where methanol adsorption at the air electrode probably occurs. Measurement of the null current (compensation) potential in the Kenrick method is suitable for determining the surface potentials of solutions where rapid surface equilibrium occurs, but it is not convenient for spread monolayers or adsorbed films that have slow time effects. 

The value of the Galvani potential Ag y) can be represented as the sum of the potential drop inside the ionic double layer Acp and the surface potentials of solution and metal, which have changed as a result... 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

Preparation of an uncontaminated surface of an aqueous solution is very difficult. Even minute traces of adsorbable organic impurities strongly influence the surface potential of water. Cleaning of the aqueous surface (e.g., by siphoning off the surface layer) is usually necessary, while for organic solutions it is usually not needed. ... 

The measurement of change in the surface potentials of aqueous solutions of electrolytes caused hy adsorption of ionophore (e.g., crown ether) monolayers seems to he a convenient and promising method to ascertain selectivity and the effective dipole moments of the ionophore-ion complexes created at the water surface. 

The impossibility of x being equal to about 1 V, as suggested by Kamieifki, " " has been demonstrated by Frumkin on the basis of a discussion of the real energies of hydration. Estimates from the variation in the solution surface potential with electrolyte molarity have yielded the value of +0.025 0.010 V.21 For methanol, the same method results in a value of -0.09 V.146 Later the authors of that investigation stated that both estimated values should be understood as the lower limits of surface potentials of water and methanol. "... 

The problem of calculating Galvani potentials now reduces to that of calculating the surface potentials of the metal and solution. 

The surface potential of a solution can be calculated, according to Eq. (10.18), from the dilference between the experimental real energy of solvation of one of the ions and the chemical energy of solvation of the same ion calculated from the theory of ion-dipole interaction. Such calculations lead to a value of -1-0.13 V for the surface potential of water. The positive sign indicates that in the surface layer, the water molecules are oriented with their negative ends away from the bulk. 

Two types of EDL are distinguished superficial and interfacial. Superficial EDLs are located wholly within the surface layer of a single phase (e.g., an EDL caused by a nonuniform distribution of electrons in the metal, an EDL caused by orientation of the bipolar solvent molecules in the electrolyte solution, an EDL caused by specific adsorption of ions). Tfie potential drops developing in tfiese cases (the potential inside the phase relative to a point just outside) is called the surface potential of the given phase k. Interfacial EDLs have their two parts in dilferent phases the inner layer with the charge density in the metal (because of an excess or deficit of electrons in the surface layer), and the outer layer of counterions with the charge density = -Qs m in the solution (an excess of cations or anions) the potential drop caused by this double layer is called the interfacial potential... 

Here, Hg(s) is the surface potential of mercury in contact with the solution phase S. The difference of metal and solution surface potentials, Hg(s) - s(Hg), is the dipolar potential difference, often written2 as g (dip) = gM(dip) - gs(dip), with the metal M here equal to Hg. 

At oxide surfaces, the surface activities of H+ and OH are not fixed in a similar way. Then the variation in surface potential with solution activity of H+ depends on the chemical and electrostatic properties of the interface. For the many oxides that are insulators, it is much more difficult to obtain a measurement of the surface-solution potential differences than it is for conductors such as Agl. Thus there is uncertainty whether the dependence of surface potential on pH is approximately Nernstian or significantly sub-Nernstian. 

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

This change has two attractive features (1) It eliminates the separation of activity coefficients into short- and long-range interactions, which cannot be evaluated separately in practice, and (2) implicitly incorporates an expected effect of surface potential on solution activity through the activity coefficient relationship of Equation 22. Table II summarizes the relevant reaction and activity coefficient terms based on the above modifications of the TLM. 

Here

work function, is the chemical potential of electrons in the metal, and Sxois the change of the metal surface potential upon contact with the solution. Hence, the modification of electronic distribution in the metal is due to the adsorbed solvent molecules, which change the surface potential of the metal, dxo- A similar concept was developed in numerous works of Trasatti (e.g.. Ref. 30). The value of Sxo at [Pg.7]

Any liquid surface, especially aqueous solutions, will exhibit asymmetric dipole or ions distribution at the surface as compared to the bulk phase. If SDS is present in the bulk solution, then we will expect that the surface will be covered with SD ions. This would impart a negative surface charge (as is also found from experiments). It is thus seen that the addition of SDS to water not only changes (reduces) surface tension but also imparts negative surface potential. Of course, the surface molecules of methane (in liquid state) will obviously exhibit symmetry in comparison to water molecules. This characteristic can also be associated to the force field resulting from induced dipoles of the adsorbed molecules or spread lipid films (Adamson and Gast, 1997 Birdi, 1989). 

Fig. 8 Results of the regression analysis of Eq. 56 for surface potential of the air-water interface with the adsorption of alkali dodecyl sulfate molecules as a function of the surfactant concentration in the bulk solution...

Calculate the critical value of the surface potential of the colloid which will just give the rapid coagulation case illustrated in Figure 7.15. Assume that the aqueous solution contains lOmM monovalent electrolyte at 25 °C. Also assume that the Hamaker constant for this case has a value of 5 x 10 J. 

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