9 The Significance of Volta and Compensation States and the Measurement of Surface
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Potentials of Monolayers B. A. PETHICA, M. M. STANDISH, J. MINGINS, C. SMART, D. H. ILES, M. Ε. FEINSTEIN, S. A. HOSSAIN, and J. Β. PETHICA Unilever Research, Port Sunlight Laboratory, Port Sunlight, Wirral, Cheshire L62 4 X N , England
The thermodynamic analysis of the Volta effect by Koenig was studied for insoluble monolayers at air/water inter faces. Capacity measurements with a frequency bridge and with vibrating plate electrodes were used to test the basic assumptions of Koenig. The compensation effect was stud ied, and the conditions to verify the compensation method for measuring changes in x potential were established for different types of spread monolayers. The field effect we reported for zwitterionic molecules is shown now to be an artefact produced by changes in the contact angle at the edge of the working interface at high monolayer pressures, with consequent changes in the geometry of the interfacial condenser. Within experimental limits,Kelvin'sassumption that the Volta and compensation potentials are equal and opposite is correct for the systems studied. O u r f a c e potential measurements have broadened our understanding of ^ both solid and liquid surfaces. The early works of Frumkin (1, 2, 3) and Guyot (4) demonstrated that adsorbed molecules can change the electrical potential at a liquid/vapor interface. This change for adsorbed and spread monolayers at aqueous interfaces has been studied by many surface chemists, including Ν. K . A d a m (5), whose contributions to surface science we honor in this volume. The method was used by Schulman, Rideal, and Alexander to investigate ionic charge distributions and the orientations of surface dipoles (6,7) and more recently to study the ionization of monolayer molecules (8,9). 123
In Monolayers; Goddard, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
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MONOLAYERS
In most of this work at the air/water interface the surface potentials were measured with an ionizing electrode i n the air above the aqueous phase. The theory of the ionizing electrode is not well understood, and it cannot be regarded as an absolute method. The vibrating plate method, as developed by Zisman (JO) and others, rests on a more secure theo retical understanding. It depends on the relatively simple determination of the potential that is applied for null current (compensation potential) in a circuit containing an oscillating capacitor made of a reference elec trode and the surface under study. Where both the Zisman and radio active electrode methods were compared directly, the same results were found (11, 12). To this extent the radioactive electrode method is reliable, although further detailed studies are desirable (13). The other method used for liquid surfaces is the flow method of Kenrick (14) i n 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 poten tials between the flowing liquids are monitored with a quadrant or other electrometer. This method has been used with good results by Randies (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. From the measurements with a form of the Zisman apparatus, we establish experimentally several features of the surface electrical states described by Koenig in his paper on the thermodynamic analysis of the Volta effect (18). Also we wish to establish conditions for reproducible and defined measurements of compensation and surface potentials. W e first define the relevant potentials and give a brief summary of Koenig's findings so that we can clarify the form in which we w i l l test some of the assumptions he isolated. W e confine the discussion to condenser states where temperature, pressure, and solution composition are constant but where the composition of the working interface may be changed by spreading an insoluble monolayer. The electrical circuit includes a halfcell, sometimes with a salt bridge. The relatively minor considerations thus introduced are fully discussed by Koenig, and for present purposes we can neglect them. Figure 1 gives the essential features for describing the Volta and compensation condenser states. In Figure l a two conducting phases, a and β, face each other at a distance χ across a non-conducting gap of,
In Monolayers; Goddard, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
9.
PETHICA
X
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ΛΙ/0.
Volta
ET AL.
1
and Compensation
125
States
Ε (3
β
(b)
(a)
Figure 1. Representation of the outer potentials (φ) in the non-conduct ing gap between the plane surfaces of two conducting phases, a and β, separated by a distance, x, and connected to a potentiometer at poten tial Ε (a) Clean surfaces, (b) surface of phase β covered by an insoluble monolayer
for example, air or paraffin oil. Phase α is a metal reference electrode, and phase β may be another metal, a semiconductor, or (in our case) an aqueous salt solution. These two phases are coupled through an appro priate circuit to a potentiometer, and a potential Ε may be applied. The surfaces of the two condenser plates carry charges ±q; their outer poten tials are denoted by ψ and ψβ. The outer potential is the work done i n bringing unit charge from infinity to a point just outside a surface where it is beyond the range of electrical double layers and other surface dipoles on the phase i n question. It is associated solely with the free charge q on the surface and as such depends on E . The Volta state corresponds to zero applied potential ( E = 0 ) ; the quantity (Δψ) = ο, where Δψ is the difference i n ψ and ψβ, is the classical Volta or contact potential. Outer potentials have sometimes been referred to as Volta potentials, but here, as i n Koenig's discussion, the Volta potential is a difference i n outer potential. The compensation state is given by ψ = ψβ, and therefore by q = 0. The value of Ε i n the compensation state is denoted by E , the compensation potential. In Figure l b the same two conducting phases are opposed, but the surface of phase β now includes a defined surface density of molecules of an insoluble monolayer. The reference electrode, phase a, is assumed unchanged by the spreading of the monolayer on β . In practice this means waiting until the spreading solvents have fully evaporated from the region α
Ε
α
α
0
In Monolayers; Goddard, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
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MONOLAYERS
of study. The Volta and compensation states are defined i n the same way as before. The value of E is changed experimentally by the presence of the monolayer. If we denote the respective systems with and without a monolayer by (1) and (2), the Volta and compensation potentials for the two systems can be written ( Δ ψ ι ) ^ , ( Δψ ) υ = Ο, (-ΕΟ)Ι, d (£0)2The surface potential ( Δ Υ ) for the insoluble monolayer is then usually defined as A V = (£0)2 ~" (£0)1. W e write χ as the drop i n electrostatic potential across the dipole layer at an interface and denote the absence and presence of a monolayer by suffixes (1) and (2). For this system &V = (χο)2 — (χο)2, where the subscript zero means that the values of χ are those i n the compensated state (23). 0
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= 0
2
a n
OSCILLATOR
Vib
Figure 2.
Schematic for measuring the potentials between a vibrating plate electrode and an aqueous electrolyte. VP—vibrating plate; Vib—vibrator; PA—preamplifier; FSDA—frequency se lective detector amplifier; POT—potentiometer; OS—oscilloscope. Dotted line represents a Faraday cage.
Koenig (18) has analyzed the thermodynamic relations between the potentials described above. H e points out that certain long-standing assumptions attributed to Kelvin (19), Bridgman (20) and Lorentz (21) are strictly non-thermodynamic and that these assumptions have not been
In Monolayers; Goddard, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
9.
PETHICA E T A L .
Volta
and Compensation
127
States
experimentally tested directly. It is sufficient to state that the assumptions of Bridgman and Lorentz concerning Volta and compensation potentials can be shown to contain the Kelvin assumption—namely that (Δψ) = ο is equal to — E . In this communication we test the assumption by noting that it requires that for a given surface χ should not change with variation of surface charge (18). The χ potential itself is not measurable (23), but the change in χ at a given surface caused by spreading an insoluble mono layer is measurable. Hence the Kelvin assumption w i l l require that i n gen eral
