Monolayers Volta potential

The charge density, Volta potential, etc., are calculated for the diffuse double layer formed by adsorption of a strong 1 1 electrolyte from aqueous solution onto solid particles. The experimental isotherm can be resolved into individual isotherms without the common monolayer assumption. That for the electrolyte permits relating Guggenheim-Adam surface excess, double layer properties, and equilibrium concentrations. The ratio u0/T2N declines from two at zero potential toward unity with rising potential. Unity is closely reached near kT/e = 10 for spheres of 1000 A. radius but is still about 1.3 for plates. In dispersions of Sterling FTG in aqueous sodium ff-naphthalene sulfonate a maximum potential of kT/e = 7 (170 mv.) is reached at 4 X 10 3M electrolyte. The results are useful in interpretation of the stability of the dispersions. 

Figure 3.29 shows density distributions for a number of lipid monolayers on water, as obtained by MD simulation ). We see that DPPC and GLCB have a broader interface, which is a result of their bigger head groups. The thickness of the interface is less for hydrophobic surfaces. It cannot be seen from these pictures that water molecules occasionally penetrate the hydrophobic core, but video animations showed this clearly. Various other dynamic parameters (rates of various internal motions) could be established. The water dipole contribution to the Volta potential V was also assessed. 

Before discussing experiments a note on nomenclature is appropriate. In the literature Volta potentials are often called surface potentials but this term has other meemings as well, so we shall not use it. The usual symbol is AV, but in line with our convention (sec. 3.4.1) the appropriate s3mibol is V , i.e. it is the Volta potential of the monolayer minus the same for the blemk at the other side of the barrier. The latter is not zero and depends on the orientation of water molecules at the interface, and in the presence of electrolytes, a double layer may form, giving rise to a non-zero y/°- In sec. II, we discussed the relevant measurement and gave results for various electrolytes. In sec. II.3.9 we concluded that for pure water, according to the best experiments presently available, >0 (the -potential is the potential of water with respect to water vapour caused by the spontaneous polarization of the Interface). This means that the water dipoles at the surface are preferentially oriented with their negative sides "out". The value of x is not... 

Figure 3.76. Stearic acid monolayers surface pressure (lower curves) and Volta potentials (upper curves). Influence of bivalent electrolytes. (Redrawn from E.D. Goddard, J.A. Ackilli, J. Colloid Set 18 (1963) 585.)...

For charged monolayers additional Information can be obtained from Volta potentied measurements. The data In figs. 3.83 and 84 eire not so suitable for that because different buffers were used (phosphate for pH 6-9 and citrate for pH 3-7) which are known to adfect V " see ref. J. 

Let it first be repeated that is the Volta potential of the monolayer minus the same of the carrier electrolyte, and not that of pure water. In fig. 11.3.75 (sec. 11.3.1 Of) it was shown that most simple electrolytes make the water surface more negative. The reason is that in electrolytes such as alkali chlorides and nitrates, the anions are usually more easily dehydrated than the cations, so that they enrich the surface (KF is an exception). Creation of this negative potential is mainly determined by the anion the bigger it is, the larger the effect. Cation specificity is only a minor effect. For simple carboxylates, like K2CO3 or KHCO3 the surface must also be negative with respect to the bulk. The observation that V ° < 0 for the three... 

Fatty amines appear to be somewhat more difficult monolayer-formers than the corresponding fatty acids. They are charged at low pH but uncharged at high pH. The counterions are anions and this can cause Interference when buffers are needed. We have already mentioned that buffers also affect the Volta potential. 

E.D. Goddard, Ionizing Monolayers and pH Effects, Adu. Colloid Interface Sci. 4 (1974) 45-78. (Older review but not dated because it contains much basic information on surface pressures and Volta potentials for ionized monolayers.)... 

Besides these specific methods, the general arsenal of techniques described in sec. 3.7 remains available. So, optical and Volta potential measurements cure often invoked to obtain structural information on the monolayers. These techniques do not basically differ from the corresponding ones for Langmuir monolayers. However, surface rheology differs drastically because for Gibbs monolayers transport to and from the bulk is possible. Differences start to appear if the molecules are not very small and therefore diffuse with time scales comparable (or shorter) than those of the measurements. Therefore this theme will be devloped separately before surfactant monolayers are discussed, see sec. 4.5. 

Figure 4.14. Volta potentials for Gibbs monolayers of n-butanol ( ) and n-heptanol (o) as a function of surface concentration. Temperature, 25°C. (Redrawn after Posner et al., loc. cit.)...

The surface (Volta) potential AV is an experimentally measurable quantity in monolayers on a water/air interface or, which is more representative for half a membrane, on a water/oil interface. 

Investigations of mixed monolayers of chlorophyll with plastocyanine revealed some slight reversible changes of the Volta potential. The magnitude of the effect depended on the illumination time and possibly points to the presence of an electron-exchange reaction between chlorophyll and plastocyanine ... 

Figure 7-75. Iron modified by 0.7 monolayers of octadecyltrichlorosilane (n-OTS). (a) SEM map (b) Volta potential map bright area surface covered by OTS dark area surface not covered (Stratmann et al., 1995).

Figure 7-76. Volta potential map of iron modified by 1 monolayer of dimethylhexadecylsilanol. (a) immediately after scratching the surface (b) surface scratched after 20 h corrosion in a humid S02-containing atmosphere (Stratmann et al., 1995).

Figure 22 Map of the Volta potential of one monolayer of octadecylsilanol on iron after expostue to 100% relative humidity for 343h. Area 5 x 5mm [87].

Figure 24 (a) Map of the Volta potential of iron modified by one monolayer of... 

The Significance of Volta and Compensation States and the Measurement of Surface Potentials of Monolayers... 

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