Surface excess, Gibbs

The above provides a means of showing how the total excess charge on the solution side of the interface q the excess charge due to cations F+ and the excess charge due to anions F, vary with potential in a solution of fixed concentration of electrolyte. On the basis of this approach to the electrocapillary curves it has been shown that the Gibbs surface excess for cations is due solely to electrostatic forces (long-range coulombic), and this is reflected in the fact that the electrocapillary curves for different cations and... 

Gibbs surface excess (free energy) gram... 

D. K. Chattoray and K. S. Birdi, Adsorption and Gibbs Surface Excess, Plenum Press, New York, 1984, Chaps. 5 and 6. 

The parameter nf always has positive values, whereas the parameters and Fj can have positive or negative values. Parameter Fj is called the Gibbs adsorption or Gibbs surface excess. 

However, the value of T depends on the position of the Gibbs surface. By common convention, this position is selected so that for one of the components (with the index j = 0), the value of T defined by Eq. (10.21) will become zero. The solvent is chosen in this capacity when one of the phases in contact is a solution. Once the position of the Gibbs surface has been fixed, one can unambiguously determine the Gibbs surface excesses of the other components. The adsorption of a component j is thus defined relative to the component y = 0 (relative Gibbs surface excess... 

More commonly used is another definition of Gibbs surface excesses, according to which r, is equal to the amount of substance j that must be added to the system (with a constant amount of the substance j = 0) so that the composition of the bulk phases will remain unchanged when the interface area is increased by unity. This definition can also be used when chemical reactions take place in the surface layer. In the case discussed here, the two definitions coincide. The set of surface excesses of all components is sometimes called the surface phase (in contrast to the real surface layer or interphase). 

It has been reported that the sonochemical reduction of Au(III) reduction in an aqueous solution is strongly affected by the types and concentration of organic additives. Nagata et al. reported that organic additives with an appropriate hydro-phobic property enhance the rate of Au(III) reduction. For example, alcohols, ketones, surfactants and water-soluble polymers act as accelerators for the reduction of Au(III) under ultrasonic irradiation [24]. Grieser and coworkers [25] also reported the effects of alcohol additives on the reduction of Au(III). They suggested that the rate of the sonochemical reduction of Au(III) is related to the Gibbs surface excess concentration of the alcohol additives. 

Experiment 2 Saturate distilled water with a rare gas and compare the intensity of the signal with that from air. The luminosity will be enhanced in the rare gas saturated solutions. For any gas atmosphere, add small amounts of volatile water-soluble solutes (e.g. alkyl series alcohols) and quantify the quenching of sonoluminescence as a function of both bulk quencher concentration and surface excess. Good correlation between the extent of quenching and the Gibbs surface excess should be observed. Explain the changes in sonoluminescence intensity when a rare gas atmosphere is used and the quenching of volatile solutes, in terms of simple thermodynamics. 

If nia and nlfj denote the total moles of the i-th component in the two phases of the idealized system, then the Gibbs surface excess rni of the i-th component can be defined as... 

In Section 6.4.2 we will find that T represents the Gibbs-surface excess, i.e., T=N/A -N°/A, where is the number of molecules that would have been there if there had been no double layer, and N is the actual number of molecules in the interfacial region. However, when the bulk concentration of the spedes is small, i.e., tfi — 0, then the number of adsorbed molecules tends to f, i.e., f — N/A. 

A purely thermodynamic approach to adsorption in the double layer allows one to obtain the Gibbs surface excess for both cations and anions. Describe an approach for breaking up the T into diffuse layer and contact-adsorbed contributions. Be careful in making the assumptions. (Bockris)... 

In describing adsorption on an electrode, it is common to write 0 for the fraction of the surface covered. However, in purely thermodynamic analyses, the symbol r is used for the Gibbs surface excess. Describe the difference in meaning between these two quantities and the conditions under which they may tend to become nearly equal. (Bockris)... 

Quantitative determinations of the thicknesses of a multiple - layered sample (for example, two polymer layers in intimate contact) by ATR spectroscopy has been shown to be possible. The attenuation effect on the evanescent wave by the layer in contact with the IRE surface must be taken into account (112). Extension of this idea of a step-type concentration profile for an adsorbed surfactant layer on an IRE surface was made (113). and equations relating the Gibbs surface excess to the absorbance in the infrared spectrum of a sufficiently thin adsorbed surfactant layer were developed. The addition of a thin layer of a viscous hydrocarbon liquid to the IRE surface was investigated as a model of a liquid-liquid interface (114) for studies of metal extraction ( Ni+2, Cu+2) by a hydrophobic chelating agent. The extraction of the metals from an aqueous buffer into the hydrocarbon layer was monitored kinetically by the appearance of bands unique to the complex formed. 

As a counterpeud to the previous section consider now solutions of sparingly soluble substances, consisting of uncharged, non-polymeric molecules. As long as the solutions remain dilute (x. x ) the Gibbs surface excess r " or r " now... 

Inhibition of Pancreatic Lipase on Oil Drop Pre-poisoned with Orlistat Tiss et al. [117] studied the partitioning of orhstat between the core and the surface of an oil drop, as a function of the orhstat concentration in the oil. The molar fraction of orhstat at the oil drop surface was eshmated using the Gibbs surface excess equation. With these data it becomes possible to express orhstat con-... 

Fig. 2 Comparison of SO4, Cl , Br , and 1 adsorption at the Au (111) electrode surface from 0.1 M HCIO4 + 10 M K2SO4 and 0.1 M KCIO4 + 10 M KCl, KBr, and KI solutions, (a) Cyclic voltammograms at u = 10 mV s (b) Gibbs surface excess versus potential, (c) Gibbs energy of adsorption versus potential. The standard state corresponds to the surface coverage / = 1 ion per cm and bulk concentration c = 1 mol dm [36].

To describe these research observations and data he realized that it was essential to write books on the subject of surface and colloid chemistry. His first book on surface science was pubhshed in 1984 Adsorption and the Gibbs Surface Excess, Chattorraj, D.K. and Birdi, K.S., Plenum Press, New York. This book remains the only one of its kind in the present decade. Further publications include Lipid and Biopolymer Monolayers at Liquid Interfaces, K.S. Birdi, Plenum Press, New York, 1989 Fractals — In Chemistry, Geochemistry and Biophysics, K.S. Birdi, Plenum Press, New York, 1994 Handbook of Surface and Colloid Chemistry, CRC Press, Boca Raton, FL, 1997 (CD-ROM, 1999), and Self-Assembly Monolayer, Plenum Press, New York, 1999. Surface and colloid chemistry has remained his major research interest throughout these years. 

These are the usual Gibbs surface excesses. Note that there is no volume term in Eq. (79). Integrating Eq. (79) we have... 

In general, the choice of the position of the Gibbs dividing surface is arbitrary. It is possible to define quantities which are invariant with respect to the choice of its location. If Fj or Fj, are the relative adsorption and F and Ff the Gibbs surface excess concentrations of components i and 1, respectively, then the relative adsorption of component i with respect to component 1 is defined by... 

Cosorption Lines Contours of equal surface activity as measured by the Gibbs surface excess concentrations plotted on phase diagrams. See p 131 of reference 9. 

---