Adsorption Gibbs energy

The adsorption coefficient or / is a function of the standard Gibbs adsorption energy alone when the molecules do not interact and the adsorption occurs without limitation, i.e. at very low electrode coverage. This dependence is expressed by the relationship... 

RT is the effective standard Gibbs adsorption energy and includes the... 

The rate of asphaltene adsorption is greater than desorption rate for studied concentration. Gibbs adsorption energy values define character of adsorption as physical, not chemical. Rate of asphaltene adsorption is less then resins (in two orders) reported earlier (Balabin Syunyaev, 2008). Parameters of resin adsorption from benzene solutions on quartz... 

Thus we can obtain the dependence of the adsorbed amount on the differential coadsorption Gibbs energy in a similar way as for the differential coadsorption enthalpy measured by calorimetry. Practically, ArG g can be calculated by numerical derivation of the plot AG = f(n ). In the case of the adsorption of a single component, the equations for the calculation of the Gibbs adsorption energy are exactly the same, we have just to take i equal to one. 

The differences between the chemical Gibbs adsorption energies of the three salts are of the order of 0.5 kT. This is comparable to the differences in —RTlnf for these salts. Surprisingly, the specificities both in surface charge and ccc disappear above around 65°C. 

The quantitative relationship between the degree of adsorption at a solution iaterface (7), G—L or L—L, and the lowering of the free-surface energy can be deduced by usiag an approximate form of the Gibbs adsorption isotherm (eq. 9), which is appHcable to dilute biaary solutions where the activity coefficient is unity and the radius of curvature of the surface is not too great ... 

The term G T, a,, A/, ) is the Gibbs free energy of the full electrochemical system x < x < X2 in Fig. 5.4). It includes the electrode surface, which is influenced by possible reconstructions, adsorption, and charging, and the part of the electrolyte that deviates from the uniform ion distribution of the bulk electrolyte. The importance of these requirements becomes evident if we consider the theoretical modeling. If the interface model is chosen too small, then the excess charges on the electrode are not fuUy considered and/or, within the interface only part of the total potential drop is included, resulting in an electrostatic potential value at X = X2 that differs from the requited bulk electrolyte value < s-However, if we constrain such a model to reproduce the electrostatic potential... 

When the adsorbed components are electrically charged, then the partial molar Gibbs energy of the charged component depends on the charge of the given phase, and thus the chemical potentials in the above relationships must be replaced by the electrochemical potentials. The Gibbs adsorption isotherm then has the form... 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

An entirely different approach to equilibrium adsorption is to assume that adsorbed layers behave like liquid films, and that the adsorbed molecules are free to move over the surface. It is then possible to apply the equations of classical thermodynamics. The properties which determine the free energy of the film are pressure and temperature, the number of molecules contained and the area available to the film. The Gibbs free energy G may be written as ... 

The problem has been treated theoretically by the use of the Gibbs adsorption isotherm, which has been used with success in treating the interfaces between liquids and gases (30). One of the most easily measurable properties of a liquid is its surface tension, and changes in this quantity can be determined with great accuracy. The surface tension of a liquid is numerically equal to its surface energy, as also are changes in these quantities. 

Adsorption at liquid surfaces can be monitored using the Gibbs adsorption isotherm since the surface energy, y, of a solution can be readily measured. However, for solid substrates, this is not the case, and the adsorption density has to be measured in some other manner. In the present case, the concentration of adsorbate in solution will be monitored. In place of the Gibbs equation, we can use a simple adsorption model based on the mass action approach. 

---