Proteins Gibbs adsorption isotherm

The most explicit recognition of macromolecular energies as surface energies of solvent excess or deficit is in the work of Blank (28, 29), who showed the essential equivalence of Gibbs adsorption isotherms and the changes in the energy of alternate protein states. 

A number of important questions revolve around the central one of reversibility of protein adsorption. Are proteins, once adsorbed, able to desorb Why are proteins so difficult to remove from a surface What happens in the situation where there is competitive adsorption between different proteins or between proteins and other species Do the different points on an adsorption isotherm correspond to dynamic equilibria If a protein molecule can desorb, does it revert to its original solution configuration and recover its original biological activity Can the Gibbs Adsorption Equation be applied to protein adsorption Some of these questions may be effectively tackled by studies with the film balance. 

To illustrate the dependence of the mobility function d>y on the concentration of surfactant in the continuous phase, in Fig. 12 we present theoretical curves, calculated in Ref 138 for the nonionic surfactant Triton X-100, for the ionic surfactant SDS ( + 0.1 M NaCl) and for the protein bovine serum albumin (BSA). The parameter values, used to calculated the curves in Fig. 12, are listed in Table 4 and K are parameters of the Langmuir adsorption isotherm used to describe the dependence of surfactant adsorption, surface tension, and Gibbs elasticity on the surfactant concentration (see Tables 1 and 2). As before, we have used the approximation Dj Dj (surface diffusivity equal to the bulk dif-fusivity). The surfactant concentration in Fig. 12 is scaled with the reference concentration cq, which is also given in Table 4 for Triton X-100 and SDS + 0.1 M NaCl, cq is chosen to coincide with the cmc. The driving force, F, was taken to be the buoyancy force for dodecane drops in water. The surface force is identified with the van der Waals attraction the Hamaker function Ajj(A) was calculated by means of Eq. (86) (see below). The mean drop radius in Fig. 12 is a = 20 /pm. As seen in the figure, for such small drops 4>y = 1 for Triton X-100 and BSA, i.e., the drop sur-... 

To compute the distributions of changes in the Gibbs free energy (/(AG)) on protein adsorption, the Langmuir equation was used as the kernel of the adsorption isotherm equation in the form of Fredholm integral equation of the first kind (see Chapter 10). 

As in the case of fluid interfaces, the question of whether the adsorption of proteins onto solids is reversible or irreversible is very important for correct estimation of physicochemical characteristics of the process. In a reversible process, dilution of sorbate in the bulk phase should lead to spontaneous desorption of some portion of adsorbed molecules up to elimination of a transient difference in the chemical potential of the sorbate at the interface and in the solution the ascending and descending branches of the isotherm must overlap at all values of Cb. Only in this case the isotherm represents thermodynamic equilibrium, and the equilibrium constant Kads and the standard Gibbs energy of adsorption AG°ads = A/7°ads - rAS°ads can be determined. 

---