pH value and ionic strength effects on the adsorption kinetics of protein/phospholipid at the chloroform/water interface

The adsorption kinetics of β-lactoglobulin in an aqueous buffer solution in the presence of phospholipid, L-α-dipalmitoylphosphatidylethanolamine (DPPE), dissolved in chloroform, has been investigated by using a pendent drop technique. The dynamic interfacial tension for the mixed β-lactoglobulin/phospholipid layer was measured as a function of time at a constant phospholipid concentration, CDPPE = 1×10-6 M. Experimentally it has been found that increase of ionic strength of aqueous solution can enhance the adsorption rate of the protein. However, the larger pH value leads to the adsorption process being reduced. When the protein concentration was increased up to 3.8 mg l-1, the effect of the salt concentration on the adsorption can be neglected and the protein dominates principally the adsorption process. In the acid aqueous solution, pH 5, approaching equilibrium time of adsorption for protein was around 1300 s and twice longer than in the base subphase, pH 8. However, the equilibrium interfacial tension was independent on the pH value. The salt concentration and pH value effects show that both the electrostatic interaction and hydrophobic effect make the mixed lipid/β-lactoglobulin layer stable at the liquid/liquid interface.