How Proteins Stabilize Colloidal Particles by Means of Hydration Forces

The classical theory of colloidal stability, the well-known Derjaguin−Landau−Verwey−Overbeek (DLVO) theory, predicts a loss of stabilization with increasing salt concentration. An anomalous stability of the system is found at high salt concentrations when proteins cover colloidal particles. It has been demonstrated that this “non-DLVO” stability only takes place on hydrophilic systems and hydration forces are responsible for this phenomenon. In this work, different proteins (IgG, fibrinogen, myoglobin, and serum albumin) have been adsorbed onto a chloromethylstyrene (CMS) latex. The influence of the protein nature on the non-DLVO stabilization has been studied. This anomalous stabilization mechanism caused by hydration forces has been observed for all the studied proteins, although some experimental results (e.g., the critical stabilization concentration) vary for different proteins and degrees of coverage. In addition, we have shown that the orientation of the protein molecules immobilized on the CMS surface is different depending on the adsorption pH. At low pH values, when protein and polymer surfaces have different signs of charge, the macromolecules are adsorbed with a preferential orientation which differs from that obtained at higher pH values. This has been corroborated by both stability and immunoreactivity studies. Finally, we have also observed that proteins are in a dynamic state when they are adsorbed on the CMS surface. After a long time, the macromolecules tend to expose their hydrophilic areas to the aqueous medium and to hide their hydrophobic zones from solution.