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Biologically Produced Sulfur 173 stability is based. The attractive van der Waals force depends on the size of and the distance between two bodies. The repulsive electrostatic force origi- nates from the surface charge of the colloidal particles. Apart from this, structural forces (e.g., hydrogen bonding) can stabilize or destabilize colloi- dal particles depending on the nature of the particles, and adsorption of polymer material at the sulfur-solvent interface can cause steric stabiliza- tion. The origin of charge can be the presence of surface functional groups, such as carboxylic acid or amino groups. Surfaces on which proteins are ad- sorbed show a dependency of the surface charge on the pH value due to the presence of carboxylic acid and amino groups. At high pH a protein-covered surface will be negatively charged (-COO and -NH2 groups) and at low pH the surface will show an overall positive charge (-COOH and -NH3+ groups). The pH at which the overall surface charge is zero is called the point of zero charge. In the absence of specific adsorption it coincides with the iso-elec- tric point. This surface charge attracts oppositely charged ions to the surface and equally charged ions are repelled from the surface. This results in an ion concentration profile as is represented in Fig. 2, which is called the diffuse electric double layer. Presence of salt causes a screening of the electric dou- ble layer. The repulsive and attractive energies of two interfaces can be calculated as a function of their distance (Fig. 3). When salt is added to a colloidal so- lution the electrostatic repulsion can be screened, resulting in a destabiliza- tion of the colloids. Long chain polymers, such as proteins, can often adsorb to the surface of colloidal particles (because they gain adsorption energy), which can have a significant influence on colloidal stability. Adsorption of polymeric material at an interface takes place in such a way that the chains can extend from the Fig. 2 Concentration profile of positive (thick line) and negative (thin line) ions in a dif- fuse electrical double layer near a negatively charged surface. [NaCl]=0.01 mol l1PDF Image | Topics in Current Chemistry
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