Role of the Polar Properties of Water in Separation Methods

• Published in: Separation and purification reviews 2011-08, Vol.40 (3), p.163-208
• Main Authors: van Oss, Carel Jan, Giese, Rossman F.
• Format: Article
• Language: English
• Subjects: adhesion energies; Adsorption; advancing freezing fronts; affinity constants; antigens and antibodies; aqueous phase separation; Attraction; Bacteria; flocculation; Hydrophobic attraction; Hysteresis; Mathematical analysis; microbes and proteins; Phases; Separation; size of cells; Solubility; water-air interface
• ISSN: 1542-2119; 1542-2127
• DOI: 10.1080/15422119.2011.555215

The role of the polar properties of water in separation methods are presented in two parts: Part I: Properties of Water General principles are given of the three non-covalent energy types acting upon non-polar as well as polar entities when these are immersed in water: Lifshitz-van der Waals (LW), Lewis Acid-Base (AB) and Electrostatic (EL) energies. The dominant one of these three is the Lewis acid-base (AB) interaction, which in biochemical and other organic interactions are about an order of magnitude stronger than the van der Waals and electrostatic energies combined. Included are the most important equations, describing these interactions when occurring in water. Part II: Forces Originating in Water and their Significance in Separation Methods * Hydrophobic attraction between particles. * Hydrophobic interaction between solute molecules immersed in liquid water is caused by the hydrogen-bonding free energy of cohesion between the water molecules which surround these particles or solute molecules. * The solubility equation is given, which links the free energy of interaction, ΔG iwi , between the solute molecules (i) immersed in water (w), and the natural logarithm of the aqueous solubility (s) of i, expressed in mol fractions. Insolubility and partial solubility in water are due to strong mutual attraction and moderate attraction among molecules, respectively. Total solubility in water is due to mutual repulsion between solute molecules. * Discusses conditions of stability versus instability (flocculation) of aqueous particle suspensions. * Discusses the hyper-hydrophobicity of the water-air interface and its interactions with: A) small water-soluble molecules (it repels them) and: (B) amphiphilic molecules such as surfactants, alcohols, proteins, etc. (it strongly attracts them). * Treats adhesion and adsorption energies between two different entities, immersed in water, such as antigens and antibodies. These energies are commonly expressed as affinity constants (K aff ), the value of which is strongly influenced by the time spent during the initial adhesion or adsorption step, where the K aff value steadily increases the longer that initial adsorption step lasts. This phenomenon is called hysteresis, or binding hysteresis. However the hysteresis effect can be avoided by reducing the initial exposure episode to time (t) approaching t = 0, to obtain K aff t→0 . * Two different aqueous phases can be formed by dissolving two different water-soluble poly