Role of the Polar Properties of Water in Separation Methods

Abstract

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 polymers, e.g., poly-(ethylene oxide) and dextran, which repel one another in aqueous solution. Such aqueous two-phase systems can be used cause the preferential migration of two different solutes to two different phases and thus to effect their separation and purification. Results of a study of multiple sub-phases forming within the middle phase of micro-emulsions are also discussed.

• Advancing freezing fronts are discussed in this Section, i.e., the rejection or engulfment of certain solutes or cells by a vertical ice column which is made to advance further vertically by continuing to cool it from below.

• The size, or rather the radius of curvature (R) of blood cells, blood proteins, bacteria and viruses, can play an important role in, e.g., the in vivo phagocytic engulfment of bacteria by leucocytes, the transformation of platelets to become “sticky”, the attachment of viruses to cells and of bacteriophages to bacteria.

Keywords:

• Hydrophobic attraction
• flocculation
• water-air interface
• adhesion energies
• antigens and antibodies
• affinity constants
• aqueous phase separation
• advancing freezing fronts
• size of cells
• microbes and proteins