ABSTRACT
It has already been stated (Chapter 2) that the equation of state is not universally recognised as a suitable method of data treatment for surface energy terms, and despite its success, it does not consider components of surface energy.2 Andrade (1987) looked forward to the possibility of more accurate interfacial predictions between biopolymers and surfaces, due to the development of the manner in which interfaces are considered. As described in Section 2.5, it has been usual to consider surface energies in terms of a polar and a dispersive component, but this is inadequate to provide a prediction of many complex interfacial phenomena. The development of an alternative theory by which to consider polar forces (as described in Section 2.5.3) allows for ionic natures to be considered by describing a surface in terms of electron donor and receptor contributions. For biological molecules, van Oss et al. (1986) defined the term yLW (LW denoting Lifshitz — van der Waals interactions; see Section 2.5.3) to describe London, Debye and Keesom forces (see Section 2.5.2.1), and a second term ySR to describe short range forces (i.e. those caused by hydrogen bonds). This approach was something of a forerunner to that described in Section 2.5.3. Van Oss et al. (1986) considered the interaction between proteins and low energy (i.e hydrophobic, to be more exact PTFE and polystyrene) surfaces, immersed in aqueous media. As described in Textbox 9.2, it is necessary to consider proteins in their dry and hydrated form, and this is because the hydrated form is the most commonly encountered, but the LW (long range) forces of the non-hydrated core will also contribute to protein binding. Using this approach, van Oss et al. (1986, 1987) found some agreement between practical observations and calculated free energies of binding. The data from van Oss et al (1986, 1987) are summarised in Table 9.3, from which it can be seen that a significant difference exists between DNA and RNA (dry state), as RNA (as well as IgG and HSA) are predominantly Lewis bases (i.e ye ) whilst DNA is essentially a Lewis acid (i.e. y®). It has been argued (van Oss et al., 1987) that the hydrated forms of these bio-molecules will have local orientation of the surrounding water molecules such that this Lewis acid — Lewis base behaviour of the dry state will persist, to some extent into the hydrated state (see DNA results in Table 9.3). It follows that the binding of DNA (mostly y®) to cellulose membranes (mostly ye ) will be strong, whilst the binding of RNA and most proteins (mostly y e ) will be weaker. Additions of cosolvents (e.g. methanol) or ionic additives, will alter the electron donor — electron receptor nature of materials, and thus alter binding. Such changes can be predicted to some degree, based upon interfacial considerations.
