General and Historical Introduction

Carel Jan van Oss

PREAMBLE

Water is the most polar1 liquid known to Man. At room temperature (20 °C) its total free energy of cohesion, Gcohesion=−145.6 mJ/m2, consisting of GvanderWaals=−43.6 mJ/m2 and GPolar=−102 mJ/m2. Thus the van der Waals part of the free energy of cohesion of liquid water represents only 30% and the polar part represents 70% of the total. This was already known since Fowkes (1963, 1964, 1965). In addition, with respect to the interaction energies between non-polar molecules (e.g., alkanes), when these are immersed in water, the combining rules for apolar interactions in such cases generally causes GvanderWaals to be rather small, leaving mainly the polar free energy of attraction of 102 mJ/m2, which thus represents close to 100% of the total free energy of interaction in water among non-polar molecules or particles.

As was only realized much later, these 102 mJ/m2, representing the polar (in this case the hydrogen-bonding) free energy of cohesion of water, also happen to be the sole driving force for the hydrophobic effect. Notwithstanding these new data and probably mainly due to a continuing indecisiveness as to which forces were apolar and which were polar (see Sub-section 2.3, below) no significant advances were made in this matter for about another 20 years after Fowkes (1963, 1964, 1965). Finally, based on important clarifications proposed by Chaudhury (1984) and starting in early 1985, Chaudhury and I began (mainly via long-distance telephone) to develop the combining rules which allow the quantitative expression of polar free energies in SI units (van Oss, Chaudhury and Good, 1987, 1988). The ensuing results allowed the polar free energies of interaction to be combined with the van der Waals interaction energies (and the electrical double layer interaction energies where applicable), into a complete system comprising all non-covalent interactions taking place in and with liquid water (see also van Oss, 1994, 2006).

Now, more than another two decades past 1987, this book aims to treat the combined non-covalent non-polar, polar and electrical double layer interactions taking place in and with water, from the viewpoint of all the germane physical and physico-chemical properties of liquid water.

1 SOME EXAMPLES OF POLAR FORCES INTERACTING IN THE MAMMALIAN BLOOD CIRCULATION

Essentially all repulsive as well as attractive non-covalent interactions at a colloidal scale occurring in biological systems take place in water. Some examples of such interactions in water, looking for instance at the mammalian peripheral blood circulation, include:

1. Repulsion:
The mutual, non-specific repulsion between protein molecules, which keeps them dissolved in blood serum and permits them to avoid precipitation.
The mutual, non-specific repulsion between leukocytes, platelets, etc., which keeps them in stable suspension in the blood and lymph circulation and thus prevents the formation of thrombi.
(Thus the principal constituents of blood can safely circulate in their aqueous environment.)

2. Attraction:
The specific attraction between pseudopodia of phagocytic leukocytes and bacteria that have found their way into the bloodstream allows the phagocytic cells to internalize such bacteria and destroy them.
The specific attraction between the epitope of a (foreign) antigen and the paratope of a complementary antibody molecule (immunoglobulin) triggers a series of events leading to the foreign antigen's destruction, see also Chapter 4, Section 6.
Whilst electrical double layer forces sometimes play a role in both the specific attractions and the non-specific repulsions alluded to above, polar, Lewis acid–base (AB) forces tend to accompany such electrical double layer forces and are often stronger than these, especially in high ionic strength media, such as the human blood circulation, in which electrostatic interactions are significantly attenuated. However, polar AB forces also act quite well in the absence of any electrical double layer forces, see Chapter 4, Sections 2, 3 and Chapter 7, Section 4, below.

2 EARLY EXAMPLES OF THE TREATMENT OF NON-COVALENT INTERACTIONS IN WATER

2.1 DLVO and Non-DLVO forces

In the 1940's the interaction energies between particles, immersed in a liquid were assumed to obey what is now known as the DLVO theory, after Derjaguin and Landau (1941), from the USSR and Verwey and Overbeek (1948), from the Netherlands; however, see also Hamaker (1936, 1937a, 1937b, 1937c). The classical version of this approach takes into account van der Waals attractive and electrostatic repulsive energies, acting upon such particles, as a function of interparticle distance. The classical DLVO theory does not account for polar (e.g., hydrogen-bonding) interactions. It does turn out, however that, in water, its polar (largely hydrogen-bonding) free energies of adhesion (to other entities) as well as of cohesion (within itself), usually account for approximately 90% of all polar, i.e., non-covalent interaction energies that are active in that liquid. [Even though van der Waals forces represent 30% of the cohesive interaction energies of water (see above), the van der Waals combining rules for interactions in liquids are such that van der Waals interaction energies among most biological compounds or particles, immersed in water, usually account for at most only a few % of the total, due to the van der Waals part of the combining rules given in Chapter 5, Eq. (5.1).]

It was only in the 1980's that hard experimental evidence, obtained with Israelachvili's force balance (Israelachvili, 1985, 1991), which permits the direct measurement of interaction forces in water, revealed the existence of “non-DLVO” forces which are neither due to van der Waals nor to electrical double layer forces (see also Claesson, 1986, and a more recent review article by Grasso et al., 2002).

2.2 Good's introduction of a Φ-factor and Fowkes' evaluation of a van der Waals/Non-van der Waals ratio of the surface tension of water

Meanwhile, from a theoretical point of view, in 1957 Good (see Girifalco and Good, 1957) had remarked upon the very different interfacial behavior of organic molecules when immersed in water, as compared to their interfacial behavior when immersed in various organic liquids. Thus Girifalco and Good (1957) proposed the use of a factor, Φ, of which the degree of departure from unity could be used as an indicator of the polarity of the system. It was the value of the (measured) interfacial tensions between oils and water (Girifalco and Good, 1957; see also Harkins, 1952), which we now know to be only valid for alkane–water systems and not for polar oil–water systems (see Chapter 3) and which for alkanes only reached a maximum value of around 51 mJ/m2, that allowed Fowkes (1963, 1964, 1965), using alkane–water interfacial tension values, to deduce the ratio of van der Waals to non-van der Waals contributions to the surface tension of water at 20 °C as being equal to 21.8/51.0 mJ/m2, i.e., as 30/70%. Later, using direct contact angle measurements with apolar liquids on aqueous gels of different concentrations, van Oss, Ju et al. (1989) confirmed the correctness of Fowkes' findings for the non-polar (van der Waals) component of the surface tension of water.

2.3 The three van der Waals forces: Are some of them polar?

It took, however, until 1985 before it became possible to begin to delineate with some clarity what the real difference was between apolar and polar forces in water. A remaining difficulty between knowing that there was a problem and solving it was that there are three different and well-known parts to the electrodynamic forces, together representing the sum-total of van der Waals forces. These are, in the order of their...