Acid-base surface energy parameters

Berg has provided a wide-ranging discussion of a range of methods - many described as tedious - by which acid-base interaction can be obtained. In addition to the method of Fowkes, considered above, two others are described Inverse gas chromatography and the contact angle method introduced by Good, Chaudhury and van Oss, which involves Acid-base surface energy parameters. 

Acid-base surface energy parameters D E PACKHAM Surface energy components... 

IGC can be used to determine various properties of the stationary phase, such as the transition temperatures, polymer—polymer interaction parameters, acid-base characteristics, solubility parameters, crystallinity, surface tension, and surface area. IGC can also be used to determine properties of the vapor-solid system, such as adsorption properties, heat of adsorption, interaction parameters, interfacial energy, and diffusion coefficients. The advantages of IGC are simplicity and speed of data collection, accuracy and precision of the data, relatively low capital investment, and dependability and low operating cost of the equipment. 

The surface energy parameters are defined by Eqn. 3 for the acid-base component of the work of adhesion between phases 1 and 2 ... 

Lee also proposes to use the LSER (linear solvation energy relationship) concept in its full context and he identifies that the polarity parameter of LSER is related to the spreading pressure. The remaining parameters of Kamlet-Taft LSER (acid, base and solubility parameter) have already their analogue in the van Oss-Good theory (acid, base and LW contributions to the surface tension). Thus, Lee believes that the spreading pressure should be included in new developments and in a proper evaluation of the van Oss-Good approach. 

There have been several applications of IGC to the determination of sur ce interactions (15-24). In particular, IGC was applied to several studies of natural polymers. Among them are cellulose (2, wood (26), potato starch as Amylopectin (27) and lignocellosic surfaces (2S). In these studies, die surface diermodynamic characteristic of wood fiber and its relationship to the fiber s water vapor adsorption was determined by IGC (26) Also, the surface ener, surface acid-base flee energy, enthalpy of desorption of acid-base probes, surface acid-base acceptors, and donor parameters were determined by IGC (26). Cellulose was also found by IGC to be sensitive to the presence of adsorbed water which possibly disorders its surface structure. 

Thermodynamic parameters of catalysts are highly relevant for practical applications. In particular, surface energies and acid-base properties of catalysts are of high interest since they reflect properties of active sites involved in the catalytic process and the initial adsorption step. 

Inverse gas chromatography involves the sorption of a known probe molecule (adsorbate, vapour) and an unknown adsorbent stationary phase (solid sample). IGC may be experimentally configured for finite or infinite dilution concentrations of the adsorbate. The latter method is excellent for the determination of thermodynamic properties such as surface energies and Lewis acid-base parameters. Measurements in this range are extremely sensitive due to the low concentration regime where the highest energy sites of the surface interact with the probe molecules. 

The original contribution of van Oss et al. (11) to this held is the determination of the acid-base component of the surface free energy in terms of two parameters Y (Lewis) acid parameter of surface free energy Y (Lewis) base parameter of surface free energy. According to these authors, one can write ... 

This approach introduces the concept of a Lewis acid parameter of surface energy, y+ and a Lewis base parameter of surface energy, y - This recognizes that a given solid or liquid (e.g. chloroform) may only exert acid properties, that is be monofunctional. In this case, y is zero. For a mono-functional base (e.g. triethylamine), y is zero. Some substances, such as water, show both electron donor and electron acceptor properties, so both y+ and y are finite. These are termed bifunctional . An inert molecule shows... 

Before the acid-base component of solid surface energy can be calculated, it is necessary to assume some arbitrary relation between the parameters of surface energy for one test liquid. Good chose to take... 

Koberstein lists some vaiues of the interaction parameter. Somewhat different treatments of interfaciai energies are discussed under Surface energy components and Acid-base interactions. 

The concept of surface energy components provides one route to interfacial tensions and solid surface energies and others include the Good-Girifalco interaction parameter and Acid-base interactions. 

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