Measurement of Acid-Base Interactions

Interaction between acids and bases can be measured in many ways  

Direct calorimetric methods or temperature dependence of equilibrium constants can be used to measure enthalpies and entropies of acid-base reactions. The following section gives more details on use of data from these measurements. 

Gas phase measurements of the formation of protonated species can provide similar thermodynamic data. 

Infrared spectra can provide indirect measures of bonding in acid-base adducts by showing changes in bond force constants. For example, free CO has a C—O stretching band at 2143 cm and CO in Ni(CO)4 has a C—Obandat2058 cm  

Nuclear magnetic resonance coupling constants provide a similar indirect measure of changes in bonding on adduct formation. 

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Measurement of Acid-Base Interactions in Group IIIA Containing... 

MEASUREMENT OF ACID-BASE INTERACTIONS IN GROUP IMA CONTAINING SAMPLES... 

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

For decades such adsorption had been assumed to involve dipole interactions and interacting sites were termed "polar." It is quite clear in the above studies that dipoles in the polymers and in the solid surfaces do not contribute measurably to adsorption. Even from carbon tetrachloride, the solvent most favorable to adsorption, the amount of basic polymer (PMMA) that adsorbed onto basic calcium carbonate was only 2.5% of the amount that adsorbed on the same area of silica surface. Similarly, the amount of acidic polymer (CPVC) that adsorbed onto the acidic silica from any of the six solvents was less than 0.2% of the amount that adsorbed from carbon tetrachloride or dichloromethane onto the same area of basic calcium carbonate. It is concluded that adsorption of organic acids or bases from neutral organic solvents onto inorganic solids is governed entirely by acid-base interactions and is quite independent of dipole phenomena. It is therefore proposed that heats of adsorption are actually enthalpies of acid-base interaction and should therefore be subject to the Drago correlation ... 

Inverse gas chromatography at infinite dilution appears to be a powerful tool for studying the surface properties of carbon fibres and polymer matrices. The use of alkane probes and acid/base probes allows the characterization of the surfaces in terms of their London dispersive component of surface energy and their acid/base or acceptor/donor characteristics. A strong correlation was obtained between fibre-matrix adhesion, measured by a destructive fragmentation technique, and the level of acid base interactions calculated from the chromatographic analysis. 

Dispersive and specific interactions are considered to contribute independently to the adsorption of probe molecules at the adsorbent surface. It was presented that the adhesion of the fibre-matrix interface depends clearly on the measured strength of acid/base interactions of both fibre and polymer-matrix. Fowkes [2,3] indicated also that the surface of fillers can be chemically modified to enhance acid-base interaction and increase adsorption. 

Among the cosolvents studied, methanol and acetone have received the greatest interest (36-38). Methanol may act as either a Lewis acid or a Lewis base while acetone is a weaker Lewis base and very slightly acidic ( ). The dipole moment of acetone is 2.88 Debeye compared to 1.7 Debeye for methanol. Based on these properties, Walsh, et al., (40), interpret the data of Van Alsten (37) and Schmitt (38) and present liquid phase IR measurements which show Lewis acid-base interactions in the systems methanol/acridine and acetone/benzoic acid. Supercritical solubility data of Dobbs, et al., ( ), exhibit trends which indicate the importance of acid-base interactions. Van Alsten and Schmitt present data which show that acid-base interactions are a secondary cosolvent effect superimposed on a primary effect determined by cosolvent concentration. 

In adhesion science and technology, the manifestations of acid-base interactions have been observed at both macroscopic and microscopic scales (wetting, adhesion, metallization, etc.). The development of scanning probe microscopic methods (scaiming tunneling microscopy (STM) and atomic force microscopy (AFM)) over the past decade has led to the possibility of measuring adhesion forces on the molecular scale in addition to imaging surfaces in atomic resolution. 

The above shows a good correspondence between the data measured by different methods. This was also shown by a theoretical analysis of the effect of acid-base interactions on the aggregation of PMMA. But there is as yet no universal theory for the characterization of acid-base interaction. 

Mefliods of study and data interpretation still require further work and refinement. Several experimental techniques are used, including microscopy (TEM, SEM) dynamic light scattering " using laser sources, goniometers, and digital correlators spectroscopic methods (UV, CD, fluorescence) fractionation solubility and viscosity measurements and acid-base interaction. "... 

In order to eliminate the difficulties associated with gas-phase measurements, Lewis acid-base interactions are investigated in solutions prepared with inert parent solvents. These investigations set out from the assumption that the course of the reaction in an inert solvent corresponds to the gas-phase reaction, hence the various side-effects of solvation, etc., can be neglected. 

As noted in the preceding section, IGC is an excellent tool for measurements of surface properties and of acid base interaction potentials of macromolecular solids (2,10,16). In this section the importance of acid-base interactions relative to the adhesion of PUs is considered in greater detail. IGC has been applied to a series of PU adhesives and to selected polymer substrates, allowing quantitative measurements to be made of the acid/base (electron donor-acceptor) interaction parameters applicable to the surfaces of these materials. Acid base pair-interaction parameters for substrate/PU combinations have been calculated. The bond characteristics of polymer/PU combinations have been measured, in part by conventional lap-shear procedures and in part, by the more recent constrained blister detachment method [11, 12]. Possible relationships between bond properties and acid base interactions have been considered, and a comparison of the two adhesion tests has been made. 

Vignes-Alder, M. and Brenner, H., A micromechanical derivation of the differential equations of interfacial statics. III. line tension, 7. Colloid Interface Sci., 103,11,1985. Vogler, E.A., Practical use of concentration-dependent contact angles as a measure of solid-liquid adsorption. 1. Theoretical aspects, Langmuir, 8, 2005, 1992. Vrbanac, M.D. and Berg, J.C., The use of wetting measurements in the assessment of acid-base interactions at solid-liquid interfaces, J. Adhesion Sci. TechnoL, 4, 255, 1990. 

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