Acid-base interaction, solid surface

A requirement underlying the validity of Zisman plots is that there be no specific interactions, such as acid-base interactions, between the solid surface and the probe liquids. Such interactions, however, can, in principle, be taken into account by Young s equation, provided the contact angle remains finite. Their... 

The mineral surface may be considered as a solid source of Lewis and/or BrfSnsted acidity and the reactive sites S as localized acidic or basic functional groups. Reactions involving such sites may be understood in terms of Lewis acid/base or BrfSnsted acid/base interactions ( 1, 5, 6, 8, 38). As the acidity of the reactive sites increases, increasingly weak bases are neutralized and reactive surface complexes (A S) may be formed. The term "acidity" is often used in the broad sense of the word, including both BrjSnsted and... 

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 ... 

Surfactants adsorb on solid surfaces due to hydrophobic bonding, electrostatic interaction, acid-base interaction, polarisation of rr electrons and dispersion forces. Hydrophobic bonding occurs between the hydrophobic surfactant tail and the hydrophobic solid surface (tail down adsorption with monolayer structure) or between the hydrophobic tails of the surfactant adsorbed on the hydrophilic solid surface and the hydrophobic tails of the surfactant from the liquid phase (head down adsorption with bilayer structure) [54, 55]. 

Electrostatic interactions occur between the ionic head groups of the surfactant and the oppositely charged solid surface (head down adsorption with monolayer structure) [56]. Acid-base interactions occur due to hydrogen bonding or Lewis acid-Lewis base reactions between solid surface and surfactant molecules (head down with monolayer structure) [57]. Polarisation of jt electrons occurs between the surfactant head group which has electron-rich aromatic nuclei and the positively charged solid surface (head down with monolayer structure) [58]. Dispersion forces occur due to London-van der Waals forces between the surfactant molecules and the solid surface (hydrophobic tail lies flat on the hydrophobic solid surface while hydrophilic head orients towards polar liquid) [59]. 

A study of the interaction of Lewis acids and bases (or electron acceptors and donors) in surface dynamics has led to new insight into interactions with various solid surfaces [26,64,99,100,104,110,130-142], as well as interactions at interfaces between two different substances. It is noted that the acid-base interactions of Lewis, including the orientational properties of charge transfer forces of Mulliken [143], occur between specific (or polar) groups in substances. These interactions are quite dependent on the Stockmayer degree of polarity, <5, [126] as measured by dipole moment in Eq. (58). Furthermore, it can be found that a concept of acids attract bases may be substituted... 

A further illustration of IGC as a source of data for acid/base characterization of polymers and of solid constituents of complex polymer systems, is given by Osmont and Schreiber (49), who rate the inherent acid/base interaction potentials of glass fiber surfaces and of polymers by a comparative index, based on the Drago acid/base concepts (SO). The interaction index is conveniently measured by IGC and is shown to differentiate clearly among untreated and variously silane-modified glass fiber surfaces. Conventional methods are used to determine adsorption isotherms for fiber-polymer pairs, and the IGC data ate used to demonstrate the relationship between acid/base interactions and the quantity of polymer retained at fiber surfaces. 

In nonpolar media due to the low ionization of the solute species, electrostatic attractive or repulsive forces can be ruled out as a major mechanism for adsorption. However, polar interactions have to be considered especially when polar surfaces such as oxides are involved. Recent work has shown acid-base interactions between the surface species and the solute molecules to be responsible for adsorption in nonaqueous media. Fowkes has suggested that the interaction between a solid surface and an uncharged adsorbate can be divided into two parts, dispersive interactions and polar interactions. The dispersive interactions are due to the fluctuating dipole moments created by the movement of electrons in any atom or molecule and thus occur between all atoms and molecules. Polar interactions refer to specific interactions between hydrophilic surface groups and functional groups in the adsorbate molecules. 

For better dispersion, the relative acid-base interactions of the three participants—liquid medium-adsorbate, adsorbate-solid surface, and solid surface-liquid medium—should be considered simultaneously rather than one pair at a time. 

Fowkes and Mostafa (51) demonstrated another fruitful approach to the problem of estimating surface energies of solids by separating the surface free energy into different components, of which dispersive and polar acid-base interactions dominate, thus giving the following ... 

Acid-base interactions have found numerous applieations in research dealing with adsorption of molecules of liquids on the surfaees of solids. The main focus of this research is to estimate the thermodynamic work of adhesion, determine mechanism of interactions, analyze the morphology of interfaces and various surfaee coatings, develop surface modifiers, study the aggregation of macromolecular materials, explain the kinetics of swelling and drying, understand the absorption of low molecular weight compounds in polymeric matrices, and determine the properties of solid surfaces. In addition to these, there are many other applieations. 

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. 

Acid-base interaction parameters for the materials of this research are given in Table 7.6. As anticipated by the results in Figure 7.7, and in agreement with the results shown in the preceding section, these show PVC to be predominantly acidic. ABS has a moderately basic surface and Xenoy is an amphoteric solid with nearly equal AN and DN indices. All of the PU systems function as both donors and acceptors, with PU5 and PU6 skewed to acidity, PUl and PU4 to basicity, while PU3 and (to a lesser degree) PU2 are amphoteric. 

The surface chemistry of acid-base interactions in the nanospaces of solids comparable in size to the n-oleculariimensions of the reactants themselves is a new and fascinating subject. It has been approached from various angles attempts were made to extend the concepts of classical chemistry, but new rules had to be developed. The consequence is that there is still no consensus regarding the significance of experimental results and their relationship to the theoretical notions. 

For example, little is known about acid-base interactions at solid/solid interfaces. It has long been recognized that molecular interactions across the interface between condensed phases may be split into physical and chemical terms. Physical interactions (vander Waals forces) contribute to the non-ideality of fluids and have been traditionally considered for interfaces. However, modern theories explain interfacial phenomena on solid surfaces, such as adhesion or wetting, in terms of chemical interactions. Moreover, the Lewis definition of acidity is so comprehensive that it can easily be accepted that most chemical interactions at solid surfaces may be effectively described as acid-base interactions [15]. continuously growing literature reinterprets interactions at solid/solid interfaces in terms of acid-base properties. For example, their role was shown in relation to solid/solid adhesion in film-substrate or fber-matrix systems as well as in wood and paper processing,... 

Similar to homogeneous one-phase systems, the acidity of solid surfaces manifests itself only in relation to a base that must be present the strength of the acid-base interaction therefore depends on the particular base involved. This is true for both types of acid sites, Bronsted or Lewis. The reverse can be said, of course, for surface basic sites. 

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