Handbook of Solvents - George Wypych - ChemTech - Ventech!

10
Acid-base Interactions
10.1 GENERAL CONCEPT OF ACID-BASE INTERACTIONS
George Wypych
ChemTec Laboratories, Inc., Toronto, Canada
Acid-base interactions have found numerous applications in research dealing with
adsorption of molecules of liquids on the surfaces 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 surface 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 applications.
Several techniques are used to determine and interpret acid-base interactions. These
include: contact angle, inverse gas chromatography, IGC, Fourier transform infrared, FTIR,
and X-ray photoelectron spectroscopy, XPS. These methods, as they are applied to solvents
are discussed below.
Contact angle measurements have long been used because of common availability of
instruments. In recent years, they have been developed from simple optical devices to the
present day precise, sophisticated, computer-controlled instruments with sufficient precision.
Van Oss and Good 1,2 developed the basic theory for this method. Their expression for
surface free energy is used in the following form:
LW AB LW
+ −
γ = γ + γ = γ + 2 γ γ
[10.1.1]
where:
γ LW
Lifshitz-van der Waals interaction
γ AB
acid-base interaction
γ +
Lewis acid parameter of surface free energy
γ -
Lewis base parameter of surface free energy.
The following relationship is pertinent from the equation [10.1.1]:
AB + −
γ = 2 γ γ
[10.1.2]
A three-liquid procedure was developed 1-3 which permits the determination of the
acid-base interaction from three measurements of contact angle: