Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 8

chemical background. Briefly speaking the problem to detect real contact angles is
here more focused on the solid surface free energy that is assumed to be considered
as a result of two main independent contributes [168-170] (11):
γ TOT

+ γ AB , (11)
where LW stands for the Lifschitz-van der Waals dispersive components and AB
are the Acid-Base ones. The AB components may be further expressed by follow-
ing two main different approaches. The first is the one supported by Fowkes [170]
that makes a link between the W ADH , intended as a free energy term, and the en-
tropic contribution (temperature) to the H AB enthalpy calculated following Drago
[171] accordingly to (12):

= γ LW

2 γ LW H AB ), (12)

where f is a parameter expressing the entropic component and N is the number
of the acid-base interactions involved on the interface. The second (13) has been
provided by Good-van Oss-Chaudury (GvOC) [172] and works directly on the free
energy term, due its need of contact angle values determined at a known assigned
temperature.

γ LW W ADH = + fN( −

sv

lv

γ lv γ sv ). (13)
Such approaches therefore depend upon the contact angle data that one may col-
lect by submitting the same solid surface to several liquids, featured by different
polar and dispersive characteristics. While the Fowkes version appears difficult to
be applied due to the necessity to evaluate contact angles at different temperatures
the GvOC seems apparently easier to be used. Equation (13) in principle let one
to calculate the LW contribution, the electron-donor γ − (Lewis base) and electron
acceptor γ + (Lewis acid) of a solid surface by measuring the contact angle values
with three liquids of known surface free energy components. When indeed applied
on polymer surfaces some Authors [173, 174], by comparing the results provided
by inverse gas chromatography or ζ potentials, found discrepancies with the re-
sults provided by GvOC. In particular they noticed a constant overestimation of the
basic component of the surface free energy that were labeling these polymers sur-
faces as basically featured contrarily to experimental data. The Authors of GvOC
Theory got aware of this problem and suggested that the observed influence could
have been ascribed to the effects of monopolar basics [175] but this hint has not
been fully accepted. Della Volpe and Siboni [176-181] highlighted the fact that the
source of misunderstandings was probably lying on a wrong choice of referenc-
ing fluids, known as the triplet . In particular they stressed that the choice made by
the original GvOC to compare acidic and basic components to water caused a sys-
tematic ill-conditioned situation by which the obtained experimental results appear
wholly shifted to basic features. These Authors, recommending that only a direct
comparison among same featured components (i.e., acidic with acidic, basic with
basic) of different materials let to GvOC to be correctly applied, proposed a matrix

2 ( γ LW

γ lv γ sv +

γ LW W ADH = +

sv

lv