Influence of the Processes Parameters on the Properties of The ...

Chapter 3.
Analytical Methods and Designs <strong>of</strong> Experiments
tension causes low wetting properties. A high solid surface energy (mostly hydrophilic), on <strong>the</strong> o<strong>the</strong>r hand,
means that <strong>the</strong> interfaces between <strong>the</strong> solid material and <strong>the</strong> air are not favourable in a <strong>the</strong>rmodynamic sense.
Therefore, high surface energy solids are easily wetted by liquids (cf. Figure 3.20). Wetting <strong>of</strong> a solid
eliminates <strong>the</strong> solid-air interface in favour <strong>of</strong> <strong>the</strong> solid-liquid interface. Interactions between <strong>the</strong> solid surface
and <strong>the</strong> liquid are resulting in a lowering <strong>of</strong> energy state, which is a more favourable state [Lazghab et al.,
2005; Shaw, 1992].
Figure 3.20: Wetting <strong>of</strong> hydrophilic and hydrophobic samples.
6.2.1 Surface Tensions <strong>of</strong> Liquids
Different liquids may be used in surface tension measurements but in general chosen liquids have
relatively low viscosity and low volatility. Van der Waals forces result to <strong>the</strong> addition <strong>of</strong> different
components: d dispersive and nd non dispersive or attractive forces (polar forces and hydrogen bonds). We
can write: = d + nd .
In some apolar molecules, <strong>the</strong>re exist important intermolecular forces but no permanent dipolar
moment. When two molecules are in <strong>the</strong> proximity, instantaneous dipoles interact via London forces. In
polar molecules, permanent attractive forces create special arrangement increasing stability <strong>of</strong> <strong>the</strong> couple
(Debye and Keesom forces). Interactions between apolar molecules and polar surface are limited to <strong>the</strong>se
attractive interactions and nd L polar forces are constituted by <strong>the</strong> sum <strong>of</strong> <strong>the</strong>se two forces. A special attention
must be paid when a hydrogen atom is linked to an electronegative atom. The electronegative atom exerts an
important attractive force on <strong>the</strong> unique hydrogen atom and forms hydrogen bonds. Also, even if <strong>the</strong>se types
<strong>of</strong> relationships fit for <strong>the</strong> dispersion forces, and possibly also for some polar (Debye and Keesom forces),
<strong>the</strong>y are not so good for acid/base and hydrogen bonding.
For this, Oss [2006, 1994] have proposed o<strong>the</strong>r combinations <strong>of</strong> surface energies. The argument is
that <strong>of</strong>ten, <strong>the</strong> polar (Keesom and Debye) forces are weak, and can be included in <strong>the</strong> dispersive
contribution. The "combined" contribution is denoted by LW (Lifschitz-van der Waals). In addition, <strong>the</strong>re is
a short-range interaction that is caused by acid-base interactions (hydrogen bonding is a type <strong>of</strong> Acid-Base).
In that case, we write: = LW + AB .
There are several common methods described in <strong>the</strong> literature for measuring <strong>the</strong> surface tension <strong>of</strong>
liquids. The surface tensions <strong>of</strong> liquids can be directly determined by measuring <strong>the</strong> surface force with a Du
Noüy ring or a Wilhelmy plate for example. The determination <strong>of</strong> surface energy is necessary indirect In
literature, <strong>the</strong>re are several methods to measure <strong>the</strong>ir contact angle with a liquid [Petrie, 2000]. By using a
given model, we only can estimate <strong>the</strong>ir surface energy.
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