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

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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. - 78 -
Chapter 3. Analytical Methods and Designs <strong>of</strong> Experiments 6.2.1.1 Du Noüy Ring Method In this method, a clean platinum ring (cf. Figure 3.21) is placed under <strong>the</strong> surface <strong>of</strong> <strong>the</strong> test liquid, and <strong>the</strong> liquid is slowly moved downward until <strong>the</strong> ring or plate breaks through <strong>the</strong> liquid surface. The force is recorded, and by means <strong>of</strong> appropriate conversion factors, <strong>the</strong> surface tension <strong>of</strong> <strong>the</strong> liquid is calculated. 6.2.1.2 Wilhelmy Plate Method Figure 3.21: The Du Noüy’ ring method. A second method to measure <strong>the</strong> liquid surface tension involves <strong>the</strong> use <strong>of</strong> a pre-weighed plate and <strong>the</strong> measurement <strong>of</strong> wetting forces. The level <strong>of</strong> <strong>the</strong> liquid is raised until contact between <strong>the</strong> liquid surface and <strong>the</strong> plate is registered. Contact between <strong>the</strong> liquid and <strong>the</strong> plate induces a change <strong>of</strong> working forces on <strong>the</strong> plate, which is measured by <strong>the</strong> tensiometer. There are 3 forces acting on <strong>the</strong> plate: <strong>the</strong> force due to (1) vertical gravity, (2) wetting and (3) buoyancy. By using a pre-weighed plate, <strong>the</strong> tensiometer can exclude <strong>the</strong> gravity force and by extrapolating <strong>the</strong> measured forces back to zero depth <strong>of</strong> immersion, <strong>the</strong> buoyancy force can also be excluded. The only force left, <strong>the</strong> wetting force (F w ), is <strong>the</strong>n easily measured by <strong>the</strong> tensiometer [Mykhaylyk et al., 2003; Gaonkar and Neuman, 1984]. A high surface energy (platinum plate) is used in this approach, with <strong>the</strong> assumption that <strong>the</strong> contact angle (θ) liquid-platinum plate is 0° (Figure 3.22). As a consequence, <strong>the</strong> liquid-vapour surface tension can be calculated as follows: F w = γ LV L cos θ, where γ LV is <strong>the</strong> liquid-vapour surface tension, L <strong>the</strong> wetted length <strong>of</strong> <strong>the</strong> plate (twice <strong>the</strong> width and length <strong>of</strong> <strong>the</strong> plate) and θ <strong>the</strong> plate-liquid contact angle [Mykhaylyk et al., 2003; Gaonkar and Neuman, 1984]. Figure 3.22: The Wilhelmy’ plate method with a platinum plate. Its biggest advantage is that <strong>the</strong> liquid surface tension is analyzed at a fixed point, resulting in a static determination method. After <strong>the</strong> plate is immersed in <strong>the</strong> liquid and equilibrium has settled, <strong>the</strong>re is no movement <strong>of</strong> <strong>the</strong> plate or liquid which results in a higher accuracy. Fur<strong>the</strong>rmore <strong>the</strong> Wilhelmy’ plate method allows determining <strong>the</strong> surface tension <strong>of</strong> viscous liquids [Mykhaylyk et al., 2003; Gaonkar and Neuman, 1984]. - 79 -
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