Contact angle measurements are of fundamental importance in a range of industrial and everyday processes such as flotation, painting i. In the flotation process a solid block of the powdered mineral to be floated is often studied using a wide range of collector i. It has been shown that y c is a reasonable approximation for y sv. In this experiment y c will be determined for methylated hydropho- bic soda glass. Both advancing 0 A and receding 0 R angles will be measured in order to estimate the degree of hysteresis in each case.
Use the values of both 0 A and 0 R , as well as the average, to produce several Zisman plots. Experimental details Contact angle measurement The contact angles can be measured by observing the TPL through a microscope that has a rotating eyepiece and cross-hairs. The eyepiece is used to measure the angle of the cross-hairs on a protractor scale. By tilting the microscope slightly the reflected image of the liquid droplet can also be observed and the double-angle 20 can be meas- ured, which increases the accuracy.
An illustration of this type of appa- ratus is given in Figure 2.
Contact angle cell Side view Binocular microscope Figure 2. The glass plate which must be cleaned in the manner described later is then positioned on the upraised table and a small beaker of the liquid to be used is placed in the base of the cell.
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The cell is then sealed and left to equilibrate for 5 min. About 0. A droplet of the liquid is then slowly forced out and allowed to equilibrate.
Since the needle is left in the droplet during measurement, the droplet must be of sufficient size that the region near the TPL line is not affected by the needle changing the shape of the droplet. The drop volume should then be slowly increased until the maximum angle is obtained just before the TPL moves forwards. This has to be repeated several times to obtain the maximum angle. Measure the contact angle with the TPL at different positions on the plate.
Take the average value to give the advancing contact angle 0 A. Follow a similar procedure to measure the receding angle 0 R but slowly withdraw liquid into the syringe and measure the minimum angle just before the TPL moves.
Be careful and always wear safety glasses. Warm, concentrated NaOH solution is very harmful to the eyes. The contact angle of water on clean glass should be very low; otherwise, further cleaning is required. The cleaned glass is very easily contaminated by finger grease and exposure to laboratory air. For these reasons the samples have to be prepared just before they are used in the cell and must only be handled using clean tweezers.
Methylated hydrophobic glass is prepared simply by exposing a clean, dry plate to the vapour of highly reactive trimethylchlorosilane Me 3 SiCl for about 1 minute in a fume cupboard. Simply place the plate with the polished surface exposed in a large, clean beaker con- taining a smaller beaker of liquid Me 3 SiCl. Loosely cover the large beaker. The Me 3 SiCl reacts vigorously with water as well as the surface silanol groups on glass Figure 2. Table 2. The resulting monolayer of methyl groups is chemically attached and completely alters the wetting properties of the surface.
Again, to prevent contamination these plates must be handled only with twee- zers and stored in cleaned, sealed containers. Liquids used to determine the critical surface tension of methylated glass Measure advancing and receding contact angles on methylated glass plates using the liquids in the order given in Table 2. For all these liquids except water prepare the syringe and cell in a fume cupboard and seal before measuring contact angles in the labo- ratory use caution. Use the same methylated plate for the first two liquids and a second plate for the other three.
In each case rinse the plate with clean ethanol and blow dry when changing liquids. Plot out cos 0 A , cos 0 R and cos 0 A v against the corresponding y LV values to estimate the value of y c. What do you think are the causes of the contact angle hysteresis observed in this experiment? What would you expect for the wetting properties of these liquids on untreated, clean glass? Is the value you obtained for y c reasonable for this type of surface? Thermodynamics of Adsorption Derivation of the Gibbs adsorption isotherm. Determination of the adsorption of surfactants at liquid interfaces.
Laboratory project to determine the surface area of the common adsorbent, powdered acti- vated charcoal. This equation is based on the entropy associated with a component in a mixture and is at the heart of why we generally plot measurable changes in any particular solution property against the log of the solute concentration, rather than using a linear scale. Generally, only substantial changes in concentration or pressure produce significant changes in the properties of the mixture. For example, consider the use of the pH scale.
The real concentration gradient of solute near the interface may look like Figure 3. When the solute increases in concentration near the surface e. Figure 3. Let us now examine the effect of adsorption on the interfacial energy y. This situation is illus- trated in Figure 3. Thus, although the total free energy of the system increases with the creation of new surface, this process is made easier as the chemical potential of the selectively adsorbed component increases i.
This is the Gibbs surface tension equation. Basically, these equations describe the fact that increasing the chemical potential of the adsorb- ing species reduces the energy required to produce new surface i. This, of course, is the principal action of surfactants, which will be dis- cussed in more detail in a later section. But the latter appears not to be possible, be- cause the chemical potentials are dependent on the concentration of each component.
However, for dilute solutions the change in p for the solvent is negligible compared with that of the solute. Note that for con- centrated solutions the activity should be used in this equation. Experi- mental measurements of y over a range of concentrations allows us to plot y against lnci and hence obtain T l5 the adsorption density at the surface. The validity of this fundamental equation of adsorption has been proven by comparison with direct adsorption measurements.
However, care must be taken to allow equilibrium adsorption of the solute which may be slow during measurement. Finally, it should be noted that 3. Question: Consider which form of the isotherm would apply to an ionic surfactant solution made up in an excess of electrolyte. Determination of surfactant adsorption densities Typical results obtained for the variation in surface tension with sur- factant log concentration, for a monovalent surfactant, are given in Figure 3. These results show several interesting features.
As we will see in Chapter 8 another method for determining the surfactant head group area is afforded by the Langmuir trough tech- nique. The surface is fully packed with surfactant molecules, although y still continues to fall. This, apparently odd, situation arises because the chemical potential of the surfactant continues to increase with its con- centration see Equation 3. However, at the cmc a sharp transition occurs which apparently cor- responds to zero adsorption i.
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How can this be so? If we examine properties of the bulk solution in this region, we find that at this same concentration there is a sharp transition in a wide range of properties, such as conductivity, osmotic pressure and turbidity see the following chapter. What is in fact happening is that the surfactant molecules are forming aggregates, usually micelles, and that all the additional molecules added to the solution go into these aggregates and so the concentration of monomers remains roughly constant.
That is, both dy and dlnC are effectively zero, and the plot should strictly stop at the cmc value, since although we are adding surfactant molecules, we are not increasing their activity or concentration.
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The precise nature of these aggregates is discussed in the following chapter. The crystals within a clay domain can be represented by a three-plate crystal model in which one crystal separates the other two crystals to produce a slit-shaped pore, where the crystals overlap. This situation is illustrated in Figure 3.
The surface separation in the slit-shaped pore is determined by the crystal thickness. For an illite a fine-grained mica with a surface area of 1.
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The stability of clay domains within a soil is a crucial feature for agricultural production because the permeability of a soil to aqueous electrolyte solutions depends on this stability. Swelling of these domains reduces permeability. The interaction of clay crystals within a domain depends upon the DLVO repulsive pressure in the slit-shaped pores and the balance between repulsive pressure [P R ] from counterion hydration and the attractive pres- sure [P A ] generated by van der Waals forces and the recently discovered ion-ion correlation attraction between the counterions in the confined space of the overlap pores see Kjellander et al.
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However, when the crystal Figure 3. In dilute solutions the repulsive pressure, in the slit-shaped pores, is sufficient to release the platelets from the shallow potential minimum and the domains start to swell. However, in accord with DLVO theory, the swelling pressure, in the slit-shaped pores can be reduced by increasing the electrolyte concentration. Studies on soils have shown that there is a nexus between saturated permeability [zero suction], sodicity and electrolyte concentration.
The con- centration, obtained by diluting the electrolyte, at which there is a first discernible decrease in permeability, called the 'threshold concentration', corresponds to the start of the swelling of the clay domains.