Interfacial phenomena

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  • 1. , ) inside the volume of liquid ) in the surface layer gas liquid

2. The surface tension of most liquids decreases with increasing temperature 3. Measurement of surface tension by Stalagmometer drop-weight methods n0, 0, 0 number of droplets, density and surface tension of water, n, , of investigated liquid 0 0 0 n n = 4. grh 2 1 = Where g is gravity = 9,8 m/s2 For zero contact angle, 5. For the rise of a liquid up a narrow capillary N.B. In practice, the capillary rise method is only used when the contact angle is zero, owing to the uncertainty in measuring contact angles correctly 6. Involuntary surface phenomena Cohesion is the interaction between moleculars inside one phase (homogeneous system). Adhesion is the interaction between moleculars inside of the different phases Heterogeneous formation of a new phase Spreading of the liqid on the surface of other liquid Formation of spherical drops 7. Absorption is the process of arbitrary absorption of the substance by volume Chemisorption - chemical interaction adsorbent with adsorbate 8. Difference between Adsorption and AbsorptionDifference between Adsorption and Absorption 9. POSITIVE AND NEGATIVE ADSORPTION 10. In certain cases - solutions of electrolytes, sugars, etc. - small increases in surface tension due to negative adsorption are noted. Here, because the solute-solvent attractive forces are greater than the solvent-solvent attractive forces, the solute molecules tend to migrate away from the surface into the bulk of the liquid. 11. Spreading Adhesion and cohesion The work of adhesion between two immiscible liquids is equal to the work required to separate unit area of the liquid-liquid interface and form two separate liquid-air interfaces (Figure: Work of adhesion (a) and of cohesion (b), and is given by the Dupre equation 12. Spreading of one liquid on another When a drop of an insoluble oil is placed on a clean water surface, it may behave in one of three ways: 1. Remain as a lens, as in Figure 4.16 (non-spreading). 2. Spread as a thin film, which may show interference colours, until it is uniformly distributed over the surface as a 'duplex' film. (A duplex film is a film which is thick enough for the two interfaces - i.e. liquid-film and film-air - to be independent and possess characteristic surface tensions.) 3. Spread as a monolayer, leaving excess oil as lenses in equilibrium, as in Figure 4.17. 13. Harkins defined the term initial spreading coefficient (for the case of oil on water) as Substituting in the Dupre equation, the spreading coefficient can be related to the work of adhesion and cohesion 14. Yungs equation Cos = s-g - l-g /l-g = rhgd/2cos 15. Adhesional wetting In adhesional wetting, a liquid which is not originally in contact with the solid substrate makes contact and adheres to it. In contrast to spreading wetting, the area of liquid-gas interface decreases. Immersionai wetting In immersional wetting, the solid, which is not originally in contact with the liquid, is immersed completely in the liquid. The area of liquid-gas interface, therefore, remains unchanged 16. Surface tension of aqueous sodium chloride solutions at 20C 17. Surface activity Materials such as short-chain fatty acids and alcohols are soluble in both water and oil (e.g. paraffin hydrocarbon) solvents. The hydrocarbon part of the molecule is responsible for its solubility in oil, while the polar COOH or -OH group has sufficient affinity to water to drag a short-length non-polar hydrocarbon chain into aqueous solution with it. If these molecules become located at an air-water or an oil-water interface, they are able to locate their hydrophilic head groups in the aqueous phase and allow the lipophilic hydrocarbon chains to escape into the vapour or oil phase 18. Figure shows the effect of lower members of the homologous series of normal fatty alcohols on the surface tension of water. The longer the hydrocarbon chain, the greater is the tendency for the alcohol molecules to adsorb at the air-water surface and, hence, lower the surface tension. A rough generalisation, known as Traube's rule, is that for a particular homologous series of surfactants the concentration required for an equal lowering of surface tension in dilute solution decreases by a factor of about 3 for each additional CH2 group. If the interfacial tension between two liquids is reduced to a sufficiently low value on addition of a surfactant, emulsification will readily take place, because only a relatively small increase in the surface free energy of the system is involved. 19. SURFACE ACTIVITY OF DRUGS Even small drug molecules are frequently amphiphilic, and therefore also generally surface active. This means that the drug tends to accumulate at or close to an interface, be it a gas/liquid, solid/liquid or liquid/liquid interface. This surface activity frequently depends on the balance between electrostatic, hydrophobic and van der Waals forces, as well as on the drug solubility. Since the former balance depends on the degree of charging and screening, the surface activity, and frequently also the solubility of the drug, it often depends on the pH and the excess electrolyte concentration. the surface activities of drugs may contribute to their biological action, although the relationship between surface activity and biological effect is less straightforward.

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