Water interface, cohesive forces

Replacement of gas by the nonpolar (e.g., hydrocarbon) phase (oil phase) has been sometimes used to modify the interactions among molecules in a spread film of long-chain substances. The nonpolar solvent/water interface possesses an advantage over that between gas and water in that cohesion (i.e., interactions between adsorbed molecules) due to dipole and van der Waals s forces is negligible. Thus, at the oil/water interfaces, the behavior of adsorbates is much more ideal, but quantitative interpretation may be uncertain, in particular for the higher chains, which are predominantly dissolved in the oil phase to an unknown extent. The oil phase is poured on the surface of an aqueous solution. Thus, the hydrocarbon, such as heptane or decane, forms a membrane a few millimeters thick. It is thicker than the adsorbed monolayer. Owing to the small difference in dielectric constant between the air and a hydrocarbon oil, the... 

Cohesive Forces in Monomolecular Films at an Air-Water Interface... 

Determination of the force-area curves at the oil-water interface is usually carried out in a vessel of constant surface area. The area available to each molecule is reduced by adding more molecules to the interface. A method similar to that for air-water surfaces has also been described (7,8). In general, the oil-water interface reduces greatly the cohesion between the chains, and the pressure measured is due only to kinetic agitation and electrical repulsion. The lack of cohesion causes the pressure to be higher than for the same film at the air-water interface (Fig. 6). 

From the previous discussions, the residual oil was pulled and stripped from the rock surfaces. As shown in a 2D glass-etched model (see Figure 6.24), the residual oil after waterflood became isolated oil droplets. The polymer solution pulled the oil into oil columns. These oil columns became thinner and longer to form oil threads as they met the residual oil downstream. The oil upstream flowed along these oil threads to meet the residual oil downstream so that an oil bank was built. In the process of residual oil flowing along the oil threads, because of the cohesive force of the oil/water interfaces, it was also possible to form new oil droplets, which flowed downstream and coalesced with other oils. Now we are ready to discuss the role viscoelasticity plays. 

Fig. 8.1 Schematic diagram of the contact angle formed between a glass wall and a liquid where the adhesive force on a molecule near the air water interface and wall is and the cohesive force with the bulk,/e. The vector sum of...

The curvature of the interface depends on the relative magnitudes of the adhesive forces between the liquid and the capillary wall and the internal cohesive forces in the liquid. When the adhesive forces exceed the cohesive forces, 9 lies in the range 0° < 9 < 90° when the cohesive forces exceed the adhesive forces, 90° < 9 < 180°. When 9 > 90°, the cos 9 term is negative, resulting in a convex meniscus towards the vapor phase and the liquid level in the capillary falling below the liquid level in the container (capillary depression). This occurs with liquid mercury in glass where 9 = 140° and also with water in capillary tubes coated internally with paraffin wax. Thus, liquid mercury is used in the evaluation of the porosity of solid adsorbents in the mercury injection porosimetry technique (see Section 8.5). 

Although Zlsman has appeared never to attribute a specific fundamental meaning to his Yc> others have considered Y o be the surface tension that the solid would have were Its cohesive forces the same as those acting across the solid-liquid Interface. Thus for a hydrocarbon solid, which has been supposed to Interact only through dispersion forces both cohesively and across the solid-liquid Interface, Yq should be the actual surface tension of the solid-vapor interface. This Is essentially what Is observed Yc for PE Is approximately that measured for a high molecular weight liquid hydrocarbon. On the other hand, if water Is considered to Interact with a hydrocarbon only through dispersion... 

Water possesses the largest cohesive forces and consequently, the largest surface tension of the commonly encountered liquids. The molecules which form biological membranes have unique properties which allow them to reduce the surface tension at a cell s aqueous interface, and thereby increase the surface area of the cell and its internal organs, e.g., the Golgi apparatus. 

N.L. Gershfeld, Cohesive forces in monomolecular films at an air-water interface, Advan. Chem. Ser. 84, 115 (1958). 

If the adhesive forces between the liquid and solid molecules are less than the cohesive forces between the liquid molecules, the solid surface will tend to push the liquid away. For example, if mercury replaces water in the glass tube in Figure 2.19, the mercury-to-air interface would curve upward and the mercury in the glass tube would be lower than that outside h < 0). 

The most effective emulsion and foam stabilizers are aerosol systems containing fluorocarbon propellants as surfactants. These are believed to form an oriented polymolecular structure at the propellant-water interface for optimum stability Sanders has found [90] that the surfactants must have a low solubility in both phases and have the ability to remain in the interfacial region. Hydrocarbon and fluorocarbon chains are not freely miscible and this perhaps explains the unusual behaviour of the surfactants in these systems. Addition of long-chain alcohols or acids enhance stability of the fluorocarbon emulsions and a hypothetical structure of the interfacial region has been proposed (Fig. 8.16). Davis et al. [91] have investigated the stability of fluorocarbon emulsions intended as artificial blood substitutes. Perfluorocarbon oils tended to produce unstable emulsions while oil phases such as perfluorotributylamine or per-fluorotetrahydrofuran formed more stable systems. These authors also refer to the possibility that as fluorocarbon-hydrocarbon mixtures have positive excess free energies, cohesive and adhesive forces between surfactant and oil phase will result. 

The molecules of a liquid are held together by attractive forces. The sum of aU attractive forces on any molecule present in the bulk of a liquid averages zero. The net force (also known as cohesion force) on a surface molecule is a non-zero quantity in the direction towards the bulk (Figure 2.1). This is the force that must be counteracted to increase the surface area the energy consumed by this process is called surface energy. The unbalanced forces on the interface cause it to contract to a minimum surface area value. Therefore, water droplets are spherical because a sphere has the minimum surface area of all shapes for a given volume. Surface tension and surface free energy of a liquid are equal while the same is not true for a solid surface. 

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