Spreading Behavior of Oils

If the oil spreads over the substrate, then the surface tension of the substrate liquid will necessarily decline. This observation will, however, not of itself distinguish between the two types of spreading behavior that may occur. The simplest involves duplex film formation where the oil completely spreads to form a thick oil film with properties of oil in bulk. The measured surface tension of the film will then equal the sum of the oil-air and substrate-oil surface tensions. Addition of further drops of oil simply produces an iuCTease in thickness of this film. The thickness of the oil film (and even its presence) may be determined by ellipsometry (see, e.g., references [53,56]). 

The theoretical basis of these generalizations together with actual experimental observations are reviewed in detail in Chapter 3. 

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FIGURE 3.11 Schematic illustration of possible equilibrium wetting, and therefore spreading, behaviors of oil that emerges into air-water surface. 

As we will show in Chapter 4, the entry and spreading behavior of antifoam oils is central to their mode of action. In particular, such oils must enter the air-foaming liquid surface if they are to function. It has been shown that it is not necessary that they spread over that surface [15]. However, it has also been shown [34-36] that the presence of spread films over the air-foaming surface influences the stability of the relevant pseudoemulsion films, the emergence into that surface, and therefore the effectiveness of an antifoam. The relevance of the entry and spreading behavior of oils to antifoam action therefore justifies the detailed review given here. 

Since most eommercially effective antifoams are oil based, a chapter is devoted to the entry and spreading behavior of oils and the role of thin film forces in determining that behavior. The book reviews the mode of action of antifoams, including theories of antifoam mechanisms and the role of bridging foam films by particles and oil drops. It also addresses issues related to the effect of antifoam concentration on foam formation by air entrainment and the process of deactivation of mixed oil-particle antifoams during dispersal and foam generation. 

The direct demonstration of a surfactant film in the airways is relatively recent (2-4), although a surface-active film had been inferred from physiological (5) and electron microscopic studies (6) many years before. The surface tension in large airways has been measured directly with a bronchoscope from the spreading behavior of oil droplets placed onto the tracheal walls or bronchi of anesthetized sheep and horses (7,8). A surface tension of approximately 32 mN/m has been recorded at the mucus-air interface in these animals. This relatively low surface tension suggests the presence of a surface film in large airways, because proteins, surface polymers of blood cells, polysaccharides, and other biopolymers, all have... 

It has been known for many years " °° ° that some correlation exists between the spreading behavior of the oils and their antifoam activity. The valne of the spreading coefficient ... 

The classical spreading-entering theories do not predict the behavior of oil drops, because they do not take into account the role of the pseudoemulsion film as an intermediate step (Figure 22b) between the oil drop (a) and oil lens (c) or spread oil layer (d) configurations. If this film is stable, it provides an energy barrier that prevents the coalescence of bubbles. If this film is unstable that is, the barrier is small, oil drops can enter and spread on the bubble surfaces, and the oil can act as defoamer. 

Many feel that emulsification is the second most important behavioral characteristic of oil after evaporation. Emulsification has a very great effect on the behavior of oil spills at sea. As a result of emulsification, evaporation of oil spills slows by orders of magnitude, spreading slows by similar rates, and the oil rides lower in the water column, showing different drag with respect to the wind. Emulsification also significantly affects other aspects of a spill. Spill countermeasures are quite different for emulsions as they are hard to recover mechanically, to treat, or to bum. 

SPREADING BEHAVIOR OF TYPICAL ANTIFOAM OILS ON AQUEOUS SURFACES... 

Hydrocarbon oils often form the main ingredient of antifoams used for the foam control of aqueous systems. As we will describe in Chapter 4, the entry and spreading behavior of such oils is central to both their mode of action and their effectiveness. It is therefore appropriate to review that behavior here. 

The spreading behavior of PDMS oils on the surfaces of surfactant solutions is of course the property of relevance for antifoam behavior. It is only in the past 20 years or so that significant systanatic observations of that behavior have been made. Table 3.3 sununarizes most of those observations. Similar observations with fully formulated PDMS-based antifoams (see, e.g., reference [12]) are not included but are considered as appropriate in Chapter 4. 

Dong [73] report a more detailed study of the spreading behavior of PDMS oils on non-ionic alcohol ethoxylate snrfactant solutions. Both the molecular weight of the PDMS (as indicated by the viscosity) and the molecnlar strncture of the surfactants were varied. The stndy was confined to micellar solutions. A summary of the spreading pressures and coefficients is given in Table 3.3. 

Finally, we should note that antifoam oils such as PDMSs and polyperfluoralkyl-siloxanes are used for controlling the foam of non-aqueous liquids. There appears to be a relative absence of studies of the spreading behavior of such oils on the relevant non-aqueous substrates equivalent to those concerning aqueous surfactant solutions. 

The possibility that the spreading coefficient of PDMS oils is modified by the addition of hydrophobic particles had been explored by Povich [209]. Here the initial spreading coefficient for PDMS was shown to slightly decrease upon the addition of hydrophobed silica (despite the effectiveness of this silica in promoting the antifoam behavior of the oil). Povich [209] attributed this to adsorption of oil-soluble surface-active impurities on the silica. Hydrophobed silica has also been shown to be without significant effect on the spreading behavior of a liquid paraffin [43,71]. 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

Wettabihty is defined as the tendency of one fluid to spread on or adhere to a soHd surface (rock) in the presence of other immiscible fluids (5). As many as 50% of all sandstone reservoirs and 80% of all carbonate reservoirs are oil-wet (10). Strongly water-wet reservoirs are quite rare (11). Rock wettabihty can affect fluid injection rates, flow patterns of fluids within the reservoir, and oil displacement efficiency (11). Rock wettabihty can strongly affect its relative permeabihty to water and oil (5,12). When rock is water-wet, water occupies most of the small flow channels and is in contact with most of the rock surfaces as a film. Cmde oil does the same in oil-wet rock. Alteration of rock wettabihty by adsorption of polar materials, such as surfactants and corrosion inhibitors, or by the deposition of polar cmde oil components (13), can strongly alter the behavior of the rock (12). 

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... 

In summary, the results of our thin film drainage study as well as our investigation of oil spreading mechanisms and frequency dependence of dynamic interfacial tension all suggest that the C 2 0S system, which displays the m.ost unstable foam behavior in the presence of oil, should not perform as effectively as the Ci6A0S system in oil displacement experiments in porous media. 

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