Spreading coefficient foams

The Spreading process is governed by the spreading coefficient S defined as in equation 4 (30) where c is the surface tension of the foaming medium, C the surface tension of the defoamer, and C. the interfacial tension between them. 

Spreading Coefficient. The spreading coefficient is defined as the difference of the surface tension of the foaming medium cry, the surface tension of the defoamer aj, and the interfacial tension of both materials a /. 

In the study of the mechanism of heterogeneous defoaming along with the spreading coefficients the so-called enter coefficient (destruction coefficient) is used to estimate the instability of aqueous foam films... 

Defoaming ability of antifoams with different spreading coefficients in OP-10 and sulphonol foaming systems... 

Finally we should comment that it is necessary to employ in the calculation of the spreading coefficient (which is often used as a stability criterion) accurately measured values of the various tensions operative in the pseudoemulsion film to determine whether oil is spreading or nonspreading in the three phase foam structure. 

Once the oil is present at the interface, and it bridges the lamellar liquid and the gas phase, it would be predicted (from thermodynamics) to spread spontaneously over a foam if its spreading coefficient , 5, is positive (55). For an oil—foam system S is given, for unit surface areas, by... 

Spreading and Entering Coefficients. From thermodynamics, a defoamer would be predicted to spread as a lens over a foam if its spreading coefficient is positive, (17, 45). The spreading coefficient, S, for an oil—foam system is given by... 

The mechanisms for foam sensitivity to oils can also be compared to the results from core-flood experiments in which foams were made to flow through porous rock in the presence of residual oil. Holt and Kristiansen (26, 27, 56) studied foams flowing in cores under North Sea reservoir conditions and found that the presence of residual oil could reduce the effectiveness of flowing foams. They compared their results with the spreading and entering coefficients and found foam sensitivity to be correlated with the (oil) spreading coefficient. 

If an antifoam is to be effective it must be able to enter the film that makes up the foam bubbles and spread across the film surface. Equations 1 and 2 define the entering coefficient, E, and the spreading coefficient, 5, of an antifoam with respect to a particular foaming medium. 

The entering and spreading processes are governed by the entering coefifi-cient and the spreading coefficient S defined in equations (4) and (5), respectively (7), where af is the surface tension of the foaming medium, the surface tension... 

In the case of the macroemulsified oil system, the important role of the so-called pseudo-emulsion film (formed between the air/water interface and an approaching oil droplet) on the stability of the aqueous foaming system was emphasized.(Figure 2.7). Clearly, the entering and spreading coefficients are thermodynamic properties which determine whether the particular configuration of the oil droplet is energetically favourable and they cannot predict the fate of the oil droplet under the dynamic conditions which exist within... 

As briefly discussed earlier, a widely accepted mechanism for antifoaming action is that first the oil drop enters the air/water interface, and in a secondary step, begins to spread over the foam film, so causing rupture. An entering coefficient ( ) and a spreading coefficient (S) have been defined in terms of the change in free energy when the oil droplet enters the interface or spreads at the surface. These are defined as follows ... 

If fi > 0 and 5 < 0, then an oil drop will form a lens, which may be in equilibrium with the original air-foaming liquid surface (i.e., partial wetting) or that surface after incorporation of material from the oil drop (i.e., pseudo-partial wetting). However, formation of oil lenses on the equilibrium liquid surface does not preclude the possibility that the initial spreading coefficient S is positive. If 5 > 0 and S < 0, then pseudo-partial wetting will occur and the oil drop may, for example, spread to form a duplex film, which is, however, unstable. That film will disproportionate to form oil lenses in equilibrium with oil-contaminated air-water surfaces The spreading of a drop of benzene on distilled water to form lenses in equilibrium with a benzene monolayer exemplifies this type of behavior. 

The initial spreading coefficient is defined by Equation 3.5 and is a measure of the tendency of a duplex film to form on the surface of the foaming liquid free of any contamination by the oil. We can therefore write... 

Robinson and Woods [9] produced perhaps one of the earliest experimental studies of antifoam mechanism. The study concerned the effect of various undissolved oils on the foam behavior of both aqueous and non-aqueous solutions of surfactant The oils included alkyl phosphates, alcohols (including diols), fatty add esters, and PDMS. The solutions were of aerosol OT (AOT or sodium diethylhexyl sulfosucci-nate) in either ethylene glycol or triethanolamine and sodium alkylbenzene sulfonate in water. Many quoted entry and spreading coefficients, however, violate Equations 3.11 and 3.12, which implies that these coefficients were non-equilibrium (i.e., initial) values where the relevant liquids are not mutually saturated. Robinson and Woods [9] observed that for these systems, wherever < 0, no antifoam effect is found. This then represents some evidence that a positive value of the initial entry coefficient is necessary for antifoam action. 

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