Foam entering coefficient

Foam stability in the presence of oil can be described from thermodynamics in terms of the spreading and entering coefficients S and E respectively. These coefficients are defined as follows ... 

These agents may operate via a number of mechanisms, but the most common ones appear to he those of entry and/or spreading. The defoamer must first of all he insoluble in the foaming liquid for these mechanisms to function. Second, the surface tension of the defoamer must be as low as possible. The interfacial tension between defoamer and foamer should be low. but not so low that emulsification of the defoamer may occur. Third, the defoamer should be dispersible in the foaming liquid. It was first shown in I fM8 that thermodynamically the entry of the defuamcr droplet into a bubble surface occurs when the entering coefficient has a positive value. The physics of bubbles is described in entry on Foam. 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

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... 

If an insoluble agent is dispersed in the liquid interior of foam lamellae, then, from thermodynamics, it can penetrate or enter the gas—liquid interface if the entering coefficient, E, is positive (originally defined (55) as a rupture coefficient, R). E is defined, for unit surface areas, as... 

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 spreading and entering coefficients have correlated with foam sensitivity to oil in a number, but not in all, of the cases. As already noted, the degrees of foam sensitivity to oil (Figure 2) observed in microvisual experiments have been compared to the thermodynamic predictions based on S and E in Figure 8 (37, 40, 47). In these comparisons, the predictions were not always borne out Depending on the oil studied, on the order of one-half of the surfactant solutions predicted to produce foams of type C actually produced foams of type B. The predictions of which surfactants would produce foams of type A were much better for the heavier oils than for the lighter oils. Even quantitative measurements of lamella rupture frequency in the microvisual experiments showed that satisfactory correlations with 5 or were not obtained (37, 40, 47). 

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. 

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 ... 

Some authors use a coefficient to describe the spreading of aqueous solution over oil, for example, Sw = 7°0 — 7°F — 7OF (22, 27). The process described is equivalent to the reverse of entering, and E = —Sw. Thus the region marked Type A in Figure 8 could be considered to represent the conditions under which foaming solution will spread over oil, as illustrated in Figure 6a. 

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... 

It is generally accepted that if oils, in the form of dispersed drops, have the potential to act as antifoams, they must be able to enter (or emerge into or adhere to ) the surfaces of foaming liquids. The classic entry coefficients, and E , should all therefore be... 

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