Spreading and Entering Coefficients

Oil droplet size is thought to be quite important to the effectiveness of crude oils at destabifizing foams, with smaller droplets being the more effective [105, 116-118]. A number of microvisual and coreflood studies suggest that 

The thin liquid films bounded by gas, on one side, and by oil, on the other side, denoted air-water-oil (A/W/O) or oil-water-air (O/W/A) are referred to as pseudoemulsion films [58]. 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 [58, 70, 107] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [65]. This is done based on disjoining pressures in the films, which may be measured experimentally [105], or calculated from electrostatic and dispersion forces [107]. The pseudoemulsion model has been applied to both bulk foams and foams flowing in porous media. 

In summary, many foams are completely unstable in the presence of oil, many others are moderately most unstable in the presence of oil and some foams are 

Isenberg, C. (1978) The Science of Soap Films and Soap Bubbles, Tieto, Clevedon. 

(1980) Introduction to Colloid and Surface Chemistry, 3rd edn, Butterworths, London. 

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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 ... 

Since information about the instability of asymmetric films provided from the spreading and enter coefficients is incomplete, it has been proposed [541,542] to use generalised spreading coefficients 5s on the basis of Eqs. (3.157-3.160). By definition the generalised spreading coefficient is [541]... 

Comparison of the spreading and entering coefficients with the stability of non-symmetric films has been considered by Lobo and Wasan [52]. The results are presented in Table 9.4. 

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. 

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 antifoaming action can be rationalised [28] in terms of the balance between the entering coefficient E and the Harkins [29] spreading coefficient S, which are... 

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. 

Entering Coefficient. A measure of the tendency for an insoluble agent to penetrate, or enter , an interface (usually gas—liquid or liquid— hquid). Entering is thermodynamically favored if the entering coefficient is greater than zero. When equilibria at the interfaces are not achieved instantaneously, reference is made to the initial and final (equihbiium) entering coefficient. See also Spreading Coefficient. 

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 ... 

For practical purposes, if the contact angle is greater than 90° the liquid is said not to wet the solid (if the liquid is water one speaks of a hydrophobic surface) in such a case drops of liquids tend to move about easily and not to enter capillary pores. If 8 = 0, (ideal perfect wettability) Eq. (A.4.3) no longer holds and a spreading coefficient, Sls(V). reflects the imbalance of surface free energies. 

Pollutants emitted by various sources entered an air parcel moving with the wind in the model proposed by Eschenroeder and Martinez. Finite-difference solutions to the species-mass-balance equations described the pollutant chemical kinetics and the upward spread through a series of vertical cells. The initial chemical mechanism consisted of 7 species participating in 13 reactions based on sm< -chamber observations. Atmospheric dispersion data from the literature were introduced to provide vertical-diffusion coefficients. Initial validity tests were conducted for a static air mass over central Los Angeles on October 23, 1968, and during an episode late in 1%8 while a special mobile laboratory was set up by Scott Research Laboratories. Curves were plotted to illustrate sensitivity to rate and emission values, and the feasibility of this prediction technique was demonstrated. Some problems of the future were ultimately identified by this work, and the method developed has been applied to several environmental impact studies (see, for example, Wayne et al. ). 

Ross and McBain [30] suggested that, for efficient defoaming, the oil drop must enter the air/water interface and spread to form a duplex film at both sides of the original film. This leads to a displacement of the original film, leaving an oil film which is unstable and can easily break. Ross [28] used the spreading coefficient [Eq. 

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. 

For a successful defoaming process AG of both steps must be <0 which is the case if the entering and the spreading coefficient (defined by the surface) are positive (>0). 

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... 

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