Surfactants, effects drop soluble

The analysis given for the surfactant effect on the thinning rate has shown that a flux from inside the emulsion drops is much less effective than the surfactant present in the homogeneous phase. It will be shown below, however, that almost all surfactants are usually soluble in both liquid phases of an emulsion so that obviously the distribution coefficient will be the parameter which controls the efficiency of a surfactant with respect to film thinning. 

The Gibbs-Marangoni effect would also explain Bancroft s rule when making an emulsion of two phases, the one in which the surfactant is most soluble will become the continuous phase. If the surfactant is in the droplets, a y-gradient as depicted in Figure 2.12b would never develop, and the drops would be prone to coalescence. Hence, surfactants with an HLB value >7 tend to produce O/W emulsions those with HLB < 7, W/0 emulsions. 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

For flow through porous media studies, the sandpacks used as porous media were flushed vertically with carbon dioxide for an hour to replace interstitial air. Distilled water was pumped and the pore volume (PV) of the porous medium was determined. By this procedure, the trapped gas bubbles in the porous media can be easily eliminated because carbon dioxide is soluble in water. For determining the absolute permeability of the porous medium, the water was pumped at various flow rates and the pressure drop across the sandpack as a function of flow rate was recorded. After the porous medium was characterized, the mixed surfactant solutions of known surface properties were injected. This was followed by air injection to determine the effect of chain length compatibility on fluid displacement efficiency, breakthrough time and air mobility in porous media. 

Thin liquid films in foam and emulsion systems are usually stabilised by soluble surfactants. During the formation of such films the flow-out process of liquid disturbs the surfactant equilibrium state in the bulk and film surfaces. The situation of drainage of a surfactant containing liquid film between two oil droplets is shown in Fig. 3.15. (after Ivanov Dimitrov 1988). Here j" and are the bulk fluxes in the drops and the film, respectively, j and j are the fluxes due to surface diffusion or spreading caused by the Marangoni effect, respectively. 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

For DOC, it can be seen that the results of Williams (1967), for example, show an extra 2.1 g m DOC in the microlayer. If the thickness of the water film obtained with the screen device is taken to be —200 pm, the surface excess of DOC can be calculated as 2.1 X 200 X 10 = 4.2 X 10" g m . A reasonable lower limit to take for the molecular weight of this extra organic material in the surface film is that of a relatively short-chain acid or alcohol with —14 carbon atoms, equivalent to —170 g mole carbon. Using this minimum value, the area per molecule in the ambient type of films sampled by Williams (1967) can be calculated as > — 170/4.2 X lO" X 6.02 X 10 3 = > 70 A. It can be seen from Fig. 1 that for all surface film types except gaseous films, such an area per molecule has no effect on the surface tension of seawater, as measured by the spreading drop method, or on the damping of capillary waves. Moreover, only relatively water-soluble surfactants remain in the gaseous state at film pressures of —10 N m" ... 

In 1987, Eley et al. (72) used the pendant-drop retraction method in the study of film compressibilities of crude oil/water interfaces for three crudes. These varied in asphaltene content. Libya (Brega) had 0.46 g/L, Kuwait 3.7 g/L, and Tia Juana (Venezuela) 5.94 g/L. They added a dispersant containing a nonionic oil-soluble surfactant, and observed increased film compressibilities. The concentration of effective dispersant correlated with the asphaltene con-... 

These results are complicated by the fact that most experiments with drops involve the presence of surfactants which serve to stabilize droplets and aid in their formation. However, surfactant dynamics and their solubility also vary with temperature. Since the effect of surfactant adsorption can be more dramatic than that of temperature, the surfactant dynamics can greatly modify the effect of temperature gradients. Indeed anomalous thermocapillary flow was... 

To sum up, the following mechanism is proposed to account for the observed effects in IFT and oil droplet flattening phenomenon. As shown in Figure 4, mixed micelles in equilibrium with surfactant monomers are formed by the water-soluble and oil-soluble species in the bulk aqueous solutions. During equilibration, the surfactant monomers transfer to the water/oil interface and then to the interior of the oil drop resulting in a reduction of IFT. 

The equilibrated and nonequilibrated oil/brine/surfactant systems differed in their oil displacement efficiency. The equilibrated oil rather than the equilibrated aqueous phase of the surfactant solution is responsible for the high oil displacement efficiency of dilute surfactant systems containing no alcohol. The oil soluble fraction of petroleum sulfonate is more effective in lowering the interfacial tension and in promoting the flattening of oil drops. Almost 94% oil recovery was achieved in sandpacks by a low concentration ( 0.1%) surfactant plus alcohol formulation when used in place of brine flooding. 

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