Interfacial tension at the oil-water

The scope of the present review is to emphasize that thermodynamics can explain the above experimental observations. The next section (Section 2), which is based on ref. [10], will be concerned with the effects of HLB (denoted in what follows h) on the interfacial tension and on the stability of macroemulsions, the goal being to explain the observations of Boyd et al. [5] and of Berger et al. [4]. Section 3, which is based on ref. [11], will examine the effect of temperature on the interfacial tension at the oil-water interface by assuming that no microemulsion or emulsion is formed, as well as its effect on the stability of emulsions. Shinoda and Saito s observations regarding the equality of the two inversion temperatures will be thus explained. Finally, the Bancroft rule [8,9], and some of the violations of this rule, will be examined in the spirit of ref. [12],... 

Figure 13. Interfacial tension at the oil -water interface in a bicontinuous inicroemulsion system (filled squares) as a function of the volume fraction of oil in the microemulsion. In all cases, the volume fraction of surfactant is 0.01. The system consists of SDS, 1-pentanol, cyclohexane, water, and 0.3 M NaCl. Also shown are the interfacial tensions at the flat surface between the O/W droplet microemulsion phase and the excess oil phase (filled circles) and between the W/O droplet microemulsion phase and excess water phase (open circles) in two-phase systems.

P7-28a An understanding of bacteria transport in porous media is vital to the efficient operation of the water flooding of petroleum reservoirs. Bacteria can have both beneficial and harmful effects on the reservoir. In enhanced microbial oil recovery, EMOR, bacteria are injected to secrete surfactants to reduce the interfacial tension at the oil-water interface so that the oil will flow out more easily. However, under some circumstances the bacteria can be harmful, by plugging the pore space and thereby block the flow of water and oil. One bacteria that has been studied, Leuconostoc mesenteroides, has the unusual behavior that when it is injected into a porous medium and fed sucrose, it greatly... 

Figure 1.32. Non-equilibrium interfacial tension at the oil-water interface system water + hexane, containing palmitic acid, of which the concentration c is indicated. The drawn curves relate to a model interpretation involving diffusion. (Redrawn from J. van Hunsel, G. Bleys and P. Joos, J. Colloid Interface Set 114 (1986) 432.)...

Adamson (51) proposed a model for W/0 microemulsion formation in terms of a balance between Laplace pressure associated with the interfacial tension at the oil/water interface and the Donnan Osmotic pressure due to the total higher ionic concentration in the interior of aqueous droplets in oil phase. The microemulsion phase can exist in equilibrium with an essentially non-colloidal aqueous second phase provided there is an added electrolyte distributed between droplet s aqueous interior and the external aqueous medium. Both aqueous media contain some alcohol and the total ionic concentration inside the aqueous droplet exceeds that in the external aqueous phase. This model was further modified (52) for W/0 microemulsions to allow for the diffuse double layer in the interior of aqueous droplets. Levine and Robinson (52) proposed a relation governing the equilibrium of the droplet for 1-1 electrolyte, which was based on a balance between the surface tension of the film at the boundary in its charged state and the Maxwell electrostatic stress associated with the electric field in the internal diffuse double layer. 

The apparent pKa for ionizing groups on stable surfaces is estimated by many techniques. In early studies, Peters (11) and Danielli (12) measured the changes in interfacial tension at the oil-water interface as a function of pH. They found that the apparent pKa for carboxylic acids adsorbed to a surface was higher than that for carboxlic acids dissolved in solution. Later Schmidt-Nielson (13) and Mattson and Volpenhein (14) titrated oleate soaps and found apparent pKa values of 7.8 and 8.0. 

In this paper the use of electroacoustic techniques involving the application of a sonic field and the detection of an electric field, for monitoring coalescence of water droplets in non-polar media will be discussed. This technique was used to evaluate the rate and extent of dewatering in oil continuous emulsions when surface active chemicals were added. The results showed that a combination of an oil soluble demulsifier and water soluble surfactant was substantially more effective in causing droplet coalesence than the individual components. An explanation for these findings were based on studies of time-dependent interfacial tensions at the oil/water interface and electrokinetic properties. The results indicated that a direct relationship exists between the adsorption behavior at the oil/water interface (apparent rate of spreading) and emulsion stability. 

Marasperse lignosulfonates can produce stable emulsions. These emulsions are resistant to pH and eletrolyte contents. It has been observed that sodium hydroxide enhances the surface activity of aqueous lignosulfonate solutions, which in turn lowers the interfacial tension at the oil/water interface, thereby rendering the emulsions more stable. The emulsions stabilized by lignosulfonates are also resistant to mechanical agitations and to large temperature variations. 

Most single chain surfactants do not sufficiently reduce interfacial tension at the oil/water interface to form MEs, furthermore they may lack the right molecular attributes (i.e., HLB) to act as cosolvents. To overcome such a hurdle, cosur-factant/cosolvent molecules are introduced to sufficiently lower the oil/water interfacial tension, fluidize the rigid hydrocarbon region of the interfacial film, and induce ideal curvature of the interfacial film. Typically, molecules with small to medium hydrocarbon chains (C3-C8) with a polar head group (hydroxyl, amine group, sulfoxide, or -oxides) that can effectively diffuse between the immiscible phases and the interfacial film are used [11]. 

In micro-emulsion polymerization, an initiator, typically water-soluble, is added to the aqueous phase of a thermodynamically stable microemulsion containing swollen micelles. The polymerization starts from this thermodynamically stable, spontaneously formed state and relies on high quantities of surfactant systems, which possess an interfacial tension at the oil/water interface close to zero. Furthermore, the particles are completely covered with surfactant because of the utilization of a high amount of surfactant. Initially, polymer chains are formed only in... 

It is a well-known adage that oil and water do not mix. However, it will be shown that, by changing the interfacial forces at the oil-water boundary, one can indeed disperse oil in water (or vice versa). At the oil-water interface there exists interfacial tension (IFT), which can be measured by some of the methods mentioned earlier (e.g., by drop weight, pendant drop, or Wilhelmy plate). 

All molecules that, when dissolved in water, reduce surface tension are called surface-active substances (e.g., soaps, surfactants, detergents). This means that such substances adsorb at the surface and reduce surface tension. The same will happen if a surface-active substance is added to a system of oil-water. The interfacial tension of the oil-water interface will be reduced accordingly. Inorganic salts, on the other hand, increase the surface tension of water. 

PVA can lower the surface tension of water, reduce interfacial tension at an oil/water interface and enhance tear film stability. These together with ease of sterilization, compatibility with a range of ophthalmic dmgs and an apparent lack of epithelial toxicity have led to the widespread use of PVA as a drag delivery vehicle and a component of artificial tear preparations. 

In Fig. 9c, the effects of different surface tension values on the equilibrium are examined. By decreasing the interfacial tension, the Laplace term becomes less significant than the contribution given by the entropy of mixing, and therefore ripening is decreased and stability is enhanced. Theoretically, in a system with zero surface tension at the oil/water interface, the total monomer chemical potential is given solely by the entropic terms, and it is always stable. 

Properties of Component Phases The composition and physicochemical properties of both the oil and aqueous phases influence the size of the droplets produced during homogenization (52). Variations in the type of oil or aqueous phase will alter the viscosity ratio, ri ,/ri(-, which determines the minimum size that can be produced under steady-state conditions. The interfacial tension of the oil-water interface depends on the chemical characteristics of the lipid phase, e.g., molecular structure or presence of surface-active impurities, such as free fatty acids, monoacylglycerols, or diacylglycerols. These surface-active hpid components tend to accumulate at the oil-water interface and lower the interfacial tension, thus lowering the amount of energy required to disrupt a droplet. 

Emulsifiers are a single chemical substance, or mixture of substances, that lower the tension at the oil-water interface (interfacial tension) and have the capacity for promoting emulsion formation and short-term stabilisation. 

From the foregoing considerations it will be apparent that the lower the surface tension at the oil/water interface, the smaller will be the size of the drops of oil formed in an emulsion. Donnan s drop-number method is a simple and convenient means of determining or comparing oil/detergent solution interfacial surface tension. The apparatus is shown in Fig. 9.12, in which a pipette A of about 5 ml capacity is provided with a capillary tube B... 

After water flooding, residual oil is believed to be in the form of discontinuous oil ganglia trapped in the pores of rocks in the reservoir. The two major forces acting on an oil ganglion are viscous forces and capillary forces, the ratio of which is represented by the capillary number. At the end of the secondary oil recovery stage, the capillary number is around 10 . To recover additional oil, the capillary number has to be increased to around 10" —10, which can be achieved by decreasing the interfacial tension at the oil/brine interface. Surfactants are used for this purpose. 

Micro-emulsion is another variant of emulsion polymerisation. Such emulsions are thermodynamically stable systems including swollen monomer micelles dispersed in a continuous phase. In general, they require fairly large concentrations of surfactants to be produced compared with the other dispersed polymerisation systems. Hence, the interfacial tension of the oil/water is generally close to zero. Polymers with ultra-high molecular weight, i.e. above 10 g/mol, can be obtained, as can copolymers with a very well-defined, homogenous composition. Whereas polymerisation can take 24-48 h in the normal emulsion process, it proceeds at a fast rate in micro-emulsion, as total conversion can be obtained in less than 30 min. Polymer particles of very small size (diameter < 100 nm) and narrow distribution can be obtained by this process. 

Investigations of the effects of oil-soluble surfactants on the emulsification of paraffins in aqueous surfactant solutions led to the proposal that the formation of interfacial complexes at the oil-water interface could increase the ease with which emulsions could be formed and, possibly, explain the enhanced stability often found in such systems (Figure 9.9). By definition, an interfacial complex is an association of two or more amphiphilic molecules at an interface in a relationship that will not exist in either of the bulk phases. Each bulk phase must contain at least one component of the complex, although the presence of both in any one phase is not ruled out. The complex can be distinguished from such species as mixed micelles by the fact that micelles (and therefore mixed micelles) are not adsorbed at interfaces. According to the Le Chatelier principle, the formation of an interfacial complex will increase the Gibbs interfacial excess F/ [Eq. (9.2)] for each individual solute involved, and consequently, the interfacial tension of the system will decrease more rapidly with increasing concentration of either component. 

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

At a given NaCI concentration, an increase in temperature resulted in an increase in interfacial tension. In contrast, for a narrow range of CaCI concentrations, interfacial tensions decreased with increasing temperatures. Changes of the amphiphile at the oil/water interface accounted for some of the experimental observations. Since the extent of oil desaturation is dependent on interfacial tension, the tension data could be used to assess the ability of surfactants to reduce oil saturations in the reservoir for application of surfactants and foams to thermal recovery processes. 

For results where comparisons could be made, the interfacial tension behavior was practically independent of the type of heavy oil used. Interfacial tensions strongly depended on the surfactant type, temperature, and NaCI and CaCI2 concentrations. Changes in the structure of the amphiphile at the oil/water interface is affected by these variables and accounted for some of the experimental observations. 

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