Tension at the oil-water interface

On a microscopic scale (the inset represents about 1 - 2mm ), even in parts of the reservoir which have been swept by water, some oil remains as residual oil. The surface tension at the oil-water interface is so high that as the water attempts to displace the oil out of the pore space through the small capillaries, the continuous phase of oil breaks up, leaving small droplets of oil (snapped off, or capillary trapped oil) in the pore space. Typical residual oil saturation (S ) is in the range 10-40 % of the pore space, and is higher in tighter sands, where the capillaries are smaller. 

P. Dunlop and co-workers, "Aqueous Surfactant Solutions Which Exhibit Ultra-Low Tensions at the Oil-Water Interface," Paper presented at the... 

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.

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. 

P7-28i An understanding of bactoria transport in porous media is vital to the efficient operation of the watar 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 interfacia] 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 mesentroides, 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. 

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... 

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. 

P Dunlap, PM Wilson, C Brandner. Aqueous surfactant solutions which exhibit ultralow tensions at the oil-water interface. J Colloid Int Sci 60 473—479, 1977. 

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... 

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