Interfacial tension ultra-low

T. Sottmann and R. Strey Shape Similarities of Ultra-Low Interfacial Tension Curves in Ternary Microemulsion Systems of the Water-AUcane-CiEj Type. Ber. Bunsenges Phys. Chem. 100, 237 (1996). 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

Finally, the phase diagram of Figure 11c illustrates the formation of three liquid phases in which the middle phase is a microemulsion. Since such systems typically have ultra-low interfacial tensions, they can form fine dispersions (droplet radius less than pore radius) with only a minimum of shear (68). 

Wang, D.-M., Cheng, J.C., Yang, Z.-Y, Li, Q., Wu, W.-X., Yu, H.-Y., 2001a. Successful field test of the first ultra-low interfacial tension foam flood. Paper SPE 72147 presented the SPE Asia Pacific Improved Oil Recovery Conference, Kuala Lumpur, 6-9 October. 

The driving force for microemulsion formation is the ultra-low interfacial tension which is normally obtained by using two surfactants one which is predominantly... 

The extensive research on microemulsions was prompted by two oil crises in 1973 and 1979, respectively. To optimise oil recovery, the oil reservoirs were flooded with a water-surfactant mixture. Oil entrapped in the rock pores can thus be removed easily as a microemulsion with an ultra-low interfacial tension is formed in the pores (see Section 10.2 in Chapter 10). Obviously, this method of tertiary oil recovery requires some understanding of the phase behaviour and interfacial tensions of mixtures of water/salt, crude oil and surfactant [4]. These in-depth studies were carried out in the 1970s and 1980s, yielding very precise insights into the phase behaviour of microemulsions stabilised by non-ionic [5, 6] and ionic surfactants [7-9] and mixtures thereof [10]. The influence of additives, like hydro- and lyotropic salts [11], short- and medium-chain alcohols (co-surfactant) [12] on both non-ionic [13] and ionic microemulsions [14] was also studied in detail. The most striking and relevant property of micro emulsions in technical applications is the low or even ultra-low interfacial tension between the water excess phase and the oil excess phase in the presence of a microemulsion phase. The dependence of the interfacial tension on salt [15], the alcohol concentration [16] and temperature [17] as well as its interrelation with the phase behaviour [18, 19] can be regarded as well understood. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

As the latter is comparatively simple to use it can be regarded as the most suitable method to measure low and ultra-low interfacial tensions. In the following the general features of interfacial tensions in microemulsion systems are presented. The dramatic decrease of the water/oil interfacial tension upon the addition of surfactant, the correlation of interfacial tension and phase behaviour, the variation of the water/oil interfacial tension with the respective tuning parameter and the scaling of the interfacial tension will be discussed in detail. All data presented have been determined using the spinning drop technique [17]. 

From the above, it is clear that a pre-requisite of low water/oil interfacial tensions is the complete saturation of the water-rich and oil-rich phases as well as the water/oil interface by surfactant molecules. Of course, this pre-requisite is fulfilled if one of the phases considered is a microemulsion. Furthermore, since the pioneering work of Lang and Widom [81] it is known that if a system is driven through phase inversion the interfacial tensions may become ultra-low. However, about 20 years ago, a number of experimental investigations were devoted to clarifying the origin of the ultra-low interfacial tensions [15, 17, 39, 71, 81-85]. In order to understand this correlation between phase behaviour and interfacial... 

Whereas Winsor III systems exhibit ultra-low interfacial tensions between the three phases and also very high solubilisation capacity, Winsor I systems have higher interfacial tensions and much lower solubilising power. At the transition between the two types of microemulsion systems, an intermediate behaviour can be found which is called supersolubilisation [47,70]. The uptake of oils into surfactant aggregates is usually enhanced by one to two orders of magnitude compared to effective micellar systems, but interfacial tension reduction is still moderate. The transition point can be adjusted by varying the salinity or organic components. 

The optimum formulation is a surfactant system with which maximum oil recovery can be achieved. For that purpose, the interfacial tension has to be as low as possible and the oil solubilisation in the microemulsion as large as possible [15,115,121,123,127,140,142-144]. In general, formulation is a concept that tunes the properties of a water-oil-surfactant system such that it can be used for the certain application (see Chapter 3). Extensive studies on the optimum formulation for EOR and various other applications have shown that many variables have to be considered to achieve an ultra-low interfacial tension at relatively low surfactant concentration, or the occurrence of a single-phase bicontinuous microemulsion at high surfactant concentration [15, 143, 144]. 

Salinity Salinity plays at least two important roles, namely it maintains the integrity of the reservoir and it balances the physicochemical environment so that surfactant formulation stays close to optimal. Thus, ultra-low interfacial tension and oil solubilisation are very sensitive to salinity. Mixing of the surfactant slug with connate water may alter the surfactant formulation mainly due to dilution and to the incorporation of new electrolytes to the formula. Adsorption and desorption of electrolytes, particularly divalent cations, onto or from solid materials such as clay, will also change the salinity of the aqueous phases to some extent and may cause surfactant precipitation, which is however not always an adverse effect [151]. In order to attenuate the undesirable salinity effects on formulation, surfactants able to tolerate salinity changes [109], high salinity [150] and the presence of divalent ions [112] maybe used. 

The reason for the optimal degreasing performance of the microemulsion in the three-phase region is the ultra-low interfacial tension between water and oil aab- Figure 10.11 shows the variation of water plus fat mass fraction a = 0.15 was chosen, which is the least fat mass fraction required for spinning drop measurements. 

In Fig. 10.12(a), the phase behaviour in the short float is shown. The three-phase state of the respective systems is located near the degreasing temperature T = 30°C. Efficient degreasing is a result of the ultra-low interfacial tension between water and fat. Upon diluting the short float with pure water the salt mass fraction in the water phase is effectively reduced from e = 0.21 to e = 0.07. Sodium chloride belongs to the group of lyotropic salts. When the salt mass fraction is reduced the hydration of the surfactant head... 

Figure 10.12 Schematic of the variation of the phase behaviour during the degreasing process. In the short float the ultra-low interfacial tension between water and oil ensures efficient degreasing. Upon reducing the salt mass fraction the phase behaviour shifts to higher temperatures. At the degreasing temperature now an oil-in-water microemulsion coexists with an oil-excess phase. Shearing induces the formation of a stable macroemulsion that prevents the depositing of the fat on the skin and ensures the transport of the fat away from the skin. Note that only the Gibbs triangles correspond to the real experimental conditions. The T-y cuts are shown for clarity.

Efficient degreasing was found to be closely connected to the three-phase state and hence to the ultra-low interfacial tension between water and oil [170]. The so far unidentified mechanism of degreasing of animal skins could be understood and explained. Correlation of results obtained from phase behaviour measurements and degreasing experiments revealed that Eusapon OD shows the best degreasing performance and lead to the clarification of the four-step process of degreasing as shown in Fig. 10.13 [171]. The first step is the penetration of the surfactant into the skin. In a second step the natural fat is solubilised. A microemulsion phase coexists with a fat- and a water-excess phase and the interfacial tension between water and oil is ultra-low. On the surface of the skin dilution of the microemulsion with pure water, i.e. reduction of the salt concentration in the float, leads to the formation of a stable emulsion via shearing. The stable emulsion prevents the deposition of the fat on the skin and enables the transport of the natural fat away from the skin. 

Microemulsions form spontaneously and exhibit nano-disperse structures. In contrast to emulsions there is no additional energy input necessary for the production of a microemulsion. The formation is thermodynamically favoured due to the ultra-low interfacial tension between the oil and water domains. The microemulsified fuels are in principle thermodynamically stable for an unlimited period of time only the chemical stability of the single components could be a limiting factor. A further advantage of microemulsions in contrast to emulsions is the fact that the water content can be adjusted over a broad range. Therefore, the combustion process can be customised to specific needs. An important criterion for a microemulsion to be used as fuel is that the one-phase region extends over a wide temperature range (Fig. 11.4). Mixtures of ionic and non-ionic surfactants, which exhibit almost temperature-invariant phase behaviour by optimal composition, are suitable to meet these standards. 

Equation (515) is known as Vonnegut s equation and it is valid on the assumption that the drop is in equilibrium and its length is larger than four times its diameter (/ > 4r ). The spinning drop tensiometer method is widely used for measuring liquid-liquid interfacial tension, and is especially successful for examination of ultra-low interfacial tensions down to l(T6mNnr1. In addition, it can also be used to measure interfacial tensions of high viscosity liquids when precise temperature control is maintained. 

When the value of the interfacial tension is significantly less than 1 mN m 1, then we consider the measurement of ultra-low interfacial tension, which is common in liquid-liquid emulsification processes when effective surfactant solutions are used. The dynamic spinning drop tensiometer method is especially suitable for this purpose. Ultra-low interfacial tension measurement is important in the chemical industry because the cleaning of solid surfaces of dirt, grease, and oil the formulation of stable emulsions the recovery of petroleum, and other applications often rely on lowering the interfacial tension between immiscible liquids to ultra-low values by the use of surfactants. 

The appearance of ultra-low interfacial tension determines the use of such microemulsion systems for enhancing the degree of oil pool recovery. Microemulsion systems, sometimes also referred to as the micellar solutions, are pumped into the secondary satellite holes located at a certain distance from the production oil well. Water containing the required amount of electrolyte is pumped further into these satellite wells. While penetrating the oil pool, this microemulsion with substantial surfactant content, washes off the oil and forces it towards the production well [25,26]. 

Several micellar-polymer flooding models as applied to the EOR are discussed in [237]. It is noted that the co-solvent ordinarily used in this process considerably influences not only the microemulsion stabilisation, but also the removal of impurities in the pores of the medium. The idea of using an alkali in micellar-polymer flooding is discussed in [238] in detail. The alkali effect on the main oil components was studied aromatic hydrocarbons, saturated and unsaturated compounds, light and heavy resin compounds and asphaltenes. It is demonstrated that at pH 12 surfactants formed from resins allow to achieve an interfacial tension value close to zero. For asphaltenes, such results are achieved at pH 14. In the system alkali solution (concentration between 1300 to 9000 ppm)/crude oil at 1 1 volume ratio a zone of spontaneous emulsification appears, which is only possible at ultra-low interfacial tensions. 

Microemlusion system applications span many areas including EOR, soil and aquifer decontamination and remediation, wood treatment, foods, pharmaceuticals (drug delivery systems), cosmetics and pesticides [6,107,144-148]. Some of these are listed in Table 3.6. The widespread interest in microemulsions and use in these different industrial applications are based mainly on their high solubilization capacity for both hydrophilic and lipophilic compounds, their large interfacial areas, the ultra-low interfacial tensions achieved when they coexist with excess aqueous and oil phases and their long-term stability. 

K Shinoda, M Hanrin, H Kunieda, H Saito. Principles of attaining ultra-low interfacial tension The role of hy-drophile—lipophile balance of surfactant at oil/water interface. Colloids Surfaces 2 301—314, 1981. 

JC Noronha, DO Shah. Ultra—low interfacial tension, phase behavior and microstructure in oil/brine/surfactant/alcohol systems. AIChE Symp Ser, No. 212, 78 42—57, 1982. 

The importance of the properties of the interfacial region for spontaneous emulsification was first demonstrated by Gad (4), who observed that when a solution of lauric acid in oil was carefully placed on an aqueous alkaline solution, an emulsion spontaneously formed at the interface. The reason for this spontaneous emulsification is the formation of a mixed film of lauric acid and sodium laurate (produced by partial neutralization of the acid by alkali) which produces an ultra-low interfacial tension. 

Figure 4.1. Schematic representation of spontaneous emulsification (a) interfacial turbulence (b) diffusion and stranding (c) ultra-low interfacial tension...

With the use of specific surfactants, and with appropriate physico-chemical conditions, the interfacial tension of a crude oil/water interface can drop as low as lO"" mN/m (6). As will be discussed below, such ultra-low interfacial tensions are very important in technologies such as enhanced oil recovery processes for recovering crude oil from depleted oil wells. 

Enhanced oil recovery research in the 1970s led to the development of empirical correlations that numerically describe the conditions for attaining ultra-low interfacial tension and maximum oil mobilization. The correlation, the surfactant affinity difference (SAD), is a measure of the difference between the standard chemical potentials or the Gibbs free energy of surfactant in the oil and water phase, as follows ... 

Figure 11.11. Schematic representation of the role of ultra-low interfacial tension on the formation of the oil bank...

Microemulsions are stable emulsions of hydrocarbons and water in the presence of surfactants and co-surfactants. They are characterized by spontaneous formation, ultra-low interfacial tension, and thermodynamic stability. 

Chan, K.S. and Shah, D.O., The Molecular Mechanism for Achieving Ultra-low Interfacial Tension Minimum in a Petroleum Sulfonate/Oil/Brine System., J. Dispersion Sci. Technol., 1(1) 55-95 (1980). 

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