Oil/water interfacial tension

Templeton obtained data of the following type for the rate of displacement of water in a 30-/im capillary by oil (n-cetane) (the capillary having previously been wet by water). The capillary was 10 cm long, and the driving pressure was 45 cm of water. When the meniscus was 2 cm from the oil end of the capillary, the velocity of motion of the meniscus was 3.6 x 10 cm/sec, and when the meniscus was 8 cm from the oil end, its velocity was 1 x 10 cm/sec. Water wet the capillary, and the water-oil interfacial tension was 30 dyn/cm. Calculate the apparent viscosities of the oil and the water. Assuming that both come out to be 0.9 of the actual bulk viscosities, calculate the thickness of the stagnant annular film of liquid in the capillary. 

Gel emulsions were applied successfully for the first time in aldol additions of DHAP to phenylacetaldehyde and benzyloxyacetaldehyde as model aldehydes catalyzed by RAMA [24]. The first interesting observation was that the stability of RAMA in water-in-oil gel emulsions improved by 25-fold compared to that in dimethylformamide/water l/4v/v co-solvent mixture. The reported experimental data concluded that both the highest enzymatic activities and equilibrium yields were observed in water-in-oil gel emulsion systems with the lowest water-oil interfacial tension attained with the most hydrophobic oil component (i.e. tetradecane, hexadecane, and squalane). 

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]. 

Figure 1.13 Schematic representation of the water/oil interfacial tension = 0.50, the interfacial tension (Xat, decreases from 50 mN m 1 to values as low as 1 0-4 mN m 1. After having crossed the monomeric solubility 70 of the surfactant in the water- and oil-rich phase, 70, where the microemulsion phase (c) exist in form of a lens (right). (From Ref. [26], reprinted with permission of Elsevier.)...

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... 

Figure 1.14 Schematic phase prism (a) and interfacial tensions (b) as function of temperature for the system water-oil-non-ionic surfactant. The minimum of the water/oil interfacial tension crab at T is a consequence of the phase behaviour. Increasing the temperature the aqueous phases separates into the phases (a) and (c) at the critical endpoints cepp whereas the phases (b) and (c) merge into a single oil-rich phase at cepa. Thus, the interfacial tensions

Figure 1.15 Water/oil interfacial tension crab (plotted on log-scale) as function of the relevant tuning parameter, (a) Variation of crab with temperature T, exemplarily shown for the water-n-octane-C- oE4 system [17]. (b) Variation of crab with the composition of the amphiphilic film 8yi in the quaternary system hbO-n-octane-fS-CsG-i-CsEo at T = 25°C [90]. Both systems show that the water/oil interfacial tension runs through a distinct minimum in the middle of the three-phase region. The full line is calculated considering the bending energy difference between a curved amphiphilic film in the microemulsion and the flat film of the macroscopic interface [96].

In Fig. 1.16, the variation of the water/oil interfacial tension with temperature is shown for four representative systems, namely water-n-octane-C6E2, CgEj, Q0E4 and Ci2E5. In... 

Figure 1.16 Temperature dependence of the water/oil interfacial tension

Figure 1.23 Variation of the water (shown as hollow symbols) and n-octane (shown as filled symbols) diffusion coefficients DA and Dg [115], the length scale [25], the mean curvature H and the water/oil interfacial tension (jat, as function of the temperature for the system hbO-n-octane-CnEs. Note that at the mean temperature of the three-phase body f the diffusion of water and oil molecules is equal (points to bicontinuity), the length scale runs through a maximum, the curvature change sign and the water/oil interfacial shows an extreme minimum.

The variation of the interfacial tension as a function of T for the Eusapon OD system shows the typical V-shape. The full curve corresponds to a theoretical description in terms of bending energy [164,165], The minimum of the interfacial tension correlates well with the mean temperature of the system and is located at interfacial tension between water and oil near the degreasing temperature corresponds to aab = 0.43 mN m. Although the interfacial tension between water and triolein is high compared to efficient microemulsion systems, it is still two orders of magnitude lower than the pure water oil interfacial tension (50 mN m 1). 

Fig. 6.5.8 Influence of the water-oil interfacial tension (-y) on the equilibrium product yield and initial reaction rate (v°) for the RAMA-catalyzed aldol addition of DHAP (30 mM) to phenylac-etaldehyde (50 mM) in water/CiaEa/oU 90/4/6w/w gel emulsion systems at 25 °C...

There has been a spate of recent activity associated with the formation and flow of aqueous droplets in channels surrounded by immiscible oil [10]. The typical configuration is similar to that used for flow cytometry in which a sample flow is injected into a co-fiowing sheath flow. In this chip-based manifestation, however, photolithography is used to fabricate a four-way intersection of channels and the sheath fluid is immiscible. Therefore, the water-oil interfacial tension results in the formation of droplets. Typically, the aqueous sample flow enters the intersection head-on and the two side channels... 

II. INTERACTIONS WITH LIQUID OILS A. Reduction of Water-Oil Interfacial Tension... 

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