Oil-water boundary

Interphase Free-Radical Copolymerization at the Oil-Water Boundary. . 168... 

To elaborate a theory of interphase copolymerization at an oil-water boundary the necessity arises to consider initially the growth of an individual polymer chain near the surface separating the organic and water phases. By the model introduced in paper [74], molecules of only one of the monomers are presumed to be solved inside either of these two phases. A theoretical examination of the formation of macromolecules turns out here to be substantially simpler, since their chemical structure under such an approximation is the same as that of a traditional block copolymer. 

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). 

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

The understated conclusion that the results are consistent with the formulae derived from a direct consideration of an electrochemical equilibrium coupled with the need for electroneutrality belies the fundamental confirmation that liquid junction potential at the oil/water boundary are small a fact only possible to derive because of the very careful minimisation of all other effects in the work of Karpfen and Randles. 

The loss of emulsified water due to the effect of temperature is shown in Fig. 6. Once again, the slope of the exponential functions illustrates the rate of water loss. No significant relationship was found between the properties of the boundary of the creams (surfactant quantity accumulated on the oil/water boundary, wetting) and the evaporation rate of the water phase. 

For a particle in isolation, the charges at the particle-nonpolar fluid interface create an electric field in the oil that asymptotically resembles the electric field of a dipole (Figure 4.29). This field practically does not penetrate into the water phase, because it is reflected by the oil-water boundary owing to the relatively large dielectric constant of water. For a single particle, the respective electrostatic problem is solved in Ref. [355]. The asymptotic behavior of the force of electrostatic repulsion between two such particles-dipoles (Figure 4.29) is [355] ... 

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

We consider an oil-water two-phase system, which contains an ionic surfactant i. If we vary the phase-boundary potential either by externally applying the voltage using two electrodes or by adjusting the solution composition of potential determining ions, the concentration of i in each phase varies accordingly, keeping the total amount of i in the system, m, constant. The condition of the latter is... 

Figure 4 illustrates the dependence of on Aq for the case when r = 1 at several different values of [Fig. 4(a)] and when = 0.5 and at several different values of r [Fig. 4(b)]. From Fig. 4(a), one can see that takes a maximum around y = 0, i.e., Aq The volume ratio affects strongly the value of as shown in Fig. 4(b), which is ascribed to the dependence of the equilibrium concentration on r through Eq. (25). This simple example illustrates the necessity of taking into account the variation of the phase-boundary potential, and hence the adsorption of i, when one tries to measure the adsorption properties of a certain ionic species in the oil-water two-phase systems by changing the concentration of i in one of the phases. A similar situation exists also in voltammetric measurements of the transfer of surface-active ions across the polarized O/W interface. In this case, the time-varying thickness of the diffusion layers plays the role of the fixed volume in the above partition example. The adsorption of surface-active ions is hence expected to reach a maximum around the half-wave potential of the ion transfer. 

Very finely disperse solids, which are adsorbed at the liquid/liquid interfaces, forming films of particles around the disperse globules. Certain powders can very effectively stabilize against coalescence. The solid s particle size must be very small compared with the emulsion droplet size and must exhibit an appropriate angle of contact at the three-phase (oil/water/solid) boundary [141]. 

Karpfen, F. M., and J. E. B. Randles, Ionic equilibria and phase-boundary potentials in oil-water systems, Trans. Faraday Soc.f 49, 823 (1953). 

The model for desorption from an oil-water multilaminate is shown in Figure 5. Only the boundary and initial conditions change from the earlier diffusion problem. Both source and receptor compartments are now maintained under sink conditions. At time zero, each oil layer contains initial concentration PC of solute and the concentration of each aqueous layer is C. To determine the amount... 

This expression was suggested previously by de Gennes and Taupin7 on the basis of scaling arguments. Experimentally, one can measure three different interfacial tensions in the three-phase system, namely, at the microemulsion—excess water phase boundary (yMw), at the microemulsion—excess oil phase boundary (yMo), and between the excess oil and excess water phases (yow). On the basis of intuitive arguments, it has been suggested8 that... 

At the liquid-liquid interface between a hydrocarbon oil and water under mixing, the molecules encounter unbalanced attraction forces, pull inwardly, and contract as other molecules leave the interface for the interior of the bulk liquid. As a result, spherical droplets are formed. Customarily, the boundaries between a liquid and gas and between two liquids are the surface and the interface, respectively. The interfacial tension (or interfacial free energy) is defined as the work required to increase the interfacial area of one liquid phase over the other liquid phase isothermally and reversibly. Moving molecules away from the bulk to the surface or interfacial surface requires work (i.e., an increase in free energy). Water molecules and hydrocarbon oil molecules at the interface are attracted to the bulk water phase as a result of water-water interaction forces (i.e., van der Waals dispersion y and hydrogen bonding y ), to the bulk oil phase due to the oil-oil dispersion forces, y 1, and to the oil-water phase by oil-water interactions, y )W (i.e., dispersion forces). As mentioned in Chapter 3, the oil-water dispersion interactions are related to the geometric mean of the water-water and oil-oil dispersion interactions. The interfacial tension is written as ... 

Adsorption and oil-water potentials. Dean, Gatty, and Rideal2 discuss the thermodynamics and the mechanism of the establishment of interfacial potential differences by the adsorption of ions, or by the adsorption or spreading of a film containing dipoles. They show that, provided that one or more of the charged components can pass the phase boundary and come into equilibrium on both sides, the adsorption of the interfacial film will not by itself change the phase boundary potential. For the electrochemical potentials of those charged components which can pass the boundary are equal, at equilibrium, in the two phases, i.e. 

It was discussed that the structure created by the ternary system oil/water/ nanoparticle follows the laws of spreading thermodynamics, as they hold for ternary immiscible emulsions (oil 1 /oil 2/water) [114,116,117]. The only difference is that the interfacial area and the curvature of the solid nanoparticle has to stay constant, i.e., an additional boundary condition is added. When the inorganic nanoparticles possess, beside charges, also a certain hydrophobic character, they become enriched at the oil-water interface, which is the physical base of the stabilizing power of special inorganic nanostructures, the so-called Picker-... 

Capillary waves — Capillary waves are triggered by thermal fluctuation and recovered by -> surface tension on an interface between liquid phases rather than by gravity [i]. They propagate along the interface [ii], and they distort the sharp boundary at immiscible oil water interfaces. The frequency of capillary waves, which has been determined with light scattering measurements [iii, iv], is predicted to evaluate time-dependent local surface tensions without any contact with the surface. 

Commentary on Ionic equilibria and phase-boundary potentials in oil-water systems,... 

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