The Hydrocarbon-Water Interface

When sufficient amounts of hydrocarbon chains are mixed with water, macroscopic phase separation takes place. The saturation value of hydrocarbon 

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The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

During the past few years, the determination of the interfacial properties of binary mixtures of surfactants has been an area in which there has been considerable activity on the part of a number of investigators, both in industry and in academia. The Interest in this area stems from the fact that mixtures of two different types of surfactants often have interfacial properties that are better than those of the individual surfactants by themselves. For example, mixtures of two different surface-active components sometimes reduce the interfacial tension at the hydrocarbon/water interface to values far lower than that obtained with the individual surfactants, and certain mixtures of surfactants are better foaming agents than the individual components. For the purpose of this discussion we define synergism as existing in a system when a given property of the mixture can reach a more desirable value than that attainable by either surface-active component of the mixture by itself. 

Figure 9.2 Calculation of the surface packing parameter. Via x I, which determines to some extent the form of the aggregate. For V/(a x 1) c. 1, bilayers are preferentially formed when this ratio is less or more than one, spherical aqueous and reverse micelles are formed, respectively. Self-assembly may be described in terms of the curvature that exists at the hydrocarbon-water interface.

Desorption from the hydrocarbon is a critical part of the growth cycle of petroleum-degrading bacteria. Petroleum is a mixture of thousands of different hydrocarbon molecules. Any particular bacterium is only able to use a part of the petroleum. As the bacteria multiply at the hydrocarbon/water interface of a droplet, the relative amount of nonutilizable hydrocarbon continually increases, until the cells can no longer grow. For bacteria to continue to multiply, they must be able to... 

Rosenberg, M. Rosenberg, E. (1985). Bacterial adherence at the hydrocarbon-water interface. Oil and Petrochemical Pollution, 2, 155-62. 

Case A. Transfer of hydrocarbon from water to hydrocarbon. The transfer of a hydrocarbon solute of surface area A from water to hydrocarbon removes the hydrocarbon-water interface (w = — yHc/w) and creates a hydrocarbon-vacuum and a water-vacuum interface (w = yHC + yw) formation of a cavity in the hydrocarbon solvent creates a hydrocarbon-vacuum interface (w = ync) transfer of the hydrocarbon solute to the hydrocarbon, and collapse of the water-vacuum cavity, loses two hydrocarbon-vacuum surfaces and a water-vacuum surface (w = — 2yHC — yw). The free energy of transfer, AGtrans, is given by A X Sw i.e.,... 

Figure 3.25 Cyclopentane hydrate formation from a water droplet (a) initial contact, (b) hydrate shell formation around the water droplet, (c) depressions formed on the hydrate shell, (d) conversion of interior water to hydrate, indicated by darkening, (e) almost completely converted hydrate. (From Taylor, C.J., Adhesion Force between Hydrate Particles andMacroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, Master s Thesis, Colorado School of Mines, Golden, CO, (2006). With permission.)...

A conceptual picture of the proposed mechanism for hydrate film growth at the hydrocarbon-water interface based on the above experimental results is given... 

Taylor, C.J., Adhesion Force between Hydrate Particles and Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, Masters Thesis, Colorado School of Mines, Golden, CO (2006). 

Note that since Oq = (4n) (3v) /M, and the tail volume of an amphiphile is known, the only parameters which occur in the distribution eqn (4.6) are M and y, the interfacial tension of the hydrocarbon-water interface. 

Consider first a spherical micelle (fig. 1). Here the radius R, hydrocarbon core volume V and surface area a per amphiphile at the hydrocarbon-water interface are related by... 

Secondary hydrocarbon migration generally occurs through water-saturated sedimentary rocks, i.e. through rocks that are water-wet. As water is generally considered a perfect wetting fluid (Schowalter, 1979), the contact angle 0 in Equation 4.14 for hydrocarbon-water-rock systems can be taken to be zero. If, in addition, the hydrocarbon-water interface is assumed to be spherical, then Equation 4.14 becomes identical with Equation 4.13. 

Because BLM made of pure lipid or oxidized cholesterol In common salt solutions are nonconducting, the physical properties of BLM are with one exception similar to those of a liquid hydrocarbon layer of equivalent th ckness. The Interfaclal tension of BLM Is less than 5 dynes cm, which Is approximately one order of magnitude lower than that of the hydrocarbon/water Interface. This low Interfaclal tension Is due to the presence of polar groups at the Interface. BLM have negligible permeability for Ions and most polar molecules. Permeability to water Is comparable to that of biological membranes. The permeability to water of Chlorophyll BLM, as determined by an osmotic flew method. Is 50 pm s, which Is in-the range of phospholipid BLM but six times larger than that of oxidized cholesterol BLM. 

Voglino, L., McIntosh, T.J., and Simon, S.A. Modulation of the binding of signal peptides to lipid bilayers by dipoles near the hydrocarbon-water interface. Biochemistry, 37, 12241, 1998. 

For the solubilization of highly insoluble hydrocarbons by POE nonionics into water, the rate of solubilization has been found (Carroll, 1981, 1982) to be directly proportional to the surfactant concentration above the CMC, and to increase with the polarity and decrease with the molecular weight of the oil. The rate is also strongly temperature dependent in the region of the cloud point (Section IIIB below), increasing rapidly as that temperature is approached. The mechanism suggested involves diffusion of the micelles to the hydrocarbon-water interface, where they dissociate and adsorb as monomers. This adsorption produces concerted desorption from the interface of an equivalent amount of monomeric surfactant, but in the form of micelles containing a quantity of solubilizate. 

Some Effects of Hydrocarbon Solvent Structure on the Phase Behavior of Distearoyl Lecithin Monolayers at the Hydrocarbon/Water Interface... 

Emulsion polymerization is known to be a method of carrying out polymerization in a disperse system in which water is usually the dispersion medium. In order to ensure the stability of an emulsion containing 30-60% of the monomer, emulsifiers are used. They are compounds of diphilic type surfactants which decrease the surface tension at the hydrocarbon-water interface. This decrease facilitates the emulsification of the monomer in water and favours the stabilization of the emulsion. 

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