Interfacial water-hydrocarbon

Water invariably occurs with petroleum deposits. Thus, a knowledge of the properties of this connate, or interstitial, or formation water is important to petroleum engineers. In this chapter, we examine the composition of oilfield water water density, compressibility, formation volume factor and viscosity solubility of hydrocarbons in water and solubility of water in both liquid and gaseous hydrocarbons and, finally, water-hydrocarbon interfacial tension. An unusual process called hydrate formation in which water and natural gas combine to form a solid at temperatures above the freezing point of water is discussed in Chapter 17. 

The definition of interfacial tension given in Chapter 8 also applies to water-hydrocarbon systems. 

Interfacial Tension of Water-Hydrocarbon Gas — Interfa-cial Tension of Water-Hydrocarbon Liquid Exercises 467... 

Adsorption can be measured by direct or indirect methods. Direct methods include surface microtome method [46], foam generation method [47] and radio-labelled surfactant adsorption method [48]. These direct methods have several disadvantages. Hence, the amount of surfactant adsorbed per unit area of interface (T) at surface saturation is mostly determined by indirect methods namely surface and interfacial tension measurements along with the application of Gibbs adsorption equations (see Section 2.2.3 and Figure 2.1). Surfactant structure, presence of electrolyte, nature of non-polar liquid and temperature significantly affect the T value. The T values and the area occupied per surfactant molecule at water-air and water-hydrocarbon interfaces for several anionic, cationic, non-ionic and amphoteric surfactants can be found in Chapter 2 of [2]. 

When nonpolar compounds are suspended in water their relative insolubility causes them to associate, diminishing the water-hydrocarbon interfacial area (a hydrophobic effect). This association is greater in water than in methanol and brings the reactive partners into close proximity, increasing the rate of reaction. Any additive that increases the hydrophobic effect will increase the rate. ... 

Animal studies with structurally related polynuclear aromatic hydrocarbons (PAHs), such as benzo(d )py-rene, benz(it)anthracene, and 3-MC, confirmed that intestinal transport readily occurs primarily by passive diffusion after oral dosing. From the partitioning parameters, the rate-limiting step involves solvation of transfer species in the interfacial water at the phospholipid surface. 

The formation of three different condensed phases in the water-hydrocarbon-surfactant system allows one to measure the surface tension at the three interfaces, and to study the a(T) dependence at them (VI-19). Due to the dehydration of surfactant molecules, the interfacial tension at the aqueous solution - microemulsion interface, ow.me, increases with temperature, while the interfacial tension at the microemulsion - oil interface, a0.me, drops until a complete vanishing of this interface occurs. For the hydro carbon-water... 

In the other case, the possibility of the adsorption of the organic liquid component at the interface between liquid water and a low energy solid is more difficult to exclude. Such adsorption would give rise to a film pressure denoted by 7Ti3 2- In this case, however, it may be argued that the solid-liquid water ihterfacial tension would be of the same order of magnitude as water-hydrocarbon liquid interfacial tensions. Hence, an appreciable decrease in interfacial tension could not be effected by an adsorption process. 

The molecular structure of the interfacial region is obtainable from computer simulations (Benjamin 1996). Michael and Benjamin treated the water/hydrocarbon (Michael and Benjamin 1995) and water/nitrobenzene (Michael and Benjamin 1998) interfaces by molecular dynamics simulation. In the former system two hydrocarbons were treated n-nonane and pseudononane that consisted of globular molecules with the same mass and potential functions as the long chain actual n-nonane. The mean width 5ws of the interfacial region is related to the macroscopic interfacial tension Xws by the approximate expression ... 

Our simulations show that the interfacial region has a narrow water-hydrocarbon interface about 4 A thick, as has been found in previous simulation of micelles [9, 27]. The Na" " ions and carboxylate head groups make up a characteristic electrostatic double layer around these micelles. In the polarizable models the Na" ions do not come in contact as much with the headgroups as in the original nonpolarizable model, in better agreement with experiment. 

Figure 5 Images of core, interfacial, and aqueous regions of ionic micelles, (a) It shows a classical micelle cartoon that represents the dynamic nature of the course and interface, but may over emphasize the minimization of water-hydrocarbon contact, (b and c) The results of united-atom (thin lines) and all-atom (thick lines) molecular dynamic simulations of decyltrimethylanunonium bromide, DeTABr containing 29 or 30 surfactants. COM = center of mass. (Structure 5(a) Reproduced from Ref. 71. American Chemical Society, 1991. Structures 5(b) and 5(c) Reproduced from Ref. 72. American Chemical Society, 2008.)...

The interfacial tension can undergo significant changes if the polarity of the medium is altered, such as in the stability/coagulation transition caused by the addition of water to hydrophobic silica dispersions in propanol or ethanol [44,52,53]. Also, the addition of small additives of various surface-active substances can have a dramatic effect on the structure and properties of disperse systems and the conditions of transitions [14,16,17,26]. The formation and structure of stable micellar systems and various surfactant association colloids, such as microemulsion systems and liquid crystalline phases formed in various multicomponent water/hydrocarbon/surfactant/alcohol systems with varying compositions and temperatures, have been described in numerous publications [14-22,78,79,84-88]. These studies provide a detailed analysis of the phase equilibria under various conditions and cover all kinds of systems with all levels of disperse phase concentration. Special attention is devoted to the role of low and ultralow values of the surface energy at the interfaces. The author s first observations of areas of stable microheterogeneity in two-, three-, and four-component systems were documented in [66-68],... 

Using this expression for water—hydrocarbons interfacial tension (y h) die dispersive surface energy component of water may be found. As basic components for hydrocarbons should be zero, the dispersive component is the surface tension of the hydrocarbon. Measuring yu, Yw> and YwH die only unknown is y which turns to be... 

The most effective emulsion and foam stabilizers are aerosol systems containing fluorocarbon propellants as surfactants. These are believed to form an oriented polymolecular structure at the propellant-water interface for optimum stability Sanders has found [90] that the surfactants must have a low solubility in both phases and have the ability to remain in the interfacial region. Hydrocarbon and fluorocarbon chains are not freely miscible and this perhaps explains the unusual behaviour of the surfactants in these systems. Addition of long-chain alcohols or acids enhance stability of the fluorocarbon emulsions and a hypothetical structure of the interfacial region has been proposed (Fig. 8.16). Davis et al. [91] have investigated the stability of fluorocarbon emulsions intended as artificial blood substitutes. Perfluorocarbon oils tended to produce unstable emulsions while oil phases such as perfluorotributylamine or per-fluorotetrahydrofuran formed more stable systems. These authors also refer to the possibility that as fluorocarbon-hydrocarbon mixtures have positive excess free energies, cohesive and adhesive forces between surfactant and oil phase will result. 

The success of this equation was shown from the start (Fowkes, 1964) as it was illustrated that experimental liquid-liquid interfacial tension data for ten mercury-hydrocarbon mixtures and for eight water-hydrocarbon mixtures resulted to more or less unique values for the dispersion contribution of mercury and water, respectively (see Example 3.3). Moreover, these values are in agreement with those estimated from theoretical considerations. 

J. Chowdhary and B. M. Ladanyi,/. Phys. Chem. B, 110,15442 (2006). Water-Hydrocarbon Interfaces Effect of Hydrocarbon Branching on Interfacial Structure. 

M. N. D. S. Cordeiro, Molecular Simulation, 29, 817 (2003). Interfacial Tension Behaviour of Water/Hydrocarbon Liquid-Liquid Interfaces A Molecular Dynamics Simulation. 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

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]. 

Sulfonates for Enhanced Oil Recovery. The use of hydrocarbon sulfonates for reducing the capillary forces in porous media containing cmde oil and water phases was known as far back as 1927—1931 (164,165). Interfacial tensions between 10 and 10 N/m or less were estabUshed as necessary for the mobilization and recovery of cmde oil (166—169). 

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