Water/decane interface

Figure 1.33. Test for the pristine state of the water-decane interface. The Interfaclal tension is measured during compression and expansion cycles (a) meticulously purified decane, (b) commercial decane sample. (Redrawn from R. Miller, P. Joos and V.B. Falnerman, Adv. Colloid Interface Set 49 (1994) 249.)...

Figure 4.37. Computed interfaciai tension at the water-decane interface in the presence of CjQ - p SOgNa". Influence of branching. Temperature 25 C. (Redrawn after van Os et al., loc. cit.)...

Relaxation behaviour of the water-decane interface commercial decane ( ), specially purified decane ( ) according to Miller et al. (1994c)... 

Using the ADSA technique (cf. Rotenberg et al. 1983, Cheng et al. 1990), the adsorption of HA at the water/decane interface was studied (Miller et al. 1993a). At low concentrations an induction time is detectable (Fig. 5.40). At higher concentrations, of 0.015 and 0.02 mg/ml, the interfacial tensions tend to the same equilibrium value. A similar phenomenon was also observed at the water air interface. Even at extremely high HA concentrations of about 50 mg/ml the equilibrium surface tension values are not less than about 51 mN/m, a veilue which has already been reached at a concentration as low as 1 mg/ml. 

One peculiarity should be mentioned in the dependencies given in Fig. 5.38, which can be also seen in Fig. 5.40 for the two lower HA concentrations at the water/decane interface. Just after the formation of a fresh surface, the surface tension is higher than that of pure water/air or water/decane interface. This effect, which is much higher than the accuracy of the methods... 

Interfacial relaxations of HA at the water/decane interface are performed with the same technique (Miller et al. 1993c, d). The results for three subsequent square pulse disturbances of the adsorption layer of 0.02 mg/ml HA are shown in Fig. 6.22. 

Figure Bl.14.13. Derivation of the droplet size distribution in a cream layer of a decane/water emulsion from PGSE data. The inset shows the signal attenuation as a fiinction of the gradient strength for diflfiision weighting recorded at each position (top trace = bottom of cream). A Stokes-based velocity model (solid lines) was fitted to the experimental data (solid circles). The curious horizontal trace in the centre of the plot is due to partial volume filling at the water/cream interface. The droplet size distribution of the emulsion was calculated as a fiinction of height from these NMR data. The most intense narrowest distribution occurs at the base of the cream and the curves proceed logically up tlirough the cream in steps of 0.041 cm. It is concluded from these data that the biggest droplets are found at the top and the smallest at the bottom of tlie cream.

Every liquid interface is usually electrified by ion separation, dipole orientation, or both (Section II). It is convenient to distinguish two groups of immiscible liquid-liquid interfaces water-polar solvent, such as nitrobenzene and 1,2-dichloroethane, and water-nonpolar solvent, e.g., octane or decane interfaces. For the second group it is impossible to investigate the interphase electrochemical equilibria and the Galvani potentials, whereas it is normal practice for the first group (Section III). On the other hand, these systems are very important as parts of the voltaic cells. They make it possible to measure the surface potential differences and the adsorption potentials (Section IV). 

Monte Carlo and molecular dynamics calculations of the density profile of model system of benzene-water [70], 1,2-dichloroethane-water [71], and decane-water [72] interfaces show that the thickness of the transition region at the interface is molecu-larly sharp, typically within 0.5 nm, rather than diffuse (Fig. 4). A similar sharp density profile has been reported also at several liquid-vapor interfaces [73, 74]. The sharpness of interfaces thus seems to be a general characteristic of the boundary between two stable phases and it is likely that the presence of supporting electrolytes would not significantly alter the thickness of the transition region at an ITIES. The interfacial mixed solvent layer [54, 55], if any, would probably have a thickness comparable with this thin inner layer. 

As shown in Figure 3.9, the L2 phase is able to solubilize a very large amount of a hydrocarbon such as decane or hexadecane. In fact, a composition containing up to 75% decane and water/surfactant/cosurfactant proportions corresponding to the L2 phase is still clear, fluid and isotropic, forms spontaneously, and is thermodynamically stable. The structure of this microemulsion can be (to some extent) regarded as a dispersion of tiny water droplets (reverse micelles) in a continuous phase of the hydrocarbon. The surfactant and cosurfactant are mainly located at the water/oil interface. This type of system is often referred to as a w/o microemulsion. 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilisation of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the PITs of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One was the lamellar liquid crystalline phase La (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilisation of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C using n-decane as the hydrocarbon. At this temperature, which is near the PIT of the water/decane/C Es system, the existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evident. These results can be utilised to optimise surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and co-surfactants. 

The adsorption of the anionic surfactant sodium dodecyl sulphate (SDS), probably the most frequently studied surfactant and often used as model substance at the air/water and at the decane /interface is given in Fig. 1.5. The surface and interfacial tension have been plotted as a function of SDS concentration in the aqueous phase. From the slope of the tangents to the curves in Fig. 1.5 the interfacial excess concentration (adsorption density) F at different interfacial tensions can be calculated directly using Gibbs fundamental adsorption isotherm (see section 2.4.1),... 

Upon the contact of a water drop with the flat water/soap solution in n-decane interface, a liquid hydrocarbon film was formed which is surrounded by Newton rings that may be seen in reflected light. Upon thinning, the film itself changes its color from white to grey and then to black. 

A similar effect of the appearance of a negative potential shift is also observed when FeEP is replaced by Co EP or by metalless porphyrins. This means that a redox reaction at the interface is catalyzed only by porphyrins having as a central atom a transition metal which is capable of accepting electrons. The influence of the nature of the central atom in the porphyrin on the Volta potential value and the reaction rate was also observed for other redox reactions at water/octane, water/decane, water/chlo-roform and water/dichloroethane interfaces [57-59]. 

The molecular orientation of surfactants has been determined by SHG the first detailed study was performed [79] for the adsorption of sodium 1-dodecylnaphtalene-4-sulfonate (SDNS) at the aqueous/decane and aqueous/CCU interfaces the results showed that the molecular orientation depends on the nature of the nonaqueous phase. Recent advances have used SHG to probe simfactants containing an aromatic head group at the water/dichloroethane interface in an electrochemical cell [80] ... 

Replacement of gas by the nonpolar (e.g., hydrocarbon) phase (oil phase) has been sometimes used to modify the interactions among molecules in a spread film of long-chain substances. The nonpolar solvent/water interface possesses an advantage over that between gas and water in that cohesion (i.e., interactions between adsorbed molecules) due to dipole and van der Waals s forces is negligible. Thus, at the oil/water interfaces, the behavior of adsorbates is much more ideal, but quantitative interpretation may be uncertain, in particular for the higher chains, which are predominantly dissolved in the oil phase to an unknown extent. The oil phase is poured on the surface of an aqueous solution. Thus, the hydrocarbon, such as heptane or decane, forms a membrane a few millimeters thick. It is thicker than the adsorbed monolayer. Owing to the small difference in dielectric constant between the air and a hydrocarbon oil, the... 

The work of adhesion is influenced by the orientation of the molecules at the interface. For example, with the help of Table A.4.1 and Eq. (A.4.8), the work of adhesion of n-decane-water (corresponding to a paraffinic oil-water system) and of glycerol-water can be computed to be 40 10 3 J nr2 and 56x 10 3 J nr2, respectively. It requires more work to separate the polar glycerol molecules (oriented with the OH groups toward the water) from the water phase than the nonpolar hydrocarbon molecules. For paraffinic oils Woo is about 44 mj nr2, for water Www is 144 mj nr2, and for glycerol Woo is 127 mJ nr2. 

From the solubility data of n-decane in water, the enthalpy for the process n-decane (H2O) - n-decane (pure) at 25°C has been estimated by Boddard et al. ( ) to be -5.85 kJ moT. Substracting this value from the calculated AH°(25°c) values for CioBMG and C12BMT, in Tables III and IV, the AH (-W) values for micellization and for adsorption at the aqueous solution/air interface at 25 C can be estimated. Values are shown in Table V. 

In line with the Gibbs adsorption equation (equation 3.33 in chapter 3), the presence of thermodynamically unfavourable interactions causes an increase in protein surface activity at the planar oil-water interface (or air-water interface). As illustrated in Figure 7.5 for the case of legumin adsorption at the n-decane-water interface (Antipova et al., 1997), there is observed to be an increase in the rate of protein adsorption, and also in the value of the steady-state interfacial pressure n. (For the definition of this latter quantity, the reader is referred to the footnote on p. 96.)... 

Figure 7.15 Effect of thermodynamically favourable interactions between biopolymers on protein surface activity at the planar oil-water or air-water interface. The surface pressure n reached after 6 hours is plotted against the polysaccharide concentration ( ), legumin (0.001 wt%) + dextran (Mw = 270 kDa) at / -decane-water surface at pH = 7.8 and ionic strength = 0.01 M, (Ay = -0.2 x 105 cm3 mol1) (Pavlovskaya et ah, 1993) ( ), legumin (0.001 wt%) + maltodextrin (MD6, Mw = 102 kDa) at air-water surface at pH = 7.2 and ionic strength = 0.05 M (Ay = - 0.02 x 105 cm3 mol-1) (Belyakova et ah, 1999) (A), legumin (0.001 wt%) + maltodextrin (MD10, Mw = 45 kDa) at air-water surface at pH = 7.2 and ionic strength = 0.05 M (.1 / = - 0.08 x 105 cm3 mol-1) (Belyakova et ah, 1999).

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

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