Interface hexadecane/solution

A 0.2-mm thick hexadecane layer was placed on the oxazine solution. The vibrational coherence at the hexadecane/solution interface was pump-probed in a similar manner [27]. The light pulses traveled in the hexadecane layer and experienced group velocity dispersion before arriving at the interface. This undesired dispersion... 

Similarly, Zosim, Gutnik Rosenberg (1983) have reported uranium binding by emulsan (up to 240 mg uranium (U02 + 2)/mg emulsan), a bioemulsifier produced by Acinetobacter calcoaceticus RAG-1. The intriguing aspect of this system is that in a hexadecane-water solution, the emulsan preferentially binds to the hexadecane-water interface, which effectively concentrates the complexed uranium for easy recovery. 

The interfacial tension vs. temperature curves for different systems were investigated C,g vs. water, Cjg vs. an aqueous solution with protein (BSA casein). These data showed that the freezing of n-hexadecane takes place at 18°C however, supercooling is observed down to 16.6°C. In contrast, surface tension measurements at the air-liquid interface showed no supercooling behavior. ... 

The supercooling is also observed with protein (BSA, casein, lactoglobulin) in addition to the aqneous phase-Cjg system, bnt the freezing point of hexadecane increases to 18.2°C. This indicates that the crystallization of the hexadecane is affected by the presence of surface-active molecules. The supercooling will have extensive dependence on various interfaces, such as emulsions, oil recovery, and immunological systems. The adsorption of proteins from aqueous solutions on snrfaces has been studied by neutron reflection. ... 

For solutions of typical ionic surfactants with no added salts the studies of Carroll and Ward showed that solubihzation rates were much smaller than those for nonionic surfactants, presumably because the surfactant ions adsorbed at the oil-water interface repelled the micelles of like charge in the solution. Indeed, Bolsman et al. found no measurable solubilization of n-hexadecane into solutions of a pure benzene sulfonate and a commercial xylene sulfonate. They injected small oil drops into the surfactant solutions and observed whether the resulting turbidity disappeared over time due to solubilization. Similarly, Kabalnov found from Ostwald ripening experiments that the rate of solubilization of undecane into solutions of pure SDS was independent of surfactant concentration and about the same as the rate in the absence of surfactant. That is, the hydrocarbon presumably left the bulk oil phase in this system by dissolving in virtually miceUe-free water near the interface. In similar experiments TayloC and Soma and Papadopoulos observed a small increase in the solubilization rate of decane with increasing SDS concentration. De Smet et al., who used sodium dodecyl benzene sulfonate, which does not hydrolyze, found, like Kabalnov, a minimal effect of surfactant concentration. 

We report on the use of surface viscosity measurement at the planar oil—water interface to monitor time-dependent structural and compositional changes in films adsorbed from aqueous solutions of individual proteins and their mixtures. Results are presented for the proteins casein, gelatin, oC-lactalbumin and lysozyme at the n-hexadecane— water interface (pH 7, 25 °C). We find that, for a bulk protein concentration of 10 wt%, while the steady-state tension is invariably reached after 5—10 hours, steady-state surface shear viscosity is not reached even after 80—100 hours. Viscosities of films adsorbed from binary protein mixtures are found to be sensitively dependent on the structures of the proteins, their proportions in the bulk aqueous phase, the age of the film, and the order of exposure of the two proteins to the interface. 

Figure 6 Casein addition to a 24 h old gelatin film at the n-hexadecane—water interface (pH 7, 0.005 M, 25 °C). The apparent viscosity is plotted against time t following exposure of 10" wt % gelatin solution to fresh interface. The arrow indicates the point at which 10 wt % casein is added. The two dashed lines represent the behavior of pure casein (C) and pure gelatin (G).

Zhang and coworkers (237, 238) studied aqueous solutions of polyacrylamide (PAAM) mixed with three different smfactants at the hexadecane/water interface, using a rotational torsion viscometer. The structure of the interfacial films were shown to be dependent on the shear rate. Based on the experimental results, a mechanism for PAAM-sm-... 

Mixtures of hydrocarbons with calcium alkyl phosphate particles are known to be effective antifoams in the context of domestic textile machine washing where wash cycles can last for up to 1 h and involve temperature ranges from 30°C to 95°C (see Section 8.2.4.1) [43, 44]. Under these circumstances, little or no deactivation of antifoam effectiveness is apparent. Mixtures of hydrocarbons with alkyl phosphoric acid esters also function as antifoams in this context provided the aqueous phase has a high enough pH and calcium ions are present [43, 44] so that the calcium salts can precipitate as particles in situ at the relevant hydrocarbon-water interface. This behavior is of course analogous to that shown by mixtures of oleic acid and hexadecane when dispersed in an aqueous phase under similar conditions [45]. As with the preformed calcium alkyl phosphate particles, no deactivation of antifoam effectiveness is observed in the case of in situ formation of the precipitates. Indeed, it has been observed that continuous aeration for several hours, using a circulating Ross-Miles apparatus at 90°C (see Section 2.2.3), of an aqueous solution of a blend of a commercial sodium dodecylbenzene sulfonate and an ethoxylated alcohol in the presence of mixtures of a hydrocarbon and an alkyl phosphoric acid ester (dispersed... 

To assess the activity of the extracts at the oil-water interface, we measured the interfacial tension of these extracts (at a concentration of two times the CMC) against hexadecane, in which case we obtained values close to 8 mN/m. This interfacial tension is substantially higher when compared to the 1 mN/m obtained with rhamnolipid solutions discussed in the first half of this chapter. Another point of comparison is that the interfacial tension obtained with a conventional sodium dodecyl benzene sulfonate (SDBS) surfactant against hexadecane was close to 2 mN/m. These studies suggest that these alkaline extracts are more hydrophilic than rhamnolipids and lipopeptides obtained by biological means. 

With C12E5 as the nonionic surfactant at a 1 wt% level in water, quite different phenomena were observed below, above, and well above the cloud point when tetradecane or hexadecane was carefully layered on top of the aqueous solution. Below the cloud point temperature of 31 °C, very slow solubilization of oil into the one-phase micellar solution occurred. At 35 C, which is just above the cloud point, a much different behavior was observed. The surfactant-rich L phase separated to the top of the aqueous phase prior to the addition of hexadecane. Upon addition of the oil, the L, phase rapidly solubilizes the hydrocarbon to form an oil-in-water microemulsion containing an appreciable amount of the nonpolar oil. After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase. This behavior is shown schematically in Figure 12.16. Unlike the experiments carried out below the cloud point temperature, appreciable solubilization 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. 

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