Liquid interfaces benzene-water

When a drop (water) falls to a flat interface (benzene-water) the entire drop does not always join the pool (water). Sometimes a small droplet is left behind and the entire process, called partial coalescence, is repeated. This can happen several times in succession. High-speed motion pictures, taken at about 2000 frames per second, have revealed the details of the action (W3). The film (benzene) ruptures at the critical film thickness and the hole expands rapidly. Surface and gravitational forces then tend to drag the drop into the main pool (water). But the inertia of the high column of incompressible liquid above the drop tends to resist this pull. The result is a horizontal contraction of the drop into a pillar of liquid above the interface. Further pull will cause the column to be pinched through, leaving a small droplet behind. Charles and Mason (C2) have observed that two pinches and two droplets occurred in a few cases. The entire series of events required about 0.20 sec. for aniline drops at an aniline-water interface (C2, W3). 

Supercomputers become more and more useful, and the Insights they can generate become more and more unique, as the complexity of the system modelled Is Increased. Thus Interfaclal phenomena are a very natural field for supercomputation. In addition to the examples In this volume It may be useful to mention the work of Llnse on liquid-liquid benzene-water interfaces, which he studied with 504 H2O molecules, 144 CgHg molecules, and 3700 Interaction sites. He generated over 50 million configurations In 56 hours on a Cray-lA, and he was able to quantitatively assess the sharpness of the Interfaclal density gradient, which Is very hard to probe experimentally. Similarly Spohr and Helnzlnger have studied orientational polarization of H2O molecules at a metallic Interface, which is also hard to probe experimentally. 

Williams and coworkers preliminarily reported that CO oxidation on Pt/Al203 is faster in the presence of water solvent than in the presence of ethanol [141]. We then studied CO oxidation on platinum surface in the presence of different solvents, and identified obvious solvent effects, namely, CO oxidation takes place the most easily with water solvent, the least easily with carbon tetrachloride solvent, and follows the overall trend of water > ethanol > methanol > cyclohexane > benzene carbon tetrachloride [67]. We subsequently took advantage of the solvent effect to design a diagnosing tool to pin down low-coverage CO at the liquid-solid interface, by flushing the liquid-solid interface with water and carbon tetrachloride individually [67]. 

A hint as to one of many possibly important effects of addition of small amounts of substances to a liquid on one side of a liquid-liquid interface is given by the observation of Alexander and others,4 that kephalin films at a water-benzene interface increase their surface pressure substantially when calcium ions are added to the water. This is due to more powerful anchorage of the kephalin molecules in the interface, causing additional molecules to crowd in from solution in the Benzene. The development of means of studying liquid-liquid interfaces opens a wide and extremely important field. 

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. 

Phenylgermanic acid anhydride, (CgHsGeOjaO.—Equimolecular proportions of mercury diphenyl and germanium tetrachloride in dry xylene are heated in a sealed Pyrex bulb for two days, then diluted with dry ether and filtered. The solid residue is pure phenylmercuric chloride, and the filtrate is treated with benzene, and finally with water containing a few drops of ammonium hydroxide. The granular precipitate which separates at the liquid interface is removed and dried at 115° C. The anhydride is a white, fluffy, amorphous solid, with no definite melting-point, soluble in excess of alkali, and reprecipitated by carbon dioxide, insoluble in water and organic solvents. 

Liquid-liquid interfacial tensions can in principle also be obtained by simulations, but for the time being, the technical problems are prohibitive. Benjamin studied the dynamics of the water-1,2-dichloroethane interface in connection with a study of transfer rates across the interface, but gave no interfacial tensions. In a subsequent study the interface between nonane and water was simulated by MD, with some emphasis on the dynamics. Nonane appears to orient relatively flat towards water. The same trend, but weaker, was found with respect to vapour. Water dipoles adjacent to nonane adsorb about flat, with a broad distribution the ordering is a few molecular layers deep. Fukunishi et al. studied the octane-water Interface, but with a very low number of molecules. Their approach differed somewhat from that taken in the simulations described previously they computed the potential of mean force for transferring a solute molecule to the interface. The interfacial tension was 57 11 mN m", which is in the proper range (experimental value 50.8) but of course not yet discriminative (for all hydrocarbons the interfacial tension with water is very similar). In an earlier study Linse investigated the benzene-water interface by MC Simulation S He found that the water-benzene orientation in the interface was similar to that in dilute solution of benzene in water. At the interface the water dipoles tend to assume a parallel orientation. The author did not compute a x -potential. Obviously, there is much room for further developments. 

In membrane filtration, water-filled pores are frequently encountered and consequently the liquid-solid transition of water is often used for membrane pore size analysis. Other condensates can however also be used such as benzene, hexane, decane or potassium nitrate [68]. Due to the marked curvature of the solid-liquid interface within pores, a freezing (or melting) point depression of the water (or ice) occurs. Figure 4.9a illustrates schematically the freezing of a liquid (water) in a porous medium as a fimction of the pore size. Solidification within a capillary pore can occur either by a mechanism of nucleation or by a progressive penetration of the liquid-solid meniscus formed at the entrance of the pore (Figure 4.9b). 

Deutsch11 found that aqueous solutions of indicator dyes acquire a different color when vigorously shaken with an immiscible liquid such as benzene. Thus the color of Bromothymol Blue at pH 7.4 changes from blue to yellow when treated in this manner. The change in color takes place at the interface of the emulsified droplets, and the original color is restored when the liquids separate. In another study, it was observed that a colorless solution of rhodamine in benzene turns a deep red when vigorously shaken to form an emulsion with water. When the phases later separate, the color almost completely disappears. The phenomenon is not limited to liquidj liquid interfaces, but also occurs at an air liquid interface a brownish-yellow solution of thymolsulfonaphthalein (pH 2.8) when shaken with air develops a reddish-violet foam. 

Emulsions, like foams, can also be stabilised by finely divided solids, provided the properties of the solid/liquid/liquid interface are appropriately adjusted. These properties may also determine whether an O/W or a W/O emulsion is formed. For example, shaking water and benzene together with finely divided calcium carbonate yields a benzene-in-water emulsion. On the other hand, if the calcium carbonate is made hydrophobic by treatment with oleic acid solution, a water-in-benzene emulsion results. 

Kamusewitz H, Possart W (1985) The static contact angle hysteresis obtained by different experiments for nthe system PTFE/water. Int J Adhes Adhes 5 211-215 Karpovich DS, Ray DJ (1998) Adsorption of dimethyl sulfoxide to the liquid/vapor interface of water and the thermochemistry of transport across the Interface. Phys Chem B 102 649-652 Kereszturi A, Jedlovszky P (2005) Computer simulation investigation of the water-benzene interface in a broad range of thermodynamic states from ambient to supercritical conditions. J Phys Chem B 109 16782-16793... 

Electron transfer across monolayer at liquid-liquid interface ZnPor(benzene)/R (water)... 

R= Ru(CN)6 -, Mo(CN)8 -, Fe(CN)6 -, and so forth (Table 2)) in an aqueous phase have been surveyed. At the probe microelectrode surface, ZnPor+ was oxidized to ZuFor" ". When the probe is positioned close to the benzene-water interface, ZnPor+ is reduced back to ZnPor by accepting an electron from R in the aqueous phase at the liquid-liquid interface. In the experiment, the driving force was controlled with two parameters the difference in standard potentials of the redox mediators in benzene and in water (AE ), and the interfacial potential drop (A ), which is controllable by varying the concentration ratio of a base electrolyte such as Cl04 in the two Kquids. The driving force dependence on the electron transfer rate at the liquid-liquid interface has been shown in the literature in the absence and presence of the monolayer. The existence of the monolayer lowers the electron transfer... 

Linse, R, Monte-Carlo simulation of liquid-liquid benzene-water interface, J Ghent Phys, Wol 86, (1987) p. 4177. 

Liquid-liquid interfacial tensions exist for immiscible liquid-liquid systems, e.g. water or glycols with hydrocarbons and water-alcohols. In most cases, the interfacial tension value is between the surface tensions of the two liquids involved, e.g. the value of 51 mN m reported for water-hexane is between the 18 and 72 mN m for hexane and water, respectively. The lower, compared to hexane-water, value for the interfacial tension of benzene-water (35 mN m ) is due to the higher solubility of benzene compared to hexane in water (Figure 3.9). This is due to the weak complexes formed between aromatics and water which exist because of the so-called r -electrons of the aromatic rings. For this reason, the benzene-water interface is much smaller than the hexane-water one and this is why water-benzene has a much lower interfacial tension than the more insoluble water-hexane. 

Table 3.8 Mercury-non-metallic liquid interfaces at 20 °C (except for water, heptane and benzene which are at 25 °C)...

How sharp is the interfacial region between water and an organic liquid and what is its molecular structure Broadly, three possibilities should be considered (1) the interface is sharp and flat, as assumed in continuum models (2) the interfacial region is a mixture of the two liquids and (3) the interface is a locally sharp but rough surface that fluctuates in time. Recent computer simulations of interfaces between water and benzene, " decane, nonane, hexane, dodecane, 1,2-dichloroethane (DCE), CCU, and octanol have dealt with this issue. 

P. Linse, /. Chem. Phys., 86, 4177 (1987). Monte Carlo Simulation of Liquid-Liquid Benzene-Water Interface. 

In this and many similar cases, it must be remembered that benzene and many other liquids of low water miscibihty have, in fact, a small but finite solubility and the water will rapidly become saturated with benzene. Benzene, having a lower surface tension than water, will adsorb at the water-air interface so that the surface will no longer be that of pure water but that of water with a surface excess of benzene. The surface tension of benzene-saturated water can be measured and is found to be... 

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