Mercury-water interfacial

Assume that an aqueous solute adsorbs at the mercury-water interface according to the Langmuir equation x/xm = bc/( + be), where Xm is the maximum possible amount and x/x = 0.5 at C = 0.3Af. Neglecting activity coefficient effects, estimate the value of the mercury-solution interfacial tension when C is Q.IM. The limiting molecular area of the solute is 20 A per molecule. The temperature is 25°C. 

Researches carried out in electrochemistry on solid electrodes and especially on the mercury-water interface have made a significant contribution to an understanding of interfacial phenomena. Although the electrode-water interfaces are typically... 

Figure 4. Plots of 0 vs.. (o o o and —) and 8(AG,ds)/l r vs., ( and - - -) due to triethylamine adsorption on a Hg electrode at concentrations 0.002, 0.00126, 0.001, 0.000794, 0.000631, and 0.0005 mol dm (from top to bottom). Points are experimental data reprinted from J. Electroanal. Chem., 255, M. L. Foresti el al.. Adsorption of Triethylamine at the Mercury/Water Interphase from Charge and Interfacial Tension Measurements, p. 267, Copyright 1988, with permission from Elsevier Science. Curves were calculated from Eqs. (16), (21), and (23) using the following parameters r,i = r = 1, m = 1, fej = 2.02 V , = -0.57 V...

Figure 6 shows the effect of surfactant concentration on interfacial tension and electrophoretic mobility of oil droplets (14). It is evident that the minimum in interfacial tension corresponds to a maximum in electrophoretic mobility and hence in zeta potential at the oil/brine interface. Similar to the electrocapillary effect observed in mercury/water systems, we believe that the high surface charge density at the oil/brine interface also contributes to lowering of the interfacial tension. This correlation was also observed for the effect of caustic concentration on the interfacial tension of several crude oils (Figure 7). Here also, the minimum interfacial tension and the maximum electrophoretic mobility occurred in the same range of caustic concentration (17). Similar correlation for the effect of salt concentration on the interfacial tension and electrophoretic mobility of a crude oil was also observed (18). Thus, we believe that surface charge density at the oil/brine interface is an important component of the ultralow interfacial tension.

The interfacial tension of the mercury-water system yhsw)- Compare the result with the experimental value, which is in the range 415-426 mN m at 20 °C. 

Using appropriate data from Table II-9, calculate the water-mercury interfacial tension using the simple Girifalco and Good equation and then using Fowkes modification of it. 

The diffusion current Id depends upon several factors, such as temperature, the viscosity of the medium, the composition of the base electrolyte, the molecular or ionic state of the electro-active species, the dimensions of the capillary, and the pressure on the dropping mercury. The temperature coefficient is about 1.5-2 per cent °C 1 precise measurements of the diffusion current require temperature control to about 0.2 °C, which is generally achieved by immersing the cell in a water thermostat (preferably at 25 °C). A metal ion complex usually yields a different diffusion current from the simple (hydrated) metal ion. The drop time t depends largely upon the pressure on the dropping mercury and to a smaller extent upon the interfacial tension at the mercury-solution interface the latter is dependent upon the potential of the electrode. Fortunately t appears only as the sixth root in the Ilkovib equation, so that variation in this quantity will have a relatively small effect upon the diffusion current. The product m2/3 t1/6 is important because it permits results with different capillaries under otherwise identical conditions to be compared the ratio of the diffusion currents is simply the ratio of the m2/3 r1/6 values. 

Kakiuehi et al. [84] studied the adsorption properties of two types of nonionic surfactants, sorbitan fatty acid esters and sucrose alkanoate, at the water-nitrobenzene interface. These surfactants lower the interfacial capacity in the range of the applied potential with no sign of desorption. On the other hand, the remarkable adsorption-desorption capacity peak analogous to the adsorption peak seen for organic molecules at the mercury-electrolyte interface can be observed in the presence of ionic surfactants, such as triazine dye ligands for proteins [85]. 

Electro Capillarity and the dropping Mercury Electrode. The term electro capillarity derives from the early application of measurements of interfacial tension at the Hg-electrolyte interface. The interfacial tension, y, can be measured readily with a dropping mercury electrode. E.g., the life time of a drop, tmax. is directly proportional to the interfacial tension y. Thus, y is measured as a function of y in presence and absence of a solute that is adsorbed at the Hg-water interface this kind of data is amenable to thermodynamic interpretation of the surface chemical properties of the electrode-water interface. 

The study of the interfacial liquid-liquid phase however is complicated by several factors, of which the chief is the mutual solubility of the liquids. No two liquids are completely immiscible even in such extreme cases as water and mercury or water and petroleum the interfacial energy between two pure liquids will thus be affected by such inter-solution of the two homogeneous phases. In cases of complete intersolubility there is evidently no boundary interface and consequently no interfacial energy. On addition of a solute to one of the liquids a partition of the solute between all three phases, the two liquids and the interfacial phase, takes place. Thus we obtain an apparent interfacial concentration of the added solute. The most varied possibilities, such as positive or negative adsorption from both liquids or positive adsorption from one and negative adsorption from the other, are evidently open to us. In spite of the complexity of such systems it is necessary that information on such points should be available, since one of the most important colloidal systems, the emulsions, consisting of liquids dispersed in liquids, owe their properties and peculiarities to an extended interfacial phase of this character. 

In Ref 169, some peculiarities associated with adsorption of alkyne peroxides from DM F-water solutions onto the mercury electrode in the presence of tetraethylammonium cations have been described. Polarography and electrocapillary measurements were employed as the experimental techniques. It has been shown that interfacial activity of these peroxides was determined by the species generated as a result of associative interactions between peroxides and DMF and tetraethylammonium cations. 

Consider a simple interfacial region at a mercury/solution interface. The electrolyte is 0.01 M NaF and the charge on the electrode is 10 iC negative to the pzc. The zeta potential is -10 mV on the same scale. What is the capacitance of the Helmholtz layer and that of the diffuse layer Galculate the capacitance of the interfaces. Take the thickness of the double layer as the distance between the center of the mercury atoms and that of hydrated K+in contact with the electrode through its water layer. (Bockris)... 

EXAMPLE 6.5 Estimation of Interfacial Tensions Using the Girifalco-Good-Fowkes Equation. The following are the interfacial tensions for the various two-phase surfaces formed by n-octane (O), water (W), and mercury (Hg) for n-octane-water, y = 50.8 mJ m 2 for n-octane-mercury, y = 375 mJ m 2 and for water-mercury, y = 426 mJ m 2. Assuming that only London forces operate between molecules of the hydrocarbon, use Equation (100) to estimate y d for water and mercury. Do the values thus obtained make sense Take y values from Table 6.1 for the interfaces with air of these liquids. 

Mercury Injection data revealed a porosity of 30 % and a bimodal pore size distribution with pore size maxima at 20 and 110 nm. The capillary displacement pressure (Pd) for mercury was 2.7 MPa corresponding to an equivalent value of 0.5 MPa. For the conversion from the mercury-air to the gas-water system the following parameters were used interfacial tension values of p(Hg-air) = 480 mN/m, and p(N2-water) = 70 mN/m contact angles (Hg-air) = 141°, and 6l(N2-water) = 0°. 

Table 3.1 shows some values of surface and interfacial tensions. It can be seen, for example, that mercury has a greater cohesive energy than does water, which is in turn greater than that of benzene. Thus ... 

The contact angle between two liquids and air varies,3 with an applied potential, in a manner very similar to the interfacial tension between mercury and water this is to be expected, since the angle depends on the three surface tensions meeting at the line of contact, of which the liquid-liquid tension probably varies most as the potential is changed. 

The capillary tube method can be used to determine the interfacial tension Gi2 between two immiscible, or partially miscible liquids (Fig. 12.VIII G.) The drop weight method ( 14.VIIIG) has also been used. Bartell, Case, and Brown measured the interfacial tensions between mercury and organic liquids by the capillary tube and the drop weight methods and found that the two methods gave the same results. Some values for water are also given. Values in dynes/cm. are ... 

The interfacial tension between water and mercury is 426-427 dynes/cm. in absence of oxygen, but if measured in presence of air it varies between 375 and 427. The effect of pressure on interfacial tension varies with the pressure and may be positive (increasing a) or negative withp in lb./in.2 the values of (100/or)(do /d ) at about 5000 atm. are Hg/H2O+0 74, Hg/ether+1-23, water/ether—20-73, chloroform/water—0-73, carbon disulphide/water+2 37. The interfacial tension between two liquids vanishes at the critical solution temperature.4... 

L) values for water and mercury have been determined by measuring the interfacial tension of these liquids with a number of liquid-saturated hydrocarbons. The inteimolecular attraction in the liquid hydrocarbons is entirely due to London-van der Waals dispersion forces for all practical purposes. Yjd was derived from contact angle measurements. 

Equation (645) shows that contact angle is a thermodynamic quantity, which can be related to the work of adhesion and interfacial free energy terms. When 6 values are small, the work of adhesion is high and considerable energy must be spent to separate the solid from the liquid. If 0 = 0°, then W L = 2yv if 0 = 90°, then W L = yLV, and if 0 = 180°, then W1L = 0, which means that no work needs to be done to separate a completely spherical mercury drop from a solid surface (or a water drop from a superhydrophobic polymer surface), and indeed these drops roll down very easily even with a 1° inclination angle of the flat substrate. 

Capillary Rise The tendency and process for a liquid to rise in a capillary. Example Water rises in a partially immersed glass capillary. Negative capillary rise occurs when the liquid level in the capillary falls below the level of bulk liquid, as when a glass capillary is partially immersed in mercury. Capillary rise forms the basis for a method of determination of surface or interfacial tension. 

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