Mercury-water interfacial tension

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. 

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. 

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. 

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. 

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. 

Current theories to explain hysteresis of contact angles are primarily based on the concepts of surface roughness, surface heterogeneity, friction, and adsorption phenomena. Unintentional adsorption, or contamination—the result of inadequate experimental technique—is, however, the most frequent explanation. As all systems involving solids are subject to the reasons indicated above for hysteresis, we chose the system mercury-benzene-water, which should be affected only by adsorption phenomena, controllable under proper experimentation. An additional advantage is the fact that all interfacial tensions involved can be measured. 

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...

The surface and interfacial tensions for a series of liquids are given in the table below. From on that information, predict whether octyl alcohol will spread at the water mercury interface. Will hexane If the alcohol spreads at the water-mercury interface, what molecular orientation do you predict for the alcohol ... 

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.

A new method for the measurement of surface and interfacial tension has been developed based on a video digitising technique to measure the drop profQe of pendant and sessile drops [19]. The method gives a standard deviation of 0.5%, so the resolution, at present, is less than the maximum bubble pressure technique. It is an extremely useful technique for studying the liquid-liquid interface, and electrocapillary curves of similar shape to those obtained on mercury have been obtained for the water-nitrobenzene interface. 

Finally, estimate the interfacial tension between mercury (485 mN m" ) and water and compare the result with the experimental value (426 mN m ). The experimental interfacial tension between mercury and octane (21.8 mN m ) is 375 mN m . ... 

Problem 3.5 Interfacial tensions for Hquid-liquid systems with the Fowkes equation The interfacial tension of mercury with benzene is (at 20 °C) YugB = 357 mN m Using the values given in the table below for the surface tensions of mercury (Hg), benzene (b) and water (w), estimate using the Fowkes equation for the hquid-liquid 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. 

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. 

Despite its simplicity, many success stories have been presented. For example, as already mentioned, using hquid—hquid interfacial tension data for several different mercury—hydrocarbons and water-alkanes, we can estimate ahnosf unique contributions for the dispersion surface tension of mercury (201 mN m ) and water (21.9 mN m ) (Hiemenz and Rajagopalan, 1997 Fowkes, 1964, 1980). These values appear to be reasonable and moreover when used to predict the interfacial tension of water-mercury, quite good agreement is obtained (see Example 3.3 and Problem 3.5). In addition to the success for hquid-hquid interfaces, the Fowkes equation has been used for solid interfaces with good results. As shown in Chapter 6, when combined with the Young equation, we obtain ... 

Problem 15.7 Water and mercury liquid-liquid interfaces with classical theories The tables below present surface tensions for some hquids and interfacial tensions in mixtures with water (72.8 mN m ) and mercury (484 mN m ). AU values are at 20 °C. 

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