Tension, interfacial solutions

Lewis, as already mentioned, used a solution of sodium glycocholate and determined the adsorption of the salt by a surface of paraffin oil. The drop method just discussed, and the [Pg.42]

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

The theoretical treatments of Section III-2B have been used to calculate interfacial tensions of solutions using suitable interaction potential functions. Thus Gubbins and co-workers [88] report a molecular dynamics calculation of the surface tension of a solution of A and B molecules obeying Eq. III-46 with o,bb/ o,aa = 0.4 and... 

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. 

Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

Yaminsky and Yaminskaya [114] have used a Wilhelmy plate to directly measure the interfacial tension (and hence infer the contact angle) for a surfactant solution on... 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

Larch gum is readily soluble in water. The viscosity of these solutions is lower than that of most other natural gums and solutions of over 40% soHds are easily prepared. These highly concentrated solutions are also unusual because of their Newtonian flow properties. Larch gum reduces the surface tension of water solutions and the interfacial tension existing in water and oil mixtures, and thus is an effective emulsifying agent. As a result of these properties, larch gum has been used in foods and can serve as a gum arabic substitute. 

Dispersion is the process of wetting the surface of the metal, thereby penetrating the oil film. Surfactants can reduce the surface tension and interfacial tension of the cleaning solution at the metal—Hquid interface. As the cleaner undercuts and penetrates the oil, the cleaner breaks the oil into small droplets which then float to the surface. 

Critical Micelle Concentration. The rate at which the properties of surfactant solutions vary with concentration changes at the concentration where micelle formation starts. Surface and interfacial tension, equivalent conductance (50), dye solubilization (51), iodine solubilization (52), and refractive index (53) are properties commonly used as the basis for methods of CMC determination. 

Like the methylceUuloses, water solutions of HPC display greatly reduced surface tension. A 0.1% solution of HPC at 25°C has a surface tension of about 44 mN/m(=dyn/cm) (water is 74.1 mN/m) and interfacial tension of about 12.5 mN/m(=dyn/cm) against mineral oil. The molecular weight of the HPC has only a slight effect on the surface tension. 

Foam Production This is important in froth-flotation separations in the manufac ture of cellular elastomers, plastics, and glass and in certain special apphcations (e.g., food products, fire extinguishers). Unwanted foam can occur in process columns, in agitated vessels, and in reactors in which a gaseous product is formed it must be avoided, destroyed, or controlled. Berkman and Egloff (Emulsions and Foams, Reinhold, New York, 1941, pp. 112-152) have mentioned that foam is produced only in systems possessing the proper combination of interfacial tension, viscosity, volatihty, and concentration of solute or suspended solids. From the standpoint of gas comminution, foam production requires the creation of small biibbles in a hquid capable of sustaining foam. 

Even if the interfacial tension is measured accurately, there may be doubt about its applicability to the surface of bubbles being rapidly formed in a solution of a surface-active agent, for the bubble surface may not have time to become equihbrated with the solution. Coppock and Meiklejohn [Trans. Instn. Chem. Engrs., 29, 75 (1951)] reported that bubbles formed in the single-bubble regime at an orifice in a solution of a commercial detergent had a diameter larger than that calculated in terms of the measured surface tension of the solution [Eq. (14-206)]. The disparity is probably a reflection of unequihbrated bubble laminae. 

Inteifacial tension. A high interfacial tension promotes rapid coalescence and generally requires high mechanical agitation to produce small droplets. A low interracial tension allows drop breakup with low agitation intensity but also leads to slow coalescence rates. Interfacial tension usually decreases as solubility and solute concentration increase and falls to zero at the plait point (Fig. 15-10). 

Nielsen, A.E. and Sohnel, O., 1971. Interfacial tensions, electrolyte crystal aqueous solution, from nucleatioii data. Journal of Crystal Growth. 11, 233-242. 

A typical example of an ideal polarizable interface is the mercury-solution interface [1,2]. From an experimental point of view it is characterized by its electrocapillary curve describing the variation of the interfacial tension 7 with the potential drop across the interface, 0. Using the thermodynamic relation due to Lippmann, we get the charge of the wall a (-a is the charge on the solution side) from the derivative of the electrocapillary curve ... 

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. 

The log of the reciprocal of the bulk concentration of surfactant (C in mol/ L) necessary to produce a surface or interfacial pressure of 20 raN/m, log( 1 / On= 20 i e > a 20 mN/m reduction in the surface or interfacial tension, is considered a measure of the efficiency of a surfactant. The effectiveness of surface tension reduction is the maximum effect the surfactant can produce irrespective of concentration, (rccmc = [y]0 - y), where [y]0 is the surface tension of the pure solvent and y is the surface tension of the surfactant solution at its cmc. 

The surface and interfacial tension of a great number of ester sulfonates has been measured by Stirton et al. [26-28,30]. The values of the surface tension of 0.2% solutions at 25 °C are in the range from 25 to 50 mN/m and from 2 to 20 mN/m for the interfacial tension. In the group with the same number of C atoms the pelargonates and laurates have the lowest values. Among the esters of the same a-sulfo fatty acid, the surface and interfacial tension decreases with increasing molecular weight of the alcohols. Surface tension values also depend on the cation. For the alkali salts the values decrease from lithium to sodium to potassium. 

For long-chain alcohol esters it is interesting to see that the interfacial tension between a 0.01 wt % aqueous solution and octane or xylene has a minimum for ester sulfonates with a total 22 carbon atoms in the fatty acid chain and the ester chain [60]. The balance in length between the two chains has only a poor effect. Thus, a-sulfonated fatty acid esters with a total number of 22-26 carbon atoms in the molecule have excellent interfacial activities. To attain the same magnitude in the interfacial tension between linear alkylbenzenesulfonate (LAS) solution and octane, the required concentration of LAS is 0.1 wt %. This is 10 times the concentration needed for a-sulfonated fatty acid esters [60]. 

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