The Surface Tension of Solutions

In the case of pure liquids the surface phase is naturally of the same composition as the hulk phase for solutions on the other hand the composition of the surface phase need not necessarily be identical with that of the underlying solution. In general the addition of a solute to a solvent will affect the surface tension of the latter and since the energy of the system strives towards a minimum there will be a tendency if the solute lowers the surface tension of the solvent for the former to accumulate at the surface. If the solute elevates the surface tension of the solvent the reverse action occurs and the surface phase will be poorer in solute than the bulk phase. This enrichment or impoverishment of the surface phase does not continue indefinitely, for a point is reached when the action is balanced by the return from the enriched to the impoverished phase due to diffusion, the rate of which is proportional to the difference in osmotic pressures of the solute in the two 

The exact mathematical treatment for the calculation of the excess or deficiency of solute in the superficial phase was first made by Gibbs and independently a year later by J. J. Thomson, and Gibbs equation may be regarded as the fundamental basis for the thermodynamical treatment of interfacial phases. 

Gibbs equation, which is perfectly general, may be deduced readily from the potential functions of Gibbs Thermodynamics, p. 221) and Duhem Le Potential Thermodynamique, Paris 1886), or in the following manner. 

AT is a vessel which encloses a system in equilibrium. IT is a closed surface within the vessel which contains a volume F. 

For any infinitesimal change in a reversible system, the change in energy may be written 

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The influence of the presence of alcohols on the CMC is also well known. In 1943 Miles and Shedlovsky [117] studied the effect of dodecanol on the surface tension of solutions of sodium dodecyl sulfate detecting a significant decrease of the surface tension and a displacement of the CMC toward lower surfactant concentrations. Schwuger studied the influence of different alcohols, such as hexanol, octanol, and decanol, on the surface tension of sodium hexa-decyl sulfate [118]. The effect of dodecyl alcohol on the surface tension, CMC, and adsorption behavior of sodium dodecyl sulfate was studied in detail by Batina et al. [119]. 

Values of the surface tension of solutions of Paludrine as a function of concentration have been recorded (202). 

In the previous sections we have noted that the hypothesis of a unimolecular Gibbs layer for solutions of liquids of markedly different internal pressures together with the equation of Gibbs leads to values for molecular areas and thicknesses which are not at all unreasonably different from those determined by means of X-ray measurements, or from a study of insoluble substances on the surface of water, but cannot be said to be identical within the limits of experiment. In one respect, however, such soluble films differ from the insoluble films which we shall have occasion to examine in the next chapter the surface tension of solutions which according to the Gibbs adsorption equation... 

The results show that for a mixed solution, varies with the surface tension of solution. This is reasonable because the denseness of surface molecular packing is not the same at different 7 t the lower the O, the denser the packing and the greater the molecular interaction. This is most obvious in syetem of large negative j3,-values and less obvious or even vague in the case of weak interactions (such as in CyFNa C to SNa system). 

Spremulli, G. H. 1942. A study of the effects of time, buffer, composition, specifications, and ionic strength on the surface tension of solutions of /3-lactoglobulin. Pub. 510. University of Michigan Microfilms, Ann Arbor, Michigan, p. 130. 

Addition of alkali suppresses the hydrolysis, and when sufficient alkali has been added for complete suppression of hydrolysis, the adsorbed layer consists of neutral soap. This is probably the state of affairs at the maxima of surface tension in Fig. 32. More alkali is required to reach the maximum with the stronger solutions, because more alkali is needed to suppress hydrolysis completely. The maximum surface tensions in Fig. 32 are probably very near to the surface tension of solutions of neutral soap only of the concentration indicated on each curve. The subsequent slow fall of tension, as more alkali is added, is probably due to a salting out of the soap by the alkali, an increase in escaping tendency caused by the presence of comparatively large amounts of another solute. It would be interesting to find whether addition of neutral salt, in addition to the small amount of alkali needed to reach the maximum, produces a fall in tension similar in amount to that given by additional alkali. 

The surface tension seems more likely to be a function of solvent compositions, but is negligibly dependent on the solution concentration. Different solvents may contribute different surface tensions. However, not necessarily a lower surface tension of a solvent will always be more suitable for electrospinning. Generally, surface tension determines the upper and lower boundaries of electrospinning window if all other variables are held constant. The formation of droplets, bead and fibers can be driven by the surface tension of solution and lower surface tension of the spinning solution helps electrospinning to occur at lower electric field [57],... 

Surfactants and block polymers useful for lowering the surface tension of solutions have two components the hydrophobe, which has a lower surface tension and is usually insoluble in aqueous solutions, and the hydrophile, which is the more compatible component. The lowering of surface tensions of solutions provides evidence of the degree of surface activity of the hydrophobe but is a less reliable way of inferring surface activity compared with direct surface tension measurement of the hydrophobic material, because surface tension lowering depends also on the concentration of surfactant used, the type and relative proportions of hydrophobe and hydrophile, the overall molecular weight of the surfactant, and the solvent used. Nevertheless, even the best surfactant of a given class cannot perform beyond certain limits, and these limits offer a useful measure of surface activity. 

That the surface tensions of solutions of d- and /-optical isomers aredififerent seems doubtful. Surface tensions of normal alkanes (paraffin hydrocarbons) containing n atoms of carbon are given by a —l4-6 log ( —3)+ll 52 within experimental error. 

A difficulty encountered in the measurement of the surface tension of solutions is that it is often different when measured by so-called dynamic methods (vibrating jets, etc.), in which the value for a freshly-formed surface is measured rapidly, and when measured by so-called static methods (capillary rise, etc.), which determine the value for a surface which has been in existence for some time. The difference is due to the fact that the composition of the surface is different from that in the bulk of the solution, and in a fresh surface a change of concentration occurs, which, as it involyes diffusion, usually occurs slowly, so that rapid measurements give results different from those which deal with a surface which has come into equilibrium. In capillary active solutions, the surface is enriched in solute, whilst in capillary inactive it is usually richer in solvent. In the case of electrolyte solutions, the surface layer is considered to consist of a unimolecular layer of solvent molecules. The thermodynamic theory was established by Gibbs, and indicates that when the solute... 

In some cases (dyes, etc.) the adsorption of solute in the surface may be so great that a solid film may be formed. The surface tensions of solutions containing hybrid ions may be greater or less than that of water.3 The change of surface tension of a solution with time was found by McBain, Ford, and Mills to be very slow, many days being needed to attain the niaximum effect,... 

Values closer to 2-12 are found if allowance is made for dissociation in calculating M. This would indicate that no change in association is produced by the presence of the electrolyte in the water. The Eotvos constants of binary mixtures seem to depend on the concentration and temperature. The effect of temperature is either (i) normal, when d[a MvY>mdt is about 2-1, or (ii) abnormal, when this coefficient is less than 2-1 ljut increases with temperature from these results, conclusions have been drawn as to the molecular weights of dissolved substances. Light (including ultraviolet) has no influence on the surface tension of solutions. ... 

In order to measure the surface tension of solutions containing surfactants, the maximum bubble pressure, pendant drop and Wilhelmy plate (immersed at a constant depth) methods are suitable capillary rise, ring, mobile Wilhelmy plate, sessile drop and drop weight methods are not very suitable. These methods are not recommended because surfactants preferably adsorb onto the solid surfaces of capillaries, substrates, rings, or plates used during the measurement. In a liquid-liquid system, if an interfacially active surfactant is present, the freshly created interface is not generally in equilibrium with the two immiscible liquids it separates. This interface will achieve its equilibrium state after the redistribution of solute molecules in both phases. Only then can dynamic methods be applied to measure the interfacial tension of these freshly created interfaces. 

Szyszkowski (1908-1909) carried out precise measurements of the surface tension of solutions containing various carboxylic acids ranging from butyric to caproic, as well as their isomers. He managed to find an empirical relationship that described with high precision all of his results ... 

First, we note that the line shape of the plot of surface tension for UI in Fig. 2C differs from that of I" and II The surface tension of aqueous solutions of III decreases abruptly at a concentration of 0.003 mM whereas the surface tensions of solutions of I" " and IIdecrease gradually with increasing concentration. The adsorption isotherm of III cannot be described by the Langmuir model of adsorption. The limiting area occupied by III" ( < 28 A /molec) is also substantially less than either I (55 A /molec) or II (85 A /molec). 

Jones G, Ray J (1939) A theoretical and experimental analysis of the capillary rise method for measuring the surface tension of solutions of electrolytes. J Am Chem Soc 59 187... 

Impurities in the technical-grade gum lower both the surface tension and the interfacial tension. Presumably, the same impurities are responsible for the dispersant properties of the technical-grade gum. The surface tension of solutions of the technical-grade gum decreases as the pH is lowered addition of electrolytes also lowers the surface tension. The solution pH of technical-grade products is 4.0 to 4.5 (9, 25). The pH values of solutions of commercially purified preparations will vary with the method of purification (21). 

Peper [195] found that fatty acids reduce the surface tension of solutions of sodium dodecylbenzene sulfonate containing calcium chloride. The effect was found to be least pronounced for the fatty acid that forms the least soluble calcium soaps and the best antifoam. It is difficult to reconcile this finding with a Marangoni spreading mechanism for antifoam action (see Section 4.4.2). 

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