Surface Tensions of Solutions

The capillary-rise method is used to study the change in surface tension as a function of concentration for aqueous solutions of / butanol and sodium chloride. The data are interpreted in terms of the surface concentration using the Gibbs isotherm. 

If a body of material is homogeneons, the valne of any extensive property is directly proportional to the quantity of matter contained in the body  

It is known however that the values of extensive properties of bodies often thought of as homogeneous are not always independent of the surface area. In fact liquid bodies with surfaces are in general not entirely homogeneous, for the value of a given intensive quantity (say 3i/) in the region of the surface may at equilibrium deviate from the value that this quantity has in the bulk of the solution (Qy). We can write 

This quantity represents the excess, per unit area of surface, of the quantity Q over what Q would be for a perfectly homogeneous body of the same magnitude with Qv = Qy throughout see Fig. 1. In some cases may be negative, corresponding to a deficiency rather than an excess. 

If the body has surfaces of several different kinds (such as a crystal with different kinds of crystal faces or a liquid with part of its surface in contact with the air and the remainder of its surface in contact with sohd or other liquid phases), the parenthesized quantity in Eq. (2) is independent of surface positional coordinates only within the bound-aries of each kind of surface. Each kind of surface will have in general a different and we write in such a case 

Work is done when any solution is diluted in a reversible manner. Let v be the volume and tt the osmotic pressure of the solution. The maximum work done when the solution is diluted isothermally by d-y is tt dv. If we now allow for the fact that the surface of the solution is increased by da during the dilution (which we may suppose to be performed by isothermal distillation from a large volume of the solvent), we must subtract the work yda due to the surface tension. Hence the free energy of the solution diminishes during dilution by 

This equation states that the osmotic pressure of a solution 

As — is positive for all solutions, u and are always opposite in sign. The surface tension of a solution will therefore increase Digitized by Microsoft  

In dilute solutions we may apply van t Hoff s law tt = RT. c. Substituting this in equation (5), we obtain 

Equations (5) and (6) have been confirmed qualitatively by several investigators. A summary of the literature will be found in Freundlich s book. The equations have recently been tested in a quantitative manner by W. C. M C. Lewis. He investigated the surfaces between two non-miscible liquids, such as mercury and an aqueous solution, to which the same relationships apply. In some cases he obtained an entire confirmation of equation (6). In other cases, particularly when electrolytes were present, he found deviations, which he was able to explain, however, as being due to electrocapillarity. 

---

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]. 

SOL.7. 1. Prigogine, Sur la tension superficieUe des solutions de molecules de dimensions differentes (On surface tension of solutions of molecules of different sizes), J. Chim. Phys. 47, 35-40 (1950). 

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

Sucrose (20) was also used for the reaction of telomerization with 1 [19, 49, 50]. Applying specific conditions, sucrose octadienyl ethers were obtained with an average degree of substitution of 4.7-5.3. These products are practically insoluble in water, clear or almost colorless, and present a viscosity of 1500-2000 cPs at 25°C [39]. These properties confer to these products the possibility of being employed as emulsifiers or defoaming agent [49]. Minimum surface tension of solution of substituted sucrose in water is 25-28 mN/m whatever the degree of substitution... 

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). 

A Multipurpose Apparatus to Measure Viscosity and Surface Tension of Solutions 212... 

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. 

---