Effects on the Surface Tension

6 Ionic Effects on the Surface Tension The surface layer of the aqueous solutions, of thickness lnm, has air (or dilute water vapors) on the one side and bulk water on the other. Ions may be positively or negatively sorbed in this layer, depending on whether they decrease or increase the snrface tension, a, of water. According to the Gibbs adsorption law  

The values of exhibit some clear trends that are independent of the arbitrary assumption of the values for Na (and/or Cl ) since they pertain to sequences among cations separately from those among anions. One feature is obvious negative values of are rare, meaning that most (small) ions are desorbed from the surface layer and their concentration in it is lower than in the bulk solution 0 for 0). Outstanding 

As mentioned, cations with L 0 are repelled from the surface (Cj 0), and if k k, they are repelled more than the anions. This causes a charge imbalance in the surface layer leading to the establishment of an electric double layer. The surface potential of electrolyte solutions over that of pure water (with respect to vacuum/air/ dilute water vapor), A A f, was measured as a function of the electrolyte concentration. The available ISlS/ values at 1M MX, for M=, Na , and NH4 with a variety 

TABLE 2.12 Ionic Static Permittivity Decrements, 5uj/dm mol [130] and Surface Tension Increments, dff/dc/mN-m -moP -dm year, in Aqueous Solutions at 20-30°C 

This means that the surface potential is dominated by the ion sorption/desorption at the surface and shows the practical value of the splitting of the electrolyte molar k into the individual ionic values. 

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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. 

HI5. Hurd, R. M., Schmid, G. M., and Snavely, E. S., Electrostatic fields their effect on the surface tension of aqueous salt solutions, Science 135, 791 (1962). 

Such an explanation cannot however account for the abnormally high values of the adsorption of simple salts at a water-oil interface, as found by Lewis. If we are to accept these results (though the emulsion method is certainly the least satisfactory of those used for the present purpose) some secondary effect must be looked for, such as the increased solubility of oil in water when emulsified and in the form of small particles. (The mean radius in one experiment was found to be about 4 x 10 cms., a size at which Hulett and Ostwald (see page 166) found a notable increase in the solubility of calcium and barium sulphates.) It is probable however that the chief error introduced was the disturbance brought about by the effect on the surface tension of the free charges of the salt ions (Lewis, Zeit. Physikal. Chem. Lxxiii. 129, 1910). (See Ch. VII.)... 

If the original liquids are again partially miscible, and the added component soluble in either the mutual solubility may be increased if so the interfacial tension will probably diminish whatever may be the effect on the surface tensions of the two pure liquids. Clearly, if sufficient of the third component be added to make the two phases completely soluble the interfacial tension must disappear altogether. 

Tables IV-XVI show that the tetraalkylammonium salts have a large effect on both solvents in the binary solvent mixture, especially the larger tetraalkylammonium bromides, i.e., (n-C3H7)4NBr and (n-C4Hg)4NBr. This can be seen from consideration of the boiling temperature alone. This observation is also borne out by the surface tensions and solubilities at 25°C of the individual salts studied, the results of which are tabulated in Table XVII in water, in ethanol, and in an ethanol-water mixture at x = 0.206. For the higher homologs of the R4NBr series, these salts exert a large effect on the surface tensions of the solvent systems studied and show a marked increase in their solubility in ethanol.

The most widely applied LC method for the separation of peptides and proteins is reversed-phase LC (RPLC). A typical mobile-phase composition is an acetonitrile-water gradient with a fixed concentration of TFA (typically 0.05-0.5%). TEA acts as an ion-pairing agent enhancing the retention of peptides and proteins, but also masks secondary interactions with the silica-based stationary phase. TFA is a volatile additive, but due to its ion-pairing properties and effect on the surface tension, it may significantly suppress the ESI response in positive-ion mode. 

Values of Yq are often taken to be the surface tension of the pure components, Y and have also been obtained by iterative procedures. Figure 4a shows a typical plot of Y as a function of x for a binary slag and the individual x Yi contributions have also been included. These methods work well for certain slag mixtures but break down when surface-active constituents, such as P205 are present. These components migrate preferentially to the surface and cause a sharp decrease in the surface tension and consequently only very small concentrations are required to cause an appreciable decrease in Y. Thus some unreported or undetected impurity could have a marked effect on the surface tension of the slag and thereby produce an apparent error in the value estimated by the model. In this respect surface tension differs from all the other physical properties which are essentially bulk properties. 

Hydrophobic Effect on the Surface Tension and Interfacial Tension... 

For DOC, it can be seen that the results of Williams (1967), for example, show an extra 2.1 g m DOC in the microlayer. If the thickness of the water film obtained with the screen device is taken to be —200 pm, the surface excess of DOC can be calculated as 2.1 X 200 X 10 = 4.2 X 10" g m . A reasonable lower limit to take for the molecular weight of this extra organic material in the surface film is that of a relatively short-chain acid or alcohol with —14 carbon atoms, equivalent to —170 g mole carbon. Using this minimum value, the area per molecule in the ambient type of films sampled by Williams (1967) can be calculated as > — 170/4.2 X lO" X 6.02 X 10 3 = > 70 A. It can be seen from Fig. 1 that for all surface film types except gaseous films, such an area per molecule has no effect on the surface tension of seawater, as measured by the spreading drop method, or on the damping of capillary waves. Moreover, only relatively water-soluble surfactants remain in the gaseous state at film pressures of —10 N m" ... 

Stable spreading behavior of type 1 results when the additive has only a small effect on the surface tension, lv > deposits a... 

For example, carbon atoms located on branch sites will contribute approximately two thirds as much to the character of a surfactant molecule as one located in the main chain (56). The above-mentioned surfactants [35a-h], [37a-h] follow this general trend. In addition, it has generally been found that the presence of alkyl groups attached to the nitrogen seems to have little effect on the surface tension reduction of a surfactant. This general trend also holds for the y values of the compounds [35a-h], [37a-h]. 

Koberstein, J.T., Jalbert, C. Molecular weight dependence and end-group effects on the surface tension of poly(dimethylsiloxane). Macromolecules 26, 3069-3074 (1993)... 

In a spraying process, a liquid is forced through an orifice (the spray nozzle) to form droplets by the application of hydrostatic pressure. The effect of surfactants and/or polymers on the droplet size spectrum of a spray can be described in terms of their effects on the surface tension. Since surfactants lower the surface tension of the liquid, one would expect that their presence in the spray solution would result in the formation of smaller droplets. However, when considering the role of surfactants in droplet formation, one should consider the dynamics of surfactant adsorption at the air/liquid interface. In a spraying process, a fresh liquid surface is continuously being formed. The surface tension of this... 

Because of the mobility of molecules at fluid interfaces, it is not surprising to find that temperature can have a large effect on the surface tension of a liquid (or the interfacial tension between two hquids). An increase in surface mobility due to an increase in temperature will clearly increase the total entropy of the surface and thereby reduce its free energy, AG. Since the surface tension has been thermodynamically defined as... 

The surface tension has only been reported for PILs with alkylammonium cations, and the values are given in Table 3. Changes to either the cation or anion structures had a similar effect on the surface tension, indicating that both ions are present at the surface and influence the surface tension. Similar behavior has previously been noted by Bagno et al. to occur for AILs. ... 

The synthesis of surfactants in the series of fluoroalkyltetrazoles 102, 103 and the study of their effect on the surface tension of m-xylene was described by Read... 

Bhatia QS, Pan DH, Koberstein JT (1988) Preferential surface adsorption in miscible blends of polystyrene and poly(vinyl methyl ether). Macromolecules 21 2166-2175 171. Jalbert C, Koberstein JT, Yilgor I, Gallagher P, Krukonis V (1993). Molecular weight dependence and end-group effects on the surface tension of poly(dimethylsiloxane). Macromolecules 26 3069-3074... 

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