Effect on surface tension

There is also the possibility of having surface tension affected directly by the presence of an electrostatic field. To some extent this will be a matter of definition since the outward pressure due to a surface charge could be defined as an apparent effect on surface tension. Hurd, Schmid, and Snavely (H15) measured the surface tension of water and water solutions when fields up to 0.7 V/micron were applied across the air-solution interface. The results showed a reduction in surface tension of less than 1 %. These data must not be considered conclusive, however, because insufficient details are reported to permit assessment of the exact nature of the electrostatic field applied or of the validity of a number of corrections that had to be applied but were reported to be very large and difficult to apply. 

Temperature and molecular weight have a significant effect on surface tension (Table 2.3). For example, in the normal hydrocarbon series, a rise in temperature leads to a decrease in the surface tension, but an increase in molecular weight increases the surface tension. A similar trend, that is, an increase in molecular weight causing an increase in surface tension, also occurs in the acrylic series and, to a lesser extent, in the alkylbenzene series. 

Until now, we have avoided considering the above topics by intentionally excluding solutes of variable concentration from our consideration of surfaces. Now, the effects of such solutes are our specific interest. We are especially concerned with a particular class of solutes that show dramatic effects on surface tension. These are said to be surface active and are often simply called surfactants. Our primary emphasis here is on the relationship between adsorp-... 

Heat treatment of milk has little effect on surface tension except that sterilizing treatments cause an increase of a few dynes cm-1 coinciding with grain formation (Nelson 1949). This effect undoubtedly results from denaturation and coagulation of the proteins so that they are no longer effective surface-active agents. 

Thompson et al. (24) also verified that the addition of trace minerals to potato process effluents has little effect on surfactinproductionby B. subtilis ATCC 21332. In fact, the addition of com steep liquor had a detrimental effect on biosurfactant production, whereas the addition of trace minerals had no effect on surface tensions. On the other hand, growth rates were marginally higher with added nutrients. 

Lutskii (1280, 1277, see C.A. 49, 15324h) shows that in/ramolecularly H bonded substances have lower surface tensions than do the isomers that have interaction between molecules. For example, at 131 C the surface tensions of o-, m-, and / -methoxyphenol are 28.8, 33.6, and 34.2 dynes/cm. For the three dimethoxyphenols, the values are all 26 0.5 dynes/cm. This same ortho effect on surface tension (as well as on viscosity, density, and boiling point) is illustrated with many types of compounds in the papers quoted. 

The Saam-Cole approach has several approximations, among which are the neglect of the sohd-adsorbate interaction and curvature effects on the adsorbate chemical potential, and curvature effects on surface tension in symmetrical and asymmetrical states, while modeling the multilayer region. Here, a more accurate version of the above approach has been introduced and tested for explaining the reversibihty of adsorption in MCM-41. For fluid molecules inside a cyhndrical pore of radius R, the incremental potential function has been expressed as [4,6,7]... 

Example Excluded Volume Effects on Surface Tension... 

Pegram LM, Record MT Jr (2007) Hofmeister salt effects on surface tension arise from partitioning of anions and cations between bulk water and the air-water interface. J Phys Chem B 111 5411-5417... 

The effect on surface tension by surfactant adsorption from the bulk solution (Gibbs effect) and by diffusion along an interface (Marangoni effect) is often referred to as the combined Gibbs-Marangoni effect (Figure 11.7). 

For light scattering (bottom curve), the change in solution turbidity indicates the appearance of a scattering species of significantly greater size and optical properties than the monomeric solute. The effect on surface tension (middle curve) has already been introduced, but will be discussed in more detail below. These and many other types of measurement serve as evidence for the formation of aggregates or micelles in solutions of surfactants at relatively well-defined concentrations. 

The surface tension data seem to point toward a trend that could be summarized as follows In general, systematically prolonging the cation alkyl chain reduces surface tension as long as the anion is kept the same. Once the anion is changed for a different type, especially one with larger size or longer alkyl chain, additional anion-specific effects on surface tension have to be considered. This picture is consistent with previously reported results on the effects of both anion size as well as alkyl chain length on surface tension [1, 5, 24]. 

Another possible mechanism involves increases in the surface tension of the droplets as a result of high concentrations of co-eluting compounds (Mallet 2004). This effect would reduce the rate of solvent evaporation, and thus also the probability that the droplet will reach a sufficiently small size that ion evaporation can occur, and also increase the difficulty of ion evaporation itself via the term in the rate constant, see Equations [5.6-5.7]. Another effect that seems to be particularly important for biological sample extracts arises from the presence of involatile solutes, believed (King 2000) to cause ionization suppression not only via a strong effect on surface tension but also via co-precipitation of analyte as the droplets shrink in size. 

A rough calculation supports this explanation [22]. In order to have a substantial effect on surface tension, the surface concentration of surfactant must be close to its saturation value, which for anionic surfactants corre-... 

The solution properties of ordinary salts, such as NaCl, in water are rather simple. One can dissolve at a given temperature a specific amount of NaCl, giving a saturated solution (approximately 5 mol/L). Similarly, the solution characteristics of methanol or ethanol in water are also simple and straightforward. These alcohols mix with water in all proportions. However, the solution behavior of surfactant molecules in water is much more complex. Besides the effect on surface tension, the solution behavior is found to be dependent on the charge of the surfactant. Surfactant aqueous solutions manifest two major forces that determine the solution behavior. The alkyl part being hydrophobic would tend to separate out as a distinct phase, while the polar part tends to stay in solution. The difference between these two opposing forces thus determines the solution properties. The factors that one has to consider are the following ... 

Surface tension is responsible for the shape of liquid droplets, which can be illustrated by dewdrops on a blade of grass (Fig. 6.14). When amphiphile molecules are spread on the water surface, the last layer of water molecules recovers molecular partners above them so that the surface tension is relaxed (Fig. 6.15A). The amphiphiles exert a pressure on the water surface that is proportional to their effect on surface tension. A simple relationship can thus link the surface tension and the surface pressure 3i=yo y, where Ji is the surface pressure... 

In this equation, Aq is the surfactant head group area (i.e., the area taken up on the fluid surface by a single molecule), and K d is a rate constant for surfactant adsorption to the interface. By substituting the Langmuir equation into the previous equation for L(c) and then integrating, we can obtain an expression for the surface tension that includes the adsorption rate constant and the surfactant concentration. In this way, we can relate the addition of a particular surfactant to its effects on surface tension. 

As a model liquid three different polymers polyvinylpyrrolidone (PVP) of different molecular weights are used. These polymers are made available by BASF SE. Luvitec K17 has a mean molecular weight of 9000 Da, Luvitec K30 has 49,000 Da and Luvitec K90 has 1,250,000 Da. The letter K is an abbreviation for the fsT-value, a value that is proportional to the intrinsic viscosity and therefore proportional to the mean molecular weight. Applications of these polymers are found in pharmaceutical and cosmetic industry, because they are not toxic and have a universal solubility, ranged from hydrophilic solvent to hydrophobic solvents [1]. The effect on surface tension is described as low and measurements show a variation between 63 and 67 mN/m, changing with the fsT-value of the polymer. In addition to these solutions glycerin-water mixtures of different mass fractions are used to compare the effect of higher viscosities. 

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