Surface tension impurity effects

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettahle monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal hahit in particle formation. Minimize surface roughness in milhng. 

The effect of an impurity on the surface energy is often discussed in terms of the surface activity of the impurity BJb, defined as the slope of the surface tension or energy versus composition at infinite dilution ... 

The interfacial surface tension may be markedly affected by a surface reaction occurring in the presence of traces of impurities. A good example of such an effect is to be noted in the alteration of the interfacial surface tension between an oil containing a fatty acid and water containing acid or alkali. For acid solutions the interfacial surface tension remains constant and almost independent of the Ph of the aqueous phase. As the solution becomes more alkaline the carboxyl groups of the acid commence to dissociate and the interfacial surface tension falls rapidly. (Hartridge and Peters, Proc. Boy. Soc. A, Cl. 348, 1922, see p. 252.)... 

As for other types of fluid particle, the internal circulation of water drops in air depends on the accumulation of surface-active impurities at the interface (H9). Observed internal velocities are of order 1% of the terminal velocity (G4, P5), too small to affect drag detectably. Ryan (R6) examined the effect of surface tension reduction by surface-active agents on falling water drops. 

Decanolc acid was purchased from Aldrich Chemicals, Gold Label, and from BDH, special pure grade. BE was obtained from Shefford Chemicals. It was distilled and kept over molecular sieves. The preparation of NaDec and Its purification are the same as In previous studies (16). The purity of NaDec was checked by surface tension. A minimum In surface tension Is observed which disappears when NaDec Is prepared at a pH higher than 11. This suggests that the Impurity In NaDec Is most probably free decanolc acid. In consideration of the previous discussion (16), NaDec was prepared at a pH of 9.2 and used as such, since a trace of decanolc acid should have less effect on the thermodynamic properties than an excess of NaOH. 

Thus, melting of a crystalline substance without superheating is a superficial effect. Pre-melting phenomena are apparently also related to the formation of liquid films on the surfaces of crystals, if not to other incidental causes (for example, impurities), and are not pertinent to Frenkel s theory. Heterophase fluctuations are quite large where the difference between two phases and the surface tension between them tend to zero—near the critical point and near the Curie point. The first case is commonly known, the second was earlier investigated quantitatively in Landau s fine work [13, 14]. 

The formation of an adsorbed surface layer is not an instantaneous process but is governed by the rate of diffusion of the surfactant through the solution to the interface. It might take several seconds for a surfactant solution to attain its equilibrium surface tension, especially if the solution is dilute and the solute molecules are large and unsymmetrical. Much slower ageing effects have been reported, but these are now known to be due to traces of impurities. The time factor in adsorption can be demonstrated by measuring the surface tensions of freshly formed surfaces by a dynamic method for example, the surface tensions of sodium oleate solutions measured by... 

An interesting effect arises when the surfactant is contaminated with surface-active impurities. A pronounced minimum in the surface tension-log c plot is observed at the cmc, which would seem to be an apparent violation of the Gibbs equation, suggesting a desorption (positive dy/d[logc] value) in the vicinity of the cmc. The minimum in fact arises because of the release below the cmc of the surface-active impurities on the breakup of the surfactant micelles in which they were solubilised. 

Properties (pure anhydrous) density of solid, 1.71 g/cc, density of liquid 1.450 g/cc at 20C, viscosity, liquid 1.245 cP, surface tension 80.4 dynes/cm at 20C, fp -0.41C, bp 150.2C. Soluble in water and alcohol. (Solutions) pure hydrogen peroxide solutions, completely free from contamination, are highly stable a low percentage of an inhibitor such as acetanilide or sodium stannate, is usually added to counteract the catalytic effect of traces of impurities such as iron, copper, and other heavy metals. A relatively stable sample of hydrogen peroxide typically, decomposes at the rate of approximately 0.5% per year at room temperature. 

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. 

Yest- Yexpt)/Yexpt ) was ca.+ 10%. Undoubtedly, much of the uncertainty arises from experimental errors, where the effect of unreported surface active impurities and the nature of the gaseous atmosphere could have a marked effect on the value of surface tension. Another major source of error is the amount of Fe203 present in the slag, and this investigation has shown clearly that Fe203 is very surface active. Few investigators report the (Fe3+/Fe2+) ratio which is dependent upon, (i)P02, ) T and (iii) the... 

Unfortunately, good surface tension data in this region are difficult to obtain, since traces of impurities adsorbed from the air or present in the solvent or in the surfactant can markedly affect the results. Second, there are only a few studies in the literature on this region of the surface tension-concentration curves, since investigators of the effect of surfactants on the surface tension of solvents generally are interested in the region where surfactants show the maximum effect, rather than the region where they show little effect. 

Overviews of attempts to quantify the effect of impurities and to derive criteria for the purity of a liquid system for interfacial studies was given by Lunkenheimer (1984) and Miller (1987). It was shown that the absence of a minimum in a surface tension isotherm, y as a function of log(Co) is the criterion most frequently used to judge the purity of a surfactant. On the other hand, it had been shown by Krotov Rusanov (1971) that this is not sufficient. Under certain conditions such a minimum can disappear by some compensating effects as well as by addition of electrolyte (Weiner Flynn 1974). In addition, there are surfactants which do not form micelles or are not sufficiently soluble so that higher concentrations cannot be reached. 

Soluble substances exist which can immobilise the surface even of large bubbles present in water at extremely low concentrations. The problem of the effect of residual mobility of a bubble surface loses its meaning if these impurities are contained in water. However, their influence on surface mobility can be hampered at retarded adsorption kinetics. At a given surface tension decrease due to impurities a critical bubble dimension exists. For bubbles exceeding the critical size a residual surface mobility is present. Eq. (10.40) interrelates the critical bubble size with the surface tension drop. On the basis of Eq. (10.45) it was shown that residual mobility is important even for highly contaminated river water at high Reynolds numbers (cf. Section 10.2.7). 

It is well known that a small amount of impurity can profoundly affect the nucleation rate, however, it is impossible to predict the effect prior. The presence of additives can either enhance or inhibit the solubility of a substance. Enhanced solubilities would lead to lower supersaturations and lower growth rates. If it is postulated that the impurity adsorbs on the crystal surface, then two opposing effects come into play—the presence of an additive would lower the surface tension and lead to higher growth rates, however, the impurity adsorption blocks potential growth sites and lowers nucleation rates. Thus, the effect of impurities is complex and unpredictable. 

The position of point A in the curve shown in Fig. 1.7 is characteristic for a sufficient purity of the surfactant and solvent. The point is located at a definite surface tension and concentration (CMC). As a rule, a minimum in the y(c) or y(log c) plot is evidence of traces of highly surface-active impurities which affect the results [147]. In chapters 2 to 4 the effect of lateral molecular interaction with increasing alkyl chain length, the effects of changes in orientation and conformation, essentially shown by polymeric surfactants such as proteins, may also lead to significant changes in the adsorption behaviour. 

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