Surface tension and concentration

The surface tension data in the case of a homologous series of alcohols and acids show a simple relation to the alkyl chain length (Figure 3.3). It is observed that each addition of the -CH2- group gives values of concentration and surface tension such that the value of concentration is lower by about a factor 3. 

How can Eq. (3.52) be verified For verification, the two variables — concentration and surface tension — need to be determined independently. One way is to use radioactively labeled dissolved substances. The radioactivity close to the surface is measured. /Temi tiers (3H, 14C, 35S) are suitable because electrons only travel a short range, i.e., any recorded radioactivity comes from molecules from the interface, or close below [48],... 

After the creation of a fresh surface (or an almost bare surface) the adsorption increases until equilibrium has been reached. Liquid flow lateral to the surface creates concentration and surface tension gradients which induce additional surface flow. This effect is called the Marangoni-Gibbs effect. 

There is no specificity of the transport stage of microflotation at the condition (10.33) in comparison with a solid sphere. However let us emphasise that the surface retardation is caused by the dynamic state of the adsorption layer, i.e. by the change of the surface concentration and surface tension along the bubble surface. 

When drops or bubbles approach each other their diffusion layers overlap. This leads to local changes in the surface concentration and surface tension which causes liquid to flow into or out of the thick liquid interlayer. The dynamic adsorption layer and its diffusion layer deviate from electroneutrality and contain charges of opposite sign. The charged dynamic adsorption layer and the oppositely charges diffusion layer as a whole are electroneutral. This ensemble can be called the secondary electrical double layer. 

The effect of the drying process parameters (amount of latex fed in, air pressure for spraying the latex, consumption of heat transfer agent, temperature of heat transfer agent, latex concentration and surface tension of the latex) on the particle size of PVC latex obtained by continuous polymerisation of vinyl chloride using 2-3% sodium alkylmonosulphonate is studied. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

Table 3 Characteristic dissolution time (td) and shape parameter values of potassium chloride embedded in white beeswax, containing surfactants at 10% w/w concentration and surface tension of aqueous solutions of surfactants and contact angles of free film samples prepared by melting from wax together with surfactants...

Li and coworkers modeled the surface-bulk pardtitming of SDS in mixed SDS/NaCl particles using Kohler theory and chemical properties of the surfactant, allowing the solute concentration and surface tension of the aerosol to vary as the organic partitiOTied between the gas-aerosol interface and the particle bulk [257]. This partitioning was found to affect both the Raoult and Kelvin terms, leading to a net increase in Sc-... 

Figure 5 shows the relationship between free surfactant concentration and surface tension for the methylmethacrylate system above. As shown in Figure 5, the free surfactant concentration can be determined from knowledge of the surface tension and a surfactant titration curve for the system in question. 

Surface tension of 0.7% HPMC solution decreases on addition of SDS until CAC is reached [Taylor, Thomas, Penfold 2007]. Below CAC there is no interaction between HPMC and SDS, and both HPMC and SDS molecules adsorbs separately at the surface which makes surface tension of HPMC/SDS solution lower than surface tension of corresponding SDS solution. Above CAC, SDS binds to adsorbed HPMC molecules via hydrophobic interaction rather than to adsorb to the surface. Consequently, there is no more decrease in surface tension of HPMC/SDS solution on increasing SDS concentration, and surface tension of HPMC/SDS solution becomes higher than surface tension of corresponding SDS solution. Binding of SDS to HPMC molecules continues until PSP is reached. Above PSP surface tensions of corresponding HPMC/SDS and SDS solutions are equal, since all HPMC molecules are solubilized with SDS, and only SDS molecules are adsorbed at the surface. 

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

The CMC of commercial AOS and other surfactants at 40°C has been determined by Gafa and Lattanzi [6] who plotted the surface tension of aqueous surfactant solutions against concentration. The surface tensions were determined with the ring method according to du Nouy. Table 5 gives their CMC values in mmol/L and the surface tension at the CMC in mN/m. Table 5 also contains CMC values of isomerically pure sodium alkyl sulfates, sodium alkylbenzene-sulfonates, sodium hydroxyalkanesulfonate, and sodium alkenesulfonates at 40°C, taken from the literature [39 and references cited therein]. 

Micelles tend to aggregate, and there are many ways to measure their concentration, including surface tension measurements. The midpoint of the concentration range over which micellar aggregation occurs is called the critical micellar concentration (CMC). Below the CMC, added bile-salt molecules dissolve in the form of monomers above the CMC, added bile-salt molecules form micelles, leaving the monomeric concentration essentially constant. The pH at which CMC formation occurs is called the critical micellar pH, (CMpH). Table 1.1 lists values for some of the bile acids mentioned in this review. 

If one adds an inorganic salt, such as NaCl, instead of detergent, then no foam is formed. Foam formation indicates that the surface-active agent adsorbs at the surface, and forms a TLF (consisting of two layers of amphiphile molecules and some water). This has led to many theoretical analyses of surfactant concentration (in the bulk phase) and surface tension (consequent on the presence of surfactant molecules at the surface). The thermodynamics of surface adsorption has been extensively described by the Gibbs adsorption theory (Chattoraj and Birdi, 1984). 

Mass transfer to the liquid phase around a slug can be treated with the thin concentration boundary layer assumption through Eq. (1-63). Van Heuven and Beek (VI) completed these calculations for a slug with viscous and surface tension forces negligible (Eod > 100, M < 10 ). The results can be represented by... 

The conditions for synergism in surface tension reduction efficiency, mixed micelle formation, and Surface tension reduction effectiveness in aqueous solution have been derived mathematically together with the properties of the surfactant mixture at the point of maximum synergism. This treatment has been extended to liquid-liquid (aqueous solution/hydrocarbon) systems at low surfactant concentrations.) The effect of chemical structure and molecular environment on the value of B is demonstrated and discussed. 

EXAMPLE 7.4 Determination of Surface Excess Concentration from Surface Tension Data. The slope of the 25°C line in Figure 7.15 on the low-concentration side of the break is about -16.7 mN m 1. Calculate the surface excess and the area per molecule for the range of concentrations shown. How would Figure 7.15 be different if accurate measurements could be made over several more decades of concentration in the direction of higher dilution Could the data still be interpreted by Equation (49) in this case ... 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

Physical interferences are generally considered to be effects associated with such properties as change in viscosity, and surface tension can cause significant inaccuracies, especially in samples that may contain high dissolved solids, or acid concentrations, or both. If these types of interferences are operative, they must be reduced by dilution of the sample or utilization of standard addition techniques, or both. 

Solutions of highly surface-active materials exhibit unusual physical properties. In dilute solution the surfactant acts as a normal solute (and in the case of ionic surfactants, normal electrolyte behaviour is observed). At fairly well defined concentrations, however, abrupt changes in several physical properties, such as osmotic pressure, turbidity, electrical conductance and surface tension, take place (see Figure 4.13). The rate at which osmotic pressure increases with concentration becomes abnormally low and the rate of increase of turbidity with concentration is much enhanced, which suggests that considerable association is taking place. The conductance of ionic surfactant solutions, however, remains relatively high, which shows that ionic dissociation is still in force. 

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