Aqueous phase surface tension

Table 9.3 Latex surface polarity and polymer/aqueous phase interfacial tensions...

This model indicates that the HETP value primarily is a function of surface tension and liquid viscosity. The flow parameter and vapor density also show minor influences. This correlation is applicable to systems with a liquid-phase surface tension above 4 dyne/cm and less than 33 dyne/cm and a liquid viscosity of at least 0.08 cps but not greater than 0.83 cps. It does not apply to aqueous systems, interacting compounds, or systems with a relative volatility of 3 or greater. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Phosphorus-containing surfactants are amphiphilic molecules, exhibiting the same surface-active properties as other surfactants. That means that they reduce the surface tension of water and aqueous solutions, are adsorbed at interfaces, form foam, and are able to build micelles in the bulk phase. On account of the many possibilities for alteration of molecular structure, the surface-active properties of phosphorus-containing surfactants cover a wide field of effects. Of main interest are those properties which can only be realized with difficulty or in some cases not at all by other surfactants. Often even quantitative differences are highly useful. 

The mechanisms that affect heat transfer in single-phase and two-phase aqueous surfactant solutions is a conjugate problem involving the heater and liquid properties (viscosity, thermal conductivity, heat capacity, surface tension). Besides the effects of heater geometry, its surface characteristics, and wall heat flux level, the bulk concentration of surfactant and its chemistry (ionic nature and molecular weight), surface wetting, surfactant adsorption and desorption, and foaming should be considered. 

The use of low flow rates allows the production of ions from mobile phases containing high percentages of water which, as noted previously, usually presents a particular problem because of its high surface tension and conductivity. The production of electrospray spectra from aqueous solution is possible at flow rates of less than 50 tLlmin ... 

After only a small percentage of the monomer has been converted to polymer (in the presence of emulsifier), the initially low surface tension of the aqueous emulsion rises rather abruptly, indicating a decrease in the soap concentration in the aqueous phase of the emulsion. The soap concentration is then too low to maintain micelles, which may therefore be abandoned as a locus for further polymerization beyond this point. As additional evidence of the depletion of soap in the aqueous phase, monomer droplets are no longer stable, and upon discontinuing agitation a supernatant monomer layer is readily formed. 

Separation of phases is sometimes a problem, especially in the case of mixtures of alkaline aqueous solutions and organic liquids. Addition of materials that decrease the surface tension, filtration of the emulsion to remove interfacial contamination, and the use of sedimentation centrifuges should be considered if the separation time is too long. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

Kakiuchi et al. [75] used the capacitance measurements to study the adsorption of dilauroylphosphatidylcholine at the ideally polarized water-nitrobenzene interface, as an alternative approach to the surface tension measurements for the same system [51]. In the potential range, where the aqueous phase had a negative potential with respect to the nitrobenzene phase, the interfacial capacity was found to decrease with the increasing phospholipid concentration in the organic solvent phase (Fig. 11). The saturated mono-layer in the liquid-expanded state was formed at the phospholipid concentration exceeding 20 /amol dm, with an area of 0.73 nm occupied by a single molecule. The adsorption was described by the Frumkin isotherm. 

Retention in HIC can be described in terms of the solvophobic theory, in which the change in free energy on protein binding to the stationary phase with the salt concentration in the mobile phase is determined mainly by the contact surface area between the protein and stationary phase and the nature of the salt as measured by its propensity to increase the surface tension of aqueous solutions [331,333-338]. In simple terms the solvopbobic theory predicts that the log u ithn of the capacity factor should be linearly dependent on the surface tension of the mobile phase, which in turn, is a llne2u function of the salt concentration. At sufficiently high salt concentration the electrostatic contribution to retention can be considered constant, and in the absence of specific salt-protein interactions, log k should depend linearly on salt concentration as described by equation (4.21)... 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

At concentrations above their aqueous solubility, the so-called c.m.c., low-molar-mass biosurfactants form micelles in the aqueous phase. Micelles are spherical or lamellar aggregates with a hydrophobic core and a hydrophilic outer surface. They are capable of solubilising nonpolar chemicals in their hydrophobic interior, and can thereby mobilise separate phase (liquid, solid or sorbed) hydrophobic organic compounds. The characteristics for the efficiency of (bio)surfactants are the extent of the reduction of the surface or interfacial tension, the c.m.c. as a measure of the concentration needed to bring about this reduction, and the molar solubilisation ratio MSR, which is the number of moles of a chemical solubilised per mole of surfactant in the form of micelles [96]. 

Water molecules at the air-water interface experience unbalanced attraction for both water and the air phases [227-229]. This is a manifestation of the polar nature of water in contact with a nonpolar phase (i. e., the air). The water molecules are drawn together, resulting in a phenomenon called surface tension . The contact area between the water and the nonpolar phase is a region of relatively high interfacial tension and the system will naturally tend to minimize such contact. This polar structure of water will also make the aqueous medium relatively inhospitable to nonpolar, neutral (i.e., uncharged) molecules [230-234]. 

The spreading pressure defines the lowering of surface tension at the aqueous-solid phase (i. e., adsorbate-solution) interface ... 

Reactions carried in aqueous multiphase catalysis are accompanied by mass transport steps at the L/L- as well as at the G/L-interface followed by chemical reaction, presumably within the bulk of the catalyst phase. Therefore an evaluation of mass transport rates in relation to the reaction rate is an essential task in order to gain a realistic mathematic expression for the overall reaction rate. Since the volume hold-ups of the liquid phases are the same and water exhibits a higher surface tension, it is obvious that the organic and gas phases are dispersed in the aqueous phase. In terms of the film model there are laminar boundary layers on both sides of an interphase where transport of the substrates takes place due to concentration gradients by diffusion. The overall transport coefficient /cl can then be calculated based on the resistances on both sides of the interphase (Eq. 1) ... 

In this modification, the ionic micelle has been considered as the charged phase, which has difficulties from the thermodynamic viewpoint. The precise measurement of the surface tension of aqueous sodium dodecyl sulfate solutions revealed the cotlnuous decrease of surface tension above the cmc and indicated that the charged phase separation model is not correct (27). ... 

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