Minimum surface tension

The extrapolation of curves such as shown in Figure 11 to E = 0 gives the surface tension at or above the CMC as it would have been obtained by a static measurement. The CMC for this C14-Ci6 AOS is 0.2% and its minimum surface tension is 35.1 dyne/cm. 

The physicochemical data underline the striking influence of the dicyclopentadienyl unit on the properties of these silicone surfactants. In comparison to conventional products [7], the critical micelle formation concentration was lowered for up to two orders of magnitude whereas the minimum surface tension reached rose only slightly. The data collected indicate that the type of surfactant has been changed from the initial "effective" to a more "efficient" one. 

Minimum surface tension at a 1 1 cationic/anionic ratio has been observed in the system containing SDS and dodecyltrimethyl ammonium bromide (3). In addition, 1 1 association has been observed for with long-chain sulfonates (1,2). The... 

C gDAO with SDS respectively. For LDAO/SDS ratio greater than 1, these mixed solutions are turbid and blrefrlngent addition of SDS results in production of filament-like structures. When the molar amount of SDS is equal to or greater than that of the LDAO, the solutions become Isotropic and clear. AT 3 1 LDAO/SDS molar composition, the change in pH reaches its maximum value. Minimum surface tension is reached when the LDAO content is still in excess, unlike the cases of Cj 2 C] 4 DA0. This observation... 

Sucrose (20) was also used for the reaction of telomerization with 1 [19, 49, 50]. Applying specific conditions, sucrose octadienyl ethers were obtained with an average degree of substitution of 4.7-5.3. These products are practically insoluble in water, clear or almost colorless, and present a viscosity of 1500-2000 cPs at 25°C [39]. These properties confer to these products the possibility of being employed as emulsifiers or defoaming agent [49]. Minimum surface tension of solution of substituted sucrose in water is 25-28 mN/m whatever the degree of substitution... 

In mixed sur-factant systems, the interactions betMeen sur-factants a-f-fects the tendency -for monolayer formation. At concentrations above the CMC, the surface tension may very slOMly increase or decrease. The surface tension at the CMC is close to the minimum surface tension which a surfactant system can attain. Therefore, in terms of surface tension reduction, the surfactant concentration required to attain a specified surface tension below the CMC and the surface tension at the CMC are indicative of the usefulness of a system. 

The higher the negative deviation from ideality in monolayer formation, the lower the concentration required to attain a given surface tension below the CMC. The higher this deviation for micelle formation, the lower the CMC. Since the CMC is where the surface tension approximately levels out at near a minimum value, the minimum surface tension in such a system represents the relative enhancement of monolayer formation over micelle formation. This relative favorability of aggregate formation is often an important factor in many applications, as will be further discussed in this article. 

The minimum surface tension that can be attained, regardless of the amount of agent required... 

Table 4.8. Synthesis of azo groups-containing polyacrylamides by precipitation polymerization in tert-butyl Alcohol and the surface activity of the products. CMC = critical micelle Concentration, omm = minimum surface tension...

To analyze the surface activity of one component, the minimum surface tension (7min) can be calculated as ... 

Figure 2 presents surface tension and E values obtained for fermented broth at different initial pH values in CS media. For all the pH values tested, a decrease in surface tension of the medium was noted before inoculation (62.5 dyn/cm) by the microorganism activity. These results indicate that the tested bacteria have the ability to produce biosurfactant at pH values ranging from 5.0 to 8.0 The minimum surface tension values of 30.1 and 31.5 dyn/cm were obtained when the medium s initial pH was 7.0 and 8.0, respectively. 

Thompson et al. s (24) description of the kinetics of B. subtilis ATCC 21332 cultured in potato process waste streams medium presents the same correlation between biomass and surface tension. The minimum surface tension occurred near the onset of stationary phase. This behavior, as stated by these investigators, indicates a production of growth-associated surfactin. 

McBain1 has drawn attention to the existence of a number of cases in which strongly surface active substances first lower the tension of water considerably, but as the concentration increases further, the surface tension either becomes nearly constant or shows a slight increase. The cases for which the evidence is most conclusive are aqueous solutions of soaps or other paraffin chain salts and of some dyestuffs. The minimum surface tension often occurs in quite dilute solution, e.g. with dodecyl sodium sulphate2 it is about 0 006 N the exact concentration of the minimum, however, depends on the purity of the water used as solvent. 

We noted early on that the driving force in chemistry is the decrease of free energy (cf. Eq, IB). Thus, a. system will change spontaneously in the direction of minimum. surface tension. This leads to two observations ... 

If the CMC exceeds the solubility of the surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum... 

Since liquid surface tension is strongly dependent on temperature, it can be controlled by controlling the liquid temperature. This technique was utilized [101,102] to control the breakup of a water jet. Furlani et al. [103] have conducted a linear analysis of a jet subject to a spatially periodic variation of surface tension imposed along its length. It is shown that as the jet approaches breakup it swells at the points of maximum stuface tension, and necks at the points of minimum surface tension. A periodic variation of temperature can induce a time-harmonic modulation of the surface tension cr of the jet, which has an equatimi of state of the form cr(7) = (To - P(T - To) where jS is a property constant. Instability of an evaporating jet is considered by Saroka et al. [104], who showed that the evaporation increases the growth rate of instability. 

FIGURE 17.14 Effect of surfactant concentration on the surface tension. The minimum surface tension achieved is different for different surfactants depending on their structural attributes. The minimum surface tension achieved is in the order of a[Pg.444]

As seen in the figure, beyond the minimum surface tension achieved, further addition of surfactants in the system would not result in any change in surface tension. The excess surfactant added beyond the concentration of minimum surface tension (known as critical micelle concentration or CMC) would go into the bulk-forming micelles, as indicated in the figure. It can be seen that the adsorption of the surfactant at the interface occurs only up to the point of minimum surface tension and beyond this, the adsorption stops and micellization begins. 

Sharma wrote that, ... fluorinated surfactants are about ten times more effective than silicones and 50-100 times more effective than hydrocarbon surfactants [53]. In Sharma s study, an increased hydrophilic portion of nonionic fluorinated surfactant shifted the HLB (hydrophile-lipophile balance) and resulted in an increased CMC and an increase in minimum surface tension. The increased surface tension for the more hydrophilic nonionic fluorinated surfactant was attributed to a lower surface excess and higher surface area occupied by each surfactant molecule at the air-liquid interface, both indicating less dense packing at the interface see Table 2. 

There are two additional aspects of cell morphology that must be considered cell size and cell shape. In the case of low density foams, p 160 kg/m (10 Ib/ft ), the cell geometry should be such that the cellular surface area is minimized, since this is the condition for minimum surface tension. Figure 3(a) illustrates the pentagonal-dodecahedrol (a figure with twelve plane faces) cell... 

Figure 2 shows, with mass fraction of 0.28%, SDS solution had a minimum surface tension of 28.89 mN/m, meanwhile, the surface tension of the fluorocarbon surfactant AS-3 solution would be able to reduce to 20 mN/m with mass fraction of 0.03%. Based on Gibbs Theory, surfactant solution tended to stay on lower surface energy state (Crawford, 2001). AS-3 molecule contained amine groups, so it formed hydrogen bonds in aqueous solution, reduced the surface tension of the solution. 

Reid and coworkers studied the morphology of aqueous oleic acid/NaCl aerosols using Raman spectroscopy [10]. It appeared that, rather than spreading to form a uniform surface film, the oleic acid formed a lens on the NaCl droplet. They also measured the surface tension of bulk aqueous oleic acid/NaCI mixtures using the Wilhelmy plate method. The minimum surface tension in these mixtures was 25.1 mN m, at a concentration of 0.3 mM oleic acid/2.5 M NaCl. 

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