Second critical micelle concentration

Fig. 3 Mechanism for the synthesis of a hexagonal silica thin film by dip coating. This is zoomed in at the substrate-bulk solution interface. The dotted line indicates the second critical micelle concentration, above which the micelles form cylindrical micellar rods. These rods then begin to assemble at the air-liquid interface and the liquid-substrate interface. (From Ref. l)...

Lake [117] used kinetic dialysis to examine the existence of cdc. The cdc of the sodium decanoate-sodium perfluorooctanoate mixed-micelle system was found to occur at a specific surfactant concentration and mole fraction. Ben Ghoulam et al. [118] determined the demixing diagram of the Neos Ftergent (a branched alkylbenzenesulfonate by surface tensiometry) and measured second critical micelle concentrations. However, a critical demicellization concentration was not observed. 

Figure 20 shows the plot of the surface tension vs. the logarithm of the concentration (or-lg c-isotherms) of sodium alkanesulfonates C,0-C15 at 45°C. In accordance with the general behavior of surfactants, the interfacial activity increases with growing chain length. The critical micelle concentration (cM) is shifted to lower concentration values. The typical surface tension at cM is between 38 and 33 mN/m. The ammonium alkanesulfonates show similar behavior, though their solubility is much better. The impact of the counterions is twofold First, a more polarizable counterion lowers the cM value (Fig. 21), while the aggregation number of the micelles rises. Second, polarizable and hydrophobic counterions, such as n-propyl- or isopropylammonium and n-butylammonium ions, enhance the interfacial activity as well (Fig. 22). Hydrophilic counterions such as 2-hydroxyethylammonium have the opposite effect. Table 14 summarizes some data for the dodecane 1-sulfonates.

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. 

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

Critical micelle concentrations for a range of PEO-PBO, PBO-PEO (here the block order reflects the synthesis sequence), PEO-PBO-PEO, PBO-PEO-PBO and cyclo-PBO-PEO ring copolymers are listed by Booth et al. (1997). Polymerization of PEO second (PBO-PEO diblock) leads to a broader PEO block length distribution, which in turn leads to a difference, for example, in surface tension (this is illustrated for PE04iPB08 and PB08PE041 by Booth et al. 

Kinetics of oxidation of iron(II) by the surfactant complex ions d.v-chloro/bromo (dodecylamine)bis(ethylenediamine)cobalt(III) have been reported. The second-order rate constant remains constant below the critical micelle concentration (cmc), but increases with cobalt(III) concentration above the cmc. The rate of reaction was not affected by the added hydrogen ions. It is suggested that the reaction proceeds by an inner-sphere mechanism.74... 

Luisi has shown that membrane material itself can be formed autocatalytically in an experiment to investigate the base catalysed ester hydrolysis of hydrophobic ethylcaprylate [27], Hydrolysis occured initially at the aqueous-organic interface where the products were micelle-forming sodium caprylate and ethanol. Once the critical micelle concentration, or cmc, was reached an exponential increase in hydrolysis was observed. The rate of hydrolysis in this second phase was almost 1000 times greater than in the initial phase suggesting that a catalytic mechanism had been activated. Luisi and co-workers hypothesized that once the cmc had been reached hydrolysis occurred within the micelles and, as the reactants were then constrained within a more hydrophobic environment, the increased rate was due to autocatalysis. Below pH 7 the micelles reorganized into unstable vesicles, in the order of 150 nm across as verified by freeze-fracture electron microscopy. 

This study is a continuation of our previous investigations, in which the aggregation phenomena of surfactant molecules (amphiphiles) in aqueous media to form micelles above the critical micelle concentration (c.m.c.) has been described based on different physical methods (11-15). In the current literature, the number of studies where mixed micelles have been investigated is scarcer than for pure micelles (i.e., mono-component). Further, in this study we report various themodynamlc data on the mixed micelle system, e.g., ci H25soi4Na (NaDDS) and sodium deoxycholate (NaDOC), enthalpy of micelle formation (by calorimetry), and aggregation number and second virial coefficient (by membrane osmometry) (1 6). 

Single surfactants lower the interfadal tension y, but in most cases the critical micelle concentration (cmc) is reached before y is close to zero. The addition of a second surfactant of a completely different nature (i.e., predominantly oil-soluble, such as an alcohol) then lowers y further and very small, even transiently negative, values may be reached [9]. This is illustrated in Figure 15.7, which shows the effect of addition of the cosurfactant on the y-log curve. It can be seen that addition of cosurfaclant shifts the whole curve to low y-values, while the cmc is shifted to lower values. 

Micelle formation has been studied for sodium salts of fatty acids containing terminal double bonds using electrical conductivity . Here two critical micelle concentration points were observed of which the first point at 0.044 moles litres" was critical in terms of the number average degree of polymerisation of the polymers produced. At concentrations up to the second point the molecular weight change was significantly smaller. The photopolymerisation of acrylamide in reverse micelles was found to be first order with respect to monomer concentration whilst the order was found to depend upon the oil concentration in the... 

Inspection of Figure 2.8 shows that 8 millimolar SDS would equal the critical micellization concentration. In the first experiment, the concentration gradient of the SDS molecules (or ions, rather) would thus have been as assumed. In the second experiment, both compartments would have contained the same concentration of free SDS, the concentration gradient being merely be due to SDS in micellar form. The micelles clearly have a larger hydrodynamic radius than free SDS species, hence the slower diffusion. 

A number of control experiments were performed to establish that the copurification observed between HOS and HAS was not the result of nonspecific interactions (30). In the first experiment, HOS and HAS were expressed in different cells, and the detergent-solubilized cell lysates were then combined and incubated together prior to protein purification. Again, HOS and HAS were found to copurify. In a second experiment, / -dodecyl p-D-maltoside was used to solubilize HOS and HAS instead of the detergent Triton X-100, and this change was found to have no effect on the interaction between HOS and HAS. Because -dodecyl P-D-maltoside has a lower micelle number as well as a lower critical micelle concentration (cmc), it seems unlikely that the copurification observed is an artifact of the two enzymes being trapped in the same micelle. 

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