Critical micelle activity

In this work, the critical micelle activity, cma, which is the activity of the surfactant at the cmc, is introduced and used Instead of the cmc to Investigate the free energy of micelle formation. It is found that upon the addition of an extra methylene group into the hydrocarbon chain, an approximately 3-fold reduction in cma is observed, irrespective of the hydrophilic head group. The effect of added electrolyte on cmc is also examined by the use of cma. 

Application of Activity at cmc. The above consideration suggested us to propose a new treatment for ionic micelle formation. According to thermodynamics, the micelle-monomer equilibrium is achieved when the chemical potential of surfactant in the micelle is equal to that in the bulk solution. The free energy of micelle formation can be represented by the use of the critical micelle activity, cma, which is the activity of surfactant at the cmc, as... 

In conclusion, micelle formation of an ionic surfactant is found to take place at a constant activity,i.e., critical micelle activity, cma, irrespective of counter ion concentration. 

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

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.

The curve shown in Fig. 6 for sodium dodecyl sulfate is characteristic of ionic surfactants, which present a discontinuous and sharp increase of solubility at a particular temperature [80]. This temperature is known as the Krafft temperature. The Krafft temperature is defined by ISO as the temperature [in practice, a narrow range of temperatures] at which the solubility of ionic surface active agents rises sharply. At this temperature the solubility becomes equal to the critical micelle concentration (cmc). The curve of solubility vs. temperature intersects with the curve of the CMC vs. temperature at the Krafft temperature. 

In addition to their poor solubility in water, alkyl phosphate esters and dialkyl phosphate esters are further characterized by sensitivity to water hardness [37]. A review of the preparation, properties, and uses of surface-active anionic phosphate esters prepared by the reactions of alcohols or ethoxylates with tetra-phosphoric acid or P4O10 is given in Ref. 3. The surfactant properties of alkyl phosphates have been investigated [18,186-188]. The critical micelle concentration (CMC) of the monoalkyl ester salts is only moderate see Table 6 ... 

The basic mechanism for surfactants to enhance solubility and dissolution is the ability of surface-active molecules to aggregate and form micelles [35], While the mathematical models used to describe surfactant-enhanced dissolution may differ, they all incorporate micellar transport. The basic assumption underlying micelle-facilitated transport is that no enhanced dissolution takes place below the critical micelle concentration of the surfactant solution. This assumption is debatable, since surfactant molecules below the critical micelle concentration may improve the wetting of solids by reducing the surface energy. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

Aminophthalate anion Atmospheric pressure active nitrogen Analyte pulse perturbation-chemiluminescence spectroscopy Arthromyces rasomus peroxidase Ascorbic acid Adenosine triphosphate Avalanche photodiode 5-Bromo-4-chloro-3-indolyl 2,6-Di-t< r/-bu(yl-4-mclhyl phenol Bioluminescence Polyoxyethylene (23) dodecanol Bovine serum albumin Critical micelle concentration Calf alkaline phosphatase Continuous-addition-of-reagent Continuous-addition-of-reagent chemiluminescence spectroscopy Catecholamines Catechol... 

A similar multiphase complication that should be kept in mind when discussing solutions at finite concentrations is possible micelle formation. It is well known that for many organic solutes in water, when the concentration exceeds a certain solute-dependent value, called the critical micelle concentration (cmc), the solute molecules are not distributed in a random uncorrelated way but rather aggregate into units (micelles) in which their distances of separation and orientations with respect to each other and to solvent molecules have strong correlations. Micelle formation, if it occurs, will clearly have a major effect on the apparent activity coefficient but the observation of the phenomenon requires more sophisticated analytical techniques than observation of, say, liquid-liquid phase separation. 

Nondestructive testing (NDT), 15 747, 748 active, 17 415-416 passive, 17 416, 425 on plastics, 19 588-589 Nonelectrolytes, critical micelle concentration and, 24 122-123 Nonemulsion paints, organic titanium compounds in, 25 121-122... 

A disperse dye suspension responds somewhat differently when a surface-active solubilising agent is added. At low concentrations of this type of additive the saturation solubility of the dye remains relatively little changed, but when the critical micelle concentration of the surfactant is reached a sudden marked increase in dye solubility is observed [57]. When micelles of the surfactant begin to form in the solution, these provide a more amenable environment into which the dye molecules or dimers can transfer. Above the critical micelle concentration the increase in solubility of the dye is directly proportional to the concentration of surfactant present [1,56]. 

The surface active agents (surfactants) may be cationic, anionic or non-ionic. Surfactants commonly used are cetyltrimethyl ammonium bromide (CTABr), sodium lauryl sulphate (NaLS) and triton-X, etc. The surfactants help to lower the surface tension at the monomer-water interface and also facilitate emulsification of the monomer in water. Because of their low solubility surfactants get fully dissolved or molecularly dispersed only at low concentrations and at higher concentrations micelles are formed. The highest concentration where in all the molecules are in dispersed state is known as critical micelle concentration (CMC). The CMC values of some surfactants are listed in table below. 

Therefore, the physical meaning of the solubility curve of a surfactant is different from that of ordinary substances. Above the critical micelle concentration the thermodynamic functions, for example, the partial molar free energy, the activity, the enthalpy, remain more or less constant. For that reason, micelle formation can be considered as the formation of a new phase. Therefore, the Krafft Point depends on a complicated three phase equilibrium. 

Several variations in chemical constitution, which lead to a depression of the Krafft-Point (for example, branching of the hydrophobic part of the molecule), frequently result in diminished hydrophobicity of the molecule. At constant molecular weight, the critical micelle concentration (Cj.) is shifted with increased branching to higher concentrations, the surface activity diminishes, the tendency to adsorb at hydrophobic interfaces decreases, etc. (j, 14, 15). Therefore, the nature of the oxyethylene groups in aTkyl ether sulfates is of major importance. 

For pure nonionic EO adducts, increase in the number of oxyethylene groups in the molecule results in a decrease in the tendency to form micelles and an increase in the surface tension of the solution at the critical micelle concentration (1 ) (l. ) This change in surface activity is due to the greater surface area of the molecules in the adsorption layer and at the micellar surface as a result of the presence there of the highly hydrated polyoxyethylene chain. The reduction in the tendency to form micelles is due to the increase in the free energy of micelle formation as a result of partial dehydration of the polyoxyethylene chain during incorporation into the micelle ( 1 6) (17). 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

Effect of Structure on Activity at the Critical Micelle Concentration and on the Free Energy of Micelle Formation... 

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

FORMATION. Aqueous solutions of highly surface-active substances spontaneously tend to reduce interfacial energy of solute-solvent interactions by forming micelles. The critical micelle concentration (or, c.m.c.) is the threshold surfactant concentration, above which micelle formation (also known as micellization) is highly favorable. For sodium dodecyl sulfate, the c.m.c. is 5.6 mM at 0.01 M NaCl or about 3.1 mM at 0.03 M NaCl. The lower c.m.c. observed at higher salt concentration results from a reduction in repulsive forces among the ionic head groups on the surface of micelles made up of ionic surfactants. As would be expected for any entropy-driven process, micelle formation is less favorable as the temperature is lowered. 

At their critical micelle concentrations, surface active agents (such as sodium dodecyl sulfate, Triton X-100, lysolecithin, and bile salts) self-associate into spherical or rod-shaped structures. Because dilution to below the c.m.c. results in rapid disassembly or dissolution of these detergent micelles, micelles are in dynamic equilibrium with other dissolved detergent molecules in the bulk solution. 

Perfluorocarbons bearing a polar hydrophilic head are very active surfactants. Indeed, the presence of fluorine atoms strongly lowers the critical micelle concentration (CMC) of an amphiphilic compound. Moreover, fluorination generally has important effects on micellization phenomena, especially on the size and shape of formed micelles. 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

In mixed surfactant systems, physical properties such as the critical micelle concentration (cmc) and interfacial tensions are often substantially lower than would be expected based on the properties of the pure components. Such nonideal behavior is of both theoretical interest and industrial importance. For example, mixtures of different classes of surfactants often exhibit synergism (1-3) and this behavior can be utilized in practical applications ( ).In addition, commercial surfactant preparations usually contain mixtures of various species (e.g. different isomers and chain lengths) and often include surface active impurities which affect the critical micelle concentration and other properties. 

The addition of an alkyl alcohol to the aqueous solution of an ionic surfactant greatly influences the surface activity of the surfactant. The critical micelle concentration (cmc) of the surfactant becomes lower in presence of alkyl alcohol, and the surface tension of the aqueous solution at cmc reaches a much lower value (1-4). 

The anionic surfactant, sodium dodecylsulfate, SDS, was obtained from Merck, Darmstadt, Federal Republic of Germany. It has a stated purity of 99.99% and was used without further purification. Surface tension measurements gave no minimum in the surface tension at the critical micelle concentration, indicating that the sample did not contain highly surface active impurities. 

Octadecanol was recrystallized from hexane after fractionation by vacuum distillation, and its purity was checked by gas-liquid chromatography. Dodecylammonium chloride was recrystallized from a mixture of ethanol and water, and its purity was confirmed by the fact that it had no minimum near the critical micelle concentration on the surface tension vs concentration curve. Hexane was distilled after passing through an activated alumina column. Water was distilled from alkaline permanganate solution of distilled water after refluxing for one day. The purity of hexane and water was confirmed by the value of the interfacial tension between them. 

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