CMC values

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

FIG. 6 Gibbs plot CMC values for LAS homologs measured by surface tension at 38°C in 0.01 M Na2S04. 

FIG. 7 CMC values for C12 LAS phenyl isomers measured by conductivity at 38 °C in deionized water. 

The CMC of lauryl sulfates in aqueous solution decreases in the order of Li+ > Na+ > K+ > Cs+ > N(CH3)4+ > N(C2H5)4+ as shown in Tables 13 and 14. Calcium salts have still lower CMC values but higher values than those of the magnesium salts. 

Alcohol sulfates commonly have free alcohol and electrolytes as impurities. Other hydrophobic impurities can also be present. A method suitable for the purification of surfactants has been proposed by Rosen [120]. Consequently, commercial products have CMCs that deviate from the accepted reference values. This was demonstrated by Vijayendran [121] who studied several commercial sodium lauryl sulfates of high purity. The CMC was determined both by the conductimetric method and by the surface tension method. The values found were similar for both methods but while three samples gave CMC values of 7.9, 7.8, and 7.4 mM, close to the standard range of 8.0-8.2 mM, three other samples gave values of 4.1, 3.1, and 1.7 mM. The sample with a CMC of 7.9 mM was found to have a CMC of 8.0 mM with no detectable surface tension minima after purification and recrystallization. This procedure failed in all other cases. 

It has been found that the CMC values are higher and the micelle aggregation numbers smaller than those of the corresponding nonionic surfactants. The CMC increases with increasing EO chain, which is, according to the authors, opposite to the results for sodium alkyl ether sulfate. 

C,4—C20 AOS surfactants were laboratory-prepared by Tuvell et al. [2]. Table 3 shows the CMC values of these single-carbon-cut AOS surfactants and of reference compounds, their areas per molecule at the water-air interface inferred from plots of surface tension vs. In (concentration), and the surface tension at the CMC, all at 23°C. The area of the molecule is proportional to the equilibrium adsorptivity, which in turn is taken as a comparative measure of the surface activity of the molecule. Tuvell et al. [2] argue that the greater the... 

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]. 

From the data in Table 5 it can be seen that the CMC depends on the type of surfactant and, within a series of similar or homologous compounds, on the chain length, i.e., molecular weight of the compound. For the three AOS compounds it is seen that the CMC of the lowest molecular weight product is about twice the value of the other two products. The CMC values of the commer-... 

Table 5 also shows that the CMC values of the three sulfated alcohol (AS) samples are in line with those of the isomerically pure sulfates. The surface tension values at the CMC of the AS samples are less than those observed with the other commercial samples, including the AOS compounds. 

Their results in Fig. 2 show that LAS-AOS mixtures which are rich in LAS have smaller CMC values than LAS alone. This indicates that the addition of small amounts of AOS promotes the formation of mixed micelles in the solution. A minimum in the CMC plot is observed for mixtures containing 20 wt%... 

Stirton et al. also studied the influence of the structure of the ester group on the CMC. They found that for a-sulfopalmitates and a-sulfostearates the esters of secondary alcohols, like isopropyl, isobutyl, and secondary butyl esters, have higher CMC values than the esters of primary alcohols [30]. 

Bistline and Stirton compared the CMC values of ester sulfonates with cyclic ester groups [54]. The phenyl esters have higher values than benzyl and cyclohexyl esters. The influence of the structure of the ester group decreases with increasing chain length of the hydrophobic fatty acid group. The cyclic esters of a-sulfostearic acid, for example, have nearly the same CMC values. 

At the end of the 1960s, Subba Rao et al. examined the influence of the interface on the CMC values [56]. They found a decrease in the CMC at the oil-water interface compared with the air-water interface. The CMC decreased by about 10% in the presence of heptane and by about 30-40% in the presence of benzene. The solubilization of the hydrocarbon in the micelle interior results in an increase in the micelle size and a slight change in the curvature of the micelle surface. The electrical potential and hence the electrical work of... 

Compared with sodium w-alkyl sulfates the ester sulfonates with the same number of carbon atoms in the hydrophobic chain have lower CMC values because the methyl group also contributes to the micellization. 

Because there are never chain length pure surfactants in technical applications, the CMC of mixtures of different ester sulfonates is important. Fabry and Giesen showed that the CMC value of C16 a-methyl ester sulfonate is lower than the value of a C16/C18 a-ester sulfonate. There is the same tendency for C16 a-disalt and C16/C18 a-disalt. For the C16/C18 mixtures the ester group has no influence on the CMC. The methyl ester has nearly the same values as the ethyl and the /-propyl ester [59]. 

Okane et al. measured the CMC values of a-sulfonated fatty acid higher alcohol esters. These molecules can be regarded as double-chain amphiphiles, but the CMC values are about three to six orders of magnitude larger than expected for double-chain amphiphiles that can spontaneously form vesicles in water [60]. 

Fujiwara et al. used the CMC values of sodium and calcium salts to calculate the energetic parameters of the micellization [61]. The cohesive energy change in micelle formation of the a-sulfonated fatty acid methyl esters, calculated from the dependency of the CMC on the numbers of C atoms, is equivalent to that of typical ionic surfactants (Na ester sulfonates, 1.1 kT Ca ester sulfonates, 0.93 kT Na dodecyl sulfate, 1.1 kT). The degree of dissociation for the counterions bound to the micelle can be calculated from the dependency of the CMC on the concentration of the counterions. The values of the ester sulfonates are also in the same range as for other typical ionic surfactants (Na ester sulfonates, 0.61 Ca ester sulfonates, 0.70 Na dodecyl sulfate, 0.66). 

Limit Concentration of Monomers in Solution In the calcic environment considered, the CMC values of surfactants are low. For example, sulfonate and Genapol solutions reach their CMC at 30 ppm (Table II). The surfactant solutions injected in practice at concentration of about one or several percent are thus generally used well above their CMC. Under such conditions, the predominant fraction of each surfactant is the micellar form whose composition (xj) is practically equal to the initial proportion of products (i.e. alpha 1 for sulfonate). At this concentration level of products, very small proportions of monomer species coexist, the limit concentrations of which are respectively ... 

The adsorption plateaus on this solid, determined with each of the surfactants (Table II) and the individual CMC values, were used to calculate the adsorption constants input in the model. Figure 3 compares the total adsorption (sulfonate + NP 30 EO) of the pseudo-binary system investigated as a function of the initial sulfonate fraction of the mixtures under two types of conditions (1) on the powder solid, batch testing with a solid/liquid ratio, S/L = 0.25 g/cc (2) in the porous medium made from the same solid, for which this solid ratio is much higher (S/L = 4.0 g/cc). 

It was mentioned previously that the narrow range of concentrations in which sudden changes are produced in the physicochemical properties in solutions of surfactants is known as critical micelle concentration. To determine the value of this parameter the change in one of these properties can be used so normally electrical conductivity, surface tension, or refraction index can be measured. Numerous cmc values have been published, most of them for surfactants that contain hydrocarbon chains of between 10 and 16 carbon atoms [1, 3, 7], The value of the cmc depends on several factors such as the length of the surfactant chain, the presence of electrolytes, temperature, and pressure [7, 14], Some of these values of cmc are shown in Table 2. 

Key questions in these treatments are the constancy of a (or P) and the nature of the reaction site at the micellar surface. Other questions are less troubling for example the equations include a term for the concentration of monomeric surfactant which is assumed to be given by the cmc, but cmc values depend on added solutes and so will be affected by the reactants. In addition submicellar aggregates may form at surfactant concentrations near the cmc and may affect the reaction rate. But these uncertainties become less important when [surfactant] > cmc and kinetic analyses can be made under these conditions. In addition, perturbation of the micelle by substrate can be reduced by keeping surfactant in large excess over substrate. 

Surface acoustic waves (SAWs), acoustic wave sensors and, 22 270 Surface-active agent(s), 12 33. See also Surfactant entries cmc values of, 24 121t general classification of, 24 144-153 nonionic, 10 665 organic esters as, 10 519 Surface-active molecules, 12 1 foaming and, 12 3... 

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. 

They have studied also the N dependence of the thermodynamic parameters of micellization, calculated from the cmc values and their temperature dependence, as shown in Figure 1, where AGm is... 

To understand micelle formation quantitatively, critical micelle concentrations (cmc) have to be determined for a large number of surf actants ( 5 ). When the cmc values of the surfactants with the same hydrophilic group (a homologous series) are examined, a nearly 3-fold decrease in cmc is observed for nonlonlc and zwitterionlc surfactants (1,2) upon the addition of a methylene group into the hydrocarbon chain, whereas, a 2-fold or only 1.8-fold reduction in cmc can be observed for univalent (1,2) and blvalent( ) ionic surfactants,respectively. 

The capillary-rise method was employed to measure the surface tension of aqueous solutions of disodlum alkyl phosphate at 25 °C. The cmc values of the solutions were obtained from the discontinuity in the surface tension - concentration curves(7). 

M. For nonionic surfactant, we used Newcol 1102, 1103 and 1105. These surfactants contain dodecanol ethoxylate. The last digit in the Newcol number represents the ethylene oxide (EO) number. The CMC values for pure dodecanol ethoxylate ( ) with EO number from 3 to 5 are in the concentration range of 0.001-0.003%. C value in... 

For anionic surfactant, we used sodium dodecyl sulfate (SDS). The CMC values were measured by conductance method. The CMC values were taken from the breaks of curves from plots of K/C versus N 5. Where K is the specific conductance, C is molar concentration and N is the equivalence. Figure 1 shows the CMC values of SDS at 25 °C. The curve showing in the lower left side represents data taken from literature for pure SDS. The curve showing in the upper right side represents measurements for our impure sample. Table I shows some values of (dy/dC) (C-Cq) for pure SDS at 25 °C. The values for NaCl concentrations of 0.03 M to 0.50 M are not far from constant. Therefore, in this concentration region, t is also considered to be proportional to M. 

The contribution to t by emulsified oil in our experiments is considered negligible in the nonlonlc surfactant solutions due to the very low CMC values. In SDS solutions, the emulsification occurs at the very beginning when no NaCl is added to the solution and the turbidity Introduced by emulsification does not change with time. When NaCl is added to SDS solutions, the CMC becomes low and emulsification becomes unimportant as it will be shown in the following sections. 

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