Critical micelle concentration coefficients

An interesting change of the UV-absorbances with electrolyte concentration was observed for A18 and T18, as shown in Fig. 5. The molar extinction coefficient of A18 decreased by about 7% at 0.09 mM, and that of T18 about 10% at 0.16 mM. These concentrations may correspond to the critical micelle concentration, since the cmc observed from the surface tension measurements were about 0.1 mM for both A18 and T18. 

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. 

Estimation is easier and less time-consuming because use is made of empirical relationships between the BCF and physicochemical properties of the compound, such as water solubility (S) [42-48], Km, (solid organic carbon/water partition coefficient) [48], Kmw (membrane water partition coefficient), iipw (liposome water partition coefficient) [49], critical micelle concentration (CMC) [45], steric factors, molecular weight [47,48], and others. The most common regression method is the estimation of BCF from the octanol-water partition coefficient (Kovl) [18,42,44-48,50,51],... 

Surfactant solutions critical micelle concentration distribution of reactants among particles surfactant aggregation numbers interface properties and polarity dynamics of surfactant solutions partition coefficients phase transitions influence of additives... 

Partition coefficients of surfactants have been reported to remain constant below the critical micelle concentration (CMC), and to increase with concentration above the CMC (2,9,10). The effect of surfactant concentration in the aqueous phase (C ) on K was investigated with Makon 14 (14 mol% ethylene oxide, NPEj ), the results are given in Fig. 2. These data indicate a CMC of about O.lg/1, or 12 piM, in close agreement with the value obtained by surface tension measurements (our data and ref. 22). In subsequent determinations of Kp, C was just below the CMC to minimize the effects of micellization (15,23). 

Enhanced HOC solubility in surfactant systems generally has been quantified by a distribution coefficient that only considers HOC partitioning to surfactant micelles that exist above the critical micelle concentration (CMC). Although surfactants can form a mobile micellar pseudophase that leads to the facilitated transport of solubilized HOCs, they also can be adsorbed by the solid matrix and thereby lead to HOC partitioning to immobile sorbed surfactants and, thus, enhanced HOC retardation. Therefore, the effectiveness of a remediation scheme utilizing surfactants depends on the distribution of an HOC between immobile compartments (e.g., subsurface solids, sorbed surfactants) and mobile compartments (e.g., water, micelles). 

Table 2. Surfactant critical micelle concentrations (CMC) and micellar partition coefficients (Kmu) for phenanthrene under various solution chemisty conditions.a ... 

Effect of Radiation Dose on Micellar Properties. Figure 1 shows the concentration dependence of the micellar diffusion coefficient at 40° as determined by quasi-elastic light scattering (QELS) for solutions subjected to radiation doses of up to 4.56 Mrad. Limiting diffusion coefficients, D0>were obtained by extrapolation of data for dilute solutions (<0.05%) to zero concentration, the critical micelle concentration (CMC) being negligibly low for this poloxamer ( 1 ). 

If the chemical is surface active, for example an alkyl benzene sulfonate used in detergents, it will form micelles above a critical micelle concentration (CMC). This is effectively a solubility limit for such substances and it is essential that the test conditions be below the CMC, otherwise the BCF will be underestimated. Finally it should be noted that actual concentrations in the water may differ considerably from nominal concentrations deduced by adding a known mass of chemical to a known volume of water, because much of the chemical may sorb to the walls of the tank and to pumps and filters. Further, substances of relatively high air-water partition coefficients will evaporate appreciably from solution especially as a result of aeration. For these reasons actual concentration measurements are essential, and nominal values should not be trusted. 

Ka <1, where k is the Debye screening length and a is the radius of the micelle, diffusion coefficients at the critical micelle concentration (cmc), Dcmc. decrease with decreasing ionic strength, i.e., Ka. With further decrease in m from 0.24 to 0.18, the Dcmc value does not decrease. This fact suggests that the drag of ionic atmosphere reaches maximum when xa becomes ca. 0.2. 

In this study we have measured tracer diffusion coefficients of octadecyltrimethylammonium chloride (CigTAC) micelles in water and aqueous NaCl solutons at 35 °C. Due to its low critical micelle concentration (cmc), the smallest Ka value is as low as 0.18. We have also determined tracer diffusion coefficients for C14TAB micelles at 35 °C. 

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). 

Schick and Fowkes (11) studied the effect of alkyl chain length of surfactants on critical micelle concentration (CMC). The maximum lowering of CMC occurred when both the anionic and nonionic surfactants had the same chain length. It was also reported that the coefficient of friction between polymeric surfaces reaches a minimum as the chain length of paraffinic oils approached that of stearic acid (12). In order to delineate the effect of chain length of fatty acids on lubrication, the scuff load was measured by Cameron and Crouch (13). The maximum scuff load was observed when both hydrocarbon oil and fatty acid had the same chain length. Similar results of the effect of chain length compatibility on dielectric absorption, surface viscosity and rust prevention have been reported in the literature (14-16). 

The totality of micelles represents a colloidal phase, into which a substance is dissolved in the aqueous phase partitions. The capacity of the micellar phase to solubilize a solute can therefore be expressed as a partition coefficient A n,. Hence, a linear relationship can be expected between the concentration of substance solubilized by micelles and the concentration of the surfactant Cs in the system. Because only micelles contribute to the solubilizing effect but not the monomeric surfactant molecules, the critical micelle concentration Ccmc must be subtracted from the total of the surfactant concentration. The resulting total concentration of solute in the micellar solution is then ... 

One of the main mechanisms of surfactant-related FOR is reducing IFT, which is closely linked to water and oil solubilization (phase behavior). Several mechanisms for ultralow IFT have been proposed. Rosen (1978) proposed that for ultralow IFT to occur, not only the oil/water interface adsorbs surfactant molecules, but also a third phase exists. Chan and Shah (1981) proposed that surfactant concentrations in water and oil phases must reach the critical micelle concentration, and the partition coefficient must equal 1. Chen et al. (1999a) proposed that the ultralow IFT in an AS or ASP system is attributed to the synergy between surfactant and in situ generated soap. 

Gao, Z., Wasylishen, R.E., and Kwak, J.C.T., An NMR paramagnetic relaxation method to determine distribution coefficients of solubilization in micellar systems, J. Phys. Chem., 93, 2190, 1989. Treiner, C., The partitioning of neutral solutes between micelles and water as deduced from critical micelle concentration determinations, in Solubilization in Surfactant Aggregates, Christian, S.D. and Scamehorn, J.R, Eds., Marcel Dekker, New York, 1995, chap. 12. 

Vigon B.W. and Rubin AJ. (1989) evaluated surfactant selection and dose optimization through evaluation of parameters such as surface tension, cmc. extent of hydrophilic-hydrophobic balance, solubilizaton efficiency, and partition coefficient. Chu W. and So W.S. (2000) has studied the surfactant aided soil washing of some contaminated non-chlorinated aromatic dyes. They concluded that dye concentration increased slightly until the surfactant monomers in the bulk solution was saturated the washing performance was 1 1 proportional to the monomer concentration. Above the effective critical micelle concentration (as the result of the sorption of surfactant monomer onto soil), the soilwashing performance increased linearly with increasing available surfactant micelles in the aqueous phase. 

Addition of a block copolymer, A-B, to immiscible blend of homopolymers A and B reduces the interfacial tension coefficient similarly as addition of a surfactant affects emulsions. Thus, the idea of the critical micelle concentration, CMC, and the limiting value of the interfacial tension coefficient, can be applied to polymer... 

Addition of a compatibilizer reduces Vj2 to the level corresponding to the critical micelles concentration, CMC [Utracki and Shi, 1992 Tang and Huang, 1994], The radius of the dispersed drop, R, follows the same titration curve as the interfacial tension coefficient, Vj2. 

Figure 5.28 Correlation of cerebrovascular permeability with critical micelle concentration and air/water partition coefficient. Adapted from [27].

---