Critical micelle concentration surface pressure

Table 17 shows the CMCs of sodium alcohol propoxysulfates at 20°C determined from surface tension measurements by the maximum bubble pressure [127] and Table 18 shows the critical micelle concentrations of sodium pro-poxylated octylphenol and propoxylated nonylphenol sulfates. Surface tension... 

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. 

For solutions of AEg with different distributions of hydrocarbon chain lengths, the Y log C curves appear to be different than mono-component system. The surface pressure at critical micelle concentration (iTcjic) AEg with a long hydrocarbon chain (C gEg) is Increased by adding the short AEg, but the effect is not significant if the hydrocarbon chain is in a wide distribution (i.g. coconut fatty radical) (Figure 2,3,4). As for the efficiency of surface tension reduction there is a synergestic effect for the mixed... 

Changes in surface pressure with time or concentration can be used to measure various fundamental properties. Changes in surface pressure versus protein concentration curves can be used to determine the excess surface concentration (critical micelle concentration), defined as the amount of protein at the surface divided by the surface area. Indicates minimum amount of protein needed to form an emulsion. 

Bioaccumulation All classes of surfactant are active surface tension depressants. At the critical micelle concentration (CMC) abrupt changes occur in the characteristic properties of surfactants such that surface and interfacial tensions in an aqueous system are at their minimum while osmotic pressure and surface detergent properties are significantly increased. The CMC for most surfactants is reached around 0.01% (18, 19). These effects have an impact on the potential for bioaccumulation of the pesticide, and in the organisms monitored the presence of Dowanol and nonylphenol increased the accumulation of fenitrothion and aminocarb at least 20-300% respectively, over the accumulation obtained in their absence (20). In effect, these adjuvants... 

Methods. All experiments were performed at 25°C. Critical micelle concentrations were determined using the maximum bubble pressure method on a SensaDyne 6000 surface tensiometer. Dry nitrogen was used as the gas source for the process and was bubbled through the solution at a rate of 1 bubble/sec. Cmc s measured using the Wilhemy plate method were in agreement with those obtained from the bubble tensiometer however, the bubble pressure method was used since it is less susceptible to error due to impurities and the nitrogen environment makes pH control easier. 

The physical properties of surface active agents differ from those of smaller or nonamphipathic molecules in one major aspect, namely, the abrupt changes in their properties above a critical concentration. This is illustrated in Fig. 1, in which a number of physical properties (surface tension, osmotic pressure, turbidity, solubilization, magnetic resonance, conductivity, and self-diffusion) are plotted as a function of concentration. All these properties (interfacial and bulk) show an abrupt change at a particular concentration, which is consistent with the fact that above this concentration, surface active ions or molecules in solution associate to form larger units. These association units are called micelles and the concentration at which this association phenomenon occurs is known as the critical micelle concentration (cmc). 

In dilute aqueous solutions, surfactants have normal electrolyte or solute characteristics and are formed at the interface. As the surfactant concentration increases beyond the well-defined concentrations (i.e., critical micelle concentration, c.m.c.), the surfactant molecules become more organized aggregates and form micelles. At the c.m.c., the physicochemical characteristics of the system (osmotic pressure, turbidity, surface tension, and electrical conductivity) are suddenly changed, as shown in Figure 4.19. 

The efficiency of a surfactant in reducing surface tension can be measured by the same quantity that is used to measure the efficiency of adsorption at the liquid-gas interface (Chapter 2, Section HIE), pC20, the negative log of the bulk phase concentration necessary to reduce the surface tension by 20 dyn/cm (mN m-1). The effectiveness of a surfactant in reducing surface tension can be measured by the amount of reduction, or surface pressure, IIcmc, (= To Ycmc) attained at the critical micelle concentration, since reduction of the tension beyond the CMC is relatively insignificant (Figure 5-3). 

We have seen in Figure 2-15 that the surface tension of a solution of an individual surfactant decreases steadily as the bulk concentration of surfactant is increased until the concentration reaches a value known as the critical micelle concentration (CMC), above which the tension remains virtually unchanged. The surface tension at the CMC is therefore very close to the minimum tension (or maximum surface pressure) that the system can achieve. The surface pressure at this point, I ICmc, is therefore a suitable measure of the effectiveness of a surfactant in reducing surface tension (Figure 5-3). 

FIGURE 2.8 The effect of micelle formation on some solution properties, (a) Schematic picture of micelle formation, (b) Osmotic pressure, surface tension, and turbidity of solutions of sodium dodecyl sulfate (SDS) as a function of concentration (approximate). CMC = critical micellization concentration. 

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated Surfactants.

A) Graphs of surface tension as well as osmotic pressure plotted versus concentration of sodium lauryl sulfate, demonstrating critical micelle concentration at about 0.21 percent. B) Representation of a sodium lauryl sulfate micelle. 

Micelles. Dilute aqueous solutions of surface-active substances have normal physical properties, but at a higher concentration (characteristic for each substance) there occurs an abrupt change in surface tension, osmotic pressure, and electrical conductivity. These changes are due to the formation of a new, dispersed phase which takes the form of aggregates named micelles. These are often roughly spherical the hydrocarbon chains are in the interior of the sphere and the hydrophilic groups occupy the outside of the micelles in contact with the solvent water. The lowest concentration at which micelle formation occurs is called the critical micelle concentration, an abrupt transition point. 

Figures 4-6 were generated by means of the following assumptions. The volume fraction of the dispersed phase is 0.5, its density Pdp is 0.9 g/cm, the surfactant is sodium dodecylsulfate with a molecular weight Ms = 288.4 g/mol, a surface coberage Og = 0.50 nm per molecule, and a critical micelle concentration cmc = 10 M, the volume of a single particle is v and the pressure inside is Pd = 10 N/m, the interfacial tension between dispersed phase and continuous ...

However, at a particular concentration, the critical micelle concentration (c.m.c.), the surface tension curve abruptly becomes flat, and no amount of additional surfactant will reduce the surface tension further. The reason for this change in solution behaviour, which can also be detected by light scattering, osmotic pressure, conductivity, or from diffusion coeffldent measurements, is the formation of micelles. 

It was found that during the formation of PVP, the vapor pressure of the droplet did not follow Raoult s law [30]. The standard assumption was that at a constant temperature, a higher concentration of dissolved PVP leads to a decrease of the vapor pressure of the solvent. However, the surface of the droplets, and therefore, the evaporation rate increased considerably at mass firactions of 5 m% PVP with no further changes at higher concentrations. One reason can be that the polymer assembles at the surface and reduces the sxuface tension. Since the drying rates did not increase at fractions higher than 5 m% PVP, the critical micelle concentration can be the reason why further addition of polymer will not increase the concentration at the surface, but will only generate new micelles inside the droplet that do not contribute to the evaporation process. 

The critical micelle concentration of a surfactant solution can be determined by measuring surface tension, electric conductivity, osmotic pressure, light scattering, viscosity, dye solubilization, and other physical properties [107] (see Chapter 9). The cmc in aqueous solution can be determined also from a change in the chemical environment indicated by the -NMR chemical shift (5) [140), plotted as a function of inverse surfactant concentration (Fig. 6.14). The intersection in the lines indicates the cmc. 

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