Reduction of the CMC

For Maximum Reduction of the CMC. Select surfactant pairs with the largest negative (3M values (strongest attractive interaction in mixed micelle formation). When interaction between the two surfactants is weak, i.e., (3M is a small negative number, select a pair with approximately equal CMC values. The surfactant with the smaller CMC should always be used in larger quantity than the one with the larger CMC. 

Effect of Sodium Chloride Concentration. Figure b compares interfacial tensions of several different surfactant concentrations verses n-undecane in the presence of 0.1 M sodium chloride with values obtained without salt. Salt reduces the interfacial tension at all surfactant concentrations. Aqueous potassium oleate has a critical micelle concentration of 0.001 M (13). It could be inferred from Figure b that 0.001 M sodium oleate with no added salt is below the cmc, because of the high interfacial tension. If so, the much lower interfacial tension in the presence of 0.1 M sodium chloride stems from reduction of the cmc expected in the presence of added salt (lb). 

Fungal cutinases show no free SH groups but have 4 Cys residues, indicating that they are in disulfide linkage [119]. The reaction of the native enzyme with DTE was extremely slow but in the presence of SDS at its CMC rapid reduction could be observed [102]. Reduction of the disulfide bridge resulted in irreversible inactivation of the enzyme and the protein tended to become insoluble. CD spectra of cutinase in the 205-230 nm region, before and after DTE reduc-... 

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. 

Additions of substances for surface tension depression (ocenol, PEG, CMC TS.20, see Table 1 of Part I) induced a significant reduction of the mass transfer coefficient (by... 

Chemical modifications of the antigens were achieved by oxidation of the monosaccharide residues with periodate and reduction of the carboxyl groups of the uronic acid residues by the carbodiimide and borohydride methods, Fig. (IOC). The periodate oxidation was performed by a procedure described in the literature [37]. The reduction of the uronic acid residues of the antigen was performed by the carbodiimide (CMC) and sodium borohydride method [38], The oxidized and reduced types of antigens no longer reacted with the antibodies, Fig. (IOC). 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

Most detergents contain electrolytes, e.g. sulphate, bicarbonate, carbonate or citrate and the presence of these electrolytes increases the adsorption of anionic surfactants at the gas/liquid interface as already mentioned. This leads to a reduction of the surface tension at an equal solution concentration [7] and to a strong decrease of the cmc. The effect can be of several orders of magnitude. Similar to this are the effects of mixtures of surfactants with the same hydrophilic group and different alkyl chain length or mixtures of anionic and non-ionic surfactants as they are mostly used in detergency [8]. Mixtures of anionic and non-ionic surfactants follow the mixing rule (eqn. 3) in the ideal case ... 

More drastic changes in the CMC and N are observed when additives are present in the micelle-forming surfactant - water systems. The addition of ionic species (i.e. electrolytes) usually results in an increase in the aggregation number and a reduction in the CMC. Table III (and Table II) present some data which illustrate this effect. Depending upon the concentration, the presence of water miscible organic molecules can either enhance or inhibit micelle formation. 

At low emulsifier concentrations near the CMC, an increase in the degree of agitation results in a reduction of the emulsifier used for the formation of polymer particles (like micelles). This is because the monomer droplets become smaller as the degree of agitation is increased, and so the amount of emulsifier adsorbed onto the surfaces of the monomer droplets increases in proportion to the increased surface area of the monomer droplets. This brings about a decrease in the number of polymer particles produced, and so a decrease in the rate of polymerization. 

Kinetic curves of the polymerization of these monomers in the presence of both emulsifiers with their initial concentration close to CMC are shown in Fig. 9. As can be seen the initial polymerization rate, at the same rate of radical formation (the same concentrations of initiator and the same temperature), decreases from MA to BA in accordance with the decrease of the chain propagation rate constant in these monomers (Bagdasar yan, 1966). Subsequently, however, the polymerization rate of the MA becomes much lower than the rate of other monomers, a fact which apparently is associated with the colloid behavior of the system (reduction of the number of polymerization centers as a result of flocculation). 

It has been found that a combination of surfactants produces a slower drainage and an improved foam stability. For example, mixtures of anionic and nonionic surfactants or anionic surfactant and long-chain alcohol produce much more stable films than the single components, but this could be attributed to several factors. For example, the addition of a nonionic surfactant to an anionic surfactant causes a reduction in the cmc of the anionic. The mixture can also produce a lower... 

CMC), the concentration at which the monomeric form, in which the surfactant exists in very dilute solution, aggregates to form a surfactant cluster known as a micelle (Chapter 3). Above this concentration the surface tension of the solution remains essentially constant since only the monomeric form contributes to the reduction of the surface or interfacial tension. For concentrations below but near the CMC the slope of the curve is essentially constant, indicating that the surface concentration has reached a constant maximum value. In this range the interface is considered to be saturated with surfactant (van Voorst Vader, 1960a) and the continued reduction in the surface tension is due mainly to the increased activity of the surfactant in the bulk phase rather than at the interface (equation 2.17). For ionic surfactants in the presence of a constant concentration of counterion, this region of saturated adsorption may extend down to one-third of the CMC. 

Dioxane, ethylene glycol, water-soluble esters, and short-chain alcohols at high bulk phase concentrations may increase the CMC because they decrease the cohesive energy density, or solubility parameter, of the water, thus increasing the solubility of the monomeric form of the surfactant and hence the CMC (Schick, 1965). An alternative explanation for the action of these compounds in the case of ionic surfactants is based on the reduction of the dielectric constant of the aqueous phase that they produce (Herzfeld, 1950). This would cause increased mutual repulsion of the ionic heads in the micelle, thus opposing micellization and increasing the CMC. 

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). 

The reduction of the tension at an interface by a surfactant in aqueous solution when a second liquid phase is present may be considerably more complex than when that second phase is absent, i.e., when the interface is a surface. If the second liquid phase is a nonpolar one in which the surfactant has almost no solubility, then adsorption of the surfactant at the aqueous solution-nonpolar liquid interface closely resembles that at the aqueous solution-air interface and those factors that determine the efficiency and effectiveness of surface tension reduction affect interfacial tension reduction in a similar manner (Chapter 2, Section IIIC,E). When the nonpolar liquid phase is a saturated hydrocarbon, both the efficiency and effectiveness of interfacial tension reduction by the surfactant at the aqueous solution-hydrocarbon interface are greater than at the aqueous solution-air interface, as measured by pC2o and IIcmc, respectively. The replacement of air as the second phase by a saturated hydrocarbon increases the tendency of the surfactant to adsorb at the interface, while the tendency to form micelles is not affected significantly. This results in an increase in the CMC/C2o ratio. Since the value of rm, the effectiveness of adsorption (Chapter 2, Section IIIC), is not affected significantly by the presence of the saturated hydrocarbon, the increase in the... 

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