Alkyl chain length, critical micelle

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

The industrial pro cess is b ased on the Fischer synthesis and commercial development work started some 25 years ago. Once a route using coconut-based alcohols had been established, full-scale plants became viable and alcohol blends are now used to control hydrophobicity of the molecules produced. Thus, products based on C8 alcohols are water soluble and those based on C18 are virtually insoluble. The critical micelle concentration (CMC) values of pure alkyl glucosides and CulCu alkyl polyglucoside (APG) are comparable with those of a typical non-ionic surfactant and decrease with increasing alkyl chain length. 

Area ( s) Occupied by a Surfactant Molecule at a Polystyrene-Water Interface at the Critical Micelle Concentration Effect of Alkyl Chain Length... 

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). 

Table 4 Critical micelle concentration (CMC) and static surface (Yen,) tension of alkyl glycosides with different head group and alkyl chain lengths...

The influences both of the alkyl chain length and of the terminal amino acid sidechain on the physicochemical properties are shown in Table 6. The study was limited to the water-soluble compounds in which the surface tension reduction of the aqueous solutions was determined by the Du Noiiy ring technique at room temperature as a function of log (C). Parameters studied were critical micelle concentration (cmc, determined from the break point of the surface tension vs. concentration curves), Ycmc (the surface tension at the cmc), and the minimum area per surfactant molecule at the air/aqueous solution... 

The formation of a significant number of micelles is determined by the concentration range, which is called the critical micelle concentration (CMC). Above the CMC, there appears an equilibrium between individual monomers and micelles, and this system is thermodynamically stable. The CMC is the most important parameter defining micellar solutions, and its value depends on a number of factors, the most important being monomer structure, solvent properties, temperature, and concentration. The value of the CMC for micelles depends mainly on the alkyl chain length and, to a lesser extent, on the structure of the hydrophilic part. 

The results of Fig. 2.15 clearly show distinct inflection points which may be identified with the critical micelle concentration (cmc). However, the aggregation numbers of these association units are much lower (in the region of 9-12) than those encountered with micellar surfactants (which show aggregation numbers of 50 or more depending on the alkyl chain length). These lower aggregation numbers cast some doubt on micelle formation and a continuous association process may be envisaged instead. 

In fact, solubility near the KP is almost equal to the critical micelle concentration (CMC). The magnitude of KP is dependent on the chain length of the alkyl chain (Figure 3.7). 

Skin safety of niosomes was tested in a number of studies. As an example, the toxicity of polyoxyethylene alkyl ether vesicles containing Ci2-i8 alkyl chains and 3 and 7 oxyethylene units was assessed by measuring the effect on proliferation of cultured human keratinocytes [47]. It was found that the length of either polyoxyethylene headgroup or alkyl chain had only a minor influence on keratinocyte proliferation. However, the ether surfactants were much more toxic than esters tested in this study. The concentrations of ether surfactants required to inhibit cell proliferation by 50% were 10-fold lower than for ester surfactants. Neither the HLB nor the critical micelle concentration values or cholesterol content affected keratinocyte proliferation. 

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