CMC/C20 ratio

A convenient way of measuring the relative effects of some structural or microenvironmental factor on micellization and on adsorption is to determine its effect on the CMC/C20 ratio, where C20 (Chapter 2, Section HIE) is the concentration of surfactant in the bulk phase that produces a reduction of 20 dyn/cm in the surface tension of the solvent. An increase in the CMC/C2o ratio as a result of the introduction of some factor indicates that micellization is inhibited more than adsorption or adsorption facilitated more than micellization a decrease in the CMC/C2o ratio indicates that adsorption is inhibited more than micellization or micellization... 

Some CMC/C20 ratios are listed in Table 3-5. The data show that for single-chain compounds of all types listed the CMC/C20 ratio ... 

The decrease in the CMC/C20 ratio with increase in temperature (10-40°C) presumably occurs either because the size of the hydrophilic group decreases as a result of dehydration with this change or because the surface area of the micelle increases with this change. 

In general, then, ionic surfactants (both anionic and cationic) with a single straight-chain hydrophobic group, in distilled water against air at room temperature, show low CMC/C20 ratios of 3 or less, while POE nonionics under the same conditions show ratios of about 7 or more. Increase in the electrolyte content of the solution causes the CMC/C20 ratios of ionics to approach those of nonionics. Zwitterionics have CMC/C20 ratios intermediate between those of ionics and POE nonionics. 

Without using the tables, place the following compounds in order of decreasing CMC/C20 ratios. Use if values are approximately equal. 

The factors that affect the CMC/C20 ratio have also been previously discussed (Chapter 3, Section VA). We have seen that ... 

Some of these factors affect Ym and the CMC/C2o ratio in parallel fashion (i.e., they increase both or decrease both) some in opposing fashion. When the effects are parallel, we can readily predict the resulting change in the effectiveness of surface tension reduction when they are opposed, it is difficult to do so. Thus, increase in the length of the hydrophobic group in ionic surfactants has little effect on either Ym or the CMC/C20 ratio, and we can therefore expect that an increase in the length of the hydrophobic group will have little effect on their effectiveness of surface tension reduction. 

On the other hand, the introduction of some branching into the hydrophobic group increases the CMC/C20 ratio but has little effect on Ym. We can therefore expect that the introduction of branching into the hydrophobic group will make the surfactant a more effective surface tension reducer. This is seen in the isomeric p-dodecylbenzene-sulfonates (Figure 5-4), where the isomers with branched alkyl chains, although less efficient reducers of the surface tension than the isomer with the straight alkyl chain, reduce the surface tension to lower values than does the latter. 

The critical micelle concentration (c.m.c.) of the material increases with decrease in pH of the solution below 5, as is to be expected as the ratio of to B- increases. The increase in the c.m.c. is somewhat greater than the increase in the C20 value with pH decrease as shown by the cmc/C2o ratio, indicating somewhat greater inhibition of micellization than of adsorption at the aqueous solution/air interface as the BHVB- ratio increases. This may reflect some steric inhibition of micellization resulting from the increased size of the protonated hydrophilic head. 

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