Solutions of soaps and other long-chain colloidal electrolytes

Solutions of soaps and other long-chain Colloidal electrolytes. The surface tension of soaps has been very extensively studied,1 but for the most part the results in the literature are discordant far beyond the usual error of measurement of surface tension. In general the surface tension diminishes rapidly with increasing concentration, reaching a steady, or nearly steady, low value after a certain concentration is reached this concentration is naturally lower the longer the hydrocarbon chain. The variation between the results obtained by different experimenters, and even by the same experimenter under different conditions, may 

Addition of alkali suppresses the hydrolysis, and when sufficient alkali has been added for complete suppression of hydrolysis, the adsorbed layer consists of neutral soap. This is probably the state of affairs at the maxima of surface tension in Fig. 32. More alkali is required to reach the maximum with the stronger solutions, because more alkali is needed to suppress hydrolysis completely. The maximum surface tensions in Fig. 32 are probably very near to the surface tension of solutions of neutral soap only of the concentration indicated on each curve. The subsequent slow fall of tension, as more alkali is added, is probably due to a salting out of the soap by the alkali, an increase in escaping tendency caused by the presence of comparatively large amounts of another solute. It would be interesting to find whether addition of neutral salt, in addition to the small amount of alkali needed to reach the maximum, produces a fall in tension similar in amount to that given by additional alkali. 

So-called neutral soap solutions are more complex systems than at first appears, and the fairly small amounts of hydrolysis in the interior of the solution are very much magnified by the great difference in adsorbability between a neutral soap and an acid soap, or free fatty acid. A close approximation to the true surface tension-concentration curve of neutral soap can probably be obtained by plotting the maxima of tension shown in Fig. 32 against the concentration on each curve. It does not seem possible as yet to plot the true surface tension-concentration curve for acid soap, as the concentration of acid soap, and even the ratio of acid to soap molecules in any compound which may be formed in the interior, is unknown. 

According to Lottermoser and Baumgurtel,3 the final surface tension of soap solutions is reached in a few minutes Ekwall, however, records a fall lasting at least two hours in solutions more dilute than N/20,000. 

Paraffin-chain salts, similar in general constitution to the soaps but containing a strongly dissociated end group such as a sulphonic acid or a quaternary ammonium atom, are not subject to hydrolysis, and might be expected to behave in a simpler manner. The surface-tension measurements of Adam and Shute,1 R. C. Brown,2 and Lottermoser and others8 indicate, however, a curious, very slow attainment of the final surface tension in solutions so dilute that there are few, if any, ionic micelles present in the interior. The tension may take several days to reach the final value and when the final tension is reached it appears to be independent of the concentration, at least for solutions over 0-003 per cent., i.e. of the order N/10,000. The amount of this final tension depends somewhat on the nature of the end group it is usually about 30 dynes per cm. 

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Solutions of soaps and other long-chain colloidal electrolytes... 

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