Structure and aggregation in dilate solution of surface active agents

Summary Critical micelle concentrations (CMC) are shown to depend on chain length. All straight chain saturated surface active agents of equal ion length have approximately the same CMC. Thus a C 13 fatty acid soap, a C 12 sulfonate, a C 11 sulfate, and a C 12 ammonium chloride have CMC values of 0.010–0.014M. Values of CMC are not changed to any extent by substitution near the hydrophilic head of one, two, or three groups, even as large as hydroxyethyl, in place of the amine hydrogens in the cationic detergents. However substitution with dihydroxypropyl groups has a marked effect on association in C 12 but not in the C 16 series, indicating that the relative lengths of the two chains must be considered as an important factor in association. This is well illustrated in the dialkylsulfosuccinate series, and in a tetradecane sulfate series in which the −SO 4 Na group was progressively moved down the chain, particularly when the branched chain compounds are compared with the corresponding straight chain detergents of length equal to the maximum length from the charged head to the ultimate carbon atom. Introduction of double bonds causes a small but definite increase in CMC whereas polar substitution in the chain results in a marked increase in CMC. Possible micelle structures are discussed in the light of these association phenomena, and it is concluded that the assignment of a definite size and shape to micelles appears at the present time to be slightly premature. However if one includes the concept of relative order‐disorder in the micelle as one of the factors, in addition to chain length, type of detergent, and environment, which are important in micelle structure, it is possible to explain partially the apparent marked differences which have been reported for micelles of different surface active agents.