Study On The Correlations Between Critical Micelle Concentration And Molecular Energies And Structure Of Surfactants

The cmc values of a series of cationic, anionic, zwitterionic and nonionic surfactants were obtained from literatures. Various energy items were calculated using Hyperchem 8.0, which were correlated with cmc values by simple and stepwise regression analysis using SAS system for Windows V8 respectively. The results indicated that when Log(cmc) values were correlated individually with the energy items, the highest absolute correlation coefficient, ?r?=0.7509, 0.7195, 0.7803 are given by LogP of cationic, zwitterionic and nonionic surfactants and ?r?=0.6432 is given by binding energy of anionic surfactants, where the coefficients for LogP, surface area, volume, and core-core interaction are negative and others are positive. However by stepwise regression analysis, the total energy, isolated atomic energy, as well as the core-core interaction terms are involved in the final linear correlation equation of cationic surfactants and the one of anionic surfactants includs LogP, hydration energy, binding energy, core-core interaction and heat of formation. Besides, the final equation of zwitterionic surfactants embodys LogP only and LogP, binding energy, volume, heat of formation, with total energy are contained in the one of nonionic surfactants. Moreover, when put the energy items mentioned above into the corresponding regression equation respectively, new Log(cmc) values were received and the overall correlation coefficient is 0.8676, 0.8203, 0.7195, 0.9485 with the literature.In addition the effects of molecular structure on the cmc values were examined for amphoteric surfactants, alkylcarboxylbetaines, including single long alkylcarboxybetaines (CnB, n=12,14,16,18) and double long alkylcarboxybetaines(diC12B). The cmc values were determined by surface tension method and steady-state fluorescence probe method. The results show that for the single long alkylcarboxybetaines Log(cmc) decreases linearly with increasing n, the carbon atom number in the long alkyls, or following Log(cmc)=A-Bn. But Log(cmc) value of the double long alkylcarboxylbetaine (diC12B) deviates from the linear relationship. Although the hydrophobicity of diC12B, which has 24 alkyl carbon atoms, is much larger than that of C18B, its cmc value is greater than that of C18B. It is implied that double alkyl chain inhibits formation of micelles. Further study indicates that diC12B has a small micelle aggregation number, nearly 12 at a concentration of 2 mM, which is quite smaller than that of CnB. It is suggested that the presence of double long alkyls in diC12B may not be beneficial to formation of spherical micelles. Thus only small micelles can be formed at low concentration. Meanwhile the monolayer formed by adsorption of diC12B at oil/water interface may be planar instead of convex or concave, which is very good for obtaining ultralow IFT.