NICELLIXATION OF DIMETHYL-%-ALKYLAMINE OXIDES
3575
Calorimetiric Studies of the Micellization of Dimethyl-n-alkylamine Oxides1
by L. Benjamin M i a m i Valley Laboratories, T h e Procter and Gamble Company, Cincinnati, Ohio
(Received FebruaTy 10, 1964)
Heat of solution measurements have been made with a series of dimethyl-n-alkylamine oxides (DC,AO) in water. The derived partial molal enthalpy values, R2, allow estimates to be made of the standard frce energies of micellization (AF,') to furnish the corresponding The well-known regular decrease in A F m o with increasing chain entropy changes, AS,'. length is seen to be due to a parallel decrease in AHm0since the entropy of micellization is essentially independent of chain length. The chain lengths studied are CS, CS, Clot and Clz. Below the critical micelle concentration (c.m.c.) considerable heats of dilution are found with the shorter chain length compounds while 142 values are more nearly constant for the Clo and Clz compounds. In all cases B2is found to be essentially constant above the c.m.c., even up to the point of separation of a mesomorphic phase (30-505G w./w.). The positive values of AH,' are interpreted in terms of hydrophobic bonding present during micellization. Data obtained for DC1,AO in dilute HCl solutions, where the surfactant is cationic, show a negative value for AH,' and indicate a lower entropy change for the ionic species than for the nonionic species. Comparison with published data for ionic surfactants shovvs that this is generally true arid is interpreted as resulting from the disrupting influence of ionic head groups on the ordering of water around the monomer.
Introduction Considerable attention has been given in the last decade to a study of the thermodynamics of surfactant solutions, the objective being to understand the factors which govern the formation of micelles. Standard frele energies (AFm") and enthalpies (AH, ") of micellization have been derived from critical micelle concentrations (c.m.c.) and their temperature dependence, respectively. I n addition, calorimetric AHm and heat capacity values have been obtained for certain ionic surfw tants. 2-G It has been concluded from these studies that the entropy increase during micellization contributes largely to the free energy change and overrides the unfavorable enthalpy term. Structural changes in the solvent, associated with loss of hydration of the hydrocarbon chain when the surfactant enters the micelle, are thought to be iresponsible for the positive AS,' values, and, thus, micelle formation is partially a result off what has been termed hydrophobic bonding. A statistical mechanical treatment of the latter has recently been presented.6
The present w0.t-k concerns calorimetric measurement of AH,' values for a homologous series of nonionic surfactants, the dimethyl-n-alkylamine oxides (DC,AO), and the application of such data to the thermodynamics of micellization in these systems. Other calorimetric data are available only for ionic surfactants where interpretation is somewhat complicated by varying electrical interactions. 2-5 Following a consideration of nonionic micellization according to the law of mass action, the experimental data are interpreted particularly in terms of the thermodynamic
(1) Paper presented at the Kendall Award Symposium, Division of Colloid Science of the Aimerican Chemical Society, Philadelphia, Pa., April, 1964. (2) (a) E. D. Goddard and G. C. Benson, Trans. Faraday SOC., 52,409 (1956); (b) E. D. Godditrd, C. A. J. Hoeve, andG. C. Benson. J . P h y s . Chem., 61, 593 (1957). (3) P. White and G . C . Benson, J . Colloid Sci., 13, 584
