The Effect of a Terminal Double Bond on the Micellization of a Simple Ionic Surfactant ESTEL D. SPRAGUE, DAVID C. DUECKER, AND C. E. LARRABEE, JR. Department of Chemistry, University of Cincinnati, CincinnatL Ohio 45221 Received June 3, 1982; accepted August 9, 1982 Conductance measurements have been employed to determine the critical micellization concen- tration (CMC) of sodium 10-undecenoate from 5 to 30°C and of sodium undecanoate from 10 to 30°C. The CMC for sodium 10-undecenoate was found to be about a factor of 2 greater than that for sodium undecanoate from l0 to 30°C, although this factor exhibited a small, but consistent, increase with increasing temperature. INTRODUCTION Unsaturation in simple hydrocarbons tends to increase their water solubility. Careful measurements have shown that the intro- duction of one double bond is essentially equivalent to removing one CH2 group from a fully saturated chain in its effect on the standard free energy for transfer of the hy- drocarbon from aqueous solution to pure liq- uid hydrocarbon at 25°C (1, 2). On the other hand, relatively little is known about the ef- fect of unsaturation on the micellization of unsaturated surfactants (3). Early measure- ments indicate, for example, that incorpo- ration of a double bond near the center of the hydrocarbon chain in long-chain surfac- tants has an effect on the critical micelliza- tion concentration (CMC) equivalent to the removal of 1 to 1.5 CH2 groups from a sat- urated chain (2, 4). Thus the CMC values for potassium oleate and potassium elaidate are roughly a factor of 3 higher than that for potassium stearate at 55-60°C. These results demonstrate that a double bond is less hy- drophobic, or more hydrophilic, than its sat- urated counterpart and raise the interesting question whether a terminal double bond on a simple, ionic surfactant might have some tendency to act as a second headgroup. If so, 0021-9797/83/040416-06503.00/0 Copyright© 1983by Academic Press,Inc. All rightsof reproduction in any form reserved. this should be reflected in the value of the CMC in aqueous solution. The aggregation behavior of molecules with two ionic headgroups, the bolaform sur- factants (5), has been examined. Docosane- 1,22-bis(trimethylammonium bromide), with ionic groups on the ends of a 22-carbon chain, has nearly the same CMC as the sur- factant with only a 14-carbon chain and one such headgroup (6). Even more striking is the fact that dodecane- 1,12-bis(trimethylam- monium bromide) has been shown by a va- riety of techniques not to exhibit critical micellization behavior. The smoothly vary- ing solution properties indicate the forma- tion of small (possibly only dimers or tri- mers) loose aggregates (6). Since the presence of two genuine headgroups alters the aggre- gation behavior so dramatically, a significant effect upon introduction of a terminal double bond seems possible. To our knowledge, a direct test of this possibility has not been made before. The unsaturated surfactant we have ex- amined is sodium 10-undecenoate. Our in- terest in this particular material derives from the fact that it can be made to polymerize under certain conditions. Small polymers (~ 10-mers) have been prepared in this lab- oratory by exposing dilute, aqueous solutions 416 Journalof Colloid and Interface Science, Vol. 92, No. 2, April 1983