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Observed 13C NMR chemical shift changes with respect to their single micelles upon mixed-micelle formation of potassium dodecanoate and short-chain potassium carboxylates (hexanoate up to an including decanoate) are described in all but one case to increasing distances between the apolar ends of the long amphiphile chains as compared with its single micelle. Only for dodecanoate-hexanoate micellar systems can a different conformational equilibrium of the dodecanoate chain not be excluded. Furthermore, recently observed solvent effects upon mixing of n-alkanes of different chain lengths are compared with both the decanoate and nonanoate chemical shift changes upon mixing with the dodecanoate amphiphiles. This leads to the conclusion that the former detergents are mainly subject to increased intermolecular chain packing. Observed effects for the octanoate and heptanoate are not as pronounced, and these surfactants should be considered as borderline cases, while the hexanoate undergoes conformational changes toward more extended forms. Finally, it is observed that maximally ca. 1 equiv of hexanoate or ca. 2 equiv of heptanoate can be incorporated into micelles of potassium dodecanoate. At higher percentages of short-chain surfactants, these maximally incorporated mixed micelles coexist with short-chain monomers up to the concentration where the free short surfactants reach their critical micelle concentration and may form micelles. We postulate that a statistical distribution of dodecanoate molecules in short-chain micelles is attained.

The triplet fine structures in the I3C NMR spectra of carbons in the a-position to nitrogen in several n-alkyltrimethylammonium bromides (TAB's), DMPC, and DPPC have been studied in different aggregational states under conditions of enhanced proton noise decoupling. Under the same conditions, the signals of the hydrophobic tails of the lecithins could also be studied better than hitherto, mainly by virtue of relatively narrow signals. It is shown that relatively small changes in packing and lateral motions can be detected. Intercalation of several n-alkyltrimethylammonium bromides in lecithin vesicles causes no decrease of the lipid -Nf(CH3)3 mobility around the CH2-CH2 head-group linkage nor a decrease in acyl chain mobility. Moreover, no changes in acyl chain kinking are detected. On the other hand, the incorporated TAB molecules are forced by the lecithin molecules toward severely restricted head-group and tail mobilities. For those combinations of PC and TAB's where the TAB I3C NMR signals are detectable, a change in the conformational equilibrium toward more extension is found. A packing model for the incorporation of TAB's in PC vesicles is presented which probably has a rather general validity. The behavior of mixed micelles of Pc's and TAB's, originating from enhancing the latter's concentration, is also described. In these systems, mobilities and chain kinking are increased with respect to the vesicular state.

CTAB/TTAB mixed micelles, studied by Lindman et al.,lS correspond nicely with those of the 1:l mixed micelles of decanoate and dodecanoate of this study. In retrospect, the CTAB/TTAB mixed micelles form a special case of the more general situation as presented.J. Phys. Chem. 1982Chem. , 86, 2528Chem. -2533 Acknowledgment. This investigation has been supported by the Netherlands Foundation for Chemical Research (SON) 13C NMR measurements of dioctanoyl-L-a-lecithin micellar solutions detect an almost complete visibility of the intrinsic magnetic nonequivalent behavior of the two lipid acyl chains. This is an extension of recently published observations. According to the latter, the chemical shift data are interpreted in terms of effectively different lengths of the sn-1 and sn-2 acyl chains due to a bending near the C2 carbon atom of the sn-2 chain. Mixed micellar systems of DOPC and several n-alkyltrimethylammonium bromides show a difference in effective chain lengths of the constituent detergent types. 13C NMR shieldings are observed for n-alkyl detergent fragments which are longer than both acyl chains of the lipid molecules. At an effective chain length difference of seven carbon atoms a sizeable contribution of extra gauche conformers with respect to their single micelles occur for these n-alkyl surfactants. For smaller differences decreasing van der Waals interactions (i.e., decreasing molecular packing) participate almost exclusively leading to chain separation. The deshieldings observed for the n-alkyl segments situated directly between neighboring lecithin chains indicate conformational changes toward more extended forms, as compared with their single micellar solutions, rather than increasing van der Waals interactions. The lipid molecules do not undergo measurable conformational changes upon mixed micelle formation but are only subject to increased molecular packing. This may indicate that conformational changes are of minor importance for solubilizing micelle bound hydrocarbon-like compounds.

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