13C nuclear magnetic resonance spectra of diastereomeric C-24 alkyl sterols have been assigned. Differences in the chemical shifts of side-chain carbons permitted the determination of the absolute configuration at C-24 in several sterols since these chemical shifts are insensitive to structural changes remote from the asymmetric centre. An unknown sterol from Tetraselmissuecica has been identified as (24R)-24-methylcholest-5-en-3β-ol and the configuration assigned from 1H nmr data to the sterol from Phaeodoctylumtricornutum has been confirmed. The utility and potential of this method in characterising new sterols and their biological precursors is discussed.

The enthalpies of micellization of the surfactant series benzyl(2-acylaminoethyl)dimethylammonium chlorides, RABzMe(2)Cl, have been determined by calorimetry and conductivity measurements in the temperature range 15-75 degrees C. Here R stands for an acyl group containing 10-16 carbon atoms and A, Bz, and Me stand for NH(CH(2))(2)N(+), benzyl, and methyl groups, respectively. The enthalpy of micellization, DeltaH(mic) degrees , and the critical micelle concentration, cmc, were calculated directly from calorimetric data. The free energy of micellization, DeltaG(mic) degrees , was obtained from the cmc and the conductance-based degree of counterion dissociation. There is an excellent agreement between DeltaG(mic) degrees calculated from the data of both techniques, but the DeltaH(mic) degrees , the entropy of micellization, values differ. The dependence of the thermodynamic parameters of micellization on the chain length of the hydrophobic group and on the temperature has been analyzed by considering the delicate balance between the factors that contribute to micelle formation, including transfer of the surfactant hydrocarbon chain from the aqueous environment to the micelle, with concomitant release of the solvating water molecules, and the effect of temperature on the structure of water. DeltaG(mic) degrees is more negative, that is, more favorable for RABzMe(2)Cl than for the structurally related alkylbenzyldimethylammonium chlorides. This is attributed to direct and water-mediated H bonding between the amide groups of molecules of the former series.

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Aggregation of the following surfactant series in D2O has been studied by 1 H and 13 C NMR spectroscopy: RCONH(CH2)2N + (CH3)3Cl -, where RCO is an acyl group containing 10-16 carbon atoms. Micelle formation has been followed by measuring observed chemical shifts, δobs, and apparent transverse relaxation times, 1/T2*, of the surfactant discrete groups as a function of surfactant concentration, below and above its critical micelle concentration, cmc. Plots of δobs and/or 1/T2* versus [surfactant] are sigmoidal and were fitted to a model based on the mass-action law. A modified computation procedure was introduced in order to calculate the following: cmc; the equilibrium constant of micelle formation, K; the micelle aggregation number, N agg; and the chemical shifts of the monomer, δmon, and the micelle, δmic, respectively. The modification introduced permits simple and accurate calculation of the above-mentioned micellar parameters from the same set of experimental data. NMR-based cmc and N agg values are in excellent agreement with those previously determined by independent techniques. K and Nagg increase as a function of increasing the length of the surfactant hydrophobic tail. Gibbs free energies of micellization, ∆G°mic, were calculated and divided into contributions from the CH2 groups in the hydrophobic chain and from the (terminal CH3 + headgroup). Both quantities agree with those previously calculated from conductivity data. The contribution of the (terminal CH3 + headgroup) to ∆G°mic shows the importance to micellization of direct and/or water-mediated H-bonding of the surfactant amide group. Comparison of (δmic -δmon) with data in bulk solvents (CDCl3 and CD3OD/D2O, respectively) shows that the monomers are probably not fully exposed to D2O below the cmc, in agreement with previous NMR investigations of cationic surfactants.

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