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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.

Specific ion effects in surfactant solutions affect the properties of micelles. Dodecyltrimethylammonium chloride (DTAC), bromide (DTAB), and methanesulfonate (DTAMs) micelles are typically spherical, but some organic anions can induce shape or phase transitions in DTA(+) micelles. Above a defined concentration, sodium triflate (NaTf) induces a phase separation in dodecyltrimethylammonium triflate (DTATf) micelles, a phenomenon rarely observed in cationic micelles. This unexpected behavior of the DTATf/NaTf system suggests that DTATf aggregates have unusual properties. The structural properties of DTATf micelles were analyzed by time-resolved fluorescence quenching, small-angle X-ray scattering, nuclear magnetic resonance, and electron paramagnetic resonance and compared with those of DTAC, DTAB, and DTAMs micelles. Compared to the other micelle types, the DTATf micelles had a higher average number of monomers per aggregate, an uncommon disk-like shape, smaller interfacial hydration, and restricted monomer chain mobility. Molecular dynamic simulations supported these observations. Even small water-soluble salts can profoundly affect micellar properties; our data demonstrate that the -CF3 group in Tf(-) was directly responsible for the observed shape changes by decreasing interfacial hydration and increasing the degree of order of the surfactant chains in the DTATf micelles.

Noninvasive techniques such as FT-IR and (1)H NMR spectroscopy have been employed to investigate the solubilization of formamide, FA, and its aqueous solution, FA-water, by sodium 1,4-bis(2-ethylhexyl)sulfosuccinate, AOT, in heptane or isooctane reverse micelles, respectively. Partially deuterated FA (FADH) was used in the FT-IR experiments and nu(OD), n(ND) were analyzed. Also, the nu(C=O) band of FA was investigated. For AOT, the changes of the SO(3)(-) group's symmetric, nu(s), and asymmetric, nu(a), bands were also studied. The results are showing that FA is interacting strongly with the Na+ counterions of the surfactant through electrostatic interactions maintaining their hydrogen bond network present in the FA bulk. Accordingly, partially deuterated FA is "frozen" inside the aggregates and it is possible to detect, by FT-IR technique, the cis and trans isomers. Curve fitting of the nu(OD) (in the FA-water mixture) band requires use of two peaks because the band is asymmetric, not because the solubilizate molecules are present in layers of different structure. The chemical shifts of the (1)H bound to N and C of FA were studied by (1)H NMR. The comparison of the chemical shift of AOT in reverse micelles with FA and the FA-water mixture in the polar core of the aggregate shows that there is a strong preferential solvation of Na+ by FA (through electrostatic interaction) and the AOT's sulfonate group by water (through hydrogen bond interaction).

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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.

The solvation of six solvatochromic probes in a large number of solvents (33-68) was examined at 25 degrees C. The probes employed were the following: 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (RB); 4-[(E)2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePM; 1-methylquinolinium-8-olate, QB; 2-bromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr, 2,6-dichloro-4-(2,4,6-triphenyl pyridinium-1-yl) phenolate (WB); and 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr(2), respectively. Of these, MePMBr is a novel compound. They can be grouped in three pairs, each with similar pK(a) in water but with different molecular properties, for example, lipophilicity and dipole moment. These pairs are formed by RB and MePM; QB and MePMBr; WB and MePMBr(2), respectively. Theoretical calculations were carried out in order to calculate their physicochemical properties including bond lengths, dihedral angles, dipole moments, and wavelength of absorption of the intramolecular charge-transfer band in four solvents, water, methanol, acetone, and DMSO, respectively. The data calculated were in excellent agreement with available experimental data, for example, bond length and dihedral angles. This gives credence to the use of the calculated properties in explaining the solvatochromic behaviors observed. The dependence of an empirical solvent polarity scale E(T)(probe) in kcal/mol on the physicochemical properties of the solvent (acidity, basicity, and dipolarity/polarizability) and those of the probes (pK(a), and dipole moment) was analyzed by using known multiparameter solvation equations. For each pair of probes, values of E(T)(probe) (for example, E(T)(MePM) versus E(T)(RB)) were found to be linearly correlated with correlation coefficients, r, between 0.9548 and 0.9860. For the mercyanine series, the values of E(T)(probe) also correlated linearly, with (r) of 0.9772 (MePMBr versus MePM) and 0.9919 (MePMBr(2) versus MePM). The response of each pair of probes (of similar pK(a)) to solvent acidity is the same, provided that solute-solvent hydrogen-bonding is not seriously affected by steric crowding (as in case of RB). We show, for the first time, that the response to solvent dipolarity/polarizability is linearly correlated to the dipole moment of the probes. The successive introduction of bromine atoms in MePM (to give MePMBr, then MePMBr(2)) leads to the following linear decrease: pK(a) in water, length of the phenolate oxygen-carbon bond, length of the central ethylenic bond, susceptibility to solvent acidity, and susceptibility to solvent dipolarity/polarizability. Thus studying the solvation of probes whose molecular structures are varied systematically produces a wealth of information on the effect of solute structure on its solvation. The results of solvation of the present probes were employed in order to test the goodness of fit of two independent sets of solvent solvatochromic parameters.

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