A 2H NMR study of order and dynamics in the cubic I1 phase of the dodecyltrimethylammonium chloride/water system

The micellar cubic phase I₁ of the dodecyltrimethylammonium chloride (C₁₂TACl)/water system has been studied by ²H NMR spectroscopy. Relaxation rates of selectively deuteriated surfactant were measured at several frequencies down to 0.5 MHz in the temperature range 25-61°C. The results are interpreted within models for the reorientational dynamics in I1 phases and it is concluded that the remaining order after surface diffusion and aggregate rotation is small and does not vary substantially with temperature. The results are discussed in relation to the two prevailing models for the structure of these phases as suggested (separately) by Fontell and Vargas. At 0°C the ²H NMR spectrum from perdeuteriated n-dodecane solubilized in this phase shows static quadrupole splittings with a magnitude that is in good agreement with the results obtained from the analysis of the relaxation data.     

Intermediate phases in the dodecyltrimethylammonium chloride/water system

Two intermediate phases have been found in the concentration range between the hexagonal and concentrated cubic phases in the binary system dodecyltrimethylammonium chloride (C₁₂TACl)/water. This region in the phase diagram was studied by means of ²H NMR of specifically deuteriated surfactant as well as by ¹⁴N NMR. Below 35°C, an intermediate phase with non-cylindrical aggregates is formed in the concentration range 80 to 84 wt% surfactant, X-ray data from this phase can be indexed to a centred rectangular lattice. In addition, there is a uniaxial phase with a reduced quadrupole splitting. The aggregates comprising the centred rectangular phase were analysed by means of bandshape analysis of the NMR spectra and by small angle X-ray scattering.     

Thermodynamic and NMR study of aggregation of dodecyltrimethylammonium chloride in aqueous sodium salicylate solution

The complex aggregation processes of dodecyltrimethylammonium chloride (DTAC) have been studied in dilute solutions of sodium salicylate (NaSal) by isothermal titration calorimetry and electrical conductivity at temperatures between 278.15 K and 318.15 K. A structural transformation that was dependent on the concentrations of DTAC and NaSal was observed. The micellization process in dilute solutions of DTAC has been subjected to a detailed thermodynamic analysis and shown to occur at considerably lower critical micelle concentrations than reported for DTAC in water and NaCl solutions. Gibbs free energy, Δ _mic G ^o, and entropy, Δ _mic S ^o, were deduced by taking into account the degree of micelle ionization, β, estimated from conductivity measurements. From the temperature dependence of the enthalpy of micellization, Δ _mic H ^o, the heat capacities of micellization, D_mic c_p^o {\Delta_{{{\rm mic} }}}c_p^o were determined and discussed in terms of the removal of large areas of non-polar surface from contact with water upon micellization. The process is exothermic at all temperatures, indicating, in addition to the hydrophobic effect, the presence of strong interactions between surfactant and salicylate ions. These were confirmed by ¹H NMR spectroscopy and diffusion NMR experiments. Salicylate ions not only interact with the headgroups but also insert further into the micelle core. At c _NaSal/c _DTAC > 2.5, the structural rearrangements occur even at relatively low concentrations of NaSal.     

1H NMR spectroscopic study of theert-butyl chloride—aluminum bromide cationic initiating system

The interaction oftert-butyl chloride with aluminum bromide in methylene dibromide at −30°C leads to the formation of two types of adducts, which give signals with δ 2.4 and 3.2 in the¹H NMR spectra in addition to that of free alkyl halide. these signals are attributed to a polarized complex (PC) and ion pair (IP), respectively. An excess of AlBr₃ shifts the equilibria toward IP. The latter contains more AlBr₃ than the polarized complex. Based on the spectral data, we calculated the limiting values of some equilibrium constants. The ability of AlBr₃ to solvate counterions is consistent with the results of isobutylene polymerization under the action of the initiating Bu^tCl−AlBr₃ system at different ratios of the starting concentrations [AlBr₃]₀/[Bu^tCl]₀. An increase in this ratio results in both the acceleration of polymerization and an increase in the relative role of chain transfer reactions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2217–2222, November, 1998.     

A potentiometric and 51V NMR study of the aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine system

The speciation in the quaternary aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine (Ah) system has been determined from quantitative 51V NMR measurements and potentiometric data (glass electrode). The study was performed in 0.150 M Na(Cl) medium at 25 degrees C. Data were evaluated with the computer program LAKE, which is able to treat combined EMF and NMR data. The pKa values for Ah were determined as 8.06, 6.72 and 2.64. In the ternary H+/H2VO4-/Ah system, two complexes, (H+)p(H2VO4-)q(Ah)r, for which (p, q, r) values of (0, 1, 1) and (1, 1, 1) with log beta(0,1,1) = 2.52 +/- 0.03 and log beta(1,1,1) = 9.40 +/- 0.05 (pKa = 6.88), respectively, explain all data. The errors given are 3sigma. In the quaternary H+/H2VO4-/H2O2/Ah system, eight complexes were determined in addition to all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and four with a ratio 1:2:1 (X = peroxo ligand). VX2Ah2- and VX2Ah- (pKa = 8.19) are the main complexes and predominate in the pH range 5 to 9. Three additional minor species have also been found but their compositions could not be determined owing to their small amounts. Equilibria are slow, significant decomposition of peroxide occurs only in acidic solutions. Data in the pH range 5 to 10 have been used for the LAKE calculations. Chemical shifts, compositions, and formation constants for the eight quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams. Structural proposals for VX2Ah2- and VX2Ah- are made from 1H and 13C NMR measurements.     

Phase behavior of the palmitic acid/palmitin system. A 2H NMR study

The phase behavior of mixtures of palmitic acid (PA) and 1-monohexadecanoyl-rac-glycerol, palmitin, was studied by phase contrast microscopy and deuterium solid-state NMR. At pH 5, mixtures remained precipitated as lumps in solution. The NMR spectrum of the perdeuterated PA (PAd31) at 300 K exhibited a shape and quadrupolar splittings, deltav, characteristic of lipids embedded in a gel phase. The alkyl chains remained in a trans conformation with their long molecular axis oriented at about 15 degrees with respect to the bilayer normal. However, gauche defects were shown to occur at the end of the alkyl chain. At 330 K, the system underwent a phase transition to a hexagonal phase followed by an isotropic phase at 340 K. Upon cooling to 330 K, the spectrum in the hexagonal phase was oriented at 0 degrees showing that the cylinders were oriented with their long axis parallel to the field. Up to 11 positions (from 15) of PAd31 could be assigned. At pH 7 and 9 at room temperature, the mixtures were fully dispersed in a viscous solution of vesicles. The system underwent a phase transition at 320 K from a gel phase to a fluid phase with the bilayer normal oriented at 90 degrees with respect to the field. Analogous experiments performed with PA selectively labeled on carbon C2 allowed for the assignment of deltav for that position and suggested different conformations of the headgroup in the gel and fluid or hexagonal phases. The implications of these findings for the bio-availability of these fatty acids, in the understanding of the contribution of hydroxyl and carboxyl groups in the membrane formation, and for the production of simple self-oriented systems are discussed.     

13C and 2H NMR study of structure and dynamics in banana B2 phase of a bent-core mesogen

In this paper, the difficulty in orienting the B(2) phase of the banana mesogen 1,3-Phenylene-bis 4-[4-(10-undecenyloxy)-benzoyloxy] benzoate (Pbis11BB) in a relatively high magnetic field is reported based on some observations using both (13)C and (2)H NMR. (2)H NMR spectra recorded for the two labeled isotopomers of Pbis11BB in the isotropic and B(2) phases are shown here. Preliminary results on the deuteron spin-spin relaxation (T(2)) data are reported at 61 MHz in order to underline the peculiar slow dynamics of banana-shaped liquid crystals (BLC), and these results are discussed in the framework of recent studies on similar BLC. The molecular structure and dynamics in the B(2) and crystalline phases are also studied by (13)C solid-state NMR techniques. The results also point to the slow dynamics in the B(2) phase of Pbis11BB. In particular, two-dimensional MAS exchange experiment has been performed to shed light on the molecular conformation structure of the five-ring banana core in the crystalline phase of Pbis11BB, and to compare with that of quantum mechanical calculations reported in the literature.     

A deuteron NMR study on order and phase behaviour of liquid-crystalline copolymers

Liquid-crystalline (LC) block copolymers of lamellar morphology were prepared via anionic polymerization of polystyrene-block-1,2-polybutadiene and subsequent introduction of a deuterated mesogen by polymer analogous reaction. Deuteron NMR measurements show that the first-order nematic/isotropic transition in the block copolymer is changed to a critical behaviour under the influence of shear. In the nematic phase the order parameter is reduced with respect to the homopolymer, and there is no evidence of an isotropic boundary layer of the LC-block at the nematic/isotropic interface.     

Double quantum 1H MAS NMR studies of hydrogen-bonded protons and water dynamics in materials

Two-dimensional double quantum (DQ) 1H MAS NMR was used to investigate different proton environments in a series of alkali (Na, K, Rb, Cs) [Nb6O19]8- Lindqvist salts, with the water and hydrogen-bound intercluster protons being clearly resolved. Through the analysis of the DQ 1H NMR spinning sideband pattern, it is possible to extract both the mean and distribution of the motionally averaged intramolecular homonuclear 1H-1H dipolar coupling for the different water environments and the intercluster protons. Motional order parameters for the water environments were then calculated from the averaged dipolar couplings. The influence of additional intermolecular dipolar couplings due to multispin interactions were simulated and discussed.     

Ultrafast hydration dynamics in the lipidic cubic phase: Discrete water structures in nanochannels

We report here our studies of hydration dynamics of confined water in aqueous nanochannels (approximately 50 A) of the lipidic cubic phase. By systematically anchoring the hydrocarbon tails of a series of tryptophan-alkyl ester probes into the lipid bilayer, we mapped out with femtosecond resolution the profile of water motions across the nanochannel. Three distinct time scales were observed, revealing discrete channel water structures. The interfacial water at the lipid surface is well-ordered, and the relaxation dynamics occurs in approximately 100-150 ps. These dynamically rigid water molecules are crucial for global structural stability of lipid bilayers and for stabilization of anchored biomolecules in membranes. The adjacent water layers near the lipid interface are hydrogen-bonded networks and the dynamical relaxation takes 10-15 ps. This quasi-bound water motion, similar to the typical protein surface hydration relaxation, facilitates conformation flexibility for biological recognition and function. The water near the channel center is bulklike, and the dynamics is ultrafast in less than 1 ps. These water molecules freely transport biomolecules near the channel center. The corresponding orientational relaxation at these three typical locations is well correlated with the hydration dynamics and local dynamic rigidity. These results reveal unique water structures and dynamical motions in nanoconfinements, which is critical to the understanding of nanoscopic biological activities and nanomaterial properties.     

Molecular dynamics study of the properties of capsaicin in an 1-octanol/water system

Molecular dynamics simulations were performed to study the behavior of capsaicin in an 1-octanol/water system at 298 K and 1 bar. Capsaicin is the pungent chemical found in chili pepper that stimulates our sensory system resulting in a burning, pain sensation. In the first step toward investigating the activity of capsaicin, we have used two molecular representations for capsaicin based on the OPLS force field: all-atom and united-atom models. The octanol/water mixture was selected as a model system to determine the hydrophobic and hydrophilic properties of capsaicin by analyzing equilibrium, structural, and dynamic properties from the simulations. Our simulations showed that capsaicin preferentially partitions to the octanol phase, with its hydrocarbon segment oriented with that in octanol, while the polar part remains exposed to the aqueous phase. The simulations with the all-atom and united-atom models yielded similar results.     

1H and 13C NMR study of vinyl chloride telomers with carbon tetrachloride

The ¹H and ¹³C NMR spectra of some vinyl chloride telomers have been analysed.     

2H NMR study on the lyomesophases of the system hexaethylene glycol dodecyl methyl ether/water Temperature dependence of quadrupole splittings

Abstract

The liquid-crystalline phases of the system hexaethylene glycol n-dodecyl methyl ether (C₁₂E₆C₁)/water are studied by deuteron NMR spectroscopy. Information about the molecular orientation is derived from the quadrupole splittings of two selectively deuteriated derivatives of C₁₂E₆C₁, one deuteriated at the α-position of the alkyl chain, the other at the methoxy group. The temperature dependence of the quadrupole splittings reveals a continuous decrease of orientational order on approach to the macroscopic phase transformations. This behaviour is explained by an increase of defects and fluctuations in the microstructure of the mesophases.     

Structure and dynamics of micelles and cubic phase structures with ethoxylated phytosterol surfactant in water

The self-assembly of a sterol ethoxylate surfactant with 30 oxyethylene units in water was studied by 1H NMR self-diffusion measurements in a wide concentration range in the micellar region (0-25 wt %). The data showed that the surfactant aggregates do not interact by hard sphere interactions but rather a strong concentration dependence of the diffusion coefficient was noted which was explained by polymer scaling theory. In the cubic phase (30-65 wt %), the self-diffusion data from water, from surfactant, and from free polyoxyethylene suggest spherical micelles, although water diffusion was much restricted due to binding to the surfactant headgroup. From X-ray measurements in the cubic phase, the unit cell size was calculated, and together with surfactant self-diffusion measurements the exchange dynamics between free and aggregated surfactant was obtained.     

Study of water dynamics and distances in paramagnetic solids by variable-temperature two-dimensional 2H NMR spectroscopy

A recently proposed two-dimensional (2)H NMR experiment is used to measure the (2)H (spin I=1) quadrupolar and paramagnetic shift anisotropy interactions in powdered CuCl(2).2D(2)O as a function of temperature. The principal components of the quadrupolar and paramagnetic shift anisotropy tensors and the Euler angles describing the orientations of the tensors in the molecular frame are determined at each temperature. For this purpose an analytical approach is introduced to extract desired parameters from motionally averaged two-dimensional line shapes where the averaging is introduced by rapid 180 degrees flips around C(2) axes of D(2)O molecules. This approach can be readily applied to study various materials containing water of crystallization. It is also clearly shown that the rapid continuous rotation of D(2)O molecules around their C(2) axes is not taking place in the studied solid in the range of temperatures between 209 and 344 K. Once the paramagnetic shift anisotropy of a deuterium atom is measured accurately it is used to estimate the distance between deuterium and the nearest copper atom bearing an unpaired electron. Excellent agreement is found between structural parameters obtained in this study and those provided by neutron and x-ray diffraction, showing that the paramagnetic shift anisotropy is a sensitive probe of distances in paramagnetic solids.     

An eight-degree-of-freedom quantum dynamics study for the H2+C2H system

An eight-degree-of-freedom (8DOF) time-dependent wave-packet approach has been developed to study the H(2)+C(2)H-->H+C(2)H(2) reaction system. The 8DOF model is obtained by fixing one of the Jacobi torsion angle in the nine-degree-of-freedom AB+CDE reaction system. This study is an extension of the previous seven-degree-of-freedom (7DOF) computation [J. Chem. Phys. 119, 12057 (2003)] of this reaction system. This study shows that vibrational excitations of H(2) enhance the reaction probability, whereas the stretching vibrational excitations of C(2)H have only a small effect on the reactivity. Furthermore, the bending excitation of C(2)H, compared to the ground-state reaction probability, hinders the reactivity. A comparison of the rate constant between the 7DOF calculation and the present 8DOF results has been made. The theoretical and experimental results agree with each other very well when the present 8DOF results are adjusted to account for the lower transition state barrier heights found in recent ab initio calculations.     

1H NMR and DFT study of proton exchange in heterogeneous structures of pyridine-N-oxide/HCl/DCl/H2O

Moderately narrow 1H NMR signals were observed in the solid-phase obtained from pyridine-N-oxide (PyO)...HCl solutions in acetonitrile/H2O after heterogeneous phase separation. High-resolution 1H NMR spectra are compared with those of crystalline PyO...HCl and PyO...DCl. It is concluded that partially resolved peaks in 1H NMR spectra of solids are related with heterogeneity of the spin system and the presence of different mobile H-bond clusters containing PyO, HCl, DCl and water molecules. Some part of non-bonded water or HCl molecules is captured in the cavities of crystalline samples. The attribution of the 1H NMR signals was based on the density functional theory calculation of proton magnetic screening tensor of the most expected H-bond structures in the 6-311G** basis taking into account the solvent effect by the polarized continuum model.     

NMR study of polymer surfactant interaction in the system HPMC/SDS/water

The interaction between the nonionic water soluble polysaccharide hydroxypropylmethyl cellulose (HPMC) and the low molecular weight amphiphile sodiumdodecyl sulphate (SDS) has been studied by NMR¹H-chemical shift measurements and by self-diffusion NMR measurements. The polymer concentration has been kept sufficiently dilute to avoid coil overlap and the SDS composition range goes from zero up to well above the normal CMC point. Although a different fraction was used, the present results agree well with previous results for the same system obtained by techniques other than NMR and show very clear break points that can be related to the polymer surfactant interaction. Furthermore, it can be inferred from the chemical shift measurements that the structure of the micellar clusters are similar whether polymer is present or not. From a combination of chemical shift and self diffusion measurements it is also found that neither the size nor the shape of the clusters seem to change significantly in the composition interval investigated.     

Orientational order and dynamics of fluorescent probes in aligned lyotropic phases by fluorescence depolarization spectroscopy. Perylene in a nematic discotic phase. A comparison with 2H NMR

Abstract

The orientational order and dynamics of perylene in the nematic discotic phase (N_dII) of the potassium laurate/KCl/decanol/water system has been investigated by fluorescence depolarization (FD) spectroscopy. The order parameters of perylene-d ₁₂ in the same phase had been previously determined by ²H NMR. FD experiments were performed with the phase-modulation technique in the temperature range between 10°C and 50°C. At each temperature we could determine <P ₂> and <P ₄> for the axis of the transition moments of perylene, together with the rotational correlation time Θ_R. All these quantities assume reasonable values and show regular trends as functions of temperature. In particular, <P ₂> is in good agreement with that determined by ²H NMR.