Field-Dependent NMR Relaxation Study of Aggregation and Dynamics in Dilute to Concentrated Micellar Decylammonium Chloride Solutions

Physicochemical characterization of green sodium oleate-based formulations. Part 3. Molecular and collective dynamics in rodlike and wormlike micelles by proton nuclear magnetic resonance relaxation

HYPOTHESIS

Sodium oleate (NaOL) self-aggregates in water forming rodlike micelles with different length depending on NaOL concentration; when KCl is added wormlike micelles form, which entangle giving rise to a viscoelastic dispersion. It is expected that aggregates with different size and shape exhibit different internal and overall molecular motions and collective dynamics.

EXPERIMENTS

Two low viscosity NaOL/water and two viscoelastic NaOL/KCl/water formulations with different NaOL concentration (0.23 and 0.43 M) were investigated by ¹H fast field cycling NMR relaxometry over broad temperature and Larmor frequency ranges, after a first screening by ¹H and ¹³C NMR spectroscopy at high frequency.

FINDINGS

The analysis of the collected data indicated that fast conformational isomerization and rotation of NaOL about its long molecular axis and lateral diffusion of NaOL around the axis of the cylindrical aggregates are slightly affected by the aggregate shape and length. On the other hand, fluctuations of the local order director are quite different in the fluid and viscoelastic systems, reflecting the shape and size of the aggregates. Quantitative information was obtained on activation energy for fast internal and overall motions, correlation times and activation energy for lateral diffusion, and coherence length for collective order fluctuations.

Dodecan-1-aminium sulfate trihydrate

The asymmetric unit of the title salt, 2C12H28N+·SO4 2−·3H2O, contains two n-dodecylammonium cations, one sulfate anion and three water molecules. In the crystal, N—H...O hydrogen bonds link the cations and anions into layers parallel to (100). These layers are further connected through O—H...O hydrogen-bonding interactions involving the sulfate ions and the isolated water molecules. The three-dimensional structure can also be considered as the superposition of thin inorganic layers of SO4 2− anions and thick layers of alkylammonium cations perpendicular to the c axis.

Growth Behavior, Geometrical Shape, and Second CMC of Micelles Formed by Cationic Gemini Esterquat Surfactants

Micelles formed by novel gemini esterquat surfactants have been investigated with small-angle neutron scattering (SANS). The growth behavior of the micelles is found to differ conspicuously depending on the length of the gemini surfactant spacer group. The gemini surfactant with a long spacer form rather small triaxial ellipsoidal tablet-shaped micelles that grow weakly with surfactant concentration in the entire range of measured concentrations. Geminis with a short spacer, on the other hand, form weakly growing oblates or tablets at low concentrations that start to grow much more strongly into polydisperse rodlike or wormlike micelles at higher concentrations. The latter behavior is consistent with the presence of a second CMC that marks the transition from the weakly to the strongly growing regime. It is found that the growth behavior in terms of aggregation number as a function of surfactant concentration always appear concave in weakly growing regimes, while switching to convex behavior in strongly growing regimes. As a result, we are able to determine the second CMC of the geminis with short spacer by means of suggesting a rather precise definition of it, located at the point of inflection of the growth curve that corresponds to the transition from concave to convex growth behavior. Our SANS results are rationalized by comparison with the recently developed general micelle model. In particular, this theory is able to explain and reproduce the characteristic appearances of the experimental growth curves, including the presence of a second CMC and the convex strongly growing regime beyond. By means of optimizing the agreement between predictions from the general micelle model and results from SANS experiments, we are able to determine the three bending elasticity constants spontaneous curvature, bending rigidity, and saddle-splay constant for each surfactant.

Correlation between the geometrical shape and growth behaviour of surfactant micelles investigated with small-angle neutron scattering

The correlation between the growth behaviour and geometrical shape for CTAB-rich mixed micelles formed by the cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB) and the anionic surfactant sodium octyl sulphate (SOS) has been investigated with small-angle neutron scattering (SANS). Small tablet-shaped micelles formed by CTAB are found to grow only weakly in size with increasing surfactant concentration. The extent of growth becomes increasingly stronger as the fraction of SOS is increased. At higher fractions of SOS, a rather weak growth at low surfactant concentrations is followed by a sharp increase in aggregation numbers beyond a certain surfactant concentration. Such an abrupt transition from weakly to strongly growing micelles has been observed in the past for several micellar systems and is usually referred to as the second critical micelle concentration. The growth behaviour has been rationalized from a theoretical point of view by means of employing the recently developed general micelle model. The theory excellently predicts micellar growth behaviours as well as the observed correlation between the geometrical shape and micellar growth. In accordance, both width and length are found to slightly increase for weakly growing tablet-shaped micelles. On the other hand, strongly growing micelles that are observed above the second cmc display a completely different behaviour, according to which the length increases considerably while the width of the micelles decreases. Most interestingly, by means of optimizing the agreement between the general micelle model and experimentally determined aggregation numbers, we are able to determine the three bending elasticity constants: spontaneous curvature, bending rigidity and saddle-splay constant.

Explaining the growth behavior of surfactant micelles

The growth behavior of surfactant micelles has been investigated from a theoretical point of view. It is demonstrated that predictions deduced from the spherocylindrical micelle model, which considers micelles that are only able to grow in the length direction, are inconsistent with experimental measurements. Accordingly, the rise in aggregation numbers above a certain concentration, roughly corresponding to the second critical micelle concentration, appears to be much stronger than predicted by the spherocylindrical micelle model. On the other hand, predictions deduced from the general micelle model, which considers micelles that are able to grow with respect to both width and length, show excellent agreement with experimental observations. The latter theory is based on bending elasticity and it is demonstrated that the associated three parameters spontaneous curvature, bending rigidity and saddle-splay constant may all be determined for a micellar system from experimental measurements of the aggregation number as a function of surfactant concentration. The three parameters turn out to influence the appearance of a micellar growth curve rather differently. In accordance, the location of the second cmc is mainly determined by the saddle-splay constant and the bending rigidity. The shape of the growth curve, when going from the region of weakly growing micelles at low surfactant concentrations to strongly growing micelles above the second cmc, is mainly influenced by the bending rigidity.

Penta-decyl-ammonium methyl sulfate

In the crystal of the title compound, C₁₅H₃₄N⁺·CH₃SO₄⁻, the cations and anions are joined together via strong N—H⋯O hydrogen bonds into layers parallel to (001).

n-Tridecyl-amine chloride monohydrate

In the title compound, C₁₃H₃₀N⁺·Cl⁻·H₂O, the C₁₃H₂₇ alkyl chain is in an all-trans conformation. In the crystal, inter­molecular N—H⋯Cl, N—H⋯O and O—H⋯Cl hydrogen bonds connect the components into layers parallel to (010), with the alkyl chains oriented approximately perpendicular to these layers.

n-Dodecyl-ammonium bromide monohydrate

In the title compound, C₁₂H₂₈N⁺·Br⁻·H₂O, the ionic pairs formed by n-dodecyl­ammonium cations and bromide anions are arranged into thick layers; these layers are linked in a nearly perpendicular fashion [the angle between the layers is 85.84 (5)°] by hydrogen-bonding inter­actions involving the water mol­ecules. The methyl­ene part of the alkyl chain in the cation adopts an all-trans conformation. In the crystal structure, mol­ecules are linked by inter­molecular N—H⋯Br, O—H⋯Br and N—H⋯O hydrogen bonds.

Architecture of the hydrophobic and hydrophilic layers as found from crystal structure analysis of N-benzyl-N,N-dimethylalkylammonium bromides

The molecular and crystal structures of N-benzyl-N,N-dimethylalkylammonium bromides monohydrates with chain length n=8-10 have been determined. The crystals are isostructural with the N-benzyl-N,N-dimethyldodecylammonium bromide monohydrate. The structures consist of alternated hydrophobic and hydrophilic layers perpendicular to [001]. The attraction between N+ of the cation head-groups and Br- anions is achieved through weak C_H...Br interactions. The water molecules incorporated into ionic layers are donors for two O_H...Br hydrogen bonds and serve as the acceptors in two weak interactions of C_H...O type. The methylene chains, with the slightly curved general shape, have the extended all-trans conformation. The mutual packing of the chains in the hydrophobic layers is governed by weak C_H...pi interactions.

Headgroup Motion in a Lyotropic Lamellar Phase by Deuterium and Nitrogen-14 Relaxation Studies

Deuterium (2H) and nitrogen-14 (14N) NMR spectroscopy were used to investigate the molecular dynamics of a lyotropic liquid crystal. Deuterium spectral densities of motion for the C1 deuterated site on the chain of the molecule decylammonium chloride (DACl) at the Larmor frequency 61.4 MHz and those for the (14)N at the headgroup (NH(3)(+)) at 28.9 MHz are analyzed quantitatively in the lamellar phase of the DACl-d(11)/water binary system to shed light on the headgroup dynamics. The motional model used is the small step rotational diffusion for reorientations plus internal rotations of the methylene group in the strong collision limit. The tumbling motion of the long axis of the DACl molecule in the aggregates seems to be as rigorous as the molecular spinning motion, likely due to the proposed motional model. The similarity of deuterium spectral densities from the C1 and C2/C3 sites may indicate a relatively rigid unit of C1-C2-C3 in the backbone.

Collective and local molecular dynamics in the lyotropic mesophases of decylammonium chloride: 1H and 2H NMR study

The collective and individual dynamics of decylammonium chloride (DACl) molecules in water environment were investigated as a function of surfactant concentration and temperature. In the presence of water the DACl forms a variety of self-assembled structures, ranging from isotropic micellar systems to lyotropic liquid crystalline phases of hexagonal, nematic, and lamellar types. In order to characterize the complex molecular dynamics that occur in the DACl-water system, we applied 1H and 2H NMR techniques that cover the whole frequency range between 1 kHz and 30 MHz. The slow molecular dynamics were studied by 1H NMR fast-field-cycling T1 measurements and pulse-frequency dependence of 2H NMR transverse relaxation time, performed by means of the Carr-Purcell-Meiboom-Gill sequence. We detected a well-expressed contribution of order director fluctuations, i.e., layer undulations, with characteristic omega(-1)(L) frequency dependence of T(-1)(1) in the lamellar phase. Its presence indicates a relatively weak impact of interactions between neighboring DACl layers. The frequency dependence of proton T(-1)(1) in the hexagonal phase exhibits a different type of frequency dispersion, T(-1)(1) approximately omega(-1.32)(L). The increase in the exponent is explained with the quasi-one-dimensional character of fluctuations in elongated cylinders. Further, the T1 and T2 relaxation times of deuterons selectively attached to the C2 and C7 segments of the hydrocarbon chains of DACl were measured at a Larmor frequency of 30.7 MHz, providing quantitative information about local molecular dynamics.

Molecular dynamics of n-dodecylammonium chloride in aqueous solutions investigated by 2H NMR and 1H NMR relaxometry

Molecular dynamics in n-dodecylammonium chloride/water solutions for concentrations of 34 and 45 wt% was studied by 2H NMR and by 1H NMR dispersion of spin-lattice relaxation in the 2 kHz-90 MHz frequency range. The system exhibits a number of lyotropic liquid crystalline phases, which differ in symmetry and involve motions characterized by a wide frequency scale. The analysis of 2H NMR lineshapes of selectively deuterated DDACl molecules gave us an evidence for local trans-gauche conformational changes in the chains, whereas the dispersion of spin-lattice relaxation times T1 explored by fast field cycling method revealed fast local motions, translational diffusion and collective molecular dynamics of the chains. In particular, we have found that the order director fluctuation mechanism in smectic and nematic phases dominates spin-lattice relaxation below 1 MHz and that local motions and translational diffusion are responsible for the spin-lattice relaxation in the higher Larmor frequency range.