Structural phase transformations induced by the addition of decanol to the hexagonal phase of the binary sodium decyl sulphate/water system

The hexagonal phase of the sodium decyl sulphate/water system transforms into a lamellar phase on the introduction of decanol. This transformation occurs in several steps. We present here a study of the sequence of the corresponding phase transformations. The various phases are identified according to their textures by optical microscopy. It can be seen that, as the decanol/soap ratio increases the two dimensional hexagonal phase is followed by two dimensional rectangular phases before the one dimensional lamellar phase is reached. The symmetries of the structures of the phases and the shapes of their aggregates of amphiphilic molecules were determined by small angle X-ray and neutron scattering studies (SAXS and SANS). Two rectangular phases with cmm and pgg symmetries show up successively between the hexagonal and lamellar phases. The shape of the aggregates evolves along the sequence in an unexpected manner. In the two dimensional hexagonal phase, the aggregates are cylinders with an isotropic circular section at low decanol/soap ratio, which become anisotropic as this ratio increases, i.e. the aggregates become ribbon-like aggregates. The aggregates keep this shape in the rectangular phases, with changes of size, and also, most probably, in the lamellar phase near the two dimensional rectangular phases where it can be seen that the lamellae are fragmented. The local organization of decanol and sodium decyl sulphate molecules within the ribbon-like aggregates was also investigated by SANS and deuteron magnetic resonance (DMR). The SANS studies show that the two molecules are not distributed uniformly within the aggregates, while the DMR measurements show that both amphiphilic molecules stay anchored at the amphiphile/water interface by their polar heads. It can then be inferred that the decanol molecules are preferably in the regions of lowest interfacial curvature and the sodium decyl sulphate molecules are in the regions of highest interfacial curvature. This study shows that addition of decanol in the hexagonal phase induces deformation of the cylinders into ribbons by a local flattening of the interface. This deformation, which starts already deep within the hexagonal phase, is not associated with a change of symmetry of the structure. It also appears that a further deformation of the ribbons into lamellae is not needed for the rectangular phase to change into the lamellar phase when the decanol content increases. Thus, in this sequence of phases, the symmetry of the aggregates and those of their organization are not necessarily related.     

Structural fluctuations in the lamellar phase of sodium decyl sulphate/decanol/water

We present a neutron scattering study of oriented samples for the lamellar phase of the ternary mixture sodium decyl sulphate/1-decanol/water. Diffuse scatterings are observed, around the Bragg reflections and away from them, which show that the structure of this lamellar phase deviates from the periodic stacking of infinite homogeneous lamellae of water and amphiphilic molecules usually proposed for the structure of lamellar phases. The nature of this deviation evolves with the soap/decanol ratio, according to the location of the sample in the lamellar domain of the phase diagram. In the middle of the domain the deviation relates to the organization of the lamellar stacking, without apparent modification of the structure of the lamellae of amphiphiles. Moving away from the middle, for higher soap/decanol ratios, the structure of the lamellae appears to be randomly perturbed, eventually by the presence of a few water regions piercing them. When the boundary of the lamellar domain is approached, for still higher soap/decanol ratios, the density of these peturbations increases and they start to be correlated over limited distances, within the lamellae and from lamella to lamella. The local symmetry of these short range correlations is such that these perturbations may be seen as structural fluctuations which may be seen as precursors of the transformation of the lamellar phase into a neighbouring phase on the phase diagram. This phenomenon is discussed briefly in relation to the structural fluctuations of the relative concentrations of sodium decyl sulphate and decanol within the aggregates.     

Evolution of cylindrical structures in the system sodium decylsulphate/decanol/water

The binary mixture sodium decylsulphate/water exhibits a hexagonal structure for a degree of hydration of about 50 per cent by weight. In this structure the amphiphiles build infinitely long cylinders which are organized on a 2D lattice of space group p6m. If, in such a mixture, sodium decylsulphate is progressively substituted by decanol, the hexagonal structure transforms itself successively into two rectangular structures. In these rectangular structures the amphiphiles also build cylinders, but these are organized on 2D lattices with space groups cmm and pgg, respectively. This evolution of cylindrical structures in the ternary mixture is also marked by a growth of the mean normal section of the cylinders. An analysis of these results is proposed, in which the growth is accompanied by a change in the normal section, from circular for the hexagonal structure to non-circular for the rectangular structures; this change occurs abruptly at the hexagonal-rectangular transition and is stabilized by an important fluctuation of the relative concentrations of the two amphiphiles within the cylinders.     

Reversed micellar solutions in the system sodium octanoate/decanol/water: Model calculations and dynamic light scattering measurements

Geometrical calculations of aggregate sizes in the reversed micellar solution phase of the system water/sodium octanoate/decanol at 20 °C have been tested by dynamic light scattering studies. The autocorrelation functions were interpreted in the simplest possible way (monodisperse aggregates, Stokes-Einstein diffusion equations) since the geometrical model does not account for detailed changes in shapes or micellar interactions. The model predicts the main features of micellization in these solutions, i. e., the micelles grow continuously as the concentration of water or the molar ratio water/octanoate increases, association begins at quite low concentrations of water and the surrounding decanolic solvent behaves as pure decanol which is saturated with water.     

Evolution of cylindrical structures in the system sodium decylsulphate/decanol/water

Abstract

The binary mixture sodium decylsulphate/water exhibits a hexagonal structure for a degree of hydration of about 50 per cent by weight. In this structure the amphiphiles build infinitely long cylinders which are organized on a 2D lattice of space group p6m. If, in such a mixture, sodium decylsulphate is progressively substituted by decanol, the hexagonal structure transforms itself successively into two rectangular structures. In these rectangular structures the amphiphiles also build cylinders, but these are organized on 2D lattices with space groups cmm and pgg, respectively. This evolution of cylindrical structures in the ternary mixture is also marked by a growth of the mean normal section of the cylinders. An analysis of these results is proposed, in which the growth is accompanied by a change in the normal section, from circular for the hexagonal structure to non-circular for the rectangular structures; this change occurs abruptly at the hexagonal-rectangular transition and is stabilized by an important fluctuation of the relative concentrations of the two amphiphiles within the cylinders.     

Temperature dependence of the density of calamitic and discotic nematic lyotropic liquid crystals containing sodium lauryl sulphate/decanol/water

Experimental results for the temperature dependence of density at normal pressure for two compositions of sodium lauryl sulphate/decanol/water solutions, exhibiting either a calamitic or a discotic lyotropic nematic phase at room temperature, are presented. Within the limits of experimental precision (±1 ×10^-5 g cm^-3), the systems show no jump in density at the nematic to isotropic phase transition. Over the studied temperature range, the mean thermal expansion coefficients were also evaluated.     

Characterization of structural transitions in the SLS/decanol/water system

The ternary system sodium-dodecylsulphate (SLS)/decanol/water has been investigated at three different water contents and varying ratios of cosurfactant to surfactant by means of polarized optical microscopy,²H-NMR quadrupole splittings and small angle x-ray scattering. Upon addition of decanol a hexagonal phase transforms into a lamellar phase. For the highest water content of 0.65 no intermediate two-phase regions are detected but nematic phases are formed between. The lamellar phase at low cosurfactant content is very sensitive to changes of temperature and seems to be a so-called defective one with curved interfaces. From the scaling behavior it is concluded that the building units seem to be ribbons of increasing width on addition of cosurfactant or amphiphilic substance. By reaching a decanol mole fraction of 0.4 a classical lamellar phase with well-known behavior is formed. During these transformations the position of the first diffraction maximum changes gradually irrespective of phase transitions. The maximum mole fraction of cosurfactant the lamellar phase of our system can incorporate is 0.77.     

Nuclear Magnetic Resonance and X-Ray Study of a Rectangular Phase

The rectangular phase in the lyotropic system sodium decyl sulphate (SdS)–decanol–water was studied by²H NMR and small-angle X-ray scattering. To investigate the variation in unit cell dimensions, the shape of the ribbons, and molecular segregation within the ribbons with composition, nine samples were prepared. The aspect ratio of the ribbon cross section was found to be independent of composition, ρ = 1.42 ± 0.02, whereas the aspect ratio of the unit cell varied with composition,b/a= 2.7–3.0. In general, the molecular distribution of SdS and decanol is highly nonuniform; the decanol concentration is 2.5 to 3.0 times higher in the central lamellar region of the ribbons than in their curved edges, while SdS accumulates in the curved regions. On addition of SdS, a vertical compression of the unit cell occurs (adecreases andbremains unchanged). However, the absolute dimensions of the ribbons remain constant and the added SdS accumulates in the central lamellar region of the ribbons. Addition of decanol yields horizontal expansion of the unit cell (adecreases andbincreases) and the absolute dimensions of the ribbon cross section increase slightly. The added decanol distributes evenly in the aggregate.     

Phase equilibria and structures in the system glycerol,sodium dodecyl sulfate and decanol

The phase equilibria and structures in the system glycerol, sodium dodecyl sulfate and decanol were determined using visual observation, optical microscopy, and low angle X-ray diffraction.The results showed the presence of an isotropic liquid alcohol solution of the surfactant and the glycerol, a lamellar liquid crystalline phase and a small region of an isotropic glycerol solution.     

Partial Phase Diagrams in the System Water/Formamide/Sodium Dodecyl Sulfate/Decanol

The phase regions were determined in the systems of sodium dodecyl sulfate, decanol and a polar solvent. The latter was a) water, b) water and formamide (1/1 weight ratio) and c) formamide

The water system showed an aqueous micellar solution, a decanol solution with inverse micelles as well as a lamellar and a hexagonal liquid crystal. The formamide system gave a formamide solution, a lamellar liquid crystal and a decanol solution while the system with water plus formamide in a 1/1 ratio still retained the hexagonal liquid crystal.     

Molecular segregation and aggregate shape in a lyotropic rectangular phase

The microstructure of the rectangular phase in the system sodium decylsulphate/decanol/water is investigated by means of deuterium NMR. By analysing the lineshape from selectively deuteriated decylsulphate and decanol, we separate the effects of (i) the shape anisotropy of the aggregate cross-section and (ii) the inhomogeneous distribution of the two surfactants within the aggregate. The aspect ratio of the cross-section is determined to 1.39±0.01, substantially smaller than previous estimates. We find no evidence for anisotropic growth of the aggregate cross-section in the hexagonal phase, as previously suggested. Rather, the aggregate shape appears to change abruptly at the hexagonal-rectangular phase transition with little change (with temperature) thereafter. The distribution of decylsulphate and decanol within the aggregates of the rectangular phase is highly non-uniform; the decanol concentration is 3 times higher in the central lamellar region than in the curved edges, while the decylsulphate accumulates in the curved regions. This molecular segregation can be rationalized in terms of electrostatic interactions within and between the aggregates.     

Nematic-isotropic interfacial tension anisotropy of a calamitic lyotropic liquid crystal

In a planar oriented sample of a calamitic nematic lyotropic system (mixture of sodium lauryl sulphate/water/decanol), isotropic pretransitional domains appear at the nematic to isotropic transition. The domains are oblong in shape with the long axis along the orientational direction. We show experimental evidence that this oblong shape is determined by the nematic-isotropic interfacial tension anisotropy. Two uniparametric models of simple angular dependences for the interfacial tension are tested. Using the differential system obtained from the Young-Laplace condition at the nematic-isotropic interface, the domain shape can be numerically calculated for each value of the interfacial tension anisotropy. By processing the values of the transmitted light through both an isolated isotropic domain and its surrounding nematic zone, we obtain the anisotropy of the interfacial tension as the main fitting parameter. An estimation of the ratio of the extreme values for the interfacial tension is given.     

Partial molar enthalpies in “inverted micellar” solutions of water and sodium octanoate in decanol

Summary The relative molar enthalpy of solutions with three constant molar ratios of water to sodium octanoate in the isotropic decanol-rich solution phase, L₂, of the system water — sodiumn-octanoate -n-decanol have been determined by means of calorimetry at 2° C. A method of calculating the relative partial enthalpies of the components from these data is described. It is the thermic interaction of the sodium octanoate that dominates the properties throughout the extent of the phase studied. Where the decanol content is highest, the partial enthalpies of the sodium octanoate and the water increase sharply while the partial enthalpy of the decanol remains relatively constant throughout the extent of the phase. This agrees well with the general view that aggregation into inverted micelles takes place as a continuous process and that the properties of the aggregates remain constant.

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Zusammenfassung Die relative molare Enthalpie von Lösungen mit drei konstanten Molverhältnissen Wasser/Natriumoktanoat in der isotropen dekanolreichen Lösungsphase, L₂, in dem System von Wasser-NatriumN-Oktanoat —N-Dekanol wurde kalorimetrisch bei 25°C bestimmt. Außerdem wurde ein Verfahren angegeben, um die relativen partiellen Enthalpien der Komponenten aus diesen Daten zu berechnen. Die thermische Wechselwirkung des Natriumoktanoates bestimmt die Eigenschaften der untersuchten Phase. Wo die Dekanolgehalte am größten sind, nehmen die partiellen Enthalpien von Natriumoktanoat und Wasser noch zu, während die partielle Enthalpie von Dekanol relativ konstant bleibt, Das stimmt gut überein mit der allgemeinen Auffassung, daß die Aggregation zu inverted micelles durch einen kontinuierlichen Prozeß geschieht und daß die Eigenschaften der Aggregate konstant bleiben.

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With 5 figures and 3 tables     

Phase behavior of sucrose monoalkanoate in a water-long-chain alcohol system

Phase diagram of a water/sucrose monododecanoate (SE)/hexanol system was determined at 30°C. Aqueous micellar, reverse micellar, normal hexagonal liquid crystalline, and lamellar liquid crystalline phases appear in the phase diagram. The change in interlayer spacing and interfacial section area of surfactant in the liquid crystalline phases was investigated by small-angle x-ray scattering. Upon addition of water, the section area and the radius of cylindrical aggregates are almost constant in a hexagonal liquid crystal, whereas the distance between each cylinder is separated on the water-SE axis. The interlayer spacing slightly decreases or is almost unchanged on the surfactant-hexanol axis, because alcohol molecules penetrate into the palisade of bilayers. Although the average section area decreases with increasing alcohol content, each section area of SE and alcohol molecules are kept constant. Since the interfacial section area of alcohol is less than the section area of hydrocarbon chain, the phase transition from lamellar liquid crystal to reverse micelle occurs in an alcohol-rich region.     

An NMR study of translational diffusion and structural anisotropy in magnetically alignable nonionic surfactant mesophases

The diffusion of both water and surfactant components in aqueous solutions of the nonionic surfactant "C12E6"--which includes hexagonal, cubic, lamellar, and micellar mesophases--has been studied by pulsed-field-gradient NMR. Diffusion coefficients were measured in unaligned samples in all of these phases. They were also obtained in the hexagonal and lamellar phases in oriented monodomain samples that were aligned by slow cooling from the micellar phase in an 11.7 T magnet. Measured water and soap diffusion coefficients in the NMR-isotropic cubic and (high-water-content) micellar phases as well as diffusion anisotropy measurements in the magnetically aligned hexagonal phase were quantitatively consistent with the constituent structures of these phases being identical surfactant cylinders, with only the fraction of surface-associated water varying with the water-soap molar ratio. The values of the water and soap diffusion coefficients in the oriented lamellar phase suggest an increase in defects and obstructions to soap diffusion as a function of increasing water content, while those in the low-water-content micellar phase rule out the presence of inverse micelles.     

The lamellar phase of the sodium dodecylsulfate/decanol/toluene/formamide system: NMR and phase studies of solubilization

The phase behavior of the system sodium dodecylsulfate/decanol/toluene/ formamide was investigated and pseudo-ternary diagrams established. In particular, the effect of varying the amount of toluene in the system on the stability of the lamellar phase region was studied in detail. Deuterium NMR and low-angle x-ray diffraction measurements showed that more toluene was distributed between the surfactant chains as the amount of decanol in the system is reduced, resulting in a more disordered dynamic structure in the bilayers. Similarly, increased formamide content lead to greater penetration of the toluene into the bilayer and more disorder. Both factors were found to be instrumental in decreasing the stability of the lamellar structure.     

Supermolecular self organization via molecular aggregation. Non-discotic three chain diols displaying a hexagonal columnar phase

A new class of mesogen formed by three chain diols is described. These compounds consist of a central part to which three aliphatic chains of various length (n = 4-14) are attached at one end and two hydroxy groups at the other. X-ray scattering, dilatometric measurements, dielectric and Kerr relaxation studies were carried out. Both X-ray investigations, dielectric and electro-optic studies revealed that the molecules are aggregated in the isotropic fluid state and that hydrogen bonding and dipole correlation contribute to this aggregation. Columnar assemblies of up to several hundred molecules are orientationally correlated in the isotropic fluid phase. Correlated reorientations occur in the presence of external electric fields. The X-ray data show that the aggregates pack in a hexagonal way at lower temperatures giving a structure which strongly resembles that of the hexagonal disordered columnar phase. This anisotropic phase can be quenched to a glassy liquid-crystalline state. It is evident that the mesogenic properties of this new class of compounds are a function of the disc-like shape of the aggregates rather than the shapes of the individual molecules.     

Quantitative SAXS analysis of the P123/water/ethanol ternary phase diagram

The ternary phase diagram of the amphiphilic triblock copolymer PEO-PPO-PEO ((EO)(20)(PO)(70)(EO)(20) commercialized under the generic name P123), water, and ethanol has been investigated at constant temperature (T = 23 degrees C) by small-angle X-ray scattering (SAXS). The microstructure resulting from the self-assembly of the PEO-PPO-PEO block copolymer varies from micelles in solution to various types of liquid crystalline phases such as cubic, 3D hexagonal close packed spheres (HCPS), 2D hexagonal, and lamellar when the concentration of the polymer is increased. In the isotropic liquid phase, the micellar structural parameters are obtained as a function of the water-ethanol ratio and block copolymer concentration by fitting the scattering data to a model involving core-shell form factor and a hard sphere structure factor of interaction. The micellar core, the aggregation number, and the hard sphere interaction radius decrease when increasing the ethanol/water ratio in the mixed solvent. We show that the fraction of ethanol present in the core is responsible for the swelling of the PPO blocks. In the different liquid crystalline phases, structural parameters such as lattice spacing, interfacial area of PEO block, and aggregation number are also evaluated. In addition to classical phases such as lamellar, 2D hexagonal, and liquid isotropic phases, we have observed a two-phase region in which cubic Fm3m and P6(3)mmc (hexagonally close packing of spheres (HCPS)) phases coexist. This appears at 30% (w/w) of P123 in pure water and with 5% (w/w) of ethanol. At 10% (w/w) ethanol, only the HCPS phase remains present.     

Segregation of two amphiphilic molecules within nonspherical micelles: A neutron scattering study

The internal structure of the disk-like aggregate of a lyotropic nematic phase formed by two amphiphiles, potassium laurate and decanol, in the presence of water was studied by neutron scattering with contrast variation method. These experiments show that the two amphiphiles are not distributed uniformly in the aggregate. The potassium laurate concentration is higher in the rim of the disk, where the interface exhibits a semitoroidal curvature, than in its central core, where the curvature is lower. This segregation of the two amphiphiles within the aggregate may be invoked to explain the existence of finite bilayered aggregates which had been suggested to be unstable relative to infinite bilayers by previous models.     

Structural changes induced by addition of a hydrocarbon to water/amphiphile mixtures

The phase conditions in a system of water, hexadecane, sodium dodecyl sulphate, and di-ethylene glycol dodecyl ether showed theW/O microemulsions to be obtained first after destabilization of a liquid crystalline phase by addition of the hexadecane. The original lamellar liquid crystalline phase was moved towards higher surfactant/cosurfactant ratios and a new phase with inverse amphiphile cylinders in a hexagonal packing appeared.