Cubic phases in surfactant and surfactant-like lipid systems

Cubic liquid crystalline phases are common in surfactant and surfactant-like lipid systems at temperatures above the Krafft point. They are optically isotropic and very stiff. Therefore, they are often not recognized as independent phases and separated in pure state. The liquid crystalline nature is evidenced by a low-angle diffraction pattern with sharp reflections having Bragg-values above 20 Å coupled with a diffuse wide-angle reflection at 4.5 Å, proving that the hydrocarbon moiety is in a liquid state. The cubic phases occur in a variety of lipid/water systems (also with liquid organic solvents), such as simple soaps, amphiphilic lipids of biological origin, and extracts from membrane lipids. The location of the cubic phases in a phase diagram varies.The original concept of a cubic structure composed of closed globular aggregates, either of oil-in-water or water-in-oil type in face-centered array seems to be obsolete. The present structure concepts include closed anisotropic aggregates, short rod-like aggregates forming continuous networks or lamellar aggregates with zero curvature forming networks of Infinite Periodic Minimal Surfaces (IPMS). The structure is mostly primitive or body-centered cubic.     

Morphological studies of deformed polybutylene terephthalate (PBT)

PBT is a semi-crystalline thermoplastic polymer whose deformation behavior highly depends on processing parameters. This makes it a model polymer for investigating morphological changes caused by deformation on the spherulitic and lamellar level. In the neck region all states of deformation of the spherulites are observed. Even in the fibrillar phase the borders of the spherulites remain visible. The spherulitic structure is not totally destructed in the neck. The lamellar structure of the fibrillar phase significantly differs from that of the spherulitic region. The lamellae are orientated with respect to the direction of deformation and the lamellae heights are reduced distinctly. Scanning electron microscopy of fracture surfaces reveals for some samples a sharp frontier between spherulitic and fibrillar region. This leads to the conclusion that the necking process may be a phase transition between an isotropic and a highly orientated phase, as predicted for a Van der Waals network.     

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.     

Extreme swelling of a lyotropic lamellar liquid crystal

The evolution of the OBS/water/decane/pentanol lyotropic lamellar crystal is followed from 0% to more than 90% decane content. The lamellar spacing (d) varies then from 35 Å to 11000 Å. The swelling is followed with small angle x-ray scattering and the Bragg divergence in the intensity is found to disappear in moderately swollen lamellar crystal (d from 200 Å up to 800 Å) while the central scattering increases. More striking is the reappearance of the Bragg divergences observed by SAXS (d from 800 Å to 1100 Å) and for extremely swollen lamellar crystals in the angular distribution of scattered light (d from 2 000 Å to 10 000 Å). We discuss these observations along the lines of the recent models of swollen lamellar liquid crystals and in particular the apparent evolution of the dilute lamellae.     

A non-aqueous microemulsion system of ethylene glycol,sodium dodecyl sulfate,toluene, and decanol

The microemulsion liquid in the system ethylene glycol, sodium dodecyl sulfate, decanol, and toluene was investigated by determination of solubility regions and of light scattering intensity.The system without hydrocarbons showed a critical point which remained in the system during the addition of toluene to high content. No other indications of association structures was formed and the microemulsions should be considered as solutions with critical behavior.     

Spontaneous film formation from a polymer solution

An unusual continuous film formation process of lateral pentyloxy substituted poly(p-phenylene terephthalate)s (s-PPPT) and poly(carbonate) (PC) is observed. A liquid film of polymer solution creeps over the surface of water dropped into the polymer solution. By vaporization of the solvent a solid polymer film is formed on the water surface and can be removed. The driving force for the film formation mechanism is assumed by the reduction of the surface tension of water. Experiments verify this mechanism by increasing the film formation speed using a gas stream, by reducing the formation speed through lowering the surface tension by rinsing agents, and by lowering the solubility of the polymer. As expected, no effects are found by variation of the pH-value of water. Necessary conditions for the film formation process are: good solubility of the polar polymers in organic solvents having a high vapor pressure, complete phase separation, solution density higher than water density, and a surrounding gas phase unsaturated with solvent vapor.The thickness of the mechanically stable films is less than 0.5 m. The films are amorphous by microscopical, FT-IR, x-ray, and DTA investigations.     

Emulsion pathways to composite polymeric membranes for separation processes

A novel method for the preparation of selective composite membranes from emulsions is suggested. The dispersed phase is chosen to yield a polymer soluble in those components for which the membrane should be selective; the continuous phase, on the other hand, is selected to yield a polymer that is insoluble in any of the components of the mixture. Conventional emulsions (which have a maximum dispersed phase volume fraction of 0.74) or microemulsions can be employed to generate composites. However, concentrated emulsions which allow volume fractions as large as 0.99 are most suitable as precursors to selective and efficient membranes. These concentrated emulsions have the appearance of gels with a structure similar to that of foams. The relatively high permeabilities obtained with the resultant membranes are due to the small thickness of the films of the continuous phase. A concentrated emulsion of a hydrophobic (hydrophilic) monomer dispersed in a hydrophilic (hydrophobic) continuous phase is first prepared at room temperature, with suitable initiators in each phase for later polymerization, and with an appropriate dispersant in the continuous phase. To ensure the stability of the emulsion, the hydrophilic monomer is, in general, replaced by monomer plus water. On heating the gel at 50 °C, polymerization occurs in both phases and the emulsion transforms into a composite polymer membrane. As examples, composite membranes containing polystyrene as the dispersed phase and polyacrylamide as the continuous phase are used to separate toluene from cyclohexane, while other composite membranes containing acrylamide as the dispersed phase and a crosslinked polystyrene as the continuous phase are used to separate water from ethanol.Lecture presented at the Colburn Symposium, University of Delaware, October 19, 1988.     

A structural study of the mesophases of two oxyethylene alkyl ether-decane-water systems,a comparison between technical grade C12EO〈7〉 and pure C12EO6

As part of a study polyoxyethylene alkyl ethers (C _m EO _n ), water and decane, the phase diagram and the structures of the mesophases of pure C₁₂EO₆ and technical grade C₁₂EO₇ were compared. The constructed phase diagrams of the two systems show a great resemblance except for one difference: the viscous isotropic phase is only present in the C₁₂EO₆ phase diagram.The swelling behavior of the lamellar and hexagonal phases was studied with smallangle x-ray scattering. Both the lamellar and hexagonal phases showed an ideal swelling behavior and no differences between the lamellar and hexagonal phases of the two systems were detected.With freeze-fracture electron microscopy the hexagonal and lamellar phases were visualized. No differences in the textures of the lamellar phases were found, however, the micrographs of the hexagonal phases of the two systems clearly showed different textures. While in the hexagonal phase of the C₁₂EO₆ system only infinite long rods were visualized, short interrupted rods were found in the hexagonal phase of the C₁₂EO₇ system.     

Aqueous dispersion of polyurethane ionomers from hexamethylene diisocyanate and trimellitic anhydride

Polyurethane (PU) ionomers were prepared from trimellitic anhydride (TMA), poly(tetramethylene adipate) glycol (PTAd), and hexamethylene diisocyanate (HDI) in acetone. Upon neutralizing the carboxylic groups with a tertiary amine (TEA), and adding water to PU ionomer solution, followed by removing the acetone, stable aqueous PU dispersions were obtained.Effects of interionic molecular weight and nonionic hydrophilic segment, viz. monofunctional ethylene-propylene oxide ether on particle, size, emulsion viscosity, mechanical, and viscoelastic properties of the emulsion cast films were examined.     

Lamellar phases in mixtures of low molecular weight alkanols and cetyltrimethylammonium bromide (CTAB)

Partial phase diagrams showing the domains of existence of a transparent, viscous, lamellar-structured (D)-phase that transforms reversibly into fluid single phase solutions at high temperature are presented for the system: cetyltrimethylammonium bromide (CTAB), two low molecular weight alcohols, and water with and without additives. At constant temperature and with a fixed amount of surfactant, the size and location of this phase in the phase diagram depends upon three composition variables: i) the ratio of concentrations of medium chain alcohol to long chain alcohol (R), ii) the ratio of concentrations of medium chain alcohol to surfactant (R), and iii) the concentrations of small amounts (up to 10 % by weight) of additives such as ethylene glycol, propylene glycol, and dimethylformamide, as well as NaBr. Small-angle x-ray scattering measurements of these mixtures reveal a lamellar structure. The observed lamellar repeat distances range from 60 A to 290 Å and depend upon the ratiosR andR and the concentration of the additives. The mechanical and structural properties of theseD-phases can be tuned by adjustingR andR. TheD-phase-to-isotropic transition temperature can be varied from near room temperature to above 80 °C by adjustingR andR.     

Phase equilibria and structures in ternary systems of a cationic surfactant (C16 TABr or (C16 TA)2SO4), alcohol,and water

The dependence of alcohol chain length on the isothermal phase behavior of the ternary systems hexadecylrrimethylammonium bromide/alcohol/water has been investigated. A liquid crystalline phase (the normal hexagonal one) occurs in the phase diagrams along the surfactant/water axis and this phase extends in the interior of the diagrams.When the alcohol is methanol, ethanol or butanol, there is in the ternary phase diagram a continuous solution region from the water to the alcoholic corner, and in the butanol case, in addition, a small region of lamellar liquid crystalline phase in the interior of the diagram. When the alcohol chain length is increased, the continuous solution region is divided into two subregions, an aqueousL ₁ and an alcoholicL ₂. The lamellar phase occupies the center of the phase diagrams and has the capability to incorporate large amounts of water under one-dimensional swelling. On the alcoholic side of the lamellar phase occur a reversed hexagonal liquid crystalline phase and a cubic liquid crystalline phase in the octanolic system; in the decanolic system the cubic phase is missing, but instead another liquid crystalline phase, presumably with rod-structure, occurs in addition to the reversed hexagonal phase.In a decanolic system where the monovalent bromide ion is replaced by the divalent sulphate ion there are the same solution regionsL ₁ andL ₂, and phase regions with liquid crystalline normal hexagonal and lamellar structures. The lamellar phase has lost much of its capability of incorporating water. That is in analogy with the conditions in anionic systems where the counterion charge has been increased. There is no reversed hexagonal phase, but on the alcoholic side of the lamellar phase, there is the same foreign liquid crystalline phase with a presumed rod-structure as in the monovalent system.     

The first liquid crystalline diol compound which exhibits nematic,smecticA +, and smecticC + mesophases in the presence of water

A new amphiphilic ethane-1,2-diol derivative with a rodlike 2-phenylpyrimidine rigid core has been synthesized. From the combined results of differential scanning calorimetry and optical polarization microscopy a phase diagram amphiphile/water was constructed. The system exhibits a nematic phase at a very low water content, a smecticA ⁺ and a smecticC ⁺ phase at higher water concentrations. Such a phase sequence has been found for a lyotrophic system for the first time.     

The dynamik contact angle II. Investigation of the adsorption mechanism based on the measurement of the preceding contact angles

The structure of the adsorption layer at the solid/gas interface is characterized, as a function of conditioning concentration, by the measurement of preceding contact angles. The contact angles were determined tensiometrically (plate method) and cinema tographically (capillary rise method) in the system glass or mercury/n-dodecyl ammonium chloride solution/air, respectively. In the dependence of contact angle on concentration, four regions are provable. These regions correlate with the surfactants, which are bound to adsorption in a heteropolar mode or by van der Waals forces of interaction, with the formation of layer-like coverage and with bilayers. Special attention was given to the fact that loosely bound surfactants are transferred from the solid/gas interface to the liquid/gas interface and cause a reduction of the preceding contact angle.Publication No. 1077 from the Research Institut of Mineral Processing, Academy of Sciences of the GDR, Freiberg, G.D.R.     

Ringing gels: Their structure and macroscopic properties

The phase diagram of the ternary surfactant system which consists of dimethyltetradecylaminoxide, hydrocarbon, and water contains a highly elastic gel phase which borders on the micellarL ₁-phase. This gel phase is transparent, optically isotropic, and shows the ringing phenomena when it is excited to mechanical vibrations. From SANS and light-scattering measurements it is shown that this phase consists of the same spherical microemulsion droplets which are present in the adjacent micellar solution. Even in the micellar solution the droplets are fairly monodisperse and in the SANS scattering functions a second scattering maximum was observed. Both the light scattering and SANS data can be described quantitatively on the basis of hard sphere interactions between the particles. Furthermore, it is shown that elasticity and shear modulus of the gel phase, which were determined experimentally, correlate with the compressibility modulus as calculated from the scattering data. The elasticity modulus and hence the osmotic compressibility modulus are related to the Laplace pressure inside the globules. For the calculation of this pressure it is possible to take the interfacial tension, which is obtained from a dilute micellar solution against the hydrocarbon which is used for the system. The radius of the particles and the hydrocarbon content in the system can be increased when some of the dimethyltetradecylaminoxide is replaced by dimethyltetradecylphosphinoxide.     

On the stability of membrane,foam and emulsion bilayers with respect to rupture by hole nucleation

Bilayer membranes and bilayer foam or emulsion films were considered thermodynamically from a unified point of view as two-dimensional phases. The work of fluctuation formation of holes in a bilayer was determined and it was shown that the bilayer can be either stable or unstable with respect to rupture by hole nucleation. When the bilayer is in contact with a solution, its stability with respect to rupture was found to decrease with decreasing concentration of dissolved amphiphiles, and a limiting concentration was shown to exist, below which the bilayer is subject to rupture by hole nucleation. The connection is discussed between existing formulae for the work of fluctuation formation of holes in bilayers.     

Chemical heterogeneity in emulsion copolymers of carboxylic monomers

Copolymer latices of butylacrylate (BA) with acrylic and methacrylic acid (AA and MAA) were prepared by batch type emulsion polymerization, and, for comparison, copolymers with identical monomer composition were prepared by batch type solution polymerization.The distribution of the carboxylic monomers in the latex particles and the serum was studied by density gradient and sedimentation experiments with the analytical ultracentrifuge. Dynamic mechanical measurements of films of these copolymers were used to determine the storage and loss moduli as a function of temperature. From these measurements the position and extension of the glass transition range on the temperature scale is obtained. For heterogeneous emulsion copolymers with two glass transition temperatures the distribution of the carboxylic monomer units in the different copolymer phases can be determined. Electron microscopy of ultra thin cross-sections of stained films gave further insight into the film morphology.The combination of the results obtained with the different methods gives rise to the following clues: In the BA/AA latices about 40% (by weight) of the total AA used in the recipe are found in the serum as a water soluble polymer, about 50% are found to increase the glass transition temperatureT _g of the bulk of the BA copolymer and, therefore, are thought to be incorporated into the interior of the latex particles, and the remaining 10% are, conclusively, located on the particle surface.In the BA/MAA latices no water soluble copolymer could be detected in the serum, about 90% of the MAA used is found in the bulk of the copolymer, and about 10% form a second hard phase on the surface of the latex particles.Dynamic mechanical measurements on the copolymer latex films show at least two phases with different glass transition temperatures: the bulk of the copolymer with a relatively low content of (M)AA units and a glass transition range at low temperatures, and a second (M)AA rich phase with a highT _g.The latter phase forms a honeycomb-like structure surrounding the packed latex particles. That results in a three-dimensional network of polymer with a highT _g extending throughout the latex film. In spite of the fact that this phase is built from a small fraction of the total copolymer only, it has a very pronounced influence on the performance behaviour of latex films.Dedicated to Professor Dr. R. Manecke on the occasion of his 70th birthday.     

Solutions of alkali soaps and water in fatty acids XIII. Circumstances that regulate the extension of theL 2-phase and the role of the acid-soaps rich in fatty acid for the occurrence of the phase

A basic requirement for that type ofL ₂-phase which exists in the system sodium octanoate-octanoic acid-water is the formation of acid-soaps. In order for the phase to be formed at all, the temperature must lie above the melting point of the fatty acid so that a reaction in non-aqueous milieu between neutral soap and fatty acid is possible. In order to obtain the characteristic shape and complete extension of the phase in direction of high water content the temperature must be so high that also the hydrated acid-soaps occur in fluid state. On the other hand the temperature cannot be so high that the acid-soaps become unstable.At temperatures at which the phase has obtained its full extension those circumstances differs which in different regions regulate the location of the phase borders; they depend on the composition of the acid soaps and on their amounts. In that part of the phase where the molar ratio between octanoic acid and sodium octanoate lies between 2 and 3 and where one has a continuous transition from reversed to normal structure only the two acid octanoates 1 NaC₈ 2 HC₈ x H₂O and 1 NaC₈ 3 HC₈ x H₂O occur and both are at 20 °C in fluid state.At water contents from about 22 % to 40 % the hydrate-water molecules belonging to the first mentioned soap are capable of contributing actively to the formation of large aggregates of acid-soap, a process which however is counteracted by the inmixing of the latter acid-soap. This mixture of the two acid-soaps decides in this region where the border of the phase will lie in direction towards an increased content of sodium octanoate; the result is that in spite of the fact that the hydration is increased, the border is only slowly displaced towards a higher content of fatty acid. As soon as the hydration of the acid octanoates has been completed and the additional water occurs as unbound bulkwater, the location of the phase boundary will no longer be influenced by the water content — now it will be the amphiphilic composition of the acid-soaps that determines the location of the border and it remains at the molar ratio 2.5 between octanoic acid and sodium octanoate at water contents from about 40% and up to 82%.In the direction of decreasing content of neutral sodium octanoate and increased content of water theL ₂-phase both at the highest content of fatty acid and the highest contents of water will be in equilibrium with the water-richL ₁-phase; in the first mentioned region with theL ₁-phase below the lac where at the border it is saturated with octanoic acid and in the latter region with theL ₁-phase just above the lac, where the dilute sodium octanoate solution contains dissolved 1 NaC₈ 1HC₈ x H₂O. In the large central part of theL ₂-phase, from about 20 % to about 86 % of water, the location of the border is dominated by the acid octanoate 1 NaC₈ 3 HC₈ x H₂O and that makes an equilibrium with theL ₁-phase impossible; instead one has an equilibrium via a two-phase zone between the amphiphile-rich region of theL ₂-phase and its water-rich region. In the first region the location of the border is regulated by the decreasing capability of the hydrated acid octanoate 1 NaC₈ 3 HC₈ x H₂O to dissolve octanoic acid; in the latter it is regulated by the fact that 1 NaC₈ 3 HC₈ x H₂O is the most fatty acid-rich acid-soap that is formed and that the octanoic acid is very little soluble in water and in the aqueous solution of this acidsoap.The middle part of theL ₂-phase, especially the region between about 55 % and 82 % of water, constitutes a direct continuation of the liquid crystalline lamellarD-phase. The liquid crystalline character of theD-phase is lost at the transition, but the lamellar organization is retained. That the molecules at least up to a water content of about 40 % are of the original reversed type and have an elongated shape with a central part of hydrated polar groups, from which core the hydrocarbon chains extend in two opposite directions, is the reason to that they, at crowding, form transient layer-like agglomerates of tightly packed more or less parallel molecules; this facilitates the transformation to coherent double amphiphilic layers, in which all molecules lie with the hydrated polar groups outwards toward coherent domains of bulk-water, without another liquid phase occurs.     

Solutions of alkali soaps and water in fatty acids. XII. Correlations between the acid sodium octanoates in the isotropic liquidL 2-phase at water contents from 82% down to 10% and those in adjacent liquid crystalline phases

A close connection exists between the isotropic L₂-phase and the liquid crystalline lamellar D-phase. This connection is evidenced both with respect to similar compositions of their acid sodium octanoates and concerning the structure of the phases.The two-phase zone between these phases is very narrow, from 82 % down to about 55 % of water, it then widens slowly to about 40 % and thereafter very rapidly to 22 % of water; below the latter value it remains broad. The border of the L₂-phase is in the whole region from 82 % to 22 % of water located at molar ratios between octanoic acid and sodium octanoate of somewhat above 2.0; the border of the D-phase lies at the same values down to a water content of 40 %. The acid octanoate 1 NaC₈ 2 HC₈ x H₂O plays thus a considerable role in the phase transition all the way down to a water content of 22 %. As long as free bulk-water exists in both phases the structures in them remain alike; this similarity is greatest as long as the amount of bulk-water is so large that it suffices to form coherent domains. At contents of water below 40 %, that is after the disappearance of the bulk-water and when a decrease in the content of water begins to affect the hydration of the acid octanoates, the structural similarity between the phases disappears rapidly.     

Photochemically induced phase transitions in the system cetyltrimethyl-ammonium bromide-water containing N-methyl-N,N-diphenylamine

Phase transition temperatures from lyotropic liquid crystals to the isotropic micellar solution of the system cetyltrimethylammonium bromide-water (CTAB-H₂O) were measured in the presence of 0.1-0.9 weight % of N-methyl-N,N-diphenylamine (MPA). They were determined by optical and viscometric methods and were found to increase as a function of MPA concentration in solutions containing 21 and 23 weight % of CTAB. This effect was reversed when MPA was in situ photochemically converted to N-methylcarbazole, allowing photochemically induced phase transitions.     

Solution behavior of two liquid crystalline polymers of different architectures

Solutions of two different liquid crystalline polymers of high molecular weight are investigated by static and dynamic light scattering (LS), membrane osmometry and size-exclusion chromatography (SEC). Measurements in dilute solution in different solvents showed no specific behavior as formation of aggregates or chain stiffening. Large discrepancies between the LS results and the results from osmometry and SEC show that the latter methods are in the present cases not suitable for molecular weight determination. In semi-dilute solution the osmotic modulus and the time correlation function were studied. Behavior of flexible chains was observed. In one system a slight aggregation of small molecules onto longer chains was found causing less interpenetration of the chains in that solvent. At moderately high concentrations cluster formation was observed from i) a small angle excess scattering, ii) a downturn of the osmotic modulus, and iii) the appearance of a slow motion in the time-correlation function.