Gordon J. T. Tiddy

Phase behaviour of polyoxyethylene surfactants with water. Mesophase structures and partial miscibility (cloud points)

From a review of the major factors responsible for surfactant mesophase structure, a model phase diagram is deduced which shows phase structure as a function of surfactant volume fraction and micelle curvature. To test this model the phase behaviour of a series of pure polyoxyethylene surfactants (CnEOm) with water has been studied using optical microscopy over the temperature range 0–100 °C. The compounds studied were C8EO3, C8EO4, C8EO8, C8EO12, C10EO3, C12EO3-C12EO6, C12EO8, C12EO12, C14EO3, C14EO6, C16EO3, C16EO4, C16EO6, C16EO8, C16EO12, C9PhEO8 and C12(2-C10)EO10. Phase diagrams were determined for C8EO4, C12EO3-C12EO6, C12EO8, C16EO4, C16EO8 and C16EO12. With the other compounds optical microscopy was used to determine the number, sequence and type of mesophases.The mesophases observed were cubic–spherical-micelles (I1), hexagonal (H1), normal-cubic–bicontinuous (V1), lamellar (Lα) and reversed-cubic–bicontinuous (V2). Large head groups and low temperatures favour I1 and H1 phases, while Lα and reversed phases occur for small head groups and higher temperatures.There is agreement between experiment and theory for low to medium temperatures if increasing temperature is assumed to lead to a decreased surface area per molecule at the micelle surface (a). At high temperatures and low water content theoretical concepts were reconciled to practical behaviour only by assuming that increased interactions between EO groups occur at a critical water concentration. Two separate mechanisms are proposed for the lower consolute behaviour of surfactant + water solutions (the cloud point). One, involving van der Waals attractions between micelle cores, operates at low temperatures, while the second, involving intermicellar EO—EO attractions occurs at high temperatures. These two mechanisms can account for the ‘double’ cloud point phenomenon observed for surfactants with short EO groups.

Optical microscopy and nuclear magnetic resonance studies of mesophases formed at compositions between hexagonal and lamellar phases in sodium n-alkanoate + water mixtures and related surfactant systems

The mesophase structures formed in sodium dodecanoate + water mixtures have been investigated using n.m.r. spectroscopy and polarising microscopy. Four different regions were indicated by optical microscopy, while 2H and 23Na n.m.r. measurements detected only three mesophases separated by first-order phase boundaries. From the n.m.r. data we conclude that the two mesophases with highest surfactant concentrations both consist of bilayers. One is the normal lamellar (Lα) phase whilst the second, containing less surfactant and occurring over limited temperature and composition ranges, has very thin bilayers. We suggest that within this phase there is a hydrogen-bonded water network linking the bilayers. The phase formed at lowest surfactant concentration is hexagonal (H1). To account for the optical microscope and n.m.r. data, we propose that as concentration is increased the H1 phase gradually changes to a deformed hexagonal lattice (H1d). The H1d structure and the second lamellar phase are termed ‘intermediate’ phases.Optical microscopy has been used to investigated the occurrence of intermediate phases in other C8-C18 sodium and potassium alkanoates with even chain numbers. These phases are found with the C12 to C18 sodium soaps whilst the C8 and C10 sodium soaps form a cubic phase (V1). The C10 derivative also forms one intermediate phase (probably H1d). Broadly similar phase behaviour was found for the potassium soaps except that two intermediate phases and a V1 phase occur with the C10 compound, while with the C12 derivative the intermediate phases are transformed to V1 at higher temperatures. The same technique has been used to investigate mesophases of sodium n-alkyl sulphates. A single intermediate phase only forms with C10 to C14 sodium alkyl sulphates. For the hexyl derivative a V1 phase occurs, while the intermediate phase of the C8 homologue changes to V1 on heating. Only H1 and Lα occur for sodium oleyl sulphate.Intermediate phases only appear to form with longer-chain ionic surfactants. Literature data indicate that only V1 phases occur with non-ionic and zwitterionic surfactants. There is a brief discussion of possible explanations for this.

Phase structure, nuclear magnetic resonance and rheological properties of viscoelastic sodium dodecyl sulphate and trimethylammonium bromide mixtures

The phase diagram of octyltrimethylammonium bromide and sodium dodecyl sulphate (SDS) in water ( > 90 %) is reported at 298 K; the phase regions observed are liquid, two liquid, and liquid plus liquid crystal. Liquids with viscoelastic properties occur close to the SDS rich liquid boundary, and a correlation is observed between the surfactant n.m.r. linewidths and the rheological properties of these liquids. The most likely explanation of these effects is one involving the occurrence of both cylindrical and spherical micelles, but the possibility that the viscoelasticity is due to the presence of a microemulsion can not be excluded.

Surfactant–water interactions in lamellar phases. An equilibrium binding description of interbilayer forces

The importance of ‘hydration forces’ in controlling short-range interactions in colloidal and biological systems has been recognised for some time, although their origin remains controversial. Equilibrium water vapour pressures and X-ray repeat spacings for a number of polyoxyethylene alkyl ether-type non-ionic surfactant–water liquid-crystalline phases as functions of both concentration and temperature have been measured in order to investigate hydration effects in simple systems where electrostatic interactions are absent. Measurements on the lamellar phases of C12EO3 and C12EO4, and on the hexagonal phase of C12EO8 are reported. The results have been analysed, along with published data on C12EO6, in terms of force–distance relationships. Although the net repulsive force decays approximately exponentially, the decay distances are significantly longer than those reported previously for phospholipid–water systems. An alternative treatment in terms of binding of water molecules to surfactant headgroups has been proposed. Using this simple approach, which does not require layers of ‘structured water’, the current thermodynamic data have been linked with earlier n.m.r. results for the non-ionic surfactant systems.

Gel and liquid-crystal phase structures of the trioxyethylene glycol monohexadecyl ether/water system

Optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction and nuclear magnetic resonance spectroscopy have been used to study the lyotropic phases formed by trioxyethylene glycol monohexadecyl ether with water (2H2O). The phase diagram exhibits regions corresponding to lamellar (Lα), inverse cubic [V2(1), V2(2)], gel (Lβ) and isotropic liquid phases (W, L2, L3). The two V2 phases were identified by both optical microscopy and small-angle X-ray measurements. The gel phase exhibits a large melting entropy, and part of the molecule has a much restricted mobility. Low-angle X-ray data indicate that the alkyl chains are ordered, with a thickness equivalent to one hydrocarbon chain length. Taken together these data confirm a monolayer ‘interdigitated’ alkyl chain structure for the gel. In the presence of water the gel is stable (at thermodynamic equilibrium) below 40 °C. Anhydrous C16EO3 exhibits polymorphic phase behaviour. After melting the crystalline surfactant, a metastable gel structure forms on cooling and the crystalline solid reforms only after lengthy storage at low temperature. The maximum water-layer thickness of the Lα phase is larger than that of Lβ despite the expected weaker inter-layer attractions in the latter case. This implies the existence of an additional repulsion in Lα, possibly arising from elastic undulations of liquid bilayers.

Nuclear magnetic resonance technique to distinguish between micelle size changes and secondary aggregation in anionic and nonionic surfactant solutions

The presence of surfactants in large micelles gives rise to broad n.m.r. resonances because of the long correlation time for diffusion around the micelle. This has been used to investigate the structure of surfactant aggregates in systems where other evidence indicates that large micelles occur. For polyethylene oxide surfactants at the cloud point, the surfactant micelles are small and the large units are formed by secondary aggregation of small micelles. For sodium dodecylsulphate with added salt, octanol or other surfactants, large micelles are formed. The changes in micelle size indicated by changes in n.m.r. linewidths are in agreement with changes measured by the quasi-elastic light scattering technique.

Ion condensation model and nuclear magnetic resonance studies of counterion binding in lyotropic liquid crystals

It is suggested that the ion condensation hypothesis, previously used for polyelectrolyte solutions, also describes the ion binding in lyotropic liquid crystals formed in mixtures of amphiphiles and water. Counterion n.m.r. quadrupole splittings of these systems are shown to be appropriate for testing the ion condensation model. Data on 23Na+ quadrupole splittings are found to be in good agreement with the ion condensation approach. In particular, the effects of varying the water content, the concentration of added salt and the temperature are discussed. A previous conclusion concerning the thickness of the electrical double layer in lamellar liquid crystals is shown to be incorrect. The magnitude of the quadrupole splitting is estimated by means of a solution to the Poisson–Boltzmann equation obtained previously. Comparison with experimental data suggests that the field gradients at the nucleus are mainly determined by local effects.

Phase structure and rheological properties of a mixed zwitterionic/anionic surfactant system

The phase diagram of the mixed zwitterionic/anionic surfactant system hexadecyldimethyl-ammoniopropanesulphonate/sodium dodecyl sulphate/water has been determined in the aqueous region (90.0 % water) by optical microscopy and low angle X-ray scattering; the phases observed were an isotropic surfactant solution and a hexagonal liquid crystalline phase. Some aqueous solutions were found to be viscoelastic and the composition boundaries of solutions with these properties were parallel to the phase boundaries. N.m.r. was used additionally to study the structure of the viscoelastic solutions and the results are interpreted using a model which involves the existence of both normal spherical micelles and cylindrical micelles in equilibrium.

Influence of organic solutes on the self-diffusion of water as studied by nuclear magnetic resonance spectroscopy

The self-diffusion coefficient of water and of the organic solute have been measured for aqueous solutions of tertiary butanol and tertiary alkyl ammonium chlorides (CnH2n+1)4NCl, n= 1–4, as a function of solute concentration at 25 °C with the pulsed field gradient Fourier-transform n.m.r. technique. The decrease in the water diffusion coefficient with increasing solute concentration is interpreted with a model which includes the obstruction from the solute particles to the diffusion of water and the hydration of the solute particles. The decrease of the obstruction effect due to particle motion is accounted for by a correction which gives the model the proper limiting behaviour. Hydration numbers for the organic solutes have been calculated. The reduction of the water diffusion coefficient with increasing solute concentration can be explained by assuming that less than a monolayer of water molecules are associated with the surface. For solutions of (C3H7)4NCl and (C4H9)4NCl inclusion of the obstruction effects from the solute particles is essential to explain the concentration dependence of the water diffusion coefficient. The solute diffusion coefficient at infinite dilution agrees with predictions of the Stokes–Einstein equation for the hydrated solute particle.

Structure of liquid-crystalline phases formed by sodium dodecyl sulphate and water as determined by optical microscopy, X-ray diffraction and nuclear magnetic resonance spectroscopy

The lyotropic liquid crystals formed in the sodium dodecyl sulphate + water system have been studied using optical microscopy, low-angle X-ray diffraction and nuclear magnetic resonance spectroscopy. Optical microscopy reveals the presence of a mesophase at compositions between those of the hexagonal and lamellar phases. The magnitudes of water and sodium n.m.r. quadrupole splittings indicate that this phase contains rod micelles similar to those in the hexagonal phase, despite the fact that the material aligns at glass surfaces with the director normal to the surface. The X-ray measurements suggest that the phase has a deformed hexagonal structure and results from a gradual distortion of the hexagonal lattice with increasing surfactant concentration. An hypothesis to account for its formation and properties is proposed, involving hydrogen-bonded water links between micelles.