Investigations into the Mesophase Behaviour and Equilibrium Dynamics of a Family of Aerosol OT-related Surfactants
Authors:   , Fairweather Graham 
Published: November 2021

<p>Surfactants are ubiquitous, with many important applications in physical and biological sciences. Increasingly, they are being utilised for their liquid crystalline properties rather than in their traditional roles as detergents, emulsifiers or wetting agents. This is because these liquid crystalline mesophases offer a large range of different architectures with varying degrees of structural order and functionality. Presently, surfactants used in these applications are those with behaviours that are already understood due to their routine use in industry. However, the concept of producing surfactants by rational design would be a significant step forward and could potentially revolutionise the field. Such a step is not trivial due to the hierarchical and complex nature of self-assembly. In order to realise this goal, we have to understand how the various contributions to the free energy of the system can be manipulated to affect the building blocks of the system on different lengthscales. How do changes made on the sub-nanometre lengthscale alter the physical dynamics of a system as a whole?  In order to answer this question, we must gather experimental evidence for different systems using a range of complementary techniques to probe the system dynamics on different lengthscales. Here we report the characterisation of phase behaviour and the evolution of microstructure for a family of surfactants. We investigated this using a combination of small-angle X-ray spectroscopy, polarising optical microscopy and cryogenic scanning electron microscopy, producing seven new phase diagrams. We also report the flow properties of these systems, which were studied using static and dynamic rheology.  The starting point of this study is sodium bis(2-ethylhexyl)sulfosuccinate (Na-AOT), which is an industrially important surfactant that forms lyotropic liquid crystalline phases in water. This behaviour has been extensively studied previously and presents over large concentration and temperature ranges. The system has also been a source of debate and confusion in regards to a possible transition within the lamellar phase. We present a review of the literature and, with the addition of our own investigation, show that system does not undergo any of the previously proposed transitions within this region. We show that the anomalies reported in the literature can be explained by a change from a swollen and highly connected lamellar phase to a more classical ordered lamellar phase with increasing surfactant concentration. This change is a result of different intermolecular forces governing the system as the lamellar bilayer repeat distance decreases. The impressive stability of the lamellar phase is mediated through formation of different types of topological defects, which change from positive to negative Gaussian curvature as the bilayer elasticity varies. We then use Na-AOT as a base system for comparison with a family of related surfactants to detail how molecular changes alter the self-assembly of each new system.   The first comparison system discussed is the an analogue of Na-AOT with reduced branching, sodium bis(1,3-dimethylbutyl) sulfosuccinate (Na-butylAMA). The use of this system allowed us to investigate the role of the surfactant’s branching ethyl groups in the self-assembly of the Na-AOT system.We show that the less bulky tail region and reduced conformational freedom of Na-butylAMA dramatically reduces the stabilisation of the lamellar phase, with liquid crystalline phases only presenting at surfactant concentrations above 50 wt %. We also report that the phase progression of the Na-AOT system can be closely reproduced by modification of the Na-butylAMA headgroup via ion exchange. Replacing the sodium counterion with potassium restores the balance between headgroup and tail volumes and allows the stabilisation of the lamellar phase at low concentrations.  Alterations were made to the steric bulk and electronic properties of the AOT headgroup without changing the chemical functionality by replacing the sodium cation with two other Group 1 alkali metals: lithium and potassium. Cryo-SEM images show how these modifications dramatically alter the elasticity in the system. In the case of Li-AOT, this results in a significant shift of the phase boundaries to lower concentrations, with extended stability of the ordered inverted bicontinuous cubic and inverted hexagonal phases. The K-AOT system shows increased elasticity due to more negative Gaussian curvature. This drives the system to form a more disordered sponge phase, which can reversibly transition to a lamellar phase upon heating. These changes are explained in terms of the different hydration and headgroup affinities of the cations, which both alter the elastic moduli of the surfactant bilayer.  The AOT system was also studied with three biologically relevant ammonium-based counterions: ammonium (NH₄⁺), choline (Ch⁺) and acetylcholine (AcCh⁺). With the first being of comparable size to K⁺, the effects of increased polarisability and hydrogen bonding on phase behaviour were investigated. The different microstructures observed in the AOT system when using Na⁺, K⁺ and NH₄⁺ are discussed in terms of the different contributions to the Gaussian curvature, including ion pair formation, hydrogen bonding, hydration and steric constraints on molecular packing. In comparison to the other counterions investigated, choline and acetylcholine are more weakly hydrated, have greater affinity for the sulfonate anion, and provide additional steric bulk to the AOT headgroup. This removes some degrees of freedom in these systems, and hence increases bilayer rigidity, causing significant stabilisation of their lamellar phases. As a result, lamellar phases dominate both systems over nearly their entire concentration ranges.  The bilayer elasticity was further investigated by the addition of electrolyte to the lamellar phases of the NH₄-AOT and Ch-AOT systems. Both are shown to exhibit salt-induced phase transitions, forming sponge and cubic phases depending on the starting surfactant concentration. These transitions were achieved with small amounts of NaCl in the case of NH₄-AOT, due to its highly disordered microstructure. By contrast, as a result of the increased bilayer rigidity, the Ch-AOT system is shown to have a highly stable lamellar phase that can accommodate large amounts of salt.  Having elucidated the phase behaviour and microstructure of each surfactant system, the viscoelastic properties of a number of the systems were investigated. The flow properties of the lamellar and sponge phases were studied by static and dynamic rheology. The shear behaviour of the sponge phases were observed to be dependent upon concentration, with both Newtonian and shear thinning behaviours displayed. The lamellar phases of several systems demonstrated shear-dependent phase transitions. These transitions were also monitored by cryo-SEM, which showed the formation of multilamellar vesicles (onions) at a critical shear rate. Upon increasing shear, the onions decreased in size and assembled in a close-packed arrangement.   In summary, this study addresses the confusion surrounding the widely disputed lamellar phase of the Na-AOT system. The observed changes in physical properties are explained by the evolution of defect behaviours on multiple lengthscales. Through a series of molecular modifications, a range of different systems have been produced and their liquid crystalline phase behaviours and microstructures characterised. The relative stabilisation of lamellar and sponge phases are rationalised in terms of the various competing contributions to the total free energy. These phases are then manipulated through changes in temperature, electrolyte addition and shear, with the induction of several different phase transitions observed. By systematically changing the AOT cation, we have demonstrated the role of the counterion in mesophase stability and system elasticity. We have shown that through careful counterion selection, we can enhance or reduce the stabilisation of curved interfaces. This has allowed us to compile a set of considerations, which can be applied to other surfactant systems in order to better predict the potential outcomes of alterations in these systems, and is a step towards rational design in the field.</p>