Atomistic Studies of Structure and Dynamics of Aggregation in Solution: From Traditional Surfactant Micelles to Surfactant-Free Microemulsions

The properties of the micellar structures formed by surfactant molecules in aqueous solution has been a widely discussed topic for the last century. Numerous hypotheses evolved in the effort to characterize the shape, internal structure, and driving force in the emergence of these aggregates. Many of the disputes can be ascribed to the fact that precise examination would require observations on a molecular level, which is difficult to achieve with experimental techniques in such dynamic colloidal systems. Therefore, theories often necessarily rely on the interpretation of circumstantial evidence rather than direct proof. In this study, the subject is approached with atomistic molecular dynamics simulations, that provide atomistic insight into the assembly of micelles formed by the ionic surfactants n-octyltrimethylammonium bromide, n-decyltrimethylammonium bromide, n-dodecyltrimethylammonium bromide, sodium octanoate, and sodium decanoate in water. In the course of these simulations, the spontaneous aggregation of surfactant molecules from initially homogeneous distribution is sampled and data acquisition is progressed far beyond the point where dynamic equilibrium is reached. This converged stage is in each case characterized by a noticeable degree of polydispersity that describes a bell-shaped peak in a histogram of aggregation numbers. Targeted analysis of micelles close to the most probable aggregation number reveals objects of globular shape with a diffuse interfacial region. In this assembly, the molecular subunits preferably occupy certain radial positions with the apolar alkyl tails in the micelle center and the polar groups oriented towards the solvent. Compared to idealized representations, however, this ordering is significantly defused and even the terminal methyl groups have a finite probability to be located at the micelle/water interface. The internal configuration that leads to this occurrence is a combination of a few surfactant molecules constituting a radially aligned molecular scaffold and a number of tangentially attached monomers. In case of the Cₓ-trimethylammonium bromide micelles, a consequence of this mixed array is a rough aggregate surface with deep crevices that allow for significant water penetration up to a buried hydrophobic core. The anionic species form a notably smoother boundary by tighter packing and comprise a larger water-free region in the micelle center, that, however, still only makes up 40% of the micelle volume. The immediate solvent shell of the aggregates is characterized by a substantial decrease of water-water hydrogen bonds that is more pronounced in case of the anionic compounds, where the carboxylate group represents a dedicated hydrogen-bonding site. The monomer exchange rate is found to be strongly decelerated with increasing aliphatic tail length and ranges between 10⁷ s⁻¹ and 10⁹ s⁻¹ for the cationic and 10⁷ s⁻¹ and 10⁸ s⁻¹ for the anionic micelles. The acquired insights on traditional surfactant micelles provide a useful background for the characterization of aggregation in solutions without surfactants. Speculation on such surfactant-free microemulsions dates back to the 1970s but the idea was dismissed for a long time, not least due to the absence of experimental evidence. Recent studies, however, confirmed this conjecture with direct proof of micelle-like oil-in-water aggregates in the ternary system water/ethanol/octan-1-ol. The weakly associated structures occur for a certain range of compositions that constitute the so-called “pre-Ouzo region” in the monophasic part of the ternary phase diagram. The hydrotrope ethanol enables the stability of these octanol droplets on the nanometer scale by accumulating to a film at the interface similar to that formed by surfactant molecules in conventional microemulsions. A variety of aggregation numbers is observed, that encompasses a continuous spectrum of nanoordering. The smaller clusters are of globular shape and feature a roughly radial symmetric assembly with a core region composed of mostly terminal carbon units and an outer shell enriched with hydroxyl groups. This resemblance to traditional micelles fades towards larger aggregation numbers, where increasingly amorphous structures are detected. The system is governed by quick rearrangements and the mechanisms of micelle growth due to inter-particle collisions and shrinkage by decomposition into smaller aggregates play a major role. Not all of the cluster sizes that occur during these fluctuations are equally stable and certain aggregation numbers can be characterized as transitory stages during processes of swelling or depletion. The surfactant-free microemulsion exhibits excellent solubilizing properties for the apolar substances propane and squalene. For the first, the solvation free energy in the micelle interior is close to the favorable value predicted for a similarly composed binary octanol/ethanol solution, whereas the aqueous pseudo-phase poses a poor solvent. In the presence of the C₃₀ compound, octanol domains expand to enclose the large organic molecules in comparatively stable structures. This leads to significantly enhanced solubilization over a binary water/ethanol mixture of equal proportions. The primary consequence of the addition of the electrolytes NaI and LiCl is a stabilization of the aggregates via the salting-out of ethanol. Preferential adsorption of I⁻ or Li⁺ at the interface is only weakly observed due to the local accumulation of ethanol and octanol hydroxyl groups that leave little apolar surface exposed. In further analogy to traditional microemulsion systems, oil-in-water aggregation reverses to water-in-oil aggregation in excess of octanol. These two regimes are linked by a bicontinuous region, where the octanol and water pseudo-phases entwine to form a sponge-like network