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

Sebastian Schöttl
2019
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
more » ... l 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 n [...]
doi:10.5283/epub.37989 fatcat:g5sueetxefcstkoxupdnjul2ri