A series of molecular dynamics simulations have been conducted in order to study the structural and interfacial properties of dodecyl sulfate (DS − ) monolayers with Li + , Na + , Cs + and NHcations. Varying the counterion has no significant effect on the structural properties of the surfactant molecules within the different monolayers. However, the different counterions have a significant effect on the interfacial properties of the monolayer. The NH + 4 ions are the most strongly bound to the headgroup, the least hydrated at the interface, directly compete with the hydrating water molecules for hydrogen bonds with the headgroup and more frequently interact with more than one headgroup. The Cs + ions are strongly bound to the headgroup and weakly hydrated, such that they would prefer to displace water in the DS − hydration shell to interact with headgroup. Also, the Cs + ions frequently interact with more than one headgroup. In the case of the Li + ions, they interact almost as strongly with the DS − headgroups as the Na + ions, but are generally less hydrated than the Na + ions and therefore they bring less water to the monolayer interface than the Na + ions. Therefore, by changing the counterion, one can modify the interfacial properties of the aggregates, and therefore effect their ability to encapsulate drug molecules, which we discuss in further greater detail.

Large-scale molecular dynamics simulations were used to study the structural and dynamic properties of the solubilization process of testosterone propionate (TSTP) within sodium dodecyl sulfate (SDS) micelles. We observed that the TSTP spontaneously adsorb onto the SDS micelles and preferentially reside among the polar head groups of the SDS molecules. We found that the SDS micelle is slightly aspherical in size and has a surface area of ∼170 Å(2)/molecule, while the SDS+TSTP micelle is more aspherical and has a surface area of ∼156 Å(2)/molecule. The structural properties of the interior of the SDS micelle and the hydration of the SDS headgroup are largely undisturbed by the presence of the TSTP. However, there seems to be a correlation between the location of the TSTP molecules and the location of Na(+) counterions on the surface of the SDS micelle. Additionally, we also observe that the TSTP molecules diffuse on the surface of the SDS micelle and try to organize themselves such that they are approximately equidistant from one another.

In recent years, the field of nanotechnology has become increasingly prevalent in the disciplines of science and engineering due to it’s abundance of application areas. Therefore, the ability to study and characterize these materials is more relevant than ever. Despite the wealth of simulation and modeling studies of nanoparticles reported in the literature, a rigorous description of the interface of such materials is rarely included in analyses which are pivotal to understanding interfacial behavior. We propose a novel method for constructing the continuous intrinsic surface of nanoparticles, which has been applied to a model system consisting of a sodium dodecyl sulfate micelle in the presence of testosterone propionate. We demonstrate the advantages of using our continuous intrinsic surface definition as a means to elucidate the true interfacial structure of the micelle, the interfacial properties of the hydrating water molecules, and the position of the drug (testosterone propionate) within the micelle. Additionally, we discuss the implications of this algorithm for future work in the simulation of nanoparticles. Graphical Abstractᅟ

Surface tension and specular neutron reflectivity measurements have been used, for the first time to systematically study both the interfacial structure and composition of monolayers of the soluble surfactant, sodium dodecyl sulfate containing a low-dose, poorly water soluble drug, testosterone enanthate. Modelling of the specular neutron reflectivity data suggests that the hydrophobic testosterone enanthate was adsorbed in the C12 hydrophobic tail region of the surfactant monolayer, regardless of the concentration of surfactant at the interface and whether or not additional drug was added to the interface. The location of the hydrophobic drug in the tail region of the surfactant monolayer is supported by the results of classical, large-scale molecular dynamics simulations. The thickness of the surfactant monolayer obtained, in the presence and absence of drug, using molecular dynamics simulations was in good agreement with the corresponding values obtained from the specular neutron reflectivity measurements. The stoichiometry of surfactant:drug at the air-water interface at sodium dodecyl sulfate concentrations above the critical micelle concentration was determined from specular neutron reflectivity measurements to be approximately 3 : 1, and remained constant after the spreading of further testosterone enanthate at the interface. Significantly, this stoichiometry was the same as that obtained in the micelles from bulk solubilisation studies. Important insights into the preferred location of drug in surfactant monolayers at the air-water interface as well as its effect on the structure of the monolayer have been obtained from our combined use of experimental and simulation techniques.