Photo-Responsive Control of Adsorption and Structure Formation at the Air-Water Interface with Arylazopyrazoles.

Smart interfaces that are responsive to external triggers such as light are of great interest for the development of responsive or adaptive materials and interfaces. Using alkyl-arylazopyrazole butyl sulfonate surfactants (alkyl-AAP) that can undergo E/Z photoisomerization when irradiated with green (E) and UV (Z) lights, we demonstrate through a combination of experiments and computer simulations that there can be surprisingly large changes in surface tension and in the molecular structure and order at air-water interfaces. Surface tensiometry, vibrational sum-frequency generation (SFG) spectroscopy, and neutron reflectometry (NR) are applied to the study of custom-synthesized AAP surfactants with octyl- and H-terminal groups at air-water interfaces as a function of their bulk concentration and E/Z configuration. Upon photoswitching, a drastic influence of the alkyl chain on both the surface activity and the responsiveness of interfacial surfactants is revealed from changes in the surface tension, γ, where the largest changes in γ are observed for octyl-AAP (Δγ ∼ 23 mN/m) in contrast to H-AAP with Δγ < 10 mN/m. Results from vibrational SFG spectroscopy and NR show that the interfacial composition and the molecular order of the surfactants drastically change with E/Z photoisomerization and surface coverage. Indeed, from analysis of the S-O (head group) and C-H vibrational bands (hydrophobic tail), a qualitative analysis of orientational and structural changes of interfacial AAP surfactants is provided. The experiments are complemented by resolution of thermodynamic parameters such as equilibrium constants from ultra-coarse-grained simulations, which also capture details like island formation and interaction parameters of interfacial molecules. Here, the interparticle interaction ("stickiness") and the interaction with the surface are adjusted, closely reflecting experimental conditions.

Frame-guided assembly from a theoretical perspective.

The molecular self-assembly of various structures such as micelles and vesicles has been the subject of comprehensive studies. Recently, a new approach to design these structures, the frame-guided assembly, has been developed to progress toward fabrics of predefined shape and size, following an initially provided frame of guiding elements. Here, we study the frame-guided assembly in a two-dimensional membrane via computer simulations based on a single-bead coarse grained surfactant model in continuous space. In agreement with the experiment, the assembly process already starts for surfactant concentrations below the critical micelle concentration. Furthermore, upon increasing temperature, the formation process gets more delocalized. Additionally, the assembly process of the resulting membrane plane is modeled by a lattice gas model. It displays a similar phenomenology but additionally allows for the derivation of analytical mean-field predictions. In this way, a fundamental understanding of frame-guided assembly can be gained.

Non-equilibrium effects of micelle formation as studied by a minimum particle-based model.

The formation of self-assembled structures such as micelles has been intensively studied and is well understood. The ability of a solution of amphiphilic molecules to develop micelles is dependent on the concentration and characterized by the critical micelle concentration (cmc), above which micelle formation does occur. Recent studies use a lattice approach in order to determine cmc and show that the correct modeling and analysis of cluster formations is highly nontrivial. We developed a minimalistic coarse grained model for amphiphilic molecules in the continuum and simulated the time evolution via dynamic Monte Carlo simulations in the canonical (NVT) ensemble. Starting from a homogeneous system, we observed and characterized how the initial fluctuations, yielding small aggregates of amphiphilic molecules, end up in the growth of complete micelles. Our model is sufficiently versatile to account for different structures of surfactant systems such as membranes, micelles of variable radius, and tubes at high particle densities by adjusting particle density and potential properties. Particle densities and micellization rates are investigated and an order parameter is introduced so that the dependence of the micellization process on temperature and surfactant density can be studied. The constant density of free particles for concentrations above cmc, e.g., as expected from theoretical considerations, can be reproduced when choosing a careful definition of free volumes. In the cmc regime at low temperatures, different nonequilibrium effects are reported, occurring even for very long time scales.