Photoswitchable Arylazopyrazole Surfactants at the Water-Air Interface: A Microscopic Perspective.

Surfactants play an important role in modifying the properties of water-air interfaces. Here, we combine information from molecular dynamics simulations, surface tensiometry, and vibrational sum-frequency generation spectroscopy to study the interfacial behavior of photoswitchable arylazopyrazole (AAP) surfactants. This combination of the experimental techniques allows a direct relation between surface tension and surface concentration rather than just the bulk concentration.

Photo-induced drug release at interfaces with arylazopyrazoles.

Smart responsive materials have spurred the progress in high-precision drug delivery. Enormous attention has been given to characterizing drug release in bulk aqueous solutions, however, aqueous-hydrophobic interfaces are vital components of biological systems which serve as the point of entry into cells. These interfaces are involved in many key biomolecular interactions, and while the potential for drug molecules to adsorb to these interfaces is recognized, their specific role in the context of drug release remains largely unexplored.

Photoresponsive arylazopyrazole surfactant/PDADMAC mixtures: reversible control of bulk and interfacial properties.

In many applications of polyelectrolyte/surfactant (P/S) mixtures, it is difficult to fine-tune them after mixing the components without changing the sample composition, pH or the ionic strength. Here we report on a new approach where we use photoswitchable surfactants to enable drastic changes in both the bulk and interfacial properties. Poly(diallyldimethylammonium chloride) (PDADMAC) mixtures with three alkyl-arylazopyrazole butyl sulfonates (CAAP) with -H, -butyl and -octyl tails are applied and / photoisomerization of the surfactants is used to cause substantially different hydrophobic interactions between the surfactants and PDADMAC.

Charge Regulation at the Nanoscale as Evidenced from Light-Responsive Nanoemulsions.

Emulsions are indispensable in everyday life, and the demand for emulsions' diversity and control of properties is therefore substantial. As emulsions possess a high internal surface area, an understanding of the oil/water (o/w) interfaces at the molecular level is fundamental but often impaired by experimental limitations to probe emulsion interfaces in situ. Here, we have used light-responsive surfactants (butyl-AAP) that can photoisomerize between and isomers by visible and UV light irradiation to tune the emulsion interfaces.

Adsorption of CTAB on Sapphire- at High pH: Surface and Zeta Potential Measurements Combined with Sum-Frequency and Second-Harmonic Generation.

The adsorption of cetyltrimethylammonium bromide (CTABr) on sapphire- surfaces was studied at pH 10 below the surfactants' critical micelle concentration. The evolution of interfacial potentials as a function of CTAB concentration was characterized by surface and zeta potential measurements and complemented by molecular dynamic (MD) simulations as well as by second-harmonic (SHG) and vibrational sum-frequency generation (SFG) spectroscopy. The changes in interfacial potentials suggest that the negative interfacial charge due to deprotonated surface aluminols groups is neutralized and can be even overcompensated by the presence of CTA cations at the interface.

Nanoscale Effects on the Surfactant Adsorption and Interface Charging in Hexadecane/Water Emulsions.

Nanoscale properties at interfaces play a key role in the colloidal stability of emulsions and other soft matter materials where physical properties need to be controlled from the nano to macroscopically visible length scales. Our molecular level understanding of oil-water interfaces arises mostly from results at extended interfaces and the common view that emulsions are stabilized by a large number of surfactant molecules at the droplet's interface which, however, has been recently challenged. In this work, we show that the particle size and the curvature of oil droplets at the nanoscale is of great importance for the interface adsorption of dodecyl sulfate surfactants and possible counterion condensation at the charged hexadecane-water interface.

pH effects on the molecular structure and charging state of β-Escin biosurfactants at the air-water interface.

Saponins like β-escin exhibit an unusually high surface activity paired with a remarkable surface rheology which makes them as biosurfactants highly interesting for applications in soft matter colloids and at interfaces. We have applied vibrational sum-frequency generation (SFG) to study β-escin adsorption layers at the air-water interface as a function of electrolyte pH and compare the results from SFG spectroscopy to complementary experiments that have addressed the surface tension and the surface dilational rheology. SFG spectra of β-escin modified air-water interfaces demonstrate that the SFG intensity of OH stretching vibrations from interfacial water molecules is a function of pH and dramatically increases when the pH is increased from acidic to basic conditions and reaches a plateau at a solution pH of > 6.

A cyclodextrin surfactant for stable emulsions with an accessible cavity for host-guest complexation.

Although cyclodextrins (CDs) have been used as additives to small molecule surfactants, there are only a few examples of this cyclic oligosaccharide being used as a surfactant template. Herein, we present a novel ionic β-CD derivative that shows significant surface activity and stabilizes oil-in-water emulsions prepared with hexadecane as a model oil. The macrocyclic surfactant strongly reduces the oil-water surface tension with increasing concentration and shows a critical micelle concentration (CMC) around 5 mM.

Photo-Switchable Surfactants for Responsive Air-Water Interfaces: Azo versus Arylazopyrazole Amphiphiles.

Arylazopyrazoles (AAPs) as substitutes for azobenzene derivatives have gained considerable attention due to their superior properties offering / photoisomerization with high yield. In order to compare and quantify their performance, azobenzene triethylammonium (Azo-TB) and arylazopyrazole triethylammonium (AAP-TB) bromides were synthesized and characterized in the bulk (water) using NMR and UV/Vis spectroscopy. At the air-water interface, complementary information from vibrational sum-frequency generation (SFG) spectroscopy and neutron reflectometry (NR) has revealed the effects of isomerization in great detail.