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, e.g. 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 (CnAAP) with -H, -butyl and -octyl tails are applied and E/Z photoisomerization of the surfactants is used to cause substantially different hydrophobic interactions between the surfactants and PDADMAC. These remotely controlled changes affect significantly the P/S binding and allows for tuning both the bulk and interfacial properties of PDADMAC/CnAAP mixtures through light irradiation. For that, we have fixed the surfactant concentrations at values where they exhibit pronounced surface tension changes upon E/Z photoisomerization with 365 nm UV light (Z) and 520 nm green (E) light and have varied the PDADMAC concentration. The electrophoretic mobility can be largely tuned by photoisomerisation of CnAAP surfactants and P/S aggregates, which can even exhibit a charge reversal from negative to positive values or vice versa. In addition, low colloidal stability at equimolar concentrations of PDADMAC with CnAAP surfactants in the E configuration lead to the formation of large aggregates in the bulk which can be broken up by irradiation with UV light when the surfactant's alkyl chain is short enough (C0AAP). Vibrational sum-frequency generation (SFG) spectroscopy reveals changes at the interface similar to the bulk, where the charging state at air-water interfaces can be modified with light irradiation. Using SFG spectroscopy, we interrogated the O-H stretching modes of interfacial H2O and provide qualitative information on surface charging that is complemented by neutron reflectometry, from which we resolved the surface excesses of PDADMAC and CnAAP at the air-water interface, independently.

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 E and Z isomers by visible and UV light irradiation to tune the emulsion interfaces. This causes massive changes in the interface tension at the extended o/w interfaces in macroemulsions and a drastic shift in the surfactants' critical micelle concentration, which we show can be used to control both the stability and phase separation. Strikingly different from macroemulsions are nanoemulsions (RH ∼90 nm) as these are not susceptible to E/Z photoisomerization of the surfactants in terms of changes in their droplet size or ζ-potential. However, in situ second-harmonic scattering and pulsed-field gradient nuclear magnetic resonance (NMR) experiments show dramatic and reversible changes in the surface excess of surfactants at the nanoscopic interfaces. The apparent differences in ζ-potentials and surface excess provide evidence for a fixed charge to particle size ratio and the need for counterion condensation to renormalize the particle charge to a critical charge, which is markedly different compared to the behavior of very large particles in macroemulsions. Thus, our findings may have broader implications as the electrostatic stabilization of nanoparticles requires much lower surfactant concentrations, allowing for a more sustainable use of surfactants.

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

The adsorption of cetyltrimethylammonium bromide (CTA+Br-) on sapphire-c 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. However, SFG intensities from strongly hydrogen-bonded interfacial water molecules as well as SHG intensities decrease with both increasing CTAB concentration and the magnitude of the surface potential. They do not suggest a charge reversal at the interface, while the change in zeta potential is actually consistent with an apparent charge inversion. This can be qualitatively explained by results from MD simulation, which reveal adsorbed CTA+ cations outside a first strongly bound hydration layer of water molecules, where they can locally distort the structural order and replace some of the interfacial water molecules adjacent to the first layer. This is proposed to be the origin for the significant loss in SFG and SHG intensities with increasing CTAB concentration. Moreover, we propose that CTA+ can act as a counterion and enhance the occurrence of deprotonated surface aluminols that is consistent with the decrease in surface potential.

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

Saponins like ß-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 ß-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 ß-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. These changes are attributable to the interfacial charging state and to the deprotonation of the carboxylic acid group of ß-escin. Thus, the change in OH intensity provides qualitative information on the degree of protonation of this group at the air-water interface. At pH < 4 the air-water interface is dominated by the charge neutral form of ß-escin, while at pH > 6 its carboxylic acid group is fully deprotonated and, consequently, the interface is highly charged. These observations are corroborated by the change in equilibrium surface tension which is qualitatively similar to the change in OH intensity as seen in the SFG spectra. Further, once the surface layer is charge neutral, the surface elasticity drastically increases. This can be attributed to a change in prevailing intermolecular interactions that change from dominating repulsive electrostatic interactions at high pH, to dominating attractive interactions, such as hydrophobic and dispersive interactions, as well as, hydrogen bonding at low pH values. In addition to the clear changes in OH intensity from interfacial H2O, the SFG spectra exhibit drastic changes in the CH bands from interfacial ß-escin which we relate to differences in the net molecular orientation. This orientation change is driven by tighter packing of ß-escin adsorption layers when the ß-escin moiety is in its charge neutral form (pH < 4).

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. Using second-harmonic scattering, we have studied the surface charge of oil droplets in nanoemulsions where we systematically varied the particle size R between 80 and 270 nm and demonstrate that the surface charge density σ changes drastically with size: For sizes >200 nm, σ is similar to what can be expected at flat extended interfaces, while σ is dramatically reduced by almost an order of magnitude when the particle size of the oil droplet is 80 nm. Using a theoretical approach that considers counterion condensation, we quantify the nanoscale effects on the change in surface charge with particle size and find excellent agreement with our experimental result. Modeling of the experimental results also implies that the charge per particle remains constant and depends on a critical balance of surfactant adsorption and ion condensation.

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 ß-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. We demonstrate the stability of the oil-in-water emulsions and verify the possibility to address the emulsion droplet surface using molecular recognition of the ß-CD cavity at the oil-water interface.

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 E/Z 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 E/Z isomerization in great detail. In bulk water the photostationary states of >89% for E/Z switching in both directions were very similar for the surfactants, while their interfacial behavior was substantially different. In particular, the surface excess Γ of the surfactants changed drastically between E and Z isomers for AAP-TB (maximum change of Γ: 2.15 µmol/m2); for Azo-TB, the change was only moderate (maximum change of Γ: 1.02 µmol/m2). Analysis of SFG spectra revealed that strong nonresonant contributions that heterodyned the resonant vibrational bands were proportional to Γ, enabling the aromatic C-H band to be interpreted as an indicator for changes in the interfacial molecular order. Close comparison of Γ from NR with the SFG amplitude from the aromatic C-H stretch as a function of concentrations and E/Z conformation revealed substantial molecular order changes for AAP-TB. In contrast, only Γ and not the molecular order varied for Azo-TB. These differences in interfacial properties are attributed to the molecular structure of the AAP center that enables favorable lateral interactions at the air-water interface, causing closed-packed interfacial layers and substantial changes during E/Z photoisomerization.