Smart Air–Water Interfaces with Arylazopyrazole Surfactants and Their Role in Photoresponsive Aqueous Foam

Abstract

A new light-switchable azo-surfactant arylazopyrazole tetraethylene glycol carboxylic acid (AAP-E4) was used as a molecular building block to functionalize macroscopic foams. AAP-E4 was studied in the bulk solution with UV/vis spectroscopy and at the interface with sum-frequency generation (SFG) as well as tensiometry. Additional foaming experiments were performed with a dynamic foam analyzer to study the role of AAP-E4 surfactants at the ubiquitous air–water interface as well as within macroscopic foam. In the bulk, it is possible to switch the AAP-E4 surfactant reversibly from trans to cis configurations and vice versa using 380 nm UV and 520 nm green light, respectively. At the interface, we demonstrate the excellent switching ability of AAP-E4 surfactants and a substantial modification of the surface tension. In addition, we show that the response of the interface is strongly influenced by lateral electrostatic interactions, which can be tuned by the charging state of AAP-E4. Consequently, the electrostatic disjoining pressure and thus the foam stability are highly dependent on the bulk pH and the charging state of the interface. For that reason, we have studied both the surface net charge (SFG) and the surface excess (tensiometry) as important parameters that determine foam stability in this system and show that neutral pH conditions lead to the optimal compromise between switching ability, surface excess, and surface charging. Measurements on the foam stability demonstrated that foams under irradiation with green light are more stable than foams irradiated with UV light.

Unexpected monolayer-to-bilayer transition of arylazopyrazole surfactants facilitates superior photo-control of fluid interfaces and colloids

“…9,10 Aqueous foams are inherently interface controlled, where the macroscopic properties can be tailored at the molecular level through structure-property relations. 2,28 Since the surface excess of an ionic surfactant such as butyl-AAP-C 4 S determines the interfacial charging state, the latter is reduced when the samples are irradiated with UV light. This reduction in surface charge density leads to a considerable reduction in the electrostatic disjoining pressure P(h), which results in coalescence and bursting of foam bubbles.…”

“…where, c (2) NR , G q , and u q are the nonresonant contribution to the second-order susceptibility, the Lorentzian linewidth, and the resonance frequency of the q th vibrational mode, respectively. In addition, A q f Ghm q a q i ¼ Ghb q i is the amplitude of the mode q, and is directly related to the surface excess G of amphiphilic molecules as well as to the orientational average of both the dynamic dipole moment m q of molecules and their Raman polarizability a q .…”

“…Such control can be achieved by tuning molecular self-assembly through the use of an external stimulus. 2,6 Here, so interfaces are of great interest because they allow fast and reversible recongurations, e.g., by de-or adsorption of molecules triggered by an external stimulus. [7][8][9][10][11] For example, tuning the properties of uid interfaces helps to develop smart foams with self-healing and adaptive functions.…”

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

“…This contribution is often used to gain qualitative 48,50,51 and in some cases quantitative information [52][53][54] on the interfacial charging state:…”

Light‐Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry

“…[52,53] To address this limitation, arylazopyrazoles (AAPs) have been recently introduced as improvedl ight-responsive molecular switches. [54][55][56] In further studies, we have investigated, with others, the properties of AAPs [57] as molecular switches and developeds everal supramolecular systemsr anging from colloidal host-guest complexes, [58,59] foams, [60] adhesives, [61] and DNA complexes. [62,63] Recently,a lso al ight-responsive peptideh ydrogel featuring hostguest interactions betweenA AP peptides and cyclodextrin vesicles was introduced by our group.…”