Photoswitchable Surfactants–Are there Alternatives to Azobenzene-Based Systems?

Introduction

Surfactants featuring a photoswitchable surface-activity have emerged as powerful materials for the manipulation of macromolecules and colloids in an aqueous environment.¹ Consisting of a hydrophobic tail, a switchable unit, and a hydrophilic, electrostatically charged or non-charged, headgroup,¹ the molecules’ geometry can be toggled in a light-responsive fashion. This entails an alteration of the surfactants’ molecular surface activity, from a strongly to a less surface-active form, leading to some consequences, which can be readily exploited to manipulate colloidal systems, macromolecules, surfaces, or solutions.

As an example, their switchable surface-activity translates into a conversion of the aggregates these surfactants form in solution, when they are present at a concentration above the critical micelle concentration (CMC). Structural investigations, thus, revealed that the surfactants form different micellar^2-8 architectures, depending on their switching state. As the presence of differently shaped aggregates, such as worm-like and spherical micelles, may affect the viscosity of the solution, photo-responsive surfactants represent a suitable platform to adjust the rheological properties of an aqueous system in a light-responsive manner.^{3, 4} If light-responsive surfactants are mixed with lipids, photo-destructible vesicles may result, whose integrity can be affected using a photo-stimulus.⁹ In addition, switchable surfactants assembling at liquid/liquid and liquid/gas interfaces, yield switchable emulsions^10-12 or foams.^{2, 13, 14}

Along with their switchable surface-activity, photo-responsive surfactants assemble at different interfaces.^{15, 16} The assembly of the surfactants, which can be studied by different surface-sensitive spectroscopic methods,^17-19 has a direct impact of the wettability of the respective surface,^{19, 20} and can e. g. be illustrated by the behaviour of water droplets on these surfaces.^{20, 21} The concomitant switching of surface energies can be exploited to control the fouling of bacteria on a surface as well, yielding e. g. tailor-made interfaces with switchable antifouling properties.²²

The surfactants’ property to adsorb at interfaces can also be exploited to manipulate micro- and nanoscale objects in water. Objects at the microscale, which are large enough to sediment, can be displaced using the light-driven diffusioosmosis (LDDO) effect.^{1, 16, 23-27} Briefly, this effect relies on the fact that light irradiation induces a local switching of the surfactant at the particle-water interface. The osmotic pressure, originating from the resulting concentration imbalance near the particle surface, is compensated by a diffusioosmotic flow generated along the surface, which in turn facilitates the particle transport.^{1, 23, 26, 27}

Smaller, nanoscale particles are kinetically stabilized, and can be directly affected in their colloidal stability by photoswitchable surfactants. It has been shown, e. g. for carbon nanomaterials,^{28, 29} that a surfactant can induce an on-demand flocculation an redispersion of a nanomaterial, depending on the switching state of the molecule. The principle of light-stimulated particle aggregation can also be employed to foster the assembly and disassembly of nanoparticulate superstructures^30-33 yielding particle supracrystals that possess, e. g. a photo-switchable magnetic,³² or catalytic³³ activity. A decoration of charged particles with surfactants affects the hydrophobicity of the particles. Photoswitching of responsive surfactants can, therefore, mediate the transfer of particles from an aqueous medium to an organic solvent vice versa.^{34, 35} In recent studies, it has also been shown that photoswitchable surfactants have a direct impact on the morphology of nanoparticles: Particles originating from phase-separated block-copolymers are influenced using light-responsive surfactants, whereby the varying affinity of the surfactant to the different blocks alters the overall structure of the nanostructures.^36-39

Surfactants that possess charged headgroups interact electrostatically with their ionic counterpart. This becomes particularly remarkable for polyelectrolytic macromolecules,^{1, 40-42} referred to as soft materials. The literature on this topic is vast, as reviewed elsewhere,¹ and involves studies, where interactions with synthetic polymers^{23, 43, 44} or macromolecules of biological origin,^45-48 such as nucleic acids,^46-48 are described. In this perspective article, we want to deduce the concepts, which will foster the design of novel light-responsive surfactants, fueled by different application scenarios. Eventually, we aim to provide a conclusion about surfactant design, which we complement with a careful outlook.

Molecular Architectures of Photoswitchable Surfactants

The Diversity of Azobenzene-Based Structures

Azobenzene is a well-known photo-switch, which has been described in many literature examples. Though being explored as a photo-responsive unit already in the 1930s,⁴⁹ azobenzenes are still a field of active research, and continuous research efforts have yielded a library of azobenzene structures that have been tuned to cover a broad range of the visible electromagnetic spectrum.⁵⁰ The peculiarity of azobenzenes is their reversible isomerization ability from an E to a Z configuration, where both isomers differ notably in their electric dipole moment ($$ ,^{1, 51} Figure 1a).

Azobenzenes are characterized by a good photostability along with advantageous switching kinetics.⁵² Beyond that, their straightforward synthetic accessibility renders azobenzene photoswitches the basis for the most abundantly reported light-responsive surfactant systems.^{1, 53}

Classically, the azobenzene-based surfactants consist of a hydrophilic headgroup and a hydrophobic backbone, into which the azobenzene switch is incorporated (Figure 2a). The switching of the system described in Figure 2a occurs, in an aqueous environment, at <400 nm and >450 nm effectively for its back reaction (note, that according to overlapping absorbance peaks between E- and Z-form there is always a relative proportion of both isomers).⁵⁴ The headgroup may be cationically, anionically or zwitterionically charged, or may consist of a non-charged polar moiety, such as a carbohydrate.¹ Alongside, more intricate molecular architectures are known, for instance bola-form or gemini surfactants, which will not be a subject of this article. In this regard, the diversity of azobenzene-based surfactants that consist of two hydrophobic tails should be emphasized, which are the basis for switchable membrane structures.⁵⁵ Such systems often form complex assemblies, which arouse great interest in their utilization as a material platform. As an example, systems containing a branched tail and a zwitterionic headgroup have been described that transform from worm-like micelles to lamellar bilayers.^{56, 57}

What is the Hallmark of an Efficient Photo-Switch?

The addition of light-responsive surfactants to colloidal systems offers an elegant possibility to render, e. g. suspensions of nano- or microscale particles, emulsions, microgels or polymers, photo-sensitive. In order to design a suitable surfactant, several key properties of the surfactant molecule have to be optimized. Briefly, the interaction of the surfactant with the desired material should differ significantly, depending on the switching state of the surfactant (Figure 3).^{1, 70, 71}

At a first glance, this concerns the kinetics of the ad- and desorption of the surfactants as well as the kinetics of their self-assembly into micelles. As the interaction between the surfactant molecules with the desired material or with themselves (by assembling into micelles) is a dynamic equilibrium, one isomer must adsorb quickly and effectively to the material, while the other one is supposed to desorb effectively. The term ‘effectively’ hereby underlines the dynamic nature of the exchange. This renders such systems not always easy to predict, as the kinetics of two isomers interacting dynamically with the material or to the corresponding micelle have to be considered, thus resulting in total 2³ =8 possible cross-correlating parameters (Figure 3).

These considerations do not yet comprise the influence of photoisomerization kinetics in the relative isomer ratio that is present in the PSS state. The ratio of both isomers controls the macroscopic structure response of the chemical system and, thus, its light response.⁴⁹ In the following, we want to deduce the concepts, that would result in the most optimal design of light-responsive surfactants, based on the applications, which is illustrated with general roadmap displayed in Figure 4.

Regarding the surfactant interactions with materials, there are basically two kinds of material interactions to discuss. The first one is based on the physical principle of assembly and aggregation changes within “soft materials,” possessing a diffuse interface, such as polymer chains, networks or brushes. The second one is the interaction of the surfactants with hard interfaces and their structural changes at a molecular scale as well as the macroscopic implications during the photoisomerization.

All the interactions are generally very complex, but they can be roughly approximated by four major key points:

A slow photoisomerization allows a precise control in light-induced manipulation, but usually, such systems do not allow a strongly pronounced manipulation in time frames of seconds or minutes, which renders them unattractive for most experimental applications. Thus, ideally both photoisomerization rate as well adsorption kinetics should be in time scales of seconds or minutes.

If the lifetime (~1/(thermal relaxation rate)) is in the same time range as the photo-isomerization process, we suggest minimizing the impact of the lifetime by increasing the illumination intensity. Some photochemical reactions may require a critical intensity, as the thermal relaxation is very fast, and effective switching is never reached. A simple approximation of the applied intensity for adjusting the desired isomer molar ratio X in PSS, with respect to the thermal relaxation, can be calculated accordingly:⁵⁴

$$ ,(1)

with X=c_E/c_Z as the isomer ratio from concentration of E and Z isomer c_E and c_Z in PSS, k_EZ and k_ZE as the rate constant of photo induced isomerization from E→Z (index EZ) and vice versa (index ZE), k_T as thermal relaxation rate constant and I as the intensity of applied light (with λ=constant). Equation 1 shows that with increasing intensity the impact of k_T decreases. Thus, in case of low lifetime (~fast thermal relaxation) we suggest increasing the light intensity as much as possible, however, at the expense of an enhanced photodegradation tendency with increasing intensity.

As a general rule of thumb, the surfactant's attractive interaction potential – i. e. its tendency to adsorb – is a result of the hydrophobic effect,⁷⁵ the surface-activity (correlating with the CMC),⁷⁶ the packing parameter as well as the polarity of the entire surfactant molecule, comprising the effective dipole moment or the head group type (non-ionic, zwitterionic, ionic).^77-80 Note, that all parameters are in strong cross-correlation with each other.

To predict the influence of such complex systems on colloidal systems, thus, in a first approximation, the three parameters should notably differ between the both isomers. However, not always are all changes achievable over the chemical structure. Hence, depending on the material interaction, a trade-off between the polarity or geometry should be aimed at while choosing the surfactant structure and the photo-switchable moiety. This is discussed in more detail in the following for the two major material types: “Soft materials” and “hard interfaces”.

Hard Interfaces

The dynamic transition between the isomers of a photo-switchable surfactant at the surface represents the fundamental physical principle for the manipulation of materials possessing a hard interface. As an example, plain surfaces can be affected with respect to their wetting behaviour,^{19-21, 94} colloids can be aggregated and dispersed on demand,^28-33 or microscale particles are transported on a solid surface in the presence of a surfactant.^{1, 26, 27} These manipulations have their fundamental physical principle on the complex interaction of surfactants with interfaces on molecular scale. For such interfacial-bulk solution interactions, there are two general key parameters which need to be tuned.

First, (i), the general molecular structure of the surfactant determines the exchange ability of the molecule between the interface and the bulk solution. Here, the size of the surfactant plays an important role. Smaller molecules can more easily align as mono- or multilayers, hemi- and full micelles at smooth or rough interfaces or at porous structures than larger ones, as they are sterically less demanding.^{15, 95-97} Their tendency to be attracted by the interface is also driven by the properties of the head group⁹⁸ but also by the chain length of hydrophobic tail,⁹⁹ which is in turn characterized by the packing parameter of the entire molecule.^{15, 100} The adsorption, however, is not only determined by the steric properties of the molecule itself. As dynamic processes allow generally a faster exchange between the interface and bulk solution, the properties affecting the ad- and desorption kinetics must be considered as well. This may include the surface potential of the interface or the counterion type of the interacting ion pairs, i. e. counterionic specific interactions.¹⁰¹

Second, (ii), the packing parameters of both isomers should be strongly different. Therefore, the photo-switchable moiety should impart a strong structural change of the entire geometry of the surfactant during the switching. As rule of thumb, an indication of the surface-active potential of surfactants is the concentration, at which the surfactant assembles from entirely water-soluble molecules into micelles, i. e., the CMC.¹⁰⁰ This number provides information, at which concentration the surfactant molecules strongly adsorbs at interfaces. Values of CMC can vary in several orders of magnitude depending on the length and structure of the hydrophobic chain length and head group.¹⁰² Generally higher chain lengths of the backbone and less polarity of the headgroup results in lower values of the CMC,¹⁰³ as these parameters increase the hydrophobicity of the entire molecule and the tendency to aggregate in comparison to be surrounded from water molecules.

Both isomers of the photo-switchable surfactant are surface-active, translating into different absolute bulk surfactant concentration. For a reversibly switching “on-off” surface activity, one isomer should always have a much lower CMC value than the corresponding isomer and ideally, they differ as much as possible between both isomers.¹ Such properties are satisfied for surfactants exhibiting an azobenzene, or azopyrazole (AAP) moiety, where the difference is approximately around one order of magnitude between E- and Z-isomer.^{60, 102} Due to the enhanced water-solubility of the Z-isomer, its CMC is higher. Empirically, the difference in CMC between E- and Z- surfactants for azobenze may be approximated as a formal difference in the alkyl chain length by three methylene groups (12.6 carbon atoms for E- to 9.7 carbon atoms for the Z-isomer) as example from a reported substance.¹⁰⁴ Discussed findings represent an overall guideline but are not universal and also strongly depend on the nature of the head group and also the position of the photo-switchable unit along the hydrophobic chain.¹⁰⁵

When it is aimed to manipulate hard surfaces, the surfactant should be added at such a concentration, the CMC of the Z-isomer (CMC_Z) is not reached yet, but that it is already above the CMC of the E-isomer CMC_E with CMC_E<c<CMC_Z (c being the working concentration), with the tendency to adjust the total surfactant concentration closer to the CMC value of the surface-active E form. If the concentration is much lower than the CMC range, the response is limited due to a slow and dynamic exchange. The same is also important for concentrations exceeding the CMC of the well water-soluble Z form, since in this case, both isomers are rather surface active at this “high” concentration range, and the light-induced surface-activity contrast between both forms is rather unpronounced.

Despite the fact, that low-CMC surfactants are quite surface-active, the utilization of surfactants with overall high CMC values are more favoured. The rate of adsorption is a product of the adsorption rate constant and the isomer concentration, k⋅c,^{15, 72} which is a simple parameter to control, to amplify the dynamics of interaction towards higher concentration. Accordingly, ionic surfactants are always a better choice, as they possess generally higher solubility, thus, increased values of CMC (~0.1–1 mM) compared to non-ionic derivates (~10⁻³ to 10⁻² mM)^106-109 at fixed hydrophobic chain lengths. However, this depends on counterion-specific interactions between head group and ions¹⁰¹ as well as strongly on the ionic strength of the bulk solution and applies merely for low ionic strength solution (Millipore water instead of a buffered solution).

Note, that the CMC for ionic surfactants strongly decreases with increasing concentrations of salt in solution. If the medium must be buffered, the only choice is restricted to non-ionic surfactants.

What molecular features does the surfactant need to exhibit to be well suited for the manipulation of hard interfaces? In order to maximize the CMC difference between two isomers, the weighting on molecular tuning should rely on alterations of via the packing parameter.¹ It can be pointed out here, that the molecular geometry can play a more important role than molecule's polarity, which can be hypothesized from comparing azobenzene vs. spiropyran surfactants (Figure 5). In detail, the azo-benzene isomerization from E to Z results in a drastic change of the angular reorientation from an effective zero angle (0°, E-form) into an acute angle (20°, Z-form), which basically changes the geometry of the hydrophobic tail from an elongated shape into a bent shape as illustrated in Figure 5b.¹⁵ For spiropyrans, however, the transition between a non-charged spiropyran to a zwitterionic merocyanine results in a rotation along the molecular axis of the surfactant (Figure 5d) and then the effective geometry is similar between both forms. Even though these both forms of the photo moiety siginificantly differ in their dipole moment (~3D for azobenzenes vs. ~10 D for spiropyrans),⁶¹ they possess similar ΔCMC values. If the polarity would dominate the tendency of self-assembly, drastic changes in the CMC difference for the spiropyran in comparison to azobenzene containing surfactants could be expected. Based on these observations, we hypothesize, that the geometry change has a much stronger impact in comparison to polarity, which causes the huge difference of CMC between E versus Z form for azobenzene and obviously not for spiropyran versus merocyanine. Further, the position and the chain length of the hydrophobic spacer and tail influences the value of the CMC, where the tail has a stronger impact on CMC.¹⁰²

Are there Other Possible Photoswitch Motifs?

Various strategies have the potential to improve the performance of photoswitchable surfactants. Taking the lessons learnt for spiropyrans and azobenzens, a selection of promising motifs and candidates will be highlighted and briefly discussed. This will include both new classes of photoswitches as well as modifications of established ones.

As previously discussed, both the polarity and the geometry differences between the two isomers are important parameters to consider, if an optimal manipulation of colloids is targeted. As shown in Figure 4, the significant alteration of these properties among the photoisomerization is the crucial lever to improve the ▵CMC. Only the combination with the photochemical properties of photoisomerization allows a comprehensive description of the photoswitchable surfactant. An ideal photoswitch converts a large quantity of absorbed photons into a chemical reaction (high quantum yield), resulting in an almost full conversion of one isomer at the PSS. This means that at given wavelength of illumination, most isomers in the E state have to be transferred into Z and vice versa at another wavelength. Only the combination of both properties (pronounced ▵CMC and almost full isomer conversion) enables a precise control of alteration between E- and Z-surfactant and, thus, into an alteration of the switchable response of colloidal systems (nano- and microparticles, soft particles, polymeric networks etc.), which in our opinion represents the most ideal photo-responsive surfactants.

Moreover, the thermal stability, absorption wavelength and molar absorption coefficient are important parameters to consider, which depend on the application scenario. For example, the molar adsorption coefficient determines the applicable exposure wavelengths and may limit the usage in biological processes (~UV light), while technical processes with a demand for high spatial resolution are benefiting from rather shorter wavelengths.

Photophysically, the absorption features, characterized by the absorption coefficient μ_a, need to be optimized to obtain an ideal response of the molecule to the incident light. This parameter constitutes an inherent molecular property and is used to be determined by default in studies, concerning molecular photoswitches. In addition to the actual absorption characteristics, in a colloidal system, also the scattering properties must be considered. Along with the targeted manipulation, such as the swelling and shrinking of a microgel, the aggregation of nanomaterials, or the formation of micelles from fully dissolved species inter alia, the switching process of a surfactant changes the system from a non-scattering into a scattering sample vice versa. Hence, also the scattering coefficient μ_s of the respective colloidal system has to receive attention. It must be mentioned, that the absorbance of the non-scattering state requires precise engineering, so that the spectral absorption and the intensity loss due to scattering do not interfere with each other. This will avoid total absorbance and ensure a homogeneous photoreaction throughout the material. The structure-property relationship of both absorption wavelength and molar absorption coefficient of photoswitches as well as the chemical engineering of surfactants are discussed comprehensively elsewhere.^{110, 111}

As discussed in the previous section, both increasing the polarity and geometry change upon the switching process determines the performance of the photoswitchable surfactant. Azobenzenes have long been established as photoswitches and can therefore be considered a benchmark system. To improve their performance, the change of the molecular polarity upon E to Z transition can be tuned. Improving this polarity change, however, represents a challenging task. Altering the substitution pattern can address this issue, yet will almost always alter the electronic structure of the azobenzene, and with it the photochemistry. Particularly, the thermal stability of the Z state is well known to rapidly decline throughout substitution with that push-pull patterns.^{112, 113} Focus of the past decade was shifting the absorption into the visible, and even the IR region.⁵⁰ Further, the applicability in water was a focus.¹¹⁴ With this momentum, various substitution patterns have been investigated. Garmshausen et. al reported the replacement of one benzene moiety in the azobenzene by an tetracyanocyclopentadienide (Figure 6a).¹¹⁵

When the second benzene moity is substituted with an ammonium group, a photoswitchable zwitterion is formed. It shows a comparable photochemical behaviour to frequently used azobenzenes, and the absorption maxima (λ_max E=390 nm and λ_max Z=470 nm in acetonitrile) are well separated. Illumination of the E-isomer with 405 nm results in a PSS of 91 % of the Z-Form, and a reversion upon illumination with light greater than 500 nm of 94 % E-isomer. The half-life time is reported to be 150 min.¹¹⁵ Alkyl substitution at the nitrogen could access novel surfactant types. These would be expected to alter the geometry of the head group rather than their polarity, which could be beneficial for the interaction with hard materials.

Another very potent variation of the azobenzene is the azobenzene-difluoroboron complex (azo-BF₂) switch (Figure 6b).¹¹⁶ Here, a difluoroboron is coordinated to phenylhydrazones. They can be isomerized between the E isomer, which is thermodynamically stable, and the Z isomer. These switches can show excellent PSSs and very high photoisomerization quantum yields (PSSE−Z=97 %Z, PSSZ−E=80 %E, ΦE−Z=0.48, ΦZ−E=0.67 in dichloromethane – DCM). The half-life of the Z form was approximately 12.5 h in deoxygenated and 0.5 h in regular DCM. These represent convenient photosystems, which can, upon a selective alkylation at the two benzene rings, yield surfactants, which experience a large change in their geometry upon switching. This feature would render the surfactants well suited for an interaction with hard materials.

Alongside azobenzenes, various other motifs show an E−Z isomerisation (Figure 7).¹¹⁴ Even the merocyanine can be modified to exhibit solely an E−Z isomerisation. Other moieties are stiff stilbenes (Figure 7a),¹¹⁷ indigo and thioindigo^{118, 119} (Figure 7b) as well as hemi-indigoid^{120, 121} (Figure 7c) and aryliminoindigoid^{122, 123} (Figure 7d) and hydrazone^124-127 (Figure 7e) photoswitches. As these structures promise a pronounced geometry change, surfactants derived therefrom would interact well with hard materials.

Multiresponsive chromism is in our opinion a promising strategy to achieve a significant change in both geometry and/or polarity. Here, a set of equilibria are coupled, which can be of thermal of photochemical nature, and used to maximize the alteration of the molecular properties. In fact, the conversion between spiropyrans and merocyanine is one of the most complex examples for multi-responsive chromism. It does not only comprise an electrocyclization, but also a E−Z- isomerization as well as an acid-base equilibrium.⁶¹ This makes it such a potent switch for photoswitchable surfactants.¹²⁸ However, these features render it also vulnerable to external interference, e. g. solvent polarity or pH. In particular, water is a challenging environment. Alongside spiropyran, a variety of motifs are incorporating both E−Z-isomerisation with electrocyclization, e. g. fulgides, fulgimides,^129-135 chromenes, naphthopyrans, dihydroazolenes and stiff diarylethens.^{136, 137} These interconnections result not only a significant geometric changes, but also in more rigid structures of the photoproduct.

An alternative strategy to maximise geometric change among switching is the coupling of multiple E−Z-isomerisations, e. g. by combination of multiple azo-unites in one molecular moity (Figure 8).^{138, 139} High conversions to the Z,Z configuration are achievable, but only if the two photosiwchable units are π-decoupled. This can be achieved throughout a dihedral angle of 90° as it is shown in the example by Bléger et al.¹³⁸ Here, illumination with 368 nm results in 95 % Z,Z configuration. In asymmetric molecules, where the azobenzene moieties are not identical¹⁴⁰ or combined with other switches,¹⁴¹ it is possible to address both switches independently. Cross-conjugation, e. g. by substituting the ortho positions is an alternative approach to decouple the azo units. Using this pattern even photoresponsive foldamers are reported.^{142, 143} The switching states of photosurfactants derived from this moiety would tremendously differ in their geometry and could, thus, be highly interesting possessing a high ΔCMC.

Moreover, the combination of photochemical with a thermal equilibrium, such as a protonation/deprotonation and thermal equilibrium in one molecule can be achieved. Donor–acceptor Stenhouse adducts (DASAs) are a new and promising class of photoswitches, first described in 2014.^{144, 145} Visible light induces a E−Z-photoisomerization followed by a thermal cyclisation and proton transfer (Figure 9). This cascade of reactions ends in a significant change in geometry and a formation of zwitterion. Thereby, this class combines the preferable properties of azobenzenes and spiropyrans. In addition, they are synthetically straightforward to access. Consequently, first applications to functionalise polymers, e. g. in microgels or in liquid crystals have been reported.^{146, 147} Incorporating this photoswitch into a surfactant structure would therefore result in surfactants whose isomers differ in both geometry as well as polarity, which renders the respective surfactant an ideal candidate for the manipulation of both hard and soft materials.

The photochemical electrocyclization of diarylethenes results in a conversion of a hexatriene into a cyclohexadiene. This is accompanied by a relocalisation of double bonds, which has been used to stabilise or destabilise, respectively, the charges in (hetero) aromatic moieties (Figure 10). Gurke et al. incorporated a thiozolon moiety into a diarylethenes allowing for the modulation of the pKa.¹⁴⁸ A negative charge is well stabilized in the aromatic thiazolon. Upon photoisomerization, the aromatic stabilization of vanishes, which goes along with a significant increase of the pKa. The ring closure can be addressed with UV light, leading to an almost full conversion into a thermally stable isomer. Upon illumination with visible light reverses the reaction. Azulene moiety can be used in a similar fashion to control a protonation, forming a cation.¹⁴⁹ Incorporating this moiety into a surfactant could result in structures whose switching states differ in the polarity of a surfactant by controlling a protonation state. This could be highly interesting for the remote control of soft materials.

Conflict of Interests

The authors declare no conflict of interest.