Recognition competes with hydration in anion-triggered monolayer formation of cyanostar supra-amphiphiles at aqueous interfaces

The triggered self-assembly of surfactants into organized layers at aqueous interfaces is important for creating adaptive nanosystems and understanding selective ion extraction. While these transformations require molecular recognition, the underlying driving forces are modified by the local environment in ways that are not well understood. Herein, we investigate the role of ion binding and ion hydration using cyanosurf, which is composed of the cyanostar macrocycle, and its binding to anions that are either size-matched or mis-matched and either weakly or highly hydrated. We utilize the supra-amphiphile concept where anion binding converts cyanosurf into a charged and amphiphilic complex triggering its self-organization into monolayers at the air–water interface. Initially, cyanosurf forms aggregates at the surface of a pure water solution. When the weakly hydrated and size-matched hexafluorophosphate (PF6−) and perchlorate (ClO4−) anions are added, the macrocycles form distinct monolayer architectures. Surface-pressure isotherms reveal significant reorganization of the surface-active molecules upon anion binding while infrared reflection absorption spectroscopy show the ion-bound complexes are well ordered at the interface. Vibrational sum frequency generation spectroscopy shows the water molecules in the interfacial region are highly ordered in response to the charged monolayer of cyanosurf complexes. Consistent with the importance of recognition, we find the smaller mis-matched chloride does not trigger the transformation. However, the size-matched phosphate (H2PO4−) also does not trigger monolayer formation indicating hydration inhibits its interfacial binding. These studies reveal how anion-selective recognition and hydration both control the binding and thus the switching of a responsive molecular interface.


Instrumentation
All measurements are performed at ambient temperature 21.0 ± 2 °C, atmospheric pressure, and relative humidity 31 ± 7 %. Surface pressure-area (Π-A) isotherms were performed on a custom Teflon Langmuir trough with an area of 144.5 cm 2 equipped with movable Delrin barriers (KSV NIMA, Finland). Surface pressure was measured by the Wilhelmy plate method and controlled using KSV software (KSV NIMA, Finland). Prior to each experiment, the presence of extraneous surface-active contaminants was checked by sweeping the surface of the aqueous solution with the barriers until no significant change in the surface pressure (< 0.1 mN / m) was observed.

Surface Pressure-Area Isotherms.
Surface pressure-area (Π-A) isotherms of cyanosurf on neat water and salt solutions were performed on a computer-controlled Langmuir trough (KSV), this custom Teflon trough (168 mm x 85 mm) was equipped with two movable Delrin barriers (KSV NIMA, Finland) for symmetrical compression of the cyanosurf. Prior to each measurement, the trough needed to be cleaned with copious amounts of ethanol and Milli-Q water. The presence of organic contamination was checked by sweeping the surface of the subphase solution with a fast compression of the Delrin barriers at 227 mm per minute per barrier to ensure there was no significant rising of the surface pressure (<0.1 mN / m).
The cyanosurf solutions were deposited dropwise using the Hamilton syringe method and surface pressure was measured using Wilhelmy paper plates (Whatman, Ashless). After spreading, a 10-minute delay was applied to ensure solvent evaporation. Delrin barriers compressed symmetrically at a speed of 5 mm/min/barrier until the collapse of the monolayer was reached. For S7 IRRAS experiments, once the desired surface pressure was reached, the barriers are oscillated slowly at a rate of 1 mm / min / barrier both forward and backward to maintain the surface pressure.

Infrared Reflection−Absorption Spectroscopy
To probe the surface signature and organization of cyanosurf on the surface of subphase solutions, infrared reflection−absorption spectroscopy (IRRAS) spectra were collected using a Fourier transform infrared spectrometer (Perkin Elmer). The FT-IR spectrometer is equipped with a liquid nitrogen-cooled MCT (HgCdTe) detector and modified with a breadboard setup using two gold mirrors fixed at an incidence angle of 48° to collect the reflectivity off the cyanosurf. The IRRAS spectra were plotted as reflectance−absorbance (RA), which is given as RA = −log (Rc/R0), where Rc is the reflectivity of the cyanosurf surface and R0 is the reflectivity of the subphase solution (i.e., either water, ClO4 -, or PF6 -). An average of 300 scans over full range (450-4000 cm -1 ) was collected using unpolarized light in the single-beam mode for each spectrum. Π-A isotherms and IRRAS spectra shown here are the result of averaging at least three spectra using the average function in Origin software (Origin 9, Northampton, MA).

Brewster Angle Microscopy
A custom-built Brewster angle microscope setup was used to simultaneously record Brewster angle microscopy (BAM) images and Π−A isotherms. The He-Ne laser source (1.5 mW, Research Electro-Optics, Boulder, CO) emitted polarized light at 543 nm with linear polarization. The light was filtered by a Glan polarizer for p-polarized purification before reaching to the aqueous surface. The incoming angle was first fixed at ∼53° for the aqueous solution surface. The reflected beam was collected after going through a 10× infinity-corrected super-long working distance objective lens (CFI60 TU Plan EPI, Nikon Instruments, Melville, NY) and collimated by a tube lens (MXA22018, Nikon Instruments; focal length 200 mm) before going into a back-illuminated EM-CCD camera (iXon DV887-BV, Andor Technology USA, Concord, MA; 512 × 512 active pixels with 16 μm × 16 μm pixel size). The BAM images were processed using ImageJ software77 and cropped from their original size to show the region of highest resolution. The dark regions of the images corresponded to the aqueous surface of subphase, whereas the light blue and bright regions of the images corresponded to the cyanosurf domains with low and high (aggregates) coverage, respectively.

Sum Frequency Generation Spectroscopy
The detail of the broadband SFG spectrometer set up used for this study was previously reported. S3,S4 Briefly, a regenerative Ti:sapphire amplifier (Spitfire Ace, Spectra-Physics) seeded with a sub-50 fs 800 nm pulse from a Ti:sapphire oscillator provides an ∼4 W beam of 75 fs pulses and 1 kHz repetition rate. The amplified beam is then directed through a 50:50 beam splitter. One half of the beam is needed to pump an optical parametric amplification system (TOPAS-C, Light Conversion), which is coupled to a non-collinear difference frequency generator (NDFG, Light Conversion) to generate the broadband infrared beam. The other half of the beam intensity is spectrally narrowed to a fwhm of 12 cm −1 by an etalon (SLS Optics, United Kingdom) and is used as the visible 797 nm beam. The IR and the visible beams are co-propagating and fall on the sample surface at an angle from the surface normal of 60 and 50 degrees, respectively. The IR beam is focused on the sample surface with a CaF2 lens (15 cm FL) and the visible beam is focused approximately 1 cm after the surface with a BK7 lens (25 cm FL). The sum frequency signal is collected in the reflected direction by a spectrometer (IsoPlane SCT 320, Princeton Instruments) and an LN2 CCD (PyLoN, 1340 × 400 pixels, Princeton Instruments). All the spectral measurements are done in SSP polarization combination (for SF, vis, IR, respectively). The data for the OH spectral region was obtained with a 60 sec integration time and 5 averages. The spectra were then normalized to the non-resonant spectrum of a gold mirror (Thor labs) after background subtraction. Figure S6. The IRRAS spectrum of surface cyanosurf molecules on water is shown as a black trace. The spectrum of surface cyanosurf molecules with 10 mM of the sodium phosphate at an MMA of 150 Å 2 / molecule is shown as an orange trace. Figure S7. IRRAS spectra of surface cyanosurf molecules on water is shown as a black trace for (a, b). The spectra of the surface cyanosurf molecules with 10 mM of the sodium salts of (a) Cl -(green) and (b) H2PO4 -(orange) at a MMA of 150 Å 2 / molecule S9 Figure S8. Langmuir titration curve showing normalized IRRAS peak intensity for the ClO4asymmetric vibration (1110 cm -1 ) obtained from the spectra of cyanosurf molecules on solutions with ClO4at different concentrations. IRRAS data collected at a MMA of 150 Å 2 / molecule. Least-squares regression fitting was based on the equation associated with formation of a 2:1 cyanosurf-anion complex. Large uncertainties in the IRRAS intensity data are due to the inherent variability in the IRRAS spectral response and signal-tonoise factors. Reported error (±0.21 × 10 3 M -1 ) corresponds to one standard deviation from the spectral fits.