Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces

Abstract

Surfactants containing pH-switchable, carboxylic acid moieties are utilized in a variety of environmental, industrial, and biological applications that require controlled stability of hydrophobic droplets in water. For nanoemulsions, kinetically stable oil droplets in water, surface adsorption of the anionic form of the carboxylic acid surfactant stabilizes the droplet, whereas a dominant surface presence of the neutral form leads to destabilization. Through the use of dynamic light scattering, ζ-potential, and vibrational sum frequency scattering spectroscopy (VSFSS), we investigate this mechanism and the relative surface population of the neutral and charged species as pH is adjusted. We find that the relative population of the two surfactant species at the droplet surface is distinctly different than their bulk equilibrium concentrations. The ζ-potential measurements show that the surface concentration of the charged surfactant stays nearly constant throughout the stabilizing pH range. In contrast, VSFSS shows that the neutral carboxylic acid form increasingly adsorbs to the surface with increased acidity. The spectral features of the headgroup vibrational modes confirm this behavior and go further to reveal additional molecular details of their adsorption. A significant hydrogen-bonding interaction occurs between the headgroups that, along with hydrophobic chain–chain interactions, assists in drawing more carboxylic acid surfactant to the interface. The charged surfactant provides the stabilizing force for these droplets, while the neutral surfactant introduces complexity to the interfacial structure as the pH is lowered. The results are significantly different than what has been found for the planar oil/water studies where stabilization of the interface is not a factor.

In Situ Detection of Chemical Compositions at Nanodroplet Surfaces and In-Nanodroplet Phases

“…Vibrational sum frequency generation scattering is anticipated to be an analytical technique for the characterization of interfacial molecular species. ,− To assess our in situ VSFS technique, we have collected polarized VSFS spectra from droplet surfaces for solutions of methanol, ethanol, 1-propanol, and 1-butanol solutions in 0.5 M NaCl. These normalized spectra collected at VVH polarization are shown in Figure ; corresponding spectra at other polarizations can be found the Supporting Information along with all fitting parameters.…”

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

“…Figure e shows VSFSS spectra, collected in the SSP polarization scheme, of nanoemulsions stabilized by SDS/PNVA in the amide CO stretching region. ,− Again, the PNVA concentration was fixed at 5 mM, while the SDS concentration varied from 0.1 (orange), 0.5 (green), 1 (blue), to 2.5 (purple) mM. The carbonyl spectral region has been notoriously difficult to probe due to a highly intense nonresonant response that obscures any resonant responses from the species of interest. , In order to measure the resonant CO responses from PNVA, the visible pulse was carefully detimed from the IR pulse by ∼500–600 fs, to remain within the vibrational lifetime of CO stretching resonance . This detiming approach allows for the resonant response to be measured, while minimizing any unwanted overlapping contributions. − Its presence indicates that as the polymer adsorbs to the interface, it does so with the carbonyl mode adopting a net orientation perpendicular to the interface as opposed to solely random in orientation.…”

“…The carbonyl spectral region has been notoriously difficult to probe due to a highly intense nonresonant response that obscures any resonant responses from the species of interest. 46,47 In order to measure the resonant CO responses from PNVA, the visible pulse was carefully detimed from the IR pulse by ∼500−600 fs, to remain within the vibrational lifetime of CO stretching resonance. 48 This detiming approach allows for the resonant response to be measured, while minimizing any unwanted overlapping contributions.…”

Phase State and Thermodynamic Properties of Proxy Sea Spray Aerosol Interfaces Derived from Temperature-Dependent Equilibrium Spreading Pressure

“…Other studies have calculated the thermodynamic properties of 3D to 2D spreading for PA on different salt solutions, ,,,, but none have considered the thermodynamic properties of carboxylate–metal interactions for proxy SSA interfaces. However, carboxylate–metal interactions such as contact ion pairing and solvent sharing have been well documented by spectroscopic investigation, , demonstrating a need for further thermodynamic elucidation. ,,,,− Therefore, we can exploit our ESP values to obtain the thermodynamic properties of PA–Na + interactions given the following equations. The enthalpy (ΔΔ H PA‑Na + ), entropy (ΔΔ S PA‑Na + ), and Gibb’s free energy (ΔΔ G PA‑Na + ) of PA–Na + interactions can be calculated using eqs –. Our PA and NaCl model system is only one example of this, but this approach could be applied to other atmospherically relevant systems.…”