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We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned near the liquid, but out of direct contact. We observe a parabolic dependence of second harmonic intensity on applied potential. Based on standard theory, we associate this dependence with the response of the diffuse layer water molecules to changes in interfacial potential profile. Taking the literature value for this response leads to the unexpected conclusion that the electric fields within the diffuse layer are opposite in sign from those originating from the electrodes. This conclusion implies that the traditional continuum-based models of interfacial screening lack the complexity necessary to properly describe the potential profile of the liquid water-vapor interface. Effects such as overscreening in the topmost interfacial layer and extended correlations in the interfacial hydrogen bonding network may play a role in governing the response of the water interface to external fields.

We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid out of contact. We observe a parabolic dependence of second harmonic intensity on applied potential. This dependence is reminiscent of bulk-phase electric field induced second harmonic (EFISH) but more complicated because it combines the second-order response of the topmost water layer and the potential dependent response of the interfacial electrical double-layer. Based on the literature values for these contributions, we derive a physical interpretation of our measurements that reveals new insight into the response of the neat water interface to external electric fields. Specifically, we find that the net dipolar orientation of water molecules within the double-layer is primarily responsive to the internal fields generated by the excess surface concentrations of OH- and H3O+ that arise to screen the external potential. Notably, this interpretation implies that the orientational response of water dipoles at the interface can actually oppose the direction of the external field, a subtle effect that is not captured by traditional models.

The composition and lifetime of sea spray aerosols are driven by the molecular and biological complexity of the air–seawater interface. We explore in situ the surface properties of marine algal blo...

We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid. We observe a parabolic dependence of second harmonic intensity on applied potential. This dependence is reminiscent of bulk-phase electric field induced second harmonic (EFISH) but more complicated because it combines the second-order response of the topmost water layer and the potential dependent response of the interfacial electrical double-layer. Based on the literature values for these contributions, we derive a physical interpretation of our measurements that reveals new insight into the response of the neat water interface to external electric fields. Specifically, we find that the net dipolar orientation of water molecules within the double-layer is primarily responsive to the internal fields generated by the excess surface concentrations of OH- and H3O+ that arise to screen the external potential. Notably, this interpretation implies that the orientational response of water dipoles at the interface can actually oppose the direction of the external field, a subtle effect that is not captured by traditional models.

We use second harmonic generation spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid. We observe a parabolic dependence of second harmonic intensity on applied potential, indicating that water’s net interfacial dipole responds linearly. We also observe a minimum intensity when the potential is tuned to a specific positive value. Interpreting this minimum based on the macroscopic electrostatic potential profile yields misleading physical conclusions because it neglects the internal bias exerted on molecular orientations by the excess surface concentrations of OH- or H3O+. We thus find that water’s net interfacial dipole orientation is primarily responsive to the effects of these ionic species rather than the external electric field.

We explore in situ the surface properties of marine algal blooms of diatom monocultures by utilizing surface techniques of Brewster angle microscopy (BAM) imaging, vibrational sum frequency generation spectroscopy (SFG), and infrared reflection absorption spectroscopy (IRRAS). Over the course of the bloom, the marine algae produce surface-active biogenic molecules that temporally partition to the topmost interfacial layers and are selectively probed through surface imaging and spectroscopic measurements. BAM images show morphological structural changes and heterogeneity in the interfacial films with increasing density of surface-active biogenic molecules. Film thickness calculations quantified the average surface thickness over time. The image results reveal an ~5 nm thick surface region in the late stages of the bloom which correlates to typical sea surface nanolayer thicknesses. Our surface-specific SFG spectroscopy results show significant diminishing in the intensity of the dangling OH bond of surface water molecules consistent with organic molecules partitioning and replacing water at the air-seawater interface as the algal bloom progresses. Interestingly, we observe a new broad peak appear between 3500 cm-1 to 3600 cm-1 in the late stages of the bloom that is attributed to weak hydrogen bonding interactions of water to the surface-active biogenic matter. IRRAS confirms the presence of organic molecules at the surface as we observe increasing intensity of vibrational alkyl modes and the appearance of a proteinaceous amide band. Our work shows the often overlooked but vast potential of tracking changes in the interfacial regime of small-scale laboratory marine algal blooms. By coupling surface imaging and vibrational spectroscopies to complex, time-evolving, marine-relevant systems, we provide additional insight into unraveling the temporal complexity of sea spray aerosol compositions.

We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of l-phenylalanine (Phe) with lipid monolayers of zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air–water interface. The measured VSFG spectra in the CH stretch region due to the lipid and Phe, and the OH stretch region due to interfacial water molecules were analyzed. These results in combination with surface pressure studies reveal that the Phe molecules at acidic pH of 5.6 intercalate into DPPC monolayers, and replace some interfacial water molecules. Consequently, there is a decrease in the VSFG intensity in the OH stretch region in the Phe subphase, and a concomitant increase in the surface pressure of the DPPC monolayer. The exclusion of the water molecules is controlled by both the bulk concentration of Phe, and the surface concentration of DPPC. In contrast, at the neutral pH of 7.3 there is an increase in the VSFG intensity due to the interfacial wat...

We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular-level understanding of interaction of anionic surfactant sodium dodecyl sulfate (SDS), in low bulk concentration at the micromolar level, with lipid monolayer zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air–water interface. These results are different from those reported at higher bulk concentration of SDS at millimolar level. At very low concentration neither DPPC nor SDS produces any VSFG signal in the CH stretch region in the water subphase, but with the same concentration DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction leads to polar ordering of DPPC molecules with enhancement of VSFG intensity in the CH vibrational region of the hydrophobic tails. The interaction between the lipid and SDS molecules is influenced by concentrations of both lipid and SDS. Hydrophobic interactions between long alkyl chains of SDS and DPP...

The structure and orientation of interfacial morpholine molecules have been investigated, using vibrational sum-frequency generation (VSFG) spectroscopy—a nonlinear surface specific technique. The VSFG spectra with SSP and PPP polarizations have been measured in the CH (2800–3000 cm–1) and the OH (3000–3750 cm–1) stretch regions at the air–morpholine and air–solution interfaces. The vibrational frequencies in the CH stretch region could be observed in VSFG spectra, implying presence of morpholine molecules at the interfaces with net polar orientation. The intensities of the CH stretch bands get enhanced at the air–solution interface of morpholine solution in millimolar concentration, in comparison to that at the air–morpholine interface which is attributed to increase in polar orientation of interfacial morpholine molecules induced by water molecules. In pure morpholine, the most predominant conformation of molecules is equatorial chair, both in the bulk and at the air–morpholine interface. But in aqueous...

The photodissociation dynamics of benzoyl chloride at 235 nm has been investigated and compared with that of 2-furoyl chloride. Atomic Cl and molecular HCl channels have been detected in benzoyl chloride by employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl((2)PJ=3/2, relative quantum yield 0.70 ± 0.15) and Cl*((2)PJ=1/2), show isotropic angular distribution and bimodal translational energy distributions. The predominant high kinetic energy channel contributes 72% to the C-Cl bond scission and arises from the S1 state having nπ* character of benzoyl chloride. However, the low-energy Cl and HCl channels originate from the ground electronic state. The most plausible mechanism of HCl formation is proposed based on molecular orbital calculations. In contrast to benzoyl chloride, the HCl channel is not observed in 2-furoyl chloride on excitation at 235 nm, and this is attributed to an energy constraint.

The effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, and cetyltrimethylammonium bromide (CTAB), a cationic surfactant, at an air–water interface have been investigated using vibrational sum-frequency generation (VSFG) nonlinear optical spectroscopy. The work is focused on understanding the temporal evolution of aggregation behavior of the catanionic system of CTAB and SDS at the charged air–water interface. For the mixed surfactants, the VSFG intensity of the OH stretching bands decreases, whereas that of the CH stretching of alkyl chains increases with time. For the 1:1 ratio of surfactants, the VSFG intensity of the OH stretch vanishes much earlier than the complete growth of the CH stretching modes. Thus, the polar ordering of interfacial water molecules is faster than the time-evolution of alkyl chains of the catanionic system. The temporal growth of the complex between surfactants CTAB and SDS exhibited an induction time (up to ∼200...

The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersonic beam by resonance enhanced multiphoton ionization (REMPI), using time-of-flight (TOF) mass spectrometry. The study is focused on the nascent state of the primary chlorine atom, formed on excitation of the (n, sigma*) transition of the molecule around 235 nm. Dissociation of PCl(3) and the REMPI detection of chlorine atoms are performed, using the same laser around 235 nm. The photofragments, namely, Cl((2)P(3/2)) and Cl*((2)P(1/2)), are probed, using the 2+1 REMPI scheme in the 234-236 nm region. We have determined the photofragment speed distribution, the recoil anisotropy parameter beta, and the spin-orbit branching ratio for chlorine atom elimination channels. Polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies. The anisotropy parameters for Cl and Cl* are characterized by values of 0.0 +/- 0.05 and 0.20 +/- 0.05, respectively. Two components, namely, the fast and the slow, are observed in the speed distribution (P(v)) of Cl and Cl* atoms, formed from different potential energy surfaces. The average translational energies for the Cl and Cl* channels for the fast component are 29.7 and 30.6 kcal/mol, respectively. Similarly, for the slow component, the average translational energies for the Cl and Cl* channels are 9.5 and 9.1 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of an impulsive model, for the fast component, and a statistical model, for the slow component. Apart from the chlorine atom elimination channel, molecular chlorine (Cl(2)) elimination is also observed in the photodissociation of PCl(3). The observation of the molecular chlorine in the dissociation process and the bimodal translational energy distribution of the chlorine atom clearly indicate the existence of a crossover mechanism from the initially prepared state to the ground state.