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 ∼2000 s), followed by a rapid growth (∼30 s) and then a slow growth for hours. The effects of different compositions of the surfactants on the induction time and the adsorption kinetics have been investigated. Our results on VSFG are supported by measurements, employing the surface pressure-time (π–t) kinetics and Brewster angle microscopy (BAM). The nature of π–t curves and its dependence on composition of surfactants is qualitatively similar to VSFG temporal profiles, except for the absence of the rapid adsorption growth in the former. This difference in the adsorption kinetics is explained based on formation of surfactant domains, as detected by BAM, at the air–water interface.

There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water–air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO4 2– > H2PO4 – > NO3 – > NO2 – while harboring very weak interactions with the halides in the order I– > Cl– ≈ Br–. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins’s rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl–, Br–) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.

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 bloom diatom monocultures by utilizing surface techniques of Brewster angle microscopy (BAM) imaging, vibrational sum-frequency generation (SFG) spectroscopy, 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 of a productive bloom over time. The image results reveal an ∼5 nm thick surface region in the late stages of the bloom, which correlates with 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 band appear between 3500 and 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 an increasing intensity of vibrational alkyl modes and the appearance of a proteinaceous amide band over time. 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.

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.

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 DPPC are responsible for an increase in the conformational order of the alkyl chain of DPPC with a decrease in the gauche defect and increase in trans conformer. Similarly, the orientation and concentration of interfacial water molecules of DPPC monolayer at SDS subphase are controlled by concentration of both SDS and DPPC. The VSFG results are complemented by the surface-pressure measurements.