Effects of Dye Addition on the Rheological Properties of Aqueous Polymer Solutions.

In many commercial applications, polymer-dye interactions are frequently encountered from food to wastewater treatment, and while shear rheology has been well characterized, the extensional properties are not well known. The extensional viscosity ηE and relaxation time λE are the extensional rheological parameters that provide valuable insights into how aqueous polymers respond during deformation, and this study investigated the effect of dyes on the extensional rheology of three different aqueous polymer solutions (e.g., anionic, cationic, and neutral) paired with two different dye salts (e.g., anionic and cationic) using drop pinch-off experiments. We have found that the influence of dyes on the pinch-off dynamics is complex but generally leads to a decrease in, for example, the apparent extensional relaxation time. We have utilized the dripping-onto-substrate method to probe the uniaxial deformation of widely used polymers such as xanthan gum (XG), poly(diallyldimethylammonium chloride) (PDADMAC), and poly(ethylene oxide) (PEO) as the anionic, cationic, and neutral polymers, respectively, paired with either fluorescein (Fl) or methylene blue (MB) as the anionic and cationic dyes, respectively. Polymer-dye pairs with opposite charges (e.g., XG-MB and PDADMAC-Fl) displayed a pronounced decrease in pinch-off times, but even PEO, which is a neutral polymer, resulted in decreased pinch-off times, which was restored by the addition of NaCl. The pinch-off times for the Boger fluid (mixture of poly(ethylene glycol) and PEO), however, were surprisingly uninfluenced by dyes. These results showed that not only did the small addition of dyes strongly decrease the polymer relaxation times, but the relative importance of the dye salts on the polymer pinch-off dynamics was also different from that of pure salts such as NaCl.

Determining the Surface pKa of Perfluorooctanoic Acid.

Perfluorooctanoic acid (PFOA) is an environmentally prevalent and persistent organic pollutant with toxic and bioaccumulative properties. Despite the known importance of perfluorinated pollutants in the global environment, molecular-level details of the physicochemical behavior of PFOA on aqueous interfaces remain poorly understood. Here, we utilized two surface-specific techniques, vibrational sum frequency generation spectroscopy (SFG) and surface tensiometry, to investigate the pH-induced structural changes of PFOA and octanoic acid (OA) and determined the apparent pKa at the air-water surface. The SFG spectra and surface activity model were investigated over a wide range of pHs. With the surface tension measurements, the surface pKa values for OA and PFOA are determined to be 3.8 ± 0.1 and 2.2 ± 0.2, respectively. These results could provide insights into improved remediation of PFOAs and may impact climate modeling of perfluorinated alkyl chain molecules.

Quantification of anion and cation uptake in ice Ih crystals.

While ice has very low solubility for salts compared to water, small amounts of ions are doped into ice crystals. These small ion dopants can alter the fundamental physical and chemical properties of ice, such as its structure and electrical conductivity. Therefore, these results could have a direct impact on the chemical reactivity of ice and ice surfaces. Here, we examine the influence of the uptake of three salts-ammonium chloride (NH4Cl), sodium chloride (NaCl), and ammonium sulfate [(NH4)2SO4]-on ice Ih formation using capillary electrophoresis. Using both cation and anion modes, we observed and quantified the uptake of individual ions into the ice. Our results indicate that anions have a higher propensity for uptake into ice Ih crystals.

Lipid-Specific Direct Translocation of the Cell-Penetrating Peptide NAF-144-67 across Bilayer Membranes.

The cell-penetrating peptide NAF-1 has recently emerged as a promising candidate for selective penetration and destruction of cancer cells. It displays numerous membrane-selective behaviors including cell-specific uptake and organelle-specific degradation. In this work, we explore membrane penetration and translocation of NAF-1 in model lipid bilayer vesicles as a function of lipid identity in zwitterionic phosphatidylcholine lipids mixed with anionic phosphatidylserine, phosphatidylglycerol, and phosphatidic acid lipids. By monitoring the digestion of NAF-1 using the protease trypsin located inside but not outside the vesicles, we determined that the translocation of NAF-1 was significantly enhanced by the presence of phosphatidic acid in the membrane compared to the other three anionic or zwitterionic lipids. These findings were correlated to fluorescence measurements of dansyl-labeled NAF-1, which revealed whether noncovalent interactions between NAF-1 and the bilayer were most stable either at the membrane/solution interface or within the membrane interior. Phosphatidic acid promoted interactions with fatty acid tails, while phosphatidylcholine, phosphatidylserine, and phosphatidylglycerol stabilized interactions with polar lipid headgroups. Interfacial vibrational sum frequency spectroscopy experiments revealed that the phosphate moiety on phosphatidic acid headgroups was better hydrated than on the other three lipids, which helped to shuttle NAF-1 into the hydrophobic region. Our findings demonstrate that permeation does not depend on the net charge on phospholipid lipid headgroups in these model vesicles and suggest a model wherein NAF-1 crosses membranes selectively due to lipid-specific interactions at bilayer surfaces.

Lower degree of dissociation of pyruvic acid at water surfaces than in bulk.

Understanding the acid/base behavior of environmentally relevant organic acids is of key relevance for accurate climate modelling. Here we investigate the effect of pH on the (de)protonation state of pyruvic acid at the air-water interface and in bulk by using the analytical techniques surface-specific vibrational sum frequency generation and attenuated total reflection spectroscopy. To provide a molecular interpretation of the observed behavior, simulations are carried out using a free energy perturbation approach in combination with electronic structure-based molecular dynamics. In both the experimental and theoretical results we observe that the protonated form of pyruvic acid is preferred at the air-water interface. The increased proton affinity is the result of the specific microsolvation at the interface.

Interfacial Vibrational Dynamics of Ice Ih and Liquid Water.

Insights into energy flow dynamics at ice surfaces are essential for understanding chemical dynamics relevant to atmospheric and geographical sciences. Here, employing ultrafast surface-specific spectroscopy, we report the interfacial vibrational dynamics of ice Ih. A comparison to liquid water surfaces reveals accelerated vibrational energy relaxation and dissipation at the ice surface for hydrogen-bonded OH groups. In contrast, free-OH groups sticking into the vapor phase exhibit substantially slower vibrational dynamics on ice. The acceleration and deceleration of vibrational dynamics of these different OH groups at the ice surface are attributed to enhanced intermolecular coupling and reduced rotational mobility, respectively. Our results highlight the unique properties of free-OH groups on ice, putatively linked to the high catalytic activities of ice surfaces.

Molecular hydrophobicity at a macroscopically hydrophilic surface.

Interfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atom toward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle.

Comparative Adsorption of Acetone on Water and Ice Surfaces.

Small organic molecules on ice and water surfaces are ubiquitous in nature and play a crucial role in many environmentally relevant processes. Herein, we combine surface-specific vibrational spectroscopy and a controllable flow cell apparatus to investigate the molecular adsorption of acetone onto the basal plane of single-crystalline hexagonal ice with a large surface area. By comparing the adsorption of acetone on the ice/air and the water/air interface, we observed two different types of acetone adsorption, as apparent from the different responses of both the free O-H and the hydrogen-bonded network vibrations for ice and liquid water. Adsorption on ice occurs preferentially through interactions with the free OH group, while the interaction of acetone with the surface of liquid water appears less specific.

Structure from Dynamics: Vibrational Dynamics of Interfacial Water as a Probe of Aqueous Heterogeneity.

The structural heterogeneity of water at various interfaces can be revealed by time-resolved sum-frequency generation spectroscopy. The vibrational dynamics of the O-H stretch vibration of interfacial water can reflect structural variations. Specifically, the vibrational lifetime is typically found to increase with increasing frequency of the O-H stretch vibration, which can report on the hydrogen-bonding heterogeneity of water. We compare and contrast vibrational dynamics of water in contact with various surfaces, including vapor, biomolecules, and solid interfaces. The results reveal that variations in the vibrational lifetime with vibrational frequency are very typical, and can frequently be accounted for by the bulk-like heterogeneous response of interfacial water. Specific interfaces exist, however, for which the behavior is less straightforward. These insights into the heterogeneity of interfacial water thus obtained contribute to a better understanding of complex phenomena taking place at aqueous interfaces, such as photocatalytic reactions and protein folding.

Time-Resolved Sum Frequency Generation Spectroscopy: A Quantitative Comparison Between Intensity and Phase-Resolved Spectroscopy.

Time-resolved and two-dimensional sum frequency generation (TR-SFG and 2D-SFG) spectroscopies are promising tools in the experimental study of molecular dynamics, specifically at interfaces. Most implementations of TR/2D-SFG spectroscopy rely on a pump-probe scheme, where an excitation pulse excites a fraction of interfacial molecules into the first excited state of a specific vibrational mode, and a subsequent SFG probe pair detects the time-dependent changes of the surface vibrational response. In steady state SFG spectroscopy, phase-resolved detection (also known as heterodyne-detection), as opposed to SFG intensity measurements, has been shown to be useful in unraveling the steady-state response of interfacial vibrations. Here, we explore the merits of phase-resolved TR/2D-SFG spectroscopy. This purely theoretical and numerical study reveals that, for a typical response from aqueous interfaces, the intensity 2D-SFG measurements contain the same information content as phase-resolved 2D-SFG measurements. We specifically analyze the frequency-dependence of the bleach lifetime (reflecting vibrational relaxation), and the time-dependent slope of the on-diagonal features observed in a 2D spectra. We show that for different systems, the intensity-based and phase-resolved 2D-SFG measurements provide the same information and are quantitatively very similar. We investigate the effect of different lineshapes, anharmonicity, and nonresonant signal contributions, and show that none of these effects substantially change the conclusion that intensity-based and phase-resolved 2D-SFG measurements provide equivalent information.

Experimental and theoretical evidence for bilayer-by-bilayer surface melting of crystalline ice.

On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.

Probing structural features of self-assembled violanthrone-79 using two dimensional infrared spectroscopy.

Two-dimensional infrared (2D IR) spectroscopy was used to characterize the structure of a self-assembled polycyclic aromatic hydrocarbon (PAH), violanthrone-79. A local mode basis was constructed using spectroscopic and computational results of anthrone and monomer violanthrone-79. The vibrational modes in the spectral region 1550-1700 cm(-1), carbonyl stretching and in-plane ring breathing, are used as vibrational probes. The local mode basis and an electrostatic coupling model were applied to three nanoaggregate structures: parallel, antiparallel, and a chiral configuration produced by a 28° rotation from parallel. Angular disorder within each nanoaggregate configuration was also explored. This investigation is a first approach to probe self-assembled PAHs with 2D IR spectroscopy. The experimental and calculated 2D IR spectra align best when the violanthrone-79 molecules are in an anti-parallel configuration within the nanoaggregate.

Characterizing Anharmonic Vibrational Modes of Quinones with Two-Dimensional Infrared Spectroscopy.

Two-dimensional infrared (2D IR) spectroscopy was used to study the vibrational modes of three quinones--benzoquinone, naphthoquinone, and anthraquinone. The vibrations of interest were in the spectral range of 1560-1710 cm(-1), corresponding to the in-plane carbonyl and ring stretching vibrations. Coupling between the vibrational modes is indicated by the cross peaks in the 2D IR spectra. The diagonal and off-diagonal anharmonicities range from 4.6 to 17.4 cm(-1) for the quinone series. In addition, there is significant vibrational coupling between the in-plane carbonyl and ring stretching vibrations. The diagonal anharmonicity, off-diagonal anharmonicity, and vibrational coupling constants are reported for benzoquinone, naphthoquinone, and anthraquinone.

Active Bragg angle compensation for shaping ultrafast mid-infrared pulses.

Active Bragg angle compensation is demonstrated for shaping ultrafast, mid-infrared pulses. The effects of angular dispersion introduced by the acousto-optic modulator on the temporal characteristics of the pulse are measured by autocorrelating the output from the pulse shaper. The time duration of the output pulses were measured to be thirty times shorter than pulses produced with a constant frequency amplitude waveform. This approach acts to mitigate angular dispersion in Bragg-regime acousto-optic devices, thus affording the ability to shape ultrafast pulses of light with broad bandwidths that are centered at mid-IR wavelengths and longer.