Effect of salts on dynamics of water: A Raman spectroscopy study

Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties

We present concentration-dependent dynamics of highly concentrated LiBr solutions and LiCl temperature-dependent dynamics for two high concentrations and compare the results to those of prior LiCl concentration-dependent data. The dynamical data are obtained using ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE). The OHD-OKE decays are composed of two pairs of biexponentials, i.e., tetra-exponentials. The fastest decay (t1) is the same as pure water's at all concentrations within error, while the second component (t2) slows slightly with concentration. The slower components (t3 and t4), not present in pure water, slow substantially, and their contributions to the decays increase significantly with increasing concentration, similar to LiCl solutions. Simulations of LiCl solutions from the literature show that the slow components arise from large ion/water clusters, while the fast components are from ion/water structures that are not part of large clusters. Temperature-dependent studies (15-95 °C) of two high LiCl concentrations show that decreasing the temperature is equivalent to increasing the room temperature concentration. The LiBr and LiCl concentration dependences and the two LiCl concentrations' temperature dependences all have bulk viscosities that are linearly dependent on τcslow, the correlation time of the slow dynamics (weighted averages of t3 and t4). Remarkably, all four viscosity vs 1/τCslow plots fall on the same line. Application of transition state theory to the temperature-dependent data yields the activation enthalpies and entropies for the dynamics of the large ion/water clusters, which underpin the bulk viscosity.

Dynamics of Concentrated Aqueous Lithium Chloride Solutions Investigated with Optical Kerr Effect Experiments

We report the dynamics of concentrated lithium chloride aqueous solutions over a range of moderate to high concentrations. Concentrations (1-29 to 1-3.3 LiCl-water) were studied in which, at the highest concentrations, there are far too few water molecules to solvate the ions. The measurements were made with optically heterodyne-detected optical Kerr effect experiments, a non-resonant technique able to observe dynamics over a wide range of time scales and signal amplitudes. While the pure water decay is a biexponential, the LiCl-water decays are tetra-exponentials at all concentrations. The faster two decays arise from water dynamics, while the slower two decays reflect the dynamics of the ion-water network. The fastest decay (t₁) is the same as pure water at all concentrations. The second decay (t₂) is also the same as that of pure water at the lower concentrations, and then, it slows with increasing concentration. The slower dynamics (t₃ and t₄), which do not have counterparts in pure water, arise from ion-water complexes and, at the highest concentrations, an extended ion-water network. Comparisons are made between the concentration dependence of the observed dynamics and simulations of structural changes from the literature, which enable the assignment of dynamics to specific ion-water structures. The concentration dependences of the bulk viscosity and the ion-water network dynamics are directly correlated. The correlation provides an atomistic-level understanding of the viscosity.

Shining (Infrared) Light on the Hofmeister Series: Driving Forces for Changes in the Water Vibrational Spectra in Alkali-Halide Salt Solutions

The Hofmeister series is frequently used to rank ions based on their behavior from chaotropes ("structure breakers"), which weaken the surrounding hydrogen-bond network, to kosmotropes ("structure makers"), which enhance it. Here, we use fluctuation theory to investigate the energetic and entropic driving forces underlying the Hofmeister series for aqueous alkali-halide solutions. Specifically, we exploit the OH stretch infrared (IR) spectrum in isotopically dilute HOD/D₂O solutions as a probe of the effect of the salt on the water properties for different concentrations and choice of halide anion. Fluctuation theory is used to calculate the temperature derivative of these IR spectra, including decomposition of the derivative into different energetic contributions. These contributions are used to determine the thermodynamic driving forces in terms of effective internal energy and entropic contributions. This analysis implicates entropic contributions as the key factor in the Hofmeister series behavior of the OH stretch IR spectra, while the effective internal energy is nearly ion-independent.

Water Dynamics and Structure of Highly Concentrated LiCl Solutions Investigated Using Ultrafast Infrared Spectroscopy

In highly concentrated salt solutions, the water hydrogen bond (H-bond) network is completely disrupted by the presence of ions. Water is forced to restructure as dictated by the water-ion and ion-ion interactions. Using ultrafast polarization-selective pump-probe (PSPP) spectroscopy measurements of the OD stretch of dilute HOD, we demonstrate that the limited water-water H-bonding present in concentrated lithium chloride solutions (up to four waters per ion pair) is, on average, stronger than that occurring in bulk water. Furthermore, information on the orientational dynamics and the angular restriction of water H-bonded to both water oxygens and chloride anions was obtained through analysis of the frequency-dependent anisotropy decays. It was found that, when the salt concentration increased, the water showed increasing restriction and slowing at frequencies correlated with strong H-bonding. The angular restriction of the water molecules and strengthening of water-water H-bonds are due to the formation of a water-ion network not present in bulk water and dilute salt solutions. The structural evolution of the ionic medium was also observed through spectral diffusion of the OD stretch using 2D IR spectroscopy. Compared to bulk water, there is significant slowing of the biexponential spectral diffusion dynamics. The slowest component of the spectral diffusion (13 ps) is virtually identical to the time for complete reorientation of HOD measured with the PSPP experiments. This result suggests that the slowest component of the spectral diffusion reflects rearrangement of water molecules in the water-ion network.

Continuous in situ portable SERS analysis of pollutants in water and air by a highly sensitive gold nanoparticle-decorated PVDF substrate

The increasingly serious environmental pollution worldwide has posed a great threat to the ecosystem and human health, and yet the development of portable in situ monitoring techniques that are sensitive to gaseous and water pollutants remains incomplete. Herein, we report a highly active surface-enhanced Raman spectroscopy (SERS) substrate fabricated by immobilizing gold nanoparticles (AuNPs) onto a polyvinylidene fluoride (PVDF) membrane for continuous in situ SERS detection of pollutants in water and atmosphere. 4-Mercaptobenzoic acid (4-MBA) was adopted as a probe molecule to evaluate the performance of the substrate, and the results indicate that the polymer-based flexible substrate features high sensitivity, uniformity, and repeatability. The fabricated PVDF/SERS substrate was integrated with a portable Raman spectrometer operating under both passing-by and passing-through modes. The integrated system accomplishes quantitative detection and real-time online monitoring of pH in a liquid environment with a response speed of less than 10 s and the rapid SERS response to gas molecules at a low concentration within 30 s. We also demonstrated the highly sensitive detection for mainstream smoke (MS) and sidestream (SS) of cigarette smoke and verified their differences in the main constituent which contributes to the harmful secondhand smoke in public. The developed portable Raman system has excellent application prospects in online liquid and gas environmental detection.

Surface-enhanced Raman scattering of water in aqueous dispersions of silver nanoparticles

The resonance Raman effect (RRE) is a phenomenon which results in a strong selective enhancement of Raman signals from the samples. Previous studies showed that the RRE in liquid water directly corresponds to its supramolecular structure. It was also reported that the electric-field-induced orientation of water molecules on the electrode surface results in the surface-enhanced Raman scattering (SERS) effect. In this work, we show the SERS effect for water molecules in the dispersion of silver nanoparticles (AgNPs) without any external electrical field. An enhancement factor was estimated to be (4.8 ± 0.8) × 10⁶ for an excitation wavelength of 514.5 nm and for AgNPs with an average size of 34 ± 14 nm. The temperature experiment results showed a higher enhancement with temperature increase. Performed simulation studies revealed a slowdown of the mobility of the water molecules close to the surface of AgNPs.

Dissolution of a surfactant-water lamellar phase investigated by combining time-lapse polarized light microscopy and confocal Raman spectroscopy

HYPOTHESIS

While the phase behavior of aqueous surfactant solutions is usually described in term of the equilibrium microstructures of lyotropic liquid crystals, the transformations which take place when a phase turns into another one, either by changing the concentration or the temperature, are still to be elucidated. A simultaneous determination of concentration and microstructure is at order to elucidate the phase behavior under changing conditions, such as in a dissolution experiment.

EXPERIMENTS

Confocal Raman micro-spectroscopy and time-lapse polarized light microscopy are combined to study the phase transitions taking place in the dissolution of a common anionic surfactant (sodium laurylethersulfate) in water.

FINDINGS

By comparing Raman concentration profiles and polarized light images, it is found that the aqueous solution, with initial surfactant concentration of 72 wt%, undergoes a sequence of complex microstructural transformations including distortion of the initial lamellar phase, formation of an intermediate striated texture, which can be considered as a precursor of a cubic phase, and a heterogeneous hexagonal phase going through a transition region before turning into a micellar phase. The effects of the sodium counter-ion and of water confinement are also investigated by analyzing the OH-stretching bands.

Influence of Inorganic Solution Components on Lithium Carbonate Crystal Growth

Lithium-bearing brines are an increasingly attractive source of Li for extraction. One extraction mechanism is the removal of Li from the fluid phase through the precipitation of zabuyelite (Li₂CO₃). The chemistry of the brine plays an important role in this process because ions in solution can compete for the components of the Li-carbonate phase. Here we explore the effect of different brine components on the precipitation of zabuyelite using experiments and computational simulations. Crystals formed in all solutions showed morphological evidence for potential transformation from a precursor phase. Our study indicates that Ca²⁺ and SO₄ ^2- are incorporated into the precipitated zabuyelite crystals. Sulfate also interacts directly with specific surfaces on the growing crystal and is expected to form ion pairs with Li⁺ in solution. Similarly, Na⁺ appears to form ion pairs in solution with the carbonate ion, slowing nucleation of zabuyelite in the experiments. K⁺ and Cl^- may interact with the growing zabuyelite crystals but do not appear to affect zabuyelite nucleation and growth times. These experiments highlight the importance of understanding the solution chemistry on zabuyelite formation in order to predict the efficiency of extraction processes and the purity of the solids.

CdSSe nanowire-chip based wearable sweat sensor

Background

Sweat, as an easily accessible bodily fluid, is enriched with a lot of physiological and health information. A portable and wearable sweat sensor is an important device for an on-body health monitoring. However, there are only few such devices to monitor sweat. Based on the fact that sweat is mainly composed of moisture and salt which is much more abundant than other trace ions in sweat, a new route is proposed to realize wearable sweat sensors using CdSSe nanowire-chips coated with a polyimide (PI) membrane.

Results

Firstly, the composition-graded CdS_1−xSe_x (x = 0–1) nanowire-chip based sensor shows good photo-sensitivity and stress sensitivity which induces linear humidity dependent conductivity. This indicates good moisture response with a maximum responsivity (dI/I) 244% at 80% relative humidity (RH) even in the dark. Furthermore, the linear current decrease with salt increase illustrates the chip sensor has a good salt-sensing ability with the best salt dependent responsivity of 80%, which guarantees the high prediction accuracy in sweat sensing. The sensor current is further proven to nonlinearly correlate to the amount of sweat with excellent stability, reproducibility and recoverability. The wearable sweat sensor is finally applied on-body real-time sweat analysis, showing good consistence with the body status during indoor exercise.

Conclusions

These results suggest that this CdSSe nanowire-chip based PI-coated integrated sensor, combined with inorganic and organic functional layers, provides a simple and reliable method to build up diverse portable and wearable devices for the applications on healthcare and athletic status.

Electronic supplementary material

The online version of this article (10.1186/s12951-019-0480-4) contains supplementary material, which is available to authorized users.

Raman micro-spectroscopy as a viable tool to monitor and estimate the ionic transport in epithelial cells

The typical response to the lowering of plasma Na⁺ concentration and blood pressure in our body involves the release of aldosterone from the adrenal glands, which triggers the reabsorption of sodium in the kidney. Although the effects of aldosterone on this physiological mechanism were extensively studied in the past decades, there are still some aspects to be fully elucidated. In the present study, we propose for the first time a new approach based on Raman spectroscopy to monitor the ionic activity in aldosterone-treated A6 renal epithelial cells. This spectroscopic technique is capable of probing the cells through their thickness in a non-destructive and nimble way. The spectroscopic variations of the Raman bands associated to the O-H stretching of water were correlated to the variations of ionic concentration in the intracellular and extracellular fluids. The increase of Na⁺ concentration gradients was clearly visualized in the cytosol of aldosterone-treated cells. The enhancement of the Na⁺ current density induced by aldosterone was estimated from the variation of the ionic chemical potential across the intracellular space. In addition, the variation of the O-H Raman bands of water was used to quantify the cell thickness, which was not affected by aldosterone.

Inferential monitoring of chlorinated solvents through Raman spectroscopic observation of the vibrational modes of water

Recent improvements in diode laser, fiber optic, and data acquisition technology have rejuvenated interest in field applications of Raman spectroscopy in a wide range of settings. One such application involves the observation of chlorinated solvents to facilitate the practice of "monitored natural attenuation." In this context, this manuscript focuses on means to improve the sensitivity of in-situ Raman analysis of chlorinated solvents. In particular, the work explores the performance limits of a Time-Resolved Raman Spectroscopy (TRRS) system employed to observe chlorinated solvents in aqueous samples via laboratory tests conducted on both liquid standards of trichloroethylene (TCE) and simulated biodegraded field samples. Quantitative assessment of TCE in solution is carried out through both direct observation of TCE Raman functional groups (381 cm(-1) (δ skeletal), 840 cm(-1) (νCCl) and 1242 cm(-1) (δCH)) and indirect observation of the broad OH stretching (2700-3800 cm(-1)) Raman modes of water. Results from tests on simple solutions show that the TRRS system can detect TCE at aqueous concentrations as low as 70 ppm by directly monitoring the 381 cm(-1) TCE line, whereas observation of the OH stretching line of water (3393 cm(-1)) provides an indirect indication of TCE presence with nearly a 9× improvement in detection level. This unique and counterintuitive mechanism to detect the presence of chlorinated compounds in solution takes advantage of the influence of chlorine on the vibrational modes of water. This influence, which is believed to be attributed to the formation of hydrogen bonds and their resultant interactions with the solvation shell, may serve as a more sensitive and robust indication of the presence of aggregate chlorinated solvent contamination in aqueous systems. Tests performed on simulated biodegraded field samples demonstrate that the indirect detection mechanism is apparent even in complex samples representative of typical field conditions.

Active biopolymers in green non-conventional media: a sustainable tool for developing clean chemical processes

By understanding structure–function relationships of active biopolymers (e.g. enzymes and nucleic acids) in green non-conventional media, sustainable chemical processes may be developed.

Effect of hygroscopicity of the metal salt on the formation and air stability of lyotropic liquid crystalline mesophases in hydrated salt-surfactant systems

It is known that alkali, transition metal and lanthanide salts can form lyotropic liquid crystalline (LLC) mesophases with non-ionic surfactants (such as CiH2i+1(OCH2CH2)jOH, denoted as CiEj). Here we combine several salt systems and show that the percent deliquescence relative humidity (%DRH) value of a salt is the determining parameter in the formation and stability of the mesophases and that the other parameters are secondary and less significant. Accordingly, salts can be divided into 3 categories: Type I salts (such as LiCl, LiBr, LiI, LiNO3, LiClO4, CaCl2, Ca(NO3)2, MgCl2, and some transition metal nitrates) have low %DRH and form stable salt-surfactant LLC mesophases in the presence of a small amount of water, type II salts (such as some sodium and potassium salts) that are moderately hygroscopic form disordered stable mesophases, and type III salts that have high %DRH values, do not form stable LLC mesophases and leach out salt crystals. To illustrate this effect, a large group of salts from alkali and alkaline earth metals were investigated using XRD, POM, FTIR, and Raman techniques. Among the different salts investigated in this study, the LiX (where X is Cl(-), Br(-), I(-), NO3(-), and ClO4(-)) and CaX2 (X is Cl(-), and NO3(-)) salts were more prone to establish LLC mesophases because of their lower %DRH values. The phase behavior with respect to concentration, stability, and thermal behavior of Li(I) systems were investigated further. It is seen that the phase transitions among different anions in the Li(I) systems follow the Hofmeister series.

Enhanced Surface Partitioning of Nitrate Anion in Aqueous Bromide Solutions

The proximity of nitrate anions to the air-water interface is thought to strongly influence their photodissociation quantum yield, due to a reduced solvent cage effect at the water surface. Although nitrate in aqueous solution exhibits little or no surface affinity, the release of gas phase NO2 (nitrate's primary photodissociation product) has been reported to be enhanced when halides, in particular bromide, are also present in solution. Here, we use glancing-angle Raman spectroscopy to investigate whether solutions containing both nitrate and halides show different propensities for nitrate at the air-water interface. We find that bromide enhances, and chloride has little effect on (or perhaps suppresses) the surface partitioning of nitrate anions.

Water interfaces, solvation, and spectroscopy

Liquid water consistently expands our appreciation of the rich statistical mechanics that can emerge from simple molecular constituents. Here I review several interrelated areas of recent work on aqueous systems that aim to explore and explain this richness by revealing molecular arrangements, their thermodynamic origins, and the timescales on which they change. Vibrational spectroscopy of OH stretching features prominently in these discussions, with an emphasis on efforts to establish connections between spectroscopic signals and statistics of intermolecular structure. For bulk solutions, the results of these efforts largely verify and enrich existing physical pictures of hydrogen-bond network connectivity, dynamics, and response. For water at interfaces, such pictures are still emerging. As an important example I discuss the solvation of small ions at the air-water interface, whose surface propensities challenge a basic understanding of how aqueous fluctuations accommodate solutes in heterogeneous environments.

Thermodynamic properties of aqueous sodium sulfate solutions to 773 K and 3 GPa derived from acoustic velocity measurements in the diamond anvil cell

The thermodynamic properties of a 1 m Na(2)SO(4) solution have been determined to 773 K and 3 GPa from acoustic velocity measurements in externally heated diamond anvil cell using Brillouin spectroscopy. The measured acoustic velocities were inverted to obtain the density of the aqueous electrolyte solution with an accuracy of 0.3%-0.5%, and an equation of state (EoS) valid in the 293-773 K and 0.4-3 GPa range is proposed. The new EoS reproduces the experimental acoustic velocity data with a maximal deviation of 1.5% and allows deriving all thermodynamic properties of the aqueous solution, including isobaric heat capacity (C(P)), thermal expansion (α(P)), and compressibility (β) with an accuracy better than 3%-8%. The addition of dissolved sulfate species decreases the compressibility of water, consistent with the structure-maker character of SO(4)(2-) ions in solution that enhance the hydrogen-bond network of the solvent.

Experimental study of NaCl aqueous solutions by Raman spectroscopy: towards a new optical sensor

Raman spectroscopy was used to study the NaCl aqueous solutions around the solid-liquid phase transition. Special attention was devoted to the modification induced by the salt on the OH stretching band of water. Investigations were carried out in the temperature range between -21 and 10 degrees C, for concentrations from 0 to 200 g/L. We demonstrated that micro-Raman spectroscopy can be used as a marker, allowing the determination of the salt concentration of an aqueous solution with an error close to +/-5%.

Geochemistry of chemical weapon breakdown products on the seafloor: 1,4-thioxane in seawater

The long-term fate of chemical weapon debris disposed of in the ocean some 50 years ago, now sinking into marine sediments and leaking into the ocean environment, is poorly known. Direct evidence exists showing chemical weapon agents actively being released on the sea floor with detrimental effects including harm to marine life. Thus there is strong interest in determining the fate and lifetime of these materials, their decomposition products, and the affected zones around these sites. Here we study the geochemical properties of a mustard gas breakdown product, 1,4-thioxane (TO), using Raman spectroscopy. We show that TO forms a hydrate with a help-gas (a second guest added to stabilize the hydrate), such as methane or hydrogen sulfide, with the hydrate stability regime some 10 degrees C above pure methane hydrate. The temperature, pressure, and reducing conditions required for hydrate formation commonly occur at known disposal sites. The TO solubility was measured in seawater and found to vary from 0.65 to 0.63 mol/kg water between 4.5 and 25.0 degrees C. Similar to other hydrate systems, the TO solubility decreased in the presence of hydrate. A low solubility in water coupled with its ability to form a hydrate within marine sediments can greatly decrease molecular mobility and increase its lifetime. These results demonstrate how unanticipated reactions with marine sediments can occur, and how little is known of the processes controlling the environmental science of these materials.

Structural and Dynamic Properties of Concentrated Alkali Halide Solutions: A Molecular Dynamics Simulation Study

The physicochemical properties of alkali halide solutions have long been attributed to the collective interactions between ions and water molecules in the solution, yet the structure of water in these systems and its effect on the equilibrium and dynamic properties of these systems are not clearly understood. Here, we present a systematic view of water structure in concentrated alkali halide solutions using molecular dynamics simulations. The results of the simulations show that the size of univalent ions in the solution has a significant effect on the dynamics of ions and other transport properties such as the viscosity that are correlated with the structural properties of water in aqueous ionic solution. Small cations (e.g., Li+) form electrostatically stabilized hydrophilic hydration shells that are different from the hydration shells of large ions (e.g., Cs+) which behave more like neutral hydrophobic particles, encapsulated by hydrogen-bonded hydration cages. The properties of solutions with different types of ion solvation change in different ways as the ion concentration increases. Examples of this are the diffusion coefficients of the ions and the viscosities of solutions. In this paper we use molecular dynamics (MD) simulations to study the changes in the equilibrium and transport properties of LiCl, RbCl, and CsI solutions at concentrations from 0.22 to 3.97 M.

Spectroscopy of Growing and Evaporating Water Droplets: Exploring the Variation in Equilibrium Droplet Size with Relative Humidity

We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2-6 microm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from the droplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstrate that it is possible to measure the equilibrium size and composition of an aqueous droplet with variation in relative humidity. For this benchmark study we investigate the variation in equilibrium size with relative humidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueous aerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be in excellent agreement with the predictions of Köhler theory. We suggest that this approach could provide an important new strategy for characterizing the thermodynamic properties and kinetics of transformation of aerosol particles.