Water structure in glycerol: Spectroscopic and computer simulation investigation of hydrogen bonding and water clustering

Modeling of the Hydrogen Bond-Induced Frequency Shifts of the HOH and HOD Bending Modes of Water

The intramolecular bending mode of water is a possible useful probe of the hydrogen-bond situations in aqueous systems, but the behavior of its frequency and intensity should be further elucidated for better understanding on its nature and, hence, for its better utilization as a probe. Here, an analysis toward this goal is conducted by doing theoretical calculations on molecular clusters of normal isotopic and deuterated species of water and examining the correlations among the vibrational, structural, and electrostatic properties. It is shown that electrostatic interactions, particularly both of the in-plane components of the electric field along the OH bond and perpendicular to it, play a major role in controlling the hydrogen bond-induced shifts of the force constant, but additional factors, including the intermolecular structural and/or charge-transfer properties, are also important. Models of the hydrogen bond-induced shifts of the force constant are presented in a form that may be combined with classical molecular dynamics. With regard to the infrared intensity changes, it is shown on the basis of the electron density analysis that the intermolecular charge flux and polarization effect play an important role, depending on the angular characteristics of the hydrogen bond.

Structural Features of the Glassy State and Their Impact on the Solid-State Properties of Organic Molecules in Pharmaceutical Systems

This paper reviews the structure and properties of amorphous active pharmaceutical ingredients (APIs), including small molecules and proteins, in the glassy state (below the glass transition temperature, Tg). Amorphous materials in the neat state and formulated with excipients as miscible amorphous mixtures are included, and the role of absorbed water in affecting glass structure and stability has also been considered. We defined the term "structure" to indicate the way the various molecules in a glass interact with each other and form distinctive molecular arrangements as regions or domains of varying number of molecules, molecular packing, and density. Evidence is presented to suggest that such systems generally exist as heterogeneous structures made up of high-density domains surrounded by a lower density arrangement of molecules, termed the microstructure. It has been shown that the method of preparation and the time frame for handling and storage can give rise to variable glass structures and varying physical properties. Throughout this paper, examples are given of theoretical, computer simulation, and experimental studies which focus on the nature of intermolecular interactions, the size of heterogeneous higher density domains, and the impact of such systems on the relative physical and chemical stability of pharmaceutical systems.

Practical Advice on Scientific Design of Freeze-Drying Process: 2023 Update

OBJECTIVE

The purpose of this paper is to re-visit the design of three steps in the freeze-drying process, namely freezing, primary drying, and secondary drying steps. Specifically, up-to-date recommendations for selecting freeze-drying conditions are provided based on the physical-chemical properties of formulations and engineering considerations.

METHODS AND RESULTS

This paper discusses the fundamental factors to consider when selecting freezing, primary drying, and secondary drying conditions, and offers mathematical models for predicting the duration of each segment and product temperature during primary drying. Three simple heat/mass transfer primary drying (PD) models were tested, and their ability to predict product temperature and sublimation time showed good agreement. The PD models were validated based on the experimental data and utilized to tabulate the primary drying conditions for common pharmaceutical formulations, including amorphous and partially crystalline products. Examples of calculated drying cycles, including all steps, for typical amorphous and crystalline formulations are provided.

CONCLUSIONS

The authors revisited advice from a seminal paper by Tang and Pikal (Pharm Res. 21(2):191-200, 2004) on selecting freeze-drying process conditions and found that the majority of recommendations are still applicable today. There have been a number of advancements, including methods to promote ice nucleation and computer modeling for all steps of freeze-drying process. The authors created a database for primary drying and provided examples of complete freeze-drying cycles design. The paper may supplement the knowledge of scientists and formulators and serve as a user-friendly tool for quickly estimating the design space.

Enhanced Dissolution of Metal Oxides in Hydroxylated Solvents - Towards Application in Lithium-Ion Battery Leaching

The recovery of critical metals from spent lithium-ion batteries (LIBs) is rapidly growing. Current methods are energy-intensive and hazardous, while alternative solvent-based strategies require more studies on their 'green' character, metal dissolution mechanism and industrial applicability. Herein, we bridged this gap by studying the effect of dilute HCl solutions in hydroxylated solvents to dissolve Co, Ni and Mn oxides. Ethylene glycol emerged consistently as the most effective solvent, dissolving up to four times more Co and Ni oxides than using aqueous acidic media, attributed to improved chloro-complex formation and solvent effects. These effects had a significant contribution compared to acid type and concentration. The highest Co dissolution (0.27 M) was achieved in 0.5 M HCl in 25 % (v/v) glycerol in water, using less acid and a significant amount of water compared to other solvent systems, as well as mild temperatures (40 °C). This solvent was applied to dissolve battery cathode material, achieving 100 % dissolution of Co and Mn and 94 % dissolution of Ni, following what was concluded to be a mixed mechanism. These results offer a simple alternative to current leaching processes, reducing acid consumption, enhancing atomic efficiency, and paving the way for optimized industrial hydrometallurgical processes leaning to 'greener' strategies.

Adsorption of a water-soluble molecular rotor fluorescent probe on hydrophobic surfaces

Environmentally sensitive molecular rotors are widely used to probe the local molecular environment in e.g. polymer solutions, polymer glasses, and biological systems. These applications make it important to understand its fluorescence properties in the vicinity of a solid surface, since fluorescence microscopy generically employs cover slides, and measurements are often done in its immediate vicinity. Here, we use a confocal microscope to investigate the fluorescence of (4-DASPI) in glycerol/water solutions close to the interface using hydrophilic or hydrophobic cover slips. Despite the dye's high solubility in water, the observed lengthening of the fluorescence lifetime close to the hydrophobic surface, implies a surprising affinity of the dye with the surface. Because the homogeneous solution and the refractive index mismatch reduces the optical sectioning power of the microscope, we quantify the affinity with the help of a simple model of the signal vs. depth of focus, exhibiting surface and bulk contributions. The model reduces artefacts due to refractive index mismatch, as supported by Monte Carlo ray tracing simulations.

Hydrogen-Bond Configurations of Hydration Water around Glycerol Investigated by HOH Bending and OH Stretching Analysis

Toward a comprehensive understanding of the mechanism of glycerol as a moisturizer, studies on the hydrogen-bond (HB) structure of hydration water, which is known to be disordered by glycerol, are insufficient. To this aim, we evaluated the HB configurations based on the HOH bending and OH stretching spectra of the hydration water from those of glycerol/water mixtures by subtracting the contributions of bulk water and glycerol using dielectric relaxation spectroscopy. Analysis of the HOH bending band showed that hydration water-donating HBs lose the intermolecular bending coupling with increasing glycerol by replacing the water-water HBs with water-glycerol HBs. The OH stretching band provided more detailed insight into the HB configuration, indicating that the double-donor double-acceptor and double-donor single-acceptor configurations in bulk water change to a predominantly double-donor single-acceptor configuration in hydration water around glycerol. The formation of more donor HBs than acceptor HBs may be due to the steric constrains by glycerol and/or differences in the partial charge on the oxygen atom between water and glycerol.

Decontamination of water co-polluted by copper, toluene and tetrahydrofuran using lauric acid

Co-contamination by organic solvents (e.g., toluene and tetrahydrofuran) and metal ions (e.g., Cu2+) is common in industrial wastewater and in industrial sites. This manuscript describes the separation of THF from water in the absence of copper ions, as well as the treatment of water co-polluted with either THF and copper, or toluene and copper. Tetrahydrofuran (THF) and water are freely miscible in the absence of lauric acid. Lauric acid separates the two solvents, as demonstrated by proton nuclear magnetic resonance (1H NMR) and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The purity of the water phase separated from 3:7 (v/v) THF:water mixtures using 1 M lauric acid is ≈87%v/v. Synchrotron small angle X-Ray scattering (SAXS) indicates that lauric acid forms reverse micelles in THF, which swell in the presence of water (to host water in their interior) and ultimately lead to two free phases: 1) THF-rich and 2) water-rich. Deprotonated lauric acid (laurate ions) also induces the migration of Cu2+ ions in either THF (following separation from water) or in toluene (immiscible in water), enabling their removal from water. Laurate ions and copper ions likely interact through physical interactions (e.g., electrostatic interactions) rather than chemical bonds, as shown by ATR-FTIR. Inductively coupled plasma-optical emission spectrometry (ICP-OES) demonstrates up to 60% removal of Cu2+ ions from water co-polluted by CuSO4 or CuCl2 and toluene. While lauric acid emulsifies water and toluene in the absence of copper ions, copper salts destabilize emulsions. This is beneficial, to avoid that copper ions are re-entrained in the water phase alongside with toluene, following their migration in the toluene phase. The effect of copper ions on emulsion stability is explained based on the decreased interfacial activity and compressional rigidity of interfacial films, probed using a Langmuir trough. In wastewater treatment, lauric acid (a powder) can be mixed directly in the polluted water. In the context of groundwater remediation, lauric acid can be solubilized in canola oil to enable its injection to treat aquifers co-polluted by organic solvents and Cu2+. In this application, injectable filters obtained by injecting cationic hydroxyethylcellulose (HEC +) would impede the flow of toluene and copper ions partitioned in it, protecting downstream receptors. Co-contaminants can be subsequently extracted upstream of the filters (using pumping wells), to enable their simultaneous removal from aquifers.