Hydrophobic Hydration Research Articles

Apparent molar volume at infinite dilution, expansivities and electrochemical behaviours of binary system (tetramethylpiperidine 1-oxyl-based ionic liquids + water)

ABSTRACT At T = (278.15–318.15) K and atmospheric pressure, Hepler’s constant values, (∂2 V B/∂ T 2) P , and apparent molar expansivities (E φ) were calculated using the temperature dependence of apparent molar volumes (V B). Because of the increase in hydrophobic hydration of tetramethylpiperidine 1-oxyl-based ionic liquids (TILs), the Hepler’s constant values were found to change from positive to negative, indicating that the TILs under consideration changed primarily from structure-promoting to structure-breaking. The critical micelle concentration of conductivity (CMC) was calculated based on experimental conductivity, which reflects the dissociation and the interaction and aggregation behaviour of TILs.

The Effect of the Tetraalkylammonium Cation in the Electrochemical CO2 Reduction Reaction on Copper Electrode.

Aprotic organic solvents such as acetonitrile offer a potential solution to promote electrochemical CO2 reduction over the competing hydrogen evolution reaction. Tetraalkylammonium cations (TAA+) are widely used as supporting electrolytes in organic media due to their high solubility and conductivity. The alkyl chain length of TAA+ cations is known to influence electron transfer processes in electrochemical systems by the adsorption of TAA+, causing modifications of the double layer. In this work, we elucidate the influence of the cation chain length on the mechanism and selectivity of the CO2RR reaction under controlled dry and wet acetonitrile conditions on copper cathodes. We find that the hydrophobic hydration character of the cation, which can be tuned by the chain length, has an effect on product distribution, altering the reaction pathway. Under dry conditions, smaller cations (TEA+) preferentially promote oxalate production via dimerization of the CO2 ·- intermediate, whereas formate is favored in the presence of water via protonation reaction. Larger cations (TBA+ > TPA+ > TEA+) favor the generation of CO regardless of water content. In situ FTIR analysis showed that TBA+ cations are able to stabilize adsorbed CO more effectively than TEA+, explaining why larger cations generate a higher proportion of CO. Our findings also suggest that higher cation concentrations suppress hydrogen evolution, particularly with larger cations, highlighting the role of cation chain length size and hydrophobic hydration shell.

Structural and dynamic behaviour of concentrated aqueous solutions of (poly)ethylene glycols: Insight into the impact of hydrophobicity, hydrogen bonding and chain length

Understanding the delicate interplay between hydrophobicity/hydrophilicity, hydrogen bonding ability and solute size in modulating the chemico-physical properties of aqueous solutions with increasing solute concentration is a challenging task. Here, we focus on how bulk properties, such as diffusion and aggregation behaviour, are affected by the different hydration patterns observed for differently concentrated aqueous solutions of (poly)ethylene glycols, and their corresponding methoxyethers, of various lengths. Using a combined experimental and computational approach, we analyse solutions of ethylene glycol (EG), dimethoxyethane (DME), polyethylene glycol with 4 repeating units capped with either hydroxyl (PEG200) or methoxyl (PEG250DME) tails, and polyethylene glycol with 9 repeating units (PEG400). By disentangling the contributions of hydrophobicity, hydrogen bonding ability, and chain length, we observe that for the smaller solutes electristriction and hydrophobic hydration are the main phenomena occurring. Instead, as the solute becomes larger, these effects give way to the non-trivial conformational flexibility that can either favour or disfavour intermolecular interactions which lead to the formation of water-rich regions excluded from the solute.

Preferential Solvation of Cyclosporin in Aqueous Mixtures of Some Polymeric Cosolvents

Background: Cyclosporin is a cyclic peptide-drug used as immunosuppressant for the prophylaxisof transplant rejection whose physicochemical properties in mixed aqueous solvent systems isstill not well understood. The preferential solvation parameters of cyclosporin in aqueous binarymixtures of diethylene glycol monoethyl ether (DEGME), polyethylene glycol 200 (PEG 200) andpolyethylene glycol 400 (PEG 400) were computed. Methods: Reported mole fraction solubilities of cyclosporin in DEGME-aqueous mixtures,PEG 200-aqueous mixtures, and PEG 400-aqueous mixtures were processed by following theinverse Kirkwood-Buff integrals (IKBI) method as suggested by Marcus and Ben-Naim using somethermodynamic parameters reported in the literature for these aqueous-polymeric mixtures at298.15 K. Results: It is observed that cyclosporin is sensitive to preferential solvation effects in theseaqueous-polymeric binary solvent systems. The preferential solvation parameter by DEGME (δx1,3)is negative in water-rich mixtures but positive in mixtures of 0.12<x1<1.00. It is conjecturablethat hydrophobic hydration around the non-polar methyl and methylene groups of this drug thatcould be present in water-rich mixtures can significantly impact the drug solvation. Otherwise,in mixtures of 0.12<x1<1.00 in DEGME-aqueous mixtures, as well as in almost all the mixtureswith PEGs, the preferential solvation by polymeric cosolvents could be due to the acidic behaviorof cyclosporin in front of ether and hydroxyl oxygen atoms of these polymeric cosolvents. Conclusion: Cyclosporin is preferentially solvated by the polymeric solvents in almost all thestudied mixtures of these aqueous-polymeric binary solvent systems.

Study of the perturbed hydrogen-bonded water structure of the hydration shell of N-Methyl-Acetamide by Raman multivariate curve resolution spectroscopy

In this work, Raman scattering measurement combined with multivariate curve resolution (Raman-MCR) technique has been used to obtain the solute correlated spectra (SC) of N-methyl-acetamide (NMA) in an aqueous medium. The SC spectra is composed of NMA intramolecular vibrations and NMA induced-perturbations of water OH stretch vibrations. The SC spectra obtained for different mole-fractions of NMA reveal that as the mole-fraction of NMA increases the perturbed water interacting with the amide group is retained whereas it gets depleted from the hydrophobic hydration shell of the methyl group of NMA. The high wavenumber region of the OH spectrum shows that the fraction of the weakly hydrogen-bonded population is higher in the bulk water compared to perturbed water. The Raman spectra of perturbed OH spectral features due to hydration of the hydrophobic methyl group of NMA have been extracted from SC spectra obtained at low mole-fraction of NMA by the MCR technique. Its spectral feature resembles the SC-OH spectra of hydrophobic hydration of the methyl group of methanol.

Phase transition in polymer hydrogels investigated by swelling, DSC, FTIR and NMR

AbstractThe swelling and thermal behavior of single (SN) and double network (DN) hydrogels containing polyacrylamide (PAAm) and temperature sensitive poly(N, N-diethylacrylamide) (PNNDEAAm) were investigated by gravimetry and differential scanning calorimetry (DSC). Changes of the hydration of carbonyl groups and mobility of polymer network chains in the hydrogels with temperature were studied by Fourier-transformed infrared and proton nuclear magnetic resonance (1H NMR) spectroscopy. The enthalpy of dissociation of the hydrophobic interaction, minimum and maximum numbers of water molecules per monomer unit in PNNDEAAm SN hydrogel involved in the hydrophobic hydration were determined by DSC. The volume phase transition accompanied by expulsion/uptake of water and heat absorption/release in heating/cooling was exhibited by all DN hydrogels containing PNNDEAAm. In these hydrogels, above the phase transition temperature, the population of hydrated carbonyls is enriched with the free and single hydrated ones in the same way as in the aqueous solution of linear PNNDEAAm and mobility of the PNNDEAAm chains is strongly reduced. Presence of the PAAm network does not influence the phase transition temperature but strongly reduces the enthalpy of the phase transition, promotes higher degrees of hydration of carbonyls, and increases mobility of the PNNDEAAm chains in the mixed hydrogels above the phase transition temperature.

Interactions of adsorbing cosolutes with hydrophobic hydration shells.

The analysis of water density fluctuations in the hydration shell of nonpolar solutes provides insights into water-mediated interactions, especially hydrophobic interactions. These fluctuations are sensitive to small perturbations due to changes in thermodynamic conditions, such as temperature and pressure, but also to the presence of cosolutes, such as salts or small organic molecules. Herein, we investigate the effect of two classes of adsorbing cosolutes, using urea and methanol as representatives, on the fluctuations in energy and solvent density within the solvation shell of a model extended hydrophobic solute. We focus on the interactions of the cosolutes with the hydrophobic hydration shell, rather than with the solute itself, which though important remain largely unexplored. We calculate and analyze the interfacial partial molar energy of the cosolute, using a methodology based on the small system method. This approach provides correlated solvent density and energy fluctuations and allows us to decompose them into contributions due to interactions between the different components present in the solvation shell of the solute. The results show that adsorbed urea molecules interact more favorably with water than nonadsorbed urea molecules, which leads to the attenuation of interfacial density fluctuations and thus to the stabilization of the solvation shell. By contrast, the adsorbed methanol molecules interact preferably with other methanol molecules in the solvation shell, leading to a nano-phase segregated structure, which enhances interfacial fluctuations.

Outstanding Properties of the Hydration Shell around β-d-Glucose: A Computational Study.

Ab initio molecular dynamics (AIMD) simulations have been performed on aqueous solutions of four simple sugars, α-d-glucose, β-d-glucose, α-d-mannose, and α-d-galactose. Hydrogen-bonding (HB) properties, such as the number of donor- and acceptor-type HB-s, and the lengths and strengths of hydrogen bonds between sugar and water molecules, have been determined. Related electronic properties, such as the dipole moments of water molecules and partial charges of the sugar O atoms, have also been calculated. The hydrophilic and hydrophobic shells were characterized by means of spatial distribution functions. β-d-Glucose was found to form the highest number of hydrophilic and the smallest number of hydrophobic connections to neighboring water molecules. The average sugar-water H-bond length was the shortest for β-d-glucose, which suggests that these are the strongest such H-bonds. Furthermore, β-d-glucose appears to stand out in terms of the symmetry properties of both its hydrophilic and hydrophobic hydration shells. In summary, in all aspects considered here, there seems to be a correlation between the distinct characteristics of β-d-glucose reported here and its outstanding solubility in water. Admittedly, our findings represent only some of the important factors that influence the solubility.

Solubility of atenolol in aqueous propylene glycol mixtures revisited: IKBI preferential solvation analysis

ABSTRACT The preferential solvation parameters of atenolol in aqueous binary mixtures of propylene glycol (PG) were derived from reported molar solubility values after conversion to mole fractions by using the inverse Kirkwood–Buff integrals method. This drug is sensitive to preferential solvation effects in this binary system. The preferential solvation parameter by PG (δx 1,3) is negative in water-rich mixtures but positive in mixtures of 0.20 < x 1 < 1.00. It is conjecturable that hydrophobic hydration around the non-polar methyl and phenylene groups of this drug observable in water-rich mixtures could play a relevant role in drug solvation. Otherwise, in mixtures of 0.20 < x 1 < 1.00, the preferential solvation by PG could be due to the acidic behaviour of atenolol.

Probing molecular interactions of an anti-epileptic drug, sodium valproate in aqueous MgCl2 solutions

Study of the physicochemical properties of anti-epileptic drugs in the presence of cosolutes in aqueous solutions has been a subject of interest of researchers because of their interaction with biological membranes to employ their effect. Such properties provide complete information regarding solute-solute and solute-solvent type interactions. As electrolytes are essential components of biological systems, therefore the study of drug-electrolyte chemistry is important to understand the mechanism of drug action at the molecular level and hence useful in drug design and development processes. Therefore, in the current report, the density and sound velocity of an anti-epileptic drug sodium valproate (SV) in aqueous and in aqueous solutions of an electrolyte viz. MgCl2 (mB = 0.024, 0.047, 0.070 mol kg−1) at different temperatures (288.15, 298.15, 308.15, and 318.15 K) and at a pressure of 0.101 MPa were measured along with 1H NMR spectroscopy. Various volumetric and acoustical parameters such as apparent molar volume (Vφ) and isentropic compressibility (Kφ) of the solute, apparent molar volume (Vφ°) and isentropic compressibility (Kφ°) of solute at infinite dilution, apparent molar expansibility (Eφ°) and their double derivatives (∂2Vφ°/∂T2)P, transfer volumes (∆trVφ°), hydration number (nH), acoustic impedance (Z), intermolecular free length (Lf), Wada's constant (W) and Rao's constant (R) were calculated from density and sound velocity data. The variation in all these parameters with cosolute concentration indicates the existence of hydrophilic/ionic-ionic interactions at lower concentrations but hydrophobic-ionic interactions at higher concentrations of MgCl2. The 1H NMR spectroscopic results also support the existence of hydrophobic-ionic interactions at higher concentrations of MgCl2. The decrease in electrostriction but increase in hydrophobic hydration of solute molecules has also been seen with rise in temperature.

Structure of polyethylene glycol/water mixtures as a separation medium of aqueous two-phase extraction systems studied by equilibrium analysis of thiocyanato complexation of Cobalt(II)

Aqueous two-phase systems (ATPS) utilizing polyethylene glycol (PEG) have been successfully used for the separation of various organic and inorganic compounds as an environmentally benign extraction method. However, a fundamental understanding of the role of the hydration structures around PEG chains in solute partitioning in ATPS remains elusive although it has been considered that PEG molecules are involved in separation. In the present work, the equilibria of Co(II) thiocyanate complexation reaction in aqueous PEG solutions were studied by spectrophotometry with varying ethylene oxide chain length and concentration of PEG. From the formation constants of the tetrahedral and octahedral Co(II) thiocyanate complexes obtained, a microheterogeneous structure consisting of pseudo phases formed by hydration of ethylene oxide (EO) chain and terminal hydroxyl (OH) groups in the PEG molecule in addition to the bulk water phase was estimated. It was revealed that the formation of tetrahedral Co(NCS)42− complex proceeds in the EO pseudo phase by the involvement of hydrophobic ethylene groups, while the OH phase and the bulk phase inhibit the progress of the reaction. This indicates that Co(NCS)42− is stable in hydrophobic hydration structure of water around the EO moiety. The EO phase imparts hydrophobicity to aqueous PEG solutions and plays a predominant role in partition of solute molecules in PEG-based ATPS.

Hydrophobic hydration of the hydrocarbon adamantane in amorphous ice.

Hydrophobic molecules are by definition difficult to hydrate. Previous studies in the area of hydrophobic hydration have therefore often relied on using amphiphilic molecules where the hydrophilic part of a molecule enabled the solubility in liquid water. Here, we show that the hydrophobic adamantane (C10H16) molecule can be fully hydrated through vapour codeposition with water onto a cryogenic substrate at 80 K resulting in the matrix isolation of adamantane in amorphous ice. Using neutron diffraction in combination with the isotopic substitution method and the empirical potential structure refinement technique, we find that the first hydration shell of adamantane is well structured consisting of a hydrogen-bonded cage of 28 water molecules that is also found in cubic structure II clathrate hydrates. The four hexagonal faces of the 51264 cage are situated above the four methine (CH) groups of adamantane whereas the methylene (CH2) groups are positioned below the edges of two adjoining pentagonal faces. The oxygen atoms of the 28 water molecules can be categorised on the basis of symmetry equivalences as twelve A, twelve B and four C oxygens. The water molecules of the first hydration shell display orientations consistent with those expected for a clathrate-hydrate-type cage, but also unfavourable ones with respect to the hydrogen bonding between the water molecules. Annealing the samples at 140 K, which is just below the crystallisation temperature of the matrix, removes the unfavourable orientations and leads to a slight increase in the structural order of the first hydration shell. The very closest water molecules display a tendency for their dipole moments to point towards the adamantane which is attributed to steric effects. Other than this, no significant polarisation effects are observed which is consistent with weak interactions between adamantane and the amorphous ice matrix. FT-IR spectroscopy shows that the incorporation of adamantane into amorphous ice leads to a weakening of the hydrogen bonds. In summary, the matrix-isolation of the highly symmetric adamantane in amorphous ice provides an interesting test case for hydrophobic hydration. Studying the structure and spectroscopic properties of water at the interface with hydrophobic hydrocarbons is also relevant for astrophysical environments, such as comets or the interstellar medium, where amorphous ice and hydrocarbons have been shown to coexist in large quantities.

Deciphering the mechanism of γ-cyclodextrin's hydrophobic cavity hydration: an integrated experimental and theoretical study.

Cyclodextrins (CDs) are host systems with inherent capability for inclusion complex formation with various molecular entities, mostly hydrophobic substances. Host CDs are highly accommodative to water molecules as well and usually contain water in the native state. There is still an ongoing discussion on both the total number of water molecules and their preferred binding position inside the cavities of the CDs. To understand the hydration/dehydration properties of γ-CD (the largest of the three most abundant native CDs), the main experimental methods applied in this study were differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). By coupling these techniques with density functional theory (DFT) calculations we try to shed some light on the mechanism of the γ-CD hydration and to address some unanswered questions: (i) what are the preferable locations for water molecules in the macrocyclic cavity ("hot spots"); (ii) what are the major factors contributing to the stability of the water cluster in the CD interior; (iii) what type of interactions (i.e., water-water and/or water-CD walls) contribute to the stability of the water assemble; (iv) how does the mechanism of the γ-CD hydration compare with those of its α-CD and β-CD counterparts.

Mechanism of poly(N-isopropylacrylamide) cononsolvency in aqueous methanol solutions explored via oxygen K-edge X-ray absorption spectroscopy.

The cononsolvency mechanism of poly(N-isopropylacrylamide) (PNIPAM), dissolving in pure methanol (MeOH) and water (H2O) but being insoluble in MeOH-H2O mixtures, was investigated by O K-edge X-ray absorption spectroscopy (XAS). The cononsolvency emerges from the aggregation of PNIPAM with MeOH clusters, leading to the collapse of the hydrophobic hydration of PNIPAM.

Applicability of a central force water model to study adsorption in disordered hydrophobic matrices—replica Ornstein–Zernike theory

A simple central force water model to study adsorption in random Lennard-Jones-like matrices has been studied using the replica Ornstein–Zernike integral equation theory in hypernetted-chain (HNC) approximation and in HNC+bridge approximation. The structure of water in obstacle matrices of different sizes was studied, showing that the model appropriately accounts for the hydrophobic hydration. By calculating the chemical potential of water in the model adsorbent, we have constructed the adsorption isotherm. Except for the cases of highly dispersed matrices, water gets excluded from the crowded hydrophobic environment, as expected experimentally.

Formation of soy protein nanoparticles with varied loading capacity by controlled enzymatic hydrolysis and their co-assembly with Vitamin D3

In the present study, rational designed soy protein nanoparticles (SNPs) with different subunit compositions and loading capacities were fabricated via Pancreatin-induced hydrolysis taken Vitamin D3 (VD3) as a hydrophobic cargo. Further assembly behavior of SNPs caused by VD3 incorporation was also noticed and analyzed. Results showed that hydrolysis to 2.98% degree of hydrolysis (DH) (Pan1) facilitated the formation of a nanoparticle structure with hydrophobic cores originated from moderate degradation of 7 S and A subunits of 11 S. This core-shell structure had a higher loading capacity towards the VD3 and was further strengthened upon VD3 addition. Further hydrolysis to 10.96% DH (Pan2) caused further degradation of subunits and the extension of protein structure, accelerating aggregation through protein-protein interactions. The coexistence of aggregates as well as degradation fragments showed different binding patterns with VD3 and the resultant mixture was in non-uniformed size distribution. Further hydrolysis to 18.04% DH (Pan3) caused severe precipitation due to hydrophobic aggregation and the hydrophilic peptides retained had no apparent hydrophobic domain for the entrapment of VD3. Interestingly, VD3 would tend to form a hydrophobic core in situ through hydrophobic hydration with Pan3, resulting in a more ordered structure. These findings suggest that enzymatic hydrolysis could be a useful tool for the rational design of nanostructure for bioactive delivery where the co-assembly behavior calls for attention.

Pentafluorophenyl‐Substituted Proline‐Derived Chiral Urea for “On Water” Asymmetric Hydrogen‐Bonding Catalysis

AbstractThis work demonstrated the efficacy of a proline‐derived chiral urea for “on water” asymmetric hydrogen‐bonding catalysis by incorporating a hydrophobic and electron‐withdrawing pentafluoro‐phenyl substituent into the urea moiety. When employed as a catalyst, pentafluorophenyl‐substituted proline‐derived chiral urea exhibited significant enhancement of both the reaction rate and stereoselectivity when conducted “on water” compared to that of conventional organic solvents due to the hydrophobic hydration effect. Moreover, this catalyst did not display notable self‐aggregation during the “on water” hydrogen‐bonding catalysis and was confirmed to exist as a monomeric species with catalytic activity. Due to its remarkable acceleration in “on water” conditions, the catalyst load was significantly reduced. Consequently, complete conversion was achieved at room temperature within 2 hours while maintaining good enantioselectivities (up to 96% ee) even at 0.2 mol% catalyst loading in the Michael addition of dithiomalonate to nitrostyrene.

Formation of the strength of fine-grained concrete based on modified slag Portland cement

The object of research is fine-grained concrete on slag portland cement. Owing to the significant content of metallurgical industry waste in its composition, slag portland cement belongs to ecologically acceptable products. However, the insufficient rate of formation of its structure and, as a result, the main quality indicator, which is compressive strength, limits the field of use of slag portland cement. Therefore, in the studies, the goal was to increase the speed of formation of the strength of fine-grained concrete made on slag portland cement. The studies of the influence of modification of slag portland cement with water, activated by the use of the hydrophobic hydration mechanism, established the factors affecting the speed of formation and the value of compressive strength of fine-grained concrete made on slag portland cement. It has been proven that these factors include the type and amount of applied water nanomodifiers, as well as the type and amount of fine concrete aggregate. The analysis of the study results confirmed that the introduction of water activated by the mechanism of hydrophobic hydration into concrete in ultra-small doses significantly increases the rate of formation of concrete strength. Given this, the strength of the resulting modified fine-grained concrete based on slag portland cement at the age of 2 days exceeds the strength of the similar concrete without additives by 60 %, and at the age of 210 days – by 25 %. This allows us to assert the effectiveness of the identified mechanism of modification of slag portland cement. Thus, there are reasons to assert the possibility of targeted regulation of the processes of forming a strong structure of fine-grained concrete based on slag portland cement by using water activated by the mechanism of hydrophobic hydration

From Local Hydration Motifs in Aqueous Acetic Acid Solutions to Macroscopic Mixing Thermodynamics: A Quantitative Connection from THz-Calorimetry.

We report the results of THz measurements (30-440 cm-1) of aqueous acetic acid solutions over the full mixing range (XAcAc = 0-1). We recorded spectroscopic observables as a function of the acetic acid concentration in the frequency range of the intermolecular stretch at 150 cm-1 and of the librational modes at 350-440 cm-1. This allowed us to unravel changes in hydrophobic and hydrophilic hydration motifs, respectively. By means of a novel THz-calorimetry approach, we quantitatively correlated these changes in local hydration motifs to excess mixing entropy and enthalpy. We find that ΔHmix is determined by both hydrophobic and hydrophilic solvation contributions. In contrast, ΔSmix is governed by hydrophobic cavity formation. Our results further suggest that acetic acid-water mixtures are systems at the edge of phase separation due to endothermic contributions from both hydrophilic and hydrophobic solvation in a large portion of the miscibility range. Our work establishes a quantitative relationship between the balance of local hydrophobic and hydrophilic solvation motifs and the macroscopic mixing thermodynamic properties.

“On‐Water” Synthesis of Sterically Congested γ‐Ketophosphine Oxides Featuring a CF3‐ and P(O)‐Tetrasubstituted Carbon Center

AbstractThe distinctive hydrophobic hydration effect in the aqueous solution contributes to the success of the phospha‐Michael reaction between β‐CF3‐β,β‐disubstituted enones and diarylphosphine oxides, providing an access to biologically relevant γ‐ketophosphine oxides bearing a sterically highly congested CF3‐ and P(O)‐tetrasubstituted carbon center. More importantly, the resulting products could be further transformed into densely functionalized γ‐ketophosphine oxides containing a CF3‐ and P(O)‐tetrasubstituted carbon center.