Hydrophobic Media Research Articles

Exploring the parameter space of the Green–Ampt model

This study systematically investigates the entire parameter space of the Green–Ampt model for water infiltration and allows for arbitrary constant hydraulic head boundary conditions, any column inclination, hydrophilic and hydrophobic porous media, and a nonzero initial cumulative infiltration. There are five infiltration and five liquid-withdrawal scenarios which differ with respect to the direction of flow and the sign of the acceleration of the liquid. We present analytical solutions for these flow scenarios and 2D phase diagrams that show which flow scenario is predicted depending on the model parameters. All liquid infiltration scenarios, except accelerating downward infiltration (ADI), have already been reported by Green and Ampt. For liquid withdrawal, however, only decelerating downward flow has previously been reported, which is representative of drainage to the water table. We present experimental evidence that ADI may occur in hydrophobic media.

Versatility of non-native forms of human cytochrome c: pH and micellar concentration dependence

In addition to its electron transfer activity, cytochrome c is now known to trigger apoptosis via peroxidase activity. This new function is related to a structural modification of the cytochrome upon association with anionic lipids, particularly cardiolipin present in the mitochondrial membrane. However, the exact nature of the non-native state induced by this interaction remains an active subject of debate. In this work, using human cytochromes c (native and two single-histidine mutants and the corresponding double mutant) and micelles as a hydrophobic medium, we succeeded, through UV-visible spectroscopy, circular dichroism spectroscopy and NMR spectroscopy, in fully characterizing the nature of the sixth ligand replacing the native methionine. Furthermore, careful pH titrations permitted the identification of the amino acids involved in the iron binding over a range of pH values. Replacement of the methionine by lysine was only observed at pH above 8.5, whereas histidine binding is dependent on both pH and micelle concentration. The pH variation range for histidine protonation is relatively narrow and is consistent with the mitochondrial intermembrane pH changes occurring during apoptosis. These results allow us to rule out lysine as the sixth ligand at pH values close to neutrality and reinforce the role of histidines (preferentially His33 vs. His26) as the main candidate to replace methionine in the non-native cytochrome c. Finally, on the basis of these results and molecular dynamics simulations, we propose a 3D model for non-native cytochrome c in a micellar environment.

Encapsulation of Water-soluble Vitamins by Gold Nanoparticles in Hydrophobic Media

Abstract Thiolate-mPEG ligand-protected gold nanoparticles (AuNPs) are shown to encapsulate the water-soluble vitamins C (ascorbic acid), B3 (nicotinic acid), B6 (pyridoxine), and B9 (folic acid) in a hydrophobic medium, i.e., between 1000 and 3000 vitamin molecules per 6-nm-core AuNP (overall size: 19.2 ± 2 nm) in dichloromethane solution.

Proton transfer reactions in carbon nanotubes endohedrally functionalized with selected polar amino acid sidechains

We use the empirical-valence-bond (EVB) theory to investigate intramolecular proton transfer reactions between a selected set of polar amino acid sidechains and hydroxyl groups suspended inside carbon nanotubes to model the effect of hydrophobic confinement on the energetics of proton transfer involving (i) translocation of an excess protonic charge (with protonated histidine sidechain as donor) and (ii) transformation of a neutral reactant state to a charge-separated product state (with sidechains of Asp, Glu, Ser and Thr as donor). In both the cases, confinement in hydrophobic medium is found to change the associated free energies compared to their respective values in the bulk solution phase. Presence of stable hydrogen bonding within the pore is found to have a significant effect on both free energies of reaction and activation and thus governs the thermodynamic and kinetic feasibilities of these intramolecular reactions in hydrophobic confinement.

Continuous synthesis of surface-modified nanoparticles in supercritical methanol: A facile approach to control dispersibility

Surface-modified cerium oxide (CeO2) nanoparticles, dispersible in either a hydrophilic or a hydrophobic medium, are synthesized continuously in supercritical methanol using methoxy polyethylene glycol (PEG350, H(OCH2CH2)nOCH3, MW=350g/mol) or α,ω-Bis(2-carboxymethyl)polyethylene glycol (PEG600, HOOCCH2(OCH2CH2)nOCH2COOH, MW=600g/mol) as a surface modifier. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the surface modifiers inhibit the growth of the particles, resulting in smaller size particles (20–25nm) as compared to unmodified particles (35nm) synthesized in supercritical methanol or unmodified particles (91nm) synthesized in supercritical water. At a high concentration of PEG600 (0.3M), surface-modified particles with a size of 3–4nm and a low degree of aggregation are produced. An X-ray diffraction analysis reveals that the PEG-modified nanoparticles retain the CeO2 phase. Fourier transform infrared spectroscopy and a thermal gravimetric analysis indicate that the amount of modifier attached to the surface of the nanoparticles is 6.88%, when 0.3M of PEG350 is used, and 4.49%, when 0.3M of PEG600 is used. A long-term stability test (40days) revealed that the PEG350-modified CeO2 nanoparticles have good dispersibility in a hydrophobic medium (oil), while PEG600-modified CeO2 nanoparticles have good dispersibility in a hydrophilic medium (water). This indicates that the nanoparticle dispersibility in either a hydrophilic or hydrophobic medium can be controlled by adjusting the end-group functionality and chain length of the modifiers.

β-Galactosidase activity in mixed micelles of imidazolium ionic liquids and sodium dodecylsulfate: A sequential injection kinetic study

An automated methodology for the kinetic study of β-galactosidase activity in sodium dodecylsulfate (SDS)/ionic liquid (IL) mixed micelles was developed. The main objective of the work was the evaluation of mixed micelles as reaction media for the industrial synthesis of glyco-oligossacharides. Enzyme activity was evaluated by means of a model reaction with the fluorescent substrate 4-methylumbelliferyl-α-d-galactopyranoside (MUG). The assay was implemented in a sequential injection analysis (SIA) system and enzyme activity was studied in SDS/bmim [BF4] and SDS/hmim [Cl] mixed micelles with variable concentrations of both components. In order to perform a critical evaluation of the obtained results, CMC and average micellar size of SDS/hmim [Cl] mixed micelles were evaluated by fluorescence and dynamic light scattering, respectively. In the micelle characterization assays it was observed that the CMC of the mixed micelles increased with hmim [Cl] concentration up to 1molL−1. In the presence of higher IL concentrations there were no evidences of micelle formation. Regarding micellar size, it was maximum for an IL concentration of 0.09molL−1. The kinetic assays evidenced that SDS/bmim [BF4] and SDS/hmim [Cl] mixed micellar systems can led to an increase of enzyme activity. This increase is dependent on the variation of the average micellar size that occurs with the increase of IL concentration up to 0.09molL−1. It was also noticed that the most promising systems are those incorporating SDS and IL in concentrations under 50mmolL−1 and 0.5molL−1, respectively. These results evidenced that the studied ILs can modify the physico-chemical properties of the surfactant solution in a favourable way regarding β-galactosidase activity being an important achievement for the future implementation of industrial processes catalyzed by this enzyme, mainly the synthesis of glyco-oligossacharides. Indeed, surfactant/IL mixed micelles proved to be an interesting alternative to conventional organic solvents in this field enabling the implementation of the processes in a relatively hydrophobic media with enhanced enzyme activity.

Spectroscopic investigations of the molecular interaction of anticancer drug mitoxantrone with non-ionic surfactant micelles

The aim of this study was to investigate the interaction of the anticancer drug mitoxantrone with non-ionic micelles, as simple model systems of biological membranes. UV-VIS absorption spectroscopy was used to quantify the drug-surfactant micelle interactions in terms of the binding constant and the micelle-water partition coefficient of the drug. Interaction of mitoxantrone with non-ionic micelles reduces the dimerization process of mitoxantrone, the drug molecules being encapsulated into micelles as monomer. The strength of the interaction between mitoxantrone and non-ionic micelles is higher at pH10 than at pH7.4, and depends on the surfactant in the order Tween 80>Tween 20>Triton X-100. The higher partition coefficient at pH10 compared to pH7.4 suggests that at basic pH the deprotonated mitoxantrone is incorporated more efficiently into the hydrophobic medium of non-ionic micelles compared to physiological pH, when the protonated drug is predominant. These results on simple model systems miming the drug-membrane interactions contribute to the elucidation of the behaviour of the drug in vivo, as well as the possible utilization of surfactant micelles as drug carriers.

Nearly Monodisperse Silica Microparticles Form in Silicone (Pre)elastomer Mixtures

The formation of silica from a tetraalkoxysilane in a sol-gel process usually requires a highly polar, typically aqueous, medium that aids in the hydrolysis of the silane and leads to electrostatic stabilization of the growing silica particles. Formation of such silica particles in a hydrophobic medium is much more challenging. We report the formation of silica microspheres within silicone oils (hydroxy-terminated poly(dimethylsiloxane), HO-PDMS) during elastomer cure using atmospheric humidity in a one-pot and one-step synthesis. Using tetraethyl orthosilicate (TEOS) as both cross-linker and silica precursor, and aminopropyl-terminated dimethylsiloxane oligomer (AT-PDMS) as a catalytic surfactant, silica particles of low polydispersity formed near or at the air interface of the elastomer: the presence of a hydrophilic polymer, poly(ethylene glycol) (PEG), had an indirect effect on the particle formation, as it assisted with water transmission into the system, which resulted in particle formation over a wider range of parameters and facilitated silicone elastomer cure further away from the air interface. Depending on the relative humidity during cure, the sizes of particles presenting at the air interface varied from ~6-7 μm under ambient conditions (20-30%RH) to ~7-9 μm at high relative humidity (90% RH). The origin of the controlled particle synthesis is ascribed to the relative solubility of the catalyst and the efficiency of water permeation through the silicone matrix. AT-PDMS preferentially migrates to the air interface, as shown by ninhydrin staining, where it both catalyzes alkoxysilane hydrolysis and condensation, and stabilizes the growing silica particles prior to aggregation. Since reactions in the presence of this catalyst are slow, TEOS can migrate from within the pre-elastomer body to the interface faster than water can penetrate the silicone, such that the main locus of hydrolysis/condensation leading both to silica formation and elastomer cross-linking is at the air interface.

Structural Determinants of Solute Diffusion through Membranes

Membrane permeability P to a solute is governed by the solubility of that solute in a hydrophobic medium and by membrane viscosity μ {Missner and Pohl}. According to this solubility diffusion model an increase in membrane cholesterol content tends to decrease P because, it decreases μ. Using scanning electrochemical microscopy, we show here that cholesterol may increase P for one solute while it decreases P for another solute. To obtain some insight into the molecular mechanism, we repeated our planar bilayer experiments in silico. The molecular dynamics simulations revealed contrasting cholesterol effects for the resistances which headgroup and tail regions of different bilayers offer to transmembrane diffusion. If this membrane inhomogeneity is taken into account, i.e. if the solubility diffusion model is individually applied to the different membrane regions, P may still be predicted from partition coefficient and viscosity.Missner, A., and P. Pohl. 2009. 110 years of the Meyer-Overton rule: Predicting membrane permeability of gases and other small compounds. ChemPhysChem 10: 1405-1414.

Insights into a surprising reaction: The microwave-assisted direct esterification of phosphinic acids

It is well-known that phosphinic acids do not undergo direct esterifications with alcohols under thermal conditions. However, the esterifications take place under microwave (MW) irradiation due to the beneficial effect of MW. As a comparison, maximum 12-15% conversions were observed on traditional heating. It was proved experimentally that the MW-assisted esterifications are not reversible under the conditions applied that may be the consequence of the hydrophobic medium established by the long chain alcohol/phosphinic ester. Neither the thermodynamic, nor the kinetic data obtained by high level quantum chemical calculations justify the direct esterification of phosphinic acids under thermal conditions. The thermodynamic data show that there is no driving force for the reactions under discussion. As a consequence of the relatively high values of activation enthalpy (102-161 kJ mol(-1)), these esterifications are controlled kinetically. Comparing the energetics of the esterification of phosphinic acids and the preparative results obtained under MW conditions, one can see the potential of the MW technique in the synthesis of phosphinates. During our study, a series of new cyclic phosphinates with lipophilic alkyl groups was synthesized.

On the nature of inhibition performance of imidazole on iron surface

Quantum mechanical calculations based on the Density Functional Theory under periodic boundary conditions were used to construct a model for the inhibition performance of imidazole on iron surface. It is shown that, at high coverage, imidazole molecules form C–C intermolecular bonds creating a protective film, which constitutes a hydrophobic medium and, consequently, prevents adsorption of water molecules. The establishment of intermolecular bonds leads to a stabilization of the adsorption energy and this cooperative effect is the thermodynamic explanation for the film formation. These results also point out to the limitation of previous studies based on properties of isolated inhibitor molecules.

DRIFT characterization of foulants and the derived effect on the contact angles of ultrafiltration membranes

Three ultrafiltration membranes, polyether-sulfone (PES), polyacrylonitrile (PAN) and cellulose acetate (CA) have been tested in order to study fouling and the removal of natural organic matter (NOM) from three natural source waters having different physical and chemical characteristics. The physically reversible fouling depended on the system of feed water and membrane type. Physically reversible fouling was significant for the system which consisted of hydrophobic feed medium–hydrophobic membrane (Las P, Lasek S — PES) as well as for the system: hydrophilic feed medium–hydrophilic membrane (Zabie D — CA). The change of membrane properties (the contact angles) after the filtration was observed. The foulants remained after physical cleaning render the PES and PAN membranes more hydrophilic, while CA membrane became more hydrophobic. The filtration cakes extracted from the membranes were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. According to spectra the water from Zabie D contained relatively more polysaccharide-like than protein-like substances, while the opposite relations between two groups were noted for Lasek S. The water form Lasek S demonstrated highest potential for fouling towards tested membranes. Las P sample showed more humic acid-like characteristics.

Methyl Groups of Trimethylamine N-Oxide Orient Away from Hydrophobic Interfaces

The molecular orientation of trimethylamine N-oxide (TMAO), a powerful protein stabilizer, was explored at aqueous/hydrophobic interfaces using vibrational sum frequency spectroscopy (VSFS). The systems studied included the octadecyltrichlorosilane (OTS)/water interface, which represents an aqueous solution in direct contact with a hydrophobic medium. Surprisingly, the measurements revealed that the methyl groups of TMAO pointed into the aqueous phase and away from the OTS. This orientation may arise from the more hydrophilic nature of methyl groups attached to a strongly electron-withdrawing atom such as a quaternary nitrogen. Additional studies were performed at the air/water interface. This interface showed a high degree of TMAO alignment, but the dangling OH from water was present even at 5 M TAMO. Moreover, the addition of this osmolyte modestly increased the surface tension of the interface. This meant that this species was somewhat depleted at the interface compared to the bulk solution. These findings may have implications for the stabilizing effect of TMAO on proteins. Specifically, the strong hydration required for the methyl groups as well as the oxide moiety should be responsible for the osmolyte's depletion from hydrophobic/aqueous interfaces. Such depletion effects should help stabilize proteins in their folded and native conformations on entropic grounds, although orientational effects may play an additional role.

Photoreduction of Chlorothalonil Fungicide on Plant Leaf Models

Photodegradation is seldom considered at the surface of vegetation after crop spraying. Chlorothalonil, a broad-spectrum foliar fungicide with a very widespread use worldwide, was considered. To represent the waxy upper layer of leaves, tests were performed within thin paraffin wax films or in n-heptane. Laser flash photolysis together with steady-state irradiation in n-heptane allowed the determination of the photodegradation mechanisms Chlorothalonil ability to produce singlet oxygen was measured; noteworthy its efficiency is close to 100%. Additionally, chlorothalonil photodegradation mainly proceeds through reductive dechlorination. In these hydrophobic media, a radical mechanism was evidenced. Photochemical tests on wax films under simulated solar light show that formulated chlorothalonil is more reactive than pure chlorothalonil. The field-extrapolated half-life of photolysis on vegetation was estimated to 5.3 days. This value was compared to the half-lives of penetration and volatilization available in the literature. It appears that chlorothalonil dissipation from crops is ruled by both photodegradation and penetration. The relative importance of the two paths probably depends on meteorological factors and on physicochemical characteristics of the crop leaf cuticle.

The K+ hydration shell structure in non-polar and low-polar environments

Knowledge of the structure of hydrated ions in hydrophobic media is crucial for understanding the mechanism of many biological processes and is relevant to a variety of technological applications. We focus here on the hydrated shell of the K+ cation, which has been widely investigated, both experimentally and theoretically, in gas phase and in aqueous solution. Recent experimental work has also been devoted to investigate Na+ and K+ halides transfer through a water/organic interface suggesting that the traditional view of hydrated ions entering the hydrophobic phase is not valid. We report a quantum mechanical study of K+(H2O)n clusters (n=1, 8) in gas phase and in dielectric media with dielectric constants ε=2, 10 corresponding to non-polar and low-polar environments, respectively. We show that dielectric effects significantly modify the stability and size of the hydrates. In particular, our computations suggest that in non-polar media, mono- and di-hydrates (n=1, 2) are the most stable species and that hexahydrates (n=6) have two water molecules in the second solvation shell.

Positions of Polar Amino Acids Alter Interactions between Transmembrane Segments and Detergents

α-Helical transmembrane (TM) segments in membrane proteins are comprised primarily of hydrophobic amino acids that accommodate insertion from water into the nonpolar membrane bilayer. In many such segments, however, polar residues are also present for structural or functional reasons. These latter residues impair the local favorable acyl interactions required for solvation by hydrophobic media such as phospholipids in native bilayers or detergents used for in vitro characterization. Using a series of Lys-tagged designed TM-like peptides (typified by KK-YAAAIAAIAWAIAAIAAAIAA-KKK) in which single-Asn residue substitutions (from Ile or Ala) were made successively from the center of the hydrophobic region toward the C-terminus, we demonstrate that polar residues strongly alter the nature of the interaction between TM segments and the solvating detergent. Through the application of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, circular dichroism spectroscopy, and tryptophan fluorescence, we observed drastic differences in the structures of the detergent-peptide complexes that contain relatively minor sequence differences. For example, the blue shift of the Trp fluorescence (indicating local detergent solvation at this location) differs by as much as ~10 nm depending upon the position of a single Asn substitution in an otherwise identical segment. The overall results suggest that polar point mutations occurring in a biological membrane will elicit comparable effects, placing a significant refolding burden on the local protein structure and potentially leading to disease states through altered protein--lipid interactions in membrane proteins.

Correlation effects during liquid infiltration into hydrophobic nanoporous media

Correlation effects arising during liquid infiltration into hydrophobic porous medium are considered. On the basis of these effects a mechanism of energy absorption at filling porous medium by nonwetting liquid is suggested. In accordance with this mechanism, the absorption of mechanical energy is a result expenditure of energy for the formation of menisci in the pores on the shell of the infinite cluster and expenditure of energy for the formation of liquid-porous medium interface in the pores belonging to the infinite cluster of filled pores. It was found that in dependences on the porosity and, consequently, in dependences on the number of filled pores neighbors, the thermal effect of filling can be either positive or negative and the cycle of infiltration-defiltration can be closed with full outflow of liquid. It can occur under certain relation between percolation properties of porous medium and the energy characteristics of the liquid-porous medium interface and the liquid-gas interface. It is shown that a consecutive account of these correlation effects and percolation properties of the pores space during infiltration allow to describe all experimental data under discussion.

Activation of Amberlyst-70 for Alkene Oligomerization in Hydrophobic Media

We have observed that pretreating Amberlyst-70 with a C9 ketone improves, up to five times, the rate of alkene oligomerization. Physical characterization of Amberlyst-70 reveals minimal changes to the particle dimensions upon exposure to 5-nonanone. Diffuse reflectance infrared spectroscopy indicates an interaction between the ketone and the acid sites within the catalyst. While the total number of acid sites is similar before and after treatment with nonanone, microcalorimetric studies of butene adsorption and temperature programmed desorption studies of butene desorption reveal that both adsorption and desorption of butene occur more rapidly in the catalyst treated with 5-nonanone, indicating that pretreatment of the catalyst with the ketone decreases the barrier for butene transport by 5–6 kJ/mol. The peak temperature for TPD spectra of butene desorption are observed to shift from 360 K for nonanone-treated Amberlyst-70 to 376 K in the case of untreated sample. The catalytic effect of the treatment with nonanone decreases slowly with time-on-stream, but remains after 200 h on stream (with the rate still being a factor of 2 higher than for the untreated sample); however, catalytic activity can be recovered upon additional treatment with ketone.

Modulation of Ground- and Excited-State Dynamics of [2,2′-Bipyridyl]-3,3′-diol by Micelles

The binding of [2,2'-bipyridyl]-3,3'-diol (BP(OH)(2)) with ionic and neutral surfactants like cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX-100) has been studied by steady-state and time-resolved fluorescence spectroscopy. The absorption as well as emission spectra of BP(OH)(2) are highly sensitive toward the variation of surfactant concentration and hydrophobicity of the environment. The fluorescent state of the diketo form gains stability in surfactant assemblies, leading to a red-shifted emission spectra. A sharp increase in the fluorescence quantum yield near critical micellar concentration (CMC) is encountered followed by saturation. This indicates a complete encapsulation of BP(OH)(2) in the micelles. The maximum fluorescence quantum yield in anionic SDS is rationalized by the formation of cationic fluorophore at the Stern layer. The increase in quantum yield in neutral TX-100 is attributed to higher microviscosity experienced by the fluorophore in the palisade layer. A direct support in favor of this argument in TX-100 is provided by the viscosity dependence exhibited by the probe in different concentrations of sucrose solutions. CTAB exhibiting only hydrophobic effect shows least increase in qunatum yield of BP(OH)(2) among all the surfactants. Time-resolved fluorescence study of BP(OH)(2) in micelles is used as a tool to monitor the extent of micellization in the lipophilic cavity. An increase in fluorescence quantum yield as well as lifetime of BP(OH)(2) upon micellization indicates an enhanced extent of ESIDPT in hydrophobic medium.

Structure changes of electrorheological fluids based on polyaniline particles with various hydrophilicities and time dependence of shear stress and conductivity during flow

Particles of polyaniline protonated with perfluorooctanesulfonic acid provided a material with hydrophobic surface. This property enabled its perfect dispersion in silicone oil due to its good compatibility with the hydrophobic medium. In contrast, in a suspension of hydrophilic polyaniline particles doped with sulfamic acid, strong interactions of particles prevailed, which led to the formation of entangled chains of aggregated particles in suspension. The difference in structural properties of suspensions exists already in the absence of electric field and significantly influences their electrorheological behavior after application of electric field. The formation of electrorheological structure has been monitored by recording time dependences of the shear stress and the electric current passing through the flowing suspensions.