Solvent Exchange Rates Research Articles

Shear-Assisted Fabrication of Block Copolymer Agglomerates with Various Morphologies in Viscous Medium.

In this work, we have investigated the effect of laminar flow shearing on the formation of block copolymer agglomerates in viscous medium. Under a laminar flow shearing, the block copolymer solution droplets were spontaneously emulsified and were then elongated into protofibers, which in turn transformed into particles with various morphologies. Besides micro-/nanorods, which were previously reported for homopolymers, sphere and sheetlike structures were unexpectedly fabricated from block copolymers depending on the solvent quality, solvent exchange rates, and the entanglement of the polymer chains. In particular, the sheet structure, fabricated from poly(ethylene glycol)-b-polystyrene (PEG-b-PS), can be fixed by UV irradiation when photo-crosslinkable azide groups were introduced onto the polystyrene block. Surprisingly, we found that the fixed sheetlike structures show demulsification capability in tens of seconds, which may have great potential application in the separation of oil from emulsions.

Heteronuclear 1D and 2D NMR Resonances Detected by Chemical Exchange Saturation Transfer to Water

AbstractA method to detect NMR spectra from heteronuclei through the modulation that they impose on a water resonance is exemplified. The approach exploits chemical exchange saturation transfers, which can magnify the signal of labile protons through their influence on a water peak. To impose a heteronuclear modulation on water, an HMQC‐type sequence was combined with the FLEX approach. 1D 15N NMR spectra of exchanging sites could thus be detected, with about tenfold amplifications over the 15N modulations afforded by conventionally detected HMQC NMR spectroscopy. Extensions of this approach enable 2D heteronuclear acquisitions on directly bonded 1H–15N spin pairs, also with significant signal amplification. Despite the interesting limits of detection that these signal enhancements could open in NMR spectroscopy, these gains are constrained by the rates of solvent exchange of the targeted heteronuclear pairs, as well as by spectrometer instabilities affecting the intense water resonances detected in these experiments.

Heteronuclear 1D and 2D NMR Resonances Detected by Chemical Exchange Saturation Transfer to Water.

A method to detect NMR spectra from heteronuclei through the modulation that they impose on a water resonance is exemplified. The approach exploits chemical exchange saturation transfers, which can magnify the signal of labile protons through their influence on a water peak. To impose a heteronuclear modulation on water, an HMQC-type sequence was combined with the FLEX approach. 1D 15 N NMR spectra of exchanging sites could thus be detected, with about tenfold amplifications over the 15 N modulations afforded by conventionally detected HMQC NMR spectroscopy. Extensions of this approach enable 2D heteronuclear acquisitions on directly bonded 1 H-15 N spin pairs, also with significant signal amplification. Despite the interesting limits of detection that these signal enhancements could open in NMR spectroscopy, these gains are constrained by the rates of solvent exchange of the targeted heteronuclear pairs, as well as by spectrometer instabilities affecting the intense water resonances detected in these experiments.

Characterisation of the conformational preference and dynamics of the intrinsically disordered N-terminal region of Beclin 1 by NMR spectroscopy

Beclin 1 is a 450 amino acid protein that plays critical roles in the early stages of autophagosome formation. We recently reported the successful expression, purification and structural characterisation of the entire N-terminal region of Beclin 1 (residues 1–150), including its backbone NMR chemical shift assignments. Based on assigned backbone NMR chemical shifts, it has been established that the N-terminal region of Beclin 1 (1–150), including the BH3 domain (112−123), is intrinsically disordered in the absence of its interaction partners. Here, a detailed study of its conformational preference and backbone dynamics obtained from an analysis of its secondary structure populations using the δ2D method, and the measurements of effective hydrodynamic radius as well as 1H temperature coefficients, 1H solvent exchange rates, and 15N relaxation parameters of backbone amides using NMR spectroscopy is reported. These data provide further evidence for the intrinsically disordered nature of the N-terminal region of Beclin 1 and support the view that the helical conformation adopted by the Beclin 1 BH3 domain upon interaction with binding partners such as BCL-2 pro-survival proteins is likely induced rather than pre-existing.

Configuration Control in the Synthesis of Homo- and Heteroleptic Bis(oxazolinylphenolato/thiazolinylphenolato) Chelate Ligand Complexes of Oxorhenium(V): Isomer Effect on Ancillary Ligand Exchange Dynamics and Implications for Perchlorate Reduction Catalysis.

This study develops synthetic strategies for N,N-trans and N,N-cis Re(O)(LO-N)2Cl complexes and investigates the effects of the coordination spheres and ligand structures on ancillary ligand exchange dynamics and catalytic perchlorate reduction activities of the corresponding [Re(O)(LO-N)2](+) cations. The 2-(2'-hydroxyphenyl)-2-oxazoline (Hhoz) and 2-(2'-hydroxyphenyl)-2-thiazoline (Hhtz) ligands are used to prepare homoleptic N,N-trans and N,N-cis isomers of both Re(O)(hoz)2Cl and Re(O)(htz)2Cl and one heteroleptic N,N-trans Re(O)(hoz)(htz)Cl. Selection of hoz/htz ligands determines the preferred isomeric coordination sphere, and the use of substituted pyridine bases with varying degrees of steric hindrance during complex synthesis controls the rate of isomer interconversion. The five corresponding [Re(O)(LO-N)2](+) cations exhibit a wide range of solvent exchange rates (1.4 to 24,000 s(-1) at 25 °C) and different LO-N movement patterns, as influenced by the coordination sphere of Re (trans/cis), the noncoordinating heteroatom on LO-N ligands (O/S), and the combination of the two LO-N ligands (homoleptic/heteroleptic). Ligand exchange dynamics also correlate with the activity of catalytic reduction of aqueous ClO4(-) by H2 when the Re(O)(LO-N)2Cl complexes are immobilized onto Pd/C. Findings from this study provide novel synthetic strategies and mechanistic insights for innovations in catalytic, environmental, and biomedical research.

Modeling size controlled nanoparticle precipitation with the co-solvency method by spinodal decomposition.

The co-solvency method is a method for the size controlled preparation of nanoparticles like polymersomes, where a poor co-solvent is mixed into a homogeneous copolymer solution to trigger precipitation of the polymer. The size of the resulting particles is determined by the rate of co-solvent addition. We use the Cahn-Hilliard equation with a Flory-Huggins free energy model to describe the precipitation of a polymer under changing solvent quality by applying a time dependent Flory-Huggins interaction parameter. The analysis focuses on the characteristic size R of polymer aggregates that form during the initial spinodal decomposition stage, and especially on how R depends on the rate s of solvent quality change. Both numerical results and a perturbation analysis predict a power law dependence R∼s(-⅙), which is in agreement with power laws for the final particle sizes that have been reported from experiments and molecular dynamics simulations. Hence, our model results suggest that the nanoparticle size in size-controlled precipitation is essentially determined during the spinodal decomposition stage.

Insoluble layer deposition and dilatational rheology at a microscale spherical cap interface.

The dilatational properties of insoluble monolayers are important for understanding the mechanics of biological systems and consumer products, but isolating the dilatational response of an interface is challenging due to the difficulties in separating dilatation from shear and other deformation modes. Oscillations of a microscale bubble radius are useful for generating purely dilatational flows, but the current deposition methods for insoluble layers onto fluid interfaces are not easily scaled down. In this paper, we describe a miscible solvent exchange procedure for generating insoluble layers at an air-water interface pinned at the tip of a capillary tens of micrometers in diameter. We show that the amount of surfactant adsorbed at the interface can be controlled by the initial concentration dissolved in isopropanol (the starting solvent) and the volumetric flow rate of solvent exchange. Surface pressure-area isotherms and dilatational moduli are measured concurrently for three insoluble surfactants: palmitic acid (PA), dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). The isotherms measured on the microscale interface compare well with previous experiments performed on a Langmuir trough. However, the magnitudes of the dilatational moduli differ from those measured on either Langmuir trough or pendant drop apparatuses. Several possible reasons for the observed differences are discussed. A comparison of the dilatational modulus with the Gibbs elasticity is used to determine the presence of dilatational extra stresses at the interface. The isotherm and dilatational modulus of the insoluble component of the industrial surfactant Tween 80 are measured using this approach. The methods developed here also open the possibility for future study of the important role of finite size effects on microstructure formation and the resulting interfacial mechanics.

Biomembrane Fabrication by the Solvent-assisted Lipid Bilayer (SALB) Method

In order to mimic cell membranes, the supported lipid bilayer (SLB) is an attractive platform which enables in vitro investigation of membrane-related processes while conferring biocompatibility and biofunctionality to solid substrates. The spontaneous adsorption and rupture of phospholipid vesicles is the most commonly used method to form SLBs. However, under physiological conditions, vesicle fusion (VF) is limited to only a subset of lipid compositions and solid supports. Here, we describe a one-step general procedure called the solvent-assisted lipid bilayer (SALB) formation method in order to form SLBs which does not require vesicles. The SALB method involves the deposition of lipid molecules onto a solid surface in the presence of water-miscible organic solvents (e.g., isopropanol) and subsequent solvent-exchange with aqueous buffer solution in order to trigger SLB formation. The continuous solvent exchange step enables application of the method in a flow-through configuration suitable for monitoring bilayer formation and subsequent alterations using a wide range of surface-sensitive biosensors. The SALB method can be used to fabricate SLBs on a wide range of hydrophilic solid surfaces, including those which are intractable to vesicle fusion. In addition, it enables fabrication of SLBs composed of lipid compositions which cannot be prepared using the vesicle fusion method. Herein, we compare results obtained with the SALB and conventional vesicle fusion methods on two illustrative hydrophilic surfaces, silicon dioxide and gold. To optimize the experimental conditions for preparation of high quality bilayers prepared via the SALB method, the effect of various parameters, including the type of organic solvent in the deposition step, the rate of solvent exchange, and the lipid concentration is discussed along with troubleshooting tips. Formation of supported membranes containing high fractions of cholesterol is also demonstrated with the SALB method, highlighting the technical capabilities of the SALB technique for a wide range of membrane configurations.

Biomembrane Fabrication by the Solvent-assisted Lipid Bilayer (SALB) Method.

In order to mimic cell membranes, the supported lipid bilayer (SLB) is an attractive platform which enables in vitro investigation of membrane-related processes while conferring biocompatibility and biofunctionality to solid substrates. The spontaneous adsorption and rupture of phospholipid vesicles is the most commonly used method to form SLBs. However, under physiological conditions, vesicle fusion (VF) is limited to only a subset of lipid compositions and solid supports. Here, we describe a one-step general procedure called the solvent-assisted lipid bilayer (SALB) formation method in order to form SLBs which does not require vesicles. The SALB method involves the deposition of lipid molecules onto a solid surface in the presence of water-miscible organic solvents (e.g., isopropanol) and subsequent solvent-exchange with aqueous buffer solution in order to trigger SLB formation. The continuous solvent exchange step enables application of the method in a flow-through configuration suitable for monitoring bilayer formation and subsequent alterations using a wide range of surface-sensitive biosensors. The SALB method can be used to fabricate SLBs on a wide range of hydrophilic solid surfaces, including those which are intractable to vesicle fusion. In addition, it enables fabrication of SLBs composed of lipid compositions which cannot be prepared using the vesicle fusion method. Herein, we compare results obtained with the SALB and conventional vesicle fusion methods on two illustrative hydrophilic surfaces, silicon dioxide and gold. To optimize the experimental conditions for preparation of high quality bilayers prepared via the SALB method, the effect of various parameters, including the type of organic solvent in the deposition step, the rate of solvent exchange, and the lipid concentration is discussed along with troubleshooting tips. Formation of supported membranes containing high fractions of cholesterol is also demonstrated with the SALB method, highlighting the technical capabilities of the SALB technique for a wide range of membrane configurations.

Water exchange dynamics around H3O+ and OH− ions

In this letter, we report the first computer simulation of the dynamics of water exchanging between the first and second solvation shells of H3O+. Employing different rate theories for chemical reactions such as the transition state theory, the Grote–Hynes theory, the reactive flux method, and the Impey–Madden–McDonald method, we calculate the solvent exchange rates from molecular dynamics simulations that account for explicit polarization effects. In addition, we also study water exchanges around OH− and find that the corresponding time scale is much smaller than that for H3O+.

Solvent exchange and electron-spin relaxation on homoleptic acetonitrile complexes of trivalent lanthanides.

Homoleptic acetonitrile complexes [Nd(CH3CN)9][Al(OC(CF3)3)4]3, [Dy(CH3CN)9][Al(OC(CF3)3)4]3, and [Tm(CH3CN)8][Al(OC(CF3)3)4]3 have been studied in anhydrous acetonitrile by (14)N and (1)H NMR relaxation. Solvent-exchange rate constants increase from (22 ± 6) × 10(6) s(-1) (Nd(3+)) and (160 ± 40) × 10(6) s(-1) (Dy(3+)) for the nonasolvated ions to (360 ± 40) × 10(6) s(-1) (Tm(3+)) for the octasolvated ions. Electron-spin relaxation of the lanthanide ions studied is similar to that found in aqua ions. This dependence on the binding properties of the coordinating molecules is consistent with the model proposed by Fries et al. for fast electron-spin relaxation of lanthanide ions other than Gd(3+).

Solvent-assisted lipid bilayer formation on silicon dioxide and gold.

Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

NMR relaxation and exchange in metal–organic frameworks for surface area screening

We describe a robust screening technique that correlates the surface area of metal–organic frameworks to the proton T2 relaxation behavior of imbibed solvent at low field (13MHz). In frameworks with small pore sizes (<1nm) or strong solvent-framework interactions, diffusional exchange between the pore-confined and inter-particle solvent populations remains slow compared to the T2 of the pore-confined solvent, allowing for a direct porosity analysis of the T2 spectrum obtained from Laplace inversions. Increases in framework pore-size (>1nm) lead to corresponding increases in the rate of solvent exchange, as confirmed by T2 relaxation exchange (REXSY) experiments; increases in the pore size also increases the T2 of the pore-confined solvent. The combination of these two effects results in comparable rates of relaxation and exchange, which precludes the direct analysis of Laplace inversions. Thus, two- and three-site kinetics models were applied to extract porosity from relaxation decays, thereby improving the utility of the porosity screening tool.

Hydroxyl-Proton Hydrogen Bonding in the Heparin Oligosaccharide Arixtra in Aqueous Solution

Heparin is best known for its anticoagulant activity, which is mediated by the binding of a specific pentasaccharide sequence to the protease inhibitor antithrombin-III (AT-III). Although heparin oligosaccharides are thought to be flexible in aqueous solution, the recent discovery of a hydrogen bond between the sulfamate (NHSO3(-)) proton and the adjacent 3-O-sulfo group of the 3,6-O-sulfated N-sulfoglucosamine residue of the Arixtra (fondaparinux sodium) pentasaccharide demonstrates that definable elements of local structure are accessed. Molecular dynamics simulations of Arixtra suggest the presence of additional hydrogen bonds involving the C3-OH groups of the glucuronic acid and 2-O-sulfo-iduronic acid residues. NMR measurements of temperature coefficients, chemical shift differences, and solvent exchange rate constants provide experimental confirmation of these hydrogen bonds. We note that the extraction of rate constants from cross-peak buildup curves in 2D exchange spectroscopy is complicated by the presence of radiation damping in aqueous solution. A straightforward model is presented that explicitly takes into account the effects of radiation damping on the water proton relaxation and is sufficiently robust to provide an accurate measure of the proton exchange rate between the analyte hydroxyl protons and water.

Development and Noninvasive Characterization of Hormone Releasing In Situ Forming Implants

SummaryIn the human body aldosterone plays a major role in the regulation of the salt water balance and the blood pressure. To investigate the pathophysiological effects of aldosterone in a mouse model appropriate drug delivery systems, that release the drug in a constant and continuous manner, are needed. Therefore novel in situ forming implants were developed as alternative aldosterone releasing depots. The non‐invasive analytical method electron spin resonance (ESR) spectroscopy was applied to characterize in situ the implant formation by direct and continuous quantification of polymer precipitation and solvent exchange rates. Therewith influences of several key formulation parameters, such as type of the solvent and the polymer have been investigated. In this context AB di‐ and triblock copolymers of poly(ethylene glycol) (PEG) and poly(lactide‐co‐glycolide) (PLGA) were firstly explored as polymeric matrices for in situ forming implants. The phase separation kinetics and therewith the aldosterone releases were highly dependent on the hydrophilic character and the molecular weight of the used polymers.

NMR and Electron Paramagnetic Resonance Studies of [Gd(CH3CN)9]3+ and [Eu(CH3CN)9]2+: Solvation and Solvent Exchange Dynamics in Anhydrous Acetonitrile

Homoleptic acetonitrile complexes [Gd(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](3) and [Eu(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](2) have been studied in anhydrous acetonitrile by (14)N- and (1)H NMR relaxation as well as by X- and Q-band EPR. For each compound a combined analysis of all experimental data allowed to get microscopic information on the dynamics in solution. The second order rotational correlation times for [Gd(CH(3)CN)(9)](3+) and [Eu(CH(3)CN)(9)](2+) are 14.5 ± 1.8 ps and 11.8 ± 1.1 ps, respectively. Solvent exchange rate constants determined are (55 ± 15) × 10(6) s(-1) for the trivalent Gd(3+) and (1530 ± 200) × 10(6) s(-1) for the divalent Eu(2+). Surprisingly, for both solvate complexes CH(3)CN exchange is much slower for the less strongly N-binding acetonitrile than for the more strongly coordinated O-binding H(2)O. It is concluded that this exceptional behavior is due to the extremely fast water exchange, whereas the exchange behavior of CH(3)CN is more regular. Electron spin relaxation on the isoelectronic ions is much slower than on the O-binding water analogues. This allowed a precise determination of the hyperfine coupling constants for each of the two stable isotopes of Gd(3+) and Eu(2+) having a nuclear spin.

Using the water signal to detect invisible exchanging protons in the catalytic triad of a serine protease.

Chemical Exchange Saturation Transfer (CEST) is an MRI approach that can indirectly detect exchange broadened protons that are invisible in traditional NMR spectra. We modified the CEST pulse sequence for use on high-resolution spectrometers and developed a quantitative approach for measuring exchange rates based upon CEST spectra. This new methodology was applied to the rapidly exchanging Hδ1 and Hε2 protons of His57 in the catalytic triad of bovine chymotrypsinogen-A (bCT-A). CEST enabled observation of Hε2 at neutral pH values, and also allowed measurement of solvent exchange rates for His57-Hδ1 and His57-Hε2 across a wide pH range (3-10). Hδ1 exchange was only dependent upon the charge state of the His57 (k (ex,Im+) = 470 s(-1), k (ex,Im) = 50 s(-1)), while Hε2 exchange was found to be catalyzed by hydroxide ion and phosphate base (k(OH)⁻ = 1.7 × 10(10) M(-1) s(-1), K(HPO)²⁻₄ = 1.7 × 10(6) M(-1) s(-1)), reflecting its greater exposure to solute catalysts. Concomitant with the disappearance of the Hε2 signal as the pH was increased above its pK (a), was the appearance of a novel signal (δ = 12 ppm), which we assigned to Hγ of the nearby Ser195 nucleophile, that is hydrogen bonded to Nε2 of neutral His57. The chemical shift of Hγ is about 7 ppm downfield from a typical hydroxyl proton, suggesting a highly polarized O-Hγ bond. The significant alkoxide character of Oγ indicates that Ser195 is preactivated for nucleophilic attack before substrate binding. CEST should be generally useful for mechanistic investigations of many enzymes with labile protons involved in active site chemistry.

상압 건조 공정을 이용한 다공성 세라믹스 구조체 내부에 소수성 실리카 에어로겔의 합성

The synthesis behavior of nanoporous silica aerogel in the macroporous ceramic structure was observed using TEOS as a source material and glycerol as a dry control chemical additive (DCCA). Silica aerogel in the macroporous ceramic structure was synthesized via sono-gel process using hexamethyldiazane (HMDS) as a modification agent and n-hexane as a main solvent. The wet gel with a modified surface was dried at <TEX>$105^{\circ}C$</TEX> under ambient pressure. The addition of glycerol appears to give the wet gel a more homogeneous microstructure. However, glycerol also retarded the rate of surface modification and solvent exchange. Silica aerogel completely filled the macroporous ceramic structure without defect in the condition of surface modification (20% HMDS/nhexane at 36hr).

15NH/D-SOLEXSY experiment for accurate measurement of amide solvent exchange rates: application to denatured drkN SH3

Amide solvent exchange rates are regarded as a valuable source of information on structure/dynamics of unfolded (disordered) proteins. Proton-based saturation transfer experiments, normally used to measure solvent exchange, are known to meet some serious difficulties. The problems mainly arise from the need to (1) manipulate water magnetization and (2) discriminate between multiple magnetization transfer pathways that occur within the proton pool. Some of these issues are specific to unfolded proteins. For example, the compensation scheme used to cancel the Overhauser effect in the popular CLEANEX experiment is not designed for use with unfolded proteins. In this report we describe an alternative experimental strategy, where amide (15)N is used as a probe of solvent exchange. The experiment is performed in 50% H(2)O-50% D(2)O solvent and is based on the (HACACO)NH pulse sequence. The resulting spectral map is fully equivalent to the conventional HSQC. To fulfill its purpose, the experiment monitors the conversion of deuterated species, (15)N(D), into protonated species, (15)N(H), as effected by the solvent exchange. Conceptually, this experiment is similar to EXSY which prompted the name of (15)N(H/D)-SOLEXSY (SOLvent EXchange SpectroscopY). Of note, our experimental scheme, which relies on nitrogen rather than proton to monitor solvent exchange, is free of the complications described above. The developed pulse sequence was used to measure solvent exchange rates in the chemically denatured state of the drkN SH3 domain. The results were found to correlate well with the CLEANEX-PM data, r = 0.97, thus providing a measure of validation for both techniques. When the experimentally measured exchange rates are converted into protection factors, most of the values fall in the range 0.5-2, consistent with random-coil behavior. However, elevated values, ca. 5, are obtained for residues R38 and A39, as well as the side-chain indole of W36. This is surprising, given that high protection factors imply hydrogen bonding or hydrophobic burial not expected to occur in a chemically denatured state of a protein. We, therefore, hypothesized that elevated protection factors are an artefact arising from the calculation of the reference (random-coil) exchange rates. To confirm this hypothesis, we prepared samples of several short peptides derived from the sequence of the drkN SH3 domain; these samples were used to directly measure the reference exchange rates. The revised protection factors obtained in this manner proved to be close to 1.0. These results also have implications for the more compact unfolded state of drkN SH3, which appears to be fully permeable to water as well, with no manifestations of hydrophobic burial.

Modeling the mass transfers during the elaboration of chitosan‐activated carbon composites for medical applications

AbstractHydrogels composites composed of chitosan and activated carbon were prepared for medical applications using the vapor‐induced phase separation process. Since the gelation process involves mass exchanges between the polymer solution and the air, the kinetics of mass transfer were investigated through experimental and modeling approaches. Among the formulation and process parameters, gravimetric measurements exhibited that mass transfers were mostly controlled by the initial ammonia partial pressure. A nonisotherm mass‐transfer model was developed to predict the nonsolvent and solvent exchange rates, therefore, the water and ammonia concentration profiles within the sample during the process. The numerical results were successively validated with gravimetrical kinetic curves obtained in a chamber where the process parameters were controlled. The model aimed also at predicting the pH moving front along the film thickness. The gelation time could also be predicted for different operating conditions (formulation and process parameters). © 2009 American Institute of Chemical Engineers AIChE J, 2009