Thermodynamics Of Water Sorption Research Articles

Water sorption thermodynamics in glassy polymers endowed with hydrogen bonding interactions

In this contribution, we review and critically compare the results of the analyses we have previously performed on water sorption thermodynamics in a series of polyimides. The experimental investigation was performed by combining gravimetric tests and in situ vibrational spectroscopy. A non-equilibrium theory, based on a compressible lattice framework accounting for the glassy state of the polymer and for the occurrence of hydrogen bonding interactions, has been used to interpret data. Information at a molecular level gained by vibrational spectroscopy has been used to tailor the model equations. The main features of water sorption thermodynamics are well captured, qualitatively and quantitatively, by the adopted model which displays a remarkable agreement with experimental results.

Water sorption thermodynamics in poly(propylene sebacate)

Water sorption thermodynamics in poly-propylene sebacate, PPSeb, a biodegradable semi-crystalline polyester, has been studied above its glass transition temperature. Experimental results have been obtained by using gravimetric methods to determine water sorption isotherms at several temperatures, as well as by means of in-situ FTIR transmission spectroscopy, to obtain qualitative information and quantitative estimates on the self- and cross-Hydrogen Bonds occurring within the water-polymer mixture. In fact, the water-polymer system has been found to be populated by several water ‘species’, each one characterized by a particular configuration of HB interactions. Different moieties on the polymer backbone are involved in the formation of specific interactions. The experimental results have been interpreted in the light of the Non Random Hydrogen Bonding (NRHB) lattice fluid theory for mixtures, which accounts both for the presence of Hydrogen Bonding (HB) interactions and for non randomness of the lattice contacts. An excellent fitting of experimental water sorption isotherms has been obtained, from which relevant mean-field and HB interactional parameters of the model have been calculated. On that basis, NRHB model has been then used to obtain qualitative and quantitative predictions, at equilibrium, of self and cross HB interactions involving both water molecules and groups present on the macromolecules within the polymer-water mixture, which compare well with the experimental results of FTIR spectroscopy.

Hydration and swelling of amorphous cross-linked starch microspheres

Hydration of cross-linked starch microspheres, commercially available as a medical device, was investigated using a multi-method approach. We found that the uptake of water is accompanied by substantial swelling and changes of the polymer structure. Sorption calorimetry provided information about thermodynamics of water sorption, revealed presence of isothermal glass transition and absence of hydration-induced crystallization, observed in non-cross linked starch material. The changes in the surface and bulk properties of microspheres at different water–starch concentrations were investigated using synchrotron radiation X-ray scattering and analyzed using concept of fractals. The obtained information, combined with the results of differential scanning calorimetry, was used to construct a phase diagram of the studied material. Finally, hydration induced evolution of polymer structure revealed by the X-ray scattering was linked to the changes observed during swelling with optical microscopy.

Time-resolved Fourier transform infrared spectroscopy, gravimetry, and thermodynamic modeling for a molecular level description of water sorption in poly(ε-caprolactone).

Sorption of water in poly(ε-caprolactone) (PCL), with specific focus on the hydrogen-bonding interactions, has been analyzed by combining ab initio calculations, macroscopic thermodynamics modeling, and relevant features emerging from spectroscopic and gravimetric measurements. Fourier transform infrared (FTIR) data, analyzed by difference spectroscopy, two-dimensional correlation spectroscopy, and least-squares curve-fitting analysis associated with gravimetric determination of water sorption isotherm provided information on the system's behavior and on the molecular interactions established between the polymer and the penetrant. A consistent physical picture emerged pointing to the presence of two spectroscopically discernible water species (first-shell and second-shell layers) that have been quantified. Water molecules are present in the form of dimers within the polymer equilibrated with water vapor up to a relative humidity of 0.65. At higher humidities, clustering of water sorbed molecules starts to take place. The multicomponent ν(OH) band representative of absorbed water has been interpreted with the aid of ab initio calculations performed on suitably chosen model systems. The outcomes of spectroscopic analyses were interpreted at a macroscopic level by modeling the thermodynamics of water sorption in PCL based on a nonrandom compressible lattice theory accounting for hydrogen-bonding (HB) interactions. Starting from the fitting of the gravimetric sorption isotherm, the model provided quantitative estimates for the amount of self- and cross-HBs which compare favorably with the FTIR results.

Thermodynamics of water sorption in high performance glassy thermoplastic polymers.

Sorption thermodynamics of water in two glassy polymers, polyetherimide (PEI) and polyetheretherketone (PEEK), is investigated by coupling gravimetry and on line FTIR spectroscopy in order to gather information on the total amount of sorbed water as well as on the different species of water molecules absorbed within the polymers, addressing the issue of cross- and self-interactions occurring in the polymer/water systems. Water sorption isotherms have been determined at temperatures ranging from 30 to 70°C while FTIR spectroscopy has been performed only at 30°C. The experimental analysis provided information on the groups present on the polymer backbones involved in hydrogen bonding interactions with absorbed water molecules. Moreover, it also supplied qualitative indications about the different “populations” of water molecules present within the PEEK and a quantitative assessment of these “populations” in the case of PEI. The results of the experimental analysis have been interpreted using an equation of state theory based on a compressible lattice fluid model for the Gibbs energy of the polymer-water mixture, developed by extending to the case of out of equilibrium glassy polymers a previous model intended for equilibrium rubbery polymers. The model accounts for the non-equilibrium nature of glassy polymers as well as for mean field and for hydrogen bonding interactions, providing a satisfactory quantitative interpretation of the experimental data.

Effect of Hemicellulose as a Coating Material on Water Sorption Thermodynamics of the Microencapsulated Fish Oil and Artificial Neural Network (ANN) Modeling of Isotherms

In this study, effect of hemicellulose as a coating material on sorption behavior of the microencapsulated fish oil is presented. For this purpose, water sorption isotherms of the fish oil microcapsules prepared using gum arabic (GA) and gum arabic-hemicellulose (GA-HC) were determined at 10, 25, and 35 °C using the gravimetric technique. The equilibrium moisture content (EMC) data were fitted to several empirical mathematical models, namely Oswin, BET, GAB, and Freundlich equations and the novel artificial neural network (ANN) model. The GA-HC microcapsules had higher EMC values than GA, due to the sugar content of hemicellulose. The EMC of the samples were more accurately predicted by the ANN model than by the empirical models at the entire range of a w. Heat of sorption of fish oil microcapsules was then predicted using the data obtained from the ANN model and fitted to the power model (R 2 = 0.985–0.978). A linear relation existed between the differential enthalpy and the differential entropy for microcapsules, thus satisfying the enthalpy-entropy compensation theory.

Water Sorption Thermodynamics and Mass Transport in Poly(ϵ‐Caprolactone): Interactional Issues Emerging from Vibrational Spectroscopy

AbstractWater sorption and transport in poly‐(ϵ‐caprolactone) (PCL) is studied by FTIR spectroscopy in transmission mode, providing information at the molecular level on equilibrium sorption and the diffusion mechanism. Coupling of spectroscopic and gravimetric data allows the equilibrium absorbance to be correlated with the water concentration in PCL. The analysis of the ν(OH) spectral region reveals the presence of two species of absorbed water (i.e., molecules directly interacting with the carbonyl group of PCL and self‐associated water). Sorption thermodynamics are interpreted using an equation of state approach routed on a compressible lattice fluid framework, accounting for self‐ and cross‐hydrogen‐bond formation. Assuming that only the carbonyl group acts as a proton acceptor group on the polymer backbone, theoretical predictions are found to be in close agreement with the experimental results.magnified image

Water Sorption Thermodynamics in Glassy and Rubbery Polymers: Modeling the Interactional Issues Emerging from FTIR Spectroscopy

Water sorption thermodynamics has been effectively investigated in rubbery and glassy polymers using, respectively, an equilibrium lattice fluid model, originally introduced by Panayiotou et al. [Panayiotou, C.; Tsivintzelis, I.; Economou, I.G. Ind. Eng. Chem. Res.2007, 46, 2628], accounting for hydrogen bond (HB) interactions in the system (i.e., the nonrandom hydrogen bonding, NRHB, model), and an extension of this model to a nonequilibrium glassy state (i.e., nonequilibrium thermodynamics for glassy polymers (NETGP)) that follows the same line of thought adopted originally by Doghieri and Sarti [Doghieri, F.; Sarti, G.C. Macromolecules1996, 29, 7885] to develop the NETGP approach. NRHB and NETGP-NRHB models have been used to interpret water sorption thermodynamics respectively in polycaprolactone and in polyimides. Model predictions in terms of self- and cross-HB established in the system are compared with quantitative information gathered from in situ infrared spectroscopy experiments, exploiting the wealth of information provided by proper elaboration of spectroscopy data by means of 2D correlation techniques.

Non-equilibrium compressible lattice theories accounting for hydrogen bonding interactions: Modelling water sorption thermodynamics in fluorinated polyimides

In this contribution the sorption thermodynamics of water in glassy fluorinated polyimides has been interpreted by using two non-equilibrium lattice models, accounting for hydrogen bond formation, which have been developed by extending, respectively, the Panayiotou–Sanchez (PS) and the Non-Random Hydrogen Bonding (NRHB) equilibrium models to the case of glassy polymers. The procedure used to extend the equilibrium models follows the same line of thought utilized by Doghieri and Sarti in deriving the Non-Equilibrium Thermodynamics for Glassy Polymers (NET-GP) model.The approach has been found to be successful in interpreting experimental water sorption isotherms in two different perfluorinated polyimides, namely 6FDA-ODA and 6FDA-6FpDA. The models have been also found to be capable of supplying a good qualitative and quantitative estimate of the number of water–water and water/polymer hydrogen bonds, as emerged from the comparison of theoretical predictions with experimental data obtained by means of previous IR spectroscopic measurements.

Thermodynamics of water sorption in poly(ɛ-caprolactone): A comparative analysis of lattice fluid models including hydrogen bond contributions

Thermodynamics of water sorption in poly(ɛ-caprolactone) (PCL) has been interpreted by using three models based on compressible lattice fluid theories, addressing the issue of self- and cross-hydrogen bond interactions. The models, available in the literature, are of increasing complexity and consist of a compressible lattice fluid term which could account or not for non-randomness of contacts and, in the case of two of the models, of a hydrogen bonding contribution. Models were analysed and compared in terms of fitting capability of the experimental sorption isotherms and, where appropriate, of predicted amount of self- and cross-hydrogen bonds which are established in the PCL–water mixture. Results confirm that, to obtain a satisfactory fitting of data, it is necessary to explicitly account for formation of hydrogen bonds.

An Investigation of Inorganic/Ionomer Composite Electrolyte Membranes by Dynamic Vapor Sorption

Nafion-Faujasite zeolite composite membranes were prepared by solution casting at different zeolite content, ranging from 2 to 40 wt.% nominal content. A dynamic vapor sorption (DVS) apparatus was used to investigate the thermodynamics of water sorption of the composite electrolytes. The water diffusivity was estimated from the kinetics of the water adsorption process. Fitting the sorption isotherms by a multi-mode model allowed to discriminate the water states in the composites and analyze the effect of the additives on the interaction of water with the ionomer matrix. The addition of zeolites causes an increase of the mobile water population within the membrane.

Thermodynamics of water interactions with human stratum corneum. II.: Interpretation via the Guggenheim–Anderson–deBoer isotherm

Studies of adsorption on biologically derived materials typically include direct measurement of either the adsorption isotherm or the heat of adsorption, but not both. Simultaneous measurement of adsorption and heat of adsorption should provide a more reliable description of the material under study. In this context, an analysis of the thermodynamics of water sorption is presented and a multilayer heat of sorption equation is derived within the framework of the Guggenheim–Anderson–deBoer (GAB) model. This model is applied to the previously published data for water vapor sorption and heat of water vapor sorption on stratum corneum (SC) over a range of relative humidities. The GAB models effectively characterize both heat evolution and equilibrium mass uptake over a broad water activity range. The thermodynamic results suggest significant restructuring of the SC during the sorption process; the sorption data alone are not sufficient to identify this effect. The results of this study emphasize first, the importance of incorporating a multilayer approach with variable energies of interaction in modeling of water uptake by SC and second, the utility of correlating sorption and calorimetric data simultaneously.

Sorption isotherms and thermodynamics of water sorption of ready-to-use Basundi mix

Basundi mix representing powdered Basundi (an Indian traditional milk product somewhat similar to sweetened condensed milk) is a convenience product developed for enhanced shelf life at ambient temperature. Water sorption isotherms were obtained for this ready-to-use mix over the temperature range of 5–45 °C. There was definite inversion of the temperature effect at high water activity (>0.7). GAB model gave excellent agreement between experimental and predicted values. Other properties of sorbed water viz., monolayer water, number of adsorbed monolayers, density and amount of bound water and surface area of adsorption were also obtained. The net isosteric heat of sorption had strong dependence on moisture content. The enthalpy–entropy compensation suggested that the sorption mechanism involved is enthalpy driven over the entire range of moisture content studied.

Thermodynamics of water sorption on PuO 2: Consequences for oxide storage and solubility

Gibbs energies for sorption of water on plutonium dioxide are derived from equilibrium pressure-surface composition data at 298–358 K. Sorption occurs in well-defined initial steps that form variable-composition surface phases by successive addition of H 2O layers and ultimately in a continuum. Δ G 0 values vary from −24.6 to −16.1 kJ per mol H 2O as y in the initial PuO 2− y (OH) 2 y product increases from 0 to 1 and from −16.1 to −11.0 kJ per mol H 2O during formation of PuO(OH) 2 · nH 2O ( n = 0 to 1) in the second step. These results are consistent with dissociative chemisorption at 298 K and with desorption near 575 K. Gibbs energies for higher sorption steps at 298 K asymptotically approach Δ G 0 for condensation of water (−8.56 kJ mol −1) and are consistent with reversible physisorption. Respective Δ G f 298 0 and Δ H f 298 0 are −1247 and −1338 kJ mol −1 for PuO(OH) 2; corresponding values are −1488 and −1608 kJ mol −1 for PuO(OH) 2 · H 2O. Results do not support claims that all water is reversibly desorbed at 350–500 K or that dioxide solubility is enhanced by formation of Pu(OH) 4(am) on the surface.

Thermodynamics of water sorption in clay

Experimental data on desorption isotherms of three types of clay at different temperatures (40, 50 and 60°C) were determined using a static method. The results were fitted by the famous Guggenheim-Anderson-deBoer (GAB) model. The thermodynamic functions, differential heat of sorption, net integral enthalpy and entropy, were evaluated at several moisture contents. Heat of sorption decreased with the moisture content and the trend became asymptotic as the moisture content of 0.01 (d.b.) was approached.

Thermodynamics of water sorption in acrylic homonetworks and IPNs

AbstractThe equilibrium thermodynamic properties of poly(hydroxyethyl acrylate) and poly(ethyl acrylate)‐i‐poly(hydroxyethyl acrylate) hydrogels are investigated starting from the water sorption isotherms of the systems. Partial enthalpy and entropy of the sorbed water in the gel differ markedly from the values of pure water at the lowest water contents, and tend to those of liquid water as saturation is approached. The residual mixing free energy is calculated, as a means of assessing the intensity of the water‐polymer interaction. Its small positive magnitude shows that water‐polymer hydrogen bonds are labile compared to water‐water and polymer‐polymer hydrogen bonds, and thus the stability of the gel state is still mainly due to the combinatorial entropic contribution to the mixing free energy. An equation correctly describing the sorption isotherms, when combined with the thermodynamic equations, can deliver the true water‐polymer interaction parameter and its dependence on the polymer volume fraction in the gel.

Thermodynamics of water sorption by perfluorosulphonate (Nafion-117) and polystyrene–divinylbenzene sulphonate (Dowex 50W) ion-exchange resins at 298 ± 1 K

Water sorption as a function of activity (aH2O) by H+-, Li+-, N(CH3)4+- and Cs+-exchanged forms of a perfluorosulphonate ion exchanger (Nafion-117) and by 4 and 8% crosslinked Dowex 50 W resins has been investigated in order to derive sorption isotherms using the isopiestic technique. All exchangers show the sequence of water sorption as H+ > Li+ > N(CH3)4+ > Cs+. However, the H+ and Li+ forms of Nafion-117 absorb significantly larger amounts of water, while the Cs+ form sorbs much less water at all water activities compared to the corresponding ionic forms of Dowex 50W × 8, the N(CH3)+4 isotherms of the two exchangers being quite close to each other. Sorption of water by the 4 and 8% crosslinked Dowex 50W resins is found to be the same at low aH2O( < 0.55), but the 4% crosslinked exchanger sorbs more water in the higher aH2O region. The overall decrease in free energy on water sorption (–ΔGsw) computed from the isotherms follows the sequence Cs+-117 < Cs+-× 8 < N(CH3)+4-117 < N(CH3)+4-× 8 < Cs+-× 4 < N(CH3)+4-× 4 < Li+-× 8 < Li+-× 4 < H+-× 8 < H+-× 4 < Li+-117 < H+-117. Following different procedures for Nafion and Dowex resins, cationic hydration numbers (n+) evaluated reveal greater hydration of H+ and Li+ in Nafion compared to that in Dowex resins. Cs+ in Dowex resins is reasonably hydrated, but in Nafion a solvent-shared ion pair of the type SO–3(H2O)Cs+ appears to form. Swelling pressures (π) computed for the two types of exchangers follow the order Nafion-117) > Dowex 50W × 8 > Dowex 50W × 4 (any ionic form). The results are discussed in relation to the relative degrees of hydration of the (respective) ions, including the sulphonate anion, and the differences between the two types of ion exchangers.

Diffusion of water vapor through a hydrophilic polymer film

AbstractMultifilm techniques have been used to measure the diffusion coefficient of water vapor in cellophane, and the data have been compared with the integral diffusion coefficient obtained in previous work with single films. The multifilm techniques lead to a much sharper resolution of the effect of concentration on diffusion, and the maximum integral diffusion coefficient. The diffusion coefficient for water vapor in cellophane peaks at a moisture content corresponding to about 70% R. H., which is presumably the “critical concentration” discussed in previous work on the thermodynamics of water sorption by cellophane.

Thermodynamics of water sorption by polymethylstyrenesulphonic acid and its sodium salt

AbstractThe thermodynamic functions of water sorption by polymethylstyrenesulphonic acid and its sodium salt have been determined by means of calorimetric and isopiestic measurements. The plots of these functions versus water content are presented for the range from 0 to 12 moles of water per equivalent of polyelectrolyte. A qualitative interpretation of the form of these plots is given and, at the same time, a comparison is made with the same plots of the corresponding cross‐linked analogues of the polyelectrolytes mentioned above, i.e., the polystyrenesulphonic acid resins in the hydrogen and sodium states. It is found that the differences between the values of these functions for the linear and cross‐linked polyelectrolytes are rather small and that, contrary to what might be expected, these values for the linear polyelectrolytes are closer to those of the highly cross‐linked resins.