The molecular simulation of the miscibility,mechanical properties and physical cross-linking behavior of the poly(vinyl alcohol)/poly(acrylic acid) composited membranes

The miscibility and mechanical properties of poly vinyl alcohol (PVA) and poly acrylic acid (PAA)-composited membranes were studied with molecular simulation. The Flory–Huggins parameters (δ) were calculated to prove the good miscibility of PVA and PAA. The radial distribution functions of hydroxyl and carboxyl atoms and the average number of H-bonds were observed to indicate the degree of physical cross-linking between PVA and PAA. The influences of intermolecular physical cross-linking on the glass transition temperature and mechanical properties were estimated. The results revealed that the PVA/PAA membrane with a composition of 2:3 has the best plastic properties, which exhibits a good application value. All of the simulated results showed good agreement with the experimental data. It indicates that the method presented in this work has a promising application prospect.     

Modeling of composite sorption isotherm for stratum corneum

Equilibrium water sorption in stratum corneum (SC) is considered by treating it as a biocomposite with two main phases, namely, corneocytes and lipids. To validate the rule of mixtures for the individual phase sorption isotherms, a new flexible fitting model is introduced by accounting for characteristic features observed in the variations of the thermodynamic correction factors corresponding to the individual sorption isotherms. The comparison of the model fitting performance with that of the five-parameter Park's model shows a remarkably good ability to fit experimental data for different types of sorption isotherms. The effect of the lipids content on the variance of the composite sorption isotherm of stratum corneum is highlighted. The sensitivity analysis reveals that for the typical water content 20–30 wt%, which corresponds to the SC in a stable condition, the sensitivity of the composite sorption isotherm to the variation of the lipids content on dry basis is predominantly positive and sufficiently small. The good agreement observed between the experimental sorption isotherm for SC and the composite isotherm, which is based on the rule of mixtures for the individual phase sorption isotherms, yields a plausible conclusion (hypothesis) that the corneocytes–lipids mechanical interaction during unconstrained swelling of the SC membrane in the in vitro laboratory experiment is negligible.     

A study on moisture isotherms of formulations: the Use of polynomial equations to predict the moisture isotherms of tablet products

The objectives of this study were to investigate the effects of manufacturing parameters on the moisture sorption isotherms of some tablet formulations and to predict the moisture isotherms of the final formulations using polynomial equations. Three tablet formulations including a placebo and 2 drug products were prepared through wet granulation, drying, compression, and coating processes. Equilibrium moisture content of excipients and granules at 25°C with different relative humidities were determined using a dynamic moisture sorption microbalance, while such data for tablets were determined using desiccators. Moisture sorption isotherms were expressed in polynomial equations. Excipient isotherms were used to predict the moisture sorption isotherms of the 3 tablet products. Results showed that different physical properties of granules and tablets, such as particle size distribution, density, and porosity resulting from different granulation and compression conditions did not have significant effect on the moisture isotherms of the materials. Changing coating materials from a powder mixture to a film also did not change the moisture sorption characteristics significantly. The predicted moisture sorption isotherms of the formulations agreed well with the experimental results. These results show that moisture isotherms of solid pharmaceutical products manufactured with conventional processes may be predicted using the isotherms of excipients, and polynomial equations may be used as a tool for the prediction of moisture isotherms.     

Mono-component versus binary isotherm models for Cu(II) and Pb(II) sorption from binary metal solution by the green alga Pithophora oedogonia

The sorption of Cu(II) and Pb(II) by Pithophora markedly decreased as the concentration of the secondary metal ion, Cu(II) or Pb(II), increased in the binary metal solution. However, the test alga showed a greater affinity to sorb Cu(II) than Pb(II) from the binary metal solution. Mono-component Freundlich, Langmuir, Redlich-Peterson and Sips isotherms successfully predicted the sorption of Cu(II) and Pb(II) from both single and binary metal solutions. None of the tested binary sorption isotherms could realistically predict Cu(II) and Pb(II) sorption capacity and affinity of the test alga for the binary metal solutions of varying composition, which mono-component isotherms could very well accomplish. Hence, mono-component isotherm modeling at different concentrations of the secondary metal ion seems to be a better option than binary isotherms for metal sorption from binary metal solution.     

A molecular simulation of the compatibility of chitosan and poly(vinyl pyrrolidone)

The compatibility of chitosan (CS) and poly(vinyl pyrrolidone) was investigated by molecular dynamic (MD) simulations using the Flory–Huggins theory. The specific interactions in blends were studied by the radial distribution function (RDF). The Flory–Huggins interaction parameter, χ, was calculated at 298 K to assess the blend compatibility at different component ratios in the polymers. Miscibility was observed for blends with more than 50% of CS in the molar fraction, while immiscibility was prevalent at the molar fraction of CS between 10 and 50% of CS. Miscibility between poly(N-vinyl-2-pyrrolidone) (PVP) and CS polymers is attributed to the hydrogen bond formation of the –C = O group of PVP and the –CH₂OH groups of CS. This was further confirmed by MD simulations of RDFs for groups or atoms that are involved in interactions. These results are correlated well to obtain more realistic information on interactions involved as a function of blend composition.     

Water Binding in Legume Seeds

The physical status of water in seeds has a pivotal role in determining the physiological reactions that can take place in the dry state. Using water sorption isotherms from cotyledon and axis tissue of five leguminous seeds, the strength of water binding and the numbers of binding sites have been estimated using van't Hoff analyses and the D'Arcy/Watt equation. These parameters of water sorption are calculated for each of the three regions of water binding and for a range of temperatures. Water sorption characteristics are reflective of the chemical composition of the biological materials as well as the temperature at which hydration takes place. Changes in the sorption characteristics with temperature and hydration level may suggest hydration-induced structural changes in cellular components.     

Effects of temperature and concentration on the structure of ethylene oxide–propylene oxide–ethylene oxide triblock copolymer (Pluronic P65) in aqueous solution: a molecular dynamics simulation study

The effects of temperature and solution concentration on the structure of triblock polymeric surfactant (ethylene oxide)₁₉(propylene oxide)₂₉(ethylene oxide)₁₉ (Pluronic P65) have been investigated by fully atomistic molecular dynamics simulations. The Flory–Huggins interaction parameter χ, hydrogen bonding and molecular mobility in the aqueous solution of P65 were investigated covering a composition range of 0.1–0.73 (water weight fraction) and a temperature range of 273–373 K. The Flory–Huggins parameters indicated that propylene oxide (PO) segments became hydrophobic with the increase in temperature, whereas ethylene oxide (EO) segments remained hydrophilic, which caused the increase in repulsion between EO and PO segments. The intermolecular hydrogen bonds in P65 solution including water–water hydrogen bonds and water–P65 hydrogen bonds increased with the increase in solution concentration and decreased with the increase in temperature. The critical micellar temperature of Pluronic P65 predicted by Flory–Huggins interaction parameter χ and hydrogen bonding was in good agreement with experimental data.     

An examination of the moisture sorption characteristics of commercial magnesium stearate

The objective of this study was to characterize the moisture sorption of magnesium stearate and the morphological changes, if any, resulting from moisture sorption. Six samples of commercial magnesium stearate USP were examined. Moisture sorption isotherms were obtained at 25°C and 5% to 98% relative humidity (RH) using a moisture balance. Changes in crystal form resulting from moisture sorption were determined by x-ray diffraction. There were differences in the shape of the isotherm, reversibility of moisture uptake, and shape of the hysteresis loop in the isotherms of crystalline and amorphous magnesium stearates. The isotherm of crystalline magnesium stearate was almost parallel to the pressure axis until and RH of ∼80%. The isotherm of the amorphous sample was characterized by continuous uptake of water over the entire range of RH. Exposure of amorphous magnesium stearate to RH greater than 70% resulted in the formation of the trihydrate. The trihydrate was converted into the anhydrous form when heated to a temperature of 100°C to 105°C. The trihydrate could be generated by exposing the anhydrate to RH higher than 70%.     

Moisture Sorption Isotherms of Various Tobaccos

The equilibrium moisture contents of sun-cured (Kroumougrad), flue-cured (Bright Yellow—4) and air-cured (Burley-21 and Matsukawa) tobaccos were measured over a relative humidity range from 5 to 80% at 20°C. The moisture sorption isotherms of tobaccos were of sigmoid type, and classified into two groups. In a lower humidity range below ca. 40% RH, the A group (Kroumougrad and BY-4) had a smaller moisture sorption capacity than B group (Burley-21 and Matsukawa), while in a higher humidity range above ca. 50% RH the former had a larger moisture sorption capacity than the latter. By extracting with water, the moisture content of BY-4 was increased in the lower humidity range, while it decreased in the higher humidity range. However, the moisture content of Matsukawa was scarecely changed by extracting it with water. These results suggest that the differences in equilibrium moisture content with the type of curing were due to the differences in contents of water soluble com- ponents. To control the hygroscopic properties of a tobacco, therefore, the influences of the addition of sucrose and glycerol on the equilibrium moisture content were quantitatively analysed. The moisture sorption capacity of tobacco was greatly different from its nitrogen sorption capacity. The specific surface area of tobacco calculated from moisture sorption isotherm was ca. 110 times larger than the specific surface area calculated from the nitrogen sorption isotherm. Both the nitrogen and moisture sorption data should be necessary for better understanding of the complicated sorption-desorption phenomena in tobaccos.     

Equilibrium sorption of Cr6+ by a consortia of denitrifying bacteria

Summary The accumulation of metal ions on to biological materials is a promising method for removing these toxic materials from waste or ground-waters. A fractional factorial statistical experimental design was used to ascertain which experimental parameters affect the sorption of Cr⁶⁺ by a consortium of denitrifying bacteria using a minimum number of experiments. Analysis of this set of experiments found that the steady state sorption behavior could be described by Langmuir isotherms. Observed variations in the isotherm parameters were due to the imposed changes in the experimental variables. In particular, solution pH and the state of the biomass were found to have the greatest influence upon the amount of chromium that was taken up, while other factors had less significance, but were still important.     

A review of experimental measurements of effective diffusive permeabilities and effective diffusion coefficients in biofilms

Experimental measurements of effective diffusive permeabilities and effective diffusion coefficients in biofilms are reviewed. Effective diffusive permeabilities, the parameter appropriate to the analysis of reaction-diffusion interactions, depend on solute type and biofilm density. Three categories of solute physical chemistry with distinct diffusive properties were distinguished by the present analysis. In order of descending mean relative effective diffusive permeability (De/Daq) these were inorganic anions or cations (0.56), nonpolar solutes with molecular weights of 44 or less (0.43), and organic solutes of molecular weight greater than 44 (0.29). Effective diffusive permeabilities decrease sharply with increasing biomass volume fraction suggesting a serial resistance model of diffusion in biofilms as proposed by Hinson and Kocher (1996). A conceptual model of biofilm structure is proposed in which each cell is surrounded by a restricted permeability envelope. Effective diffusion coefficients, which are appropriate to the analysis of transient penetration of nonreactive solutes, are generally similar to effective diffusive permeabilities in biofilms of similar composition. In three studies that examine diffusion of very large molecular weight solutes (>5000) in biofilms, the average ratio of the relative effective diffusion coefficient of the large solute to the relative effective diffusion coefficient of either sucrose or fluorescein was 0.64, 0.61, and 0.36. It is proposed that large solutes are effectively excluded from microbial cells, that small solutes partition into and diffuse within cells, and that ionic solutes are excluded from cells but exhibit increased diffusive permeability (but decreased effective diffusion coefficients) due to sorption to the biofilm matrix.     

Effect of water content on the structural reorganization and elastic properties of biopolymer films: a comparative study

In this work, the effect of water uptake on the structural reorganization and elastic properties of three types of biopolymer films was studied. The water-biopolymer interaction for hydroxypropyl cellulose (HPC), gelatin, and cassava starch films prepared from aqueous solutions was studied and compared using Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffraction, dynamic vapor sorption (DVS), and dynamic mechanical thermal analysis with humidity generator and controller (DMTA) techniques. The FTIR spectral variations due to the water sorption were generalized into two-dimensional (2D) correlation graphs for each biopolymer, and the effect of water on the molecular conformation was compared. The water sorption isotherms were fitted with Guggenheim-Anderson-De Boer (GAB) and D'Arcy and Watt models. The water content in the mono- and multilayers predicted by both models for each biopolymer was discussed and compared. The correlation of the fitted data obtained from the sorption isotherms to the DMTA data allowed us to conclude that the elastic properties of the HPC films depended on the total water content in contrast to the elastic properties of the gelatin and cassava starch films, which decrease only with the appearance of multilayer water.     

Material properties of plasticized hardwood xylans for potential application as oxygen barrier films

Free films based on glucuronoxylan isolated from aspen wood were prepared by casting from aqueous solutions and drying in a controlled environment. Addition of xylitol or sorbitol facilitated film formation and thus examination of the material properties of these films. The mechanical properties of the films were evaluated using tensile testing and dynamic mechanical analysis in a controlled ambient relative humidity. The strain at break increased, and the stress at break and Young's modulus of the films decreased with increasing amounts of xylitol and sorbitol due to plasticization. At high amount of plasticizer, it was found that films with xylitol gave lower extensibility. Wide-angle X-ray scattering analysis showed that xylitol crystallized in a distinct phase, which we believe contributes to the more brittle behavior of these films. The effect of the plasticizers on the glass transition temperature was determined using dynamic mechanical analysis and differential scanning calorimetry. An increased amount of plasticizer shifted the glass transition to lower temperatures. The effect of moisture on the properties of plasticized films was investigated using water vapor sorption isotherms and by humidity scans in dynamic mechanical analysis. Sorption isotherms showed a transition from type II to type III when adding plasticizer. The films showed low oxygen permeability and thus have a potential application in food packaging.     

Evaluation of water binding, seed coat permeability and germination characteristics of wheat seeds equilibrated at different relative humidities

The relative binding of seed water and seed coat membrane stability were measured in two contrasting wheat (Triticum aestivum L) varieties, HDR 77 (drought-tolerant) and HD 2009 (susceptible) using seed water sorption isotherms, electrical conductivity (EC) of leachates and desorption-absorption isotherms. Analysis of sorption isotherm at 25 degrees C showed that the seeds of HDR 77 had significantly higher number of strong binding sites, with correspondingly greater amount of seed water as strongly bound water, as compared to HD 2009. Total number of binding sites was also higher in HDR 77 than HD 2009, which explained the better desiccation tolerance and higher capacity to bind water in seeds of HDR 77. EC of seed leachate in both varieties did not change with respect to change in equilibrium relative humidity (RII), indicating the general seed coat membrane stability of wheat seeds. However, absolute conductivity values were higher for HD 2009. showing its relatively porous seed coat membrane. Significantly lower area enclosed by the desorption-absorption isotherm loop in HDR 77, as compared to HD 2009 also indicated the greater membrane integrity of HDR 77. Germination and seedling vigour of HD 2009 were reduced when equilibrated over very low and very high RH. In contrast, germination and vigour in HDR 77 were maintained high, except at very high RH, indicating again its desiccation tolerance. Thus, the study demonstrated the relative drought tolerance of HDR 77, on the basis of seed water-binding characteristics and seed membrane stability. Seed membrane stability as measured by seed leachate conductivity or as area under dehydration-rehydration loop may be used as a preliminary screening test for drought tolerance in wheat.     

Batch and column studies of biosorption of heavy metals by Caulerpa lentillifera

The biosorption of Cu(II), Cd(II), and Pb(II) by a dried green macroalga Caulerpa lentillifera was investigated. The sorption kinetic data could be fitted to the pseudo second order kinetic model. The governing transport mechanisms in the sorption process were both external mass transfer and intra-particle diffusion. Isotherm data followed the Sips isotherm model with the exponent of approximately unity suggesting that these biosorption could be described reasonably well with the Langmuir isotherm. The maximum sorption capacities of the various metal components on C. lentillifera biomass could be prioritized in order from high to low as: Pb(II)>Cu(II)>Cd(II). The sorption energies obtained from the Dubinin-Radushkevich model for all sorption systems were in the range of 4-6 kJ mol(-1) indicating that a physical electrostatic force was potentially involved in the sorption process. Thomas model could well describe the breakthrough data from column experiments. Ca(II), Mg(II), and Mn(II) were the major ions released from the algal biomass during the sorption which revealed that ion exchange was one of the main sorption mechanisms.     

Does Flory–Huggins theory help in interpreting solute partitioning experiments?

The development of Flory–Huggins (FH) theory is reviewed, particularly with regard to the molecular significance of the interaction parameter that scales the contact interaction of solute and solvent. The chemical potential given by FH theory for an “idealute” solute is then compared with that provided by a more general, statistical thermodynamic approach. It is found that the FH contact term does not directly correspond to the solvation free energy. The significance of this result for the interpretation of free energies of transfer of a solute from one solvent to another is examined. It is shown that neither the earlier recommended standard free energy change for the process (using the infinitely dilute reference state, mole fraction units) nor the recently recommended FH-corrected standard free energy change provides the solvation energy desired. Instead, the standard free energy using the infinitely dilute reference state and molarity units, as long advocated by Ben-Naim, provides the desired solvation free energy. Correction of extant values, based on mole fraction units, is easily made. However, application of such results to problems of protein folding is not straightforward. © 1994 John Wiley & Sons, Inc.     

Biosorption of copper and zinc by Cymodocea nodosa

The adsorption of the two metal ions Cu and Zn in a single-component system by Cymodocea nodosa, a brown alga, under different pH conditions was investigated. The solution pH significantly affected the exhibited uptake, being maximum at a pH value of 4.5. Multi-component mixture biosorption in aqueous solutions is also reported. A comparison was made between the single-component saturation uptake and the multi-component uptakes. To evaluate the two-metal sorption system performance, simple isotherm curves had to be replaced by three-dimensional sorption isotherm surfaces. In order to describe the isotherm surfaces mathematically, three Langmuir-type models were evaluated. The isotherms indicate a competitive uptake with Cu being preferentially adsorbed. In addition, different tests were carried out to compare the process efficiency working continuously in small columns.     

Determination of Moisture Sorption Characteristics of Oat Flour by Static and Dynamic Techniques with and Without Thymol as an Antimicrobial Agent

The effectiveness of thymol as an antimicrobial agent during the determination of equilibrium moisture sorption data at high-water activities (0.50–0.98) was studied at 5, 23, and 45 °C in oat flour. The static gravimetric (SG) method (with and without added thymol) and the dynamic vapor sorption technique (DVS) were used. Microbial growth in samples conditioned in these environments at temperatures of 5 and 45 °C was null indicating no need for the use of thymol at these temperatures. However, samples confined in environments kept at 23 °C, when the SG method was used, needed addition of thymol since mold growth took place in its absence. The statistical comparison between experimental equilibrium moisture content (EMC) mean values showed that, at 45 °C, EMC values obtained using the SG technique with added thymol were significantly higher than those obtained without thymol by both SG and DVS techniques. This could indicate an interaction of thymol with food components or absorption by lipids present. Therefore, caution must be exerted when using thymol as an antimicrobial agent at elevated temperatures and high equilibrium relative humidity. Moisture adsorption isotherms for oat flour were determined using a DVS technique and no isotherm crossover with temperature, as previously reported for this product using thymol as an antimicrobial agent, was exhibited. Moisture sorption data obtained in this work by DVS can be considered more accurate than those previously reported for oat flour, since no external agent was involved during isotherm determinations.     

Thermodynamic analysis of micellization in PEO–PPO–PEO block copolymer solutions from the hydrogen bonding point of view

A thermodynamic approach is suggested to study the micellization mechanism of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers solutions from the hydrogen bonding point of view. Using Flory–Huggins theory, an association model is presented to describe hydrogen bonded (HB) chains, which are bridged by hydrogen bonds between water molecules and segments of the copolymer. The entropic change due to hydrogen bonding is formulated and the individual contribution of EO–water (EO–W) and PO–water (PO–W) hydrogen bonding to the micellization are derived respectively. Fourier transform infrared (FTIR) spectroscopy is applied to obtain the information of hydrogen bonds. During the temperature-dependent micellization of P105 block copolymer solutions, rapid disruption of PO–W hydrogen bonds is observed by FTIR and the calculated entropy relating to PO–W hydrogen bonds increases drastically compared with that of EO–W hydrogen bonds. The results demonstrate that PO–W hydrogen bonds play a dominant role in micellization.     

Prediction of band profiles of mixtures of bradykinin and kallidin from data acquired by competitive frontal analysis

The competitive adsorption isotherms of two closely related peptides, bradykinin and kallidin, were measured by frontal analysis on a Zorbax SB-C18 microbore column. An aqueous soluton at 20% acetonitrile (0.1% TFA) was used as the mobile phase. The competitive isotherm data were fitted to four different models: Langmuir, Bilangmuir, Langmuir-Freundlich, and Toth. These data fitted best to a Bilangmuir isotherm model. The influence of the pressure on the retention factors of the two peptides was found to be small and was not investigated in detail. The band profiles of large samples of the single components and of their mixtures were recorded. The overloaded profiles calculated using either the equilibrium-dispersive or POR model are in excellent agreement with the experimental profiles in all cases. Our results confirm that the competitive isotherm data derived from mixtures may suffice for a reasonably accurate prediction of the band profiles of all mixtures of the two components, provided their composition is close to 1/1.