Relative Water Vapor Pressure Research Articles

Crystallization Kinetics and X‐ray Diffraction of Crystals Formed in Amorphous Lactose, Trehalose, and Lactose/Trehalose Mixtures

ABSTRACT: Effects of storage time and relative humidity on crystallization kinetics and crystal forms produced from freeze‐dried amorphous lactose, trehalose, and a lactose/trehalose mixture were compared. Samples were exposed to 4 different relative water vapor pressure (RVP) (44.1%, 54.5%, 65.6%, 76.1%) environments at room temperature. Crystallization was observed from time‐dependent loss of sorbed water and increasing intensities of peaks in X‐ray diffraction patterns. The rate of crystallization increased with increasing storage humidity. Lactose crystallized as α‐lactose monohydrate, β‐anhydrous, and anhydrous forms of α‐ and β‐lactose in molar ratios of 5:3 and 4:1 in lactose and lactose/trehalose systems. Trehalose seemed to crystallize as a mixture of trehalose dihydrate and anhydrate in trehalose and lactose/trehalose systems. The crystal forms in a mixture of lactose and trehalose did not seem to be affected by the component sugars, but crystallization of the component sugars was delayed. Time‐dependent crystallization of lactose and trehalose in the lactose‐trehalose mixture could be modeled using the Avrami equation. The results indicated that crystallization data are important in modeling of crystallization phenomena and predicting stability of lactose and trehalose‐containing food and pharmaceutical materials. Keywords: crystallization, lactose, trehalose, crystal form, X‐ray diffraction

Surface area, porosity and water adsorption properties of fine volcanic ash particles

Our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary, despite the importance of these reactions in influencing the chemistry and dynamics of a plume. In this study, six samples of fine ash ( 500 A. All the specimens had similar pore size distributions, with a small peak centered around 50 A. These findings suggest that fine ash particles have relatively undifferentiated surface textures, irrespective of the chemical composition and eruption type. Adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption a s values. Enhanced reactivity of the ash surface towards water may result from (i) hydration of bulk ash constituents; (ii) hydration of surface compounds; and/or (iii) hydroxylation of the surface of the ash. The later mechanism may lead to irreversible retention of water. Based on these experiments, we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions. In addition, capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume. The total mass of water vapour retained by 1 g of fine ash at 0.95 relative water vapour pressure is calculated to be ~10−2 g. Some volcanic implications of this study are discussed.

Synthesis of Super-Microporous Aluminosilicate Having Excellent Water Vapor Adsorption Property as an Adsorbent for an Adsorption Heat-Pump

The synthesis of super-microporous aluminosilicate with a pore diameter of 14.6 A has been achieved, by means of synthesis under specified conditions with a skeletal material comprising TMOS and a small quantity of sodium aluminate, and a conventional alkyl-trimethylammonium halide (C10TMABr) as a template. This super-microporous aluminosilicate has an adsorption capacity of 0.2 ml/g at a relative water vapor pressure of 0.2, and thus is promising as an adsorbent for an adsorption heat-pump.

Synthetic allophane from highconcentration solutions: nanoengineering of the porous solid

AbstractThe amorphous aluminosilicate allophane was synthesized by rapid mixing of inorganic solutions with high initial concentrations (10 – 100 mmol/l) followed by hydrothermal treatment. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed homogeneous products having a hollow spherical amorphous structure with a particle diameter of 3 – 5 nm. The amorphous products had a high BET specific surface area (490 – 552 m2/g) in comparison with natural allophane and had a narrow pore-size distribution (2 – 5 nm in diameter). The results of water vapour adsorption isotherm studies showed a gradual increase over the range of relative water vapour pressure of 0.6 – 0.9 and reached a maximum of ∼85 wt.%. The synthetic allophane shows promise as an adsorbent material because of its high adsorption-desorption capacity and its unique structure.

Direct measurements of the adhesion between a glass particle and a glass surface in a humid atmosphere

An atomic force microscope has been used to measure the adhesion between individual silica-glass particles and a glass substrate in the presence of water vapour. It was found that the adhesion between the surfaces was not significantly altered, when compared with the dry case, at relative water vapour pressures of less than 0.6. Above this level of water vapour, the magnitude of the adhesion between the surfaces increased by approximately an order of magnitude. The transition of behaviour at a relative water vapour pressure of 0.6 corresponds to the formation of a capillary annulus having a critical Kelvin radius of approximately 2 nm. These findings are in good agreement with previous research data for the interaction of mica surfaces in water vapour. Force–distance data recorded as the particle and surface are separated from one another indicate the presence of a capillary neck between the surfaces. The form of these force–distance data indicates that the separation occurs under non-equilibrium conditions that more closely resemble the expected interaction under constant volume conditions (for the capillary neck). The results of this study also indicate the important role of equilibration time for the surfaces when not in contact. Successive force scans with only short equilibration times when the surfaces are separated result in the development of larger than expected capillary interaction forces. The results are relevant to the interactions between particles in a powder bed under flow.

Kinetics of Water Adsorption/Desorption on Bituminous Coals

The kinetics of water adsorption and desorption on nine bituminous coals of various ranks have been studied, mainly at 26 °C. To quantify the sorption kinetics, isobaric water uptake vs time data have been fitted by an empirical stretched-exponential model. This model provides good fits to water uptake data throughout the entire equilibration process at relative water vapor pressures (relative to saturation pressure) in the range of 0−0.9 for all coals investigated. This represents a distinct advantage oversimplified Fickian models which are valid for only a portion of the data within the short and long time limits. A flow rate kinetic parameter has been introduced to quantify the rate of water transfer from outside the coal particle to the intraparticle pore structure. Most of the bituminous coals investigated in this study have similar flow rates at relative pressures in the range of 0.2−0.9. However, for two coals of rank approaching that of semianthracites, the flow rate is significantly less than for all other bituminous coals investigated. It is believed this difference is caused by differences in the coal structure. Variations between coals in the flow rate at low relative pressures (0−0.2) appear to be caused by variations in the primary adsorption site density which is largely determined by the coal oxygen content. The density of oxygen containing functional groups also influences the flow rate at low relative pressures during water desorption.

X-ray Diffraction Study of Water Confined in Mesoporous MCM-41 Materials over a Temperature Range of 223−298 K

X-ray scattering measurements on water confined in the cylindrical pores of MCM-41 with different pore sizes C10 (diameter = 21 A) and C14 (28 A) have been performed over a temperature range of 223−298 K. Both samples were sealed in glass capillaries at relative water vapor pressures p/p0 = 0.3 and 0.6 under monolayer and capillary-condensed adsorption conditions, respectively. The X-ray radial distribution functions showed the presence of a distorted tetrahedral-like hydrogen-bonded network of water in both pores, characterized by peaks at ∼2.8, ∼4.2, and ∼4.9 A, with non-hydrogen-bonded H2O−H2O interactions at ∼3.3 A and H2O−Si interactions at ∼3.8 A between water and the silica wall. With decreasing temperature, the number of the hydrogen-bonded H2O−H2O interactions at 2.8 A increases, accompanied by the shifts of the 2.8 and 4.9 A peaks to shorter distances and of the 4.2 A peak to larger distances, for the capillary-condensed samples, showing a tendency to form more tetrahedral-like hydrogen-bonded w...

Characteristics of water in polypyrrole films

Polypyrrole films containing different dopant ions were electrochemically synthesized and the interaction between polypyrrole and water was evaluated by means of sorption isotherm, cluster function and d.s.c. It was found that the water sorbed in the polypyrrole behaved as a non-freezing bound water which was rather isolated and prevented from clustering over the whole experimental range of the relative water vapour pressure from 0.1 to 0.95. This might be associated with the nature of the conductive polymers having a π-conjugated system where the delocalization of carbonium charges would lower the ionic polarity and hydrophilicity of the polymeric chains.

Study on intelligent humidity control materials: Water vapor adsorption properties of mesostructured silica derived from amorphous fumed silica

The intelligent humidity control materials were synthesized using fumed silica and quaternary alkylammonium surfactant (decyl-, dodecyl-, tetradecyl- and hexadecyl-trimethylammonium chlorides) as a liquid-crystal template under hydrothermal conditions. X-ray diffraction and transmission electron micrographs indicate that a homogeneous hexagonal structure of SiO2 was formed for those products. The mesostructured products had high B.E.T. surface areas between 960 and 1300 m2/g, with uniform mesopore diameters of 2–4 nm. These physical quantities were controlled by varying the size of the organic template. Water vapor adsorption isotherms for these materials possess a sharp increase in adsorption when the relative water vapor pressure is at 40–60%. Over this range of pressures, the maximum amount of adsorbed water content is between 40 and 90%. These silicates have the potential to be effectively used as humidity control elements in construction materials.

Water Vapor Adsorption Properties of Ti-Containing Mesoporous Silica.

Materials enabling intelligent humidity control were synthesized using fumed silica, tetraethylorthotitanate and quaternary alkylammonium surfactant (decyl-, dodecyl-, tetradecyl and hexadecyltrimethylammonium chlorides) as a liquid-crystal template under hydrothermal conditions. X-ray diffraction profiles and transmission electron micrographs indicated that a homogeneous hexagonal structure of SiO2-TiO2 was formed from those products. The mesostructured products had high BET surface areas from 540 to 863m2/g, with uniform pore diameters of 1.5-3nm. These physical quantities were controlled by varying the size of the organic template. Water vapor adsorption isotherms for these materials experienced a sharp increase in adsorption when the relative water vapor pressure (P/P0) was in the range of 0.4-0.6. Over this range of pressures, the maximum amount of adsorbed water was between 20 and 50mass%. The isotherms were classified as type V in the IUPAC notation, showed an unusually large hysteresis. The large hysteresis could be explained by the difference in contact angle between adsorption and desorption. For both the 2nd and 3rd. water vapor adsorption-desorption isotherms, any morphological change was not observed in the isotherms, meaning that these materials have high durability in their humidity control ability. A small shift of the broad Q4 29Si-MAS/NMR spectra indicated that a wider distribution of the Si-O-Si angles in the SiO4 tetrahedra when titanium was incorporated into the silica matrix.

Interaction of local anesthetic heptacaine homologs with phosphatidylcholine bilayers: spin label ESR study

Local anesthetic monohydrochlorides of [2-(alkoxy)phenyl]-2-(1-piperidinyl)ethyl esters of carbamic acid (CnA, n=2, 3, 4, 6, 8, 10, 12 is the number of carbon atoms in the alkyloxy substituent) increase the probability of formation of gauche isomers p g and decrease the effective energy difference between gauche and trans conformation E g in egg yolk phosphatidylcholine (EYPC) acyl chains, as determined by electron spin resonance spectroscopy using dipalmitoylphosphatidylcholines labeled with the paramagnetic dimethyloxazolidinyl group on the 12-th or 16-th carbon atoms of their sn-2 acyl chain, and oriented EYPC bilayers hydrated at 81% relative water vapour pressure. CnAs also increase the hydration of EYPC in non-oriented bilayers at the same relative water vapour pressure. At the molar ratio of CnA:EYPC=0.4:1, the maximum effect on p g, E g and hydration has been observed for intermediate alkyloxy chain lengths n≈4÷6.

Solvatochromic study of water sorption in polymer films

A solvatochromic method was used to characterize the water sorption behavior of polymer films. Four different dyes were dissolved in various solid polymers and the relative water vapor pressure in the atmosphere was changed. Evaluated solvatochromic parameters depended not only on the amount of sorbed water but also on the kind of polymer. A linear correlation between the ET(30) value and Taft's multiparameter was confirmed in both the dried and the water sorbed polymers. The correlation equation was almost identical to that obtained by other researchers in liquid solvents. These results suggest that the dissolved dyes behave as indicators of microenvironmental changes in the solid polymer film. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1681–1691, 1997

Relative Vapor Pressure Measurements Using a Frequency-Modulated Tunable Diode Laser, a Tool for Water Activity Determination in Solutions

Two-tone frequency modulation spectroscopy (TTFMS) is applied to the determination of water activity using an 820-nm GaAlAs diode laser. By measuring the relative light absorption of water vapor over a solution and over a pure water reference, the water activity in the solution is determined. Fast, nonintrusive, and selective measurements are demonstrated. The detection limit for measuring a relative water vapor pressure change was 0.3%. The application of laser modulation spectroscopy to the determination Of water activity is discussed, and suggestions on increasing the sensitivity and accuracy are presented.

Nonideal effects of protein aggregation/protein activity on the sorption of water vapor by proteins: A thermodynamic linkage study

The evaluation of water vapor sorption isotherms for a diverse group of proteins, over a wide range of hydration levels using nonlinear regression analysis, strongly suggests that the relative vapor pressure is thermodynamically linked to protein aggregation/protein activity. The observed decrease in the relative vapor pressure of water as the protein concentration increases is liken into account by a model with water associated with, or trapped by, protein molecules as they form aggregates

Oxidation of SO2 in thin water films containing NH3

Abstract The reaction between NH 3 and SO 2 in thin water films was studied with a focus on the formation of sulfate. The reactions were conducted at 5 and 60% relative humidities or partial pressure of water vapor and at 0.04, 0.16 or 0.20 atm O 2 for temperatures between 20 and 50°C using a flow reactor and static flask reactors. In the presence of water vapor, the oxidation of SO 2 to sulfate was observed only when NH 3 was present. The product was primarily (NH 4 ) 2 SO 4 . Conversion of as much as 81% of the SO 2 to sulfate was observed after 10 min of reaction when the initial concentrations of SO 2 and NH 3 were 1 ppm and the relative humidity was 60%. Conversion was only 1–3% when NH 3 and/or water vapor were absent from the system. The surface of the reactor was found to influence the reaction by providing a place for a film of liquid water to form, and for the first several hours of reaction in the film, th e rate of sulfate formation was observed to be faster than oxidation in liquid water. In general, these results suggest that thin water films on environmental surfaces play a greater role in the deposition of atmospheric gases and the accumulation of their reaction products on surfaces than previously considered.

Dependence of the C Constant in the Brunauer-Emmett-Teller Equation on Water Pre-Adsorbed on Activated Carbon

The dependence of the C constant in the BET equation on the amount of pre-adsorbed water has been investigated by low-temperature nitrogen adsorption on activated carbon containing different amounts of water pre-adsorbed at the same relative water vapour pressure. It is suggested that the rapid decrease in the surface area as determined by the BET method in comparison with the C value in cases of low degrees of sorption volume filling is associated with blocking of part of the carbon texture and the parallel formation of extremely narrow micropores.

Adsorption and Low Temperature Solidification of Water on the Modified Surface of Silica

Several surface modified ultrafine silica powders have been prepared by a dehydration-condensation reaction between a hydroxy (silanol) group on the surface of silica and a normal-alkyl chain group of different alcohols. By comparing the adsorption isotherms of water molecules on the differently modified silica surfaces, the one-to-one correspondence in water adsorption was found between water molecules and residual surface silanol groups within the range of relative water vapor pressure (P/P0) in the formation of monomolecular adsorption layer on the silica surface. It was found that such a long chain of alkyl groups as normal-octyl or dodecyl groups can make the silica surface hydrophobic preventing from capillary condensation at an interparticle space, even at a small degree of alkyl group substitution (0.3-0.4 groups/nm2) due to a great increase of an effective molecular occupation area of substituted alkyl chain groups. It was evidentially pointed out that the bound water of at least two to four molecular layers adsorbed on the silica surface exists in an energetically more stable state due to structurization of water molecules in the layers than normal water being depressed in its freezing point.

Prediction of moisture transfer in mixtures of solids: transfer via the vapor phase

A mathematical method has been developed and tested to predict the final relative water vapor pressure in a closed system for a multicomponent mixture of solids knowing the initial water content for each component. From the final relative pressure and individual sorption-desorption isotherms, it is then possible to estimate the extent to which moisture redistributes via the vapor phase among the various components. The model utilizes any equation which describes the moisture sorption-desorption isotherm of each component over the entire range of relative pressures and includes as variables the initial dry weight and moisture content of each component, head space volume, and temperature. Experimental studies were carried out in a vacuum rack assembly linking various solids via the vapor state and measuring relative pressures of water vapor before and after redistribution using a sensitive pressure transducer at a known temperature and head space volume. Both the mathematical simulations and experiments were carried out with various combinations of corn starch, microcrystalline cellulose, gelatin capsules, and silica gel. Pharmaceutical situations where such information might be useful are discussed.

Adhesion between surfaces in undersaturated vapors—a reexamination of the influence of meniscus curvature and surface forces

The pull-off force between molecularly smooth mica surfaces has been measured as a function of the relative vapor pressure of cyclohexane, n-hexane, and water. For the experiments with water both normal mica (with surface potassium ions) and ion-exchanged mica (covered with hydrogen ions) have been used. The results of an earlier study (L. R. Fisher and J. N. Israelachvili, Colloids Surf. 3, 303 (1981)) have been shown to lead to erroneous conclusions due to the use of a rolling and shearing spring. Here a nonrolling and virtually shear-free double cantilever spring has been used. At high relative vapor pressures ( p/ p s ≳ 0.7) the pull-off force for all vapors is dominated by the Laplace pressure in a capillary-condensed annulus. There is a small but noticeable contribution from a solid-solid interaction across the annulus, but surface deformations do not appear to affect the measured pull-off force. As the relative vapor pressure decreases from ∼0.7 to 0, the pull-off force varies smoothly and approaches its value in the “dry” state. There is no difference in behavior between water and the nonpolar liquids other than can be explained in terms of the solid-solid contribution to the adhesion. It is concluded that such measurements do not give any immediate information on the validity of the bulk surface tension for very high ( r < 4 nm) curvature systems, at least not in the absence of a complete treatment of the surface deformation problem.

Ice formation in hardened cement paste, part III — Slow resaturation of room temperature cured pastes

Ice formation in partly dried and very slowly resaturated room temperature cured mature Portland cement pastes has been measured in the temperature range from +10° C to −60° C. It is demonstrated that there is an aging effect on the ice formation pattern. During resaturation the amount of non-frozen water increases with time and thereby the amount of micropores. Furthermore, there are indications that also the amount of non-freezable water increases. The water content at resaturation decreases irrecoverably with increasing severity of the previous desorption, but it is almost constant when desorption has been carried out to equilibrium at relative water vapor pressures below 0.58.