Freezing Point Depression Data Research Articles

Freeze Concentration of Aqueous [DBNH][OAc] Ionic Liquid Solution

In the present work, freeze crystallization studies, as a novel concentration method for aqueous 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) ionic liquid solution, were conducted. In order to find the appropriate temperature and composition range for freeze crystallization, the solid–liquid equilibrium of a binary [DBNH][OAc]–water compound system was investigated with differential scanning calorimetry (DSC). Results of this analysis showed that the melting temperature of the pure ionic liquid was 58 ℃, whereas the eutectic temperature of the binary compound system was found to be −73 ℃. The activity coefficient of water was determined based on the freezing point depression data obtained in this study. In this study, the lowest freezing point was −1.28 ℃ for the aqueous 6 wt.% [DBNH][OAc] solution. Ice crystal yield and distribution coefficient were obtained for two types of aqueous solutions (3 wt.% and 6 wt.% [DBNH][OAc]), and two freezing times (40 min and 60 min) were used as the main parameters to compare the two melt crystallization methods: static layer freeze and suspension freeze crystallization. Single-step suspension freeze crystallization resulted in higher ice crystal yields and higher ice purities when compared with the single-step static layer freeze crystallization. The distribution coefficient values obtained showed that the impurity ratios in ice and in the initial solution for suspension freeze crystallization were between 0.11 and 0.36, whereas for static layer freeze crystallization these were between 0.28 and 0.46. Consequently, suspension freeze crystallization is a more efficient low-energy separation method than layer freeze crystallization for the aqueous-ionic liquid solutions studied and, therefore, this technique can be applied as a concentration method for aqueous-ionic liquid solutions.

Combining Freezing Point Depression and Self-Diffusion Data for Characterizing Aggregation.

The colligative property freezing point depression is evaluated as a means for estimating the extent of aggregation for solutions of poly(ethylene oxide) alcohol (C10E6) nonionic surfactant in cyclohexane. Combined with additional measurements of self-diffusion coefficients, it is shown that both unaggregated C10E6 as well as reverse micelles are significantly present for the entire range of measured C10E6 concentration (0.048-2.35 mol kg-1). A change in speciation near 0.2 mol kg-1 is indicated by the results from both freezing point depression and self-diffusion coefficient measurements. It is shown that average reverse micelle radii and aggregation numbers obtained from the ratio of solvent and C10E6 self-diffusion coefficients are consistent with prior reported results. However, unreasonably small radii for the reverse micelles as well as for the cyclohexane were obtained from analysis of the results by the Stokes-Einstein equation using additional measured solution viscosities. The concentration of reverse micelles and unaggregated C10E6 was calculated from the freezing point depression results using the aggregation numbers obtained from ratio of self-diffusion coefficients. These concentrations indicate that the reverse micelles become smaller in average size and increase in number with increasing temperature without an increase in unaggregated C10E6.

Differential scanning calorimetry determination of phase diagrams and water activities of aqueous carboxylic acid solutions

We have investigated the solid-liquid equilibria phase diagrams and water activities of the several carboxylic acid/water binary systems using differential scanning calorimetry (DSC) method. These mixtures were formed by water and low molecular weight carboxylic acids (formic acid, acetic acid, propionic acid and butyric acid). Water activities of the investigated solutions were obtained from the freezing point depression data in the range of the ice melting envelope. The experimental results indicated that all the aqueous carboxylic acid systems present a eutectic point in their respective phase diagrams. The activity coefficients of two components in the liquid phase were used to adjust the parameters of the Wilson and NRTL models. For all of the investigated systems, the best correlations of the solid–liquid equilibrium data have been determined by using the NRTL model. Finally, the phase behaviors of the systems were compared to the ideal state and the influence of the chemical structure of the carboxylic acids on the magnitude of the deviation from the ideal state was discussed.

Traceable Thermodynamic Quantities for Dilute Aqueous Sodium Chloride Solutions at Temperatures from (0 to 80) °C. Part 1. Activity Coefficient, Osmotic Coefficient, and the Quantities Associated with the Partial Molar Enthalpy

We present fully traceable two-parameter Huckel equations (with parameters B and b1) for the activity coefficient of sodium chloride and for the osmotic coefficient of water in aqueous NaCl solutions at temperatures from (0 to 80) °C. These equations apply within experimental error to all thermodynamic data available for these solutions at least up a molality of 0.2 mol·kg–1. In our previous study (J. Chem. Eng. Data 2016, 61, 286–306), these equations were successfully tested against the literature results of electrochemical, isopiestic, and cryoscopic measurements usually in the temperature range from (0 to 25) °C. There, a constant value was employed for B, whereas a linear model with respect to the temperature was utilized for b1. The linear model was determined from the values of b1 at 0 °C and at 25 °C obtained from freezing-point depression data and from isopiestic and cell-potential difference data, respectively. In the present study, these two b1 values are utilized alongside the constant value o...

Nitrate Concentration near the Surface of Frozen Aqueous Solutions.

Photolysis of nitrate plays an important role in the emission of nitrogen oxides from snow and ice, which affects the composition of the overlying atmosphere. In order to quantify these reactions, it is necessary to know how much nitrate is available for photolysis near the surfaces of snow and ice. The concentration of nitrate excluded from frozen solutions of nitric acid, sodium nitrate, and magnesium nitrate was measured with attenuated total reflection infrared spectroscopy. Liquid water and nitrate were observed at and near the bottom surface of frozen aqueous solutions during annealing from -18 to -2 °C. At -2 °C, the nitrate concentration was determined to be ∼1.0 mol/L for frozen NaNO(3) and Mg(NO(3))(2) solutions and ∼0.8 mol/L for frozen HNO(3) solutions. At lower temperatures, nitrate concentration ranged from 1.6 to 3.7 mol/L. Ideal thermodynamics overestimates nitrate concentration at colder temperatures where the brine is highly concentrated for all solutions. The nitrate concentration at ice surfaces is well described by bulk freezing point depression data close to the melting point of ice and for nitric acid at colder temperatures. Effects of temperature and counterions and implications for modeling snow chemistry are discussed.

Determination of the Pitzer Interaction Parameters at 273.15 K from the Freezing-Point Data Available for Solutions of Uni-Univalent Electrolytes

A novel calculation method (see the article in Ind. Eng. Chem. Res. 2014, 53, 5608–5616) was applied in this work to evaluate the ion interaction parameters for the Pitzer model from freezing points of aqueous solutions of pure electrolytes. The freezing-point depression data from aqueous solutions of salts consisting of chloride, bromide, nitrate, chlorate, perchlorate, formate, and acetate ions as anions and lithium, sodium, and potassium ions as cations were used in the present study. The literature data from the research group of Scatchard (J. Am. Chem. Soc. 1932, 54, 2676−2695; 1933, 55, 4355−4362; and 1934, 56, 805−811) were available for the calculations. Additionally, the parameters for salts of ammonium ion with chloride, bromide, iodide, and nitrate ions were determined but the data of this group (J. Am. Chem. Soc. 1932, 54, 2696−2705) are probably not reliable in these cases. The parameter values obtained using this method for solutions of the other salts than those of ammonium salts predict more accurately the experimental data than the ordinarily used Pitzer parameters and their temperature derivatives. Up to a molality of 0.5 mol·kg–1 in the former case, the errors are usually less than ±0.001 K, but in the latter case, they can be as high as ±0.02 K. The ordinarily used parameter values have been most often determined using calorimetric measurements.

Determination of the Pitzer Interaction Parameters at 273.15 K from the Freezing-Point Data Available for NaCl and KCl Solutions

A novel calculation method is presented in this work to evaluate the ion interaction parameters for the Pitzer model from freezing points of aqueous solutions of pure electrolytes. The freezing-point depression data from aqueous solutions of sodium chloride and potassium chloride are used to epitomize this method. The parameter values obtained using this method predict more accurately the experimental data than the most often used values for the Pitzer method up to a molality of 1.5 mol·kg–1. The parameters obtained by the freezing-point technique are associated with the solutions which are below the temperature of 273.15 K. It is verified in the present study that the temperature dependences of these parameters between the two temperatures are, however, practically negligible at molalities less than 0.5 mol·kg–1. In addition to the freezing-point data existing in the literature, the validity of the parameters was also studied using the literature results from concentration cells without transference.

Selected abiotic factors that influence raw cow milk freezing point depression

Freezing point depression (FPD) is an important property of milk that is influenced primarily by milk components connected to osmotic pressure. Under certain conditions it is possible to detect the addition of water to milk. It is necessary to have the right FPD limit in legislation for milk quality control. The aim of this study was to improve the estimation procedure of this limit. Apart from factors related to dairy cow nutrition, cattle breed and milk yield, it is important to take into account CO2 (6%), water steam evaporation and pasteurization under technological conditions. Bulk milk samples (1, 30, 6, 6, 10, 1 according to experiment) from Holstein and Czech Fleckvieh breed (1:1) were used in the experiments and technologically treated. The effects of water addition (water saturated and unsaturated by CO2), carbon dioxide evaporation and pasteurization (80 °C for 22 min) were quantified. Pasteurization aggravation of FPD was -0.00394 ± 0.00171 ºC (P < 0.001). Aggravation due to carbon dioxide evaporation could be -0.00383 ± 0.00095 ºC (P < 0.001) depending on practice. Increase in FPD is recorded after milking during technological procedures of milk storage, mixing, pumping, transport shaking and warming. During FPD shift, the acuteness of FPD data sets increases. This fact should be considered in the process of deriving standard raw cow milk FPD limits. Similar experimental analysis of milk FPD technological shifts has not been performed in this way until now.

The refined e-NRTL model applied to CO 2–H 2O–alkanolamine systems

The electrolyte NRTL (e-NRTL) model by Chen (1982) and Chen and Evans (1986) is perhaps the most commonly used activity coefficient based thermodynamic model for industrial systems. It has been shown by Bollas et al. (2008) that the original e-NRTL model is inconsistent for systems with multiple cations and/or anions, in the same work the model equations for the so-called refined e-NRTL model were given. In this work the refined e-NRTL model is applied to CO 2–H 2O–alkanolamine systems. The interaction parameters of the refined e-NRTL model are regressed to partial pressure of CO 2, binary vapour–liquid-equilibrium, freezing point depression data and excess enthalpy data. The model is in the end used to predict partial pressures and speciation for the CO 2–H 2O–MEA and CO 2–H 2O–MDEA systems.

Extended UNIQUAC model for thermodynamic modeling of CO 2 absorption in aqueous alkanolamine solutions

The extended UNIQUAC model [K. Thomsen, P. Rasmussen, Chem. Eng. Sci. 54 (1999) 1787–1802] was applied to the thermodynamic representation of carbon dioxide absorption in aqueous monoethanolamine (MEA), methyldiethanolamine (MDEA) and varied strength mixtures of the two alkanolamines (MEA–MDEA). For these systems, altogether 13 interaction model parameters are adjusted. Out of these parameters, 11 are temperature dependent. All the essential parameters of the model are simultaneously regressed to a collective set of data on the single MEA and MDEA systems. Different types of data are used for modeling and they cover a very wide range of conditions. Vapor–liquid equilibrium (VLE) data for the aqueous alkanolamine systems containing CO 2 in the pressure range of 3–13,000 kPa and temperatures of 25–200 °C are used. The model is also regressed with the VLE and freezing point depression data of the binary aqueous alkanolamine systems (MEA–water and MDEA–water). The two just mentioned types of data cover the full concentration range of alkanolamines from extremely dilute to almost pure. The experimental freezing point depression data down to the temperature of −20 °C are used. Experimental excess enthalpy ( H E ) data of the binary MEA–water and MDEA–water systems at 25, 40, 65 and 69 °C are used as well. In order to enhance the calculation of the infinite dilution activity coefficients of MEA and MDEA, the pure alkanolamines vapor pressure data in a relevant temperature range (up to almost 230 °C) are included in the parameter estimation process. The previously unavailable standard state properties of the alkanolamine ions appearing in this work, i.e. MEA protonate, MEA carbamate and MDEA protonate are determined. The concentration of the species in both MEA and MDEA solutions containing CO 2 are predicted by the model and in the case of MEA compared to NMR spectroscopic data. Using only one set of parameters for correlation of different thermodynamic properties, the model has represented the experimental data with good precision.

Phase equilibria for petroleum reservoir fluids containing water and aqueous methanol solutions: Experimental measurements and modelling using the CPA equation of state

Thermodynamic inhibitors, such as methanol, are widely used to reduce the risks associated with gas hydrate formation. The work presented in this communication is the result of a study on the phase equilibria of petroleum reservoir fluids in the presence of aqueous methanol solutions. Experimental hydrate dissociation data, for systems composed of methane/water/methanol and natural gas/water/methanol, in addition to experimental freezing point depression data for aqueous methanol solutions, are reported. A statistical thermodynamic approach, with the cubic-plus-association equation of state (CPA-EoS), is employed to model the phase equilibria. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The thermodynamic model was used to predict the hydrate dissociation conditions of methane and natural gases in the presence of distilled water or methanol aqueous solutions. Predictions of the developed model are validated against independent experimental data and the data generated in this work. A good agreement between predictions and experimental data is observed, supporting the reliability of the developed model.

Freezing Point of Milk: A Natural Way To Understand Colligative Properties

A laboratory experiment is presented in which the freezing point depression is analyzed using milk as solution. The nature of milk as a mixture of different solutes makes it a suitable probe to learn about colligative properties. The first part of the experiment illustrates the analytical use of freezing point measurements to control milk quality, regarding adulteration by addition of water and lactic fermentation. The second part deals with molecular weight determination from freezing point depression data, using powdered milk as solute. The effective molar masses of whole milk and skim milk are determined and compared.

Estimating the Hydrate Safety Margin in the Presence of Salt and/or Organic Inhibitor Using Freezing Point Depression Data of Aqueous Solutions

Developing systematic methods for controlling and monitoring gas hydrate risks along the pipeline and/or downstream conditions could provide the solution for future challenges. In this paper, the possibility of predicting the hydrate suppression temperatures of fluids from freezing point depression data of aqueous solutions containing different concentrations of salt and/or non-hydrate-forming organic inhibitor is investigated by developing a general correlation. The developed correlation considers only the changes in the freezing point of the aqueous solution, and there is no need for the analytical composition of the aqueous solution. Since measurement of the freezing point for the aqueous phase is much easier than measuring the hydrate dissociation point, such a relation can reduce the experimental costs. A well-proven thermodynamic model is used for generating pseudoexperimental data of hydrate suppression and freezing point depression for 160 different aqueous solutions containing various salts and/or organic inhibitors over a wide range of concentrations in order to develop this correlation. To examine the reliability of this model in predicting the freezing point depression, some experimental freezing point depression data are measured and compared with literature data and the predictions of the model. Independent data are used to examine the reliability of this method. The predictions of this approach are in acceptable agreement with the independent experimental data, demonstrating the reliability of this predictive method.

On the behaviour of tin-containing species in cryolite–alumina melts

Tin oxide (SnO2) dissolves in cryolite–alumina melts forming tetravalent tin species, but under reducing conditions it is reduced to divalent tin and, further, to metallic tin. Freezing point depression data for SnF2 and SnO in molten cryolite (Na3AlF6) corresponded to the formation of one and two new particles, respectively. By anodic dissolution of tin into cryolite-alumina melts divalent as well as tetravalent tin was formed, depending on current density and composition. During electrolysis of SnO2-based oxygen-evolving anodes, the condensate above the melt contained both divalent (SnF2) and tetravalent (SnO2) tin species.

Experimental system for one-dimensional freezing of undisturbed soil profiles

An inexpensive and easy-to-handle setup for freeze-thaw experiments was developed. The system mimics field conditions, with a relatively deep monolith of undisturbed soil and a soil-air interface as an upper boundary condition. The setup includes a freezing device for vertical freezing of a soil monolith and transducers at several depths in the soil monolith for continuous measurement of unfrozen water content (TDR), temperature and radial temperature differences. The setup makes rudimentary control of boundary conditions and sophisticated monitoring of soil water and heat conditions possible. To study the performance of the system, soil temperatures and water contents in a clay soil monolith were measured during two freeze-thaw cycles. The setup was shown to be useful in terms of simulating freeze-thaw cycles in a lysimeter placed in the laboratory. One-dimensional heat flows could be simulated, with a thermal error (horizontal vs vertical heat flows) during freezing of about 5%. The setup was used to test the hypothesis that the similitude between freezing/thawing and drying/wetting holds for unsaturated clay soils. The results indicated a good agreement between water retention curves calculated from freezing-point depression data and from measurements, using pressure plates.

Thermodynamics of alkanolamine-water solutions from freezing point measurements

The water freezing point has been measured in several amine-water systems. The water activity coefficient derived from these measurements by regression with the nonrandom two-liquid (NRTL) model shows strong interactions in the MDEA-water binary system near 0°C. The apparent solution nonideality sequence in the amine-water system at low temperature (about 0°C) and dilute amine solution (up to 13 mol%) is methyldiethanolamine (MDEA) ≃ diglycolamine (DGA) > dimethylmonoethanolamine (DMMEA) > triethanolamine (TEA) > diethanolamine (DEA) > monoethanolamine (MEA). The combined regression of freezing point depression data and total vapor pressure data predicts a significant temperature dependence of the solution properties. At 120°C, the expected solution nonideality sequence is DEA > DGA ≃ MEA > MDEA. The parameters for activity coefficients from regression of these data should be useful for predicting acid gas solubility in these systems

Solute/solvent interaction corrections account for non-ideal freezing point depression.

A new highly accurate curve-fitting technique for looking at freezing-point depression data was proposed by Fullerton et al. (Biochem. Cell Biol., in press). The method involve plotting mass solvent to mass solute ratio (Mw/M(s)) vs. 1/delta T (i.e. the inverse change in freezing point). A measured molecular weight and a solute/solvent interaction parameter (called I value) are inferred from the resultant linear plot. The accuracy of the molecular weight method was first demonstrated with the monomers of ethylene glycol, glycerol, propanol, mannitol, glucose and sucrose to show a mean molecular weight error of 0.02% with root mean square (RMS) error 0.9%. The RMS error (0.9%) is our best estimate of the molecular weight measurement accuracy for the method applied to a monomer. This error is consistent with the experimental precision (approximately 1%) which implies no systematic error. Non-ideality is described with a single constant, I. Polyethylene glycol (PEG) polymers of increasing length (vendor designation 200 to 10,000 Da) were analyzed to show monotonically increasing non-ideality (I values of 0.12 to 3.67) with increasing molecular weight. The measured molecular weights agreed with the end-point titration value for the three smallest polymers (where the number of polymeric units was less than or equal to 7). The method underestimates the vendor molecular weights for longer polymers. This disagreement is assigned to segmental motion (internal entropy) of longer, more flexible, PEG molecules.

Structure-function relationships in an antifreeze polypeptide. The role of neutral, polar amino acids.

An alanine-rich, alpha-helical antifreeze polypeptide (AFP) from the winter flounder and seven analogs with variations in the arrangement of neutral, polar amino acids were synthesized. Circular dichroism studies determined that all of the peptides, except for one containing a proline residue, were essentially 100% alpha-helical. Freezing point depression data, analyzed by three methods, showed that rearrangement of polar residues resulted in moderate to complete loss of anti-freeze activity. It was observed that ice crystals grow as hexagonal bipyramids in dilute solutions, with a constant c to alpha axis ratio of about 3.3. Above a critical threshold concentration, which may depend on the AFP to ice binding constant and reflect the onset of cooperative interactions, growth ceases until the temperature is lowered to the freezing point. We conclude that a specific arrangement of both threonine and asparagine (or aspartic acid) residues is critical for maximal activity and that the AFPs probably bind to the pyramidal faces of ice with a specific orientation. These conclusions are consistent with a recent report (Knight, C. A., Cheng, C. C., and DeVries, A. L. (1991) Biophys. J. 59, 409-418) that a similar AFP adsorbs to the [2021] pyramidal planes of ice in dilute solution.

Partial molar volumes and activity coefficients of the water in aqueous polyol solutions and the osmotic pressures of these solutions

The freezing-point depression and density of aqueous polyol (alditol) solutions were measured, and the osmotic pressure and the partial molar volume of the water of these solutions were calculated. The osmotic pressures calculated from the freezing-point depression data were compared with those calculated with van't Hoff's equation and fairly good agreement was found. The partial molar volumes of the water in the solutions were equal or almost equal to the molar volume of pure water up to the highest concentrations examined. Also, the activity coefficient of the water was unity or almost unity up to the highest concentrations examined.

Copper(I) complex formation in chloride melts. Raman spectroscopic and cryoscopic study

Raman spectra of equimolar MCl—CuCl melts (M = Na, K, Rb and Cs) at 500°C consist of one single band with a frequency increasing from 255 to 268 cm −1 when going from Na to Cs. For KCl—CuCl melts no change in band position was observed either with changes in temperature (200–600°C) or with compositional changes (0.250 < X CuCl, < 0.667). CuCl—AlCl 3 melts (0.33 < X CuCl, < 0.67, t = 300 and 400°C) show a copper chlorine vibration at 220 cm −1. The Al—Cl stretching and bending vibrations are shifted towards lower frequencies, relative to the vibrations of AlCl 4 − in alkali chloride-aluminium chloride melts. Also the intensity ratio between 308 and 342 cm −1 bands (the strongest for Al 2Cl 7 − and AlCl 4 −, respectively) is lower compared to MCl—AlCl 3 melts of similar AlCl 3 mole fraction. This suggests the formation of [CuAlCl 4] species with bent CuClAl bridging and a distortion in the tetrahedral configuration of the chloride around aluminium. For CuCl dissolved in basic MCl—AlCl 3 (M = K, Cs) melts, both Raman spectra and freezing point depression data indicate formation of linear CuCl 2 − complexes. Two liquid phases (essentially CuCl and KAlCl 4) were formed when CuCl was added to neutral KCl—AlCl 3 melts. In acidic MCl—AlCl 3—CuCl melts, a 220 cm −1 band was observed arising from a copper chloride vibration as in the CuCl—AlCl 3 melts. The density of the equimolar KCl—CuCl melt is 2.436 and 2.389 g cm −3 at 421 and 556°C, respectively.