Chilled Mirror Dew Point Technique Research Articles

Hydromechanical behavior and prediction of unsaturated loess over a wide suction range

Subgrade loess in arid and semi-arid regions subjected to high-suction conditions owing to low relative humidity and deep groundwater levels. Understanding the hydromechanical behavior of unsaturated compacted loess over a wide suction range is critical for resolving infrastructure problems in such areas. In this study, the water retention behavior of loess was investigated by imposing or measuring suction (s) using the axis translation technique, vapor equilibrium technique, and chilled mirror dew point technique. Triaxial tests were also performed to study the mechanical behavior of compacted loess under different net cell pressures (σ3n).The soil-water retention curves obtained using the different techniques are consistent. The degree of saturation and water content decreases significantly when s 0.5 MPa, indicating that undrained triaxial tests can be considered as those under constant suction. For the same σ3n, specimens show strain-hardening, shrinkage, and drum-shaped shear failure under low-s conditions and strain-softening, dilatancy, and oblique section splitting under high-s conditions. The failure deviator stress and cohesion of the compacted loess specimens increase with increasing s over the full s range (0–299.37 MPa). An equation to predict the shear strength of unsaturated loess over a wide s range is proposed. The intersection of the capillary water retention curve and adsorption water retention curve is set as a reference point (sR), where capillary degree of saturation is applicable for s ≤ sR and adsorption degree of saturation is added for s > sR. The predicted and measured shear strengths of the compacted loess specimens are in good agreement.

Phase equilibria of (water + [EMIM] bromide) and (water + [EMIM] tosylate)

This work presents a detailed phase equilibria study on the aqueous solutions of two hydrophilic ILs that are solid at room temperature: 1-ethyl-3-methylimidazolium bromide ([EMIM][Br]) and 1-ethyl-3-methylimidazolium tosylate ([EMIM][TOS]). The melting temperatures of the pure ILs were measured to discriminate between existing literature data that show some disparity. Solid-liquid equilibrium (SLE) data for (water + [EMIM][Br]), so far lacking in the literature, were determined by a dynamic method over the major portion of the entire composition range. Using a chilled mirror dew point technique, extensive and accurate vapor-liquid equilibrium (water activity) measurements were further conducted for the aqueous solutions of both ILs. The temperature and concentration dependences of the activity data were correlated simultaneously by an extended NRTL model, whereby establishing a common description of pertinent equilibria and related thermodynamic behavior of these systems. Good performance of this model was verified against other experimental data from the literature and this work. In particular, correct predictions of experimental SLE data proved the model to extrapolate well beyond the temperature range of data underlying the correlation. Excess thermodynamic properties calculated from the model revealed that these systems exhibit large negative deviations from Raoult's law, a strongly exothermic mixing and, except for (water + [EMIM][TOS]) in the water-rich region, positive excess entropies. Comparing the two ILs, the observed deviations from ideality are always more pronounced for [EMIM][Br], which indicates that [Br]– is a more hydrophilic anion than [TOS]–. The observed pattern of thermodynamic behavior was rationalized on the molecular level by means of simple MD simulations. They showed that while there are more H-bonds formed between water and [TOS]–, these bonds are weaker than those between water and [Br]–.

Thermophysical properties of aqueous solutions of the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid

Thermophysical behavior of the binary system [water+1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid (IL)] was thoroughly characterized through systematic measurements of (vapor+liquid) equilibria (water activity aw), mixing enthalpy, density, viscosity, and refractive index. The measurements were performed in the entire composition range and/or specifically in the highly dilute IL region, at T=298.15K or as a function of temperature in the range from (288.15 to 318.15)K. Effective experimental methods minimizing IL sample consumption, using flow arrangements, instrument couplings and high degree of automation were preferably employed. In particular, the aw determination based on the chilled-mirror dew point technique and implemented by an AquaLab 4TE instrument was identified as a generally superior approach to study VLE of (water+IL) systems. Excess thermodynamic properties (Gibbs free energy, enthalpy, heat capacity, and volume) and property deviations from the linear mixing rule (viscosity, refractive index) were evaluated, Padé approximants being used to correlate adequately their complex composition dependences. The extensive aw data were processed by a two-step procedure fitting first the temperature dependence at each isopleth and subsequently the composition dependence at each isotherm. Good estimates could be thus obtained for derivative thermal properties (enthalpy, heat capacity). Alternatively, the water activity and excess enthalpy data were correlated simultaneously by a NRTL-type model, providing their compact, thermodynamically consistent and adequate representation. Despite small absolute values of excess Gibbs free energy (GE), the system is revealed to be highly nonideal, the small GE resulting from close compensation of its large enthalpy and entropy contributions. Large endothermic effects and an enhanced increase of entropy upon mixing found for this system indicate relative weakness of interactions between unlike molecules and a massive structure breakage in the solution. Positive values of excess volume and negative values of viscosity and refractive index deviations found in the major part of the composition range corroborate this general energetic and structural pattern, although the situation appears to be more complicated in the highly dilute IL region, where these properties congruently exhibit a sign inversion.

Water Activities and Osmotic Coefficients of Aqueous Solutions of Five Alkylaminium Sulfates and Their Mixtures with H2SO4at 25oC

Alkylaminium sulfates are frequently detected in ambient aerosols, and are believed to be important in the nucleation of new particles in the atmosphere, despite the comparatively low gas phase concentrations of amines. In this study, water activities and osmotic coefficients have been measured, using a chilled mirror dew point technique, of aqueous mixtures of sulfuric acid and the following alkylaminium sulfates: methylaminium, ethylaminium, dimethylaminium, diethylaminium, and trimethylaminium sulf-ate. The samples were prepared by mixing solutions of the five corresponding amines and aqueous sulfuric acid and determining the exact aminium to sulfate molar ratios by ion chromatography. The results were correlated using an extended Zdanovskii–Stokes–Robinson equation to enable concentration/water activity rela-tionships to be calculated over the entire composition range from pure aqueous sulfuric acid to pure aqueous aminium sulfate. Water activities and osmotic coefficients for aminium:sulfate ratios o...

Suction of Some Polyethylene Glycols Commonly Used for Unsaturated Soil Testing

In the laboratory, solutions of polyethylene glycols (PEGs) are used to control and apply suction in soils using the osmotic technique. In this study, the suctions of PEGs with molecular weights (Mr) of 1500, 4000, 6000, and 20 000 were measured at 25°C using the chilled-mirror dew-point technique. The mass ratio of PEG–water mixtures was varied between 0.09 to 1.09 g of PEG/g of water to cover a wide range of suction from 0.09 MPa to 12.2 MPa. The results showed that, in general, PEGs with low Mr generated greater suctions than those with higher Mr. The difference between suctions of PEGs 6000 and 20 000 was found to be minor. Concurrent with previous reports, analysis of the test results in terms of molality and molarity versus suction showed that PEGs with high M r generated greater suctions than those with lower Mr. For any PEG, both molal and molar concentrations tend to generate similar suctions at low concentrations, whereas the density effect of PEG–water mixtures was found to be significant at high concentrations in that suctions corresponding to molar concentrations were far greater than their molal series counterparts. In general, good agreements were noted between the suctions measured in this study and those reported in the literature for similar PEG types. New equations were proposed that independently related mass ratio, molality, and molarity to suction of the PEGs studied. Additionally, relationships were established between the measured % Brix values and suctions of the PEGs studied.

Assessing Errors and Accuracy in Dew‐Point Potentiometer and Pressure Plate Extractor Meaurements

Soil water potential can be determined quickly with a WP4 device, which uses a chilled mirror dew point technique. The instrument can be used to check the soil water potential of samples drained in a porous plate extractor (PPE). However, soil water potential measured in both devices on the same soil sample may not be in agreement, mainly in the wet‐end of the water retention curve. This work investigates this problem, and further evaluates how equilibrating soil samples in a PPE with different permeable material at the sample bottom (filter paper [FP], polyester fabric [PE], and synthetic knitwear [SK]) affects the final water potential. Soil samples (soil fraction <2 mm) of an Ultisol and an Oxisol were drained at 0.5 and 1.5 MPa pressure in a PPE and the water potential of these samples was measured in a WP4. Another fraction of the same soil was used to obtain a WP4 water retention curve. The permeable materials caused large differences in water retention at 0.5 MPa pressure (0.006 g g−1 in the Oxisol and 0.021 g g−1 in the Ultisol), and the FP allowed more water drainage than PE and SK, indicating the latter two should be avoided when using a PPE to determine water extraction. In both soils, the water potential measured in the WP4 was lower than −0.5 MPa for samples under 0.5 MPa pressure and greater than −1.5 MPa for samples under 1.5 MPa pressure. The increase of residual (fitted minus measured) in the WP4 water retention curve indicated that above −0.7 MPa the precision of WP4 measurements decreased drastically.

Errors in water retention curves determined with pressure plates: Effects on the soil water balance

Pressure plates apparatus are very common experimental devices utilized to measure the soil water retention curve. Many studies have demonstrated the lack of reliability of pressure plates apparatus when they are used to measure the soil water retention curve in the dry range, due to low plate and soil conductance, lack of hydrostatic equilibrium, lack of soil–plate contact and soil dispersion. In this research, we investigated measurements of soil water retention curves obtained with a combination of Stackman’s tables, pressure plates apparatus and the chilled-mirror dew point technique. Specifically, the aim of this research was: (a) to investigate the differences in the measured soil water retention curves by the different experimental methods, (b) evaluate relationships between the experimental differences and soil texture, (c) analyze the effect of experimental differences on hydraulic properties parameterization and (d) investigate the effects of the different parameters set on water transport computation. The results showed differences in measurements made by the combination of Stackman’s tables and Richards’ pressure plates apparatus as compared to the dew point method, for fine textured soils, while no significant differences were detected for coarse textured soils. Computed cumulative drainage and evaporation displayed lower values if soil water retention curves were obtained from data obtained with the Stackman’s tables and Richards’ pressure plates apparatus instead of the dew point method. In soils, where the soil water retention curve was measured with traditional methods (Stackman’s tables and Richards’ pressure plates apparatus) average cumulative drainage was 173mm, with respect to a combination of methods including the dew point methods, where the average cumulative drainage was 184mm. Average cumulative evaporation was 77mm for the traditional methods, while it was 91mm, for the combination of methods. Overall, when simulation models are used for studies related to solute transport, polluted soil remediation, irrigation management and others, erroneous measurement of the SWRC for fine textured soils, may lead to erroneous computation of the soil water balance.

Total suction measurement of unsaturated soils with a device using the chilled-mirror dew-point technique

The total suction of a soil consists of two components: matric suction and osmotic suction. There are different techniques for measuring the total, matric and osmotic suction. The total suction of the soil is related to the water vapour pressure in the air space of the soil, and can be measured by devices capable of measuring relative humidity. A range of devices is available to measure relative humidity. In this paper, a device using the chilled-mirror dew-point technique with a selectable temperature environment was used to measure the relative humidity of a soil sample. Two residual soils of Singapore were used in the evaluation of the device for total suction measurement. The residual soils were compacted at various water contents and with three compaction efforts to provide soil samples of various suction values. The total suction of the compacted soil was measured and compared with its matric and osmotic suctions as independently measured using the null-type axis-translation apparatus and the pore fluid squeezer respectively. The time to equilibrium to obtain the total suction of the soil samples varied between a couple of minutes to about 18 min. The test results show that total suctions obtained using the device are always greater than the sum of the matric and osmotic suctions measured independently. The discrepancy increases as suction increases and is attributed to the limitations of the device for measuring the relative humidity of the soil samples accurately. However, by accounting for the discrepancy, the device can be used as a rapid means of determining the total suction of unsaturated soils.

Total suction measurement of unsaturated soils with a device using the chilled-mirror dew-point technique

The total suction of a soil consists of two components: matric suction and osmotic suction. There are different techniques for measuring the total, matric and osmotic suction. The total suction of the soil is related to the water vapour pressure in the air space of the soil, and can be measured by devices capable of measuring relative humidity. A range of devices is available to measure relative humidity. In this paper, a device using the chilled-mirror dew-point technique with a selectable temperature environment was used to measure the relative humidity of a soil sample. Two residual soils of Singapore were used in the evaluation of the device for total suction measurement. The residual soils were compacted at various water contents and with three compaction efforts to provide soil samples of various suction values. The total suction of the compacted soil was measured and compared with its matric and osmotic suctions as independently measured using the null-type axis-translation apparatus and the pore f...