Density, Viscosity, and Excess Properties of Binary Mixtures of 2

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Density, Viscosity, and Excess Properties of Binary Mixtures of 2‑(Methylamino)ethanol with 2‑Methoxyethanol, 2‑Ethoxyethanol, and 2-Butoxyethanol from 293.15 to 353.15 K Xiaoqin Shi, Chenxu Li, Hui Guo, and Shufeng Shen* School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P. R. China Downloaded via NOTTINGHAM TRENT UNIV on August 9, 2019 at 19:30:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Blends of amines with alkoxyethanols have been potentially used as alternative absorbents to conventional aqueous amines for energy-efficient CO2 capture, due to the lower specific heat capacity and enthalpy of vaporization of organic solvents than water. In this work, densities and viscosities were measured at atmospheric pressure for binary liquid mixtures of 2-(methylamino)ethanol (MAE) + 2-methoxyethanol (EGME), MAE + 2-ethoxyethanol (EGEE), and MAE + 2-butoxyethanol (EGBE) over the entire composition range and at temperatures of 293.15−353.15 K. The excess molar volume and viscosity deviations were also calculated and fitted by the Redlich−Kister equation. The calculated values of density and viscosity from the proposed correlations are in good agreement with the experimental values, with average absolute relative deviations within 0.06 and 1.23% for density and viscosity, respectively. The variations of excess or deviation properties with temperature and components were also discussed in terms of intermolecular interactions and structural effects.

1. INTRODUCTION

Single-component absorbents such as ionic liquids and secondary alkanolamines have been proposed for CO2 capture as alternatives to aqueous amines.16−18 The absence of any additional solvent has some potential advantages over the aqueous primary amines; nevertheless, poor mass transfer of CO2 between gas and liquid phases was observed due to the low diffusivity of solutes and the super high viscosity of CO2loaded solutions. Accordingly, nonaqueous blends have recently been developed based on the replacement of water by organic solvents, with the purpose of maintaining good mass transfer performances yet partly avoiding their disadvantages.19,20 Using organics as solvents can provide several advantages including lowering the energy consumption, corrosiveness, and degradation. These solvents include volatile alcohols (e.g., methanol, ethanol, and butanol),21−24 low volatile alcohols and glycols (e.g., benzyl alcohol and glycols) or their mixtures,25−27 glycol ethers,7,28 dimethylformamide,29 and ionic liquids.30 Compared with water and other organics,

The increasing concentration of carbon dioxide (CO2) in the atmosphere has been considered to be one of the major contributors to climate change. It is widely accepted that CO2 capture, utilization, and storage (CCUS) plays an important role in reducing CO2 emissions.1 Currently, there are many available technologies to capture CO2 from various sources, including chemical absorption, adsorption, membrane separation, and direct air capture.2−4 As for solvent absorption, aqueous monoethanolamine (MEA) is the most widely used absorbent due to its cheap price and high reactivity with CO2.5 However, high energy consumption for solvent regeneration is currently major obstacle for implementation of large-scale CCUS globally.6,7 The reasons for this problem are mainly due to three aspects: (1) water as a solvent in the blends is the weakest point due to its large specific heat capacity and enthalpy of vaporization compared with other organic solvents, (2) large heat of absorption and desorption for the reaction of primary amines with CO2, and (3) severe degradation and corrosion problems.7−9 To overcome this issue, several alternative absorbents have been developed for energy-efficient CO2 capture.10−15 © XXXX American Chemical Society

Received: April 25, 2019 Accepted: July 29, 2019

A

DOI: 10.1021/acs.jced.9b00364 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemical Samples Used in This Work chemical name

chemical formula

molar mass/(g mol−1)

CAS no.

2-(methylamino)ethanol

C3H9NO

75.11

109-83-1

2-methoxyethanol

C3H8O2

76.09

109-86-4

2-ethoxyethanol

C4H10O2

90.12

110-80-5

2-butoxyethanol

C6H14O2

118.17

111-76-2

purity (mass fraction)a

analysis methodb

water contentc (mass %)

Industrial

0.9925

GC

0.26

Industrial

0.9966

GC

0.09

Industrial

0.9913

GC

0.03

Industrial

0.9982

GC

0.04

source Aladdin Inc Aladdin Inc Aladdin Inc Aladdin Inc

a

The purity in mass fraction was provided by the supplier. bThe analysis method for mass fraction was provided by the suppliers. GC: gas chromatography. cThe Karl Fischer method was used for the determination of the water content.

Figure 1. Relative deviations [1000 × (ρlit − ρexp)/ρlit] between the experimental density results and the literature values for pure components at T = 293.15−353.15 K: (a) EGME: black ■, ref 32; red ●, ref 33; green ▲, ref 34; blue ▼, ref 35; (b) EGEE: black □, ref 34; red ○, ref 36; green △, ref 38; blue ▽, ref 39; pink ◇, ref 40; (c) EGBE: black ■, ref 42; red ●, ref 43; green ▲, ref 44; blue ▲, ref 45; pink ◆, ref 46; and (d) MAE: black □, ref 47; red ○, ref 48; green △, ref 49; blue ▽, ref 50; pink ◇, ref 51.

Knowledge of the physicochemical properties of nonaqueous liquid mixtures is of utmost importance in understanding the molecular interactions between the components and designing any new advanced absorption systems for the CO2 capture process. In this study, we have experimentally measured the volumetric and viscometric properties of pure MAE, EGME, EGEE, EGBE, and their binary mixtures MAE (1) + alkoxyethanol (2) at mass fractions of MAE w1 = 0.1− 0.9, at temperatures from 293.15 to 353.15 K with an interval of 5 K and atmospheric pressure of 101.3 ± 0.5 kPa. The

2-alkoxyethanols such as 2-methoxyethanol (EGME), 2ethoxyethanol (EGEE), and 2-butoxyethanol (EGBE) have attractive properties such as low volatility, low specific heat capacity (about 2.2−2.4 kJ K−1 kg−1), and low enthalpy of vaporization.31 2-(Methylamino)ethanol (MAE), as a secondary amine, will remain competitive in the future due to its effective desorption and fast absorption kinetics. Here, the main focus of this work is to study the binary blends of MAE with 2-alkoxyethanols. B



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Figure 2. Relative percent deviations [100 × (ηlit − ηexp)/ηlit] at T = 293.15−353.15 K of experimental viscosity results of pure components with literature values: (a) EGME: black ■, ref 32; red ●, ref 33; green ▲, ref 34; blue ▼, ref 36; pink ◆, ref 37; (b) EGEE: black □, ref 34; red ○, ref 36; green △, ref 40; blue ▽, ref 41; (c) EGBE: black ■, ref 35; red ●, ref 36; green ▲, ref 45; and (d) MAE: black □, ref 47; red ○, ref 50; green △, ref 51; blue ▽, ref 52.

2.2. Sample Preparation. The blend systems MAE + EGME, MAE + EGEE, and MAE + EGBE were prepared by dissolving different masses of MAE in EGME, EGEE, and EGBE in volumetric flasks at room temperature. The weight of chemicals was measured by an electronic analytical balance (OHAUS, CP214) with a precision of ±0.1 mg. Before measurement, the samples were degassed by ultrasound. 2.3. Density Measurement. Densities of pure solvents and binary blend mixtures of MAE with 2-alkoxyethanols were measured by a digital oscillating tube densimeter (DMA4100, Anton Paar) having a stated precision of ±10−4 g cm−3. Duplicate measurements with a refilling cell were carried out at temperatures ranging from 293.15 to 353.15 K. The standard uncertainty in temperature measurements was found to be 0.01 K. Before each series of measurements, calibration should be done by using dry air and deionized water. 2.4. Viscosity Measurement. The viscosities were measured at temperatures of 293.15−353.15 K with an interval of 5 K. A digital rolling-ball microviscometer (Lovis 2000ME, Anton Paar) was used with the precision up to 1.0%. The ball’s rolling time in a liquid-filled calibrated capillary of 1.59 mm diameter with three inductive sensors was measured at a defined angle. The temperature was controlled with an

excess molar volume and viscosity deviations for the abovementioned systems were also calculated at the same conditions. The correlated Redlich−Kister equations were used to represent the properties of the binary blends as a function of temperature and mole fraction of the components.

2. EXPERIMENTAL SECTION 2.1. Materials. 2-(Methylamino)ethanol (MAE, 99.25% GC purity, CAS no. 109-83-1), 2-methoxyethanol (EGME, 99.66% GC purity, CAS no. 109-86-4), 2-ethoxyethanol (EGEE, 99.13% GC purity, CAS no. 110-80-5), and 2butoxyethanol (EGBE, 99.82% GC purity, CAS no. 111-76-2) were purchased from Aladdin Reagent, China. S3 viscosity standard (Lot no. 15101) was purchased from CANNON Instrument Company. N26 viscosity standard (Lot no. 2122308077) was purchased from Sigma-Aldrich, China. All of the reagents were used without further purification. The water content in the studied solvents was determined by an Automatic Karl Fischer moisture titrator (Shanghai Anting Electronic Instrument, ZSD-2). The detailed information of the chemicals such as the stated purity by the suppliers, CAS registry number, water content, and sources is given in Table 1. C



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Table 2. Experimental Data of Density ρ for MAE(1) + EGME(2), MAE(1) + EGEE(2), and MAE(1) + EGBE(2) Binary Blends at Mass Fractions of MAE w1, Temperature T, and Pressure P = 101.3 kPaa w1 %

T/K 293.15

298.15

303.15

308.15

313.15

318.15

323.15

328.15

333.15

338.15

343.15

348.15

353.15

0.9320 0.9312 0.9291 0.9268 0.9247 0.9202 0.9152 0.9125

0.9272 0.9264 0.9246 0.9224 0.9203 0.9160 0.9111 0.9085

0.9224 0.9217 0.9200 0.9179 0.9159 0.9117 0.9070 0.9044

0.9175 0.9169 0.9154 0.9133 0.9114 0.9074 0.9028 0.9003

0.9125 0.9120 0.9107 0.9087 0.9069 0.9031 0.8986 0.8962

0.9075 0.9071 0.9060 0.9041 0.9024 0.8987 0.8944 0.8921

0.8972 0.8999 0.9043 0.9060 0.9075 0.9101 0.9122 0.9125

0.8924 0.8953 0.8998 0.9016 0.9032 0.9059 0.9081 0.9085

0.8876 0.8905 0.8953 0.8971 0.8988 0.9017 0.9040 0.9044

0.8827 0.8858 0.8907 0.8926 0.8944 0.8974 0.8998 0.9003

0.8778 0.8810 0.8861 0.8881 0.8899 0.8931 0.8956 0.8962

0.8728 0.8761 0.8814 0.8835 0.8854 0.8887 0.8914 0.8921

0.8706 0.8757 0.8848 0.8890 0.8931 0.9012 0.9091 0.9125

0.8662 0.8714 0.8806 0.8848 0.8890 0.8971 0.9050 0.9085

0.8618 0.8670 0.8763 0.8806 0.8848 0.8929 0.9009 0.9044

0.8573 0.8626 0.8720 0.8763 0.8805 0.8888 0.8968 0.9003

0.8528 0.8582 0.8676 0.8720 0.8763 0.8846 0.8927 0.8962

0.8483 0.8537 0.8633 0.8677 0.8720 0.8803 0.8885 0.8921

ρ/g cm−3 0 10.0 30.0 40.0 50.0 70.0 90.0 100

0.9647 0.9632 0.9599 0.9572 0.9545 0.9491 0.9432 0.9401

0.9602 0.9587 0.9556 0.9529 0.9504 0.945 0.9392 0.9362

0.9555 0.9542 0.9513 0.9486 0.9461 0.9409 0.9353 0.9323

0.9509 0.9497 0.9469 0.9443 0.9419 0.9369 0.9313 0.9284

0.9462 0.9451 0.9425 0.9400 0.9377 0.9327 0.9273 0.9245

0 10.0 30.0 40.0 50.0 70.0 90.0 100

0.9295 0.9316 0.9348 0.9360 0.9371 0.9388 0.9401 0.9401

0.9250 0.9272 0.9305 0.9318 0.9329 0.9348 0.9362 0.9362

0.9205 0.9227 0.9262 0.9276 0.9288 0.9308 0.9322 0.9323

0.9159 0.9182 0.9219 0.9233 0.9246 0.9267 0.9282 0.9284

0.9113 0.9137 0.9176 0.9191 0.9204 0.9226 0.9243 0.9245

0 10.0 30.0 40.0 50.0 70.0 90.0 100

0.9004 0.9050 0.9136 0.9175 0.9214 0.9292 0.9368 0.9401

0.8962 0.9009 0.9095 0.9136 0.9175 0.9252 0.9329 0.9362

0.8920 0.8968 0.9055 0.9095 0.9135 0.9213 0.9290 0.9323

0.8878 0.8926 0.9014 0.9055 0.9094 0.9173 0.9250 0.9284

0.8835 0.8884 0.8973 0.9014 0.9054 0.9133 0.9211 0.9245

MAE(1) + EGME(2) 0.9415 0.9368 0.9405 0.9358 0.9381 0.9336 0.9357 0.9313 0.9334 0.9291 0.9286 0.9244 0.9233 0.9193 0.9205 0.9165 MAE(1) + EGEE(2) 0.9066 0.9019 0.9092 0.9046 0.9132 0.9088 0.9147 0.9104 0.9161 0.9119 0.9185 0.9143 0.9203 0.9162 0.9205 0.9165 MAE(1) + EGBE(2) 0.8792 0.8749 0.8842 0.8800 0.8931 0.8890 0.8973 0.8932 0.9013 0.8972 0.9093 0.9053 0.9171 0.9131 0.9205 0.9165

a Standard uncertainties u are u(T) = 0.02 K, u(P) = 0.5 kPa and the combined expanded uncertainties Uc are Uc(w1) = 0.002, Uc(ρ) = 0.0010 g cm−3, with a 0.95 level of confidence (k = 2). w1 is referred to as the mass fraction of MAE in liquid binary mixtures, and w1 = 100% denotes the pure MAE from the supplier.

large deviations. A series of Ubbelohde capillary viscometers by manually measuring the efflux time and the given viscometer constants were used in the literature.33,35−37,40,41,45,47,51,52 In this work, a rolling-ball measuring principle was applied to measure the time using three inductive sensors. 3.1. Density. The experimental data of density for the blends of MAE with EGME, EGEE, and EGBE at temperatures from 293.15 to 353.15 K are given in Table 2. The mass fraction of MAE in these mixtures covers the whole composition range. The influence of temperature and concentration of MAE on density is presented in Figure 3. It is noted that the density values of pure chemicals at a specific temperature are in the order: EGME > MAE > EGEE > EGBE. The density values decrease with an increase in temperature for pure components as well as liquid binary blends. Moreover, it can be seen that the densities increase with the increasing mass fraction of MAE in the blends of MAE with EGEE or EGBE, whereas they decrease in the blends of MAE with EGME. The excess molar volume VE of the binary mixtures has been calculated from the experimental density values of pure components and binary blends using eqs 1 and 2. The excess properties can be correlated with a Redlich−Kister equation (eqs 3 and 4) as follows:

accuracy of 0.01 K. Six measurements without the refilling cell were carried out to report the viscosity data in average. S3 and N26 viscosity standards were used to calibrate the viscometer before daily measurement. The relative uncertainty for viscosity measurements was found to be within 5%.

3. RESULTS AND DISCUSSION Experimental densities and viscosities, obtained from the measurements for liquid pure compounds and standard samples, have been presented in the Supporting information (SI) Table S1 together with the literature values.32−52 We have found a good agreement between the experimental density data obtained in this work and the literature data for all chemicals with an average absolute relative deviation (AARD) within 0.20%, as shown in Figure 1. Among them, large deviations of density are from EGEE compared with the work of Carvalho et al.,40 which may be due to the difference of water content and purity for this solvent. For viscosity measurements, we have found negligible deviations (about 0.12% AARD) between our data and the reference data for standard S3 and N26 samples. It can also be seen from Figure 2 that viscosity data of other pure chemicals in the present work are in lines with most of the published data with AARD within 2.0%. However, significant deviations (>5.0%) were also observed for several references. The difference of measuring methods and the materials used may result in the observed D



Article

Ak = ak 0 + ak1 (T − 273.15)

where ρ is the density of the binary mixtures of MAE with alkoxyethanols, g cm−3; V and VE are the mole volume and excess molar volume of binary mixtures, respectively; Vi, xi, and Mi are the molar volume, mole fraction, and molecular weight for component i (i.e., 1 stands for MAE and 2 stands for EGME, EGEE, or EGBE), respectively. The number of coefficients k used in eq 3 for each mixture system was determined by applying an F-test at a confidence level of 99.5%. Ak are the adjustable parameters, which are dependent on temperature. In this work, it assumes that each Ak has a linear relationship with temperature by fitting parameters ak using the least-squares method to give a goodness of fit test by the so-called chi-square minimization.53−55 The calculated data of VE for all of the three systems at different temperatures are presented in SI Table S2, and the representative curves are shown in Figure 4. The excess molar volume VE is negative for all of the three systems over the whole mole fraction range of MAE at all of the experimental temperatures and becomes more negative with an increase in temperature for each binary system. The VE can represent the nonideal behavior of binary mixtures, which mainly depends on the intermolecular interactions and the structural packing effects from interstitial accommodation between components due to their differences in free volumes and molar volumes.53,54 The negative VE indicates that the volumes of binary mixtures are less than the weighted mole-fraction averaging of the molar volume of the pure components, possibly due to strong intermolecular hydrogen bond interaction between MAE and alkoxyethanol molecules having a OH group and ether oxygen. It is worth noting that the minimum VE for the blends of MAE + EGME is about x1 ≈ 0.33, whereas it is x1 ≈ 0.40 for the other mixtures, where the most closest intermolecular bonding takes place. It is also found that the effect of temperature on VE values for the binary blends of MAE(1) + EGEE or EGBE(2) is much greater than that for the blends of MAE(1) + EGME(2), especially for the blends at x1 = 0.30−0.50. This may be attributed to the occupation of intermolecular voids by some MAE molecules among the alkoxyethanol molecules with a long carbon chain, resulting in reducing the volume of blend mixtures to a larger extent. The parameters in the Redlich−Kister equation from all experimental densities obtained by the least-squares regression method are listed in Table 3. The calculated results of density and VE are also presented as solid lines along with the experimental data in Figures 3 and 4. It can be found that the calculated and the experimental data match very well and the average absolute relative deviation (AARD, %) is within 0.01%, as shown in the SI Tables S3−S5. 3.2. Viscosity. The measured viscosity values of three binary mixtures at different temperatures and the whole mass fraction range of MAE are listed in Table 4. An approximate linear increase in viscosity is observed with increasing mass fraction of MAE, whereas an exponential decrease with increasing temperature is observed, as shown in Figure 5. It is found that the viscosities of pure components at a constant temperature are in the order: MAE > EGBE > EGEE > EGME. The addition of alkoxyethanol solvents into MAE can significantly decrease the viscosity of the systems, which facilitates the mass transfer as a chemical absorbent. This phenomenon may also show the existence of molecular interactions in the studied binary blends at all temperatures.

Figure 3. Density of mixture system solutions at different temperatures T and mass fractions of MAE w1: (a) MAE + EGME mixtures, (b) MAE + EGME mixtures, and (c) MAE + EGME mixtures. w1: ■, 0%; ●, 10%; ▲, 30%; ▼, 40%; ◆, 50%; ◀, 70%; ▶, 90%; and ★, 100%; solid lines: calculated from eqs 1 to 4.

x1M1 + x 2M 2 V

(1)

V E = V − (x1V1 + x 2V2)

(2)

ρ=

n

V E = x1x 2 ∑ Ak (x1 − x 2)k k=0

(4)

(3) E


Figure 4. Excess molar volume VE against mole fraction of MAE (x1) for MAE + EGME (a), MAE + EGEE (b), and MAE + EGBE (c) binary mixtures. Symbols: experimental results; solid lines: calculated values from eq 3.

The viscosity can be expressed using eqs 5−8, and viscosity deviations (Δη) can also be calculated from the experimental data of pure components and binary systems, similar to the VE calculation.

η = x1η1 + x 2η2 + Δη

(5)

Δη = η − (x1η1 + x 2η2)

(6)

F

a

AARD(%) =

A4

A3

A2

1 n

n

∑i = 1

ρi

− ρi

ρi exp

exp

mod

a00 a01 a10 a11 a20 a21 a30 a31 a40 a41

A0

A1

temperature parameters

parameters

× 100%.

0.020 3.80 × 0.063 1.19 × 0.240 4.50 × 0.150 2.82 × 0.371 6.94 ×

−0.664 −3.69 × 10−3 0.449 2.46 × 10−3 −0.948 −7.76 × 10−3 −0.425 −3.03 × 10−3 1.392 1.11 × 10−2 10−3

10−3

10−3

10−3

10−4

standard error

values derived 0.9943

R

2

MAE(1) + EGME(2) system 0.0060

AARD/%

a

−0.712 −7.51 × 10−3 7.50 × 10−2 2.90 × 10−3 −0.266 −1.34 × 10−3 −0.318 −2.35 × 10−3

values derived

7.23 × 1.35 × 3.21 × 6.01 × 3.61 × 6.76 × 0.075 1.40 ×

10−3

10 10−4 10−2 10−4 10−2 10−4

−3

standard error

0.9990

R

2

MAE(1) + EGEE(2) system 0.0027

AAD/%

a

−0.577 −7.84 × 10−3 0.261 8.99 × 10−4 −0.110 −2.45 × 10−3 −0.502 2.95 × 10−3

values derived

Table 3. Fitted Parameters of Density (ρ) in eqs 1−4 for MAE(1) + EGME(2), MAE(1) + EGEE(2), and MAE(1) + EGBE(2) Systems

7.07 1.32 3.63 6.80 3.46 6.48 8.46 1.58

× × × × × × × ×

10 10−4 10−2 10−4 10−2 10−4 10−2 10−3

−3

standard error

0.9987

R2

MAE(1) + EGBE(2) system 0.0023

AARD/%a

Journal of Chemical & Engineering Data Article



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Table 4. Experimental Data of Viscosity η for MAE(1) + EGME(2), MAE(1) + EGEE(2), and MAE(1) + EGBE(2) Binary Blends at Mass Fractions of MAE w1, Temperature T, and Pressure P = 101.3 kPaa w1 %

T/K 293.15

298.15

303.15

308.15

313.15

318.15

323.15

328.15

333.15

338.15

343.15

348.15

353.15

0.9016 1.0534 1.4464 1.6543 1.9198 2.4926 3.2085 3.6187

0.8367 0.9708 1.3126 1.4915 1.7169 2.2022 2.7980 3.1344

0.7788 0.8975 1.1958 1.3507 1.5447 1.9581 2.4556 2.7358

0.7270 0.8322 1.0936 1.2281 1.3952 1.7485 2.1696 2.4042

0.6810 0.7743 1.0036 1.1208 1.2649 1.5703 1.9280 2.1262

0.6397 0.7223 0.9241 1.0263 1.1516 1.4153 1.7215 1.8893

1.0163 1.1952 1.6075 1.8339 2.0911 2.6491 3.2931 3.6187

0.9358 1.0927 1.4494 1.6433 1.8621 2.3319 2.8656 3.1344

0.8646 1.0021 1.3125 1.4795 1.6669 2.0653 2.5120 2.7358

0.8014 0.9231 1.1934 1.3380 1.4992 1.8394 2.2181 2.4042

0.7452 0.8526 1.0890 1.2148 1.3545 1.6467 1.9674 2.1262

0.6952 0.7896 0.9970 1.1071 1.2283 1.4807 1.7563 1.8893

1.4333 1.6665 2.1291 2.3515 2.5680 2.9892 3.4048 3.6187

1.3025 1.5035 1.8983 2.0861 2.2678 2.6186 2.9594 3.1344

1.1887 1.3631 1.7005 1.8606 2.0144 2.3090 2.5913 2.7358

1.0889 1.2402 1.5306 1.6680 1.7989 2.0477 2.2842 2.4042

1.0012 1.1331 1.3841 1.5025 1.6143 1.8263 2.0255 2.1262

0.9235 1.0389 1.2563 1.3585 1.4550 1.6369 1.8061 1.8893

η/mPa s 0 10.0 30.0 40.0 50.0 70.0 90.0 100

1.6855 2.0860 3.2825 3.9846 4.9641 7.3639 10.982 13.356

1.5218 1.8656 2.8687 3.4442 4.2337 6.1294 8.8772 10.634

1.3805 1.6775 2.5230 3.0002 3.6445 5.1567 7.2860 8.6122

1.2578 1.5156 2.2335 2.6317 3.1626 4.3844 6.0536 7.0700

1.151 1.3755 1.9883 2.3239 2.7662 3.7621 5.0880 5.883

0 10.0 30.0 40.0 50.0 70.0 90.0 100

2.0397 2.5475 3.8867 4.7081 5.7205 8.1899 11.494 13.356

1.8189 2.2491 3.3558 4.0209 4.8277 6.7513 9.2524 10.634

1.6309 1.9982 2.9204 3.4644 4.1155 5.6360 7.5651 8.6122

1.4703 1.7851 2.5604 3.0095 3.5406 4.7600 6.2672 7.0700

1.3323 1.6036 2.2600 2.6339 3.0707 4.0578 5.2549 5.8830

0 10.0 30.0 40.0 50.0 70.0 90.0 100

3.2477 4.0158 5.7477 6.6716 7.6323 9.7129 12.071 13.356

2.834 3.4690 4.8618 5.5908 6.3415 7.9292 9.6737 10.634

2.491 3.0199 4.1584 4.7392 5.3311 6.5635 7.8933 8.6122

2.2044 2.6487 3.5843 4.0536 4.5277 5.4982 6.5251 7.0700

1.9645 2.3387 3.1121 3.4998 3.8831 4.6567 5.4611 5.883

MAE(1) + EGME(2) 1.0571 0.9746 1.2537 1.1471 1.7798 1.6010 2.0640 1.8429 2.4347 2.1559 3.2565 2.8395 4.3231 3.7086 4.9526 4.2143 MAE(1) + EGEE(2) 1.2122 1.1078 1.4476 1.3125 2.0061 1.7914 2.3204 2.0569 2.6844 2.3617 3.4925 3.0302 4.4532 3.8119 4.9526 4.2143 MAE(1) + EGBE(2) 1.7593 1.5843 2.0769 1.8559 2.7255 2.4013 3.0447 2.6672 3.3581 2.9264 3.9830 3.4364 4.6208 3.9482 4.9526 4.2143

a

Standard uncertainties u are u(T) = 0.02 K, u(P) = 0.5 kPa, the combined expanded uncertainty Uc is Uc(w1) = 0.002, and the relative expanded uncertainty Uc,r is Uc,r(η) = 0.05, with a 0.95 level of confidence (k = 2). w1 is referred to as the mass fraction of MAE in liquid binary mixtures, and w1 = 100% denotes the pure MAE from the supplier. n

Δη = x1x 2 ∑ Bi (x1 − x 2)i i=0

alkoxyethanol molecules decreases. The absolute values of Δη are in the order: MAE + EGME > MAE + EGEE > MAE + EGBE. The negative Δη values also indicate the presence of dispersion forces or weak interactions between the unlike molecules in these mixtures.54,55 MAE has strong selfassociation through intermolecular hydrogen bonding, resulting in high viscosity, and alkoxyethanols may undergo less selfassociation, especially for EGME. The decreasing viscosity of the mixtures suggests the weakening of self-association of MAE in alkoxyethanols. These deviations in alkoxyethanol solvents can also be ascribed to structural effects. The viscosity deviations (Δη) have been fitted to the Redlich−Kister polynomials (eqs 7 and 8). The regression parameters are summarized in Table 5. The calculated results of viscosity and Δη are also presented as solid lines along with the experimental data in Figures 5 and 6. A good agreement is observed. It can also be seen from the SI Tables S3−S5 that the AARDs for all systems are within 1.23%.

(7)

3

Bi =

∑ bij (T − 273.15) j j=0

(8)

where η is the viscosity of binary mixtures of MAE with alkoxyethanols, mPa s; ηi and xi are the viscosity and mole fraction for component i (i.e., 1 stands for MAE and 2 stands for EGME, EGEE, or EGBE), respectively. It is noted that the number of coefficients (i and j) used in eqs 7 and 8 for each mixture system was determined by applying an F-test at a confidence level of 99.5%. The adjustable parameters Bi in the Redlich−Kister equation are assumed to be dependent on the power of a temperature-related polynomial. The parameters bij were assessed by chi-square minimization to optimize the fit. The calculated data of Δη for three binary systems are also presented in the SI Table S2, and the representative plots are shown in Figure 6. The values of Δη are found to be negative for all of the systems over the whole mole fraction range of MAE at the investigated temperatures and show a negative minimum at around x1 ≈ 0.60 for all of the mixtures. The curves of Δη against x1 become more flat with increasing temperature for each binary system. It is worth noting that the more negative Δη as the carbon chain length of the

4. CONCLUSIONS Experimental data for the density and viscosity of three binary systems (MAE + EGME, EGEE, or EGBE) have been measured over the temperature range of 293.15−353.150 K and composition range from 0 to 100 mass % MAE. The calculated VE and Δη values are negative over the entire range of composition at the experimental temperature range. The VE G



Article

Figure 6. Plot of deviation in viscosity (Δη) versus mole fraction of MAE(x1) for MAE + EGME (a), MAE + EGEE (b), and MAE + EGBE (c) binary mixtures. Symbols: experimental results; solid lines: calculated results from eq 7.

Figure 5. Viscosity of mixture system solutions at different temperatures T and mass fractions of MAE w1: (a) MAE + EGME mixtures, (b) MAE + EGEE mixtures, and (c) MAE + EGBE mixtures. w1: ■, 0%; ●, 10%; ▲, 30%; ▼, 40%; ◆, 50%; ◀, 70%; ▶, 90%; and ★, 100%; solid lines: calculated from eqs 5to 8.

structural packing between unlike molecules. The calculated density and viscosity values from the proposed Redlich−Kister correlations as a function of temperature and mole fraction of components were in good agreement with the experimental data, with AARD values within 0.1 and 1.23%, respectively. These fundamental data will be useful for developing alkoxyethanol-based absorbents in carbon capture processes.

values become more negative, whereas the Δη values become less negative with increasing temperature. The excess and deviation properties also vary with the increasing carbon chain length of the alkoxyethanol molecules. The reason for the strong dependence of excess and deviation properties on temperature is probably due to intermolecular interactions and H


a

AARD(%) =

B3

B2

1 n

n

∑i = 1

ηi exp

ηi exp − ηi mod

b00 b01 b02 b03 b10 b11 b12 b13 b20 b21 b22 b23 b30 b31 b32 b33

B0

B1

temperature parameters

parameters

× 100%.

0.37 2.59 × 5.53 × 3.66 × 1.46 0.102 2.18 × 1.45 × 1.80 0.126 2.69 × 1.78 × 3.47 0.243 5.19 × 3.44 ×

−26.91 1.14 −1.70 × 10−2 8.69 × 10−5 −10.60 0.491 −7.75 × 10−3 4.08 × 10−5 −3.32 0.162 −2.67 × 10−3 1.46 × 10−5 1.25 −3.27 × 10−2 2.70 × 10−4 −5.42 × 10−7 10−3 10−5

10−3 10−5

10−3 10−5

10−4 10−4 10−6

standard error

values derived

MAE(1) + EGME(2) system 0.9983

R

2

1.23

AARD/%

a

−25.14 1.06 −1.59 × 10−2 8.08 × 10−5 −9.30 0.427 −6.72 × 10−3 3.54 × 10−5 −0.785 5.35 × 10−2 −1.01 × 10−3 5.90 × 10−6 3.22 −0.123 1.71 × 10−3 −8.26 × 10−6

values derived 0.33 0.02 4.92 × 3.26 × 1.46 0.102 2.19 × 1.45 × 1.643 0.115 2.46 × 1.63 × 3.42 0.239 5.11 × 3.39 × 10−3 10−5

10−3 10−5

10−3 10−5

10−4 10−6

standard error 0.9983

R

2

MAE(1) + EGEE(2) system 0.65

AARD/%

a

−18.33 0.801 −1.22 × 10−2 6.28 × 10−5 −6.99 0.322 −5.07 × 10−3 2.67 × 10−5 −2.47 0.109 −1.66 × 10−3 8.54 × 10−6 1.30 −6.03 × 10−2 9.95 × 10−4 −5.56 × 10−6

values derived 0.26 0.018 3.90 × 2.58 × 1.34 0.094 2.00 × 1.33 × 1.28 0.089 1.91 × 1.26 × 3.12 0.218 4.67 × 3.09 × 10−3 10−5

10−3 10−5

10−3 10−5

10−4 10−6

standard error 0.9978

R2

MAE(1) + EGBE(2) system

Table 5. Fitted Parameters of Viscosity (η) in eqs 5−8 for MAE(1) + EGME(2), MAE(1) + EGEE(2), and MAE(1) + EGBE(2) Systems

0.44

AARD/%a

Journal of Chemical & Engineering Data Article

I



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Article

(12) Shen, S.; Bian, Y.; Zhao, Y. Energy-efficient CO2 capture using potassium prolinate/ethanol solution as a phase-changing absorbent. Int. J. Greenhouse Gas Control 2017, 56, 1−11. (13) Tao, M.; Gao, J.; Zhang, W.; Li, Y.; He, Y.; Shi, Y. A novel phase-changing nonaqueous solution for CO2 capture with high capacity, thermostability, and regeneration efficiency. Ind. Eng. Chem. Res. 2018, 57, 9305−9312. (14) Li, Y.; Cheng, J.; Hu, L.; Liu, J.; Zhou, J.; Cen, K. Phasechanging solution PZ/DMF for efficient CO2 capture and low corrosiveness to carbon steel. Fuel 2018, 216, 418−426. (15) Guo, H.; Li, H.; Shen, S. CO2 capture by water-lean amino acid salts: absorption performance and mechanism. Energy Fuels 2018, 32, 6943−6954. (16) Heldebrant, D. J.; Koech, P. K.; Ang, M. T. C.; Liang, C.; Rainbolt, J. E.; Yonker, C. R.; Jessop, P. G. Reversible zwitterionic liquids, the reaction of alkanol guanidines, alkanol amidines, and diamines with CO2. Green Chem. 2010, 12, 713−721. (17) Barzagli, F.; Mani, F.; Peruzzini, M. A comparative study of the CO2 absorption in some solvent-free alkanolamines and in aqueous monoethanolamine (MEA). Environ. Sci. Technol. 2016, 50, 7239− 7246. (18) Barzagli, F.; Lai, S.; Mani, F. A novel class of single-component absorbents for reversible CO2 capture under mild conditions. ChemSusChem 2015, 8, 184−191. (19) Barzagli, F.; Lai, S.; Mani, F.; Stoppioni, P. Novel non-aqueous amine solvents for biogas upgrading. Energy Fuels 2014, 28, 5252− 5258. (20) Sada, E.; Kumazawa, H.; Han, Z. Q.; Matsuyama, H. Chemical kinetics of the reaction of carbon dioxide with ethanolamines in nonaqueous solvents. AIChE J. 1985, 31, 1297−1303. (21) Fu, K.; Rongwong, W.; Liang, Z.; Na, Y.; Idem, R.; Tontiwachwuthikul, P. Experimental analyses of mass transfer and heat transfer of post-combustion CO2 absorption using hybrid solvent MEA−MeOH in an absorber. Chem. Eng. J. 2015, 260, 11−19. (22) Yu, Y. S.; Lu, H. F.; Zhang, T. T.; Zhang, Z. X.; Wang, G. X.; Rudolph, V. Determining the performance of an efficient nonaqueous CO2 capture process at desorption temperatures below 373 K. Ind. Eng. Chem. Res. 2013, 52, 12622−12634. (23) Chen, S.; Chen, S.; Zhang, Y.; Qin, L.; Guo, C.; Chen, J. Species distribution of CO2 absorption/desorption in aqueous and non-aqueous N-ethylmonoethanolamine solutions. Int. J. Greenhouse Gas Control 2016, 47, 151−158. (24) Liu, F.; Jing, G.; Zhou, X.; Lv, B.; Zhou, Z. Performance and mechanisms of triethylene tetramine (TETA) and 2-amino-2-methyl1-propanol (AMP) in aqueous and nonaqueous solutions for CO2 capture. ACS Sustainable Chem. Eng. 2018, 6, 1352−1361. (25) Guo, C.; Chen, S. Y.; Zhang, Y. C.; Wang, G. B. Solubility of CO2 in nonaqueous absorption system of 2-(2-aminoethylamine)ethanol + benzyl alcohol. J. Chem. Eng. Data 2014, 59, 1796−1801. (26) Zheng, C.; Tan, J.; Wang, Y. J.; Luo, G. S. CO2 Solubility in a mixture absorption system of 2-amino-2-methyl-1-propanol with glycol. Ind. Eng. Chem. Res. 2012, 51, 11236−11244. (27) Barbarossa, V.; Barzagli, F.; Mani, F.; Lai, S.; Stoppioni, P.; Vanga, G. Efficient CO2 capture by non-aqueous 2-amino-2-methyl-1propanol (AMP) and low temperature solvent regeneration. RSC Adv. 2013, 3, 12349−12355. (28) Barzagli, F.; Giorgi, C.; Mani, F.; Peruzzini, M. Reversible carbon dioxide capture by aqueous and non-aqueous amine-based absorbents: A comparative analysis carried out by 13C NMR spectroscopy. Appl. Energy 2018, 220, 208−219. (29) Murshid, G.; Ghaedi, H.; Ayoub, M.; Mjalli, F. S.; Garg, S. Volumetric properties of non-aqueous binary mixture of diethanolamine (DEA) and dimethylformamide (DMF). J. Environ. Chem. Eng. 2018, 6, 6390−6398. (30) Xiao, M.; Liu, H.; Gao, H.; Olson, W.; Liang, Z. CO2 capture with hybrid absorbents of low viscosity imidazolium-based ionic liquids and amine. Appl. Energy 2019, 235, 311−319. (31) CRC Handbook of Chemistry and Physics, 90th ed. (CD-ROM, version 2010); Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2010.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00364. Comparison of the density and viscosity of S3, N26, pure EGME, EGEE, EGBE, and MAE between experimental and literature data; the calculated values of excess molar volume, viscosity deviations, density, and viscosity for binary mixtures of MAE(1) + EGME(2), MAE(1) + EGEE(2), and MAE(1) + EGBE(2) at various mole fractions of MAE and temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Tel: +86 311 88632183. Fax: +86 311 88632183. ORCID

Shufeng Shen: 0000-0003-0625-133X Funding

The authors would like to acknowledge the Key Program of Hebei Provincial Natural Science Foundation (Grant No. B2018208154), Training Program for Talent Engineers of Hebei Province (Grant No. A2017002022), and Program for Hundred Outstanding Innovation Talents in Hebei Universities (SLRC2019051) for financial support. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Vitillo, J. G.; Smit, B.; Gagliardi, L. Introduction: Carbon capture and separation. Chem. Rev. 2017, 117, 9521−9523. (2) Mumford, K. A.; Wu, Y.; Smith, K. H.; Stevens, G. W. Review of solvent based carbon-dioxide capture technologies. Front. Chem. Sci. Eng. 2015, 9, 125−141. (3) MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C. S.; Williams, C. K.; Shah, N.; Fennell, P. An overview of CO2 capture technologies. Energy Environ. Sci. 2010, 3, 1645−1669. (4) Sanz-Pérez, E. S.; Murdock, C. R.; Didas, S. A.; Jones, C. W. Direct Capture of CO2 from Ambient Air. Chem. Rev. 2016, 116, 11840−11876. (5) Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652−1654. (6) Budzianowski, W. M. Explorative analysis of advanced solvent processes for energy efficient carbon dioxide capture by gas−liquid absorption. Int. J. Greenhouse Gas Control 2016, 49, 108−120. (7) Guo, H.; Li, C. X.; Shi, X. Q.; Li, H.; Shen, S. F. Nonaqueous amine-based absorbents for energy efficient CO2 capture. Appl. Energy 2019, 239, 725−734. (8) Heldebrant, D. J.; Koech, P. K.; Glezakou, V.-A.; Rousseau, R.; Malhotra, D.; Cantu, D. C. Water-lean solvents for post-combustion CO2 capture: fundamentals, uncertainties, opportunities, and outlook. Chem. Rev. 2017, 117, 9594−9624. (9) Vega, F.; Sanna, A.; Navarrete, B.; Maroto-Valer, M. M.; Cortés, V. J. Degradation of amine-based solvents in CO2 capture process by chemical absorption. Greenhouse Gases: Sci. Technol. 2014, 4, 707− 733. (10) Wang, L. D.; Yu, S. H.; Li, Q. W.; Zhang, Y. F.; An, S. L.; Zhang, S. H. Performance of sulfolane/DETA hybrids for CO2 absorption: Phase splitting behavior, kinetics and thermodynamics. Appl. Energy 2018, 228, 568−576. (11) Barzagli, F.; Mani, F.; Peruzzini, M. Novel water-free biphasic absorbents for efficient CO2 capture. Int. J. Greenhouse Gas Control 2017, 60, 100−109. J



Article

(32) Guo, H.; Li, H.; Shen, S. Monoethanolamine+2-methoxyethanol mixtures for CO2 capture: Density, viscosity and CO2 solubility. J. Chem. Thermodyn. 2019, 132, 155−163. (33) Li, X. X.; Zhang, M.; Wu, X. X.; Wang, Y. W.; Fan, G. C.; Ma, X. X. Thermodynamic and spectroscopic studies for aqueous solution of ethylene glycol monomethyl ether. J. Mol. Liq. 2011, 158, 92−96. (34) Awasthi, A.; Awasthi, A. Intermolecular interactions in formamide + 2-alkoxyethanols: Viscometric study. Thermochim. Acta 2012, 537, 57−64. (35) Ć wikliń s ka, A.; Kinart, C. M. Thermodynamic and physicochemical properties of binary mixtures of nitromethane with {2-methoxyethanol + 2-butoxyethanol} systems at T = (293.15, 298.15, 303.15, 308.15, and 313.15) K. J. Chem. Thermodyn. 2011, 43, 420−429. (36) Begum, Z.; Sandhya Sri, P. B.; Karuna Kumar, D. B.; Rambabu, C. Thermodynamic, ultrasonic and FT-IR studies on binary liquid mixtures of anisaldehyde and alkoxyethanols at different temperatures. J. Mol. Liq. 2013, 178, 99−112. (37) Cocchi, M.; Marchetti, A.; Sanna, G.; Tassi, L.; Ulrici, A.; Vaccari, G. Kinematic viscosities of ternary mixtures containing ethane-1, 2-diol, 2-methoxyethanol and water from −10 °C to 80 °C. Fluid Phase Equilib. 1999, 157, 317−342. (38) Ohji, H.; Ogawa, H.; Murakami, S.; Tamura, K.; Grolier, J. E. Excess volume and excess thermal expansivities for binary mixtures of 2-ethoxyethanol with non-polar solvents at temperatures between 283.15 K and 328.15 K. Fluid Phase Equilib. 1999, 156, 101−114. (39) Haldar, P.; Das, B. Viscosities of some tetraalkyammonium bromides in 2-ethoxyethanol at 308.15 K. Can. J. Chem. 2005, 83, 499−504. (40) Carvalho, P. J.; Fonseca, C. H. G.; Moita, M. C. J.; Santos, A. F. S.; Coutinho, J. A. P. Thermophysical properties of Glycols and Glymes. J. Chem. Eng. Data 2015, 60, 3721−3737. (41) Islam, M. N.; Monirul, M.; Yeasmin, M. N. Viscosity of aqueous solutions of 2-methoxyethanol, 2-ethoxyethanol and ethanolamine. J. Chem. Thermodyn. 2004, 36, 889−893. (42) Sharma, B.; Sharma, R.; Das, C. Conductance studies of tetrabutylammonium bromide, sodium, bromide and sodium tetraphenylborate in 2-butoxyethanol (1) + water (2) mixture at (298.15, 303.15, 308.15 and 313.13)K. J. Chem. Eng. Data 2012, 57, 3375−3384. (43) Chiou, D. R.; Chen, S. Y.; Chen, L. J. Density, viscosity and refractive index for water + 2-butoxyethanol and + 2- (2butoxyethoxy)ethanol at various temperatures. J. Chem. Eng. Data 2010, 55, 1012−1016. (44) George, J.; Sastry, N. V. Densities, excess molar volumes at T = (298.15 to 313.15) K, speeds of sound, excess isentropic compressibilities, relative permittivities and deviations in molar polarizations at T = (298.15 and 308.15) K for methyl methacrylate + 2-bbutoxyethanol or dibutylether +benzene, toluene, or p-Xylene. J. Chem. Eng. Data 2004, 49, 1116−1126. (45) MacNeil, J. H.; Palepu, R. Viscosities and densities of binary liquid mixtures of 2-butoxyethanol with aniline and N-Alkylaniline. Thermochim. Acta 1989, 149, 275−286. (46) George, J.; Sastry, N. V. Densities, excess molar volumes, viscosities, speed of sound, excess isentropic compressibilities and relative permittivities for CmH2m+1 (OCH2CH2)nOH (m = 1 or 2 or 4 and n = 1) + benzene + toluene + (o−, m− and p−) xylenes + ethylbenzene and cyclohexane. J. Chem. Eng. Data 2003, 48, 977− 989. (47) García-Abuín, A.; Gomez-Diaz, D.; Navaza, J. M.; Rumbo, A.; Yaşaroğlu, O. Density, speed of sound, viscosity and excess properties of N-Ethyl-2-pyrrolidone + 2-(Methylamino)ethanol [or 2(Ethylamino)ethanol] from T = (293.15 to 323.15) K. J. Chem. Eng. Data 2015, 60, 795−800. (48) Maham, Y.; Teng, T. T.; Hepler, L.; Mather, A. Volumetric properties of aqueous solutions of monoethanolamine, mono- and dimethylethanolamines at temperatures from 5 to 80 °C I. Thermochim. Acta 2002, 386, 111−118.

(49) Venkat, A.; Kumar, G.; Kundu, M. Density and surface tension of aqueous solutions of (2- (Methylamino)- ethanol + 2-Amino-2methyl-1-propanol) and (2-(Methylamino)-ethanol + N-Methyldiethanolamine)from (298.15 to 323.15)K. J. Chem. Eng. Data 2010, 55, 4580−4585. (50) Kumar, S.; Sagar, S.; Gupta, M. A thermodynamic and 1H NMR spectroscopy study of binary mixtures of polyethlene glycol butyl ether (PEGBE) 206 with 1-butanol and 2-(Methylamino)ethanol (MAE). J. Mol. Liq. 2016, 214, 306−312. (51) Á lvarez, E.; Gomez-Diaz, D. G.; Dolores La Rubia, A.; Navaza, J. Densities and viscosities of aqueous ternary mixtures of 2(Methylamino)ethanol and 2-(Ethylamino)ethanol with Diethanolamine, Triethanolamine, N-Methyldiethanolamine or 2-Amino-1propanol from 298.15 to 323.15 K. J. Chem. Eng. Data 2006, 51, 955−962. (52) Li, J. L.; Mundhwa, M.; Tontiwachwuthikul, P.; Henni, A. Volumetric properties, viscosities and refractive indices for aqueous 2(Methylamino)ethanol solutions from (298.15 to 343.15) K. J. Chem. Eng. Data 2007, 52, 560−565. (53) Zhao, T. X.; Zhang, J. B.; Guo, B.; Zhang, F.; Sha, F.; Xie, X. H.; Wei, X. H. Density, viscosity and spectroscopic studies of the binary system of ethylene glycol + dimethyl sulfoxide at T = (295.15 to 323.15)K. J. Mol. Liq. 2015, 207, 315−322. (54) Roy, M. N.; Sinha, B.; Dakua, V. K. Excess molar volumes, viscosity deviations and ultrasonic speeds of sound of binary mixtures of 2-butanone with some alkoxyethanols and amines at 298.15 K. J. Mol. Liq. 2007, 136, 128−137. (55) Ouaar, F.; Negadi, N.; Bahadur, I.; Phadagi, R.; FeddalBenabed, B.; Negadi, L. Volumetric, acoustic and transport properties of mixtures containing dimethyl sulfoxide and some amines or alkanolamines: Measurement and correlation. J. Chem. Thermodyn. 2018, 121, 187−198.

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Density, Viscosity, and Excess Properties of Binary Mixtures of 2

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