Density and Volumetric Properties of Aqueous Solutions of

Article pubs.acs.org/jced

Density and Volumetric Properties of Aqueous Solutions of Trimethylamine N‑Oxide in the Temperature Range from (278.15 to 323.15) K and at Pressures up to 100 MPa Dmitriy M. Makarov,* Gennadiy I. Egorov, and Arkadiy M. Kolker G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Ivanovo 153045, Russia S Supporting Information *

ABSTRACT: Densities of aqueous solutions of trimethylamine N-oxide (TMAO) were measured over the concentration range (0.0875 to 4.3251) mol·kg−1 at temperatures (278.15, 288.15, 298.15, 308.15, and 323.15) K and pressures (0.101, 10, 25, 50, 75, and 100) MPa. Volumetric properties such as apparent molar volume of TMAO, Vϕ,2, molar isothermal compression, KT,m, molar isobaric expansion, EP,m, and internal pressure, Pint, of its aqueous solutions were calculated depending on concentration, temperature, and pressure. The volumetric partial properties of ∞ ∞ TMAO at infinite dilution in water (V∞ 2 , KT,2, and EP,2) were also determined. The results were discussed from the standpoint of solute−solute and solute−solvent interactions.

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INTRODUCTION High concentrations of organic osmolytes (tens and hundreds of mmol·L−1) enter into the composition of the cytosol of all living organisms from bacteria to human. The osmolytes are of great importance for cellular volume homeostasis and are able to cancel the influence of environmental stressors.1 Depending on the osmolytes’ nature, they have different effects on protein stability, that is, the osmolyte can cause either protein folding or unfolding. Trimethylamine N-oxide (TMAO) is an organic osmolyte inhibiting, as it was shown in vitro, the proteins unfolding in the presence of chemical denaturants,2 under the influence of high or low temperatures,3,4 and particularly efficiently under high pressures.5−7 For example, living organisms inhabiting the depth of the ocean experience extremely high hydrostatic pressure up to 100 MPa. Organic substances contributing to normal biochemical functioning of the cell under such severe conditions are called ≪piezolytes≫. In vivo it was shown8,9 that TMAO belonged to piezolytes and its concentration in the cells of marine organisms increased linearly with the ocean depth. At present, there are different views on the molecular mechanism of proteins stabilization in the presence of TMAO.10 However, recently, two opposite mechanisms are mainly under consideration: direct and indirect. It is well known that TMAO molecules are mainly excluded from the protein surface.11 According to indirect mechanism TMAO stabilizes proteins by the alteration of water structure.2,12−14 Conversely, the direct mechanism implies the immediate interaction of the osmolyte with the protein15 or unfavorable interactions with the protein backbone.16−18 At present, the © XXXX American Chemical Society

mechanism of protein stabilization by TMAO, a fortiori under high hydrostatic pressures, is still unclear. The above examples underline the importance of understanding of the intermolecular interactions of TMAO with water and getting insight into structural peculiarities of their solutions for explanation for stabilizing affect of this osmolyte.19 It should be noted that the data on densities and volumetric properties of TMAO aqueous solutions are rather limited. Earlier, the densities of these solutions over wide concentration range were obtained at T = 298.15 K only.19−22 In the work by Krakowiak et al.,23 the temperature dependences of some volumetric properties at infinite dilution were under investigation. In literature sources, there are no data on the volumetric properties of the solutions at high pressures at all.

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EXPERIMENTAL SECTION Trimethylamine N-oxide dihydrate (assay ≥0.99 mass fraction, Fluka) was used without any additional purification. Double distilled and degassed water with conductivity less than 1.5 × 10−6 S·cm−1 was used for solutions preparation. Aqueous binary solutions within the concentration range (0.0875 to 4.3251) mol·kg−1 were prepared gravimetrically with accuracy of 5 × 10−5 g. The uncertainty of concentration determination was about 1 × 10−4 mol·kg−1. For all measurements only makeup solutions were used. The densities of solutions at atmospheric pressure (p = 0.101 MPa) were measured using Anton Paar DMA 4500 densimeter Received: October 22, 2014 Accepted: March 10, 2015

A

DOI: 10.1021/je500977g J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Densities of TMAO Aqueous Solutions, ρ, at Different Temperatures and p = 0.101 MPaa

with U-shaped vibrating tube. The densimeter was calibrated with dry air and bidistilled water over the temperature range under investigation. The temperature in the measuring cell was controlled by the built-in Peltier thermostat. The density of every solution was measured at least three times and the reproducibility of the measurements is 1 × 10−5 g·cm−3 for the density and 0.01 K for the temperature. The density measurements were carried out at five temperatures (278.15, 288.15, 298.15, 308.15, and 323.15) K. The combined uncertainty of density determination was not lower than 3 × 10−5 g·cm−3. The measurements of compressibility coefficients, k, were conducted with a device described earlier24 and using modified Adam’s piezometer. The piezometer volume was calibrated at p = 0.101 MPa and T = 298.15 K using a well-known value for water density.25 The repeatability of k values in a set of four measurements for water at 100 MPa was within 5 × 10−5. The compressibility coefficients were determined at five different pressures (10, 25, 50, 75, 100) MPa and over the temperature range (278.15 to 323.15) K. The pressure was measured with pressure gauge MP-2500 (Shatkovsky Instrument Factory, Russia) with an uncertainty better than 0.002·p. The uncertainty in temperature was 0.02 K. Maximum combined uncertainty of k determination did not exceed 1 × 10−4. The detailed description of the work technique was performed earlier.24,26

mol·kg

288.15 K

298.15 K

308.15 K

323.15 K

0.99996 1.00016 1.00036 1.00055 1.00075 1.00095 1.00148 1.00211 1.00289 1.00401 1.00605 1.01169 1.01641

0.99909 0.99928 0.99946 0.99963 0.99982 1.00000 1.00047 1.00102 1.00171 1.00270 1.00448 1.00940 1.01358

0.99704 0.99722 0.99740 0.99757 0.99774 0.99791 0.99835 0.99886 0.99949 1.00040 1.00201 1.00644 1.01020

0.99403 0.99421 0.99438 0.99454 0.99471 0.99488 0.99531 0.99580 0.99641 0.99727 0.99877 1.00283 1.00627

0.98803 0.98820 0.98837 0.98853 0.98870 0.98886 0.98927 0.98975 0.99034 0.99115 0.99262 0.99637 0.99945

concentration dependences Vϕ,2= f(m), second degree polynomials were fitted to the m dependencies and then extrapolated to zero TMAO concentration at every measured temperature and pressure Vϕ ,2 = V 2∞ + bV m + cV m2

(4)

where bv (limiting slope) and cv are adjustable parameters; they could be considered as volumetric virial coefficients of solute− solute interactions, pairwise and triple ones, accordingly.27,28 Calculated partial molar volumes of TMAO at infinite dilution and adjustable coefficients in eq 4 are presented in Table 3. In parentheses, the standard uncertainties for coefficients bv and cv and the combined expanded uncertainties for V∞ 2 values are shown in Table 3. Thermal Coefficients and Internal Pressure. Molar isobaric expansion was calculated as

(1)

⎛ ∂V ⎞ M ⎛ ∂ρ ⎞ EP , m = ⎜ m ⎟ = αPVm = − 2 ⎜ ⎟ ⎝ ∂T ⎠ P , m ρ ⎝ ∂T ⎠ P , m

Densities of TMAO aqueous solutions at p = 0.101 MPa and T = (278.15 to 323.15) K are shown in Table 1 and densities at the same temperatures but at p = (10 to 100) MPa are listed in Table 2. Apparent molar volumes of TMAO in water, Vϕ,2, were determined from the density data with the following equation: (ρ − ρ1) M2 − 1000 ρ mρρ1

278.15 K

m is the molality of TMAO aqueous solution. Standard uncertainties u are u(m) = 0.0001 mol·kg−1, u(T) = 0.02 K and the combined expanded uncertainty Uc is Uc(ρ) = 0.00003 g·cm−3 (0.95 level of confidence).

where v0, ρ0 and v, ρ are specific volumes and densities at p0 = 0.101 MPa and at p = (10, 25, 50, 75, and 100) MPa, respectively. Thus, the densities at pressures different from the atmospheric were calculated from the following equation: ρ0 ρ= (1 − k) (2)

Vϕ ,2 =

g·cm−3

a

RESULTS Density and Apparent Molar Volume. The compressibility coefficients of TMAO aqueous solutions were determined at constant temperatures and pressures. The expression for the calculation of these coefficients is the following: (ρ − ρ0 ) (v0 − v) = v0 ρ

−1

0.0000 0.0875 0.1707 0.2465 0.3259 0.4018 0.5862 0.7963 1.0392 1.3729 1.9226 3.2591 4.3251

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k=

ρ

m

(5)

where αp is the coefficient of isobaric expansibility, Vm is the molar volume, M is the average molar mass of the solution. EP,m values were calculated as derivatives of the density temperature dependences the polynomials are fitted to the data at every temperature and pressure studied. Molar isothermal compression was calculated with the following expression:

(3)

where ρ1 is the density of water, m is the solution molality, M2 is the solute molar mass. The values of apparent molar volumes of TMAO in water at p = 0.101 MPa and T = (278.15 to 323.15) K are shown in Supporting Information Table S1. Vϕ,2 values at the same temperatures and p = (10 to 100) MPa are presented in Supporting Information Table S2. The limiting partial molar volume of TMAO (or standard molar volume), V∞ 2 , corresponds to its apparent molar volume at infinite dilution, V∞ ϕ,2. The values were obtained from the

⎛ ∂V ⎞ M ⎛ ∂ρ ⎞ = κTVm = 2 ⎜ ⎟ KT , m = −⎜ m ⎟ ρ ⎝ ∂p ⎠T , m ⎝ ∂p ⎠T , m

(6)

where κT is the coefficient of isothermal compressibility. For KT,m calculation ρ = f(p)T,m functions at every temperature and composition the polynomials are fitted to the data and further differentiated. Internal pressure can be expressed as B



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Table 2. Densities of TMAO Aqueous Solutions, ρ, at Different Temperatures and Pressuresa m

p

ρ

mol·kg−1

MPa

g·cm−3

0.0000

10 25 50 75 100 10 25 50 75 100 10 25 50 75 100 10 25 50 75 100 10 25 50 75 100 10 25 50 75 100 10 25 50 75 100 10 25 50 75 100 10 25 50 75 100

0.1707

0.3259

0.5862

1.0392

1.3729

1.9226

3.2591

4.3251

278.15 K 1.00478 1.01188 1.02320 1.03392 1.04408 1.00510 1.01210 1.02324 1.03379 1.04378 1.00543 1.01231 1.02331 1.03370 1.04357 1.00605 1.01275 1.02350 1.03363 1.04326 1.00725 1.01368 1.02397 1.03368 1.04296 1.00819 1.01444 1.02442 1.03385 1.04287 1.00995 1.01588 1.02537 1.03437 1.04301 1.01517 1.02033 1.02870 1.03677 1.04464 1.01959 1.02440 1.03212 1.03949 1.04685

288.15 K 1.00367 1.01042 1.02121 1.03147 1.04123 1.00398 1.01063 1.02125 1.03135 1.04095 1.00427 1.01083 1.02131 1.03127 1.04074 1.00481 1.01120 1.02147 1.03121 1.04048 1.00585 1.01201 1.02188 1.03128 1.04022 1.00674 1.01271 1.02231 1.03146 1.04016 1.00834 1.01405 1.02322 1.03197 1.04030 1.01289 1.01805 1.02633 1.03424 1.04180 1.01687 1.02169 1.02939 1.03673 1.04378

298.15 K 1.00147 1.00799 1.01844 1.02838 1.03786 1.00177 1.00819 1.01848 1.02827 1.03761 1.00205 1.00839 1.01855 1.02822 1.03741 1.00257 1.00879 1.01873 1.02819 1.03718 1.00356 1.00958 1.01917 1.02830 1.03699 1.00438 1.01025 1.01960 1.02851 1.03698 1.00583 1.01148 1.02048 1.02906 1.03722 1.00993 1.01513 1.02344 1.03132 1.03874 1.01344 1.01832 1.02618 1.03357 1.04054

308.15 K 0.99836 1.00475 1.01497 1.02471 1.03400 0.99864 1.00494 1.01502 1.02462 1.03377 0.99893 1.00515 1.01510 1.02458 1.03359 0.99944 1.00553 1.01528 1.02456 1.03339 1.00040 1.00627 1.01572 1.02468 1.03325 1.00117 1.00692 1.01614 1.02490 1.03326 1.00254 1.00808 1.01698 1.02543 1.03348 1.00631 1.01145 1.01969 1.02751 1.03499 1.00952 1.01435 1.02207 1.02950 1.03664

323.15 K 0.99231 0.99861 1.00868 1.01827 1.02742 0.99259 0.99881 1.00875 1.01820 1.02722 0.99287 0.99901 1.00883 1.01818 1.02709 0.99335 0.99937 1.00901 1.01819 1.02694 0.99429 1.00014 1.00949 1.01841 1.02692 0.99502 1.00075 1.00990 1.01864 1.02700 0.99638 1.00193 1.01079 1.01923 1.02730 0.99989 1.00510 1.01338 1.02132 1.02888 1.00281 1.00783 1.01577 1.02338 1.03078

a m is the molality of trimethylamine N-oxide. Standard uncertainties u are u(m) = 0.0001 mol·kg−1, u(T) = 0.02 K, u(p) = 0.002·p, and the combined expanded uncertainty Uc are Uc(ρ) = 0.0001 g·cm−3 (0.95 level of confidence).

⎞ ⎛ ⎛ ∂p ⎞ ⎛ ∂U ⎞ − p⎟⎟ Pint = −⎜ ⎟T , m = −⎜⎜T ⎜ ⎟ ⎝ ∂V ⎠ ⎠ ⎝ ⎝ ∂T ⎠V , m

Relative average uncertainty in the determination of molar isothermal compressions, molar isobaric expansions, and internal pressures did not exceed 1 × 10−4 cm3·mol−1 MPa−1, 1 × 10−4 cm3·mol−1 K−1, and 30 MPa, accordingly. The uncertainties were assessed by the application of propagation of errors.32

(7)

where (∂p/∂T)V,m = (EP,m/KT,m) is the isochoric thermal pressure coefficient; U is the molar internal energy of the substance. In eq 7 the internal pressure is taken as a negative value because Pint is an average force parameter which tends to approach and bind the structural units of a liquid phase system.29−31

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DISCUSSION Concentration Dependences of Volumetric Properties. In Figure 1, the concentration dependences of density of C



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Table 3. Limiting Partial Molar Volumes of TMAO in Aqueous Solutions, Coefficients bv and cv, from Equation 4, at Various Temperatures and Pressures P

T

MPa

K 278.15

288.15

0.1 10 25 50 75 100

72.88 72.96 73.08 73.26 73.46 73.64

(0.05) (0.10) (0.10) (0.09) (0.09) (0.10)

73.07 73.14 73.26 73.41 73.59 73.74

(0.05) (0.09) (0.10) (0.09) (0.09) (0.09)

0.1 10 25 50 75 100

−0.79 −0.71 −0.64 −0.58 −0.56 −0.60

(0.01) (0.03) (0.02) (0.02) (0.01) (0.02)

−0.60 −0.56 −0.53 −0.52 −0.57 −0.61

(0.01) (0.02) (0.03) (0.02) (0.01) (0.02)

0.1 10 25 50 75 100

0.037 0.022 0.011 0.003 0.000 0.006

(0.002) (0.007) (0.005) (0.004) (0.002) (0.005)

0.017 0.007 0.001 0.000 0.009 0.018

(0.003) (0.005) (0.007) (0.004) (0.002) (0.004)

298.15 3 −1 V∞ 2 in cm ·mol 73.27 (0.05) 73.34 (0.09) 73.45 (0.09) 73.60 (0.09) 73.74 (0.10) 73.88 (0.09) bv in cm3·kg·mol−2 −0.48 (0.01) −0.50 (0.01) −0.55 (0.01) −0.57 (0.02) −0.63 (0.02) −0.67 (0.01) cv in cm3·kg2·mol−3 0.006 (0.003) 0.011 (0.003) 0.019 (0.003) 0.019 (0.004) 0.027 (0.004) 0.034 (0.002)

308.15

323.15

73.53 73.61 73.69 73.80 73.91 74.04

(0.05) (0.09) (0.09) (0.10) (0.09) (0.09)

74.03 74.08 74.13 74.18 74.24 74.31

(0.05) (0.09) (0.09) (0.09) (0.09) (0.11)

−0.45 −0.49 −0.50 −0.56 −0.59 −0.66

(0.01) (0.01) (0.02) (0.02) (0.02) (0.01)

−0.46 −0.47 −0.50 −0.54 −0.61 −0.69

(0.01) (0.01) (0.01) (0.01) (0.01) (0.04)

(0.002) (0.003) (0.004) (0.004) (0.004) (0.003)

0.023 0.023 0.024 0.027 0.034 0.044

(0.003) (0.003) (0.002) (0.003) (0.003) (0.008)

0.012 0.019 0.019 0.028 0.030 0.038

f(m)T,p functions and becomes most pronounced at 100 MPa. As temperature increases the extreme moves to the region of lower TMAO concentrations (see insertions in Figure 1 and Table 2). In Figures 2 and 3, the concentration dependences of apparent molar volume of TMAO in aqueous solution at various temperatures and pressures are shown. As one can see, Vϕ,2 values decrease almost linearly with concentration increases at all conditions studied. Temperature and pressure (at low temperatures) act on the apparent molar volume in the same direction, that is, their rising causes Vϕ,2 increasing. At higher temperatures this anomaly disappears and baric coefficient (∂Vϕ,2/∂p)T changes its sign (from positive to negative) at TMAO concentration increasing (Figure 3b). Pairwise coefficients (Table 3) reflect the volume changes resulting from interaction of two hydrated solute molecules in water. Negative value of bv points to hydrophobic character of interactions between TMAO and water. The decrease in the coefficients absolute values with temperature (at pressures up to 50 MPa) and pressure (at 278.15 K) testify the weakening of TMAO−TMAO interaction and strengthening the hydration of TMAO. As is shown in Figures 4 and 5, the concentration dependences of molar isobaric expansion of TMAO aqueous solutions it is an increasing function at all temperatures and pressures studied. The opposite influence of pressure on molar isobaric expansion (0 < m < 4) is observed at different temperatures, that is, the derivative ∂EP,m/∂p has different sign at 278.15 K and 323.15 K (Figure 5). In Figures 6 and 7, the concentration dependences of molar isothermal compression of TMAO aqueous solutions are shown. The increasing of TMAO content in solution leads to the compression decreasing under all pressures and temperatures studied. The inversion observed at atmospheric pressure (Figure 6) over the concentration range 2 < m < 3 shifts to the

Figure 1. Concentration dependences of densities of TMAO aqueous solutions, ρ, at 278.15 K (a) and 323.15 K (b) and at various pressures: ■, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa.

TMAO aqueous solution are presented at 278.15 K (a) and 323.15 K (b) at all studied pressures. At low pressure the density values increase steadily with TMAO concentration rising. But as pressure increases broad minimum appear on ρ = D



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Figure 2. Concentration dependences of apparent molar volumes of TMAO in aqueous solutions, Vϕ,2, at 0.101 MPa (a) and 100 MPa (b) and at various temperatures: ■, 278.15 K; ●, 288.15 K; ▲, 298.15 K; ▼, 308.15 K; ◆, 323.15 K. Points are experimental data; lines were calculated with eq 4.

Figure 3. Concentration dependences of apparent molar volumes of TMAO in aqueous solutions, Vϕ,2, at 278.15 K (a) and 323.15 K (b) and at various pressures: □, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa. Points are experimental data; lines were calculated with eq 4.

obtained in this work at the same temperature are between the values from literature. The alteration of molar isobaric expansion of TMAO at ∞ infinite dilution in water, EP,2 = (∂V2∞/∂T)P, at various temperatures and pressures is shown in Figure 10. Under all state parameters studied, E∞ P,2 values are positive and increase with temperature rising and pressure decreasing. The qualitative information on TMAO hydration can be obtained from Hepler’s equation40 based on the following expression:

region of higher TMAO concentrations with pressure increase up to 100 MPa. The latter causes the disturbance of water structure and, as a rule, results in the shifting of the inversion to lower solute concentrations.33−35 Internal pressure and cohesive energy density a measure of total energy of intermolecular interaction, are equivalent at atmospheric pressure for nonpolar liquids with van der Waals interactions as the only type of intermolecular forces.36 Similar approach was applied to polar substances too. For such systems, the nonspecific component of total energy of intermolecular interaction was considered to be proportional directly to internal pressure and molar volume Unsp = PintV.37−39 The temperature rising from 278.15 K to 323.15 K leads to the increasing of the internal pressure absolute value and, consequently, to the strengthening of van der Waals interactions in the solution (Figure 8). At 278.15 K, the dependence of internal pressure on external one is determined by TMAO concentration. So up to m = 2 mol·kg−1, the absolute value of internal pressure increases with external pressure increase whereas at higher TMAO concentrations |Pint| decreases. Partial Molar Properties at Infinite Dilution. In Figure 9, temperature dependences of partial molar volumes of infinite diluted solutions of TMAO in water are plotted at various pressures. In this figure, the comparison with literature data at atmospheric pressure is also shown. The largest difference is 19,23 observed for V∞ at T = 2 values obtained in recent works 298.15 K (0.3%). The values of limiting partial molar volumes

⎛ ∂ 2V ∞ ⎞ ⎛ ∂CP∞,2 ⎞ 2 ⎟ ⎟ = −T ⎜ ⎜ 2 2 ⎝ ∂T ⎠T ⎝ ∂T ⎠ p

(8)

(∂E∞ P,2/∂T)p

2 =(∂2V∞ 2 /∂T )p

The positive or negative values of derivative can point out to the water structure making or breaking, respectively, by the solute molecules. For solutions under investigation at 298.15 K, the positive value of (∂E∞ P,2/ ∂T)p = (0.5 ± 0.4)·10−3 cm3·mol−1·K−2 was obtained that meant the water structure making by TMAO molecules. However, it was revealed earlier23 that TMAO is a destructor of −3 water structure with (∂E∞ cm3· P,2/∂T)p = (− 0.20 ± 0.36)·10 −1 −2 mol ·K , though this value was within the error of its determination. Partial molar isothermal compressions of TMAO at infinite ∞ dilution in water, K∞ T,2 = −(∂V2 /∂P)T, are shown in Figure 11 as functions of temperature at various pressures. Under all state parameters studied these values are negative and increase both with temperature and pressure. E



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Figure 6. Concentration dependences of molar isothermal compressions of TMAO aqueous solutions, KT,m, at 0.101 MPa (open symbols) and 100 MPa (closed symbols) and at various temperatures: ■, 278.15 K; ●, 288.15 K; ▲, 298.15 K; ▼, 308.15 K; ◆, 323.15 K.

Figure 4. Concentration dependences of molar isobaric expansion of TMAO aqueous solutions, EP,m, at 0.101 MPa (a) and 100 MPa (b) at various temperatures: ■, 278.15 K; ●, 288.15 K; ▲, 298.15 K; ▼, 308.15 K; ◆, 323.15 K.

Figure 7. Concentration dependences of molar isothermal compressions, KT.m, of TMAO aqueous solutions at 278.15 K (a) and 323.15 K (b) at various pressures: □, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa.

Figure 5. Concentration dependences of molar isobaric expansion of TMAO aqueous solutions, EP,m, at 278.15 K (open symbols) and 323.15 K (closed symbols) at various pressures: ■, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa.

If one supposes that nh value does not change significantly with pressure up to 100 MPa then the isothermal compression at infinite dilution can be expressed as following:

Limiting partial molar volume can be introduced as the sum of two contributions41−43 V 2∞

= Vint + ΔV = Vint + nh(Vh − V1)

KT∞,2 = K int + nh(KT , h − KT ,1)

(9)

(10)

where Kint is the intrinsic isothermal compression of the solute molecule; KT,h is the partial molar isothermal compression of the water in hydration shell; KT,1 is the partial isothermal compression of the bulk water. The intrinsic isothermal compression of the solute molecule, Kint, is an extremely small value and can be neglected.44 So far as the limiting partial isothermal compression of TMAO in water is negative, the term (KT,h − KT,1) < 0 and,

where the first term, Vint, is the intrinsic molar volume and the second one, ΔV, is the contribution coming from the water structure changes because of solute−solvent interactions inclusive of H-bonds, electrostriction, hydrophobic hydration, and long-range effects; nh is the hydration number; Vh is the molar volume of the water in hydration shell; V1 is the molar volume of the bulk water. F



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Figure 11. Temperature dependences of molar isothermal compressions of TMAO at infinite dilution in water, K∞ T,2, at various pressures: □, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa.

Figure 8. Concentration dependences of internal pressure, Pint, of TMAO aqueous solutions at 278.15 K (open symbols) and 323.15 K (closed symbols) at various pressures: ■, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa.

water structure as the pressure increase changed the structure of pure water. The negative value of isothermal compression, observed over the wide range of temperatures and pressures, is typical for aqueous electrolyte solutions 46 and zwitterionic compounds.44,34 In such systems, the compression is the result of electrostriction. In the solutions under investigation, the ratio of K∞ T,2/KT,1 decreases in absolute value with temperature and pressure increase and, consequently, KT,h/KT,1 ratio increases, which means a weakening of compression of water around TMAO molecules.

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CONCLUSIONS In this work, new data of the density of aqueous solutions of trimethylamine N-oxide were obtained at pressures (0.101 to 100) MPa and temperatures (278.15 to 323.15) K. From the experimental data, various molar (Vϕ,2, EP,m, KT,m) and partial ∞ ∞ (V∞ 2 ,EP,2, KT,2) parameters as well as internal pressure, Pint, were calculated at all pressures and temperatures studied. It was revealed that the concentration dependences of density of TMAO aqueous solutions had minima under pressures higher than 75 MPa. Values of apparent molar volume of TMAO in aqueous solution increased with pressure growth when temperature and TMAO concentration were not too high. According to thermodynamic criteria considered it can be concluded that TMAO in water reveals hydrophobic properties and acts as the substance making the structure of water. The values of K∞ T,2 are negative under all temperatures and pressures studied.

Figure 9. Temperature dependences of partial molar volumes of TMAO at infinite dilution, V∞ 2 , at various pressures: □, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa; ○, data by Krakowiak et al.;23 △, data by Aronn et al.;20 ◇, data by Rösgen et al.19 All literature data were obtained at atmospheric pressure.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 10. Temperature dependences of molar isobaric expansion of TMAO at infinite dilution in water, E∞ P,2, at various pressures □, 0.101 MPa; ●, 10 MPa; ▲, 25 MPa; ▼, 50 MPa; ◆, 75 MPa; ◀, 100 MPa.

Apparent molar volumes of TMAO in aqueous solutions at different temperatures and pressures. This material is available free of charge via the Internet at http://pubs.acs.org.

consequently, KT,h < KT,1 and Vh > V1 under all state parameters studied. It implies that around TMAO molecules less compressible environment, consisting of water molecules, forms as compared with the bulk water. Similar conclusions were drawn by Towey et al.45 In this work, on the basis of neutron diffraction and computer simulation experiments, it was established that small amounts of TMAO influenced the

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*E-mail: [email protected]. Funding

This work was supported by the Russian Foundation for Basic Research (project 12-03-97525-r_center_a) and grant of the President of the Russian Federation (No. MK-1288.2013.3). The density measurements were carried out with equipment of G



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Device Interlaboratory scientific center “The upper Volga region center for physical-chemical researches”.

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Notes

The authors declare no competing financial interest.

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Density and Volumetric Properties of Aqueous Solutions of

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