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Interactions of glycine with polyethylene glycol studied by measurements of density and ultrasound speed in aqueous solutions at various temperatures
Fluid Phase Equilibria 300 (2011) 155–161
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Interactions of glycine with polyethylene glycol studied by measurements of density and ultrasound speed in aqueous solutions at various temperatures Melike Sahin, Zerin Yesil, Merve Gunel, Sadife Tahiroglu, Erol Ayranci ∗ Chemistry Department, Akdeniz University, 07058 Antalya, Turkey
a r t i c l e
i n f o
Article history: Received 22 July 2010 Received in revised form 21 October 2010 Accepted 4 November 2010 Available online 12 November 2010 Keywords: Apparent molar volume Apparent molar isentropic compression Hydrophobic hydration Glycine Polyethylene glycol
a b s t r a c t Density and ultrasound speed were measured accurately for binary glycine–water and ternary glycine–water–polyethylene glycol 400 (PEG400) solutions at (293.15, 298.15, 303.15 and 308.15) K. The data were used in evaluating thermodynamic properties as apparent molar volumes (V∅ ) and apparent molar isentropic compressions (KS˚ ) of glycine in water and in PEG400 solutions. Infinite dilution o o values of these parameters, V∅ , and KS˚ , were obtained from their plots as a function of molality and have been utilized in obtaining transfer volumes and transfer compressions of glycine from water to PEG400 solutions of various molalities. Apparent molar isobaric expansions were determined from the tempero ature dependence of V∅ and V∅ values. All the results were interpreted in terms of glycine–water and glycine–water–PEG400 interactions. The partial hydrophobic character of PEG400 structure was found to play an important role in determining the transfer properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polyethylene glycols (PEGs) of various molecular weights are being used in many areas of research and technology. In many applications they are being used in conjugation with peptides. Thus, interactions of PEGs with amino acids, building stones of peptides, are important [1,2]. A useful tool for studying such interactions is volumetric properties such as apparent molar volume, isentropic compression and isobaric expansion determined from density and ultrasound speed measurements at different temperatures as proven in our earlier works [3–5]. Although the volumetric properties of PEGs and amino acids, individually, in aqueous solutions have been studied extensively [6–12], there is only limited number of reports on the interactions of PEGs with amino acids studied through volumetric properties. For example Sasahara [13] reported volume changes on glycine–PEG and l-alanine–PEG in aqueous solution where PEG was with a molar mass of 4000. In another work by Sasahara and Uedaira [14], volume and compressibility changes on mixing aqueous solutions of amino acids and PEG, again with molar mass of 4000, were studied. In the present work, we wanted to initiate a systematic work on volumetric properties of PEG–amino acid–water systems, starting with the simplest member of ␣-amino acids, glycine, and a PEG having a molar mass of 400, abbreviated as PEG400. We aimed at investigating the interactions between glycine and PEG400 through volumetric properties determined by accurate measurement of density and ultrasound
∗ Corresponding author. Tel.: +90 242 3102315; fax: +90 242 2278911. E-mail address: [email protected] (E. Ayranci). 0378-3812/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2010.11.005
speed for glycine–PEG400–water ternary systems at various temperatures in the interval from 293.15 K to 308.15 K. The general formula of PEG can be shown as HO–CH2 –(CH2 –O–CH2 )n –CH2 –OH where n is the number of repeating unit which is 7.7 for PEG400 [3]. Although there are hydrophilic –OH end groups in the structure, PEG has at least partial hydrophobic character due to presence of two –CH2 – groups against an etheric –O– group per repeating unit. Glycine is quite hydrophilic with two functional groups; an amine and a carboxylic acid group. Its isoelectric point, pI, is about 6. Thus it is found predominantly in zwitterionic form in its solutions in water. From structural information it is apparent that all kinds of interactions such as hydrogen-bonding, ion-dipole and hydrophobic interactions are expected to involve in ternary system of glycine–PEG400–water. 2. Experimental 2.1. Materials PEG400 and glycine were obtained from Sigma. Their mass fraction purity was at least 0.99 pure and they were used after drying under vacuum without any further purification. Deionized water was used in all experiments. 2.2. Density and ultrasound speed measurements Densities and ultrasound speeds were measured by an Anton Paar DSA 5000 model high precision vibrating tube digital densimeter and ultrasound speed measuring device, with automatic
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
3. Results Densities and ultrasound speeds of glycine as a function of molality in water and in 0.1, 0.4, 0.8, 1.2, 1.6 and 2.0 mol kg−1 PEG400 solutions at 293.15, 298.15, 303.15 and 308.15 K are given in Table 1 . Apparent molar volumes, V∅ , were calculated from the measured densities by the following equation [15]: V∅ =
( − ) o mo
+
M2
(1)
where o and are densities of solvent and solution, respectively, m is the molality and M2 is the molecular weight of solute. Apparent o , were determined by extrapmolar volumes at infinite dilution, V∅ olating V∅ vs concentration data to zero concentration. For glycine, in water or in aqueous PEG400, this extrapolation was made on the basis of the following empirical equation [15]: o V∅
V∅ =
+ bv m
E∅ =
∂V∅ ∂T
(3) P
where T is the temperature and P is the pressure. The slope, at any T, of the curve obtained by plotting V∅ data at any fixed concentration of solute vs T gives E∅ . Linearity of V∅ vs T plot in a certain T range implies that E∅ is constant within that T range and given by the slope of that line. Thus E∅ values at different concentrations of solute were obtained from the slopes of V∅ vs T plots. Those o vs T plots were termed as apparent molar isobaric obtained from V∅ o . expansion at infinite dilution and symbolized by E∅ Apparent molar isentropic compressions, KS˚ , were obtained from the following equation [17]:
KS˚ =
S − So mo
(a)
+ S VØ
(4)
44.1
43.8 0.0
0.1
0.2
0.3
0.4
0.5
0.6
m/(mol.kg -1 ) 45
(b) 44
43
42
41
0.0
0.1
0.2
0.3
0.4
0.5
0.6
m/(mol.kg -1 ) Fig. 1. Typical V∅ vs m plots: (a) for glycine in water at 303.15 K and (b) for glycine in 0.8 mol kg−1 PEG400 at 293.15 K.
where S and So are isentropic compressibilities of solution and solvent, respectively. So and S were obtained from Eqs. (5) and (6), respectively, using the measured ultrasound speeds; uo for solvent and u for solution: So =
(2)
where bv is an experimentally determined parameter. Linear o and b paramregression analysis of V∅ vs m data provided both V∅ v eters. Typical V∅ vs m plots are given in Fig. 1a for glycine in water at 303.15 K and in Fig. 1b for glycine in 0.8 mol kg−1 PEG400 at 293.15 K. The linear regression coefficients for these particular plots were 0.9746 and 0.8935, respectively. Apparent molar isobaric expansions, E∅ , can be obtained on the basis of the following equation [16]:
44.4
VΦ /(cm3.mol -1)
viscosity corrections. The instrument has a built-in thermostat to maintain the temperature between 0 and 70 ◦ C with a precision of ±0.005 ◦ C. The calibration of the instrument was made with degassed and bidistilled water for which density and sound velocity values were 0.998203 g cm−3 and 1482.66 m s−1 , respectively, at 20 ◦ C as given in the instruction manual of the instrument. The uncertainties of measurements were ±5 × 10−6 g cm−3 for density and 0.1 m s−1 for ultrasound speed. The instrument required a liquid volume of about 2.5 mL and measured the density and ultrasound speed simultaneously after a thermal equilibration period of about 5–10 min. Density and ultrasound speed measurements were carried out for selected solutions of glycine in water and in PEG400 solutions at various molalities and temperatures. The molalities of PEG400 solutions were 0.1, 0.4, 0.8, 1.2, 1.6 or 2.0 mol kg−1 and the selected temperatures were 293.15, 298.15, 303.15 and 308.15 K. The weighings were made on a digital balance (Scaltec SPB31) with a precision of 0.1 mg. The measured densities and ultrasound speeds were utilized in determining apparent molar volumes and isentropic compressions as described in the next section.
VΦ /(cm3.mol -1)
156
S =
1 u2o o 1 u2
(5) (6)
So, the determination of apparent molar isentropic compressions involved the usage of measured densities and ultrasound speeds in Eqs. (1), (5), (6) and then (4), successively. The dependence of apparent molar isentropic compression, KS˚ , of glycine on concentration can be given by the following empirical equation: o + bSK m KS˚ = KS˚
(7)
o where KS˚
is the apparent molar isentropic compression at infinite dilution and bSK is an experimentally determined parameter. KS˚ o and b , vs m data were regressed linearly and the parameters, KS˚ SK were determined from the intercept and slope, respectively. Typical KS˚ vs m plots are given in Fig. 2a for glycine in water at 308.15 K and in Fig. 2b for glycine in 0.8 mol kg−1 PEG400 at 293.15 K. The linear regression coefficients for these particular plots were 0.9985 and 0.9456, respectively. o , b , K o , and b V∅ v S˚ SK values for glycine in water and in solvents as PEG400 solutions of varying molalities and at the four temperatures are collectively given in Table 2. 4. Discussion o results at 298.15 K and 308.15 K, and K o results at The V∅ S˚ 298.15 K compare well with the available literature data given in
Table 1 Density and ultrasound speeds for solutions of glycine in water and in PEG400 solutions at various molalities and temperatures. T:
293.15 K −1
−3
298.15 K −1
−3
303.15 K −1
−3
308.15 K −1
Molality mol kg
g cm
u ms
g cm
u ms
g cm
u ms
g cm−3
u m s−1
H2 O
0.0000 0.0417 0.0886 0.1129 0.1823 0.2472 0.3003 0.3600 0.4148 0.4551 0.5234
0.998202 0.999527 1.001013 1.001773 1.003933 1.005939 1.007563 1.009388 1.011055 1.012279 1.014315
1482.52 1484.86 1487.38 1488.67 1492.39 1495.79 1498.59 1501.76 1504.51 1506.57 1510.15
0.997044 0.998355 0.999825 1.000576 1.002715 1.004702 1.006311 1.008114 1.009764 1.010974 1.012989
1496.72 1498.95 1501.40 1502.64 1506.24 1509.54 1512.24 1515.29 1518.05 1519.98 1523.49
0.995647 0.996945 0.998401 0.999146 1.001265 1.003235 1.004831 1.006621 1.008255 1.009454 1.011451
1509.14 1511.25 1513.61 1514.82 1518.30 1521.52 1524.16 1527.10 1529.77 1531.67 1535.12
0.994032 0.995321 0.996766 0.997506 0.999611 1.001567 1.003152 1.004927 1.006552 1.007739 1.009723
1519.83 1521.87 1524.16 1525.35 1528.73 1531.87 1534.45 1537.31 1539.94 1541.81 1545.18
0.1 mol kg−1 PEG400
0.0000 0.0500 0.0998 0.1497 0.2004 0.2500 0.2999 0.3498 0.4000 0.4498 0.4999
1.004440 1.006039 1.007597 1.009200 1.010731 1.012272 1.013774 1.015340 1.016791 1.018388 1.019792
1505.99 1508.66 1510.99 1513.39 1515.89 1518.38 1520.94 1523.60 1526.09 1528.77 1531.20
1.003163 1.004743 1.006287 1.007869 1.009387 1.010914 1.012401 1.013928 1.015387 1.016974 1.018358
1518.48 1520.99 1523.27 1525.58 1528.05 1530.49 1532.88 1535.46 1537.89 1540.50 1542.82
1.001654 1.003219 1.004752 1.006321 1.007826 1.009338 1.010815 1.012296 1.013777 1.015348 1.016723
1528.20 1530.94 1533.36 1535.89 1538.33 1540.81 1543.17 1545.70 1548.24 1550.60 1552.84
0.999935 1.001488 1.003012 1.004571 1.006065 1.007569 1.009032 1.010469 1.011977 1.013525 1.014904
1537.35 1540.03 1542.40 1544.87 1547.25 1549.66 1551.95 1554.43 1556.71 1559.21 1561.40
0.4 mol kg−1 PEG400
0.0000 0.0988 0.1521 0.2035 0.2495 0.3008 0.4045 0.4649 0.4981
1.020791 1.023866 1.025612 1.027089 1.028450 1.029998 1.033013 1.034472 1.035696
1564.90 1570.13 1573.25 1575.51 1578.51 1580.79 1585.66 1588.19 1590.06
1.019151 1.022199 1.023929 1.025397 1.026742 1.028278 1.031269 1.032713 1.033929
1572.44 1577.50 1580.55 1582.73 1585.66 1587.85 1592.62 1595.13 1596.95
1.017311 1.020338 1.022056 1.023513 1.024849 1.026373 1.029345 1.030780 1.031988
1578.64 1583.57 1586.53 1588.70 1591.54 1593.64 1598.36 1600.78 1602.58
1.015292 1.018305 1.020009 1.021458 1.022786 1.024298 1.027257 1.028685 1.029885
1583.56 1588.39 1591.29 1593.42 1596.19 1598.23 1602.88 1605.23 1606.97
0.8 mol kg−1 PEG400
0.0000 0.1000 0.1509 0.2022 0.2504 0.3006 0.3495 0.4019 0.4494 0.4984
1.038734 1.041973 1.043465 1.045082 1.046552 1.047845 1.049489 1.050908 1.052266 1.053638
1628.87 1634.07 1636.41 1639.30 1641.30 1643.84 1646.18 1648.30 1650.51 1652.65
1.036660 1.039861 1.041337 1.042940 1.044405 1.045695 1.047322 1.048734 1.050082 1.051449
1630.73 1635.89 1638.15 1640.99 1642.99 1645.44 1647.83 1649.91 1652.03 1654.16
1.034407 1.037589 1.039056 1.040649 1.042108 1.043393 1.045008 1.046417 1.047758 1.049119
1631.70 1636.74 1639.00 1641.75 1643.76 1646.15 1648.55 1650.61 1652.64 1654.77
1.032006 1.035175 1.036632 1.038220 1.039671 1.040955 1.042562 1.043966 1.045302 1.046658
1631.73 1636.66 1638.91 1641.59 1643.63 1645.94 1648.34 1650.38 1652.37 1654.47
1.2 mol kg−1 PEG400
0.0000 0.0756 0.1019 0.1476 0.2001 0.2504 0.3004 0.3517 0.4004
1.053633 1.056249 1.056865 1.058154 1.059863 1.061248 1.062798 1.063830 1.065028
1677.10 1681.83 1682.55 1684.26 1687.40 1689.00 1691.57 1692.85 1695.04
1.051132 1.053731 1.054349 1.055631 1.057330 1.058710 1.060246 1.061289 1.062487
1674.53 1679.18 1679.89 1681.60 1684.71 1686.30 1688.84 1690.12 1692.29
1.048501 1.051087 1.051705 1.052985 1.054676 1.056053 1.057583 1.058628 1.059824
1671.31 1675.85 1676.57 1678.29 1681.35 1682.89 1685.44 1686.74 1688.91
1.045750 1.048326 1.048945 1.050221 1.051908 1.053284 1.054806 1.055859 1.057049
1667.40 1671.83 1672.55 1674.29 1677.32 1678.82 1681.36 1682.67 1684.84
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
Solvent
157
158
Table 1 (Continued) T:
293.15 K −1
298.15 K
−3
−1
−3
303.15 K −1
308.15 K
−3
−1
Molality mol kg
g cm
u ms
g cm
u ms
g cm
u ms
g cm−3
u m s−1
1.6 mol kg−1 PEG400
0.0000 0.0759 0.1269 0.1528 0.2007 0.2494 0.3009 0.3509 0.4045
1.064265 1.066743 1.068553 1.069299 1.070819 1.071745 1.073557 1.074666 1.076094
1709.85 1712.69 1715.47 1716.81 1718.68 1719.52 1722.73 1724.36 1726.21
1.061483 1.063932 1.065727 1.066472 1.067988 1.068919 1.070719 1.071830 1.073258
1704.07 1707.00 1709.67 1710.95 1712.92 1713.77 1716.91 1718.50 1720.37
1.058579 1.061011 1.062799 1.063539 1.065054 1.065990 1.067781 1.068895 1.070322
1697.81 1700.78 1703.40 1704.64 1706.61 1707.56 1710.60 1712.18 1714.05
1.055569 1.057997 1.059773 1.060513 1.062026 1.062968 1.064752 1.065869 1.067297
1691.03 1694.02 1696.60 1697.80 1699.81 1700.81 1703.77 1705.34 1707.21
2.0 mol kg−1 PEG400
0.0000 0.0263 0.0750 0.1009 0.2019 0.2519 0.2992 0.3513 0.4003
1.075048 1.076117 1.077659 1.078438 1.080803 1.082446 1.083775 1.084451 1.086260
1736.20 1737.06 1739.84 1740.96 1744.29 1746.34 1747.78 1748.06 1751.10
1.071749 1.073004 1.074544 1.075315 1.077692 1.079327 1.080655 1.081346 1.083145
1727.66 1728.80 1731.17 1732.36 1735.72 1737.81 1739.29 1739.58 1742.53
1.068754 1.069802 1.071341 1.072111 1.074495 1.076125 1.077454 1.078158 1.079946
1718.74 1719.91 1722.20 1723.41 1726.82 1728.91 1730.39 1730.80 1733.63
1.065480 1.066518 1.068060 1.068828 1.071221 1.072851 1.074177 1.074896 1.076677
1709.46 1710.61 1712.84 1714.05 1717.52 1719.60 1721.08 1721.61 1724.33
Table 2 o o (cm3 mol−1 ), bv (cm3 kg mol−1 ), KS˚ (cm3 mol−1 MPa−1 ) and bSK (cm3 kg mol−1 MPa−1 ) values for glycine in water and in solvents as PEG400 solutions of varying molalities at (293.15, 298.15, 303.15 and 308.15) K. V∅ T/K
Solvent
0.1 mol kg−1 PEG400
H2 O
0.4 mol kg−1 PEG400
0.8 mol kg−1 PEG400
1.2 mol kg−1 PEG400
1.6 mol kg−1 PEG400
2.0 mol kg−1 PEG400
293.15
o
V∅ bv o KS˚ bKS
43.23 0.780 −0.0290 0.0074
± ± ± ±
0.02 0.072 0.002b 0.0006
42.12 3.95 −0.0280 0.010
± ± ± ±
0.22 0.72 0.0006 0.0019
42.92 3.01 −0.0227 0.0082
± ± ± ±
0.25 0.78 0.0006 0.0018
42.14 3.81 −0.0181 0.0077
± ± ± ±
0.24 0.72 0.0003 0.0010
40.64 11.2 −0.0193 0.0244
± ± ± ±
0.72 2.82 0.0015 0.0060
39.94 10.8 −0.0119 0.0082
± ± ± ±
0.63 2.45 0.0013 0.0050
37.06 25.22 −0.0129 0.0211
± ± ± ±
1.37 5.52 0.0010 0.0041
298.15
o V∅ bv o KS˚ bKS
43.58 0.742 −0.0266 0.0061
± ± ± ±
0.02 0.062 0.0001 0.0004
42.48 3.70 −0.0261 0.0094
± ± ± ±
0.26 0.85 0.0004 0.0016
43.20 2.97 −0.0213 0.0075
± ± ± ±
0.25 0.77 0.0006 ± 0.0018
42.54 3.48 −0.0176 0.0075
± ± ± ±
0.23 0.70 0.0004 0.0011
40.85 11.1 −0.0190 0.0238
± ± ± ±
0.70 2.76 0.0015 0.0059
40.34 10.2 −0.0121 0.0086
± ± ± ±
0.62 2.43 0.0011 0.0046
37.34 24.6 −0.0144 0.0254
± ± ± ±
1.33 5.37 0.0009 0.0036
303.15
o V∅ bv o KS˚ bKS
43.91 0.658 −0.0242 0.00458
± ± ± ±
0.02 0.053 0.0000 0.0001
42.77 3.67 −0.0247 0.0090
± ± ± ±
0.26 0.84 0.0005 0.0015
43.44 2.95 −0.0203 0.0072
± ± ± ±
0.25 0.77 0.0006 0.0017
42.76 3.41 −0.0171 0.0071
± ± ± ±
0.23 0.70 0.0003 0.0010
41.02 11.0 −0.0187 0.0231
± ± ± ±
0.69 2.73 0.0015 0.0057
40.59 9.86 −0.0123 0.0088
± ± ± ±
0.62 2.43 0.0011 0.0043
37.58 24.1 −0.0147 0.0255
± ± ± ±
1.30 5.22 0.0009 0.0037
308.15
o V∅ bv o KS˚ bKS
44.15 0.657 −0.0227 0.0038
± ± ± ±
0.02a 0.048 0.0000 0.0001
43.02 3.60 −0.0235 0.0087
± ± ± ±
0.25 0.83 0.0005 0.0015
43.63 2.98 −0.0196 0.0071
± ± ± ±
0.24 0.75 0.0006 0.0017
42.93 3.38 −0.0167 0.0067
± ± ± ±
0.23 0.71 0.0003 0.0009
41.15 10.9 −0.0184 0.0222
± ± ± ±
0.69 2.70 0.0014 0.056
40.72 9.76 −0.0125 0.0091
± ± ± ±
0.61 2.37 0.0011 0.0041
37.80 23.6 −0.0146 0.0246
± ± ± ±
1.25 5.1 0.0009 0.0035
a
o a Some of literature V∅ values are 43.19 [9,18], 43.12 [19], 43.14 [20], 43.20 [21–23], 43.46 [24], 43.26 [24,25], 43.25 [26], 43.23 [27], 43.01 [28], 43.33 [29], 43.50 [30], 43.09 [31,32], 43.30 [33,34], 42.54 [35,36], 42.40 [33], 43.22 [23], and 43.29 [37] at 293.15 K, 43.199 [38] at 298.15 K and 43.81 [9], 43.69 [19], 43.80 [33], and 43.85 [22] at 308.15 K. o b Some of literature KS˚ values at this temperature are −0.02650[9], −0.02660[10,21], −0.02700 [18,24], −0.02740 [32], −0.02709 [39], −0.02500 [40], and 0.02716 [27].
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
Solvent
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
/ (cm3.mol-1.MPa-1)
-0.02
Table 3 Apparent molar isobaric expansions of glycine in water and in PEG400 solutions of various molalities.
(a)
-0.021
K
SΦ
-0.022
-0.023
0
0.1
0.2
0.3
0.4
0.5
0.6
m /(mol.kg-1)
KSΦ / (cm3.mol-1.MPa-1)
- 0.014
(b)
- 0.015
- 0.016
- 0.017
- 0.018
0
0.1
0.2
0.3
0.4
0.5
0.6
m / (mol.kg-1) o Fig. 2. Typical KS˚ vs m plots (a) for glycine in water at 308.15 K and (b) for glycine in 0.8 mol kg−1 PEG400 at 293.15 K.
38
VΦo /( cm3.mol-1.K-1)
159
37.75
37.5
37.25
37 290
295
300
305
310
T/K o
−1
Fig. 3. A typical V∅ vs T plot for 2 mol kg
PEG400–glycine–water system.
the footnote of Table 2. All other results are being reported for the first time according to our survey. o values given in Table 2 are seen to increase regularly with V∅ temperature for glycine–water and all PEG400–glycine–water syso can best be discussed tems. The temperature dependence of V∅ on the bases of apparent molar isobaric expansion at infinite diluo . V o values are plotted as a function of temperature for tion, E∅ ∅ each system. Good linear relations were observed as seen in a typical plot of (2.0 mol kg−1 PEG400)–glycine–water system in Fig. 3. o can be taken as constant over the These linearities showed that E∅ o was determined from the slope temperature range studied. Thus E∅ o of the V∅ vs T line obtained for glycine–water or glycine–PEG400 solution systems by linear regression analysis. The same treatment was applied to V∅ vs T data at different molalities of glycine in water and in PEG400 solutions to provide E∅ values defined by Eq. (3). All o values are given in Table 3. the E∅ and E∅ o values displayed no regular trend with increasing Although, E∅ molality of PEG400 in the system, they are clearly lower for ternary systems than the binary system. This shows that the temperature o is slightly less for ternary PEG400–glycine–water dependence of V∅ systems than for binary glycine–water system. E∅ values are, in
Molality of glycine mol kg−1
E∅ cm3 mol−1 K−1
In H2 O 0 0.0417 0.0886 0.1129 0.1823 0.2472 0.3003 0.3600 0.4148 0.4551 0.5234
0.062 0.062 0.062 0.061 0.060 0.059 0.059 0.059 0.058 0.059 0.058
± ± ± ± ± ± ± ± ± ± ±
0.004a 0.004 0.004 0.004 0.004 0.003 0.003 0.003 0.003 0.003 0.003
In 0.1 mol kg−1 PEG400 0 0.0497 0.1009 0.2019 0.2498 0.3505 0.4005 0.4995
0.060 0.060 0.060 0.059 0.060 0.056 0.061 0.062
± ± ± ± ± ± ± ±
0.003a 0.003 0.003 0.003 0.003 0.004 0.004 0.004
In 0.4 mol kg−1 PEG400 0 0.0988 0.1521 0.2035 0.2496 0.3008 0.4045 0.4650 0.4981
0.047 0.046 0.048 0.047 0.048 0.048 0.047 0.046 0.046
± ± ± ± ± ± ± ± ±
0.003a 0.004 0.003 0.002 0.003 0.002 0.002 0.003 0.003
In 0.8 mol kg−1 PEG400 0 0.100 0.1509 0.2022 0.2504 0.3006 0.3695 0.4020 0.4495 0.4984
0.052 0.052 0.049 0.047 0.043 0.040 0.042 0.040 0.039 0.039
± ± ± ± ± ± ± ± ± ±
0.008a 0.006 0.006 0.006 0.004 0.004 0.004 0.004 0.004 0.003
In 1.2 mol kg−1 PEG400 0 0.0756 0.1019 0.1476 0.2001 0.2504 0.3005 0.3517 0.4004
0.034 0.037 0.029 0.028 0.029 0.027 0.030 0.025 0.025
± ± ± ± ± ± ± ± ±
0.003a 0.003 0.001 0.002 0.002 0.002 0.003 0.002 0.001
In 1.6 mol kg−1 PEG400 0 0.0760 0.1269 0.1528 0.2007 0.4494 0.3009 0.3509 0.4045
0.052 0.050 0.045 0.042 0.035 0.029 0.030 0.027 0.025
± ± ± ± ± ± ± ± ±
0.009a 0.007 0.006 0.006 0.004 0.004 0.004 0.003 0.003
In 2.0 mol kg−1 PEG400 0 0.0236 0.1009 0.2019 0.2519 0.2992 0.3513 0.4003
0.049 0.068 0.031 0.016 0.017 0.017 0.011 0.015
± ± ± ± ± ± ± ±
0.002a 0.001 0.003 0.001 0.002 0.001 0.001 0.001
a
o Infinite dilution value; E∅ .
160
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
Table 4 o , in cm3 mol−1 and isentropic compression„ in cm3 mol−1 MPa−1 of glycine from H2 O to PEG400 solutions at infinite dilution. Transfer apparent molar volumes, V∅ ,trs T/K
293.15
Transfer from H2 O to
o V∅ ,trs
KSo∅,trs
o V∅ ,trs
KSo∅,trs
o V∅ ,trs
KSo∅,trs
o V∅ ,trs
KSo∅,trs
−0.223 −0.314 −1.09 −2.59 −3.29 −6.17
0.004 0.006 0.009 0010 0.017 0.016
−0.200 −0.379 −1.04 −2.74 −3.25 −6.24
0.005 0.006 0.008 0.008 0.007 0.015
−0.230 −0.469 −1.15 −2.89 −3.32 −6.33
0.000 0.004 0.007 0.005 0.012 0.009
−0.231 −0.521 −1.23 −2.99 −3.44 −6.35
0.000 0.003 0.006 0.005 0.010 0.008
−1
0.1 mol kg 0.4 mol kg−1 0.8 mol kg−1 1.2 mol kg−1 1.6 mol kg−1 2.0 mol kg−1
PEG400 PEG400 PEG400 PEG400 PEG400 PEG400
298.15
303.15
general, seen to decrease with molality of glycine. This decrease is becoming more striking in PEG solution of higher molalities. o values given in Table 2 are all negative for both binary sysKS˚ tem and ternary systems. This means that introduction of glycine into water or PEG400 solution at any molality causes a decrease in compression which can be explained by the hydrophilic nature of glycine with two ionic centers in its zwitterionic form. Electrostriction of water in the primary hydration shell around these ionic centers results in decreased compression. On the other hand o values show a very small dependence on temperature. In genKS˚ eral, they slightly increase with T with a few exceptions (Table 2). o values to temperature is understandable The less sensitivity of KS˚ from the fact that they reflect not the volume itself but the change in volume with pressure. The change in volume with pressure is not expected to be influenced from temperature as much as the volume itself. One of the aims of the present work was to investigate the interactions between glycine and PEG400 through volumetric properties. Probably the best property is the transfer volume and compression of glycine in going from water to PEG400 solution of different molality to investigate such interactions. So the transfer o apparent molar volume at infinite dilution, V∅,trs , and the transo of glycine fer apparent molar isentropic compressions, KS∅,trs from water to PEG400 solutions are calculated from Eqs. (8) and (9), respectively: o o o V∅,trs = V∅,(in − V∅,(in aqueous X m PEG400) water)
(8)
o o o = KS˚,(in − KS˚ KS∅,trs aqueous X m PEG400) (in water)
(9)
where X is the molality of PEG400. These transfer properties are given in Table 4. It is clearly seen that all transfer volumes are negative and becoming more negative with increasing molality of PEG400, while all the transfer compression are positive and increase, in general, with increasing molality of PEG400, with a few irregularities in the general trend. This behavior can only be explained by the partial hydrophobic hydration of PEG400 in the absence of glycine [6]. When glycine is introduced into PEG400 solution, it interacts with PEG400 through ion-dipole and possibly H-bonding interactions releasing some hydrophobically hydrated water molecules around PEG400 into bulk water. Here in this interaction, ionic centers are at two end groups of glycine in zwitterionic form and the dipole is associated with the etheric –O– group of PEG400. Hydrophobically hydrated water is known to be in an icelike structure having greater volume but less compression than ordinary water [15]. So, when they are released into normal bulk water structure their volumes decrease and compression increase, as clearly observed from Table 4. This effect is less manifested in o o KS˚,trs values, again due to the fact that V∅,trs reflects directly the change in volume of glycine in going from aqueous solution o to PEG400 solutions while KS˚,trs reflects only the change in this o V∅,trs with pressure which is expected to be less significant than o V∅,trs itself.
308.15
5. Conclusion o changes linearly with temperature for It is found that V∅ glycine–water and glycine–water–PEG400 systems within the studied temperature range. So, the apparent molar isobaric expansions were constant and their numerical values were lower for o ternary systems studied than binary glycine–water system. KS∅ values were found to show less dependence on temperature than o values. Transfer volumes of glycine from water to PEG400 soluV∅ tions were found to be negative and to become more negative with increasing PEG400 concentration. The corresponding transfer compressions were in general positive and increased with PEG concentration. Transfer volumes and compressions were explained by the partial hydrophobic hydration of PEG400 in the absence of glycine.
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Interactions of glycine with polyethylene glycol studied by measurements of density and ultrasound speed in aqueous solutions at various temperatures Melike Sahin, Zerin Yesil, Merve Gunel, Sadife Tahiroglu, Erol Ayranci ∗ Chemistry Department, Akdeniz University, 07058 Antalya, Turkey
a r t i c l e
i n f o
Article history: Received 22 July 2010 Received in revised form 21 October 2010 Accepted 4 November 2010 Available online 12 November 2010 Keywords: Apparent molar volume Apparent molar isentropic compression Hydrophobic hydration Glycine Polyethylene glycol
a b s t r a c t Density and ultrasound speed were measured accurately for binary glycine–water and ternary glycine–water–polyethylene glycol 400 (PEG400) solutions at (293.15, 298.15, 303.15 and 308.15) K. The data were used in evaluating thermodynamic properties as apparent molar volumes (V∅ ) and apparent molar isentropic compressions (KS˚ ) of glycine in water and in PEG400 solutions. Infinite dilution o o values of these parameters, V∅ , and KS˚ , were obtained from their plots as a function of molality and have been utilized in obtaining transfer volumes and transfer compressions of glycine from water to PEG400 solutions of various molalities. Apparent molar isobaric expansions were determined from the tempero ature dependence of V∅ and V∅ values. All the results were interpreted in terms of glycine–water and glycine–water–PEG400 interactions. The partial hydrophobic character of PEG400 structure was found to play an important role in determining the transfer properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polyethylene glycols (PEGs) of various molecular weights are being used in many areas of research and technology. In many applications they are being used in conjugation with peptides. Thus, interactions of PEGs with amino acids, building stones of peptides, are important [1,2]. A useful tool for studying such interactions is volumetric properties such as apparent molar volume, isentropic compression and isobaric expansion determined from density and ultrasound speed measurements at different temperatures as proven in our earlier works [3–5]. Although the volumetric properties of PEGs and amino acids, individually, in aqueous solutions have been studied extensively [6–12], there is only limited number of reports on the interactions of PEGs with amino acids studied through volumetric properties. For example Sasahara [13] reported volume changes on glycine–PEG and l-alanine–PEG in aqueous solution where PEG was with a molar mass of 4000. In another work by Sasahara and Uedaira [14], volume and compressibility changes on mixing aqueous solutions of amino acids and PEG, again with molar mass of 4000, were studied. In the present work, we wanted to initiate a systematic work on volumetric properties of PEG–amino acid–water systems, starting with the simplest member of ␣-amino acids, glycine, and a PEG having a molar mass of 400, abbreviated as PEG400. We aimed at investigating the interactions between glycine and PEG400 through volumetric properties determined by accurate measurement of density and ultrasound
∗ Corresponding author. Tel.: +90 242 3102315; fax: +90 242 2278911. E-mail address: [email protected] (E. Ayranci). 0378-3812/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2010.11.005
speed for glycine–PEG400–water ternary systems at various temperatures in the interval from 293.15 K to 308.15 K. The general formula of PEG can be shown as HO–CH2 –(CH2 –O–CH2 )n –CH2 –OH where n is the number of repeating unit which is 7.7 for PEG400 [3]. Although there are hydrophilic –OH end groups in the structure, PEG has at least partial hydrophobic character due to presence of two –CH2 – groups against an etheric –O– group per repeating unit. Glycine is quite hydrophilic with two functional groups; an amine and a carboxylic acid group. Its isoelectric point, pI, is about 6. Thus it is found predominantly in zwitterionic form in its solutions in water. From structural information it is apparent that all kinds of interactions such as hydrogen-bonding, ion-dipole and hydrophobic interactions are expected to involve in ternary system of glycine–PEG400–water. 2. Experimental 2.1. Materials PEG400 and glycine were obtained from Sigma. Their mass fraction purity was at least 0.99 pure and they were used after drying under vacuum without any further purification. Deionized water was used in all experiments. 2.2. Density and ultrasound speed measurements Densities and ultrasound speeds were measured by an Anton Paar DSA 5000 model high precision vibrating tube digital densimeter and ultrasound speed measuring device, with automatic
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
3. Results Densities and ultrasound speeds of glycine as a function of molality in water and in 0.1, 0.4, 0.8, 1.2, 1.6 and 2.0 mol kg−1 PEG400 solutions at 293.15, 298.15, 303.15 and 308.15 K are given in Table 1 . Apparent molar volumes, V∅ , were calculated from the measured densities by the following equation [15]: V∅ =
( − ) o mo
+
M2
(1)
where o and are densities of solvent and solution, respectively, m is the molality and M2 is the molecular weight of solute. Apparent o , were determined by extrapmolar volumes at infinite dilution, V∅ olating V∅ vs concentration data to zero concentration. For glycine, in water or in aqueous PEG400, this extrapolation was made on the basis of the following empirical equation [15]: o V∅
V∅ =
+ bv m
E∅ =
∂V∅ ∂T
(3) P
where T is the temperature and P is the pressure. The slope, at any T, of the curve obtained by plotting V∅ data at any fixed concentration of solute vs T gives E∅ . Linearity of V∅ vs T plot in a certain T range implies that E∅ is constant within that T range and given by the slope of that line. Thus E∅ values at different concentrations of solute were obtained from the slopes of V∅ vs T plots. Those o vs T plots were termed as apparent molar isobaric obtained from V∅ o . expansion at infinite dilution and symbolized by E∅ Apparent molar isentropic compressions, KS˚ , were obtained from the following equation [17]:
KS˚ =
S − So mo
(a)
+ S VØ
(4)
44.1
43.8 0.0
0.1
0.2
0.3
0.4
0.5
0.6
m/(mol.kg -1 ) 45
(b) 44
43
42
41
0.0
0.1
0.2
0.3
0.4
0.5
0.6
m/(mol.kg -1 ) Fig. 1. Typical V∅ vs m plots: (a) for glycine in water at 303.15 K and (b) for glycine in 0.8 mol kg−1 PEG400 at 293.15 K.
where S and So are isentropic compressibilities of solution and solvent, respectively. So and S were obtained from Eqs. (5) and (6), respectively, using the measured ultrasound speeds; uo for solvent and u for solution: So =
(2)
where bv is an experimentally determined parameter. Linear o and b paramregression analysis of V∅ vs m data provided both V∅ v eters. Typical V∅ vs m plots are given in Fig. 1a for glycine in water at 303.15 K and in Fig. 1b for glycine in 0.8 mol kg−1 PEG400 at 293.15 K. The linear regression coefficients for these particular plots were 0.9746 and 0.8935, respectively. Apparent molar isobaric expansions, E∅ , can be obtained on the basis of the following equation [16]:
44.4
VΦ /(cm3.mol -1)
viscosity corrections. The instrument has a built-in thermostat to maintain the temperature between 0 and 70 ◦ C with a precision of ±0.005 ◦ C. The calibration of the instrument was made with degassed and bidistilled water for which density and sound velocity values were 0.998203 g cm−3 and 1482.66 m s−1 , respectively, at 20 ◦ C as given in the instruction manual of the instrument. The uncertainties of measurements were ±5 × 10−6 g cm−3 for density and 0.1 m s−1 for ultrasound speed. The instrument required a liquid volume of about 2.5 mL and measured the density and ultrasound speed simultaneously after a thermal equilibration period of about 5–10 min. Density and ultrasound speed measurements were carried out for selected solutions of glycine in water and in PEG400 solutions at various molalities and temperatures. The molalities of PEG400 solutions were 0.1, 0.4, 0.8, 1.2, 1.6 or 2.0 mol kg−1 and the selected temperatures were 293.15, 298.15, 303.15 and 308.15 K. The weighings were made on a digital balance (Scaltec SPB31) with a precision of 0.1 mg. The measured densities and ultrasound speeds were utilized in determining apparent molar volumes and isentropic compressions as described in the next section.
VΦ /(cm3.mol -1)
156
S =
1 u2o o 1 u2
(5) (6)
So, the determination of apparent molar isentropic compressions involved the usage of measured densities and ultrasound speeds in Eqs. (1), (5), (6) and then (4), successively. The dependence of apparent molar isentropic compression, KS˚ , of glycine on concentration can be given by the following empirical equation: o + bSK m KS˚ = KS˚
(7)
o where KS˚
is the apparent molar isentropic compression at infinite dilution and bSK is an experimentally determined parameter. KS˚ o and b , vs m data were regressed linearly and the parameters, KS˚ SK were determined from the intercept and slope, respectively. Typical KS˚ vs m plots are given in Fig. 2a for glycine in water at 308.15 K and in Fig. 2b for glycine in 0.8 mol kg−1 PEG400 at 293.15 K. The linear regression coefficients for these particular plots were 0.9985 and 0.9456, respectively. o , b , K o , and b V∅ v S˚ SK values for glycine in water and in solvents as PEG400 solutions of varying molalities and at the four temperatures are collectively given in Table 2. 4. Discussion o results at 298.15 K and 308.15 K, and K o results at The V∅ S˚ 298.15 K compare well with the available literature data given in
Table 1 Density and ultrasound speeds for solutions of glycine in water and in PEG400 solutions at various molalities and temperatures. T:
293.15 K −1
−3
298.15 K −1
−3
303.15 K −1
−3
308.15 K −1
Molality mol kg
g cm
u ms
g cm
u ms
g cm
u ms
g cm−3
u m s−1
H2 O
0.0000 0.0417 0.0886 0.1129 0.1823 0.2472 0.3003 0.3600 0.4148 0.4551 0.5234
0.998202 0.999527 1.001013 1.001773 1.003933 1.005939 1.007563 1.009388 1.011055 1.012279 1.014315
1482.52 1484.86 1487.38 1488.67 1492.39 1495.79 1498.59 1501.76 1504.51 1506.57 1510.15
0.997044 0.998355 0.999825 1.000576 1.002715 1.004702 1.006311 1.008114 1.009764 1.010974 1.012989
1496.72 1498.95 1501.40 1502.64 1506.24 1509.54 1512.24 1515.29 1518.05 1519.98 1523.49
0.995647 0.996945 0.998401 0.999146 1.001265 1.003235 1.004831 1.006621 1.008255 1.009454 1.011451
1509.14 1511.25 1513.61 1514.82 1518.30 1521.52 1524.16 1527.10 1529.77 1531.67 1535.12
0.994032 0.995321 0.996766 0.997506 0.999611 1.001567 1.003152 1.004927 1.006552 1.007739 1.009723
1519.83 1521.87 1524.16 1525.35 1528.73 1531.87 1534.45 1537.31 1539.94 1541.81 1545.18
0.1 mol kg−1 PEG400
0.0000 0.0500 0.0998 0.1497 0.2004 0.2500 0.2999 0.3498 0.4000 0.4498 0.4999
1.004440 1.006039 1.007597 1.009200 1.010731 1.012272 1.013774 1.015340 1.016791 1.018388 1.019792
1505.99 1508.66 1510.99 1513.39 1515.89 1518.38 1520.94 1523.60 1526.09 1528.77 1531.20
1.003163 1.004743 1.006287 1.007869 1.009387 1.010914 1.012401 1.013928 1.015387 1.016974 1.018358
1518.48 1520.99 1523.27 1525.58 1528.05 1530.49 1532.88 1535.46 1537.89 1540.50 1542.82
1.001654 1.003219 1.004752 1.006321 1.007826 1.009338 1.010815 1.012296 1.013777 1.015348 1.016723
1528.20 1530.94 1533.36 1535.89 1538.33 1540.81 1543.17 1545.70 1548.24 1550.60 1552.84
0.999935 1.001488 1.003012 1.004571 1.006065 1.007569 1.009032 1.010469 1.011977 1.013525 1.014904
1537.35 1540.03 1542.40 1544.87 1547.25 1549.66 1551.95 1554.43 1556.71 1559.21 1561.40
0.4 mol kg−1 PEG400
0.0000 0.0988 0.1521 0.2035 0.2495 0.3008 0.4045 0.4649 0.4981
1.020791 1.023866 1.025612 1.027089 1.028450 1.029998 1.033013 1.034472 1.035696
1564.90 1570.13 1573.25 1575.51 1578.51 1580.79 1585.66 1588.19 1590.06
1.019151 1.022199 1.023929 1.025397 1.026742 1.028278 1.031269 1.032713 1.033929
1572.44 1577.50 1580.55 1582.73 1585.66 1587.85 1592.62 1595.13 1596.95
1.017311 1.020338 1.022056 1.023513 1.024849 1.026373 1.029345 1.030780 1.031988
1578.64 1583.57 1586.53 1588.70 1591.54 1593.64 1598.36 1600.78 1602.58
1.015292 1.018305 1.020009 1.021458 1.022786 1.024298 1.027257 1.028685 1.029885
1583.56 1588.39 1591.29 1593.42 1596.19 1598.23 1602.88 1605.23 1606.97
0.8 mol kg−1 PEG400
0.0000 0.1000 0.1509 0.2022 0.2504 0.3006 0.3495 0.4019 0.4494 0.4984
1.038734 1.041973 1.043465 1.045082 1.046552 1.047845 1.049489 1.050908 1.052266 1.053638
1628.87 1634.07 1636.41 1639.30 1641.30 1643.84 1646.18 1648.30 1650.51 1652.65
1.036660 1.039861 1.041337 1.042940 1.044405 1.045695 1.047322 1.048734 1.050082 1.051449
1630.73 1635.89 1638.15 1640.99 1642.99 1645.44 1647.83 1649.91 1652.03 1654.16
1.034407 1.037589 1.039056 1.040649 1.042108 1.043393 1.045008 1.046417 1.047758 1.049119
1631.70 1636.74 1639.00 1641.75 1643.76 1646.15 1648.55 1650.61 1652.64 1654.77
1.032006 1.035175 1.036632 1.038220 1.039671 1.040955 1.042562 1.043966 1.045302 1.046658
1631.73 1636.66 1638.91 1641.59 1643.63 1645.94 1648.34 1650.38 1652.37 1654.47
1.2 mol kg−1 PEG400
0.0000 0.0756 0.1019 0.1476 0.2001 0.2504 0.3004 0.3517 0.4004
1.053633 1.056249 1.056865 1.058154 1.059863 1.061248 1.062798 1.063830 1.065028
1677.10 1681.83 1682.55 1684.26 1687.40 1689.00 1691.57 1692.85 1695.04
1.051132 1.053731 1.054349 1.055631 1.057330 1.058710 1.060246 1.061289 1.062487
1674.53 1679.18 1679.89 1681.60 1684.71 1686.30 1688.84 1690.12 1692.29
1.048501 1.051087 1.051705 1.052985 1.054676 1.056053 1.057583 1.058628 1.059824
1671.31 1675.85 1676.57 1678.29 1681.35 1682.89 1685.44 1686.74 1688.91
1.045750 1.048326 1.048945 1.050221 1.051908 1.053284 1.054806 1.055859 1.057049
1667.40 1671.83 1672.55 1674.29 1677.32 1678.82 1681.36 1682.67 1684.84
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
Solvent
157
158
Table 1 (Continued) T:
293.15 K −1
298.15 K
−3
−1
−3
303.15 K −1
308.15 K
−3
−1
Molality mol kg
g cm
u ms
g cm
u ms
g cm
u ms
g cm−3
u m s−1
1.6 mol kg−1 PEG400
0.0000 0.0759 0.1269 0.1528 0.2007 0.2494 0.3009 0.3509 0.4045
1.064265 1.066743 1.068553 1.069299 1.070819 1.071745 1.073557 1.074666 1.076094
1709.85 1712.69 1715.47 1716.81 1718.68 1719.52 1722.73 1724.36 1726.21
1.061483 1.063932 1.065727 1.066472 1.067988 1.068919 1.070719 1.071830 1.073258
1704.07 1707.00 1709.67 1710.95 1712.92 1713.77 1716.91 1718.50 1720.37
1.058579 1.061011 1.062799 1.063539 1.065054 1.065990 1.067781 1.068895 1.070322
1697.81 1700.78 1703.40 1704.64 1706.61 1707.56 1710.60 1712.18 1714.05
1.055569 1.057997 1.059773 1.060513 1.062026 1.062968 1.064752 1.065869 1.067297
1691.03 1694.02 1696.60 1697.80 1699.81 1700.81 1703.77 1705.34 1707.21
2.0 mol kg−1 PEG400
0.0000 0.0263 0.0750 0.1009 0.2019 0.2519 0.2992 0.3513 0.4003
1.075048 1.076117 1.077659 1.078438 1.080803 1.082446 1.083775 1.084451 1.086260
1736.20 1737.06 1739.84 1740.96 1744.29 1746.34 1747.78 1748.06 1751.10
1.071749 1.073004 1.074544 1.075315 1.077692 1.079327 1.080655 1.081346 1.083145
1727.66 1728.80 1731.17 1732.36 1735.72 1737.81 1739.29 1739.58 1742.53
1.068754 1.069802 1.071341 1.072111 1.074495 1.076125 1.077454 1.078158 1.079946
1718.74 1719.91 1722.20 1723.41 1726.82 1728.91 1730.39 1730.80 1733.63
1.065480 1.066518 1.068060 1.068828 1.071221 1.072851 1.074177 1.074896 1.076677
1709.46 1710.61 1712.84 1714.05 1717.52 1719.60 1721.08 1721.61 1724.33
Table 2 o o (cm3 mol−1 ), bv (cm3 kg mol−1 ), KS˚ (cm3 mol−1 MPa−1 ) and bSK (cm3 kg mol−1 MPa−1 ) values for glycine in water and in solvents as PEG400 solutions of varying molalities at (293.15, 298.15, 303.15 and 308.15) K. V∅ T/K
Solvent
0.1 mol kg−1 PEG400
H2 O
0.4 mol kg−1 PEG400
0.8 mol kg−1 PEG400
1.2 mol kg−1 PEG400
1.6 mol kg−1 PEG400
2.0 mol kg−1 PEG400
293.15
o
V∅ bv o KS˚ bKS
43.23 0.780 −0.0290 0.0074
± ± ± ±
0.02 0.072 0.002b 0.0006
42.12 3.95 −0.0280 0.010
± ± ± ±
0.22 0.72 0.0006 0.0019
42.92 3.01 −0.0227 0.0082
± ± ± ±
0.25 0.78 0.0006 0.0018
42.14 3.81 −0.0181 0.0077
± ± ± ±
0.24 0.72 0.0003 0.0010
40.64 11.2 −0.0193 0.0244
± ± ± ±
0.72 2.82 0.0015 0.0060
39.94 10.8 −0.0119 0.0082
± ± ± ±
0.63 2.45 0.0013 0.0050
37.06 25.22 −0.0129 0.0211
± ± ± ±
1.37 5.52 0.0010 0.0041
298.15
o V∅ bv o KS˚ bKS
43.58 0.742 −0.0266 0.0061
± ± ± ±
0.02 0.062 0.0001 0.0004
42.48 3.70 −0.0261 0.0094
± ± ± ±
0.26 0.85 0.0004 0.0016
43.20 2.97 −0.0213 0.0075
± ± ± ±
0.25 0.77 0.0006 ± 0.0018
42.54 3.48 −0.0176 0.0075
± ± ± ±
0.23 0.70 0.0004 0.0011
40.85 11.1 −0.0190 0.0238
± ± ± ±
0.70 2.76 0.0015 0.0059
40.34 10.2 −0.0121 0.0086
± ± ± ±
0.62 2.43 0.0011 0.0046
37.34 24.6 −0.0144 0.0254
± ± ± ±
1.33 5.37 0.0009 0.0036
303.15
o V∅ bv o KS˚ bKS
43.91 0.658 −0.0242 0.00458
± ± ± ±
0.02 0.053 0.0000 0.0001
42.77 3.67 −0.0247 0.0090
± ± ± ±
0.26 0.84 0.0005 0.0015
43.44 2.95 −0.0203 0.0072
± ± ± ±
0.25 0.77 0.0006 0.0017
42.76 3.41 −0.0171 0.0071
± ± ± ±
0.23 0.70 0.0003 0.0010
41.02 11.0 −0.0187 0.0231
± ± ± ±
0.69 2.73 0.0015 0.0057
40.59 9.86 −0.0123 0.0088
± ± ± ±
0.62 2.43 0.0011 0.0043
37.58 24.1 −0.0147 0.0255
± ± ± ±
1.30 5.22 0.0009 0.0037
308.15
o V∅ bv o KS˚ bKS
44.15 0.657 −0.0227 0.0038
± ± ± ±
0.02a 0.048 0.0000 0.0001
43.02 3.60 −0.0235 0.0087
± ± ± ±
0.25 0.83 0.0005 0.0015
43.63 2.98 −0.0196 0.0071
± ± ± ±
0.24 0.75 0.0006 0.0017
42.93 3.38 −0.0167 0.0067
± ± ± ±
0.23 0.71 0.0003 0.0009
41.15 10.9 −0.0184 0.0222
± ± ± ±
0.69 2.70 0.0014 0.056
40.72 9.76 −0.0125 0.0091
± ± ± ±
0.61 2.37 0.0011 0.0041
37.80 23.6 −0.0146 0.0246
± ± ± ±
1.25 5.1 0.0009 0.0035
a
o a Some of literature V∅ values are 43.19 [9,18], 43.12 [19], 43.14 [20], 43.20 [21–23], 43.46 [24], 43.26 [24,25], 43.25 [26], 43.23 [27], 43.01 [28], 43.33 [29], 43.50 [30], 43.09 [31,32], 43.30 [33,34], 42.54 [35,36], 42.40 [33], 43.22 [23], and 43.29 [37] at 293.15 K, 43.199 [38] at 298.15 K and 43.81 [9], 43.69 [19], 43.80 [33], and 43.85 [22] at 308.15 K. o b Some of literature KS˚ values at this temperature are −0.02650[9], −0.02660[10,21], −0.02700 [18,24], −0.02740 [32], −0.02709 [39], −0.02500 [40], and 0.02716 [27].
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
Solvent
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
/ (cm3.mol-1.MPa-1)
-0.02
Table 3 Apparent molar isobaric expansions of glycine in water and in PEG400 solutions of various molalities.
(a)
-0.021
K
SΦ
-0.022
-0.023
0
0.1
0.2
0.3
0.4
0.5
0.6
m /(mol.kg-1)
KSΦ / (cm3.mol-1.MPa-1)
- 0.014
(b)
- 0.015
- 0.016
- 0.017
- 0.018
0
0.1
0.2
0.3
0.4
0.5
0.6
m / (mol.kg-1) o Fig. 2. Typical KS˚ vs m plots (a) for glycine in water at 308.15 K and (b) for glycine in 0.8 mol kg−1 PEG400 at 293.15 K.
38
VΦo /( cm3.mol-1.K-1)
159
37.75
37.5
37.25
37 290
295
300
305
310
T/K o
−1
Fig. 3. A typical V∅ vs T plot for 2 mol kg
PEG400–glycine–water system.
the footnote of Table 2. All other results are being reported for the first time according to our survey. o values given in Table 2 are seen to increase regularly with V∅ temperature for glycine–water and all PEG400–glycine–water syso can best be discussed tems. The temperature dependence of V∅ on the bases of apparent molar isobaric expansion at infinite diluo . V o values are plotted as a function of temperature for tion, E∅ ∅ each system. Good linear relations were observed as seen in a typical plot of (2.0 mol kg−1 PEG400)–glycine–water system in Fig. 3. o can be taken as constant over the These linearities showed that E∅ o was determined from the slope temperature range studied. Thus E∅ o of the V∅ vs T line obtained for glycine–water or glycine–PEG400 solution systems by linear regression analysis. The same treatment was applied to V∅ vs T data at different molalities of glycine in water and in PEG400 solutions to provide E∅ values defined by Eq. (3). All o values are given in Table 3. the E∅ and E∅ o values displayed no regular trend with increasing Although, E∅ molality of PEG400 in the system, they are clearly lower for ternary systems than the binary system. This shows that the temperature o is slightly less for ternary PEG400–glycine–water dependence of V∅ systems than for binary glycine–water system. E∅ values are, in
Molality of glycine mol kg−1
E∅ cm3 mol−1 K−1
In H2 O 0 0.0417 0.0886 0.1129 0.1823 0.2472 0.3003 0.3600 0.4148 0.4551 0.5234
0.062 0.062 0.062 0.061 0.060 0.059 0.059 0.059 0.058 0.059 0.058
± ± ± ± ± ± ± ± ± ± ±
0.004a 0.004 0.004 0.004 0.004 0.003 0.003 0.003 0.003 0.003 0.003
In 0.1 mol kg−1 PEG400 0 0.0497 0.1009 0.2019 0.2498 0.3505 0.4005 0.4995
0.060 0.060 0.060 0.059 0.060 0.056 0.061 0.062
± ± ± ± ± ± ± ±
0.003a 0.003 0.003 0.003 0.003 0.004 0.004 0.004
In 0.4 mol kg−1 PEG400 0 0.0988 0.1521 0.2035 0.2496 0.3008 0.4045 0.4650 0.4981
0.047 0.046 0.048 0.047 0.048 0.048 0.047 0.046 0.046
± ± ± ± ± ± ± ± ±
0.003a 0.004 0.003 0.002 0.003 0.002 0.002 0.003 0.003
In 0.8 mol kg−1 PEG400 0 0.100 0.1509 0.2022 0.2504 0.3006 0.3695 0.4020 0.4495 0.4984
0.052 0.052 0.049 0.047 0.043 0.040 0.042 0.040 0.039 0.039
± ± ± ± ± ± ± ± ± ±
0.008a 0.006 0.006 0.006 0.004 0.004 0.004 0.004 0.004 0.003
In 1.2 mol kg−1 PEG400 0 0.0756 0.1019 0.1476 0.2001 0.2504 0.3005 0.3517 0.4004
0.034 0.037 0.029 0.028 0.029 0.027 0.030 0.025 0.025
± ± ± ± ± ± ± ± ±
0.003a 0.003 0.001 0.002 0.002 0.002 0.003 0.002 0.001
In 1.6 mol kg−1 PEG400 0 0.0760 0.1269 0.1528 0.2007 0.4494 0.3009 0.3509 0.4045
0.052 0.050 0.045 0.042 0.035 0.029 0.030 0.027 0.025
± ± ± ± ± ± ± ± ±
0.009a 0.007 0.006 0.006 0.004 0.004 0.004 0.003 0.003
In 2.0 mol kg−1 PEG400 0 0.0236 0.1009 0.2019 0.2519 0.2992 0.3513 0.4003
0.049 0.068 0.031 0.016 0.017 0.017 0.011 0.015
± ± ± ± ± ± ± ±
0.002a 0.001 0.003 0.001 0.002 0.001 0.001 0.001
a
o Infinite dilution value; E∅ .
160
M. Sahin et al. / Fluid Phase Equilibria 300 (2011) 155–161
Table 4 o , in cm3 mol−1 and isentropic compression„ in cm3 mol−1 MPa−1 of glycine from H2 O to PEG400 solutions at infinite dilution. Transfer apparent molar volumes, V∅ ,trs T/K
293.15
Transfer from H2 O to
o V∅ ,trs
KSo∅,trs
o V∅ ,trs
KSo∅,trs
o V∅ ,trs
KSo∅,trs
o V∅ ,trs
KSo∅,trs
−0.223 −0.314 −1.09 −2.59 −3.29 −6.17
0.004 0.006 0.009 0010 0.017 0.016
−0.200 −0.379 −1.04 −2.74 −3.25 −6.24
0.005 0.006 0.008 0.008 0.007 0.015
−0.230 −0.469 −1.15 −2.89 −3.32 −6.33
0.000 0.004 0.007 0.005 0.012 0.009
−0.231 −0.521 −1.23 −2.99 −3.44 −6.35
0.000 0.003 0.006 0.005 0.010 0.008
−1
0.1 mol kg 0.4 mol kg−1 0.8 mol kg−1 1.2 mol kg−1 1.6 mol kg−1 2.0 mol kg−1
PEG400 PEG400 PEG400 PEG400 PEG400 PEG400
298.15
303.15
general, seen to decrease with molality of glycine. This decrease is becoming more striking in PEG solution of higher molalities. o values given in Table 2 are all negative for both binary sysKS˚ tem and ternary systems. This means that introduction of glycine into water or PEG400 solution at any molality causes a decrease in compression which can be explained by the hydrophilic nature of glycine with two ionic centers in its zwitterionic form. Electrostriction of water in the primary hydration shell around these ionic centers results in decreased compression. On the other hand o values show a very small dependence on temperature. In genKS˚ eral, they slightly increase with T with a few exceptions (Table 2). o values to temperature is understandable The less sensitivity of KS˚ from the fact that they reflect not the volume itself but the change in volume with pressure. The change in volume with pressure is not expected to be influenced from temperature as much as the volume itself. One of the aims of the present work was to investigate the interactions between glycine and PEG400 through volumetric properties. Probably the best property is the transfer volume and compression of glycine in going from water to PEG400 solution of different molality to investigate such interactions. So the transfer o apparent molar volume at infinite dilution, V∅,trs , and the transo of glycine fer apparent molar isentropic compressions, KS∅,trs from water to PEG400 solutions are calculated from Eqs. (8) and (9), respectively: o o o V∅,trs = V∅,(in − V∅,(in aqueous X m PEG400) water)
(8)
o o o = KS˚,(in − KS˚ KS∅,trs aqueous X m PEG400) (in water)
(9)
where X is the molality of PEG400. These transfer properties are given in Table 4. It is clearly seen that all transfer volumes are negative and becoming more negative with increasing molality of PEG400, while all the transfer compression are positive and increase, in general, with increasing molality of PEG400, with a few irregularities in the general trend. This behavior can only be explained by the partial hydrophobic hydration of PEG400 in the absence of glycine [6]. When glycine is introduced into PEG400 solution, it interacts with PEG400 through ion-dipole and possibly H-bonding interactions releasing some hydrophobically hydrated water molecules around PEG400 into bulk water. Here in this interaction, ionic centers are at two end groups of glycine in zwitterionic form and the dipole is associated with the etheric –O– group of PEG400. Hydrophobically hydrated water is known to be in an icelike structure having greater volume but less compression than ordinary water [15]. So, when they are released into normal bulk water structure their volumes decrease and compression increase, as clearly observed from Table 4. This effect is less manifested in o o KS˚,trs values, again due to the fact that V∅,trs reflects directly the change in volume of glycine in going from aqueous solution o to PEG400 solutions while KS˚,trs reflects only the change in this o V∅,trs with pressure which is expected to be less significant than o V∅,trs itself.
308.15
5. Conclusion o changes linearly with temperature for It is found that V∅ glycine–water and glycine–water–PEG400 systems within the studied temperature range. So, the apparent molar isobaric expansions were constant and their numerical values were lower for o ternary systems studied than binary glycine–water system. KS∅ values were found to show less dependence on temperature than o values. Transfer volumes of glycine from water to PEG400 soluV∅ tions were found to be negative and to become more negative with increasing PEG400 concentration. The corresponding transfer compressions were in general positive and increased with PEG concentration. Transfer volumes and compressions were explained by the partial hydrophobic hydration of PEG400 in the absence of glycine.
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