- Email: [email protected]
Light scattering in dilute solutions of tetramethylurea in ethylene glycol
journal of MOLECULAR
LIQUIDS ELSEVIER
Journal of Molecular Liquids 80 (1999) 27-31
L i g h t scattering in dilute s o l u t i o n s o f t e t r a m e t h y l u r e a in e t h y l e n e g l y c o l I. A. Chaban a , M. N. Rodnikova b , J. Barthe[ c , L. L. Chaikov d , S. V. Kfivokhiza d , V. V. Zlaackova b a Institute ofAconstics, Shvernic str. 4, 117036 Moscow, Russia b Institute of General and Inorganic Chemistry R.AS, Leninskii pr.31, 117907 Moscow, Russia ¢ Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany d Physical Institute RAS, Leninskii pr. 53, 117333 Moscow, Russia Received 10 December 1997; accepted 8 November 1998 The light scattering intensity o f ethylene glycol-tetramethylurea (TMU) solutions was measured at 25 °C and 50 °C at low concentrations of TMU. The results show the hydrogen bond network of ethylene glycol and indicate the unattainable critical solution point. © 1999 Elsevier Science B.V. All rights reserved. 1. I N T R O D U C T I O N The concentration dependence of various physical and chemical properties of aqueous solutions of many non-electrolytes show peculiarities at low concentrations such as a maximum of the light scattering coefficient, a maximum of the asymmetry of indicatrice of scattering, a zero limit of the temperature coefficient of sound velocity, a minimum of the molar partial volume ere [1-3]. These peculiarities do not take place at the same concentration, but in a narrow concentration interval. In refs. [4-6] we have shown that these peculiarities result from the destruction of the hydrogen bond network of water at a concentration of the non-electrolyte, Co characteristic of the temperature. Two phenomena yield the network destruction: the precritical phenomena connected with the unattainable critical solution point [4,5] and phenomena induced by the appearance of non-network regions in the hydrogen bond network of water at concentrations cco. The precritical phenomena are due to the displacement of nonelectrolyte molecules from the hydrogen bond network of water to defects of this network (own defects, inclusions, surfaces and so on) in the same way as impurities in crystals are displaced to defects. This process breaks off when the network is destroyed at C=Co. As a
0167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. P11S0167-7322(98) 00012-4
28 consequence of this destruction the criticalsolution point is unattainable [4-6]. Traces of a third component may accelerate the process or also slow it down. This stratification process leads to an abrupt increase of light scattering in the solution and a maximum of the light scattering coefficient at concentration co. The temperature and concentration dependence of the light scattering coefficient at concentrations c
2. E X P E R I M E N T A L
RESULTS
Measurements at 632.8 nm (He-Ne laser) of the light scattering intensity were carried out in the Physical Institute RAS using the correlator K 7023 of "Malvern instrumentas" as figure signal accumulator [11]. The intensity of the scattered light is measured at 25 °C and 50 °C at different scattering angles with the help of the photo multiplier EM1-9863 KBI00 in one electron regime (precision 0.05 °C). The installation permits measurements in the range 20135°. Because of low scattering intensity, the measurements were only carried out at angles of 35, 45, 55, 65, 75, 85, 105, 115, 125°. At every angle the accumulation of the figure signal of PEM was carried out during 4-6 selection times of 20s. For every scattering angle the accumulation results above the mean value by more than 3.5 N v2 (N: impulse number during the selection time) were disregarded. In the samples the angle dependence of the scattering intensity exceeding the experimental precision of 7 % was not observed. The intensity was defined as a mean value for all angles, and for a removal of swills and defects of the cuvet surface and immersion cylinder the greatest and the smallest values were disregarded. The experiments were carried out three times (1,2,3 series) on the same samples with intervals of some months between the series. The results are shown in Figure 1 where the light scattering intensity is given in arbitrary units.
29
0.10
I
0.09 16
I I
II I
o.o8~
/
:,,
.~
I oo7 I i J 1~ ~ T.--"
"
0.06
11 I I IT
'
""
s ~"
oo=
°°° °°°°°°°
-'"'"
0.01
0.05
0
5
i0 0
15 =
Figure 1. Concentration dependence of the light scattering intensity in ethylene glycoltetramethylurea solutions at different temperatures. Series (1,2,3): (~ (1), 25 °C; 0 (2), 25 °C; @ (3), 25 °C; @ (3), 50 °C. Solid lines: experimental points, broken lines: backgrounds (leR scale); the pointed line is the contribution from the non-critical concentration fluctuations (right scale), the dot-and-pointlines give the contribution from the density fluctuations (left scale). 3. I N T E R P R E T A T I O N Figure I shows sharp maxima at 4 moI.%TMU. In the limits of the precision of the measurements the maximum intensitiesat 50 °C and 25 °C coincide. At a firstglance the ethylene glycol-TMU system differsfrom the corresponding pictureof aqueous solutionsof T M U [6].However, aftersubtractingthe background the picturebecomes
analogous.
30
0.04
0.03
0.02
0.01 V
0
10
,5
7o
15 =
Figure 2. Concentration dependence of the light scattering intensity in ethylene glycoltetramethylurea solutions at 0 25 °C, • 50°C obtained after background correction. To begin with, let us dwell on the background. Besides the concentration fluctuations connected with the unattainable critical solution point and fluctuations connected with the destruction of the hydrogen bond network, light scattering can also be induced by non-critical concentration fluctuations and by the density and temperature fluctuations. The intensity of light scattered by the non-critical concentration fluctuations will be proportional to concentration and will not depend on temperature. In contrast, the contribution from the density and temperature fluctuations will depend on temperature but will not depend on concentration. That gives the opportunity to separate this contribution from the contribution of the non-critical concentration fluctuations. The contribution from the non-critical concentration fluctuations is shown by the pointed line in Figure 1 (right scale). Because light scattering from the density fluctuations is much greater than that from the temperature fluctuations we can neglect the latter. Substraction from the background (broken lines) of the
31 non-critical concentration fluctuations (pointed line), yields for the contribution to the background, which are connected with the density fluctuations, two lines corresponding to two temperatures shown by dot-and-point lines in Figure 1 (left scale). The light scattering intensifies corresponding to these lines must be related to absolute temperatures. For the ethylene glycol-TMU system, the intensity relation, corresponding to the upper and lower lines shown by dot-and-point, is I'/I"=l.14. The relation of the corresponding temperatures of T'/T"ffil.08 is equal within the experimental errors. Subtraction of the background yields the curves shown in Figure 2 which a r e analogous to those for aqueous solutions of TMU [6] showing the structure analogy of TMU solutions in water and ethylene glycol as hydrogene bonded networks and the existence of the unattainable upper critical solution point.
REFERENCES 1. M.F. Vuks, Light scattering in gases, fluids and solutions. L., Pub.LGU. (1977) 320. 2. M.N. Rodnikova, L.V. Lanshina, in "Molecular physics and biophysics of aqueous systems", L., Pub.LGU. V.8 (1991) 42. 3. A.P. Kulikova, M.F. Vuks, L.V. Shurupova, in "Molecular physics and biophysics of aqueous systems", L., Pub.LGU. V.5 (1983) 43. 4. M.N. Rodnikova, L.V. Lanshina, I.A. Chaban, Dokl. Akad. Nauk SSSR 315 (1990) 148. 5. L.V. Lanshina, M.N. Rodnikova, I.A. Chaban, Russ. J. Phys. Chem. 66 (1992) 107. 6. I.A. Chaban, M.N. Rodnikova, V.V. Zhakova, Biofizika 41 (1996) 293. 7. M.N. Rodnikova, Zh. Fiz. Khim. 67 (1993) 275. 8. B.N. Kartsev, M.N. Rodnikova, V.V. Tsepulin, A.B. Razumova, Zh. Fiz. Khim. 68 (1994) 1915. 9. L. Endom, H.G. Hertz, B. Thuel, M.D. Zeidler, Bet. Buusenges. Phys. Chem. 71 (1967) 1008. 10. O.Y~LSamoilov, Stnxkaaa Vodaykh Rastvorov Elektrolitov i Gidratatsiya Ionov (Structure of aqueous solutions of electrolytes and hydration of ions.) Izdatel. Akad. Nauk SSSR, 1957. 11. S.V. Krivokhizha, O.A. Lugovaya, L.L. Chaikov, Opt. Spektrosk. 56 (1984) 381.
LIQUIDS ELSEVIER
Journal of Molecular Liquids 80 (1999) 27-31
L i g h t scattering in dilute s o l u t i o n s o f t e t r a m e t h y l u r e a in e t h y l e n e g l y c o l I. A. Chaban a , M. N. Rodnikova b , J. Barthe[ c , L. L. Chaikov d , S. V. Kfivokhiza d , V. V. Zlaackova b a Institute ofAconstics, Shvernic str. 4, 117036 Moscow, Russia b Institute of General and Inorganic Chemistry R.AS, Leninskii pr.31, 117907 Moscow, Russia ¢ Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany d Physical Institute RAS, Leninskii pr. 53, 117333 Moscow, Russia Received 10 December 1997; accepted 8 November 1998 The light scattering intensity o f ethylene glycol-tetramethylurea (TMU) solutions was measured at 25 °C and 50 °C at low concentrations of TMU. The results show the hydrogen bond network of ethylene glycol and indicate the unattainable critical solution point. © 1999 Elsevier Science B.V. All rights reserved. 1. I N T R O D U C T I O N The concentration dependence of various physical and chemical properties of aqueous solutions of many non-electrolytes show peculiarities at low concentrations such as a maximum of the light scattering coefficient, a maximum of the asymmetry of indicatrice of scattering, a zero limit of the temperature coefficient of sound velocity, a minimum of the molar partial volume ere [1-3]. These peculiarities do not take place at the same concentration, but in a narrow concentration interval. In refs. [4-6] we have shown that these peculiarities result from the destruction of the hydrogen bond network of water at a concentration of the non-electrolyte, Co characteristic of the temperature. Two phenomena yield the network destruction: the precritical phenomena connected with the unattainable critical solution point [4,5] and phenomena induced by the appearance of non-network regions in the hydrogen bond network of water at concentrations c
0167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. P11S0167-7322(98) 00012-4
28 consequence of this destruction the criticalsolution point is unattainable [4-6]. Traces of a third component may accelerate the process or also slow it down. This stratification process leads to an abrupt increase of light scattering in the solution and a maximum of the light scattering coefficient at concentration co. The temperature and concentration dependence of the light scattering coefficient at concentrations c
2. E X P E R I M E N T A L
RESULTS
Measurements at 632.8 nm (He-Ne laser) of the light scattering intensity were carried out in the Physical Institute RAS using the correlator K 7023 of "Malvern instrumentas" as figure signal accumulator [11]. The intensity of the scattered light is measured at 25 °C and 50 °C at different scattering angles with the help of the photo multiplier EM1-9863 KBI00 in one electron regime (precision 0.05 °C). The installation permits measurements in the range 20135°. Because of low scattering intensity, the measurements were only carried out at angles of 35, 45, 55, 65, 75, 85, 105, 115, 125°. At every angle the accumulation of the figure signal of PEM was carried out during 4-6 selection times of 20s. For every scattering angle the accumulation results above the mean value by more than 3.5 N v2 (N: impulse number during the selection time) were disregarded. In the samples the angle dependence of the scattering intensity exceeding the experimental precision of 7 % was not observed. The intensity was defined as a mean value for all angles, and for a removal of swills and defects of the cuvet surface and immersion cylinder the greatest and the smallest values were disregarded. The experiments were carried out three times (1,2,3 series) on the same samples with intervals of some months between the series. The results are shown in Figure 1 where the light scattering intensity is given in arbitrary units.
29
0.10
I
0.09 16
I I
II I
o.o8~
/
:,,
.~
I oo7 I i J 1~ ~ T.--"
"
0.06
11 I I IT
'
""
s ~"
oo=
°°° °°°°°°°
-'"'"
0.01
0.05
0
5
i0 0
15 =
Figure 1. Concentration dependence of the light scattering intensity in ethylene glycoltetramethylurea solutions at different temperatures. Series (1,2,3): (~ (1), 25 °C; 0 (2), 25 °C; @ (3), 25 °C; @ (3), 50 °C. Solid lines: experimental points, broken lines: backgrounds (leR scale); the pointed line is the contribution from the non-critical concentration fluctuations (right scale), the dot-and-pointlines give the contribution from the density fluctuations (left scale). 3. I N T E R P R E T A T I O N Figure I shows sharp maxima at 4 moI.%TMU. In the limits of the precision of the measurements the maximum intensitiesat 50 °C and 25 °C coincide. At a firstglance the ethylene glycol-TMU system differsfrom the corresponding pictureof aqueous solutionsof T M U [6].However, aftersubtractingthe background the picturebecomes
analogous.
30
0.04
0.03
0.02
0.01 V
0
10
,5
7o
15 =
Figure 2. Concentration dependence of the light scattering intensity in ethylene glycoltetramethylurea solutions at 0 25 °C, • 50°C obtained after background correction. To begin with, let us dwell on the background. Besides the concentration fluctuations connected with the unattainable critical solution point and fluctuations connected with the destruction of the hydrogen bond network, light scattering can also be induced by non-critical concentration fluctuations and by the density and temperature fluctuations. The intensity of light scattered by the non-critical concentration fluctuations will be proportional to concentration and will not depend on temperature. In contrast, the contribution from the density and temperature fluctuations will depend on temperature but will not depend on concentration. That gives the opportunity to separate this contribution from the contribution of the non-critical concentration fluctuations. The contribution from the non-critical concentration fluctuations is shown by the pointed line in Figure 1 (right scale). Because light scattering from the density fluctuations is much greater than that from the temperature fluctuations we can neglect the latter. Substraction from the background (broken lines) of the
31 non-critical concentration fluctuations (pointed line), yields for the contribution to the background, which are connected with the density fluctuations, two lines corresponding to two temperatures shown by dot-and-point lines in Figure 1 (left scale). The light scattering intensifies corresponding to these lines must be related to absolute temperatures. For the ethylene glycol-TMU system, the intensity relation, corresponding to the upper and lower lines shown by dot-and-point, is I'/I"=l.14. The relation of the corresponding temperatures of T'/T"ffil.08 is equal within the experimental errors. Subtraction of the background yields the curves shown in Figure 2 which a r e analogous to those for aqueous solutions of TMU [6] showing the structure analogy of TMU solutions in water and ethylene glycol as hydrogene bonded networks and the existence of the unattainable upper critical solution point.
REFERENCES 1. M.F. Vuks, Light scattering in gases, fluids and solutions. L., Pub.LGU. (1977) 320. 2. M.N. Rodnikova, L.V. Lanshina, in "Molecular physics and biophysics of aqueous systems", L., Pub.LGU. V.8 (1991) 42. 3. A.P. Kulikova, M.F. Vuks, L.V. Shurupova, in "Molecular physics and biophysics of aqueous systems", L., Pub.LGU. V.5 (1983) 43. 4. M.N. Rodnikova, L.V. Lanshina, I.A. Chaban, Dokl. Akad. Nauk SSSR 315 (1990) 148. 5. L.V. Lanshina, M.N. Rodnikova, I.A. Chaban, Russ. J. Phys. Chem. 66 (1992) 107. 6. I.A. Chaban, M.N. Rodnikova, V.V. Zhakova, Biofizika 41 (1996) 293. 7. M.N. Rodnikova, Zh. Fiz. Khim. 67 (1993) 275. 8. B.N. Kartsev, M.N. Rodnikova, V.V. Tsepulin, A.B. Razumova, Zh. Fiz. Khim. 68 (1994) 1915. 9. L. Endom, H.G. Hertz, B. Thuel, M.D. Zeidler, Bet. Buusenges. Phys. Chem. 71 (1967) 1008. 10. O.Y~LSamoilov, Stnxkaaa Vodaykh Rastvorov Elektrolitov i Gidratatsiya Ionov (Structure of aqueous solutions of electrolytes and hydration of ions.) Izdatel. Akad. Nauk SSSR, 1957. 11. S.V. Krivokhizha, O.A. Lugovaya, L.L. Chaikov, Opt. Spektrosk. 56 (1984) 381.