- Email: [email protected]
Raman spectroscopic study of water in the poly(ethylene glycol) hydration shell
Journal of
ELSEVIER
MOLECULAR STRUCTURE Journal of Molecular Structure 381 (1996) 207-212
Raman spectroscopic study of water in the poly(ethylene glycol) hydration shell 1 V. Crupi, M.P. Jannelli, S. Magazu', G. Maisano, D. Majolino*, P. Migliardo, R. Ponterio Dipartimento di Fisica dell'Universita" and Unita" INFM-GNSM Messina, Contrada Papardo, P.O. BOX 55, 98166 S. Agata ( Messina), Italy
Received 12 October 1995; accepted in final form 28 February 1996
Abstract
Raman scattering spectra of polymeric aqueous solutions of poly(ethylene glycol) vs. concentration and temperature are presented. We show that the shape of the O - H band can be reproduced by a suitable superposition of the spectra of bulk and hydration water. The relative amount of the two contributions changes with concentration and temperature, furnishing information on the number of water molecules bonded to the polymer chain. The results show that, when water concentration increases, water molecules saturate the two lone pairs of each oxygen atom of the polymer in agreement with previous adiabatic compressibility and Incoherent Quasi Elastic Neutron Scattering measurements. Keywords: Hydrogen bonding; Raman spectroscopy;water; Poly(ethyleneglycol);Hydration shell
1. Introduction Over the years the role of water in solution with polymers has been extensively studied both from an experimental and a theoretical point of view [1-4]. In particular the local structure that water takes on in poly(ethylene glycol) (PEG) [5] solutions has been the object of attention because its simple molecular structure makes this polymer a good model system for the study of the fundamental H-bond in aqueous solutions and allows us to better understand concepts like 'water structure' or definitions like 'hydrogen bonded systems'. * Corresponding author. I paper presented at the Xlth International Workshop 'Horizons in Hydrogen Bond Research', Bir~tonas, Lithuania, 9-14 September 1995.
Another important reason for the interest in H 2 0 - p o l y m e r solutions is connected with the good or poor ability of the polymer to mix with water. P E G is a polymer which has the formula O H ( C H 2 - C H 2 - O ) m - H , where m is the degree of polymerization. Concerning the conformation of the P E G chain, infrared and R a m a n analysis reveal in the crystalline state a helical conformation with two helical turns per fiber identity period which is equal to 19.3 ,~. The distance between the ether oxygen atoms is 2.88 ,~, very close to that of the oxygen atoms in water which is 2.85 ,~. Furthermore, whereas in the molten state a highly disordered helix which approaches a random coil conformation exists, in water solutions, although the well-ordered helix is absent, the nonappreciable shifts in Raman frequencies from the
0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved P H S0022-2860(96)09308-8
208
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
crystalline state indicate that the trans, gauche, trans structure is largely retained. This occurrence together with the characteristic distance of the ether oxygen atoms can justify the P E G solubility in water, from oligomers to polymers of several millions molecular weight [6], at temperatures lower than the water boiling point. Previous ultrasonic measurements [7] show that, as water concentration increases, as long as it saturates the two lone pairs of each oxygen atom, some water molecules bond themselves to the polymer end groups too. At the same time, Incoherent Quasi Elastic Neutron Scattering [8] results show that the dynamic properties of the H 2 0 molecules in this environment deeply differ from those in the bulk. The aim of the present work is to study the hydration effects of P E G and their dependence on temperature and polymer concentration by a Raman analysis of the vibrational dynamics. Raman spectroscopy represents a powerful method for studying the vibrational properties of our polymeric aqueous solutions since the O - H stretching band is quite sensitive to the structural properties of the water. We will show that the shape of the O H band can be reproduced by a suitable superposition of the spectra of bulk and hydration water. A similar model, although very simple, has been successfully applied in the study of water encaged in reverse micelles, where the water molecules can be considered partitioned in water tightly bonded between the polar heads of the ionic surfactant and bulk water [9]. The relative amount of the two contributions changes with concentration and temperature, furnishing information on the number of water molecules bonded to the polymer chain.
2. Experimental procedures and results The solutions were prepared using P E G 600 purchased from Aldrich-Chemie and double distilled deionized water at the concentrations corresponding to pure P E G 600, P E G 600 + 5 H20, P E G 600 + 10 H20, P E G 600 + 30 H20, P E G 600 + 100 H 2 0 and pure H20. Care was taken in order to obtain stable clear and dust free samples. The samples were sealed
and ample time (3 days) was allowed for equilibration. After preparation the solutions were stored at 4°C in the dark to minimize biological and (photo)chemical degradation. The investigated temperature range was (20-80)°C. The Raman spectra were detected by a SPEX Ramalog 5 double spectrometer to which a third monochromator (also made by SPEX) was added. The gratings are driven by a compudrive which allows full control of the spectrometer and to execute repeated scans to optimize the signal/noise ratio. The source was an Ar + laser working at 5145 ,~ of wavelength. Laser fluctuations were corrected by using a home made interface. We performed measurements of both polarized (VV) and depolarized (VH) Raman scattering (VH indicates the directions of electric field normal to the scattering plane), starting from a frequency shift of about 2 0 c m -1 u p t o 4 0 0 0 c m 1 with a band resolution of 5 cm -~. In the present paper, however, we limit ourselves to the discussion of the O - H stretching band. In the following the isotropic contribution of the spectra are analyzed. Such spectra are obtained from the experimental data, after a correction for local field approximation, refractive index and density, as/is(W) = Ivy(W) - 4/3IvH. In fact,/is(W), as is well known, is more directly connected with the band polarizability induced by changes in the structural properties of the system [10]. In Figs. 1 and 2 we show the evaluated isotropic spectra of the pure components (H:O and P E G 600) as a function of temperature.
3. Results and discussion Usually the O - H stretching band in water is considered as the superposition of at least four bands that arise from the presence of water employed in two hydrogen bonds and of water that has a distorted hydrogen bond [11]. However, the presence in the water isotropic spectra of an isobestic point, in which the Raman intensity is temperature independent, is clearly detected (see Fig. 1), suggesting a method of interpreting the origin of the O - H stretching band [12]. In the case o f pure P E G 600 (see Fig. 3), the O - H stretching contribution, at 20°C, is obviously
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
(o): f-20°C : =
(b): T=40°C
(d)
H10
209
T=20%
: =
PEG600
x5
&
.4
i
3000 3100 3200 3300 3400 3500 3600 3700
i
3100
Stokes shift (crn-1)
3200
i
i
3300
3400
L
i
3500
,
3600
3700
Stokes shift (cm 1)
Fig. 1. Evaluated isotropic spectra of H 2 0 pure components as
a function of temperature.
Fig. 3. O - H stretching contribution of pure P E G 600 at 20°C: (I-q) experimental data; continuous line is the best fit; broken lines are the height and width of the convoluted bands.
attributed to the O - H end groups which are the active sites for the intermolecular H - b o n d interaction. Recently such an O - H stretching contribution, in low molecular weight P E G s [13], has been decomposed into symmetric bands corresponding to w~(~ 3200 cm-1), w6(~ 3350 cm - l ) and ~ 7 ( ~ 3500 cm-1), where w, is defined [14] as the stretching of the O - H groups involved in intramolecular H-bonds (i.e. when a terminal O H is bonded to an internal monomeric unit via a H - b o n d with the oxygen atom), ~6 is defined as the stretching of the fully bonded hydroxylic O - H groups (i.e. O and H bonded respectively with H and O belonging to another polymeric unit) and finally w7 is defined as the O - H stretching in which only the H atoms are bonded. The I R absorbance in that case shows that the above mentioned three sub-bands reduce to only one (~7 in which the hydrogens of the O H groups are
bonded to the oxygens of another polymeric chain) in passing from monomeric ethylene glycol to P E G 2000 (see Fig. 3 of Ref. [13]). In our case the P E G 600 can be decomposed into two bands centered at ~3350 cm -1 and ~3500 cm -1, that correspond to the ~6 and ~ O - H vibrations. The disappearance of the w, band can be justified as due to the long distance in P E G 600 between the two O - H end groups which have a weak probability of organizing the intramolecular bond because of the polymeric steric hindrance. The isotropic bands of pure P E G 600 turn out to be almost temperature independent as can be seen by an inspection of Fig. 2. In Fig. 4 we show, as an example, the isotropic spectra of the O - H stretching band for the various water concentrations at T = 80°C. It can be seen that as the water content increases the spectral
m~~ ....... + T=20°c x T=40°C o T=60°C
.,*"
PEG600
T=80OC
"...
......."~c ~o t.m..., ~
..
...'"" . ..-"" "'"...'.. .- ......
~c ~. ,+.~.~o
".
-. .......
._~
.4
. ..
_
~
~c~o+.~.~.
.-,,
...P'
...
....
• • .,.
........... ~6oo+~.~Q ......
. ..
"'.. "..
"..
-. ". '.
...."" .............. ~e~c6oo. .............. i~ ::: .1: ".
3100
3200
3300
3400
3500
3600
3700
3100
3200
3300
3400 350~ Stokes shift (era-)
3600
3700
Stokes shift (cm I)
Fig. 2. Evaluated isotropic spectra of P E G 600 pure components as a function of temperature.
Fig. 4. Isotropic spectra of the O - H stretching band for pure H 2 0 , for pure P E G 600 and for P E G 600 at various water contents; T = 80°C.
210
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
PEG~0 + ~
PEG 600 + 5 H~O
..:::" .....[7
2¢c x5 "~ .~
5100
3200
F i g . 5. S p e c t r a o f P E G
.. ". ,. "... "..
x~
&
•....
,
i 3300
T:80Oc
.. •.
,." .,..,... ..: 80OC ...." ./' :....""
...S'" "" L i_
H~
i
3400 3500 Stokes shift (cm q)
600 + 5 H20
'"'~:::::::~
i
obtained
3600
3700
by subtracting
the pure PEG 600 contribution, at T = 20°C and T = 80°C. shape changes continuously from that of pure P E G 600 to that of pure water. In order to obtain information on the number of water molecules H - b o n d e d to the polymer chain, we adopted the structural model proposed by F r a n k - W e n [3,4] for aqueous ionic solutions and applied it to the case of polymeric aqueous solutions. Accordingly water is partitioned between the polymer and the bulk phase. Provided that the pure P E G 600 O - H stretching band is subtracted from the isotropic spectra of the aqueous solutions, following such a model, the spectral profiles result from the sum of two contributions, that of the two different kinds of water molecules: the normalized spectrum of bulk water, /bulk (cO), and that of the water bonded to the polymer, /bonded(co), obtained by subtracting the pure P E G 600 spectrum from that of P E G 600 + 5 H20. At this concentration, in fact, all the water molecules are supposed to be strongly bonded to the polymer chain over the whole temperature range, as indicated by several other experimental results [5] in which the bulk water contribution results are absent. Such a procedure's results are justifiable under the hypothesis that (i) the polymer contribution to the O - H stretching bond does not appreciably change in the presence of water and that (ii) this contribution, as in the pure polymer, is temperature independent. In Fig. 5 the spectra obtained by subtracting the pure P E G 600 spectra from those of the P E G 600 + 5 H 2 0 are shown at the two extreme temperatures T = 20°C and T = 80°C.
............ ~......, ........... ;..3100
3200
3300
~
3400 3500 Stokes shift (cm 1)
; ............... 3600
,3700
F i g . 6. S p e c t r u m o f P E G 6 0 0 + 100 H 2 0 a t T = 8 0 ° C t o g e t h e r
with the component contributions and the resulting spectrum. Finally, by applying the above depicted model, already successfully adopted for electrolytic solutions [15], the spectrum of the O - H water profile at any given concentration can be decomposed at each temperature into the sum of two contributions, i.e.: /is(W, T) = ol(T)./bonded(co, T) + [1 -- a ( T ) ] " Ibulk(co, T) where c~(T) and 1 - a ( T ) are the percentages of the 'bonded' and 'free' O H groups, respectively. In Fig. 6, as an example, the spectrum of P E G 600 + 100 H 2 0 is shown at T = 80°C together with the component contributions and the resulting spectrum. In Table 1 the values of a as a function of the number of water molecules for each polymer molecule are reported at all the investigated temperatures. F r o m the obtained values of a it is possible to obtain the number n w of H 2 0 molecules bonded in the first hydration shell of polymer's chain. In table 1 the calculated values of nw are shown together with those obtained by adiabatic compressibility measurements [16]. Despite the simplicity of Ram values are in good the applied model, the nw agreement with those calculated by compressibility techniques in the case of P E G 600 + 100 H20. It is R a m and nw c o m p r at lower water obvious that nw contents (10 H 2 0 and 30 H 2 0 ) are nearly coincident with the values of the total number of water molecules and therefore are not reported in the table. The case of P E G 600 + 100 H 2 0 deserves some
V, Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
211
Table 1 V a l u e s o f a as a f u n c t i o n o f the n u m b e r o f w a t e r m o l e c u l e s f o r e a c h p o l y m e r m o l e c u l e 20°C
40°C
60°C
a
Ram nw
compr nw
a
Ram nw
compr nw
P E G 600 + 10 H 2 0
0.91
-
-
0.85
-
P E G 600 + 30 H 2 0
0.65
-
-
0.57
-
-
P E G 600 + 100 H 2 0
0.16
32
31.8
0.15
29.8
29.7
0.14
80°C compr nw
a
Ram nw
compr nw
0.80
-
0.77
-
-
0.53
-
0.50
-
-
27
0.13
26.3
26.5
a
Ram nw
27.4
n Rarn a n d n c°mpr a r e the n u m b e r nw o f H 2 0 m o l e c u l e s b o n d e d in the first h y d r a t i o n shell o f p o l y m e r ' s c h a i n o b t a i n e d by R a m a n a n d c o m p r e s s i b i l i t y m e a s u r e m e n t s respectively.
comment: the values of c~ indicate that we are, as expected, in the presence of a low water content at the polymer active sites. However, the nw values confirm that all the active sites of P E G 600 are saturated up to 80°C. In fact the values of-Ram nw compr and nw at this temperature turn out to be equal to the number (13) of oxygen atoms of the monomeric groups ( C H z C H 2 0 ) present in P E G 600. This number increases at low temperature. This general trend can be explained considering that at low temperatures the p o l y m e r - w a t e r interaction strength allows water to bond also to the terminal groups or to f o r m a second coordination shell. As t e m p e r a t u r e increases, the interaction strength decreases and this implies the loss of water molecules m o r e weakly bonded to the polymer. This process continues until all the water molecules not closely connected (via H - b o n d s ) to the m o n o m e r oxygens break their bonds.
6
(1 a)/a 4 T=20°C
2
U// • T=80°C
0
~
I
30
~
,
,
,
60
,
,
,
90
,
120
150
N Fig. 7. D e p e n d e n c e o f the (1 - a ) / ~ p a r a m e t e r on the n u m b e r o f w a t e r m o l e c u l e s f o r e a c h P E G 600 m o l e c u l e .
Finally, in Fig. 7 we show the dependence of the ( 1 - a ) / a parameter on the number of water molecules for each P E G 600 molecule. It is possible to distinguish two regions defined as I and II in Fig. 7. In region I we observe that ( 1 - a ) / ~ slowly increases with the number of water molecules up to the value of 30, which is about the m a x i m u m number of hydrated water as evaluated in these system. In region II we observe a different growth law for (1 - a ) / a vs. N induced by the existence of bulk water in the liquid mixture.
4. Concluding remarks The present paper reports the results of a study of the hydration effects of P E G and their dependence on temperature and polymer concentration by R a m a n analysis. The results show unambiguously that, as water concentration increases, the number of water molecules tightly connected to the polymer tends to saturate the two lone pairs of each oxygen atom. Moreover some water molecules are bonded to the polymer end groups. There is also a strong indication that the interaction strength of the H 2 0 molecules bonded to the ether oxygen atoms is higher than that of the end groups, because the bonds connected to the end groups break at high temperatures. Furthermore the partitioning of water into bonded and bulk phases is supported by spectroscopic evidence. Although this argument may be questionable because intermediate states exist for water, which can give different polarization effects on the fundamental O - H stretching frequency, in this case it seems to be workable, indicating that the
212
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
experimental data are not contradictory to this simple but convincing model.
References [1] U. Dahlborg and E. Braun, Phys. Ser., 14 (1976) 253. [2] U. Dahlborg, V. Dimie and B. Ovikl, Phys. Scr., 37 (1988) 95. [3] H.S. Frank and W.Y. Wen, Discuss. Faraday Soc., 24 (1965) 133. [4] A.C. Barnes, T.W.N. Bieze, J.E. Enderby and J.C. Leyte, J. Phys. Chem., 98 (1994) 11527. [5] F.E. Bailey and J.V. Kaleske, Poly(Ethylene Oxide), Academic Press, New York, 1976. [6] S. Saeki, N. Kuwahara, M. Nakata and M. Kaneko, Polymer, 17 (1976) 685. [7] G. Maisano, D. Majolino, P. Migliardo, S. Venuto, F. Aliotta, S. Magazu', Mol. Phys., 78 (1993) 421.
[8] V. Crupi, M.P. Jannelli, S. Magazu', G. Maisano, D. Majolino, P. Migliardo, C. Vasi, lI Nuovo Cimento D, 16 (1994) 809. [9] E. Bardez, B. Larrey, X.X. Zhu, B. Valeur, Chem. Phys. Lett., 171 (1990) 362. [10] S. Bratos and G. Tarjus, Physics of Modern Materials, Vol. II, IAEA, Vienna, 1980, p. 571. [11] D.E. Irish and M.M. Brooker, in H.E. Clark and R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. II, London, 1977, p. 212. [12] G. D'Arrigo, G. Maisano, F. Mallamace, P. Migliardo, F. Wanderlingh, J. Chem. Phys., 75 (1981) 4264. [13] V. Crupi, M.P. Jannelli, S. Magazu', G. Maisano, D. Majolino, P. Migliardo, D. Sirna, Mol. Phys., 84 (1995) 645. [14] H. Graener, T.Q. Je, A. Laubereau, J. Chem. Phys., 90 (1989) 3413. [15] F. Aliotta, M.P. Fontana, G. Maisano, P. Migliardo and F. Wanderlingh, Opt. Acta, 27 (1980) 931. [16] M.P. Jannelli, S. Magazu, G. Maisano, D. Majolino and P. Migliardo, J. Mol. Struct., 322 (1994) 337.
ELSEVIER
MOLECULAR STRUCTURE Journal of Molecular Structure 381 (1996) 207-212
Raman spectroscopic study of water in the poly(ethylene glycol) hydration shell 1 V. Crupi, M.P. Jannelli, S. Magazu', G. Maisano, D. Majolino*, P. Migliardo, R. Ponterio Dipartimento di Fisica dell'Universita" and Unita" INFM-GNSM Messina, Contrada Papardo, P.O. BOX 55, 98166 S. Agata ( Messina), Italy
Received 12 October 1995; accepted in final form 28 February 1996
Abstract
Raman scattering spectra of polymeric aqueous solutions of poly(ethylene glycol) vs. concentration and temperature are presented. We show that the shape of the O - H band can be reproduced by a suitable superposition of the spectra of bulk and hydration water. The relative amount of the two contributions changes with concentration and temperature, furnishing information on the number of water molecules bonded to the polymer chain. The results show that, when water concentration increases, water molecules saturate the two lone pairs of each oxygen atom of the polymer in agreement with previous adiabatic compressibility and Incoherent Quasi Elastic Neutron Scattering measurements. Keywords: Hydrogen bonding; Raman spectroscopy;water; Poly(ethyleneglycol);Hydration shell
1. Introduction Over the years the role of water in solution with polymers has been extensively studied both from an experimental and a theoretical point of view [1-4]. In particular the local structure that water takes on in poly(ethylene glycol) (PEG) [5] solutions has been the object of attention because its simple molecular structure makes this polymer a good model system for the study of the fundamental H-bond in aqueous solutions and allows us to better understand concepts like 'water structure' or definitions like 'hydrogen bonded systems'. * Corresponding author. I paper presented at the Xlth International Workshop 'Horizons in Hydrogen Bond Research', Bir~tonas, Lithuania, 9-14 September 1995.
Another important reason for the interest in H 2 0 - p o l y m e r solutions is connected with the good or poor ability of the polymer to mix with water. P E G is a polymer which has the formula O H ( C H 2 - C H 2 - O ) m - H , where m is the degree of polymerization. Concerning the conformation of the P E G chain, infrared and R a m a n analysis reveal in the crystalline state a helical conformation with two helical turns per fiber identity period which is equal to 19.3 ,~. The distance between the ether oxygen atoms is 2.88 ,~, very close to that of the oxygen atoms in water which is 2.85 ,~. Furthermore, whereas in the molten state a highly disordered helix which approaches a random coil conformation exists, in water solutions, although the well-ordered helix is absent, the nonappreciable shifts in Raman frequencies from the
0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved P H S0022-2860(96)09308-8
208
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
crystalline state indicate that the trans, gauche, trans structure is largely retained. This occurrence together with the characteristic distance of the ether oxygen atoms can justify the P E G solubility in water, from oligomers to polymers of several millions molecular weight [6], at temperatures lower than the water boiling point. Previous ultrasonic measurements [7] show that, as water concentration increases, as long as it saturates the two lone pairs of each oxygen atom, some water molecules bond themselves to the polymer end groups too. At the same time, Incoherent Quasi Elastic Neutron Scattering [8] results show that the dynamic properties of the H 2 0 molecules in this environment deeply differ from those in the bulk. The aim of the present work is to study the hydration effects of P E G and their dependence on temperature and polymer concentration by a Raman analysis of the vibrational dynamics. Raman spectroscopy represents a powerful method for studying the vibrational properties of our polymeric aqueous solutions since the O - H stretching band is quite sensitive to the structural properties of the water. We will show that the shape of the O H band can be reproduced by a suitable superposition of the spectra of bulk and hydration water. A similar model, although very simple, has been successfully applied in the study of water encaged in reverse micelles, where the water molecules can be considered partitioned in water tightly bonded between the polar heads of the ionic surfactant and bulk water [9]. The relative amount of the two contributions changes with concentration and temperature, furnishing information on the number of water molecules bonded to the polymer chain.
2. Experimental procedures and results The solutions were prepared using P E G 600 purchased from Aldrich-Chemie and double distilled deionized water at the concentrations corresponding to pure P E G 600, P E G 600 + 5 H20, P E G 600 + 10 H20, P E G 600 + 30 H20, P E G 600 + 100 H 2 0 and pure H20. Care was taken in order to obtain stable clear and dust free samples. The samples were sealed
and ample time (3 days) was allowed for equilibration. After preparation the solutions were stored at 4°C in the dark to minimize biological and (photo)chemical degradation. The investigated temperature range was (20-80)°C. The Raman spectra were detected by a SPEX Ramalog 5 double spectrometer to which a third monochromator (also made by SPEX) was added. The gratings are driven by a compudrive which allows full control of the spectrometer and to execute repeated scans to optimize the signal/noise ratio. The source was an Ar + laser working at 5145 ,~ of wavelength. Laser fluctuations were corrected by using a home made interface. We performed measurements of both polarized (VV) and depolarized (VH) Raman scattering (VH indicates the directions of electric field normal to the scattering plane), starting from a frequency shift of about 2 0 c m -1 u p t o 4 0 0 0 c m 1 with a band resolution of 5 cm -~. In the present paper, however, we limit ourselves to the discussion of the O - H stretching band. In the following the isotropic contribution of the spectra are analyzed. Such spectra are obtained from the experimental data, after a correction for local field approximation, refractive index and density, as/is(W) = Ivy(W) - 4/3IvH. In fact,/is(W), as is well known, is more directly connected with the band polarizability induced by changes in the structural properties of the system [10]. In Figs. 1 and 2 we show the evaluated isotropic spectra of the pure components (H:O and P E G 600) as a function of temperature.
3. Results and discussion Usually the O - H stretching band in water is considered as the superposition of at least four bands that arise from the presence of water employed in two hydrogen bonds and of water that has a distorted hydrogen bond [11]. However, the presence in the water isotropic spectra of an isobestic point, in which the Raman intensity is temperature independent, is clearly detected (see Fig. 1), suggesting a method of interpreting the origin of the O - H stretching band [12]. In the case o f pure P E G 600 (see Fig. 3), the O - H stretching contribution, at 20°C, is obviously
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
(o): f-20°C : =
(b): T=40°C
(d)
H10
209
T=20%
: =
PEG600
x5
&
.4
i
3000 3100 3200 3300 3400 3500 3600 3700
i
3100
Stokes shift (crn-1)
3200
i
i
3300
3400
L
i
3500
,
3600
3700
Stokes shift (cm 1)
Fig. 1. Evaluated isotropic spectra of H 2 0 pure components as
a function of temperature.
Fig. 3. O - H stretching contribution of pure P E G 600 at 20°C: (I-q) experimental data; continuous line is the best fit; broken lines are the height and width of the convoluted bands.
attributed to the O - H end groups which are the active sites for the intermolecular H - b o n d interaction. Recently such an O - H stretching contribution, in low molecular weight P E G s [13], has been decomposed into symmetric bands corresponding to w~(~ 3200 cm-1), w6(~ 3350 cm - l ) and ~ 7 ( ~ 3500 cm-1), where w, is defined [14] as the stretching of the O - H groups involved in intramolecular H-bonds (i.e. when a terminal O H is bonded to an internal monomeric unit via a H - b o n d with the oxygen atom), ~6 is defined as the stretching of the fully bonded hydroxylic O - H groups (i.e. O and H bonded respectively with H and O belonging to another polymeric unit) and finally w7 is defined as the O - H stretching in which only the H atoms are bonded. The I R absorbance in that case shows that the above mentioned three sub-bands reduce to only one (~7 in which the hydrogens of the O H groups are
bonded to the oxygens of another polymeric chain) in passing from monomeric ethylene glycol to P E G 2000 (see Fig. 3 of Ref. [13]). In our case the P E G 600 can be decomposed into two bands centered at ~3350 cm -1 and ~3500 cm -1, that correspond to the ~6 and ~ O - H vibrations. The disappearance of the w, band can be justified as due to the long distance in P E G 600 between the two O - H end groups which have a weak probability of organizing the intramolecular bond because of the polymeric steric hindrance. The isotropic bands of pure P E G 600 turn out to be almost temperature independent as can be seen by an inspection of Fig. 2. In Fig. 4 we show, as an example, the isotropic spectra of the O - H stretching band for the various water concentrations at T = 80°C. It can be seen that as the water content increases the spectral
m~~ ....... + T=20°c x T=40°C o T=60°C
.,*"
PEG600
T=80OC
"...
......."~c ~o t.m..., ~
..
...'"" . ..-"" "'"...'.. .- ......
~c ~. ,+.~.~o
".
-. .......
._~
.4
. ..
_
~
~c~o+.~.~.
.-,,
...P'
...
....
• • .,.
........... ~6oo+~.~Q ......
. ..
"'.. "..
"..
-. ". '.
...."" .............. ~e~c6oo. .............. i~ ::: .1: ".
3100
3200
3300
3400
3500
3600
3700
3100
3200
3300
3400 350~ Stokes shift (era-)
3600
3700
Stokes shift (cm I)
Fig. 2. Evaluated isotropic spectra of P E G 600 pure components as a function of temperature.
Fig. 4. Isotropic spectra of the O - H stretching band for pure H 2 0 , for pure P E G 600 and for P E G 600 at various water contents; T = 80°C.
210
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
PEG~0 + ~
PEG 600 + 5 H~O
..:::" .....[7
2¢c x5 "~ .~
5100
3200
F i g . 5. S p e c t r a o f P E G
.. ". ,. "... "..
x~
&
•....
,
i 3300
T:80Oc
.. •.
,." .,..,... ..: 80OC ...." ./' :....""
...S'" "" L i_
H~
i
3400 3500 Stokes shift (cm q)
600 + 5 H20
'"'~:::::::~
i
obtained
3600
3700
by subtracting
the pure PEG 600 contribution, at T = 20°C and T = 80°C. shape changes continuously from that of pure P E G 600 to that of pure water. In order to obtain information on the number of water molecules H - b o n d e d to the polymer chain, we adopted the structural model proposed by F r a n k - W e n [3,4] for aqueous ionic solutions and applied it to the case of polymeric aqueous solutions. Accordingly water is partitioned between the polymer and the bulk phase. Provided that the pure P E G 600 O - H stretching band is subtracted from the isotropic spectra of the aqueous solutions, following such a model, the spectral profiles result from the sum of two contributions, that of the two different kinds of water molecules: the normalized spectrum of bulk water, /bulk (cO), and that of the water bonded to the polymer, /bonded(co), obtained by subtracting the pure P E G 600 spectrum from that of P E G 600 + 5 H20. At this concentration, in fact, all the water molecules are supposed to be strongly bonded to the polymer chain over the whole temperature range, as indicated by several other experimental results [5] in which the bulk water contribution results are absent. Such a procedure's results are justifiable under the hypothesis that (i) the polymer contribution to the O - H stretching bond does not appreciably change in the presence of water and that (ii) this contribution, as in the pure polymer, is temperature independent. In Fig. 5 the spectra obtained by subtracting the pure P E G 600 spectra from those of the P E G 600 + 5 H 2 0 are shown at the two extreme temperatures T = 20°C and T = 80°C.
............ ~......, ........... ;..3100
3200
3300
~
3400 3500 Stokes shift (cm 1)
; ............... 3600
,3700
F i g . 6. S p e c t r u m o f P E G 6 0 0 + 100 H 2 0 a t T = 8 0 ° C t o g e t h e r
with the component contributions and the resulting spectrum. Finally, by applying the above depicted model, already successfully adopted for electrolytic solutions [15], the spectrum of the O - H water profile at any given concentration can be decomposed at each temperature into the sum of two contributions, i.e.: /is(W, T) = ol(T)./bonded(co, T) + [1 -- a ( T ) ] " Ibulk(co, T) where c~(T) and 1 - a ( T ) are the percentages of the 'bonded' and 'free' O H groups, respectively. In Fig. 6, as an example, the spectrum of P E G 600 + 100 H 2 0 is shown at T = 80°C together with the component contributions and the resulting spectrum. In Table 1 the values of a as a function of the number of water molecules for each polymer molecule are reported at all the investigated temperatures. F r o m the obtained values of a it is possible to obtain the number n w of H 2 0 molecules bonded in the first hydration shell of polymer's chain. In table 1 the calculated values of nw are shown together with those obtained by adiabatic compressibility measurements [16]. Despite the simplicity of Ram values are in good the applied model, the nw agreement with those calculated by compressibility techniques in the case of P E G 600 + 100 H20. It is R a m and nw c o m p r at lower water obvious that nw contents (10 H 2 0 and 30 H 2 0 ) are nearly coincident with the values of the total number of water molecules and therefore are not reported in the table. The case of P E G 600 + 100 H 2 0 deserves some
V, Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
211
Table 1 V a l u e s o f a as a f u n c t i o n o f the n u m b e r o f w a t e r m o l e c u l e s f o r e a c h p o l y m e r m o l e c u l e 20°C
40°C
60°C
a
Ram nw
compr nw
a
Ram nw
compr nw
P E G 600 + 10 H 2 0
0.91
-
-
0.85
-
P E G 600 + 30 H 2 0
0.65
-
-
0.57
-
-
P E G 600 + 100 H 2 0
0.16
32
31.8
0.15
29.8
29.7
0.14
80°C compr nw
a
Ram nw
compr nw
0.80
-
0.77
-
-
0.53
-
0.50
-
-
27
0.13
26.3
26.5
a
Ram nw
27.4
n Rarn a n d n c°mpr a r e the n u m b e r nw o f H 2 0 m o l e c u l e s b o n d e d in the first h y d r a t i o n shell o f p o l y m e r ' s c h a i n o b t a i n e d by R a m a n a n d c o m p r e s s i b i l i t y m e a s u r e m e n t s respectively.
comment: the values of c~ indicate that we are, as expected, in the presence of a low water content at the polymer active sites. However, the nw values confirm that all the active sites of P E G 600 are saturated up to 80°C. In fact the values of-Ram nw compr and nw at this temperature turn out to be equal to the number (13) of oxygen atoms of the monomeric groups ( C H z C H 2 0 ) present in P E G 600. This number increases at low temperature. This general trend can be explained considering that at low temperatures the p o l y m e r - w a t e r interaction strength allows water to bond also to the terminal groups or to f o r m a second coordination shell. As t e m p e r a t u r e increases, the interaction strength decreases and this implies the loss of water molecules m o r e weakly bonded to the polymer. This process continues until all the water molecules not closely connected (via H - b o n d s ) to the m o n o m e r oxygens break their bonds.
6
(1 a)/a 4 T=20°C
2
U// • T=80°C
0
~
I
30
~
,
,
,
60
,
,
,
90
,
120
150
N Fig. 7. D e p e n d e n c e o f the (1 - a ) / ~ p a r a m e t e r on the n u m b e r o f w a t e r m o l e c u l e s f o r e a c h P E G 600 m o l e c u l e .
Finally, in Fig. 7 we show the dependence of the ( 1 - a ) / a parameter on the number of water molecules for each P E G 600 molecule. It is possible to distinguish two regions defined as I and II in Fig. 7. In region I we observe that ( 1 - a ) / ~ slowly increases with the number of water molecules up to the value of 30, which is about the m a x i m u m number of hydrated water as evaluated in these system. In region II we observe a different growth law for (1 - a ) / a vs. N induced by the existence of bulk water in the liquid mixture.
4. Concluding remarks The present paper reports the results of a study of the hydration effects of P E G and their dependence on temperature and polymer concentration by R a m a n analysis. The results show unambiguously that, as water concentration increases, the number of water molecules tightly connected to the polymer tends to saturate the two lone pairs of each oxygen atom. Moreover some water molecules are bonded to the polymer end groups. There is also a strong indication that the interaction strength of the H 2 0 molecules bonded to the ether oxygen atoms is higher than that of the end groups, because the bonds connected to the end groups break at high temperatures. Furthermore the partitioning of water into bonded and bulk phases is supported by spectroscopic evidence. Although this argument may be questionable because intermediate states exist for water, which can give different polarization effects on the fundamental O - H stretching frequency, in this case it seems to be workable, indicating that the
212
V. Crupi et al./Journal of Molecular Structure 381 (1996) 207-212
experimental data are not contradictory to this simple but convincing model.
References [1] U. Dahlborg and E. Braun, Phys. Ser., 14 (1976) 253. [2] U. Dahlborg, V. Dimie and B. Ovikl, Phys. Scr., 37 (1988) 95. [3] H.S. Frank and W.Y. Wen, Discuss. Faraday Soc., 24 (1965) 133. [4] A.C. Barnes, T.W.N. Bieze, J.E. Enderby and J.C. Leyte, J. Phys. Chem., 98 (1994) 11527. [5] F.E. Bailey and J.V. Kaleske, Poly(Ethylene Oxide), Academic Press, New York, 1976. [6] S. Saeki, N. Kuwahara, M. Nakata and M. Kaneko, Polymer, 17 (1976) 685. [7] G. Maisano, D. Majolino, P. Migliardo, S. Venuto, F. Aliotta, S. Magazu', Mol. Phys., 78 (1993) 421.
[8] V. Crupi, M.P. Jannelli, S. Magazu', G. Maisano, D. Majolino, P. Migliardo, C. Vasi, lI Nuovo Cimento D, 16 (1994) 809. [9] E. Bardez, B. Larrey, X.X. Zhu, B. Valeur, Chem. Phys. Lett., 171 (1990) 362. [10] S. Bratos and G. Tarjus, Physics of Modern Materials, Vol. II, IAEA, Vienna, 1980, p. 571. [11] D.E. Irish and M.M. Brooker, in H.E. Clark and R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. II, London, 1977, p. 212. [12] G. D'Arrigo, G. Maisano, F. Mallamace, P. Migliardo, F. Wanderlingh, J. Chem. Phys., 75 (1981) 4264. [13] V. Crupi, M.P. Jannelli, S. Magazu', G. Maisano, D. Majolino, P. Migliardo, D. Sirna, Mol. Phys., 84 (1995) 645. [14] H. Graener, T.Q. Je, A. Laubereau, J. Chem. Phys., 90 (1989) 3413. [15] F. Aliotta, M.P. Fontana, G. Maisano, P. Migliardo and F. Wanderlingh, Opt. Acta, 27 (1980) 931. [16] M.P. Jannelli, S. Magazu, G. Maisano, D. Majolino and P. Migliardo, J. Mol. Struct., 322 (1994) 337.