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Phase transitions in C10E5water system studied by Raman spectroscopy of the O-H band
Journal of ELSEVIER
MOLECULAR STRUCTURE Journal of Molecular Structure 348 (1995) 273-276
Phase Transitions in CloEs/Water System Studied by Raman Spectroscopy of the O-H Band Zh. S. Nickolova and J. C. Earnshawb aFaculty of Physics, Sofia University, 5, J. Bourchier Bul., 1126 Sofia, Bulgaria bDepartment of Pure and Applied Physics, The Queen’s University of Belfast, Belfast BT7 lNN, Northern Ireland
Changes in the shape of the O-H Raman band of water have been studied at the transitions to the liquid crystalline phases of CI&+/water system using a depolarization insensitive method of spectral registration and comparison. 1. INTRODUCTION Aqueous solutions of non-ionic polyoxyethelene surfactants with a chemical formula CH m ~,+l(OCH&H&,OH or C,E, for short are quite interesting with their ability to form different phases depending on temperature and concentrations. Their phase diagrams often show several anisotropic (hexagonal, lamellar) and isotropic (micellar, cubic) phases. Raman spectroscopic observations of changes in the structure and conformation of surfactants in aqueous solutions at phase transitions have attracted attention [l-5], but not all researchers have taken into account errors introduced by depolarization of the laser radiation in the anisotropic phases. The latter consist of very small oriented domains of lyotropic liquid crystals so that the incident and scattered beams in a typical Raman experiment will experience multiple reflections at the boundaries of these microcrystals and will be partially or completely depolarized. A solution of this problem has been proposed by Gaufres et al [4,5]. As the surfactant structures in the various phases are quite different the structure of water is also expected to change, especially in the more concentrated solutions where water molecules are confined in limited spaces between the organized amphiphile structures. Changes in the shape of the O-H Raman band have been recently observed by Micali et al [6] in the isothermal transition isotropic micellar phase (Ll)-continuous amphiphile phase for the aqueous solution of the non-ionic surfactant CrcE5. However, no investigations of the changes in water structure at the isotropic-anisotropic transitions of this and similar amphiphile systems have been reported. Here we concentrate on the CIOES/water system, which is interesting due to the existence of two anisotropic phases - hexagonal (HI) and lamellar (L,) - and study the changes in water structure at the phase transitions by Raman spectroscopy of the O-H band. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V SSDI 0022-2860(95)08641-2
All rights reserved
274
2. EXPERIMENTAL The non-ionic amphiphile CluE5 was provided by Nikko Chemicals Co. (Japan) and was used without further purification. Water from a Mini-Q purification system (Millipore) was used for solution preparation. Solutions were prepared by weight, thus allowing the use of a published phase diagram [7]. Before spectroscopic investigation the samples were homogenized in a centrifuge for 90 min at 1000 rpm. The iso-concentration paths which we followed to study the phase transitions micellar (Ll)-hexagonal (Hl) and micellar(LI)-lamellar (L,) phases were at concentrations of CIuE5 58% w/w and 71% w/w, respectively. The temperature was varied over the range 55 OC to 10 OC, in steps of 2 OC close to the phase transitions; the samples were thermostated to within f 0.1 OC. In order to obtain water O-H Raman spectra of the anisotropic phases which can be compared with the corresponding spectra of the isotropic phases, we recorded spectra which are linear combinations of parallel and perpendicular contributions [4]: 160 = l/4 .Z,, + 3/4 .I,,
(1)
where Z6,, in our experimental arrangement is the intensity of the spectrum recorded with an analyser oriented at 60° in respect to the polarization of the exciting laser beam. Thus errors introduced by depolarization of light due to microcrystallinity of the samples in the anisotropic phases should be eliminated. Raman scattering measurements were performed in the usual 90° geometry using a SPEX 1400 monochromator and CCD detection (ORIEL Instaspec IV). Excitation was with the 457.9 nm line of an argon ion laser. The measurements were performed at thermodynamic equilibrium allowing at least 10 min for temperature stabilization between the successive spectral recordings. The spectra of the O-H Raman band of water are not corrected for the contribution of the O-H terminal group of the amphiphile. It is not possible to use the O-H signal of pure CloEs for correction purposes in the way it was used in [6] at the transition to the anisotropic phases, because of the change in the structure of studied system. 3. RESULTS AND DISCUSSION The experimental scheme for detection of spectra of the anisotropic phases, accounting for depolarization effects by using an analyser oriented at 60° in respect to the input polarization, was tested by evaluating the relative differences: AA =
I
jjAz( v)ldv i;ZG,( “)d” “I
)
(2)
“I
where AZ is the difference between the normalized to the area of Z,, areas of the parallel and perpendicular O-H Raman spectra in the anisotropic phases, v, and v, are the boundaries of the spectral range, 3036 cm-l and 3760 cm-l, respectively. The ratio A4 for the water O-H band is less than 2.1% at 20 OCjust following the phase transition LI-HI, Fig. 1. According to [4] this is the temperature range where it is most probable that the microcrystallinity of the samples can be disturbed. However, our observations on the changes in the water Raman band showed that A4 at any of the studied temperatures in the anisotropic phases after the LI-HI and Ll-L, transitions is never higher than 3.5%. That is why we assume that the
275
microcrystallinity state expressed with the ratio hA is almost completely established in respect to the water Raman spectra. 20°C,58% w/w
3100
C&/water
3300
3500
WAVENUMBER,
3700
3000
I
I
3400
3600
3800
WAVENUMBER, cm-’
cm-’
Figure 1. An example for testing the microcrystallinity of the anisotropic phase
I
3200
Figure 2. Raman spectra (Z& of the O-H band at the transition micellar-hexagonal phase.
The evaluation of the effects of phase transitions on water structure in CluES/water system was carried out assuming one of the models for the continuous structure of water accepted in the literature [8]. According to it the continuous distribution of hydrogen bonds between water molecules can be divided in two broad classes: “bound” water (water molecules are tetra bonded and form clusters connected with strong hydrogen bonds), and all the remaining water molecules connected in a network of lower order by weaker hydrogen bonds. The molecules from the first class scatter light in a region centered at around 3250 cm-l in the O-H Raman spectra (Fig.2) while the latter has a peak around 3420 cm-l. The ratio R = I,,,,/I,,,, conveniently expresses the degree of changes in the shape of the O-H band, when the environment of the water molecules is altered due to transitions between different phases. I
1
I
2.5 2.0 R 1.51.0 0
H,
?? **
I
I
L L 2.5R
./
I I 10 20
3.0-
Ll .0
*0 0 .e .
.
0
?? .
2.0-
I I I 30 40 50 60
TEMPERATURE, ‘C Figure 3. Phase transition micellarhexagonal phase expressed by the temperature dependence of R
1
1.5- , , , , , , , 10 20 30 40 50 TEMPERATURE, ‘C
Figure 4. Phase transition micellarlamellar phase expressed by the temperature dependence of R
276
Fig.3 shows the dependence of R on temperature in the region of the transition L,-HI. The sudden change in the O-H band revealed by a stepwise decrease of R around 22 OC can be viewed as an increase in the proportion of water molecules assuming more organized, tetra bonded structure. In the hexagonal phase the mean diameter of the rod-shaped micelles increases and they are closely packed. Water fills the spaces between the micelles and obviously the number of water molecules bound to the hydrophilic head groups of CIcE5 is much higher than that in “bulk” water in the Ll phase, thus giving rise to the increase in “bound” water. We have not observed a change in the shape of the O-H band (a jump in R) at the transition LI-Lo, Fig.4. According to recent Raman studies [6] all water molecules in this composition range in the LI phase are fully bonded to the oxyethelene head groups of the surfactant, because the solution has become a predominately continuous amphiphile structure at these concentrations. In our view, the same degree of bonding to the head groups can be expected in the lamellar phase because water is “squeezed” in thin areas between the double layers of surfactant molecules and again all the water molecules are tetra-bonded. This explains why Raman O-H spectra do not change with temperature at the micellar-lamellar phase transition. 4.CONCLUSIONS The results from the investigations reported in this paper show that correct registration and interpretation of the O-H Raman spectra of the anisotropic phases of non-ionic surfactant solutions can be used successfully to analyze the transitions between the isotropic and anisotropic phases. Raman spectroscopy of the O-H band has proved to be a very informative method in the first (to the best of our knowledge) study of changes in the structure of water at phase transitions to the anisotropic phases of non-ionics. Care should be taken to interpret correctly the transitions to the lamellar phases because, as seen above, the O-H Raman spectra may not show differences between the isotropic and anisotropic phase. 5. ACKNOWLEDGMENTS One of us (Z.N.) is grateful to the Commission of the European Communities for a Mobility research grant No. ERB-CIPA-CT92-2160. Financial support by the Science Fund of Sofia University (Grant No.249/1993) is gratefully acknowledged. The authors are indebted to Prof. F.Mallamace for sending the manuscript of his paper [6] prior to publication. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
R. Fainman and D.A. Long, J. Raman Spectrosc., 3 (1975) 37 1. M. Picquart, J. Phys. Chem., 90 (1986) 243. J.-L. Bribes, R. Gaufres, S. Sportouch and J. Maillols, J. Mol. Struct., 146 (1986) 267 R. Gaufres, J.-L. Bribes, S. Sportouch, J. Ammour and J. Maillols, J. Raman. Spectrosc., 19 (1988) 249. R. Gaufres, S. Sportouch, J. Ammour and J. Maillols, J. Phys.Chem., 94 (1990) 4635. N. Micah, C. Vasi, F. Mallamace, M. Corti and V. Degiorgio, submitted. J.C. Lang and R.D. Morgan, J. Chem. Phys., 73 (1980) 5849. H.E. Stanley and J. Teixeira, J. Chem. Phys., 73 (1980) 3404.
MOLECULAR STRUCTURE Journal of Molecular Structure 348 (1995) 273-276
Phase Transitions in CloEs/Water System Studied by Raman Spectroscopy of the O-H Band Zh. S. Nickolova and J. C. Earnshawb aFaculty of Physics, Sofia University, 5, J. Bourchier Bul., 1126 Sofia, Bulgaria bDepartment of Pure and Applied Physics, The Queen’s University of Belfast, Belfast BT7 lNN, Northern Ireland
Changes in the shape of the O-H Raman band of water have been studied at the transitions to the liquid crystalline phases of CI&+/water system using a depolarization insensitive method of spectral registration and comparison. 1. INTRODUCTION Aqueous solutions of non-ionic polyoxyethelene surfactants with a chemical formula CH m ~,+l(OCH&H&,OH or C,E, for short are quite interesting with their ability to form different phases depending on temperature and concentrations. Their phase diagrams often show several anisotropic (hexagonal, lamellar) and isotropic (micellar, cubic) phases. Raman spectroscopic observations of changes in the structure and conformation of surfactants in aqueous solutions at phase transitions have attracted attention [l-5], but not all researchers have taken into account errors introduced by depolarization of the laser radiation in the anisotropic phases. The latter consist of very small oriented domains of lyotropic liquid crystals so that the incident and scattered beams in a typical Raman experiment will experience multiple reflections at the boundaries of these microcrystals and will be partially or completely depolarized. A solution of this problem has been proposed by Gaufres et al [4,5]. As the surfactant structures in the various phases are quite different the structure of water is also expected to change, especially in the more concentrated solutions where water molecules are confined in limited spaces between the organized amphiphile structures. Changes in the shape of the O-H Raman band have been recently observed by Micali et al [6] in the isothermal transition isotropic micellar phase (Ll)-continuous amphiphile phase for the aqueous solution of the non-ionic surfactant CrcE5. However, no investigations of the changes in water structure at the isotropic-anisotropic transitions of this and similar amphiphile systems have been reported. Here we concentrate on the CIOES/water system, which is interesting due to the existence of two anisotropic phases - hexagonal (HI) and lamellar (L,) - and study the changes in water structure at the phase transitions by Raman spectroscopy of the O-H band. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V SSDI 0022-2860(95)08641-2
All rights reserved
274
2. EXPERIMENTAL The non-ionic amphiphile CluE5 was provided by Nikko Chemicals Co. (Japan) and was used without further purification. Water from a Mini-Q purification system (Millipore) was used for solution preparation. Solutions were prepared by weight, thus allowing the use of a published phase diagram [7]. Before spectroscopic investigation the samples were homogenized in a centrifuge for 90 min at 1000 rpm. The iso-concentration paths which we followed to study the phase transitions micellar (Ll)-hexagonal (Hl) and micellar(LI)-lamellar (L,) phases were at concentrations of CIuE5 58% w/w and 71% w/w, respectively. The temperature was varied over the range 55 OC to 10 OC, in steps of 2 OC close to the phase transitions; the samples were thermostated to within f 0.1 OC. In order to obtain water O-H Raman spectra of the anisotropic phases which can be compared with the corresponding spectra of the isotropic phases, we recorded spectra which are linear combinations of parallel and perpendicular contributions [4]: 160 = l/4 .Z,, + 3/4 .I,,
(1)
where Z6,, in our experimental arrangement is the intensity of the spectrum recorded with an analyser oriented at 60° in respect to the polarization of the exciting laser beam. Thus errors introduced by depolarization of light due to microcrystallinity of the samples in the anisotropic phases should be eliminated. Raman scattering measurements were performed in the usual 90° geometry using a SPEX 1400 monochromator and CCD detection (ORIEL Instaspec IV). Excitation was with the 457.9 nm line of an argon ion laser. The measurements were performed at thermodynamic equilibrium allowing at least 10 min for temperature stabilization between the successive spectral recordings. The spectra of the O-H Raman band of water are not corrected for the contribution of the O-H terminal group of the amphiphile. It is not possible to use the O-H signal of pure CloEs for correction purposes in the way it was used in [6] at the transition to the anisotropic phases, because of the change in the structure of studied system. 3. RESULTS AND DISCUSSION The experimental scheme for detection of spectra of the anisotropic phases, accounting for depolarization effects by using an analyser oriented at 60° in respect to the input polarization, was tested by evaluating the relative differences: AA =
I
jjAz( v)ldv i;ZG,( “)d” “I
)
(2)
“I
where AZ is the difference between the normalized to the area of Z,, areas of the parallel and perpendicular O-H Raman spectra in the anisotropic phases, v, and v, are the boundaries of the spectral range, 3036 cm-l and 3760 cm-l, respectively. The ratio A4 for the water O-H band is less than 2.1% at 20 OCjust following the phase transition LI-HI, Fig. 1. According to [4] this is the temperature range where it is most probable that the microcrystallinity of the samples can be disturbed. However, our observations on the changes in the water Raman band showed that A4 at any of the studied temperatures in the anisotropic phases after the LI-HI and Ll-L, transitions is never higher than 3.5%. That is why we assume that the
275
microcrystallinity state expressed with the ratio hA is almost completely established in respect to the water Raman spectra. 20°C,58% w/w
3100
C&/water
3300
3500
WAVENUMBER,
3700
3000
I
I
3400
3600
3800
WAVENUMBER, cm-’
cm-’
Figure 1. An example for testing the microcrystallinity of the anisotropic phase
I
3200
Figure 2. Raman spectra (Z& of the O-H band at the transition micellar-hexagonal phase.
The evaluation of the effects of phase transitions on water structure in CluES/water system was carried out assuming one of the models for the continuous structure of water accepted in the literature [8]. According to it the continuous distribution of hydrogen bonds between water molecules can be divided in two broad classes: “bound” water (water molecules are tetra bonded and form clusters connected with strong hydrogen bonds), and all the remaining water molecules connected in a network of lower order by weaker hydrogen bonds. The molecules from the first class scatter light in a region centered at around 3250 cm-l in the O-H Raman spectra (Fig.2) while the latter has a peak around 3420 cm-l. The ratio R = I,,,,/I,,,, conveniently expresses the degree of changes in the shape of the O-H band, when the environment of the water molecules is altered due to transitions between different phases. I
1
I
2.5 2.0 R 1.51.0 0
H,
?? **
I
I
L L 2.5R
./
I I 10 20
3.0-
Ll .0
*0 0 .e .
.
0
?? .
2.0-
I I I 30 40 50 60
TEMPERATURE, ‘C Figure 3. Phase transition micellarhexagonal phase expressed by the temperature dependence of R
1
1.5- , , , , , , , 10 20 30 40 50 TEMPERATURE, ‘C
Figure 4. Phase transition micellarlamellar phase expressed by the temperature dependence of R
276
Fig.3 shows the dependence of R on temperature in the region of the transition L,-HI. The sudden change in the O-H band revealed by a stepwise decrease of R around 22 OC can be viewed as an increase in the proportion of water molecules assuming more organized, tetra bonded structure. In the hexagonal phase the mean diameter of the rod-shaped micelles increases and they are closely packed. Water fills the spaces between the micelles and obviously the number of water molecules bound to the hydrophilic head groups of CIcE5 is much higher than that in “bulk” water in the Ll phase, thus giving rise to the increase in “bound” water. We have not observed a change in the shape of the O-H band (a jump in R) at the transition LI-Lo, Fig.4. According to recent Raman studies [6] all water molecules in this composition range in the LI phase are fully bonded to the oxyethelene head groups of the surfactant, because the solution has become a predominately continuous amphiphile structure at these concentrations. In our view, the same degree of bonding to the head groups can be expected in the lamellar phase because water is “squeezed” in thin areas between the double layers of surfactant molecules and again all the water molecules are tetra-bonded. This explains why Raman O-H spectra do not change with temperature at the micellar-lamellar phase transition. 4.CONCLUSIONS The results from the investigations reported in this paper show that correct registration and interpretation of the O-H Raman spectra of the anisotropic phases of non-ionic surfactant solutions can be used successfully to analyze the transitions between the isotropic and anisotropic phases. Raman spectroscopy of the O-H band has proved to be a very informative method in the first (to the best of our knowledge) study of changes in the structure of water at phase transitions to the anisotropic phases of non-ionics. Care should be taken to interpret correctly the transitions to the lamellar phases because, as seen above, the O-H Raman spectra may not show differences between the isotropic and anisotropic phase. 5. ACKNOWLEDGMENTS One of us (Z.N.) is grateful to the Commission of the European Communities for a Mobility research grant No. ERB-CIPA-CT92-2160. Financial support by the Science Fund of Sofia University (Grant No.249/1993) is gratefully acknowledged. The authors are indebted to Prof. F.Mallamace for sending the manuscript of his paper [6] prior to publication. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
R. Fainman and D.A. Long, J. Raman Spectrosc., 3 (1975) 37 1. M. Picquart, J. Phys. Chem., 90 (1986) 243. J.-L. Bribes, R. Gaufres, S. Sportouch and J. Maillols, J. Mol. Struct., 146 (1986) 267 R. Gaufres, J.-L. Bribes, S. Sportouch, J. Ammour and J. Maillols, J. Raman. Spectrosc., 19 (1988) 249. R. Gaufres, S. Sportouch, J. Ammour and J. Maillols, J. Phys.Chem., 94 (1990) 4635. N. Micah, C. Vasi, F. Mallamace, M. Corti and V. Degiorgio, submitted. J.C. Lang and R.D. Morgan, J. Chem. Phys., 73 (1980) 5849. H.E. Stanley and J. Teixeira, J. Chem. Phys., 73 (1980) 3404.