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Partitioning of chloroform and benzene in a layer of poly(ethylene oxide) anchored on silica
Colloids and Surfaces, 36 (1989) 263-272 Elsevier Science Publishers B.V., Amsterdam
263 -
Printed
in The Netherlands
Partitioning of Chloroform and Benzene in a Layer of Poly(ethylene oxide) Anchored on Silica H. HOMMEL’, A.P. LEGRAND’, H. BALARD’ and E. PAPIRER’ ‘Laboratoire de Physique Quantique, UA C.N.R.S. 421, E.S.P.C.I., 10 rue Vauquelin, 75231 Paris Ceden 05 (France) ‘Centre de Recherches sur la Physico-Chimie des Surfaces Solides, L.P. C.N.R.S. 6601,24 Avenue du President Kennedy, 68200 Mulhouse (France) (Received 31 March 1988; accepted 16 January
1989)
ABSTRACT ESR spectroscopy of labeled poly (ethylene oxide) terminally anchored on silica and in contact with mixtures of chloroform and benzene in different proportions has been used to determine the content of both solvents in the grafted layer. If the grafting ratio is high, there is evidence for a partitioning effect and preferential adsorption of one of the solvents.
INTRODUCTION
Polymers at interfaces are very important in applications such as colloid stabilization and flocculation, adhesion, wetting and chromatography, for example. Numerous studies on model systems, as well experimental [l-5] and theoretical [ 6-91 work, increase our understanding of their behaviour. Magnetic resonance appears to be a very sensitive method for probing the conformation and dynamics of the chains at a local molecular level [ lo,11 1. It is also possible to use ESR of spin labeled molecules [ 12-151 to investigate these properties. Here we take advantage of the variation of the hyperfine coupling constant of the nitroxide spin label with the local polarity of the microenvironment to show evidence of the partitioning of chloroform and benzene in a layer of poly (ethylene oxide ) anchored on silica. When adding different amounts of a good solvent, like chloroform, to poly (ethylene oxide ) adsorbed in a flat conformation on silica in benzene, there is a displacement effect [ 161 and the chains adopt a more extended configuration. However, if the grafting ratio is high enough for the polymer to build up a homogeneous tightly packed layer like the stationary phase in chromatography [ 17,181 there is, in addition, a preferential adsorption of one of the two solvents at the interface.
0166-6622/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
264 EXPERIMENTAL
Materials
Samples with the very high grafting ratio needed for the experiments were prepared following the procedure described below. Silica
The silica was fume or pyrogenic Aerosil 300 from Degussa. Consisting of large aggregates of spheres of about 10 nm. It is different from the gels currently used in liquid chromatography, but it has the advantage of being nonmicroporous and its surface can be approximated by a plane at a molecular scale. The specific area measured by nitrogen adsorption was about 310 m2 -1 g . Polymer
The polymer was poly (ethylene oxide) (PEO) 2000 of molecular weight 1880, i.e. with about 43 repeat units, purchased from Fluka. The polydispersity of the PEO was 1.09. In addition, a sample of PEO 4000 from Fluka was used. Grafting reaction
The grafting reaction was the direct esterification of a surface silanol group by one of the two hydroxyl ends of a macromolecule. The latter had about 1.6 OH groups at the free ends. The silica was dehydrated, agglomerated with acetone (reagent pure) and dried at 150°C. For the Aerosil fume silica, chosen, the aggregates are very open and the mass fractal dimension is low [ 191. Therefore even when the silica is agglomerated the number of contact points between aggregates is small and it is easy to disperse after drying. The silica was dispersed in a PEO 2000 solution to obtain the necessary impregnation ratio of physically adsorbed polymers. The solvent was then evaporated with a rotatory evaporator. The powder was dried, degassed and placed in a container filled with nitrogen. Finally the grafting reaction took place: the powder was heated to 200°C for about 1 h. Once the grafting reaction was completed, the ungrafted polymer was removed by a long extraction (20 h) with acetone in a Soxhlet extractor. The grafting ratio was determined by pyrolysis weight loss between 25 and 750” C under oxygen. The samples of PEO 2000 had a grafting ratio, or more precisely a surface concentration, of 0.045 and 0.42 molecules/nm2. For the latter sample there was strong chain interaction. Any chains which have been attached to the sur-
265
face through both ends are included in the estimation of the grafting ratios but cannot be detected by ESR. However, no attempt was made to determine how the fraction of doubly attached chains varied with the grafting ratios. The long-term stability of the grafted polymer was not checked thoroughly [ 201. Some oxidation is indeed possible at room temperature. Here, in fact, the grafted polymers were stored in a refrigerator in the absence of light and at low temperature to minimize this risk. Spin labeling One percent of these grafted chains were then labeled at their free end by the nitroxide free radical 2,2,5,5-tetramethyl-3-pyrroline-l-oxyl-3-carboxylic acid in its acid chloride form. The chemical formula of a grafted, labeled, polymer chain is: SI-O~_(~CH,~CH,~O~~~C i>ik
a.
In addition the PEO 4000 was labeled at the free end through the same reaction, but without grafting onto silica. It was therefore possible to use this sample as a control allowing values for free (non-adsorbed) labeled chains in bulk solvent mixtures to be recorded. The molecular weight is not the same as the grafted samples, but even in this case, some conclusions could be drawn from the results. Solvents Chloroform and benzene (analytical grade) were chosen as the solvents. Mixtures of these two liquids were also prepared. The molecules have a similar size, and form organic liquids [ 211 which are completely miscible at room temperature. Some data are already available on the two-body interactions between these solvents and silica and between the solvents and poly (ethylene oxide) [ 221. The label, which has a small dipolar moment, should have more affinity for the dissymmetric chloroform molecular than for the aromatic benzene, which is more attracted by planar structures. However, as will be shown in the subsequent experiments no effect was detectable in our samples. ESRSPECTROSCOPY
Samples and apparatus All the samples were prepared in Pyrex tubes of 4 mm diameter. First the solvents remaining on the grafted silica were removed by direct pumping. Each
266
time a mixture in different proportions of benzene and chloroform was added. The whole sample was then degassed by repeated freeze-pump-thaw cycles and sealed in a vacuum. The ESR spectra were recorded on a Varian E-4 spectrometer operating in the X-band at 9.15 GHz. The temperature was regulated by a Varian E-257 temperature controller. Analysis
of the spectra
The nitroxide label has been extensively nian can be written
studied
[ 23,241. Its spin Hamilto-
H=SgB+IAS where B is the external magnetic field, S the spin of the unpaired electron and I the spin of the nitrogen nucleus, g is the gyromagnetic tensor and A the hyperfine tensor, and their principal axis are nearly parallel. If the motion is fast enough the energy levels can simply be obtained in terms of the isotropic Hamiltonian, where the tensors g and A have been replaced by the constants go=1/3*Tr(g) and A,=1/3*Tr(A). The spectrum consists of three well-defined Lorentzian lines, whose linewidths are interpreted by the Kivelson theory [ 231. The spectral lines are equally spaced, and the separation between adjacent lines is proportional to Ao, which has been measured on each sample. In fact the ESR spectra of the nitroxide free radical are sensitive not only to molecular motions, but also to the nature of the medium in which they are dissolved [ 131. The magnetic parameters are very sensitive functions of the electronic distribution in the molecule, and, therefore, are influenced by perturbations due to the environment. Thus A0 increases by more than 10% when going from hydrophobic to hydrophilic solvents. This effect can be understood qualitatively by considering the specific interactions of the polar solvents with the lone pairs of electrons on the oxygen atom. It would be of interest to choose in addition more polar liquids, but these would not be convenient for the ESR studies, and more important, the grafting bond Si-O-C would not be stable in water. RESULTS AND DISCUSSION
Observation Typical spectra of labeled PEO chains grafted onto silica with a grafting ratio of 0.42 molecules/nm2, at room temperature, in contact with mixtures of chloroform and benzene in different proportions, are shown in Fig. 1. The content by volume are respectively 100% CsHE, 80% C6H6/20% CHCl,, 50% C,H,/
261 ,5G,
r
Fig. 1. Typical spectra of labeled PEO chains grafted on silica with a grafting ratio of 0.42 molecules/nm’, at room temperature, in contact with mixtures of chloroform and benzene in different proportions. The content in benzene is indicated.
50% CHC13, 20% C&H&30% CHCl, and 100% CHC&. The first derivative of three narrow Lorentzian lines form the usual ESR spectrum. The viscosity at 20 oC of CGHsis 0.652 CP and of CHCl,, 0.58 cP. The motion of the label at the free ends depends on the local viscosity, which is also influenced by the concentration of polymer segments at this point. Some spectra exhibit a superhyperfine structure due to the presence of hydrogen in the neighbouring methyl groups when the content in chloroform is high. This result indicates a higher mobility at higher chloroform content. The parameter which is important here and which has been extracted from the spectra is the position of the resonance lines, or more precisely the distance between the peaks. The distance between the outer peaks 2Ao, as a function of
268 30.5
30
g
29.:
,
s 0 s 25
26.5
0
20
40 c
60
60
100
%
Fig. 2. Evolution of the hyperfine splitting 2A,,, between two extreme lines of the fast motion spectrum as a function of the relative concentration of benzene in the mixture of solvents, at room temperature ( 0 ) and at - 38’ C ( A ), for the sample with a grafting ratio of 0.42 molecules/rim*. 30.5
\ \
30
t
29.5 !
e 0 : 29
\
\ \\\\ ‘t t\\\\\ i\\\ f
26.5
0
20
40 c
60
60
100
%
Fig. 3. Evolution of the hyperfine splitting 2A,, between two extreme lines of the fast motion spectrum as a function of the relative concentration of benzene in the mixture of solvents for the sample with a grafting ratio of 0.045 molecules/nm2 at room temperature.
269 30.5
\ \ \ 30
.
1 \ t
\ \ \
:,
\
29.5
t \ \ \
<
\ 0
\
;:
\ 29
\
.
\
26.5
L
* 0
20
40 c
60
60
100
%
Fig. 4. Evolution of the hyperfine splitting 2A,,, between two extreme lines of the fast motion spectrum as a function of the relative concentration of benzene in the mixture of solvents for free (non-adsorbed) labeled PEO 4000 chains at a concentration of 10m3mall-’ at room temperature.
the concentration in benzene, is given in Fig. 2. There appears to be a deviation from the straight line joining the values for the pure solvents, benzene and chloroform. This deviation is outside of the experimental errors, given the accuracy of the measurements. In addition, the same experiment has been repeated at a lower temperature, - 38” C, except for pure benzene, for which the motion of the label is slow and the spectrum more complicated, because the solvent is frozen. The deviation is more marked. As the parameter A0 is not expected to depend on temperature this variation must be assigned to differences in tts composition of the mixture around the label. As a control two kinds of experiments have been carried out. Firstly, the measurement for a low grafting ratio sample is given in Fig. 3. Indeed the reported values follow a straight line significantly different from the results for the high grafting ratio sample. Secondly, the values for free (non-adsorbed) labeled PEO 4000 chains with a concentration of 10e3 mol l-l (or a total content of 6*1017 labels in the tube) in bulk solvent mixtures are given in Fig. 4. The variation is also not completely linear, but the variation remains inside experimental errors.
270
Interaction
between solvents, polymers
and silica
The polymer conformation is the result of the competition between surface attraction and chain repulsion. In a good solvent the chains are swollen. But if the affinity of the solvent for the solid surface is poor, the first effect can be hidden by a stronger adsorption of the polymer. In order to explain the obtained conformation at the interface, it appears that it is necessary to take into account the whole system and particularly the solvent-surface and polymersurface interactions [ 221. Howard and McConnel [22] have estimated by BET analysis of vapor adsorption on aerosil silica that the heat of adsorption for C&H, is 1.4 kcal mol-l and for CHCl, 1.0 kcal mol-‘. Making the assumption that the affinity of the liquid for the solid is correlated with this heat of adsorption, we expect that benzene is more strongly adsorbed on the solid surface than chloroform. Kipling and Peakall [ 211 have studied the adsorption of binary liquid mixtures of chloroform and benzene on aluminium oxide gels, and namely on boehmite. They suggest that the first adsorbed layer of solvents, held by heterogeneous forces, should have a different composition from that of the bulk liquid, whereas the second and subsequent layers, held by homogeneous forces should have substantially the same composition as the bulk liquid. Therefore the adsorption enthalpies of a chain segment which include the adsorption energies of the segment on the surface and the displacement of solvent molecules, are probably different for the different mixtures, and the conformations are influenced by the composition of the liquid from this point of view. On the other hand, it has been shown [22] that the chloroform is a better solvent than benzene for PEO as judged by intrinsic viscosity or by gel swelling, so that a preferential solvation of the polymeric chains by this solvent can be expected. Because CHCl, has a weaker surface affinity than CsHs, and because it is a better solvent of the polymer, it is expected that the chains are more stretched and extended towards the solution, the higher the content in chloroform. Both effects lead to the same result: the thickness of the adsorbed layer changes with the content of the mixture. However with the ESR spectroscopy of spin-labeled chains, at high grafting ratio this variation is not detectable at room temperature. The spectra appear simply as three narrow lines characteristic of the fast tumbling of the end segment in solution, without any indication of an immobilized fraction of labels on the surface. This is due to the high grafting ratio achieved, and the resulting strong interaction between the chains. For this technique and this temperature the layer of terminally anchored chains is similar to a homogeneous medium with an average viscosity and polarity resulting from the presence of segment and solvent molecules.
271
Partitioning of the solvents The hyperfine coupling constant A,, of the nitroxide spin label is sensitive to the local polarity of the microenvironment. In particular the splitting between the extreme lines of labels fixed to grafted chains in pure benzene and pure chloroform are respectively 29.0-t0.2 and 30.020.2 Gauss. If the composition of the binary mixture inside the grafted layer was the same as the composition of the bulk liquid, the splitting between the lines shown in Fig. 2. would be linear with concentration. The control with a low grafting ratio sample indeed shows that this variation is linear when the concentration of the layer is very small, actually much smaller than the concentration which would prevail in a random coil. In fact, in this case, the PEO spreads out on the surface and adopts a flat conformation [ 221, which is not favourable for influencing the distribution of the solvents in space. It must be emphasized that as the polymer is grafted, i.e. chemically bonded on the surface, the experiments are done with a fixed number of molecules at the interface. This situation is very different, at least in principle, from the physical adsorption case where the chains can move from the bulk solvents to the surface, and where a special isotherm should be reported for each mixture. Moreover the linear variation which is observed for the second sample indicates that the preferential affinity of the label for one of the solvents is not sufficient to induce variations of concentration in the vicinity of the grafted layer. For the high grafting ratio sample, there is an enrichment in chloroform for the intermediate range of mixtures as seen by ESR spectroscopy. This effect is still more marked at lower temperature ( - 38’ C ) where pure benzene is solid. Small differences in concentration of the mixtures inside the grafted layer and in the bulk are detectable and reach about 10% at room temperature for the mixture 50% C,H,/SO% CHC&. These differences would be lower if the thickness of the layer and the volume available for the interactions between solvents and polymer were smaller with more exchanges of molecules with the bulk. This is the situation which prevails at a low grafting ratio, where the chains lie flat on the surface. The partitioning effect appears only detectable at high grafting ratios. In the case of free (non-adsorbed) labeled PEO 4000 chains in bulk solvent mixtures, the variation is also not perfectly linear. It must be recalled that even for free chains in solution there can be a preferential solvation by the better solvent of the macromolecule [ 261. This effect is possible because the concentration is high enough inside the coil even when PEO 4000 is taken as the polymer, i.e. a higher molecular weight and therefore a somewhat lower average concentration of segments for isolated coils is chosen. The chains in solution form a microenvironment where there is an enrichment in chloroform, the better solvent of the chains, but the variation is rather small and inside the experimental errors.
212 CONCLUSION
Experimental evidence for the partitioning of two solvents in a layer of polymers anchored on a solid surface has been shown in the case of mixtures of chloroform and benzene. Even if the system chosen is not currently used in liquid chromatography, the results can be interesting for understanding the underlying mechanisms of this technique. Moreover the problem of the conformation of polymers adsorbed from a binary mixture appears highly complex because the composition of the liquid inside the chains can be different from that in the bulk [ 251. This phenomenon is certainly related to the preferential solvation by the better solvent of the macromolecule [ 261. It must be stressed that this behaviour occurs only at high local concentration of monomer units, in the case of a high grafting ratio. No polymer adsorption isotherms were carried out from these mixed solvents but it would be useful to make these experiments in the future.
REFERENCES
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
M.A. Cohen-Stuart, T. Cosgrove and B. Vincent, Adv. Colloid Interface Sci., 24 (1986) 143. C. Allain, D. Ausserre and F. Rondelez, Phys. Rev. Lett., 49 (1982) 1694. E. Pefferkorn, A. Carroy and R. Varoqui, J. Polym. Sci. Polym. Phys. Ed., 23 (1987) 1997. E. Killmann, H. Maier, P. Kaniut and N. Gutling, Colloids Surfaces, 21 (1985) 261. M. Kawaguchi, M. Mikura and A. Takahashi, Macromolecules, 17 (1984) 2063. A. Silberberg, J. Chem. Phys., 48 (1968) 2835. J. Lecourtier, R. Audebert and C. Quivoron, Macromolecules, 12 (1980) 141. P.G. De Gennes, Macromolecules, 13 (1980) 1069; 14 (1981) 1637; 15 (1982) 492; Adv. Colloid Interface Sci., 27 (1987) 189. J.M.H.M. Scheutjens and G.J. Fleer, J. Phys. Chem., 83 (1979) 1619; 84 (1980) 178. T. Tajouri, L. Facchini, A.P. Legrand, H. Balard and E. Papirer, J. Chim. Phys. Phys. Chim. Biol., 84 (1987) 243. T. Cosgrove and K.G. Barnett, J. Magn. Reson., 43 (1981) 15. I.D. Robb and R. Smith, Eur. Polym. J., 10 (1974) 1005; Polymer, 18 (1977) 500. N.N. Sistovaris, W.O. Riede and H. Sillescu, Ber. Bunsenges. Phys. Chem., 79 (1975) 882. A.T. Bullock, G.G. Cameron, I. More, and I.D. Robb, Eur. Polym. J., 10 (1984) 951. C. Friedrich, F. Laupretre, C. Noel and L. Monnerie, Macromolecules, 13 (1980) 1625. M.A. Cohen-Stuart, J.M.H.M. Scheutjens and G.J. Fleer, ACS Symp. Ser., 240 (1984) 52. D.E. Martire and R.E. Boehm, J. Phys. Chem., 87 (1983) 1045. R.E. Boehm, D.E. Martire, D.W. Armstrong and K.H. Bui, Macromolecules, 17 (1984) 400. C.H. Lochmuller and D.R. Wilder, J. Chromatogr. Sci., 17 (1979) 574. J.K. Iler, The Chemistry of Silica, Wiley-Interscience, New York, 1979. A. Halperin and S. Alexander, Eurphys. Lett., 6 (1988) 329. J.J. Kipling and D.B. Peakall, J. Chem. Sot., (1956) 4828; (1958) 3899. G.J. Howard and P. McConnell, J. Phys. Chem., 71 (1967) 2974. D. Kivelson, J. Chem. Phys., 33 (1960) 1107. P.L. Nordio, in L.J. Berliner (Ed. ), Spin Labeling Theory and Applications, Academic Press, New York, 1976, p. 5. P.G. De Gennes, J. Phys. Chem., 88 (1984) 6469. C. Strazielle and H. Benoit, J. Chim. Phys., 58 (1961) 675.
263 -
Printed
in The Netherlands
Partitioning of Chloroform and Benzene in a Layer of Poly(ethylene oxide) Anchored on Silica H. HOMMEL’, A.P. LEGRAND’, H. BALARD’ and E. PAPIRER’ ‘Laboratoire de Physique Quantique, UA C.N.R.S. 421, E.S.P.C.I., 10 rue Vauquelin, 75231 Paris Ceden 05 (France) ‘Centre de Recherches sur la Physico-Chimie des Surfaces Solides, L.P. C.N.R.S. 6601,24 Avenue du President Kennedy, 68200 Mulhouse (France) (Received 31 March 1988; accepted 16 January
1989)
ABSTRACT ESR spectroscopy of labeled poly (ethylene oxide) terminally anchored on silica and in contact with mixtures of chloroform and benzene in different proportions has been used to determine the content of both solvents in the grafted layer. If the grafting ratio is high, there is evidence for a partitioning effect and preferential adsorption of one of the solvents.
INTRODUCTION
Polymers at interfaces are very important in applications such as colloid stabilization and flocculation, adhesion, wetting and chromatography, for example. Numerous studies on model systems, as well experimental [l-5] and theoretical [ 6-91 work, increase our understanding of their behaviour. Magnetic resonance appears to be a very sensitive method for probing the conformation and dynamics of the chains at a local molecular level [ lo,11 1. It is also possible to use ESR of spin labeled molecules [ 12-151 to investigate these properties. Here we take advantage of the variation of the hyperfine coupling constant of the nitroxide spin label with the local polarity of the microenvironment to show evidence of the partitioning of chloroform and benzene in a layer of poly (ethylene oxide ) anchored on silica. When adding different amounts of a good solvent, like chloroform, to poly (ethylene oxide ) adsorbed in a flat conformation on silica in benzene, there is a displacement effect [ 161 and the chains adopt a more extended configuration. However, if the grafting ratio is high enough for the polymer to build up a homogeneous tightly packed layer like the stationary phase in chromatography [ 17,181 there is, in addition, a preferential adsorption of one of the two solvents at the interface.
0166-6622/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
264 EXPERIMENTAL
Materials
Samples with the very high grafting ratio needed for the experiments were prepared following the procedure described below. Silica
The silica was fume or pyrogenic Aerosil 300 from Degussa. Consisting of large aggregates of spheres of about 10 nm. It is different from the gels currently used in liquid chromatography, but it has the advantage of being nonmicroporous and its surface can be approximated by a plane at a molecular scale. The specific area measured by nitrogen adsorption was about 310 m2 -1 g . Polymer
The polymer was poly (ethylene oxide) (PEO) 2000 of molecular weight 1880, i.e. with about 43 repeat units, purchased from Fluka. The polydispersity of the PEO was 1.09. In addition, a sample of PEO 4000 from Fluka was used. Grafting reaction
The grafting reaction was the direct esterification of a surface silanol group by one of the two hydroxyl ends of a macromolecule. The latter had about 1.6 OH groups at the free ends. The silica was dehydrated, agglomerated with acetone (reagent pure) and dried at 150°C. For the Aerosil fume silica, chosen, the aggregates are very open and the mass fractal dimension is low [ 191. Therefore even when the silica is agglomerated the number of contact points between aggregates is small and it is easy to disperse after drying. The silica was dispersed in a PEO 2000 solution to obtain the necessary impregnation ratio of physically adsorbed polymers. The solvent was then evaporated with a rotatory evaporator. The powder was dried, degassed and placed in a container filled with nitrogen. Finally the grafting reaction took place: the powder was heated to 200°C for about 1 h. Once the grafting reaction was completed, the ungrafted polymer was removed by a long extraction (20 h) with acetone in a Soxhlet extractor. The grafting ratio was determined by pyrolysis weight loss between 25 and 750” C under oxygen. The samples of PEO 2000 had a grafting ratio, or more precisely a surface concentration, of 0.045 and 0.42 molecules/nm2. For the latter sample there was strong chain interaction. Any chains which have been attached to the sur-
265
face through both ends are included in the estimation of the grafting ratios but cannot be detected by ESR. However, no attempt was made to determine how the fraction of doubly attached chains varied with the grafting ratios. The long-term stability of the grafted polymer was not checked thoroughly [ 201. Some oxidation is indeed possible at room temperature. Here, in fact, the grafted polymers were stored in a refrigerator in the absence of light and at low temperature to minimize this risk. Spin labeling One percent of these grafted chains were then labeled at their free end by the nitroxide free radical 2,2,5,5-tetramethyl-3-pyrroline-l-oxyl-3-carboxylic acid in its acid chloride form. The chemical formula of a grafted, labeled, polymer chain is: SI-O~_(~CH,~CH,~O~~~C i>ik
a.
In addition the PEO 4000 was labeled at the free end through the same reaction, but without grafting onto silica. It was therefore possible to use this sample as a control allowing values for free (non-adsorbed) labeled chains in bulk solvent mixtures to be recorded. The molecular weight is not the same as the grafted samples, but even in this case, some conclusions could be drawn from the results. Solvents Chloroform and benzene (analytical grade) were chosen as the solvents. Mixtures of these two liquids were also prepared. The molecules have a similar size, and form organic liquids [ 211 which are completely miscible at room temperature. Some data are already available on the two-body interactions between these solvents and silica and between the solvents and poly (ethylene oxide) [ 221. The label, which has a small dipolar moment, should have more affinity for the dissymmetric chloroform molecular than for the aromatic benzene, which is more attracted by planar structures. However, as will be shown in the subsequent experiments no effect was detectable in our samples. ESRSPECTROSCOPY
Samples and apparatus All the samples were prepared in Pyrex tubes of 4 mm diameter. First the solvents remaining on the grafted silica were removed by direct pumping. Each
266
time a mixture in different proportions of benzene and chloroform was added. The whole sample was then degassed by repeated freeze-pump-thaw cycles and sealed in a vacuum. The ESR spectra were recorded on a Varian E-4 spectrometer operating in the X-band at 9.15 GHz. The temperature was regulated by a Varian E-257 temperature controller. Analysis
of the spectra
The nitroxide label has been extensively nian can be written
studied
[ 23,241. Its spin Hamilto-
H=SgB+IAS where B is the external magnetic field, S the spin of the unpaired electron and I the spin of the nitrogen nucleus, g is the gyromagnetic tensor and A the hyperfine tensor, and their principal axis are nearly parallel. If the motion is fast enough the energy levels can simply be obtained in terms of the isotropic Hamiltonian, where the tensors g and A have been replaced by the constants go=1/3*Tr(g) and A,=1/3*Tr(A). The spectrum consists of three well-defined Lorentzian lines, whose linewidths are interpreted by the Kivelson theory [ 231. The spectral lines are equally spaced, and the separation between adjacent lines is proportional to Ao, which has been measured on each sample. In fact the ESR spectra of the nitroxide free radical are sensitive not only to molecular motions, but also to the nature of the medium in which they are dissolved [ 131. The magnetic parameters are very sensitive functions of the electronic distribution in the molecule, and, therefore, are influenced by perturbations due to the environment. Thus A0 increases by more than 10% when going from hydrophobic to hydrophilic solvents. This effect can be understood qualitatively by considering the specific interactions of the polar solvents with the lone pairs of electrons on the oxygen atom. It would be of interest to choose in addition more polar liquids, but these would not be convenient for the ESR studies, and more important, the grafting bond Si-O-C would not be stable in water. RESULTS AND DISCUSSION
Observation Typical spectra of labeled PEO chains grafted onto silica with a grafting ratio of 0.42 molecules/nm2, at room temperature, in contact with mixtures of chloroform and benzene in different proportions, are shown in Fig. 1. The content by volume are respectively 100% CsHE, 80% C6H6/20% CHCl,, 50% C,H,/
261 ,5G,
r
Fig. 1. Typical spectra of labeled PEO chains grafted on silica with a grafting ratio of 0.42 molecules/nm’, at room temperature, in contact with mixtures of chloroform and benzene in different proportions. The content in benzene is indicated.
50% CHC13, 20% C&H&30% CHCl, and 100% CHC&. The first derivative of three narrow Lorentzian lines form the usual ESR spectrum. The viscosity at 20 oC of CGHsis 0.652 CP and of CHCl,, 0.58 cP. The motion of the label at the free ends depends on the local viscosity, which is also influenced by the concentration of polymer segments at this point. Some spectra exhibit a superhyperfine structure due to the presence of hydrogen in the neighbouring methyl groups when the content in chloroform is high. This result indicates a higher mobility at higher chloroform content. The parameter which is important here and which has been extracted from the spectra is the position of the resonance lines, or more precisely the distance between the peaks. The distance between the outer peaks 2Ao, as a function of
268 30.5
30
g
29.:
,
s 0 s 25
26.5
0
20
40 c
60
60
100
%
Fig. 2. Evolution of the hyperfine splitting 2A,,, between two extreme lines of the fast motion spectrum as a function of the relative concentration of benzene in the mixture of solvents, at room temperature ( 0 ) and at - 38’ C ( A ), for the sample with a grafting ratio of 0.42 molecules/rim*. 30.5
\ \
30
t
29.5 !
e 0 : 29
\
\ \\\\ ‘t t\\\\\ i\\\ f
26.5
0
20
40 c
60
60
100
%
Fig. 3. Evolution of the hyperfine splitting 2A,, between two extreme lines of the fast motion spectrum as a function of the relative concentration of benzene in the mixture of solvents for the sample with a grafting ratio of 0.045 molecules/nm2 at room temperature.
269 30.5
\ \ \ 30
.
1 \ t
\ \ \
:,
\
29.5
t \ \ \
<
\ 0
\
;:
\ 29
\
.
\
26.5
L
* 0
20
40 c
60
60
100
%
Fig. 4. Evolution of the hyperfine splitting 2A,,, between two extreme lines of the fast motion spectrum as a function of the relative concentration of benzene in the mixture of solvents for free (non-adsorbed) labeled PEO 4000 chains at a concentration of 10m3mall-’ at room temperature.
the concentration in benzene, is given in Fig. 2. There appears to be a deviation from the straight line joining the values for the pure solvents, benzene and chloroform. This deviation is outside of the experimental errors, given the accuracy of the measurements. In addition, the same experiment has been repeated at a lower temperature, - 38” C, except for pure benzene, for which the motion of the label is slow and the spectrum more complicated, because the solvent is frozen. The deviation is more marked. As the parameter A0 is not expected to depend on temperature this variation must be assigned to differences in tts composition of the mixture around the label. As a control two kinds of experiments have been carried out. Firstly, the measurement for a low grafting ratio sample is given in Fig. 3. Indeed the reported values follow a straight line significantly different from the results for the high grafting ratio sample. Secondly, the values for free (non-adsorbed) labeled PEO 4000 chains with a concentration of 10e3 mol l-l (or a total content of 6*1017 labels in the tube) in bulk solvent mixtures are given in Fig. 4. The variation is also not completely linear, but the variation remains inside experimental errors.
270
Interaction
between solvents, polymers
and silica
The polymer conformation is the result of the competition between surface attraction and chain repulsion. In a good solvent the chains are swollen. But if the affinity of the solvent for the solid surface is poor, the first effect can be hidden by a stronger adsorption of the polymer. In order to explain the obtained conformation at the interface, it appears that it is necessary to take into account the whole system and particularly the solvent-surface and polymersurface interactions [ 221. Howard and McConnel [22] have estimated by BET analysis of vapor adsorption on aerosil silica that the heat of adsorption for C&H, is 1.4 kcal mol-l and for CHCl, 1.0 kcal mol-‘. Making the assumption that the affinity of the liquid for the solid is correlated with this heat of adsorption, we expect that benzene is more strongly adsorbed on the solid surface than chloroform. Kipling and Peakall [ 211 have studied the adsorption of binary liquid mixtures of chloroform and benzene on aluminium oxide gels, and namely on boehmite. They suggest that the first adsorbed layer of solvents, held by heterogeneous forces, should have a different composition from that of the bulk liquid, whereas the second and subsequent layers, held by homogeneous forces should have substantially the same composition as the bulk liquid. Therefore the adsorption enthalpies of a chain segment which include the adsorption energies of the segment on the surface and the displacement of solvent molecules, are probably different for the different mixtures, and the conformations are influenced by the composition of the liquid from this point of view. On the other hand, it has been shown [22] that the chloroform is a better solvent than benzene for PEO as judged by intrinsic viscosity or by gel swelling, so that a preferential solvation of the polymeric chains by this solvent can be expected. Because CHCl, has a weaker surface affinity than CsHs, and because it is a better solvent of the polymer, it is expected that the chains are more stretched and extended towards the solution, the higher the content in chloroform. Both effects lead to the same result: the thickness of the adsorbed layer changes with the content of the mixture. However with the ESR spectroscopy of spin-labeled chains, at high grafting ratio this variation is not detectable at room temperature. The spectra appear simply as three narrow lines characteristic of the fast tumbling of the end segment in solution, without any indication of an immobilized fraction of labels on the surface. This is due to the high grafting ratio achieved, and the resulting strong interaction between the chains. For this technique and this temperature the layer of terminally anchored chains is similar to a homogeneous medium with an average viscosity and polarity resulting from the presence of segment and solvent molecules.
271
Partitioning of the solvents The hyperfine coupling constant A,, of the nitroxide spin label is sensitive to the local polarity of the microenvironment. In particular the splitting between the extreme lines of labels fixed to grafted chains in pure benzene and pure chloroform are respectively 29.0-t0.2 and 30.020.2 Gauss. If the composition of the binary mixture inside the grafted layer was the same as the composition of the bulk liquid, the splitting between the lines shown in Fig. 2. would be linear with concentration. The control with a low grafting ratio sample indeed shows that this variation is linear when the concentration of the layer is very small, actually much smaller than the concentration which would prevail in a random coil. In fact, in this case, the PEO spreads out on the surface and adopts a flat conformation [ 221, which is not favourable for influencing the distribution of the solvents in space. It must be emphasized that as the polymer is grafted, i.e. chemically bonded on the surface, the experiments are done with a fixed number of molecules at the interface. This situation is very different, at least in principle, from the physical adsorption case where the chains can move from the bulk solvents to the surface, and where a special isotherm should be reported for each mixture. Moreover the linear variation which is observed for the second sample indicates that the preferential affinity of the label for one of the solvents is not sufficient to induce variations of concentration in the vicinity of the grafted layer. For the high grafting ratio sample, there is an enrichment in chloroform for the intermediate range of mixtures as seen by ESR spectroscopy. This effect is still more marked at lower temperature ( - 38’ C ) where pure benzene is solid. Small differences in concentration of the mixtures inside the grafted layer and in the bulk are detectable and reach about 10% at room temperature for the mixture 50% C,H,/SO% CHC&. These differences would be lower if the thickness of the layer and the volume available for the interactions between solvents and polymer were smaller with more exchanges of molecules with the bulk. This is the situation which prevails at a low grafting ratio, where the chains lie flat on the surface. The partitioning effect appears only detectable at high grafting ratios. In the case of free (non-adsorbed) labeled PEO 4000 chains in bulk solvent mixtures, the variation is also not perfectly linear. It must be recalled that even for free chains in solution there can be a preferential solvation by the better solvent of the macromolecule [ 261. This effect is possible because the concentration is high enough inside the coil even when PEO 4000 is taken as the polymer, i.e. a higher molecular weight and therefore a somewhat lower average concentration of segments for isolated coils is chosen. The chains in solution form a microenvironment where there is an enrichment in chloroform, the better solvent of the chains, but the variation is rather small and inside the experimental errors.
212 CONCLUSION
Experimental evidence for the partitioning of two solvents in a layer of polymers anchored on a solid surface has been shown in the case of mixtures of chloroform and benzene. Even if the system chosen is not currently used in liquid chromatography, the results can be interesting for understanding the underlying mechanisms of this technique. Moreover the problem of the conformation of polymers adsorbed from a binary mixture appears highly complex because the composition of the liquid inside the chains can be different from that in the bulk [ 251. This phenomenon is certainly related to the preferential solvation by the better solvent of the macromolecule [ 261. It must be stressed that this behaviour occurs only at high local concentration of monomer units, in the case of a high grafting ratio. No polymer adsorption isotherms were carried out from these mixed solvents but it would be useful to make these experiments in the future.
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