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NMR investigation of the PEO-SDS aggregate
Colloids and Surfaces, 54 (1991) 261-266
Elsevier Science Publishers
261
B.V., Amsterdam
NMR investigation of the PEO-SDS aggregate R. Ramachandran and G.J. Kennedy’ Union Carbide Chemicals and Plastics Company Inc., Specialty
Chemicals Division,
Bound Brook, NJ 08805 (U.S.A.)
(Received 30 April 1990; accepted
16 July 1990)
Abstract The poly(ethylene oxide) -sodium dodecyl sulfate (PEO-SDS) aggregate has been investigated using W nuclear magnetic resonance (NMR) spectroscopy. SDS enriched with 13C in the C-l position was used which enabled probing SDS concentrations as low as 6*10-5 M for the first time by NMR. The NMR shift of the C-l carbon was monitored as a function of SDS concentration and polymer molecular weight. Our data suggests that there is some interaction between SDS and PEO at surfactant concentrations as low as 4*10m4 M. The upfield shift of C-l was higher in the presence of low molecular weight PEO than in high molecular weight PEO. The SDS environment in the PEO-SDS aggregate can be distinguished from that in a micelle using this technique. Results are discussed in terms of the topology of the PEO-SDS aggregate.
INTRODUCTION
The structure of a polymer surfactant aggregate has been the focus of several investigations and has been comprehensively reviewed in the literature [ 11. The poly (ethylene oxide)-sodium dodecyl sulfate (PEO-SDS ) system has been studied extensively by Saito [ 21 and Muller and Johnson [ 31 and other investigators. The pioneering work of Cabane [4] led to the description of the PEOSDS aggregate as that of a polymer wrapping around the surfactant micelles. It was established that there were two sites of the micelle where PEO replaced hydration water of the surfactant: around the polar groups and the hydrated methylene groups. Cabane’s work based on 13CNMR chemical shifts suggested that there was no difference between the SDS micellar environment and the SDS environment in a PEO-SDS aggregate. However, based on the structure of the PEO-SDS aggregate that emerged from Cabane’s work, one would expect the sulfate moieties to be in a different chemical environment when they are close to the ether oxygens than when they are free as micelles. The inherent sensitivity limitations of the 13C NMR experiment (due to the low nat,ural abundance of the 13Cisotope) coupled with the fact that Cabane’s shift of the ‘Present address: Mobil Research tory, Paulsboro, NJ 08066, U.S.A.
0166-6622/91/$03.50
and Development
0 1991-
Corporation,
Elsevier Science Publishers
Paulsboro
B.V.
Research
Labora-
262
C-l carbon was referenced to the C-10 carbon of SDS may have contributed to the inability to make this distinction between the two SDS environments. NMR has been extremely efficient in providing structural information on the state of SDS in solution [ 5-91 and on the PEO-SDS aggregate [ 41. However, NMR studies at low SDS concentrations (well below 2~10.~M) has been inhibited due to instrument limitations. We have used 13Cenriched SDS in the C-l position (closest to the sulfate moiety) to analyze NMR shifts at concentrations as low as 6~10~~ M SDS in the presence of a polymer. This has enabled us to obtain significant new data on some critical aspects of the polymer-SDS aggregate. We have examined the interactions of SDS with the nonionic poly (ethylene oxide) and with two cationic cellulosic polymers UCARE polymer JR-400 and QUATRISOFT LM-200. We report the PEO-SDS interaction as a first part of this study here and the results of the cationic hydroxyethyl cellulose-SDS interactions will be reported soon. The partitioning of SDS in the presence of PEO and the role of molecular weight of the polymer in the polymer-surfactant aggregate are discussed in this work. MATERIALS
AND METHODS
Monodisperse PEO (molecular weight 21000 and 270 000) was obtained from TOSOHAAS (Rohm and Haas Building, Philadelphia, PA) and used as received. Sodium dodecyl sulfate enriched in the C-l position with the 13C isotope was custom synthesized and obtained from MSD isotopes (Merck, Point Claire, Quebec, Canada), and used as received. All samples were made up in D,O obtained from MSD isotopes. In view of the limited amount of the expensive surfactant available we have focussed only on the NMR aspects so far and resorted to literature data and assumed the c.m.c. to be around 4. 10e3 M. The NMR data shows a sharp break around that concentration. The polymer concentration was fixed at 0.9% (wt/volume of deuterium oxide). NMR experiments were carried out in 5 mm tubes on a Bruker AM-360 NMR spectrometer operating at a 13C resonance frequency of 90.5 MHz. All chemical shift data are referenced to an external standard of TSP in D,O ( 10% wt/vol). The variation in chemical shift was measured at less than 0.002 ppm h-’ or 0.181 Hz h-l. RESULTS
AND DISCUSSION
The C-l shift of SDS as a function of concentration in the absence of polymers is shown in Fig. 1. The spectra of SDS are shown in Fig. 2 to illustrate the pntential of this approach to easily obtain NMR data at concentrations as low as 6*10V5 M SDS. It is clear that as a function of concentration we see an upfield shift of C-l and the sharp break in the shift occurs around the c.m.c. of the surfactant. The sulfate moieties are close to one another in an SDS micelle. The cluster of sulfate moieties lead to electrostatic repulsion between adjacent
263
72.40
NMR
sh,ft
---------v
1:::: 10-S
10-4
Soa
conee2i
in Molediier.
‘O-*
10-l
‘SDS
Fig. 1. NMR shifts of isotopically enriched SDS.
sulfate groups. This repulsion results in an electron enriched environment around the C-l carbon leading to the upfield shift. The higher the electrostatic repulsion on the sulfate, the greater will be the upfield shift of C-l. In this context, let us examine the C-l shift in the presence of PEO. The NMR shifts of the C-l carbon as a function of SDS concentration and in the presence of the polymer are shown in Fig. 3. PEO of molecular weight 21000 and 270 000 are referred to as PEO 21 and PEO 270, respectively. At low concentrations of SDS (below 4. 10m3M) there is a consistent upfield shift of about 0.1 ppm (9 Hz) in the presence of PEO. The shift increases in magnitude with increasing concentration of SDS. The higher upfield shift of SDS suggests that the sulfate groups of the monomer are in an electrostatic repulsive environment. The adsorption of PEO on the SDS micelle due to replacement of hydration water around the polar sulfates and the hydrated methylene groups has been shown by Cabane [4]. The consistent upfield shift of C-l indicates that aggregation between SDS and PEO occurs with the polar groups of both the polymer and the surfactant being close to one another. Most interestingly, the SDS aggregates on the polymer at concentrations as low as 4. 10e4 M SDS (where no micelles are present) can be distinguished from SDS monomers using this technique. Literature data on the C-l shift of SDS at such low concentrations in PEO-SDS aggregates is not available. Cabane’s work that postulated the PEO-SDS aggregate structure referenced all the C-l shifts to that of C-10. The assumption was made that C-10 shifts were independent of surfactant concentration which is true above 5~10~~ M. However, it is clear from Cabane’s work, that at very low concentrations of SDS (around 10W3M) the fluctuation in C-10 shift is at least 4 Hz. Cabane’s assumptions that C-10
264
SDS 7.0~10~ M 72.360 ppm
SDS 7.0~10~ M 72.377
ppm
74.0
73.0
PPM
72.0
SDS 6.0xlU2 M 71.933 ppm
SDS 6.9x10* M 71.979
ppm
1,.
I.“.J.“‘I,‘.‘l.“.II”’
74.0
73.a
PPM
72.0
71
a
Fig. 2. NMR spectra of isotopically
11,‘.
.
7a.a
.I.“.,.
71.0
1,
PCII
72.0
,,,
71
.
0
enriched SDS as a function of concentration.
shifts are independent of SDS concentration may mask some important effects that we observe at low concentrations. In our system, at an SDS concentration of 4.lop4M, we see a clear upfield C-l shift of 0.1 ppm (9 Hz) in the presence of PEO. The NMR shifts are consistent with the model that the sulfate moieties of the SDS aggregate on the polymer and are in a less electron-rich environment suggesting that the hydrated part of SDS is bound to the polymer. The consis-
265
I.4
10-3
10-4
10-5
10-Z
10-l
SDS concentration in Moledlite~. n
SDS
+
PEO 21
*
PEO 270
Fig. 3. NMR shifts of isotopically enriched SDS in the presence and absence of PEO (molecular weight 21000 and 270 000). Polymer concentration=0.9% (wt/volume of DzO).
tent upfield shift shows that the association between the polymer and the surfactant is primarily due to aggregation between the polar groups. The sulfate ions are in an electrostatic repulsive environment in the PEO-SDS aggregate. The points at which the upfield shift begins in the presence of the polymer and then levels off will correspond to the Tl and T2 regions seen in typical nonionic polymer/anionic surfactant systems. The uptield shift effect is magnified at increasing SDS concentrations. At SDS concentrations above 2. 10e3 kf, the uptield shift of C-l is much higher in the presence of the polymer. Most importantly, it is to be noted that in our study we clearly distinguish between an SDS micelle and a PEO-SDS aggregate. The C-l shift of SDS in the presence of PEO is much higher than in a micelle. This distinction has not been previously noted in the literature. The 13C enriched SDS enables us to probe such fine details of the aggregate structure. The molecular weight of the polymer does appear to play a role in the polymer-surfactant aggregate. The C-l of SDS in the presence of PEO of MW 270 000 exhibits an upfield shift but of a smaller magnitude than 21000 PEO. The higher molecular weight polymer was more viscous in solution. We can only speculate on the role of molecular weight in the PEO-SDS aggregate. The number of SDS monomers in the PEO-SDS mixed micelle can depend on the molecular weight of the polymer due to size restrictions of the polymer and the micelle. The higher shifts due to PEO 21 suggest that the number of SDS monomers around the polar groups of the polymer are higher for the smaller polymer. Muller and Johnson [ 31, however, did not see an effect of molecular weight in their NMR investigation of the F,SDS-PEO (molecular weight 7 000
266
and 20 000) system. The difference in molecular weights in their system may not have been significant to show the effect of molecular weight. CONCLUSIONS
NMR investigation of the PEO-SDS system using 13C enriched SDS enabled us to easily obtain information on the state of SDS at concentrations as low as 6.10V5 M SDS. In the case of pure SDS, we found an upfield shift of the C-l as a function of SDS concentration which is consistent with the C-l carbon being in an electron enriched environment in the presence of micelles. In the presence of PEO, the upfield shift of the C-l was much higher suggesting that polymer surfactant aggregation occurred between the polar groups of the polymer and the surfactant. The upfield shift was higher with the low molecular PEO. We speculate at this point that the number of SDS micelles on the polymer could be higher in the low molecular weight polymer. A small but consistent upfield shift was observed at SDS concentrations of 4. lop4 M in the presence of the polymer suggesting that the individual surfactant-PEO interaction cannot be completely ruled out. In contrast to what has been suggested in the literature, the higher chemical shift of C-l in the presence of the polymer shows that the SDS aggregate on PEO is in a different chemical environment than the SDS micelle and can be distinguished using NMR. We believe that NMR experiments with isotopically enriched SDS will be able to provide very good structural information on the molecule at hitherto unexplored low concentrations in solid/liquid interfaces and with polyelectrolytes. ACKNOWLEDGEMENTS
We would like to acknowledge Dr K.P. Ananthapadmanabhan, of Union Carbide Chemicals and Plastics Company Inc., Tarrytown, for his technical assistance during the course of these experiments and the Specialty Chemicals Division of Union Carbide Chemicals and Plastics Company Inc., for their support during the course of this work. REFERENCES 1 2 3 4 5 6 7 8 9
E.D. Goddard, Colloids Surfaces, 19 (1986) 255. S. Saito, Colloid Polym. Sci., 257 (1979) 266; J. Colloid Interface Sci., 30 (1969) 211; Kolloid Z., 215 (1967) 16. N. Muller and T.W. Johnson, J. Phys. Chem., 73 (1969) 2042. B. Cabane, J. Phys. Chem., 81 (1977) 1639. J. Clifford and B.A. Pethica, Trans. Faraday Sot., 60 (1964) 1483. J. Oakes, Trans. Faraday Sot., 19 (1974) 2200. H. Wennerstrom, B. Lindman and 0. Soderman, J. Am. Chem. Sot., 101 (1979) 6860. E.G. Janzen, L. Haire and G. Coulter, J. Org. Chem., 54 (1989) 2915. 0. Soderman, G. Carlstrom, U. Olsson and T. Wong, J. Chem. Sot. Faraday Trans. I,84 (1988) 4475.
Elsevier Science Publishers
261
B.V., Amsterdam
NMR investigation of the PEO-SDS aggregate R. Ramachandran and G.J. Kennedy’ Union Carbide Chemicals and Plastics Company Inc., Specialty
Chemicals Division,
Bound Brook, NJ 08805 (U.S.A.)
(Received 30 April 1990; accepted
16 July 1990)
Abstract The poly(ethylene oxide) -sodium dodecyl sulfate (PEO-SDS) aggregate has been investigated using W nuclear magnetic resonance (NMR) spectroscopy. SDS enriched with 13C in the C-l position was used which enabled probing SDS concentrations as low as 6*10-5 M for the first time by NMR. The NMR shift of the C-l carbon was monitored as a function of SDS concentration and polymer molecular weight. Our data suggests that there is some interaction between SDS and PEO at surfactant concentrations as low as 4*10m4 M. The upfield shift of C-l was higher in the presence of low molecular weight PEO than in high molecular weight PEO. The SDS environment in the PEO-SDS aggregate can be distinguished from that in a micelle using this technique. Results are discussed in terms of the topology of the PEO-SDS aggregate.
INTRODUCTION
The structure of a polymer surfactant aggregate has been the focus of several investigations and has been comprehensively reviewed in the literature [ 11. The poly (ethylene oxide)-sodium dodecyl sulfate (PEO-SDS ) system has been studied extensively by Saito [ 21 and Muller and Johnson [ 31 and other investigators. The pioneering work of Cabane [4] led to the description of the PEOSDS aggregate as that of a polymer wrapping around the surfactant micelles. It was established that there were two sites of the micelle where PEO replaced hydration water of the surfactant: around the polar groups and the hydrated methylene groups. Cabane’s work based on 13CNMR chemical shifts suggested that there was no difference between the SDS micellar environment and the SDS environment in a PEO-SDS aggregate. However, based on the structure of the PEO-SDS aggregate that emerged from Cabane’s work, one would expect the sulfate moieties to be in a different chemical environment when they are close to the ether oxygens than when they are free as micelles. The inherent sensitivity limitations of the 13C NMR experiment (due to the low nat,ural abundance of the 13Cisotope) coupled with the fact that Cabane’s shift of the ‘Present address: Mobil Research tory, Paulsboro, NJ 08066, U.S.A.
0166-6622/91/$03.50
and Development
0 1991-
Corporation,
Elsevier Science Publishers
Paulsboro
B.V.
Research
Labora-
262
C-l carbon was referenced to the C-10 carbon of SDS may have contributed to the inability to make this distinction between the two SDS environments. NMR has been extremely efficient in providing structural information on the state of SDS in solution [ 5-91 and on the PEO-SDS aggregate [ 41. However, NMR studies at low SDS concentrations (well below 2~10.~M) has been inhibited due to instrument limitations. We have used 13Cenriched SDS in the C-l position (closest to the sulfate moiety) to analyze NMR shifts at concentrations as low as 6~10~~ M SDS in the presence of a polymer. This has enabled us to obtain significant new data on some critical aspects of the polymer-SDS aggregate. We have examined the interactions of SDS with the nonionic poly (ethylene oxide) and with two cationic cellulosic polymers UCARE polymer JR-400 and QUATRISOFT LM-200. We report the PEO-SDS interaction as a first part of this study here and the results of the cationic hydroxyethyl cellulose-SDS interactions will be reported soon. The partitioning of SDS in the presence of PEO and the role of molecular weight of the polymer in the polymer-surfactant aggregate are discussed in this work. MATERIALS
AND METHODS
Monodisperse PEO (molecular weight 21000 and 270 000) was obtained from TOSOHAAS (Rohm and Haas Building, Philadelphia, PA) and used as received. Sodium dodecyl sulfate enriched in the C-l position with the 13C isotope was custom synthesized and obtained from MSD isotopes (Merck, Point Claire, Quebec, Canada), and used as received. All samples were made up in D,O obtained from MSD isotopes. In view of the limited amount of the expensive surfactant available we have focussed only on the NMR aspects so far and resorted to literature data and assumed the c.m.c. to be around 4. 10e3 M. The NMR data shows a sharp break around that concentration. The polymer concentration was fixed at 0.9% (wt/volume of deuterium oxide). NMR experiments were carried out in 5 mm tubes on a Bruker AM-360 NMR spectrometer operating at a 13C resonance frequency of 90.5 MHz. All chemical shift data are referenced to an external standard of TSP in D,O ( 10% wt/vol). The variation in chemical shift was measured at less than 0.002 ppm h-’ or 0.181 Hz h-l. RESULTS
AND DISCUSSION
The C-l shift of SDS as a function of concentration in the absence of polymers is shown in Fig. 1. The spectra of SDS are shown in Fig. 2 to illustrate the pntential of this approach to easily obtain NMR data at concentrations as low as 6*10V5 M SDS. It is clear that as a function of concentration we see an upfield shift of C-l and the sharp break in the shift occurs around the c.m.c. of the surfactant. The sulfate moieties are close to one another in an SDS micelle. The cluster of sulfate moieties lead to electrostatic repulsion between adjacent
263
72.40
NMR
sh,ft
---------v
1:::: 10-S
10-4
Soa
conee2i
in Molediier.
‘O-*
10-l
‘SDS
Fig. 1. NMR shifts of isotopically enriched SDS.
sulfate groups. This repulsion results in an electron enriched environment around the C-l carbon leading to the upfield shift. The higher the electrostatic repulsion on the sulfate, the greater will be the upfield shift of C-l. In this context, let us examine the C-l shift in the presence of PEO. The NMR shifts of the C-l carbon as a function of SDS concentration and in the presence of the polymer are shown in Fig. 3. PEO of molecular weight 21000 and 270 000 are referred to as PEO 21 and PEO 270, respectively. At low concentrations of SDS (below 4. 10m3M) there is a consistent upfield shift of about 0.1 ppm (9 Hz) in the presence of PEO. The shift increases in magnitude with increasing concentration of SDS. The higher upfield shift of SDS suggests that the sulfate groups of the monomer are in an electrostatic repulsive environment. The adsorption of PEO on the SDS micelle due to replacement of hydration water around the polar sulfates and the hydrated methylene groups has been shown by Cabane [4]. The consistent upfield shift of C-l indicates that aggregation between SDS and PEO occurs with the polar groups of both the polymer and the surfactant being close to one another. Most interestingly, the SDS aggregates on the polymer at concentrations as low as 4. 10e4 M SDS (where no micelles are present) can be distinguished from SDS monomers using this technique. Literature data on the C-l shift of SDS at such low concentrations in PEO-SDS aggregates is not available. Cabane’s work that postulated the PEO-SDS aggregate structure referenced all the C-l shifts to that of C-10. The assumption was made that C-10 shifts were independent of surfactant concentration which is true above 5~10~~ M. However, it is clear from Cabane’s work, that at very low concentrations of SDS (around 10W3M) the fluctuation in C-10 shift is at least 4 Hz. Cabane’s assumptions that C-10
264
SDS 7.0~10~ M 72.360 ppm
SDS 7.0~10~ M 72.377
ppm
74.0
73.0
PPM
72.0
SDS 6.0xlU2 M 71.933 ppm
SDS 6.9x10* M 71.979
ppm
1,.
I.“.J.“‘I,‘.‘l.“.II”’
74.0
73.a
PPM
72.0
71
a
Fig. 2. NMR spectra of isotopically
11,‘.
.
7a.a
.I.“.,.
71.0
1,
PCII
72.0
,,,
71
.
0
enriched SDS as a function of concentration.
shifts are independent of SDS concentration may mask some important effects that we observe at low concentrations. In our system, at an SDS concentration of 4.lop4M, we see a clear upfield C-l shift of 0.1 ppm (9 Hz) in the presence of PEO. The NMR shifts are consistent with the model that the sulfate moieties of the SDS aggregate on the polymer and are in a less electron-rich environment suggesting that the hydrated part of SDS is bound to the polymer. The consis-
265
I.4
10-3
10-4
10-5
10-Z
10-l
SDS concentration in Moledlite~. n
SDS
+
PEO 21
*
PEO 270
Fig. 3. NMR shifts of isotopically enriched SDS in the presence and absence of PEO (molecular weight 21000 and 270 000). Polymer concentration=0.9% (wt/volume of DzO).
tent upfield shift shows that the association between the polymer and the surfactant is primarily due to aggregation between the polar groups. The sulfate ions are in an electrostatic repulsive environment in the PEO-SDS aggregate. The points at which the upfield shift begins in the presence of the polymer and then levels off will correspond to the Tl and T2 regions seen in typical nonionic polymer/anionic surfactant systems. The uptield shift effect is magnified at increasing SDS concentrations. At SDS concentrations above 2. 10e3 kf, the uptield shift of C-l is much higher in the presence of the polymer. Most importantly, it is to be noted that in our study we clearly distinguish between an SDS micelle and a PEO-SDS aggregate. The C-l shift of SDS in the presence of PEO is much higher than in a micelle. This distinction has not been previously noted in the literature. The 13C enriched SDS enables us to probe such fine details of the aggregate structure. The molecular weight of the polymer does appear to play a role in the polymer-surfactant aggregate. The C-l of SDS in the presence of PEO of MW 270 000 exhibits an upfield shift but of a smaller magnitude than 21000 PEO. The higher molecular weight polymer was more viscous in solution. We can only speculate on the role of molecular weight in the PEO-SDS aggregate. The number of SDS monomers in the PEO-SDS mixed micelle can depend on the molecular weight of the polymer due to size restrictions of the polymer and the micelle. The higher shifts due to PEO 21 suggest that the number of SDS monomers around the polar groups of the polymer are higher for the smaller polymer. Muller and Johnson [ 31, however, did not see an effect of molecular weight in their NMR investigation of the F,SDS-PEO (molecular weight 7 000
266
and 20 000) system. The difference in molecular weights in their system may not have been significant to show the effect of molecular weight. CONCLUSIONS
NMR investigation of the PEO-SDS system using 13C enriched SDS enabled us to easily obtain information on the state of SDS at concentrations as low as 6.10V5 M SDS. In the case of pure SDS, we found an upfield shift of the C-l as a function of SDS concentration which is consistent with the C-l carbon being in an electron enriched environment in the presence of micelles. In the presence of PEO, the upfield shift of the C-l was much higher suggesting that polymer surfactant aggregation occurred between the polar groups of the polymer and the surfactant. The upfield shift was higher with the low molecular PEO. We speculate at this point that the number of SDS micelles on the polymer could be higher in the low molecular weight polymer. A small but consistent upfield shift was observed at SDS concentrations of 4. lop4 M in the presence of the polymer suggesting that the individual surfactant-PEO interaction cannot be completely ruled out. In contrast to what has been suggested in the literature, the higher chemical shift of C-l in the presence of the polymer shows that the SDS aggregate on PEO is in a different chemical environment than the SDS micelle and can be distinguished using NMR. We believe that NMR experiments with isotopically enriched SDS will be able to provide very good structural information on the molecule at hitherto unexplored low concentrations in solid/liquid interfaces and with polyelectrolytes. ACKNOWLEDGEMENTS
We would like to acknowledge Dr K.P. Ananthapadmanabhan, of Union Carbide Chemicals and Plastics Company Inc., Tarrytown, for his technical assistance during the course of these experiments and the Specialty Chemicals Division of Union Carbide Chemicals and Plastics Company Inc., for their support during the course of this work. REFERENCES 1 2 3 4 5 6 7 8 9
E.D. Goddard, Colloids Surfaces, 19 (1986) 255. S. Saito, Colloid Polym. Sci., 257 (1979) 266; J. Colloid Interface Sci., 30 (1969) 211; Kolloid Z., 215 (1967) 16. N. Muller and T.W. Johnson, J. Phys. Chem., 73 (1969) 2042. B. Cabane, J. Phys. Chem., 81 (1977) 1639. J. Clifford and B.A. Pethica, Trans. Faraday Sot., 60 (1964) 1483. J. Oakes, Trans. Faraday Sot., 19 (1974) 2200. H. Wennerstrom, B. Lindman and 0. Soderman, J. Am. Chem. Sot., 101 (1979) 6860. E.G. Janzen, L. Haire and G. Coulter, J. Org. Chem., 54 (1989) 2915. 0. Soderman, G. Carlstrom, U. Olsson and T. Wong, J. Chem. Sot. Faraday Trans. I,84 (1988) 4475.