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Langmuir 1994,10, 86-91
Interaction of ABA Block Copolymers with Ionic Surfactants in Aqueous Solution E. Hecht and H. Hoffmannl Universitat Bayreuth, Physikalische Chemie I , 95440 Bayreuth, Germany Received July 12, 1993. In Final Form: September 17, 199P
Block copolymers of the type poly(ethy1ene oxide)-poly(propy1ene oxide)-poly(ethy1ene oxide) [EO,PO EO,] form micelleswith a hydrophobic core of propylene oxide and a palisade layer of the ethylene oxide. 6'e studied the influence of sodium dodecyl sulfate (SDS) on the aggregation behavior of F127 (EOg,POsgO&) by static and dynamic light scattering, electric birefringence, and calorimetric methods. We observed that for block copolymer solutions in the range of 1-5% and increasing concentrationof SDS the light scattering passed through a deep minimum, the electric birefringence passed over a maximum, and the endothermic peak of the DSC signals which is due to the micellization of the block copolymer disappeared. The results show that SDS binds to monomers of F127 and thereby suppresses completely the formation of F127 micelles. At saturation about six molecules of SDS bind to one F127 molecule. The birefringence data indicate that the F127lSDS complex exists in a more or less extended conformation and not as a coil. The binding of SDS on F127 was also confirmed by surface tension measurements. These measurements show that the surface active block copolymers are replaced from the interfaces by SDS molecules. The binding of SDS on F127 begins at concentration far below the critical micelle concentration of SDS.
Introduction It is now well established that block copolymers of the type EO,PO,EO, behave in many ways like normal hydrocarbon surfactants. The compounds are surface active and form micelles and lyotropic liquid crystalline phases.l-1° The aggregation number of the micelles depends on the size of the propylene oxide block and the xly ratio and is for many commercially available compounds in the range of 30-70. In comparison to normal surfactants these compounds have the peculiarity that the critical micelle concentration (cmc) and their surface activity depend strongly on temperature, much more so than the classic nonionic surfactants C,E,.Mp7-9 The cmc's of the block copolymers can shift several orders of magnitudes within a small temperature range. For the commercially used systems the main shift occurs in the temperature region between 20 and 50 "C. The consequence of this is that in moderately concentrated solutions with 1%polymer the block copolymers are present in the monomer state below room temperature and are transformed into micelles at higher temperatures. Usually there is a broad temperature region of about 10" in which the transformation occur^.^ The formation of the micelles is accompanied with a large endothermic heat which can easily be determined by DSC experiments. The heat of micellization is assumed to be due to the dehydration of the propylene oxide groups even though this has not been unambiguously proven. e Abstract published in Advance ACS Abstracts, November 15, 1993. (1) Schmolka, I. R. J. Am. Oil Chem. SOC.1977,54,110. (2) Rassing, J.; McKenna, W. P.; Bandyopadhyay, S.; Eyring, E. M. J . Mol. Liq. 1984,27, 165. (3) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171. (4) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268,101. (5) Brown, W.; Schillh, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991,95,1850. (6) Yu, G.; Deng, Y.;Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J. Chem. SOC.,Faraday Trans. 1992,88,2537. (7) Linse, P.; Malmsten, M. Macromolecules 1992,25, 5434. (8) Malmsten, M.; Lindman, B. Macromolecules 1992,25, 5440. (9)Malmsten, M.; Lindman, B. Macromolecules 1992,25, 5446. (10) Almgren,M.;Bahadur,P.; Janeson,M.;Li,P.;Brown, W.;Bahadur, A. J. Colloid Interface Sci. 1992, 151, 157.
0743-7463/94/2410-0086$04.50/0
Micellar solutions of block copolymers have been investigated with many different techniques including NMR,2*3*9 static and dynamic light scattering,MJOJ1 rheologyy,Sfluorescence,12and SANS.4J3J4 As a consequence of these many investigations, much is known about the properties of the micelles. It is usually concluded from the measurements that the core of the micelles is given by the PO groups, and for globular micelles the aggregation number is determined by the length of the PO block. The core of the micelles is assumed to be free of water while the EO groups are still h ~ d r a t e d .As ~ a consequence the effective volume fraction of the micelles is generally about a factor of 2-3 larger than the real volume fraction. With increasing temperature beyond the micellization temperature, desolvation of the EO groups continues and the effective volume fraction decreases. This is reflected in the phase diagrams.15 For higher block copolymer concentrations the systems form lyotropic liquid crystals.14J5 Since the effective volume of the systems is much larger than the real volume, liquid crystal formation can occur in a concentration region where it is normally not expected for hydrocarbon surfactants. It was shown for the first time by SANS measurements that the gelation process, which is observed in some moderately concentrated block copolymer solution with increasingtemperature, is actually the formation of a cubic phase.4 Since then Mortensen et al.13J4 have obtained detailed information by SANS on the cubic phase. They showed in particular that these phases can be aligned by shear.16 Besides the cubic phases other liquid crystal phases like hexagonal and lamellar phases have also been 0b~erved.l~ These data show that block copolymers in general can form the same micellar structures as normal hydrocarbon surfactants. Micelle formation is also controlled by hydrophobic interaction (11) Rassing, J.; Attwood, D. Znt. J. Pharm. 1983, 13, 47. (12) Almgren, M.; Alsins, J.; Bahadur, P. Langmuir 1991, 7, 446. (13) Mortensen, K.; Brown, W.; Norden, B. Phys. Rev. Lett. 1992,68, 2340. (14) Mortensen, K.; Pedersen, J. S. Macromolecules, in press. (15) Wanka, G.; Hoffmann, H.; Ulbricht, W. To be submitted for
publication. (16) Almdal, K.; Bates, F. S.; Mortensen, K. J. Chem. Phys. 1992,96, 9122.
0 1994 American Chemical Society
Langmuir, Vol. 10, No.1, 1994 87
Copolymer-Surfactant Interaction
and the phase diagrams are determined by packing constants of hard sphere objects. In many applications of block copolymers the compounds are used in combination with normal surfactants. Surprisinglylittle detailed information on the interaction between block copolymer and surfactants is available.17J8 It is of course to be expected that surfactants should bind strongly on block copolymers because they are even more hydrophobic than poly(ethy1ene oxide). Many investigations have shown that surfactants like SDSbind strongly on this polymer.19-22 Indeed, Muto et al.17 found, that the cmc of SDS became less definite by the addition of Epan 420 (=L42,E03P021E03)due to interaction between both. This interaction between block copolymers and SDS is confirmed by investigations of Almgren et al.18 By 13C NMR and fluorescence quenching measurements it was shown that SDS binds strongly to L64 (E013PO~E013) respectively. The interaction and to F68(E07&'0&078), begins at concentrations well below the cmc of SDS and leads to a decrease of the aggregation number of the block copolymer. The shape of the mixed micelles is assumed to be spherical with coiled PO blocks solubilized in the interior of a SDS micelle. In this investigation we made a detailed study by various techniques in order to obtain quantitative information on the block copolymer/surfactant interaction. In particular we were interested to know how small the micelles can become in the presence of surfactants and what the polymer/surfactant complexes look like. Furthermore it was of interest whether cationic and anionic surfactants show the same binding pattern.
Experimental Section Materials. Plunomic F127, MW = 12 500 g/mol, 70 wt % EO obtained from BASF/Wyandotte and SDS research grade obtained from Serva, Germany, were used as received, CTAB (Ncetyl-N,N,N-trimethylammoniumbromide, 99 % ) supplied by Serva was recrystallized from ether/ethanol. ClDMAO (NJVdimethyl-1-tetradecanamine N-oxide) supplied by Hoechst was twice recrystallized from acetone. NaCl p.a. was obtained from Merck. All solutions were prepared in twice distilled water with pH = 5. The viscosity of all investigated solutions was not far above that of water. Surface Tension. Measurements were performed with a Lauda tensiometer TE lC, based on the method of du Noiiy. Solutionswere thermostated and stirred lOmin before measuring. After 20 s, measurements were taken with the Pt/Ir-ring until the standard deviation was below 0.2 mN/m. Afterward concentrated solution was added and stirred again. Static and Dynamic Light Scattering. To determine the mole weight of the associates, small angle light scattering was used. The refractive increment was measured with a Chromatix KMX-16 differentialrefractometer;R(6-7") was determined with a ChromatixKMX-6photometer and a He/Ne laser. All solutions were filtered through a Schleicher & Schuell filter with 0.2-pm pore size. For determining dynamic light scattering, a 30 mW He/Ne laser with a Brookhaven BI-DS 10369 photomultiplier and a BI-8000 AT advanced digital correlator was used. All solutionswere filtered through a Schleicher& Schuell filter with 0.45-pm pore size and thermostated at 25 "C for a t least 1h. On the data of the autocorrelation curve an inverse Laplace transformation was performed to obtain the distribution of relaxation times (method of non-negatively constrained least (17) Muto, S.; Ino, T.;Meguro, K. J. Am. Oil. Chem. SOC.1972,49,437. (18) Almgren, M.;van Stam, J.;Lmdblad,C.;Li, P.;Stilbs,P.;Bahadur, P.J. Phys. Chem. 1991,95, 5677. (19) Francois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym.J. 19S, 21, 165. (20) Cabane, B.; Duplemix, R. J. Phys. (Paris)1982,43,1529. (21) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19. (22) Cabane, B.; Duplessix, R. J . Phys. (Paris)1987, 48, 651.
surface tension / mN/m
50 I
I
a
45 L
40 !I
35
1
1E-06 1E-05
1E-04
1E-03
1E-02 1E-01 w t - % F127
1E+W
1E+01
Figure 1. Surface tension of F127 a t 25 "C. squares). With the scattering vector q, the equation 7
= (q2Deff)"
and the Stokes-Einstein equation
DO = k T / 6 ~ an effective hydrodynamic radius R d can be calculated, containing the structure factor S(q) = DdSCq)
Electric Birefringence. All investigated solutions showed birefringence only when an electric field was put on. Measurements were performed with a Cober high power pulse generator, Model 606, a Datalab transient recorder, DL 920, and a He/Ne laser. All solutions were thermostated a t 25 "C. Differential Scanning Calorimetry (DSC). Solid F127 and even aqueoussolutionsof it show an endothermicpeak on heating. Solid F127 melts at 50.9 "C with an enthalpy of 87.6 J/g F127. Aqueous solutions show a peak in the region of room temperature and an enthalpy of 36.7 J/g F127. Measurements were done with a Setaram Micro DSC in the temperature region from 0 to 100 "C and a scanning rate of 0.2 K/min.
Experimental Results The experimental results have to be discussed while keeping in mind that they were obtained on commercially produced samples. These samples are not pure compounds but consist of many thousands of chemically different species. The compounds have a broad molecular weight distribution and a distribution of the number of EO and PO groups. In addition there might be PEO compounds which are not linked to a PO chain. It would require timeconsuming work to fractionate the material into better defined compounds. This was not of interest in this investigation. For this reason we cannot expect to observe the behavior which is typical for well-defined surfactants where in general we can expect a sharp cmc and clear breaks from the monomer to micellar state. With the present compounds at best we can expect to observe transition concentration regions. Surface Tension Measurements Typical results of surface tension measurements are given in Figure 1for the block copolymers. In the semilog plots we usually observe two breaks (C1 and C2). It is likelythat these curves are the result of the broad molecular distribution of the compounds. Most surface active material begins to aggregate in the solution at C1while the less surface active material is not forming micelles and is increasing with the bulk concentration. It is only at C2 where all the material forms micelles. The concentration
Hecht and Hoffmann
88 Langmuir, Vol. 10, No. 1, 1994 AR(0)
surface tension / mN/m
II 42
34
1
0*1
I
11
* E06 cm
1
441
I
I11
10
1
100
[SDS] / mmoVl
Figure 2. Surface tension of 10-3 wt % ! F127 + SDS, T = 25 "C.
C2 is very sensitive and shifts with increasing temperature to smaller concentrations while C1 is not so much temperature dependent. This behavior could actually be used to fractionate the material if we would have a method which would be capable of separating the micelles from the monomers. To some degree this is possible by filtration. Similar surface tension concentration profiles are obtained for surfactant mixtures having different chain length or for mixtures of perfluoro and hydrocarbon surfactants.23 The curves which are obtained when ionic surfactants are added to solutions of block copolymers look even more complicated (Figure 2). For small. block copolymer concentration we can distinguish at least four different SDS concentration regions. For very low SDS concentration the surface tension is little affected or slightly decreasing (regionI). In this region the surfactants remain in their monomer state. They might compete with the polymer for adsorption at the surface. This does not result in a change of the surface tension. With increasing concentration the surfactants bind however on the monomers of the block copolymer. These complexes are less surface active and as a result block copolymer desorbs from the surface and the surface tension is increasing (region 11). Finally the block copolymer molecules are all saturated with surfactant molecules and now the free surfactant concentration is increasing again. With increasing concentration more surfactant molecules bind to the surface, which lowers the surface tension (region 111). When the free surfactant concentration becomes high enoughto form micelles, it will do soand the surfacetension then remains constant thereafter in region IV. It actually is not possible to say where the surfactants bind. They could bind to the EO and PO groups. We assume however that most of them will bind to the most hydrophobic groups and these are the PO groups. Since the binding of surfactant changes the environment of the methyl groups, the binding is reflected in the shift of the NMR signals of the CH3 groups.18 It seems that this pattern of the surface tension with increasing surfactant concentration is only observed as long as the polymer concentration is below the C2. For higher concentrations and in particular for higher temperature where the C2 is much lower, the surface tension/ surfactant profiles look even more complicated.
Light Scattering Data At room temperature a solution of 1wt % F127 is in the micellar state. In Figure3we show how the light scattering changes with increasing surfactant concentration. To
6,Ol
0*1
1
100
10
[SDS] / mmol/l
Figure 3. Small angle light scattering of 1 w t !% F127 + SDS/ NaCl(1:l) at T = 25 "C. R(en) I nm
8-
6-
4-
, 0 0.01
0,l
/ 1
10
100
1.000
[SDS]/ mmol/l
Figure 4. Effective hydrodynamic radius of 1 wt !% F127 + SDS/NaCl (1:l) at 8 = 90"and T = 25 O C , particle sizedistribution calculated with an inverse Laplace transformation of the autocorrelation curve.
avoid electrostatic interaction and therefore repulsion between the charged aggregates, we added NaCl in equivalent amounts to SDS. This simplifies the interpretation of the observed data. In the semilog plot we can distinguish three different regions. There is a large drop of A R ( 8 ) with small SDS concentration. Obviously there are a few large aggregates under these conditions which are very sensitiveto the presence of SDS. They are already gone with a SDS concentration of about 0.2 mM. From then on the scattering intensity changescontinuouslydown to a value which corresponds to one isolated polymer molecule. The question can be asked whether the aggregates change continuously in size or is there a stepwise decrease. This question can be answered by analyzing the data from dynamic light scattering. The distribution of particle sizes calculated from the relaxation time distribution is shown in Figure 4. At small SDS concentrations, there exist normal F127 micelles with R,ff = 11 nm. With (23) Ikeda, N.; Shiota, E.; Aratono, M.; Motomura, K. Bull. Chem. SOC.Jpn. 1989.62.410.
Langmuir, Vol. 10, No. 1, 1994
Copolymer-Surfactant Interaction aR(8) 1
An/&09 100
E06 cm
A
0 3 5 mM SDS 80 160
/i
1 \
40
I
60
*l wt-%
* 5 wt-% 40
-
n
I
,
15
20
25
35
30
40
45
50
T I "C
Figure 5. Temperature dependence of the light scatteringof 1 F127 + SDS/NaC1(1:1)at 8 = 6-7O.
wt %
increasing SDS concentration these F127 micelles disappear in favor of smaller aggregates with Refi = 5 nm. A rough estimation suggests that this aggregate contains about three to five molecules of F127. For comparison, micelles of F127 contain about 30 molecules of blockcopolymer. Above 2 mM SDS only aggregates with Refi = 1 nm are left, corresponding to aggregates of SDS adsorbed on one molecule of F127. This means that the size of the aggregates decreases with increasing SDS concentration. However, it is not a continuous decrease, but rather stepwise. At low SDS concentration micelles of F127 are in equilibrium with smaller aggregates. With increasing SDS conccentration this equilibriumis shifted toward the latter. With addition of more SDS, this aggregate is in equilibrium with monomeric F127, on which SDSis adsorbed. A t high SDS concentration this equilibrium is again shifted to the latter species. Above 2 mM SDS every block copolymermolecule is saturated with SDS, preventing further aggregation to micelles. With another increaseof the SDSconcentration, the particles grow in size. This is due to further adsorption of SDS or due to micellization of SDS. The amount of SDS which has to be added in order to suppress micelle formation of the copolymer depends on the temperature. This is obvious from the data in Figure 5 where the scattering intensity of a polymer surfactant solution is plotted against the temperature. For temperatures below 25 "C a 6.9 mM SDS concentration can suppress micelles, but with increasing temperature some of the block copolymers are transformed into bigger aggregates. At 35 mM SDS the size of the aggregates does not vary with increasing temperature.
Electric Birefringence Data Electric birefringence measurements can give very helpful and detailed information on polymer surfactant interaction. Combinations are known where both the polymer and the surfactant solutions do not give rise to electricbirefringencewhilethe polymer complex gives large signals.24 This is also the situation in the present investigation. Some results are given in Figure 6 in a plot of the electric birefringence of a 1% F127 solution against the SDS concentration. To avoid unfavorable conductivity,we do not add any NaClto the mixture. The polymer and the SDS micelles on their own do not give a signal. With increasing SDS concentration a small signal develops which first increases somewhat and levels off above 0.1 (24) Huber, G. Thesis, Universitiit Bayreuth, 1988.
n 0;001-
0,Ol
091
1
10
100
1.000
[SDS]I mmolfl Figure 6. Electric birefringence of 1 w t % ! F127 SDS and 5 wt % F127 SDS at T = 25 "C and E = 4.81 X 1@V/m(without
+
+
added NaC1).
n Figure 7. Stretched configuration of a block copolymer/SDS aggregate (zigzag = PO).
mM SDS. The situation is similar as for the light scattering data where a small decrease of the scattering intensity was detected. The main increase of the birefringence begins at 1mM SDS and reaches a maximum at about 10 mM. For higher concentrations the signal decreases again. The decrease is probably due to the increasing ionic strength of the solution which makes it more difficult to polarize the polymer surfactant complex. The maximum birefringence corresponds probably to the situation where the copolymersare completely saturated with SDS. The rotation time of the polymer surfactant complex was too short to be resolved by the available equipment (
