Amphiphilic Block Copolymers as Efficiency Boosters for Microemulsions

Transcription

1 Langmuir 1999, 15, Amphiphilic Block Copolymers as Efficiency Boosters for Microemulsions B. Jakobs, T. Sottmann, and R. Strey* Universität zu Köln, Institut für Physikalische Chemie, Luxemburger Strasse 116, D Köln, Germany J. Allgaier, L.Willner, and D. Richter Forschungszentrum Jülich, Institut für Festkörperforschung, D Jülich, Germany Received January 29, In Final Form: May 12, 1999 We demonstrate that block copolymers of the poly(ethylenepropylene)-co-poly(ethylene oxide) (PEP- PEO) type dramatically enhance the solubilization capacity of medium-chain surfactants in microemulsions, for example, in the ternary system water-n-decane-c 10E 4. The effect exhibits itself in an enormous increase of the swelling of the middle phase with an associated increase in the structural length scale of the microemulsion, while at the same time the (already ultralow) interfacial tension between water- and oil-rich phases decreases even further. Typically, the surfactant mass fraction γ 0 ) 0.13 sufficient to form the balanced one-phase microemulsion in the ternary system can be replaced by γ ) of surfactant plus polymer. If δ is the polymer mass fraction in the surfactant/polymer mixture, the overall mass fraction of polymer in the microemulsion amounts only to γ δ ) Accordingly, in this example the polymer is f B ) 24 times more efficient than the surfactant, where we define an efficiency boost factor by f B ) (γ 0 - γ (1- δ))/γ δ. The magnitude of the effect depends to some extent on the overall molar mass of the polymer but rather weakly on the relative size of the hydrophilic and hydrophobic blocks. Interestingly, the lamellar phase, which usually develops as surfactants become more efficient, is suppressed. 1. Introduction In this paper we report an efficiency boosting effect of block copolymers in microemulsions. 1 To appreciate the significance and the large magnitude of the effect, one has to recall some basic features of microemulsion systems. Microemulsions are thermodynamically stable and macroscopically isotropic mixtures of at least three components: water, oil, and surfactant. Microscopically, the surfactant forms an extended interfacial film separating water and oil on a local scale. Winsor 2 found in fivecomponent systems including ionic surfactants that there is a phase progression as the curvature of the amphiphilic film is tuned by an appropriate variable. Later Shinoda 3 demonstrated that the hydrophilic-lipophilic balance as well as the phase inversion may be achieved in ternary microemulsions with nonionic surfactants by tuning the temperature. Shinoda and Friberg 4 showed the relation of ultralow interfacial tensions to the phase inversion. In systematic studies Kahlweit and co-workers 5-7 demonstrated that the phase behavior is generic; that is, it is similar for a large variety of different systems. Strey 8 determined quantitatively the gradual change in curvature of the amphiphilic film for a suitable system. The correlation between the state of highest efficiency and the largest length scales as determined by small-angle * Corresponding author. (1) Jakobs, B.; Sottmann, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. German Patent Application No , (2) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworth: London, (3) Shinoda, K. Solvent Properties of Surfactant Solutions; Marcel Dekker: New York, (4) Shinoda, K.; Friberg, S. Adv. Colloid Interface Sci. 1975, 4, 281. (5) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654. (6) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, (7) Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993, 47, (8) Strey, R. Colloid Polym. Sci. 1994, 272, neutron scattering (SANS) was pointed out. The interrelation of surfactant efficiency and interfacial tension has been analyzed by Schechter and Wade. 9 Sottmann and Strey 10 actually measured the interfacial tension curves for more than 20 systems and obtained the bending constants of a variety of nonionic surfactants using the length scales determined separately by SANS. 11 From all these contributions it is known that with increasing hydrophobic chain length the amount of surfactant needed to form a one-phase microemulsion is systematically lowered; that is, the efficiency increases. It is a general observation that as the efficiency of the surfactant increases the lamellar phase is stabilized. As the hydrophobic chain length of the surfactant molecule exceeds carbon atoms, vast regions of the phase diagram in the balanced state are occupied by the lamellar phase. 12 Off balance, different mesophases form. We have discovered, and report here for the first time, how to increase the efficiency while suppressing the mesophases. Let us first illustrate the phenomenon. Starting from the well-known phase behavior of a ternary microemulsion system with water, n-decane, and C 10 E 4, we observed an enormous efficiency increase by adding an amphiphilic block copolymer (PEPx-PEOy). 1,13,14 For nomenclature, refer to the Experimental Section. While mixtures of two surfactants of comparable chain (9) Schechter, R. S.; Wade, W. H.; Weerasooriya, U.; Weerasooriya, V.; Yiv, S. J. Dispersion Sci. Technol. 1985, 6, 223. (10) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106, (11) Sottmann, T.; Strey, R.; Chen, S. H. J. Chem. Phys. 1997, 106, (12) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233. (13) Allgaier, J.; Poppe, A.; Willner, L.; Richter, D. Macromolecules 1997, 30, (14) Allgaier, J.; Willner, L.; Richter, D. German Patent Application No. P , (15) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, /la CCC: $ American Chemical Society Published on Web 09/09/1999

2 6708 Langmuir, Vol. 15, No. 20, 1999 Jakobs et al. Figure 1. Samples of H 2O-n-decane-C 10E 4 (R )0.422, γ ) 0.03) in Hellma flat quartz cells immersed in a thermostated water bath at 30.3 C. From left to right the content of the block copolymer PEP5-PEO5 increases: (a) δ ) 0; (b) δ ) 0.015; (c) δ ) 0.050; (d) δ ) Note the increasing phase volume of the middle phase due to polymer addition. The increasing darkness of the middle phase is a consequence of the increasing length scale and the resulting stronger light scattering. In transmitted light this results in a color change from yellowish to reddish-brown. length show small synergistic effects in microemulsions, 15 adding an amphiphilic block copolymer to a conventional microemulsion system leads to a large efficiency increase already by traces of polymer. To see this, consider Figure 1. A sample containing equal volumes of water and n-decane (φ ) 0.5; R) 0.422) and a surfactant weight fraction γ ) separates into three phases, the middle phase microemulsion taking up about 1 / 4 of the phase volume. Such situation is shown by the leftmost flat cell in Figure 1. Adding traces of PEP5-PEO5, one notes that the volume fraction of the middle phase increases dramatically (see Figure 1, the three samples on the right). Even for the highest polymer content the polymer mass fraction amounts only to γδ ) (Figure 1, rightmost cell). From this simple experiment a number of immediate observations can be made. The oil and water excess phases visible in the left test tubes are progressively swallowed by the surfactant-rich middle phase, which thereby increases in volume. The accompanying effect of increasing opalescence leads to an increasingly darker appearance of the middle phases in transmitted light. This effect is obviously connected to the increasing length scale in these systems, which leads to stronger scattering. Also, the experiment can be, and actually was, performed at constant temperature so that the hydrophilic-lipophilic balance is apparently not affected. 2. Experiment A. Phase Diagrams. The determination of the phase diagrams is carried out in a thermostated water bath with temperature control up to 0.02 K. The samples are weighed into test tubes and sealed. At constant sample composition and as a function of temperature, the occurring phases are characterized by visual inspection in both transmitted and scattered light, using crossed polarizers to detect the presence of the lamellar phase. The sample composition is given by the oil in water plus oil mass fraction R ) m B/(m A + m B), the overall mass fraction of the surfactant (or surfactant/polymer mixture) γ ) (m C + m D)/(m A + m B + m C + m D), and the mass fraction of the polymer in the surfactant/ polymer mixture δ ) m D/(m C + m D). For calculating volume fractions, the masses are replaced by the respective volumes. In this study all samples were prepared at R)0.422, corresponding to an oil/(water + oil) volume fraction φ ) 0.5. B. Materials. Water was deionized and twice distilled. As oil, n-decane was used from Sigma Aldrich (Steinheim, Germany) with a purity > 99%. The alkylpolyglycol ether surfactants (C ie j) were obtained from Fluka (Neu-Ulm, Germany) and Bachem (Bubendorf, Switzerland) with a purity > 98%. All substances were used without further purification. The amphiphilic block copolymers are similar to the C ie j surfactants, differing from these by the branched nature of the hydrophobic block and of course by the larger overall molar mass. Specifically, poly- (ethylenepropylene)-co-poly(ethylene oxide), abbreviated PEPx- PEOy, where x and y are the approximate molar masses of the blocks in kilograms per mole, were synthesized by a two-step process. 14 As an example, the model polymer used for most examinations is denoted by PEP5-PEO5. A related description of the preparation of this important class of block copolymers has been given in a different context. 16 First, a hydrophobic polyisoprene block of desired molar mass and end-capped with an OH group was prepared and then hydrogenated to PEP-OH. Second, ethylene oxide was polymerized onto the PEP-OH until the desired hydrophilic block size was obtained. The resulting block copolymers were thoroughly characterized. The procedures are described in more detail by Allgaier et al. 13,14,17 Polymers of various chain lengths both of the hydrophilic and the hydrophobic block were synthesized with rather narrow M w/m n ratios of Since some of the compounds were prepared for SANS measurements partial or full deuteration of the compounds was necessary. 17 For definitiveness, we will always refer to the molar mass on the basis of full protonation. It should be noted that deuteration only negligibly affects the results presented in this paper. The molar masses of the hydrophilic blocks varied from 1 to 50 kg/mol, and those of the hydrophobic blocks varied from 1 to 22 kg/mol. (16) Hillmyer, M. A.; Bates, F. S. Macromolecules 1996, 29, (17) Poppe, A.; Willner, L.; Allgaier, J.; Stellbrink, J.; Richter, D. Macromolecules 1997, 30, (18) Allgaier, J.; et al. Work in progress.

3 Amphiphilic Block Copolymers Langmuir, Vol. 15, No. 20, Figure 2. Sections through a phase prism at equal volumes of water and n-decane (φ ) 0.5). The well-known fish is shown for water-n-decane-c 10E 4 as hollow circles. The effect of increasing surfactant head group size (C 10E 5) and tail size (C 12E 4) is demonstrated. Note the associated temperature shifts. Adding traces of polymer PEP5-PEO5 leads to an enormous efficiency increase (full circles) at constant temperature. 3. Results and Discussion In a previous investigation the micelle formation of the model block copolymer PEP5-PEO5 was explored. 17 From the huge dimensions of the observed micelles of 30 nm radius one had hoped to have powerful amphiphiles for solubilizing efficiently water and oil. However, preliminary experiments to solubilize water and oil with polymer alone badly failed. In such cases it had earlier been useful to start with a well-defined ternary system and to add the substance under investigation in small amounts. This procedure is shown to work also here in the case of increasing amounts of polymers. A. Ternary Base System. The phase behavior of the ternary water-n-alkane-c i E j systems has been described in various connections. 5-7 A useful way to characterize these systems is as vertical sections through the phase prism at constant water/oil ratio. In these sections the coexistence curves show the well-known fish. At low temperatures a microemulsion of o/w type coexists with excess oil (denoted by 2). At high temperatures a microemulsion of w/o type coexists with excess water (2h). While at intermediate temperatures between T u and T l and lower surfactant concentrations the three-phase body occurs (3), at higher surfactant concentrations the one-phase region appears (1). The minimum surfactant concentration for complete solubilization of water and oil, that is where the three-phase and one-phase regions meet, is denoted by γ at temperature T. 5 Such a fish for C 10 E 4 is depicted by the hollow circles in Figure 2. The point (γ, T ) has the coordinates γ ) and T ) C. B. Effect of Surfactant Properties. Increasing the hydrophilic head group size by one oxyethylene (O-CH 2 - CH 2 ) unit, that is proceeding to C 10 E 5, the fish is shifted to T )49.9 C, and γ increases to γ ) This efficiency decrease is understood as a combined effect of the higher temperature and the trivial effect that C 10 E 5 has a larger molar mass than C 10 E 4, which lets γ, already for this reason, come out larger by 14%. On the other hand, on increasing the hydrophobic tail by two CH 2 groups, a significant efficiency increase is observed. The mean temperature shifts to T )18.2 C, and γ decreases to γ Figure 3. Reduction of minimum amounts of surfactant γ needed for solubilizing equal volumes of water and oil. With replacement of C 10E 4 by C 12E 4, a moderate reduction is noted (full circles), while, using polymer PEP5-PEO5, a dramatic increase in efficiency already by traces of polymer (full squares) is observed. ) This efficiency increase is clearly related to an increase in the rigidity constant κ. 10,11 Increasing the rigidity has the effect of stabilizing the lamellar phase in these systems. The lamellar phase is an ordered mesophase, which is usually observed at higher surfactant concentrations in the fish tail 19 and is a well-known feature in binary H 2 O-C i E j (i g 10) systems. 20 The ubiquitous appearance of the lamellar phase is also observed in multicomponent microemulsion systems, where it even intersects the three-phase body. 21 C. Effect of Block Copolymer. The striking phenomenon of adding the polymer PEP5-PEO5 is demonstrated by the full circles in Figure 2. Addition of polymer so that δ ) leads to a reduction of γ 0 to γ ) Proceeding to δ ) 0.119, the minimum amount of surfactant plus polymer to form a one-phase microemulsion drops to γ ) An important observation is that the addition of PEP5-PEO5 does not lead to the formation of the lamellar phase in the fish tails presented in Figure 2, where we explicitly checked for it. From a number of other systems studied, we can generalize that parallel to the efficiency increase by the block copolymers PEPx- PEOy there is a suppression of the lamellar phase. D. Efficiency Enhancement. Despite the small quantities of polymers applied, a striking efficiency enhancement was seen and documented for illustrative purposes in Figure 1. The efficiency-enhancing effect can also be quantified, if γ is plotted versus δ. This is done in Figure 3. Let us first consider the mixture of two surfactants. Replacing C 10 E 4 by C 12 E 4, one observes that γ is reduced gradually from the value of the pure surfactant C 10 E 4 to that of the pure surfactant C 12 E 4 accompanied by a monotonic decrease of the mean temperature. These variations are in agreement with known mixing rules for surfactants. 15,22 The dramatic effect of polymer addition is seen by the steep decay in Figure 3 for PEP5-PEO5. (19) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (20) Strey, R.; Schomäcker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, (21) Lekkerkerker, H. N. W.; Kegel, W. K.; Overbeek, J. T. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 206.

4 6710 Langmuir, Vol. 15, No. 20, 1999 Jakobs et al. Table 1. Properties of C 10E 4 Microemulsions with Added PEP x-peo y a M p f PEP x y δ 1/2 (0.09) (0.048) f B a M p is the molar mass of the polymer, f PEP is the mass fraction of the hydrophobic block, and x and y are the approximate block molar masses of the hydrophobic and hydrophilic blocks in kilograms per mole. δ 1/2 denotes the fraction of polymer needed to reduce the amount of surfactant γ toγ 0/2; extrapolated values are given in parentheses. The boost factor f B is a measure for how many times more efficient the polymer is than the ordinary surfactant C 10E 4. The continuation to concentrations γ < 0.03 is drawn as a dashed line, because it becomes increasingly difficult to further study the systems in test tubes due to the extremely strong light scattering by the microemulsion phase. As mentioned above in connection with Figure 1 and as is seen in Figure 2, the efficiency enhancement with polymer addition is obtained without shifting the hydrophilic-lipophilic balance temperature of about 30 C. The efficiency-boosting effect has been observed and documented 1 in a variety of microemulsion systems, including ionic AOT and temperature insensitive alkylpolyglycoside microemulsions. E. Effect of Polymer Block Size. We have performed similar experiments as shown in Figures 2 and 3 with a whole series of block copolymers. Specifically, we have measured the fish tails for PEP1-PEO1, PEP5-PEO3, PEP5-PEO5, PEP5-PEO15, PEP5-PEO30, PEP5- PEO50, PEP10-PEO10, PEP22-PEO22, and PEP22- PEO The most important results are summarized in Table 1. To characterize the polymers, the overall molar mass M p along with the fraction of PEP f PEP in the polymer is given. To compare the efficiencies of the different polymers, we determined the value of δ ) δ 1/2 at which γ ) γ 0/2 by polymer addition, where γ 0 refers to the original ternary system without polymer. In the example in Figure 3 for PEP5-PEO5, δ 1/2 ) , and for the ordinary amphiphile C 12 E 4, δ 1/2 ) The δ 1/2 values are useful direct measures of the efficiency of the polymers. F. Efficiency Boost Factor. Knowing how much surfactant (γ 0 - γ (1- δ)) has been replaced by how much of the additive γ δ, one can calculate the ratio and define an enhancement or boost factor f B ) γ 0 - γ (1 - δ) γ δ The typical enhancement due to the block copolymers is characterized by boost factors of 12 < f B < 22, if one takes the observed δ 1/2 values in the range 0.05 < δ 1/2 < 0.1 (cf. Table 1). For a given polymer the boost factor f B is observed to increase as δ increases. Specifically, for PEP5-PEO5 in Figure 3 one has 10 < f B < 25. This may be an interesting observation when discussing the origin of the effect. Another interesting observation is that the effect does not depend much on the symmetry of the block sizes. That is, also increasing the hydrophilic block leads to a substantial efficiency increase, which is a remarkable difference compared to the case for shorter chain amphiphiles (cf. Figure 2). For the ordinary surfactant C 12 E 4 one obtains f B ) 2.4 at δ 1/2. Also, here the factor increases with δ. (1) Figure 4. Increase in the characteristic length scale with decreasing interfacial volume fraction of amphiphile φ int achieved by increasing the polymer fraction δ. Note that the slope (-1.12) is steeper than the -1 recently measured for a series of 20 ternary systems. 11 Needless to say, replacing 13.2 wt % of surfactant sufficient to form the microemulsion without polymer by 3.08 wt % of the same surfactant mixed with 0.42 wt % of polymer is of attraction not only for commercial or environmental reasons but also for a variety of applications. In this connection it should again be emphasized that the efficiency enhancement does not affect the hydrophilic-lipophilic balance temperature, which is set by the ternary base system. G. Effect on Length Scales. To shed light on the microscopic origin of the effect, we have started to study the small-angle neutron scattering of these systems. We found scattering peaks typical for bicontinuous microemulsions. 24 The peak position moves to lower q with increasing δ, indicating a larger length scale. The quantitative analysis of the scattering curves yields a characteristic length d TS /2, which is roughly the diameter of an oil or water domain. 11,24 The variation of d TS /2 as a function of surfactant (plus polymer) volume fraction residing in the internal interface φ int is shown in Figure 4. The volume fraction φ int of surfactant plus polymer residing at the internal interface is obtained by subtracting the monomerically dissolved surfactant volume fraction, 11 while the polymer is assumed to be fully incorporated in the film. As a consequence of the lower amount of surfactant in the internal interface, the peak moves to low q, as is well-known from previous studies. 11 Interestingly, the length scale increases slightly more than inversely proportional to φ int. This information might be (22) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schomäcker, R. Langmuir 1988, 4, 499. (23) Jakobs, B. Dissertation, University of Cologne, work in progress. (24) Teubner, M.; Strey, R. J. Chem. Phys. 1987, 87, 3195.

5 Amphiphilic Block Copolymers Langmuir, Vol. 15, No. 20, the earlier work the adsorption of polymer is shown to reduce the rigidity of the membrane. If this would occur in our microemulsions, this would lead to less efficient microemulsion, opposite to what we have seen. For discussing the effect of block copolymers which are part of the monolayer and have loose ends that extend into the respective subphases, one should probably consider recent theoretical developments by Lipowsky and co-workers describing the effects of anchored polymers. In these works mushroom- or pancake-like conformations of the individual blocks of the polymers are discussed. However, here one will have to consider polymeric ends on both sides of the monolayer which may differently and oppositely affect curvature and bending elastic properties. Figure 5. Oil-water interfacial tension σ ab as a function of the temperature of the base system water-n-decane-c 10E 4 (full circles) and of the respective interfacial tension after addition of polymer (δ ) 0.05) (full squares). Note the reduction in the minimum by a factor of 5. useful when discussing the origin of the efficiency-boosting effect of the polymer. H. Effect on Interfacial Tension. To gain insight into the energetic nature of the effect of polymer addition, we have determined the interfacial tension between the oil- and water-rich phases with and without polymer. In Figure 5 the interfacial tension is plotted as a function of temperature. One observes the typical deep minimum as the three-phase region is traversed. 4,5,10,25,26 The moderate addition of polymer (δ ) 0.05) leads to a decrease of the minimum interfacial tension by a factor of 5 at temperatures where the microemulsion structure is bicontinuous. At temperatures above and below the three-phase region, where droplet microemulsions are known to exist in equilibrium with excess phases, the decrease is much less. I. Further Discussion. The effect of polymer adsorption on the curvature and rigidity of monolayers and bilayers has been discussed in a number of papers In (25) Bakarat, Y.; Fortney, L. N.; Schechter, R. S.; Wade, W. H.; Yiv, S. H. J. Colloid Interface Sci. 1983, 92, 561. (26) Kunieda, H.; Friberg, S. E. Bull. Chem. Soc. Jpn. 1981, 54, (27) Ji, H.; Hone, D. Macromolecules 1988, 21, (28) degennes, P. G. J. Phys. Chem. 1990, 94, (29) Brooks, J. T.; Marques, C. M.; Cates, M. E. J. Phys. II 1991, 1, 673. (30) Lipowsky, R. Colloids Surf., A 1997, 128, Conclusions The observation that the efficiency increase can be accomplished by increasing either block size or both block sizes symmetrically points to the direction that the origin of the efficiency boosting cannot be a regular mixing effect, as one would find with ordinary surfactants of longer hydrophobic chains and hydrophilic head groups. The effect is apparently related to the ability of the block copolymer to extend further into the adjacent subphases. If the polymer was mixed into the amphiphilic film, a stronger dependence on the relative block size of the polymer and an effect on the hydrophilic-lipophilic balance temperature should be observed. Also the suppression of the lamellar phase is a remarkable feature, which shows that a more curved (but large) microstructure is preferred by the mixed surfactant/polymer film. We have already seen that the efficiency-boosting effect is accompanied by a comparatively large length scale increase in the microemulsion, which is connected to a further decrease of the ultralow interfacial tension between water- and oil-rich phases. To understand this hitherto unreported effect, further phase behavior, interfacial tension, and SANS experiments are presently being performed. Acknowledgment. The authors thank B. Rathke for his photographic expertise in creating Figure 1. Also the help of S. Müller with the interfacial tension and of W. Pyckhout-Hintzen with the SANS experiments is gratefully acknowledged. LA (31) Hiergeist, C.; Indrani, V. A.; Lipowsky, R. Europhys. Lett. 1996, 36, 491. (32) Lipowsky, R. Encycl. Appl. Phys. 1998, 23, 199.