Langmuir 1!396,11, 1934-1941
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Phase Behavior of a Nonionic Microemulsion upon Addition of Hydrophobically Modified Polyelectrolyte H. Bagger-Jorgensen,”,?U. Olsson; and I. Iliopouloss Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden, and hboratoire de Physico-Chimie Macromoliculaire, Universiti Pierre et Marie Curie, CNRS URA-278,ESPCI, 10 Rue Vauquelin, F-75231 Paris Cedex 05, France Received,January 23, 1995. In Final Form: March 6, 1995@ We report on the effects on the phase behavior in the water-rich part of the ternary nonionic Surfactant system, pentaethyleneglycol dodecyl ether (C12E5)-water-decane, when adding sodium polyacrylate(PA) or hydrophobically modified polyacrylate (HMPA). The polyelectrolytes, having the same degree of polymerization, were derived from polyacrylic acid of Mw = 150 000. The HMPA contains hydrophobic octadecyl side chains randomly anchored to the polyelectrolyte backbone, with a degree of modification of either 1 or 3%of the monomer units. The PA is found to be insoluble in the lamellar liquid crystalline and the liquid L3 phases, containing infinite bilayer aggregates, while it is soluble in the liquid L1 phase containing oil-swollen normal micelles. The HMPA, on the other hand, can be solubilized in all three phases, however, strongly affecting the phase equilibria. From the difference it is concluded that while PA is nonadsorbing, HMPA adsorbs to the surfactant film by the hydrophobic “stickers”. In the presence of HMPA the surfactant films behave as effectively charged with long-rangeelectrostaticinteractions and a positive shift of the spontaneous mean curvature. The lamellar phase containing the HMPA phase separates at higher bilayer concentrations into two lamellar phases in equilibrium. Here a HMPA-rich lamellar phase with larger water spacings coexistswith a lamellar phase of smallerwater spacings essentially free of HMPA.
1. Introduction Surfactant-water-oil mixtures can form alarge variety of microstructuredliquids and liquid crystals. Here, space is divided into polar and apolar microdomains separated by a surfactant-rich dividing surface. A particularly rich phase behavior is found with nonionic surfactants of the ethylene oxide type. 1-3 Using only the minimum number of required components, balanced microemulsions,2 L3 phases, and dilute lamellar phases: as well as other oilor water-swollen lyotropic liquid ~ r y s t a l scan , ~ be formed. Due to the use of the smallest possible number of components, these systems are suitable model systems concerning general properties of nonionic self-association colloids. In this study we focus on a particular surfactantwater-oil system containing the nonionic surfactant pentaethylene glycol dodecyl ether (C12E5), heavy water (D20), and decane, upon addition of small amounts of polyacrylate(PA)or hydrophobicallymodified polyacrylate (HMPA), both with and without added electrolyte. D2O was used rather than normal protonated water because proton NMR measurements were performed on the samples. The surfactant-water-oil system corresponds to a cut through the temperature-composition phase prism at a constant C12Eddecane weight ratio of 51.9/48.1,as illustrated in Figure 1. The phase diagram of the waterrich part of this cut, redrawn from ref 6, is shown in Figure 2, drawn as temperature versus the weight percent of surfactant (W,)plus the weight percent of oil (W,,). With University of Lund.
* Universit6 of Pierre et Marie Curie. @
Abstract published in Advance ACS Abstracts, May 15,1995.
(1) Kahlweit, M.;Strey, R. Angew. Chem.,Int. Ed. Engl. 1986,24, 654. (2) Kunieda,H.; Shinoda,K. J.Dispersion Sci. Technol.1982,3,233. (3) Olsson, U.; Wennerstrom,H. Adu. Colloid InterfaceSei. 1994,49 ,113. (4) Strey,R.; Schomacker,R.; h u x , D.; Nallet, F.; Olsson,U. J.Chem. Soc., Faraday Trans. 1990,86 (121,2253. (5) Olsson, U.; Wiirz, U.; Strey, R. J. Phys. Chem.1993,97,4535. (6) Olsson, U.; Schurtenberger, P. Langmuir 1993,9, 3389.
0743-7463/95/2411-1934$09.00/0
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Figure 1. Cut in the composition-temperature phase prism, at constantCl2Es:decane ratio, giving a two-dimensional phase diagram. increasing temperature a sequence of three homogeneous phases is obtained: a liquid microemulsion phase (Ll), a lamellar liquid crystalline phase (La), and a liquid, socalled L3 phase (by some authors denoted “sponge”phase). At lower temperatures ( ~ 2 “C), 4 the microemulsion phase is in equilibrium with excess (almost pure) oil (L1+ 0). The L3 phase is in equilibrium with excess (almost pure) water at elevated temperatures. We may also note that this phase diagram is qualitatively very similar to the phase diagram of the binary water-Cl2Es system: with the exception of the L1 0 equilibrium a t lower temperatures. Recently, the microemulsion phase has been studied intensively by several different techniques.6-8 The sequence of phases occurring with increasing temperature in the phase diagram of Figure 2 follows a general trend observed in nonionic surfactant-wateroil systems and corresponds to a sequence of microstructures where the mean curvature of the surfactant monolayer decreases monotonously with increasing temp-
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(7) haver, M. Olsson,U.Langmuir 1994,10,3449. (8) haver, M. S.; Olsson, U.; Wennerstrijm, H.; Strey, R. J.Phys. I1 Fr. 1994,4, 515.
1995 American Chemical Society
Phase Behavior of a Nonionic Microemulsion 50)
1
251-=-c;y
-02
0
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ws+wo
Figure 2. Partial phase diagramof the system ClaE5-waterdecane (redrawn from ref 6), which we here refer to as the reference system. The weight ratio W$w, = 51.9/48.1,where W,is the weight percentage of C12E5 and W ois the weight percentageof decane, is kept constant. For the various phases the following notations are used: L1 is a liquid microemulsion phase similar to the normal micellar phase found in binary surfactant-water systems. LQ is a lamellar liquid crystalline phase. L3 is a liquid phase having a multiplyconnectedbilayer structure. The microstructures of the various phases are schematically illustrated in the right hand side of the figure. The L1 phase consists of discrete, oil-swollen droplets covered by a surfactant monolayer. Curvature is toward oil; i.e. (H)> 0. The lamellar phase can be described as oil-swollenbilayers stacked with one dimensionalorder. Due to the planar topology, (H)= 0. The L3 phase consists of a disordered, multiply connected bilayer, which acts as a dividing surface between two separate water labyrinths. While the average mean curvature of the bilayer midplane vanishes by symmetry, the average mean curvature of the surfactant monolayer film is negative; i.e. (H) 0.
e r a t ~ r e . At ~,~ lower temperatures, in the microemulsion phase, normal oil-swollen micelles are formed and the is toward oil and is according average mean curvature, (H), to the usual sign convention positive, i.e. (H)=- 0. In the La phase, oil-swollen bilayers are periodically stacked with one-dimensionalorder. Here, (H)= 0. Finally, in the L3 phase, the oil-swollen bilayer is multiply connected and here (H)< 0 of the surfactant monolayer.1° The various microstructures are schematically illustrated in Figure 2. In recent years, there has been an increasing interest in polymer-surfactant interactions. The phase behavior of dilute polyelectrolyte-surfactant mixtures can be generalizedin terms of the surfactant charge. It has been shown that polyelectrolytes associate strongly with oppositely charged ~urfactants.ll-~~ Electrostaticattraction between the polyion and the polar group of the surfactant or hydrophobic interaction are the main forces stabilizing (9)Strey, R. Colloid Polym. Sci. 1994,272,1005. (10)Anderson,D.; Wennerstrom,H.; Olsson,U. J.Phys. Chem. 1989, 93,4243. (11)Goddard, E. D. Colloids Surf.1986,19,301. (12)Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants Physical Chemistry;Rubingh,D., Holland, P. M., Eds.; Dekker: New York, 1991; p 189.
Langmuir, Vol. 11,No. 6,1995 1935 the polyelectrolyte/sdactantcomplex. l2 When the polyelectrolyte and the surfactant are of the same charge, electrostatic repulsion dominates and the association between the polymer and the surfactant is absent or very feeble. 11*14 Only polyelectrolytes having a very strong hydrophobiccharacter exhibit association with surfactants of the same charge.l5-lg Similarly, nonionic surfactants present a very low affinity toward polyelectrolytes, and association occurs mainly in cases where hydrophobic interactions20between polymer and surfactant are operating. When the surfactant concentration is increased, the phase behavior also becomes dependent on the geometry of the surfactant film. A suitable choice of surfactant system would then make studies of polymers in confined geometries possible. The incorporation of polymer into a lamellar phase (see schematic drawing in Figure 2) is one example. Here, the interplay between bilayer repulsion and excluded volume effects for the polymer seems to dictate the phase behavior; theoretical considerations21have shown that it should be possible to incorporate a large polymer coil into a lamellar phase when the interbilayer repulsion is strong, but not in the case of weak repulsion. Systematic experimental studies in this area have yet been rather few.22-24An important observation is that solubility may be significantly increased if the polymer can adsorb onto the surfactant film, e.g. for hydrophobicallymodified water soluble polymers.20 The same behavior has also been found with other microstructured surfactant phases, e.g. bicontinious microemul~ions.~~ In this case the size of the polymer coil compared to the water domain seems to be a crucial parameter; i.e. polymers that are larger than the water domains appear to be insoluble in the microemulsion phase.26 Surfactant systems and their mixtures of’ten show a rich phase diagram, and studies of the properties of these systems are strongly aided by a knowledge of the phase behavior. In this paper we report on the phase behavior of a water-rich nonionic surfactant system in the presence of polyelectrolyte. We compare the effects of the nonadsorbing sodium polyacrylate (PA) and its adsorbing hydrophobically modified derivative (HMPA). The reference surfactant system is well-known, and we can compare the behavior of three differentphases, namely, the micellar (L1) and the lamellar and L3 phases, all of which can be reached a t constant composition by small variations in temperature. This allows us to investigate and compare the polymer effects on different aggregate geometries. Finally, we also investigate the effects of HMPA in the presence of salt when long-rangeelectrostaticinteractions are screened. (13)Lindman, B.; Thalberg, K. In Polymer-SurfactantInteractions; Goddard, E. D., Ananthapadmanabham,K. P., Eds.; CRC Press: Boca Raton, FL, 1992;p 203. (14)Methemitis, C.; Morcellet,M.; Sabbatin, J.; Franqois,J. J.Eur. Polym. 1986,22,619. (15)Iliopoulos, I.; Wang, T. IC;Audebert, R. Langmuir 1991,7,617. (16)Goddard, E. D.; h u n g , P. S. Colloids Surf 1992,65, 211. (17)Biggs, S.;Selb, J.; Candau, F. Polymer 1993,34,580. (18)Zana, R.; Kaplun, A.; Talmon, Y. Langmuir 1993,9,1948. (19)Senan, C.; Meadows, J.; Shone, P. T.; Williams, P. A. Langmuir 1994,10,2471. (20)Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994,98,1500. (21)Brooks, J. T.;Cates, M. E. J. Chem. Phys. 1993,799(7),5467. (22)Kkkicheff, P.; Cabane, B.; Rawiso, M. J. Colloid Interface Sci. 1984,102(l), 51. (23)Ligoure, C.;Bouglet, G.; Porte, G. Phys. Rev. Lett. 1993,71(21), 3600. (24)Singh,M.;Ober,R.;Kleman,M. J. Phys. Chem. 1993,97,11108. (25)Kabalnov, A.; Olsson, U.; Thuresson, K.;Wennerstrtim, H. Langmuir 1994,10,4509. (26)Kabalnov,A.; Olsson, U.;Wennerstrom,H. Langmuir 1994,10, 2159.
Bagger-Jorgensen et al.
1936 Langmuir, Vol. 11, No. 6, 1995 50
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Figure 3. (a) Typical structure of hydrophobically modified sodium polyacrylate, denoted HMPA. z is the degree of modification, in monomer percent, by C18 tails. (b) Schematic illustration of HMPA. The negatively charged, hydrophilic backbone has hydrophobic C18 chains randomly attached. (1) is the average curve linear repeat distance between two adjacen$ hydrocarbon tails and is in our case 250 A for 1C18 and 80 A for 3C18.
3a
(27) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988,20, 577. (28) Magny, B.;Lafuma, F.; Iliopoulos, I. Polymer 1992,33,3151.
Li
2:
2. Experimental Section Materials. Pentaethylene glycol dodecyl ether (CnE5)ofhigh quality was obtained from Nikko Chemical Co. Ltd., Tokyo. Decane (99.9%)was purchased from Sigma and heavy water (DzO)from Norsk Hydro. All chemicals were used as received. Polyacrylic acid was purchased from Polysciences. The average molecular weight is, according t o the supplier, 150 000, corresponding to a contour length of about 5000 A. The modification reaction is described el~ewhere.2~The modified polymers, containing octadecyl ((218) side chains, were obtained in their sodium salt form and have the same polymerization degree as the precursor polymer. The typical structure of the modified sodium polyacrylate is illustrated in Figure 3. We denote the pure PA and the modified ones 1C18and 3C18, respectively. The numbers indicate the monomeric percentages ofhydrophobicaide chains and are in our case 1 and 3%, respectively. The distribution of the alkyl groups along the PA chain is random.28 The average curve linear repeat distance, (I), between two adjacent C18 tails is 250 A for 1C18 and 80 A for 3C18, based on a monomer segment length of 2.5 A. From measuring the pH value of modified and unmodified PA in water solution (pH % 81, we conclude that protonation is negligible; i.e. the polyelectrolyte is fully charged. Phase Diagram Determination. Studies of phase equilibria were performed using a thermostated water bath with the samples contained in sealed ampules equipped with magnet stirrers. Phase-boundary temperatures were determined by visual inspection in transmitted light, in scattered light and by observation of the samples between crossed polarizers. The temperature was controlled to within a tenth of a degree. Macroscopic phase separation was often found to be very slow in the heterogeneous regions. Samples in these regions were therefore centrifugedfor several hours, at controlled temperature, until macroscopic phase separation was achieved. The separated phases were then analyzed for their compositions. The surfactant and decane concentrations were determined by recording NMR spectra and comparing the peak areas with a spectrum from a sample with known concentrations. The polymer peaks were not possible to detect in these spectra because of the relatively low concentrations. In systems containing no added salt, the polymer concentrations were determined by atomic absorption spectrometry of the Na+ counterions. In systems containing added salt (NaCl), polymer concentrations were estimated from titration with a strong acid (HCl), using methyl orange as an indicator. A calibration curve was made from titrations of known
W+L,
L, +o
2c
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WS'WO
Figure 4. Same as figure 2 except that water is replaced by a n aqueous 0.2 w t % solution of PA. The two-phase regions obtained at higher temperatures correspond to concentrated lamellar and L3 phases, each in equilibrium with a PA solution. The lamellar and L3 phases contain essentially no PA. concentrations of polyacrylate, and the polymer concentrations in the samples were determined by interpolation on the calibration curve.
3. Results
Phase Behavior of Surfactant-Water-Oil-Polyelectrolyte. The influence of added polymer on t h e phase behavior of the surfactant-water-oil system described above was investigated. This was done by replacing water by 0.2 and 0.6 w t % of aqueous polyelectrolyte solutions a n d comparing the phase equilibria with that of the reference system (Figure 2). The resulting phase diagrams of the systems containing added polyelectrolyte, drawn as temperature versus weight percent surfactant plus oil (W, W,), a r e shown in Figures 4-6. In Figure 4 we present the system with PA, and in Figures 5a,b a n d 6a,b we present the systems containing the hydrophobically modified polyacrylates 1C18 a n d 3C18, respectively. In the system with PA (Figure 41, the microemulsion phase has a similar stability range as in the reference system, a n d also an equilibrium with excess oil is observed at lower temperatures. However, PA can be solubilized neither in the La nor in the L3 phase. Here, we instead obtain concentrated La a n d L3 phases i n equilibrium with an aqueous PA solution. In t h e 1C18 (Figure5) a n d the 3C18 (Figure 6) systems, the same sequence of phases, L1-La-L3, is observed with increasing temperature as in the reference system. Also the same equilibria, L1+ 0 at lower temperatures a n d L3 + W at higher temperatures, are observed. The relative extensions of the phases are, however, significantly altered compared to t h e reference system. A general effect with the modified polymers is that the stability ranges of the various phases a r e strongly shifted toward higher temperatures at low W, + W,. This effect becomes more
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Phase Behavior of a Nonionic Microemulsion
Langmuir, Vol. 11, No. 6, 1995 1937
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10 20 ws +wo Figure 5. Addition of 0.2 wt % 1C18 (a, left) and addition of 0.6 wt % 1C18 (b, right) to the reference system. Note that the monophasic lamellar (La)phase has a limited extension and is at higher concentrations split into two lamellar phases, L,' + L,". pronounced the higher the polyelectrolyte concentration and the higher the degree of modification. At higher W, W, a coexistence of two lamellar phases is observed. One sample in the 0.2 wt % 3C18 system, with W, W, = 30, was investigated in detail. When centrifuged at 37 "C for a long time, macroscopic phase separation into two separate lamellar phases was obtained. From IH NMR it was found that the two phases had different compositions; one was dilute in surfactant oil (W, W, % 20), while the other was concentrated (W, W, 50). The surfactant-to-oil ratios were found similar in the two phases. From atomic absorption (Na+ ions), it was found that essentially all HMPA was present in the dilute lamellar phase. The concentration of HMPA in the concentrated La phase was less than 0.03 wt %, correspondingto a partition coefficient for HMPA between the two L, phases of at least a factor of 8. Phase Behavior of Surfactant-Water-Oil-HMPA-NaCl. Adding salt to the systems containing HMPA substantially alters the phase diagrams. In Figure 7a,b, we present the phase diagrams of the systems containing 0.2 and 0.6 wt % 3C18, where the aqueous solution also contains 0.1 M NaC1. The microemulsion (L1)phase now has a finite swelling and is in equilibrium with excess brine at lower concentrations. The swelling is dependent of the HMPA concentration, as seen by comparing the two phase diagrams. The same applies to the La and L3 phases, but here the swelling is so limited that no one phase regions at all are formed in our concentration range. Three samples (W, W, = 7,10, and 301, each containing 0.2 wt % 3C18 were macroscopically separated by centrifugation at 37 "C. In all three cases a 3C18-rich, W, % 36 was optically clear lamellar phase with W, found in equilibrium with essentially pure brine.
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4. Discussion
Before discussing addition of HMPA, it is useful to discuss briefly the effects of adding a small amount of anionic surfactant (sodium dodecyl sulfate) (SDS)29,30 to the reference system. Here, it is found that replacing just a few percent of the C12E5 molecules by SDS significantly changes the phase behavior. The most striking observations is that all phase boundaries at low surfactant plus oil concentrations are strongly shifted to higher temperatures. The lamellar phase is also affected; without SDS it is slightly turbid, but upon addition of SDS the turbidity decreases dramatically. It is also noted that the temperature stability of the L3 phase is somewhat broadened when the anionic surfactant is present. The magnitude of these effects are correlated with the fraction of SDS molecules, i.e. to the surface charge density. When screening the long range electrostatic interactions by addition of 0.1 M NaC1, it was found that the phase diagram of the reference system was almost perfectly reproduced, and it was concluded that all of the above mentioned observations arose from the long range electrostatic interaction. Keeping the effect of ionic surfactant in mind, we now turn to discuss the phase behavior of the polyelectrolytecontaining systems. Our most important experimental observations can be summarized as follows: (i) PA is soluble in the L1 phase, but insoluble in the La and L3 phases. (ii) In contrast to PA, HMPA is soluble in all phases. (iii) Addition of HMPA at low surfactant plus oil concentrations strongly shifts the phase boundaries to (29) Fukuda, K.; Olsson, U.; Wiirz, U.Langmuir 1994, 10,3222. (30) Rajagopalan, V.; Bagger-JGrgensen,H.; Fukuda, K.; Olsson, U.; Jonsson, B. Manuscript in preparation.
Bagger-Jorgensenet al.
1938 Langmuir, Vol. 11, No. 6, 1995
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ws+wo %+WO Figure 6. Addition of 0.2 wt % 3C18 (a, left) and addition of 0.6 wt % 3C18 (b, right) to the reference system. Again the main features of the reference system (Figure 2) are reproduced. higher temperatures. (iv) HMPA induces a phase separation into two lamellar phases, La’ La”,at higher surfactant plus oil concentrations. (v)Addition of HMPA results in a wider temperature stability of the L3 phase. (vi) Addition of HMPA and 0.1 M NaCl results in an associative type phase separation. Below we discuss these observations separately. Addition of PA. The unmodified polyelectrolyte, PA, is soluble at the present concentrations in the L1 phase. It cannot, however, be dissolved in the lamellar phase. Since the water solubility of PA is not expected to change significantly in our temperature interval, the segregative phase separation into a surfactant- and oil-rich but PApoor phase and a PA-rich water phase must solely be attributed to the geometry of the surfactant film. The effective size of a PA molecule, compared to the thickness of the water layers, is supposed to be an important parameter in this case.26 Calculating the thickness ofthe water layers is straightforward; SANS data from the lamellar phase in the 0.2 wt % 3C18 syste0m3lare consistent with an area per head group, a, = 47 A2, a value which is commonly found for the s u r f a ~ t a n t . With ~ , ~ ~this valug the periodicity of the lamellar phase varies as D x (30 A)/@,,where 4, is the surfactant volume fraction. Defining the bilayer as consisting of the oil and the hydrocarbon tails of the surfactant, which constitute half of the surfactant ~ o l u m ewe , ~ have a bilayer thickness of 50 A. The thickness of the water layers is then db obtained from d, = D - db. Calculating the dimensions of a polyelectrolyte without added salt is, on the other hand, a formidable task. However, an estimate of the effectivemolecular size can be obtained from the charged, wormlike coil model. In the absence of excess electrolyte,
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(31) Bagger-Jorgensen, H.; e t al. To be published.
the Debye screening length is assumed to be determined by the counterions only, whose concentration is corrected for ion c o n d e n s a t i ~ n .The ~ ~ persistence length, written as a sum of an intrinsic part A, and an electrostatic part le,may then be written33
where Q is the Bjerrum length, h is the length of a monomer unit, and eP is the monomer concentration. Within this approximation, eq 1 reveals that the electrostatic persistence length A, is inversely proportional to the polyelectrolyte concentration. Inserting numerical values into eq 1gives an electrostatic persistence length of about 90 A for a 0.2 wt % PA solution. Neglecting the intrinsic part Ap, we get an end-to-end distance ( R 2 ) ~ lxl 2 1000 A, a distance which is roughly the water layer thickness in the most dilute lamellar phase. This rough calculation indicates that the effective dimension of the polyacrylate molecule is of the same order of magnitude o r possibly larger than the thickness of the water layers. This probably explains the observation that the lamellar phase is incapable of incorporating the unmodified polymer. The concentrations of the two phases in equilibrium, the PA solution and the lamellar phase, depend on the osmotic pressure exerted by the polyelectrolyte and on the interbilayer repulsion in the lamellar phase. For example, a sample with W, W, = 20 and 0.2 wt % PA phase separated into one essentially PA-free lamellar phase with W, Wo 55 in equilibrium with one water solution containing ~ 0 . wt 3 % PA. Here, the rather
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(32) Imai, N.; Ohinshi, T. J. Chem. Phys. 1959, 30, 1115. (33) Mandel, M. InEncyclopedia ofpolymerScience andEngineering; Mark, H. C., Ed.; Wiley: New York, 1988; Vol. 11;p 739.
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Phase Behavior of a Nonionic Microemulsion
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Figure 7. Same as in Figure 6 except that water is now replaced by a 0.1 M NaCl solution, i.e. addition of 0.2 wt % 3C18 in 0.1 M NaCl (a, left) and addition of 0.6 wt % 3C18 in 0.1 M NaCl (b, right). The microemulsion (L1)phase has a limited swelling and is at lower concentrations in equilibrium with excess brine. At higher temperatures we find a lamellar (La)phase concentrated in both surfactant, oil, and HMPA in equilibrium with essentially pure brine.
concentrated lamellar phase (d, x 50 A) reflects the relative weakness of the undulation force. Behavior of the HMPA System at Low Surfactant and Oil Concentration. As previously stated, a general property of the HMPA systems is that the stability region of the various phases is shifted toward higher temperatures a t lower concentrations of surfactant and oil. As previously described, a similar effect has been observed upon addition of small amounts of SDS to the surfactant film,29v30 indicating that the upturn of the phase transition temperatures at lower concentrations is an electrostatic effect. The surfactant film in this case becomes effectively charged by adsorption of HMPA. For the same polymer concentration the effect is stronger for 3C18 compared to 1C18, indicating that 3C18 adsorbs stronger to the surfactant film compared to 1C18. The electrostatic effect on the phase boundary can in this case qualitatively be easily understood. Nonionic surfactant systems are often described in terms of the flexible surface assuming the spontaneous mean to decrease monotocurvature of the surfactant film, Ho, nously with increasing temperature3(counting curvature toward oil as positive). Within this model the lower phase boundary of the microemulsion phase corresponds to the so-called emulsification f a i l ~ r e , where ~ ~ - ~spherical ~ oilswollen micelles coexist with excess oi1.6,8 Neglecting (34) Safran, S. A. In Structure and Dynamics ofStrongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S.-H., Huang, J. S., Tartaglia, P.,Eds.;Kluwer: Dordrecht,The Netherlands,
1992. (35) Safran, S. A,;Turkevich, L. A.; Pincus, P. A. J . Phys. Lett. 1984, 45, L19. (36) Safran, S. A. Phys. Rev. A 1991,43 (6), 2903.
entropy, the emulsification failure is dictated by the curvature energy only, which is independent of concentration. In the case of charged films, there is in addition to the curvature energy of electrostatic energy term which wants to reduce the radius of the spheres.37 This effect i.e. by increasing the can be balanced by decreasing Ho, temperature (a decrease in HOcorresponds to an increase of the preferred radius). Similar arguments can be applied for other aggregate geometries, and the general property is that an increase in the effective surface charge density results in an increase in the effective spontaneous mean curvature, which can be balanced by an increase in temperature to stabilize the particular aggregate curvature. The electrostatic term depends strongly on the effective surface charge density. Since the aqueous HMPA concentration in our case is kept constant, the effective charge density of the surfactant film increases with decreasing W, W,, leading to stronger electrostatic effects the lower the surfactant plus oil concentration. Addition of HMPA to the La Phase. A particularly important part of this study concerns the lamellar phase and its stability in the presence of polyelectrolyte. In the reference system, the lamellar phase swells with water by the so-calledundulation force.38 The origin ofthis force is the gain in conformational entropy of the membranes when the system is diluted and the periodic repeat distance is increased. One characteristic property of lamellar phases swollen by undulation forces, in contrast to those that swell by an electrostatic double layer force, is that
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(37) Aamodt,M.;Landgren,M.;Jonsson,B. J.Phys. Chem. 1992,96, 945. (38) Helfrich, W.Z. Naturforsch. 1978,33a, 305.
Bagger-Jorgensenet al.
1940 Langmuir, VoI. 11, No.6, 1995 they often are slightly turbid, with strong (anisotropic) scattering a t low scattering vectors.39 Dissolving HMPA, 1C18, or 3C18 in the lamellar phase results in a decrease in the turbidity. This effect is similar to that observed when small amounts of SDS are added to the nonionic lamellar phase,29indicating an introduction of a strong electrostatic interbilayer repulsion. This in turn indicates that the HMPA adsorbs onto the nonionic bilayers, making the bilayers effectively charged. An interesting observation, not seen upon addition of SDS, is the equilibrium between two lamellar phases at higher surfactant plus oil concentrations. Here, a lamellar phase having a larger water spacing contains the polymer and coexists with a lamellar phase which has a smaller water spacing and which is free of polymer. The observed behavior indicates that the polymer-coated bilayers experience an increased repulsive interaction a t shorter separations, which probably is of steric origin and associated with squeezing the polymer layer between the bilayers. (Note that we here are probably dealing with a single polymer layer forming bridges between the bilayers across water). Comparing the phase diagrams in Figures 5 and 6, we see that the minimum water spacing, dw,min, of the polymer-containing phase decreases both with an increasing degree of modification (decreasing (I)) and with an increasing polyelectrolyte concentration. The (I) dependence follows the expected trend since an increased density of hydrophobic anchors is expected to result in shorter loops and bridges and therefore a smaller effective thickness of the adsorbed polymer layer. The effect of polymer concentration is probably associated with the strong concentration dependence on the polyelectrolyte flexibility (c.f. eq 1). With increasing flexibility, the polymer layer becomes more compressible. We can compare the dw,min values with (I). In the 0.2% 1C18 and O.z% 3C18 systems, dw,min are approximately 300 and 160A, respectively, if we estimate the boundaries to be at W, W,, = 15 and 25, respectively. In the 0.2% 1C18 case this corresponds to a dw,min value which is only slightly larger than (I) while in the 0.2%3C18 case it is approximately twice the value of (I). Addition of HMPA to the L3 Phase. The L3 phase is formed at temperatures slightly higher than the La phase and has a similar extension. The L3 phase is normally stable only within a narrow temperature interval.1° HMPA mainly affects the L3 phase in two ways, namely, the swelling with water and the temperature stability. At low volume fractions no L3 phase is observed, in line with recent studies where ionic surfactant was added to a dilute, nonionic L3 p h a ~ e . However, ~ ~ , ~ ~ at higher volume fractions the phase becomes effectively broader &e. has a wider temperature range). Again, this effect is similar to when, instead of HMPA, a small fraction of SDS is added.29 It may also be noted that the effect is analogous to the dramatic increase in the so-called clouding temperature when HMPA is added to the binary water-ClzE~system.20The effect is larger for 3C18 than for 1C18 and also larger for 0.6 wt % than for 0.2 wt % samples, indicating that the broadness of the L3 phase is correlated with the effective surface charge density29and hence may be ascribed to electrostatic interactions. The increased temperature range of the L3 phase at high volume fractions may now be understood as follows: for a fixed temperature, the L3 phase is at lower surfactant and oil concentrations in equilibrium with excess water. Adding charge (ionic surfactant or adsorbing polyelectrolyte) to the bilayers results in an additional osmotic
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(39) Nallet, F. Langmuir 1991,7, 1861. (40) Hoffman,H.; Thunig, C.; Schmiedel,P.; Munkert, U. Langmuir 1994, 10, 3972.
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pressure which moves the phase boundary (L3 W L3) to a lower W, W,. At a higher concentration the L3 phase is in equilibrium with the lamellar phase. This phase boundary is less affected by addition of charge since both phases may dissolve the charged component. Hence, by shifting, only one of the two phase boundaries results in an increased temperature stability of the L3 phase. Another explanation would be that HMPA affects the spontaneous curvature of the surfactant film. At high volume fractions this is, however, less probable since then both phase boundaries of the L3 phase would be shifted, leaving the width of the L3 phase approximately unaffected. HMPA in 0.1 M NaCl. Upon addition of salt (0.1 M NaC1) the HMPA containing lamellar phase collapses and expels excess brine (Figure 7). In the system with 0.2 wt % 3C18 (Figure 7a), the collapsed lamellar phase, containing essentially all the HMPA, was at 37 "C found to have a surfactant plus oil cpcentration of W, + W, 36, correspondingto d, x 100A. The collapse of the lamellar phase can be understood in terms of attractive "bridging" interactions mediated by the polymer, resulting from bridging configurations being more favorable than nonbridging ones.41 In the absence of salt, this attractive interaction is also expected to be present; however, it is overcompensated for by the strong electrostatic repulsion, resulting in a swelling lamellar phase. A similar nonswelling, or associative, phase separation is also observed for the other aggregate geometries, i.e. the L1 and L3 phases, when adding salt, and thus appears to be qualitatively independent of the aggregate geometry. Related phase separations have been observed in aqueous mixtures of oppositely charged polyelectrolytes and ionic surfactant aggregated3 and appear to be a general behavior in the case of attractive polymer-aggregate interactions, unless compensated for by e.g. electrostatic interactions, as in the HMPA systems without salt.
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5. Conclusions In this paper we have investigated the phase behavior when a hydrophobically modified polyelectrolyte and the unmodified precursor polyelectrolyte (M,FZ 200 000) are mixed with nonionic surfactant aggregates of different geometries. PA is essentially insoluble in the lamellar and L3 phases, while being soluble in the micellar L1 phase. Here, the geometry of the surfactant aggregates determines the excluded volume interactions and hence dictates the phase equilibria. The unperturbed polyelectrolytecoil size is comparable with or larger than the size of the water domains of the lamellar and L3geometries, which penalize mixing. In the L1 phase, where the surfactant aggregates are spherical or nearly spherical micelles, PA molecules have essentially full access to all three spatial dimensions. With no or only minor restrictions on the polymer conformations, the polymer becomes soluble in this phase. HMPA, on the other hand, can be solubilized in the lamellar and L3 phases if these phases are sufficiently dilute. Here, it can be assumed that HMPA adsorbs to the surfactant bilayers. The polymer-coated bilayers become effectively charged, resulting in long range electrostatic interactions. In addition, the adsorption of polyelectrolyte also results in a positive shift of spontaneous curvature of the surfactant monolayer, which affects the phase behavior. At higher coverage, the lamellar phase becomes unstable with a transition to a L1 phase. Both the long range electrostatic interactions and the spontaneous curvature effects mimic the effects of dis(41)Rossi, G.; Pincus, P. A. Macromolecules 1989,22, 276.
Phase Behavior of a Nonionic Microemulsion solving small amounts of ionic surfactant in the nonionic surfactant film. In addition to the long range electrostatic interactions, the polymer-coated bilayers also experience a strong repulsive interaction at shorter separations, associated with the compression of the polymer layer. At higher bilayer concentrations, two lamellar phases in equilibrium are obtained, one being bilayer-rich and polymer-free while the other is bilayer-poor and contains all of the polymer. Within the composition range studied here, the minimum water spacing, of the polymer-containing lamellar phase decreases with decreasing (1) and increasing polymer concentration. The (I) dependence is consistent with shorter loops and bridges for the HMPA with shorter (1). The concentration dependence is probably associated with the general strong concentration dependence of the polyelectrolyte backbone flexibility. At higher concentrations, the polymer becomes more flexible,which is expected
Langmuir, Vol. 11, No. 6, 1995 1941 to lead to a more easily compressible polymer layer. Addition of salt (0.1 M NaC1) does not qualitatively alter the phase behavior of the PA system. However, in the HMPA systems, the presence of salt leads to a general associative type phase separation. All three phases, LI, La,and LB,show a finite swelling in the presence of 0.1 M NaCl and coexist with excess brine at lower surfactant plus oil concentrations.
Acknowledgment. This work was financially supported by STATOIL, Norway, and by the Swedish Natural Science Research Council (NFR). 1.1. acknowledges the Mission des Relations Internationales du CNRS (MDRICNRS) for the financial support of his stay in Lund. We thank Bjorn Lindman for his comments on the manuscript. LA950046J
