Polymer-Surfactant Interactions Studied by Phase Behavior, GPC, and

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Langmuir 1995,11, 1885-1892

Polymer-Surfactant Interactions Studied by Phase Behavior, GPC, and NMR Kirsti Veggeland" and Svante Nilsson Rogaland Research, P.O. Box 2503, Ullandhaug, N-4004 Stavanger, Norway Received September 16, 1994. I n Final Form: January 27, 1995@ Osmoticeffects are important both in static phase behavior and dynamicgel permeation chromatography (GPC)analysis of polymer Surfactant systems. Besides affectingthe salinity for the 11(-)/I11 phase transition in microemulsions, the water content in the middle phase will decrease when nonassociative, watersoluble polymers are added. The same systems will in a GPC column, using the surfactant solution as eluent, separate both due to osmosis and size exclusion. Association between polyethylene oxide and anionic ethoxylated surfactants disappears if more than three ethylene oxide (EO) groups are added to the surfactant head group. The phase separation of micelles and polymer into separate aqueous phases in the II(-) state occurred at lower salinity when the number of EO groups in the surfactant increased. The phase separation could qualitatively be modeled within a simple Flory-Huggins approach. With NMR self-diffusionmeasurements it was possible to study if added polymer influenced the structure of the surfactant aggregates.

Introduction Interactions between polymers and surfactants have been widely studied. Review articles and books are available, covering systems with surfactants and polymers differing in ionic character, size, and ~ h a p e . l - Most ~ studied systems so far are nonionic polymers and ionic surfactants (Po-S*) and polymers and surfactants with opposite charges (P+-S-, P--S+). In these systems, associative interactions are mainly present. The criteria and mechanisms for the interactions have been studied in great detail. From the fundamental point of view, the study of polymer-surfactant interactions was originated in biochemistry in order to study protein-surfactant interaction^.^ The industrial applications for polymersurfactant systems are many and are reviewed by Goddard.6 This work is motivated from IOR (improved oil recovery) research on chemical flooding for North Sea reservoirs. Here both polymers and surfactants must be negatively charged, (P--S-), to minimize adsorption to clay minerals. Injecting a polymer and a low concentration surfactant slug followed by a polymer drive slug is called low tension polymer flooding, LTPF.7,s To obtain knowledge about LTPF polymer-surfactant systems, our group has performed different experiments.8-10 In this work new results from phase behavior, NMR, and gel permeation chromatography (GPC) will be presented. We have used both Abstract published in Advance A C S Abstracts, May 1, 1995. (1)Goddard, E. D. In Interactions ofSurfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993,p 123. (2)Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D.; Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993;p 203. (3)Hayawaka, K.; Kwak, J. C. T. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds., Marcel Dekker: New York, 1991;p 189. (4)Saito, S. In Nonionic Surfactants; Schick, M. J., Ed., Marcel Dekker: New York, 1987;p 881. (5)Breuer, M. M.; Robb, I. D. Chem. Ind. (London) 1972,13, 530. (6)Goddard, E.D. In Interactions ofSurfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993;p 395. (7)Kalpakci, B.; Arf, T. G.; Barker, J. W.; Krupa, A. S.; Morgan, J. C.; Neira, R. D. SPEIDOE 20220 Proceedings ofthe 7th. Symposium on Enhanced OilRecouery, Tulsa, Oklahoma, April 22-25,1990,p 475. (8)Austad, T.; Fjelde, K.; Veggeland, K.; Taugbd, T. J. Petr. Sci. Eng. 1994,10, 255.(9)Veggeland, K.; Austad, T. Colloid Surf. A 1993,76, 73. (10)Taugbd, K.; von Ly, T.; Austad, T. Proceedings from the 15th IEA International Workshop on Enhanced Oil Recovery, Bergen, 1994. @

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0743-746319512411-1885$09.0010

pure chemicals and chemicals applicable for North Sea reservoirs. The chemicals most relevant for reservoir application today are ethoxylated andor propoxylated sulfonateshulfates in mixtures with biopolymers like xanthan. Little literature can be found concerning polymers and surfactants of the same charge, but some work has been published and is in progress.ll These systems show typically a segregative phase behavior where the driving force is to avoid mixing of polymer and surfactant by going to separate phases. Very few studies have been published on phase behavior of polymer-surfactant-oil system^.^,^^-^^ The phases of interest in this work are those that can be formed at low surfactant concentration. At low salinity the surfactant is dissolved in water as normal micelles in equilibrium with almost pure oil referred to as the II(-) state. By increasing the salinity a surfactant rich phase in equilibrium with almost pure oil and water salt (brine) may be found, called the I11 state or the middle phase. In the I11 state different surfactant structures are possible, closely packed discrete aggregates or bicontinuous microemulsions. Finally, a t high salinity the surfactant is dissolved in the oil phase in equilibrium with brine as reversed micelles, the II(+) state. Surfactant phase behavior studies with added polymer can describe the nature of the interaction between the surfactant and the polymer and also provide information about solubilization and compatibility. An improved understanding of the phase behavior with and without added polymer can be used for optimizing a n LTPF process. One approach to model the phase behavior of these systems is to describe the surfactant as a polymer and apply the Flory-Huggins model.16 This has been done quite successfully.17 The main reason being that both polymers and micelles are similar in that they both have a low translational entropy

+

(11)Thalberg, K.; Lindman, B. Colloid Surf. A. 1993,76,283. (12)Desai, N. In Surfactant-Polymer Interactions in Enhanced Oil Recovery Systems; PhD Thesis, University of Florida, 1983. (13)Kabalnov, A,; Olsson, U.; Wennerstrom, H.Langmuir 1994,10, 2159. (14)Pope, G. A,; Tsaur, K.; Schechter, R. S. SOC.Petr. Eng. J.1982, Dec, 816. (15)Nagarajan, R. Langmuir 1993,9,369. (16)Flory, P.In PrincipZes ofpolymer Chemistry, Cornell University Press: Ithaca, NY,1953. (17)Zhang, K.; Karlstrom, G.; Lindman, B. J.Phys. Chem. 1994,98, 4411.

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1886 Langmuir, Vol. 11, No. 6, 1995

of mixing. However, this model treats the interactions in the systems far from properly. The presence of NaCl will to some extent screen long range electrostatic interactions. We have used this Flory-Huggins approach to try to estimate the polymer effect on the 11(-)/I11 phase transition and to model the observed phase separation ofpolymer and surfactant into two aqueous phases when oil is present. To verify the aggregate structures in the different phases and also study the polymer-surfactant interactions in more detail, self-diffusion NMR studies have been performed. It is well-documented that self-diffusion coefficients are sensitive probes for surfactant systems. The diffusion of the components will be influenced by size, shape, and interactions of the surfactant aggregates. Another approach to study polymer-surfactant interactions which also includes the dynamic aspect is GPC. The method can be compared to a dynamic dialysis omitting the membrane. It provides quantitative binding ratios of surfactant binding to polymers when the interaction is associative. This we have presented in a n earlier paper.g The chromatograms will differ depending on the nature of the interactions. Structural changes of the micelles andor the polymer may complicate the interpretation of the chromatograms. Also osmotic pressure effects must be considered when studying polymer-related systems as will be seen from both the GPC and the phase behavior results. The work in this paper will be presented as follows: The results from phase behavior, GPC, and NMR studies will be given separately, then these experimental results will be compared and discussed, followed by a section with theoretical modeling of the phase behavior.

Experimental Section Chemicals. Alkylethoxysulfonates, (C&CE,S03-Na+, were supplied by Norsk Hydro. An o-xylenesulfonate, C12xySO3-, termed RL 3011, from Exxon Chemicals was extracted with hexane and purified to 96% active material. Poly(ethy1ene oxides), PE020 and PE04, with respectively average molecular weight of 20 000 and 4 000 g/mol were obtained from Merck. Dextrans, T500 and T10, were obtained from Pharmacia B, Uppsala, Sweden,having an average molecular weight of 500 000 and 10 000, respectively. Xanthan, Xc 85-11F4, was produced by Bioferm Statoil, filtered, and purified by precipitation with 2-propanol and dried. The molecular weight range was 2-3 x lo6. Poly(arylamides), purchased from Allied Colloids, UK; AF 1175A, was reported to have a very high molecular weight and a medium anionic character, and AF' 935 was of medium molecular weight and low anionic character. Isooctane of puriss quality, isobutyl alcohol of pro analysi quality, and NaCl p.a. all delivered from Merck were used. MethodsIApparatus. Phase Behavior. Samples were prepared by mixing stock solutions of polymer, surfactant, alcohol, and NaC1. The concentrations are in weight percents, and the oil to water ratio is 1 unless otherwise specified. The phase compositions after allowing time for equilibration, are analyzed with the two-phase titration method for the surfactants,ls the phenollsulfuric method for xanthan,lgand with GPC for PE020. The phase studies were performed at room temperature. NMR. 'H-NMR self-diffusionmeasurements were performed on a JEOL FX-60 or a JEOL FX-100 spectrometer operating at 60 or 100 MHz, respectively, using the FT-PGSE technique as described in more detail by Stilbs.20 With this technique one uses a 9On-r-180"-~-echo pulse sequence with two added rectangular magnetic field gradient pulses of magnitude G, separation time A, and duration time 6. The echo amplitude at (18)Reid, V. W.; Longmann, G. F.; Heinerth, E. Tenside 1967,4, 292. (19) Dubois, M.; Gilles, K. A,; Hamilton, J. K.; Rebers, P. A,; Smith, F.Anal. Chen. 1956,28,350. (20) Stilbs, P. Prog. NMR Spectrosc. 1987,19, 1.

1.5

I

I

0.04

0.08

I

0

i

I

0.12

0.16

I

0.20

polymer-conc. (wt%)

Figure 1. Stability diagram for the II(-)/III phase transition for 1.5% C1&ySO3-. Salinity as a function of added polyacrylamide (- -), xanthan (-), and PE020 (.-). time 2r is given byz1

A(2t) = A(0) exp[-2z/Tz - y2G2D6?A - 6/31]

(1)

where 2'2 is the transverse relaxation time and y is the magnetogyric ratio for the proton. The self-diffusioncoefficients D were determined by measuring the echo amplitude A as a function of 6 when keeping G and A fixed. GPC. The method and instrument setup have been described in detail in an earlier paper.g The column used is a 6 mm, Ultrahydrogel 250 water GPC column. The chromatographic setup consisted of a 600E pump, a 715 Ultra Wisp sample processor, a 410 differential refractometer, all from Waters, and an NEC, ApClV Power Mate 1 integrator. All solutions were runthrough a 0.45,~ Millipore filter prior to injection and degassed constantly with He gas. The flow rate in all experiments was 0.5 mumin. To study polymer surfactant interactions the surfactant solution is used as the eluent. To study P-S association the concentration of surfactant must be above the critical association concentration, cac. In all P-S cases, 0.01 m of surfactant in 0.01 M NaCl solutions has been used as eluent. Calibration plots, peak area vs concentration, ofthe vacant peaks for determination of the P-S binding ratio, were made. The column and the refractive index (RI) detector had a temperature of 32 "C.

Results Phase Behavior. Figure 1shows the phase behavior of 1.5%C1&ySO3- solution in equilibrium with isooctane. The II(-)/III phase transition in absence of polymer is a t 2.1%NaC1. The IIIAI(+) transition is a t 2.7% NaC1, but not shown in the figure. The addition of polyacrylamide (AF 1175A) did not shift the phase transitions significantly, whereas addition of 0.10% xanthan reduced the salinity of the II(-)/III transition to 1.9%NaC1, as can be seen from the figure. The salinity of the III/II( +) transitions was not shifted. Analysis of xanthan showed that almost all of the xanthan could be found in the aqueous phase. For both polyacrylamide and xanthan the water content of the middle phase was reduced whereas the oil content was almost constant; see Figure 2. With PE020 as added polymer a quite different result was obtained with the surfactant C1&yS03-. Instead of a small reduction in the salinity required for the formation of a middle phase, addition ofPEO2O caused the middle phase to collapse into a highly concentrated phase. In a salinity scale this collapse occurred close to the formation of a normal middle phase (Figure 1)in the absence of polymer. GPC analysis showed that about four-fifths of the original 0.04% PE020 was found in the middle phase a t that salinity. In Figure 3 the effect of adding polymer to an ethoxylated sulfonate, (C&CE3S03- is shown. The amount of oil in (21) Stejskal, E. 0.;Tanner, J. E. J. Chem. Phys. 1965,42,288.

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0.5

0

0.02

0.04

0.06

0.08

0.10

0.12

Polymer-conc. (wt%)

Figure 2. The relative volumes of the oil (01,middle (M), and aqueous (A) phases for 2.0% C12XySO3- and 2.4% NaCl as a function of added polyacrylamide (- -1 and xanthan (-).

0 0

0.04

0.08

0.12

0.16

Xanthan-conc.(wt%)

Figure 4. Stability diagram for the II(-)/I11 phase transition for 1.5%(C6)2CEzS03-and 1.5%isobutyl alcohol. Salinity as a function of added xanthan. Table 1. Molar Binding Ratios for Association Complexes of Sulfonates and PE020 surfactant mol of surfactanumol of EO unit 0.12 0.02 0 0 0.20

0.33

0

0.02

0.04

0 06

0.08

polymer-conc. (wt%)

Figure 3. Stability diagram for the II(-)/III phase transition for 1.5% (C6)zCE3S03- and 1.5% isobutanol. Salinity as a function of added polyacrylamide(- -1, xanthan (-1, and PE020 (. *I.

the microemulsion phase at the phase transition salinity was very low, 2-4%. Upon increasing salinity the oil solubilization increased rapidly. To obtain normal phase behavior without salt-induced precipitation of the surfactant, isobutyl alcohol was added. The effect of changing the alcohol concentration was tested, and the effect was mainly a parallel shift of the salinity versus the polymer concentration curve. A somewhat different phase behavior was observed for the ethoxylated sulfonates compared to the xylene sulfonate in Figure 1. Adding polyacrylamide (AF1175A) and xanthan lowered the salinity of the II(-)/I11 transition compared to the results in Figure 1. For xanthan the salinity required for a transition from a two-phase state to a three-phase state was reduced by more than 50% with a rather sharp initial decrease followed by a leveling out. The three-phase state formed a t this low salinity did not look like a n ordinary type I11 microemulsion. Adding alcohol to these systems resulted in a n increased transition salinity from the formation of a I11 state. The lower phase was very viscous and the surfactant-rich phase had not changed its visual appearance much. This indicates a separation into one polymerrich phase and one micellar phase. Addition of PE020 to (C&CEzS03- caused a similar effect as polyacrylamide, as seen in Figure 3, but PE020 was found both in the lower and the middle phase. In Figure 4 we see for the sulfonate (C&CE&03-, containing only 2 ethylene oxide (EO) groups, that the same phase separation occurred with xanthan but a t a higher polymer concentration. NMR. Micellar diffusion coefficients were measured on the oil signal of oil swollen micelles since the surfactant signal had a too rapid Tzrelaxation to be detected in a spin-echo experiment. The results in Table 2 show that addition of small concentrations of xanthan had no significant effect on the micellar diffusion rate. Addition

of PE020 also had very little effect on the micellar diffusion for the surfactants (C&CE3S03- and ( C ~ ) Z C E Z SWith ~~-. the surfactant C1aySO3-, on the other hand, a significant increase in micellar diffusion, ca. 25%, is observed on addition of PE020. This can be explained by a complex formation between PE020 and C12xySO3- which decreases the surfactant aggregation numberz5in combination with the high diffusion of free PE020. From data in the literature,22the PE020 diffusion is estimated to be 5 x m2/s. Diffusion measurements on systems in which xanthan and the surfactant had phase separated into a polymer rich and a surfactant rich aqueous phase showed that the surfactant structure was still micellar; see Table 3. There is an increase in the diffusion rate in the phaseseparated systems, but this can be attributed to the changes in concentrations of surfactant and isobutyl alcohol in the surfactant rich phase. Any structural change such as a transition from micellar to normal middle phase would have produced order of magnitude shifts.23 GPC. In Figure 5a-c chromatograms of PE020 in different solvents are shown. The polymer peak elutes a t 14.9 min in NaCl solution (Figure 5a). When the solvent, and therefore the eluent, is changed to a surfactant solution, 0.01 m C1zXySO3- a t the same salinity, the chromatogram is altered, as Figure 5b shows. The peak related to the polymer elutes at 13.5 min and is followed by a negative peak. Aloweringin retention time indicates that an aggregate larger than the pure polymer is formed in the injected solution. The vacant peak that follows shows that the surfactants have associated with the polymer. The peak area of the vacant peak is related to the complexationwith the polymer. The ratio of surfactant associated per monomer of the polymer may be found by integrating the vacant peak. For C1&ySO3- the binding (22) Brown, W.; Stilbs, P.; Johnson, R. M. J . Pol. Sci. (Polym. Phys. Ed.) 1983,20,1029. (23)Lindman, B. In Physics of Amphiphilic Layers, Springer Proceedings in Physics; Meunier, J., Langevin, D., Boccara, N., Eds.; Springer-Verlag: Berlin, Heidelberg, 1987;Vol. 21,p 357. (24)Sasaki, T.;Kushima, K.; Matsuda, K.; Suzuki, H. Bull. Chem. SOC.Jpn. 1980,53, 1864. (25) Zana, R.;Lianos, P.; Lang, J. J . Phys. Chem. 1985,89,41.

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Veggeland and Nilsson

Table 2. Micellar Diffusion Coefficients ( m 2 W with added xanthan 0.05% 0.10%

0

with added PE020:

0.14%

0.05%

2.10 x lo-" (0.98) (C&C&S03- + 0.5%NaCl 2.15 x lo-" 2.03 x (0.94) (0.94) (C&CEzS03- + 1%NaCl 5.76 x (C&C?&SO3-+ 1%NaCl 6.15 x 6.25 x (1.02) (C&CE3SOs- + 4%NaCl 9.62 x (C&CE3S03- + 4%NaCl 9.74 x (1.01) C1&ySO3- + 1.5%NaCl 4.55 x 4.34 x 10-l2 (0.95) 4.41 x (0.97) C1&yS03-+ 1.5%NaCl 5.64 x 10-l2(1.24) a The number in parantheses gives the ratio D,idDfic (no added polymer). Concentrations: 1.5%surfactant + additives as indicated above, for the surfactants (C&C&S03- and (C&CE3S03-, 1.5%isobutyl alcohol was also added. All solutions were equilibrated against

oil.

Table 3. Micellar Diffusion Coefficients (m2/sIa mic diff coeff (m2/s)

system

(C&CE3S03(C&CE2S03-

+ 3.6%NaCL + 0.04%xanthan + 1.5%NaCl + 0.14%xanthan

2.0 x 7.0 x

(2.25)

(1.05)

a The number in parantheses gives the ratio DmidDfie(no added polymer), of phase-separated samples (see text). Concentrations: 1.5% surfactant 1.5% isobutyl alcohol. All solutions were equilibrated against oil. The concentrations are overall concentrations prior to phase separation.

+

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25

Retention time (min)

Figure 6. GPC chromatogramsof 0.15%xanthan (-1 and 0.005 m (C&CE3S03- (- -1both with 0.01 m (C&CE3S03- as solvent and eluent, The flow rate was 0.5 mumin and an RI detector was used.

5

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25

Retention time (min)

Figure 5. GPC chromatogramsof (a)0.29%PE020 and 0.01 M NaCl as eluent, (b) 0.24%PE020 with 0.01 m C1&ySO3- as solvent and eluent, and (c) 0.13% PE020 with (C&CE3S03- as solvent and eluent. The flow rate was 0.5 mumin and an RI

detector was used.

ratio was found to be 0.20 f 0.02 mol of C1~yS03-/mol of EO unit. The ratio was measured for different PEO concentrations. It did not vary much with varying concentrations, but there was a slight tendency of decreasing ratio for increasing polymer concentration. This is consistent with results from Sasaki et al.24 The GPC results were different depending on the number of ethoxy groups, for the alkyl ethoxylated sulfonates. Injecting PE020 in (C&CE1S03- solution gave chromatograms similar to that of Figure 5b showing association. Inserting two additional EO groups gave a different result. For ( C ~ ) Z C E ~ Sthe O ~ -PE020-related peak did not shift in retention time, and just a small deviation from the base line is observed where other systems show negative peaks. No association is seen here. Also a chromatogram (data not shown) for an alkylethoxysulfonate with six EO groups showed no change in the retention time for PE020 and no vacant peak was observed. When the polymer and the surfactant micelles have the same charge, more complex chromtograms are obtained. Figure 6 shows 0.15% xanthan dissolved in the (C~)&E~SOSeluent, together with a chromatogram where the sample contains (C&CE$303- micelles a t a concentration lower than the eluent concentration. The peaks a t 15 min are micelles, as can be seen from the injection

of only micelles. There has been a separation of xanthan and the surfactant in the column. The last peak is NaC1. To study the separation mechanisms and interactions in more detail, systems consisting of two nonionicpolymers were used. In Figure 7a, 0.6% PE04 is the eluent and 0.5% T500 is dissolved in the eluent and the mixture injected. Two peaks occur, a t retention times characteristic for the two polymers respectively, with T500 eluting first. In Figure 7b, the sample is also 0.5%T500 dissolved in the eluent, which is 0.5% T10 in aqueous solution and 0.5%T10 in 0.5 M NaCl solution, respectively. The peaks show that the two polymers are eluting first, followed by a NaCl peak. The area of the T10 peak has decreased when NaCl is present. The polymers are also separated in the column, showing that electrostatic repulsion cannot be the main separation mechanism.

Discussion PO-S-. An associative complex between the nonionic polymer PE020 and the anionic surfactant C12XySOswas expected to be formed. Our results confirm this with all the different methods applied. From Table 1it can be seen that GPC results gave a molar binding ratio of 0.2. For the well-known system of sodium dodecyl sulfate and PEO in water, the same ratio is 0.33l and even higher when there is salt present. The difference might be caused by fewer interactions between PEO and the aromatic group than between PEO and a straight alkyl chain or just by a difference in micellar size. Table 2 shows that the diffusion coefficientof the C1ayS03- micelles with PE020 added increases compared to that of the polymer-free system. The aggregation number of the micelles or clusters in a polymer-surfactant complex is less than in ordinary micelles.z5 The self-diffusion of PE020 is also highzz and may have contributed t o the diffusion enhancement. From Figure 1 the collapse of the middle phase microemulsion when PE020 was added also verifies the association. When there is no association, watersoluble polymers will partition between the lower and middle phase, depending on their molecular size.13 One

Langmuir, Vol. 11, No. 6, 1995 1889

Study of Polymer-Surfactant Interactions example is the phase results of (C&CE3S03- with the polymers xanthan, AF’1135A, and PE020. Analysis of the phases showed that the large polymers like xanthan and the poly(ary1amide) were found solely in the lower phases whereas PE020 was distributed between the lower and the middle phase. Even though (C6)2CE3S03- is a n anionic surfactant, we saw no association with PE020 (Figure 5c). The PE020 retention times in the GPC chromatograms did not shift when the eluent was changed to the surfactant solution. This is supported by the micellar diffusion coefficientthat did not vary when polymer was added, as is seen from Table 2. However, when the number of EO groups on the surfactant decreases, association between the surfactant and PEO starts to increase, and the amount of surfactant associated with polymer increases. This is quantified in Table 1. Viscosity measurements by SaitoZ6also indicate no association when the number of EO groups of the surfactant exceeds four. The lesser degree of association when the degree of ethoxylation increases can be explained by reduced interactions between the alkyl chains and the polymer due to surfactant head group screening effects and a reduced hydrocarbodwater contact in the micelle/ cluster. Also the cmc is already low for these surfactants and there is not much free energy to gain by the system to let the polymer associate with the micelles. P--S-. Also in the phase behavior studies with the alkylethoxysulfonates and xanthan, large differences were seen when the EO number was varied. The transition from a two-phase solution to a three-phase solution for (C&CE3S03- occurred at a salinity far below the transition salinity for the polymer-free system; see Figure 3. For (C&CEzS03-, the same phase separation occurred but for a higher polymer concentration. The middle phase appeared to be different from a normal type I11 middle phase when xanthan and polyacrylamide were added. Selfdiffusion measurements showed that a phase separation between the micelles and polymers without any structural changes had occurred (Table 3). At higher salinity, the middle phase becomes a n ordinary type I11 phase with solubilized oil. A phase separation of micellar surfactant phase from a polymer-rich solution has been observed earlier both for ionic27and nonionic surfactant^.^^^^^ This segregative phase separation into two aqueous phases is analogous with the well-known polymer-polymer incompatibility in a common solvent and is discussed more in the theoretical model section. These different types of phase separations are not seen by GPC, but no association is observed for the alkylethoxysulfonates and xanthan. Systems that show a segregative behavior or repulsive interactions give quite complex chromatograms, as is seen in Figure 6. We observe a separation of the components. This is important for what we believe is one of the main mechanisms of LTPF, that the polymer moves ahead of the surfactant.8 As mentioned above, the polymer and the surfactant in Figure 6 have been separated. We believe that this dynamic separation is mainly due to a n osmotic effect which leads to a separation of both micelles and NaCl from the polymer. By studying two well-defined polymer with the same GPC-approach, the discussion below shows that this is the case. Po-Po. The phase behavior of PEO and dextran has been studied in detail.30 The polymers segregate into two phases a t high concentrations. The GPC chromatogram (26) Saito, S.J. Colloid Interface Sci. 1960,15, 283. (27) Wormuth, C. Langmuir 1991,7, 1622. (28) Gerdes, S.Personal communication. (29) Albertsson, P. A. Biochim. Biophys. Acta 1958,27, 378. (30) Gustafsson, A.; Wennerstrom, H.; Tjerneld, F. Fluid Phase Equilib. 1986,29,365.

1‘0

1‘5

._

2’0

Retention time

2‘5

(min)

Retention time (min).

Figure 7. (a)GPC chromatogram of 0.6% T500 in 0.6%PE04(aq) as solvent and eluent. The flow rate was 0.5 mumin and an RI detector was used. (b) GPC chromatograms of 0.25% T500 in 0.25% TlO(aq)(-1 as solvent and eluent and in 0.5% T10 in 0.5 M NaCl (. as solvent and eluent. The flow rate was 0.5 mumin and an RI detector was used. a)

in Figure 7a shows here a separation, indicating a n effective repulsion in the system, which is in good agreement with the reported phase behavior. Also, the Po-Po system consisting of two dextrans with different molecular weights in Figure 7b, shows that a separation takes place in the column, even when there are almost no net interactions present. In the injected sample consisting of 0.25%, T500 dissolved in the eluent, 0.5% TlO(aq1, the chemical potential of water will be lower than in the surrounding eluent of only 0.5% T10. The osmotic pressure difference between the eluent and the injected sample will then provide a mainly entropic, thermodynamic driving force for the water molecules to diffuse into the sample to gain osmotic equilibrium. To maintain a constant volume of the injected solution, other components have to diffuse out of the sample. The smallest components will have the highest rate of diffusion and for the aqueous case it leads to a peak of T10 which has been separated from T500 molecules due to osmosis and of course size exclusion. The retention time of T10 is the observed retention time for T10 also with water as eluent in the same column and with the same conditions, so a “complete” separation has taken place. To verify this dynamic separation mechanism, the same system, but with 0.5 M NaC1, is injected and the results are compared. As seen in Figure 7b, the components diffusing from the injected sample are mainly the salt ions. In a sample with several constituents, the components with the fastest diffusion will take over the response to give osmotic equilibrium a s water diffuses into the sample. As can be seen, the T10 peak has almost disappeared. If we relate these findings to the P - 4 - system discussed above, the surfactant micelles and the polymer are separated in the column. Any direct electrostatic repulsion effect, because oftheir equal charges, is negligible compared to the osmotic effect. Charged macromolecules will give a larger osmotic effect due to mobile counterions. Charged macromolecules will also influence the distribution of salt between injected sample and surrounding eluent, which is known as the Donnan effect. . Osmotic effects are also seen in the phase behavior studies. When water soluble polymers are added to surfactant-brine-oil systems, the lower phase volume increases and the volume of the middle phase decreases; see Figure 2. Water is removed from the middle phase

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1890 Langmuir, Vol. 11, No. 6, 1995

due to osmotic effects. The largest effect is seen when the polymer remains in the water phase. When the polymer partitions between phases, the changes in volume become fewer. The same has been reported for nonionic surfact a n t ~where , ~ ~ also an oil soluble polymer was tested and had the same effect on the oil phase volume. For the nonionic surfactants all phase volumes changed when polymers were added. In this study the volumes of the upper oil phase were constant.

Theoretical Modeling Effect of Polymer on the 11(-)/I11 Transition. The effect of hydrophilic polymers on the 11(-)/(III) transition can be rationalized with a simple Flory-Huggins approach. At the 11(-)/I11 transition the oil content in the brine and middle phase is low, about 2-4%, and is therefore neglected since it is low enough to be neglected in the mass balance and furthermore is a t a constant chemical potential (the latter follows from the fact that the oil phase is almost pure oil). In the II(-) state, neglecting the oil phase, the free energy, G I I ( - ~of, the system in a Flory-Huggins type of description becomes

the II(-) state were arbitrarily chosen to be zero. The advantage with this treatment is that it is not necessary to specify the type of structure in the middle phase. The standard chemical potentials of water and polymer in eq 4 will be the same as in the II(-) state, i.e. zero, since there is no special structure in the aqueous phase. In eqs 3 and 4,it has also been assumed that with high molecular weight hydrophilic polymers the aqueous phase contains practically no surfactant in the I11 phase state and that the middle phase is free of polymer. The electrostatic contributions to the free energy cannot be calculated explicitly without knowledge about the structure of the surfactant aggregates, but the dependence of the salt concentration is usually a logarithmic function of the salt concentration for a given structure. Since the polymer chains are not involved in any structural change, the coefficient describing the salt dependence, kl, is the same in eqs 2 and 4. The chemical potentials are obtained by differentiation of the free energy with respect to composition in the usual way. The chemical potential of water in the middle and aqueous phases, respectively, becomes

+XPW4Ph +X s d s h + & In & + C4dNp)In 4p + (4s/Ns)In 4s +

GII(-w%ot)

= XPS4P4S

4$11n Csalt + 4sk2 1n Csalt (2) where k is the Boltzmann constant, T is the absolute temperature, and ntotis the total number of moles in the system. Indices P, S, and W represent polymer, surfactant, and water, respectively. The interaction parameters are xps (polymer-surfactant), XPW (polymer-water), and xsw (surfactant-water) h,@s, and & are the volume fractions of polymer, surfactant, and water. Np and NS are the degrees of polymerization of the polymer and surfactant micelles. The relation between N S and the true micellar aggregation number is of course only qualitative. Two terms have been added to the standard Flory-Huggins expression, @pk1 In csaltand @& In csalt,which represent the salt dependent electrostatic free energy of the polymer, k1, and the surfactant, kz, and which will be further discussed below; csaltis the salt concentration. The free energy of the I11 phase state is, since the oil component is neglected, described as a two-phase state. Since the oil is a t a constant chemical potential and the oil content in the middle phase hardly varies on addition of polymer, the free energy contribution is almost constant and is therefore included in the standard chemical potentials. The free energy of the middle phase, G M , becomes

GMI(kT(ns+ nW,,)) = 4&" + &pWo+ x S d s & + & In & + (4S/Ns)In 4s + @sk, In Csalt (3) and the free energy of the aqueous phase, G A ~ ,

+ & In & + (4dNp)In #p + In Csalt (4)

GAd(kT(np+ nw,Aq))= xPdp&

and

The chemical potentials for the water, surfactant, and polymer components, respectively, in the II(-) state analogously become pw,II(-) = kT[ln XPW@P(l

& + 1 - & - 4S/Ns - 4p"p + - &) + X S d S ( 1 - &)

(7)

ps,II(-)= kT[(l/Ns)In 4s + (l/NS)(l-

4% + Xsw&(l - 4s) + XPS4P(1

- 4P/Np- 4s) - X P W 4 P h + k2 In cSalt(8)

and

- kTt(l/Np)In 4p + (l/Np)(l- 4p)& + X P h ( 1 - 4d + XPS4S(l - 4P) - X s d s & + k, In csalt1(9)

PP,II(-)

To decide a t which salinity the II(-)/III transition takes place, a value of the polymer concentration, which might be zero, in the aqueous phase of the I11 phase state is selected. The chemical potential of water in the aqueous phase can now be calculated directly by eq 6. Since the aqueous phase is in equilibrium with the middle phase, the chemical potentials of water in the two phases must be equal (eqs 5 and 61, pW,M

where ns and np are the number of moles of surfactant and polymer, respectively, and nW,M and nwpqare number of moles of water in the middle and aqueous phases. In eq 3 the standard chemical potentials of surfactant and water, ,uso and pw0, have been included to represent the structural change between the normal micellar structure in the II(-) state and the middle phase in the I11 phase state. Since the absolute value of the chemical potentials are of no relevance, all standard chemical potentials in

- XPS4P4SI

+

= pW,Aq

(10)

and since & @s= 1in the middle phase, it is possible to calculate the volume fractions in the middle phase. The compositions in all the phases can now be obtained and the corresponding free energies, eqs 2-4,can, apart from the salt dependent terms, be calculated. The I11 phase state is favored if GM + G A G ~ I ( - )At . the transition the two states have equal energy and the salinity a t which the

+

Study of Polymer-Surfactant Znteractions

Langmuir, Vol. 11, No. 6, 1995 1891

transition takes place can be obtained by adjusting the salinity so that the relation (11) is satisfied. Phase Separationof Polymer and Surfactantinto Two Aqueous Phases. The discussion above dealt with the effect of polymer on a phase transition that exists also in the absence of polymer. The presence of polymer can however introduce a second type of phase separation, the separation of surfactant and polymer into two different aqueous phases. To phase separate into two different phases with one phase enriched in polymer and the other in surfactant, in the micellar state, is a quite usual p h e n ~ m e n a ~ ~and J ’ ,is~completely ~ ~ ~ ~ analogous with the separation of polymer 1-polymer 2-solvent systems into two phases enriched with respect to each polymer.16 Since oil is present as a n external phase, this phase transition also results in going from a two-phase state into a threephase state. The calculations for this type of phase separation are based on eqs 7-9 with the equilibrium criteria that the chemical potential of a component is equal in all the phases that are in equilibrium with each other. That is to find conditions at which it is possible to have (12) and equivalently for the other two components, surfactant and polymer, and where not all of the volume fractions, @w,4s and +p, are equal in the two aqueous phases 1and 2. Model Parameters. To perform the calculations in practice the following set of parameters needs to be specified: k l , 122, k3, A”, pWo,X P W , XPS, X S W , N P , and N s . However, some of these can be determined by independent data in the following way. The shift in salinity of the 11(-)/I11 phase transition induced by polymers is given by eq 11 with the result that the kl term cancels out and need not be specified. The k2 and K B terms do not have to be specified independently; it is instead the difference kz - k3 which is relevant and has to be left as a fitting parameter. Of the three x parameters it is only the X P S parameter that describes the polymer-surfactant interaction that is important. The xpw and xsw parameters that give the polymer-water and surfactant-water interaction, respectively, influence the phase behavior to a lesser extent in the present case. The number of repeating units in the polymer chain, Np, is given directly by the molecular weight. The corresponding number of repeating units for the surfactant, N s , reflects the surfactant aggregation number and is in principle different in II(-) and I11 states, but for reasons given in the next section, the actual value ofNs has almost no effect on the II(-)/III transition. The standard chemical potentials of surfactant and water,psoandpw”,in the middle phase can be determined by taking the experimentally determined salinity for the II(-)/I11 transition in the absence of polymer in combination with experimental values for the composition of the middle phase, a t the transition, and calculate the values of the standard chemical potentials needed to satisfy eqs 10 and 11. To summarize, only three parameters are both important and not directly given by the theoretical model, they are k2 - K 3 , x p s , and Np. Model Predictions. Some of the above experimental features of phase behavior of surfactants and hydrophilic polymers can be understood by the rather simplistic theoretical model. An example of a theoretically calculated phase behavior is given in Figure 8. The curve gives the calculated shift in the salinity of the 11(-)/I11 transition

0

0.02

0.04

0.06

0.08

polymer-conc. (wt%)

Figure 8. Model calculation (-) of the effect of added polymer on the II(-)/II transition compared to the experimental curve (. .) of (C&CE$303- and polyacrylamide also shown in Figure 3. Parameter values are XPS = 0.2,XPW = 0.1, xsw = 0,NP = 1000,Ns = 100, and kz - k3 = 0.005.

as a function of added polymer where the parameters have been adjusted to resemble the system polyacrylamide sulfonate (C&CE3S03-. In the theoretical calculations it was found that the values of xsw and Ns have very little effect on the calculated curves. The reason for this is that the transition salinity in the absence of polymer, Csalt,O is a n experimental input parameter. Varying xsw or N s , with c,,lt,O being fixed, only changes the values of the standard chemical potentials, p,O and pw0, which are calculated as described in the theoretical section. The value ofXpw, representing the polymer-water interaction, has some effect on the curve, but it is limited to the interval 0 -= xpw 5 0.5. Of the other parameters it was found that the value of the parameter K 2 - k3, representing the electrostatic free energy difference between the middle phase and the II(-) phase per surfactant molecule, has to be very small if added polymer should have any effect on the transition salinity. The value of K z - k3 in the calculations of Figure 8 was 0.005KT. Such a low value means that the average surface charge density and curvature experienced by the surfactant head group must be very similar in the two phases. The conclusion that the surface charge density and curvature must be very similar in the two different phase states close to the II(-)/III boundary is not model dependent as might be expected. It follows from the observation that addition of 0.01-0.10% of polymer represents a small free energy change and if the polymer is to have any effect on the transition salinity it is simply not possible to have a large electrostatic free energy difference between the two phase states. The effect of polymer is to make the II(-) state energetically more unfavorable due to the polymersurfactant interaction as given by xps. Finally the two types of phase separations encountered in the experimental data above are also predicted by the theory. If the polymer-surfactant interaction is made more unfavorable, the appearance of the theoretical curve is given in Figure 9, which at least in part resembles the experimental result. The phase separation to the right of the “knee”in Figure 9 is the formation of one polymerfree micellar phase and one surfactant-free polymer phase. The driving force for this kind of phase separation is that as the polymer concentration increases it becomes energetically more unfavorable to mix polymer and surfactant in the same phase. If the polymer-surfactant interaction parameter, x p s , is made more positive, the position of the “knee” will be shifted to the left. Also this kind of phase transition is favored not only by a high polymer molecular weight but also by a large micellar aggregation number. Conditions which induce micellar growth can therefore

1892 Langmuir, Vol. 11, No. 6, 1995

. 5 .......................................... 2' 0

Veggeland and Nilsson

....... ................

-Y % z

0

-theory

..*.. 0

0.03

0.06

0.09

0.12

polymer conc. (wt%)

Figure 9. Model calculation (-1 of the effect of added polymer on the II(-)/III transition compared to the experimental curve (.*) of (Cd2CE2S03- and xanthan also shown in Figure 4. Parameter values are XPS = 0.6, xpw = 0.1, xsw = 0,N p = 1000, Ns = 100, and kz - k3 = 0.005.

also induce a phase separation of the above type. I t seems likely that changes in the micellar aggregation number are the reason for the absence of phase separation at zero salt concentration. However, to calculate salt-induced changes in the micellar aggregation number is beyond the scope of the simple model and the theoretical curve in Figure 9 is therefore ended by a dotted line.

Conclusions For nonassociative polymer surfactant systems osmotic effects are important. Water soluble polymers reduce the

salinity for a II(-)/I11 phase transition in microemulsions and decrease the water content in the middle phase due to osmosis. When the hydrophilic part of the surfactant is increased by inserting EO groups, the polymer and the surfactant separate into a micellar and a polymer phase. The middle phase collapses when a n associative polymer is added to a microemulsion. The association between ethoxylated sulfonates and polyethylene oxide decreases when the ethoxylation degree of the surfactant increases. No association is found when the number of EO groups exceeds three. The GPC method is most suitable for studying associative polymer surfactant systems. When the surfactant solution is the eluent, nonassociative polymer surfactant systems, like P--S-, separate in the GPC column due to osmosis and size exclusion. The phase separation was modeled by a Flory-Huggins approach. The transition between the two types of phase separation mechanisms was also reproduced by the model.

Acknowledgment. K. Veggeland is indebted to The Norwegian Research Council, NFR, for financial support. The work is partly funded by the state supported programme on Reservoir Utilization through advanced Technological Help (RUTH). The authors also thank Norsk Hydro for delivering the ethoxylated sulfonates and the department Physical Chemistry 1, University of Lund, for giving access to their NMR diffusion equipment. LA940741B