Langmuir 1987,3,25-31 clays represented in figure 5 and the values ki that are characteristic of the water properties represented in Figure 7, we obtain the relations between II and Ji/J? shown in Figure 8. Since Sun et a1.20have reported the values of kifor several additional water properties, II can also be related to them. Thus, we have found a general relation between the swelling pressure of a clay-water system and any property of the water within it. Such a relation would be expected only if both depend on the same factor.
25
Consequently, we conclude that interaction between the water and the adjacent surfaces of the clay layers affects the structure of the water and, thereby, ita structuresensitive properties. Since one of these properties is G,, II is also affected in accordance with eq 1. I t may be said, therefore, that the structural component of II is primarily responsible for the swelling of clays. Or,alternatively, it may be said that the swelling of clays is due to hydration of their surfaces.
Aqueous Biphase Formation in Polyethylene Oxide-Inorganic Salt Systems K. P. Ananthapadmanabhan and E. D. Goddard* Research and Development Department, Specialty Chemicals Division, Union Carbide Corporation, Tarrytown, New York 10591 Received April 22,1986. In Final Form: September 29,1986 Inorganic salts such as Na2S04,MgS04, and Na3P04have been reported to form aqueous two-phase systems with polyethylene glycols (PEGS). Recent studies show that the above phenomenon is very general in the sense that a number of inorganic salts, even certain uni-univalent salts, form aqueous two-phase systems with PEG. The relative concentration of various salts to form two-phase systems was found to depend upon the valency and hydration (size) of the ions as well as "specific" interactions of the ions with the polymer. Possible mechanisms leading to the formation of PEGinorganic salt-water aqueous two-phase systems are discussed.
Introduction tems for various bioseparations. Recently, a number of An aqueous solution of polyethylene glycol 3350 (4%)other investigators have used such systems for bioseparations6-14and to study the fundamentals of such partiwhen mixed with an aqueous solution of dextran (-5%, t i ~ n i n g . ~ ~ ~The J ~ Jmost ~ J ~widely used phase system in MW 5 X los) or an inorganic salt, such as potassium such studies has been the PEGdextran system. As phosphate or magnesium sulfate, separates into two mentioned earlier, PEG also forms two-phase systems with phases. The unique feature of this two-phase system is inorganic salts such as potassium phosphate. The use of that both phases have almost 90-95% water. Also, each PEGpotassium phosphate systems for the purification of of the phases is relatively rich in one of the solute comindustrial enzymes has been pioneered by Kula and coponents. Reported results' show that the density differworker~.~~*'~ ence between the two phases is small and the interfacial Fundamentals involved in the formation of polymertension between the two phases is small and in the range polymer-solvent two-phase systems have been examined of lo4 to lo-' mN/m. Aqueous two-phase systems of this to some e~tent.'~-~O Scott,17for example, has conducted type have been reported by Beijernick2 as early as 1896. a thermodynamic analysis of two-phase formation and has Almost 50 years later, Dobry and Boyer-Kawenoki3 condeveloped equations to predict critical conditions for phase ducted a systematic study of the miscibility behavior of separation in certain model systems. Phase-forming pairs of different polymers in aqueous and nonaqueous media. The results of their study clearly showed that incompatibility between polymers, leading to two-phase (6) Walter, H.; Brooke, D. E.; Fisher, D. Partitioning in Aqueous Two formation with the same solvent in both the phases, is a Phase Systems; Academic: New York, 1985. very general phenomenon. (7) Walter, H. In Methods of Cell Separation; Catslmpoolaa,N., Ed.; The biaqueous nature of the two-phase system and Plenum: New York, 1977; Vol. 1, p 307. (8)Johansson, G.Biochim. Biophys. Acta 1970,221, 387. difference in properties of the phases makes it possible to Taylor, P.; Barondes, 5.H. Nature (London) 1975, (9) Flanagau, S.D.; use them for the partitioning and separation of biological 254. _. 441. materials such as cells, organelles, enzymes, proteins, etc. (lO)Fisher, D. Biochem. J. 1981,196,l. Note that the use of oil-water-type two-phase systems for (11) Harris, J. M.;Case, M. G.; Hovanes, B. A. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 86. such biological separations may not, in general, be feasible (12) Matiasson, B.; Ling, T. G. I. J. Immunol. Methoda 1980,38,217. because of possible denaturation of biologically active (13) Kula,M.R.;Kroner, K. H.; Hustedt, H. Adu. Biochem. Eng. 1982, components in the nonaqueous medium. Albertsson4i6has 24, 73. (14) Kroner, K. H.;Hustedt, H.; Kula, M. R. Process Biochem. 1984, done pioneering work on the use of aqueous biphase sys170. (15) Brooke, D. E.;Seaman, G. V. F.; Tamblyn, C. H.; Walter, H. ~
(1) Albertsson, P. A. Partition of Cell Particles and Macromolecules; Wiley-Interscience: New York, 1971. (2) Beijernick, M. W. Zbl. Bakt. 1896,2,627,698; Kolloid-2. 1910, 7, 16. (3) Dobry, A.; Boyer-Kawenoki, F. J. Polym. Sci. 1947,2 (I), 90. (4) Albertsson, P.A. Biochim. Biophys. Acta 1968,27,378. (5) Albertason, P. A. Adu. Protein Chem. 1970, 24, 309.
Biophys. J. 1976,15, 142a. (16) Bamberger, S.; Seaman, G. V. F.; Brown, J. A.; Brooks, D. E. J. Colloid Interface Sci. 1984, 99 (l),187. (17) Scott, R.C. J. Chem. Phys. 1949, 17 (3), 268. (18) Tompa, H.Trans. Faraday SOC.1949,45, 1142; 1960, 46, 970. (19) Flory, P.J. Principles of Polymer Chemistry; Comell Univ.: 1953. (20) Ogston, A. G.Biochem. J. 1970,116, 171.
0743-7463/87/2403-0025$01.50/00 1987 American Chemical Society
26 Langmuir, Vol. 3, No. 1, 1987
Ananthapadmanabhan and Goddard
Table I. Inorganic Salts Forming Two-Phase Aqueous System with PEG NaOH LiS04 Na3P04 MgSOi (NH4)2S04 sodium formate (Al)2(S04)3 NaF Na2C03 ZnS04 sodium succinate alum NazSi03 CuS04 sodium tartrate Na,SO, FeS04 sodium citrate
A
characteristics of polymers with inorganic salts have not been studied systematically. In the present investigation, the aqueous two-phase formation characteristics of PEG 3350 with a number of inorganic salts have been studied systematically and phase diagrams have been generated for such systems. Results obtained have been analyzed to develop a better understanding of the mechanisms involved.
I 0’
a
I
lh
48
NazS04. PEG 2 . PHASES
2’0
2h
Results and Discussion A series of sodium salts with different anions and a series of metallic sulfates were tested for their phase-forming characteristics with PEG 3350. A list of salts which formed aqueous-two-phase systems with PEG is given in Table I. Among the uni-univalent sodium salts, hydroxide and fluoride were the only two which caused two-phase formation. Chloride, bromide, iodide, and iodate, acetate, nitrate, etc. were not effective. In a given series, substitution of Na with other cations such as K or NH4 led to comparable phase formation. It is interesting that none of these salts form two-phase systems with another widely used polymer, viz., dextran. The effectiveness of various salts to form aqueous twophase systems with PEG can be seen from the phase di-
I $2
PEG, 3350, %
2.0
0
Experimental Section Materials. Polyethylene glycols of average molecular weight 1450,3350, and 8000 used in this investigation were all products of Union Carbide Corporation. Polyethylene glycol of molecular weight 20000 was purchased from Nippon Oil Co. All the inorganic salts used were of AR grade and were purchased from Fisher Scientific Co. Methods. Initial tests to determine whether or not a particular salt will form an aqueous-two-phase system (hereafter “two phase” in this paper refers to “aqueous two phase” unless otherwise specified) were conducted by mixing a 50% solution of PEG 1450 with a saturated solution of the inorganic salt. In cases where no aqueous-two-phase formation was observed, additional amounts of solid PEG 1450 or inorganic salt were added to establish the absence of two-phase formation. Phase diagrams for systems which exhibited two-phase formation were determined in the following manner. Beginning with a particular level of the polymer and the inorganic salt where two-phase formation was found to occur, the system was titrated with water to the point where two-phase formation just ceased to occur. In all cases, after addition of water, the system was shaken throughly and then left undisturbed for further observation. Invariably, systems which separated into two aqueous phases were cloudy in their mixed state. The two separated phases were themselves transparent. In fact, instantaneous formation of a cloudy solution upon thorough shaking was a clear indication of aqueous two-phase formation. The mixed systems were left undisturbed for about 30 min for observation. If the solutions remained turbid, the samples were centrifuged to enhance the phase separation. In these tests, the true aqueous two-phase system separated out into two optically clear phases with a sharp phase boundary. If precipitation of a “solid” phase was responsible for the turbidity in the mixed state, a clear solid mass was found to settle at the bottom of the centrifuge tube. Thus, it was fairly easy to distinguish the turbidity due to solid precipitation from that due to the mixing of two aqueous phases. The concentrations of the polymer and salt were calculated a t the point where the two-phase formation just ceased to occur and these were taken as the phase boundary between the aqueous two-phase system and the homogeneous aqueous solutions.
2b
NAFORMATE
1.5
-1
P
41.0
8 3
3 0.5
0
I
8
I
I
I
16
24
32
PEG 3350,%
F i g u r e 1. (A) Phase diagram for the formation of aqueous two-phase systems in PEG-Na2S04 solutions. Phase formation occurs above the curve. (B)Phase diagrams for the formation of aqueous two-phase systems in P E G N a salt solutions. Phase formation occurs above the curve.
Na > Mg>
- ,
0
8
18 PEG 3350,%
Zn> Li
24
32
Figure 2. Phase diagrams for the formation of aqueous twephase systems in PEGmetallic sulfate solutions. Phase formation occurs above the curve.
agrams given in Figures 1 and 2. Figure 1A represents a typical phase-boundary curve with all the data points. F o r clarity reasons, the data points are not shown in Fig-
Langmuir, Vol. 3, No. I, 1987 27
Aqueous Biphase Formation Table I1 ~~
ion
lyotropic no.
5042PO4“
3.2
F OH-
c1Br-
NOSISCN-
2.0
4.8 5.8 10.0 11.3 11.6 12.5 13.3
aqueous two-phase formation
Yes Yes Yes Yes
no no no
no
ures 1B and 2. Note that for all other systems also, the boundary has been obtained by determining the transition single-phase-two-phase compositions with at least as many points and with a similar spread. The scatter in the data was also similar to that in Figure 1A. For each salt, the region below the indicated curve represents homogeneous solutions and above, a two-phase region. It is evident from Figure 1B that the higher the valency of the anion, the lower the concentration required to form a two-phase system. Thus, PO4* is more effective than OH- and SO:in “salting out” the PEG to form the two-phase system. Interestingly, this trend with valency is not seen with cations; among sulfate salts of metallic cations, for example, Na2S04is more effective than sulfates of Cu, Zn, Fe, and Mg. Multivalent cations, in general, have been suggested to interact strongly with the ether oxygens of PEG.21 In fact, polyethers, because of this complexing property, have been considered to constitute “flexible” crown ethIf multivalent ions do interact with PEG, one can expect them to “salt in” rather than “salt out” the PEG. Thus, in the case of sulfates of Cu, Zn, Fe, and Mg, the salting out tendency of S042-is offset to some extent by the metallic cations and this can account for the apparent reversal of the effectiveness of salts to form the two-phase system. Water is the main component in each phase and the dissolved components have high affinity for it. The ethylene oxide (EO) units of polyethylene glycols are known to be hydrated strongly with two to three water molecules per EO g r o ~ p . ~The ~ - high ~ ~ water solubility of polyethers has, in fact, been attributed to this factor. Similarly, ionic species in solution are known to be hydrated and the extent of hydration depends upon the valency of the ion.26 Thus, triply charged phosphate can be expected to be more effective than the doubly charged sulfate and the singly charged hydroxide in salting out polyethers because of competition for water. In the case of cations, one can expect competition between two opposing effects, namely, hydration and tendency to complex with polyether oxygen, as is seen in a series of alkali metal sulfates. According to hydration tendencies, Li should be more effective than Na, but is observed to be otherwise. This discrepancy is attributable to the strong specific interaction of Li with the EO groups. The effectiveness of multivalent ions in destabilizing colloids is expressed by the well-known Hofmeister series (21) Colwell, C. E.; Livengood, S. M. J. SOC.Cosmet. Chem. 1962,13 (5), 201. (22) Harris, J. M.;Hundley, N. H.; Shannon, T. G.; Struck, E. C. In Crown Ethers and Phase-Transfer Catalysis in Polymer Science; Mathiaa, L., Carreher, C. E., Eds.; Plenum: New York, 1984; p 371. (23) Molyneux, P. In Water; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, p 569. (24) Lin, K. J.; Parsons, J. C. Macromolecules 1969, 2, 529. (25) Maxfield, J.; Shepherd, I. M. Polymer 1975, 16, 505. (26) Bockris, J. O.,Reddy, A. K. N. Modern Electrochemistry; Plenum: New York, 1970.
251
-
1
0
1-
0.2
~
1
0.4
0.8
C B
SALT CONC.,mol/l
Figure 3. Effect of inorganic salts on the cloud point of PEO (POLYOX).Data from ref 31.
or the lyotropic ~ e r i e s . ~ ~ -The ~ O data given in Table I1 suggest that the tendency of the anions to form aqueous two-phase systems is closely related to the position of the anions in the lyotropic series. Thus, ions with relatively low lyotropic number tend to form aqueous two-phase systems with PEG. Lyotropic number can therefore be used as a predictive tool for two-phase formation. A related property of ethoxylated materials which is markedly influenced by the presence of inorganic salts is their aqueous solution cloud p0int.~l8~Ethoxylated materials in general exhibit an inverse solubility relationship with temperature. Typical results for the effects of salts on the cloud point of a high molecular weight PE0,31as well as an ethoxylated reported in the literature are reproduced in Figures 3 and 4. The data presented clearly show that the effect of ions in depressing, the cloud point essentially follows the order phosphate > citrate > sulfate > hydroxide. Among sulfate salts, the cations follow the order Na > Mg > Zn > Li. The phase diagrams for aqueous two-phase formation show that the effectiveness of various ions to form aqueous two-phase systems follows precisely the same order. Evidently, the fundamental forces involved in the two processes are the same, viz., in effect a salting out of the PEG. The inverse solubility characteristics of ethoxylated materials have been attributed to the dehydration of the ethoxylated units at higher t e m p e r a t ~ r e . ~ Ionic ~ species which hydrate strongly evidently induce the dehydration of EO units at a lower temperature. Formation of aqueous two-phase systems, on the other hand, can only involve (27) Hofmeister, F.Arch. Exp. Natur. Pharmakol. 1888,24,247; 1890, 27,395. (28) McBain, J. W. Colloid Science; Heath Boston, 1950; p 131. (29) Bruins, E. M. Proc. Acad. Sci., Amsterdam 1932, 35, 107. (30) Voet, A. Chem. Rev. 1937, 20, 169. (31) Bailey, F. E., Jr.; Callard, R. W. J . Appl. Polymer Sci. 1959, I , 56. (32) Schott, H.J. Colloid Interface Sci. 1973, 43 (l),150. (33) Durham, K. In Surface Actiuity and Detergency; Durham, K., Ed.; MacMillan: London, 1961; p 1. (34) Kjellander, R.; Florin, E. J . Chem. SOC.,Faraday Trans 1 1981, 77,2053.
28 Langmuir, Vol. 3, No. 1, 1987
Ananthapadmanabhan and Goddard
50
12
40
30
\
Na Citrate
0
0.1
0.2
0.3
0.4
(
5
SALT CONC., mol/!
Figure 4. Effect of inorganic salta on the cloud point of an ethoxylated nonionic surfactant. Data from ref 33.
partial dehydration of the polymer. Here the polymer and the salt remain strongly associated with the solvent but exclude each other by separating into two phases. The reasons for this incompatibility can only be speculative at this point. According to Flory,= when the concentration of a fairly inflexible polymer is increased in solution, the polymer molecules may undergo a transition from a random-coiltype disorderded configuration to an ordered parallelrodlike structure. Even though such a transition has not been observed for polyethylene glycols in water (as they are fairly flexible molecules), the presence of salt may change the situation considerably. Thus, increasing the concentration of a noninteracting salt at a particular level of the polymer may lead to a situation where a transition from a disordered random-coil configuration to an ordered polymer-rich phase can become energetically favorable. Such a transition should be accompanied by a marked (or a step) change in the solution properties of PEG-salt solutions. Polyme-alt compositions corresponding to levels that exist in the upper phase of a two-phase system when diluted at the same salt level do not, however, show any such marked change in viscosity characteristics. The intrinsic viscosity of PEG solutions has, in fact, been reported to decrease with increase in concentration of salts such as Na2S04and MgS0431and this indicates a collapse of the structure rather than the formation of rodlike configurations. Light-scattering measurements at different salt and polymer levels may provide better insight into the structure/configuration of the polymers under different conditions, and this type of information is necessary to test whether or not such transformation leads to phase separation in the present case. The formation of aqueous-two-phase systems, as mentioned earlier, clearly indicates the mutual exclusion of the salt and the polymer and their high affinity for the solvent. It is possible that even in homogeneous systems (below the (35) Flory, P.J. R o c . R. Soc. London, A 1966, 236, 60, 73.
phase boundary) the ions are excluded from the nearsurface region of the polymer in solution. With increase in the concentration of the polymer or the salt, the extent of exclusion will increase. Ultimately, the system could reach a state where, for entropic reasons, phase formation would become favorable. Exclusion of ions from the polymer molecule-water interface itself can occur for a number of reasons. Both the polymer and the ion are strongly hydrated in solution. Because of the hydration sheath, the near-surface region of the polymer may not be accessible to structure-making ions. Exclusion can occur also by repulsive interaction between the anions and the "anionic-like" polyether functionality. We note that even though the PEG as a whole is nonionic, the lone pair of electrons on the ether oxygen imparts an anionic character to the polymer and this is clearly evident from the binding of multivalent metallic cations by polyethers. Thus a repulsive interaction between the anions and PEG especially in the presence of nonbonding cations, like K+, NH4+, Na+, can occur in the system. Yet another mechanism which has been considered by Garvey and Robb= involves image forces arising from the dielectric discontinuity which exists at the polymer molecule-water interface. The role of image forces can be considered to be similar to that at the air-liquid interface from which ions are known to be excluded, one manifestation of this being an increase in the surface tension of salt solutions over water. In this regard, note that all the tested nonionic polymers which form aqueous two-phase systems with inorganic salts are somewhat "hydrophobic" in the sense that they all lower the surface tension of water. It is possible that all three of these factors contribute to the exclusion of the ions from the near polymer surface region. It should, however, be noted that if image forces alone were operative in the system, salts such as CaC1, or AlCl, should have caused aqueous-two-phaseformation. Since the latter is contrary (36) Garvey, M. J.; Robb, I. 0.J.Chem. Soc., Faraday Trans. 1 1979, 75, 993.
Langmuir, Vol. 3, No. 1, 1987 29
Aqueous Biphase Formation
o 30
10
a?
g-m 3
1
I
I
1.o
0.5
0 0
16
6
Figure 5. Effect of temperature on aqueous two-phase formation
I
I
2.0
2.5
NASALT CONC., mOl/l
24
PEG 3350,%
I
1.5
Figure 6. Setschenow plots for PEG-sodium salt aqueous two-phase systems.
in PEGsodium sulfate solutions.
to the experimental results, in the case of P E G s a l t systems, other factors, such as those referred to above, can also be assumed to play a role. The experimentally observed order of effectiveness among various anions supporta this conclusion. The entropic and enthalpic changes accompanying two-phase formation are complex. A polymer in its dissolved state in water will have several H bonds with the solvent. Thus, while the enthalpic contributions favor dissolution, the loss of entropy of bonded water molecules tends to reduce it. Hydration of the ions upon salt addition also will involve similar entropic and perhaps stronger enthalpic changes. Under conditions when the system separates into two phases, there will be a reduction in the polymer-solvent contact in the polymer-rich phase. Consequently, the enthalpic factors would oppose the two-phase formation, while the entropic factors will favor it. In fact, our estimates of entropy contributions using the equation A S = -R& In & i
where x i = mole fraction, = volume fraction, and R = universal gas constant, shows that AS2-,,h > A S,-. In other words, the entropic factors favor demixing in these systems. The above considerations imply that raising the temperature should favor two-phase formation. In order to test this hypothesis and thus to develop better insight into the molecular mechanisms involved in aqueous two-phase formation, the temperature dependence of phase formation was investigated. Phase diagrams for the PEG 3350Na2S04system at three different temperatures are given in Figure 5. It is seen that at higher temperatures, aqueous two-phase formation occurs at lower levels of the polymer and the salt. Evidently, the delicate balance of entropic and enthalpic changes resulting from the solute-solvent interactions determines the final state of the system. It is clear that the molecular mechanisms involved in two-phase formation are complex and also that additional information on water activity, hydration, etc. is required for development of a quantitative model of the phenomenon. Nonetheless, since the phenomenon is essentially due to the effect of an electrolyte on the solubility of a nonelectrolyte, the possibility of using Setschenow's equation3' to characterize the phase boundary was exam@J~
# 3
11 0
I
I
I
0.4
0.8
1.2
SULFATE SALT CONC., mol/i
Figure 7. Setachenow plots for PEG-metallic sulfate aqueous
two-phase systems.
ined. This equation in its conventional form relates the solubility of the nonelectrolyte in the presence (S) and absence (So) of salt to the electrolyte concentration (C,) as log (S/S,) = KC, where K is the electrolyte-nonelectrolyte interaction parameter. Results given in Figures 6 and 7, in which the logarithm of the PEG concentration required to achieve phase separations is plotted against the salt concentration, show that the aqueous two-phase formation boundary can be characterized by Setschenow-type behavior over a wide concentration range. Interestingly, among sodium salts, the higher the valency of the anion, the larger is K . The absence of such dependence in the opposite case of varying cation, constant anion (viz., sulfate) can be attributed to the specific interaction of the metallic cations with the ether oxygen of PEG mentioned earlier. In any case, the value of K can be taken as an indication of the effectiveness of various salts to cause phase separation. The Setschenow equation was originally proposed as an empirical equation to describe the solubility of nonelectrolytes in electrolyte solutions. It is of interest that rig(37) Setachenow, J. 2.Phys. Chem. 1889, 4, 117.
30 Langmuir, Vol. 3, No. 1, 1987
Ananthapadmanabhan and Goddard
"i 0.7
7
0.6
-
8 y
8
0.5-
0.4-
>
5a
NeS04 Conc. Corrected tor Bound Water
0.3 -
3350
4
NA~SO Conc. ~
12
8
16
20
24
28
32
36
PEG, YQ
0.24
Figure 9. Effect of molecular weight of PEG on aqueous twophase formation in PEG-sodium sulfate system. I
I
4
6
I
I
I
I
12
16
20
24
PEG 3350,%
ei
orous thermodynamic deductions to acount for this phenomenon have led to equations that are similar in form to this equation.38 The K value in such cases has been shown to be related to the activity coefficient of the organic molecule. In this respect, an examination of the phase diagram suggests that the activity of the polymer along the phase boundary is probably reasonably constant. If this is indeed the case, the activity of Na2S04should also be constant along the phase boundary. Deductions concerning the activity of the electrolyte along the phase boundary were therefore made in the following manner. Each EO unit of the polymer has been suggested to bind two or three water molecules. Such binding will result in a reduction in the free water content and consequently in an increase in the effective concentration of the salt. The effective Na2S04concentration a t the phase boundary was estimated on the assumption that three molecules of water are associated with each EO group. Then, with the published information on the activity coefficients of Na2S04,39 the activity of Na2S04was calculated. The results plotted in Figure 8 show that the variation in calculated activity of Na2S04along the phase boundary is relatively small. This is analogous to the formation of a precipitate under conditions where the activity of the precipitating species remains constant in the phase-separation region. In view of the uncertainties involved in the assumptions, viz., the amount of water bound to the polymer and the dependence of the latter on polymer concentration and in the determination of activities at high salt levels, the observed "constancy" over a wide range of the phase diagram is notable. A factor that has not been considered in the above calculations is the exclusion effect. As discussed earlier, sulfate can be excluded from the interfacial region for reasons other than solvation. Such exclusions will result in an increase in the activity of Na2S04in the bulk solution and the overall effect will be higher activity at higher polymer levels. Thus, the small apparent decrease in calculated Na2S04activity a t high polymer levels may be caused, at least in part, by neglect of this factor. From a practical viewpoint, these calculations suggest that if the
8
(38) Long, F. A,; McDevit, W. F. Chem. Reu. 1952,52, 119. (39) Harned, H.S.;Owen, B. B. The Physical Chemistry of Electrolyte Solutions;Reinhold New York, 1950; p 415.
PEG M.W.
5-
Figure 8. Diagram illustrating the almost constant activity of sodium sulfate along the aqueous two-phase formation boundary.
a
I;
1450
4-
3350
3-
fl
ds
2-
20,000
0
1 2
3
4
5
6
7
N A ~ S CONC., O~ %
Figure 10. Effect of molecular weight of PEG on the effective concentration of EO units for aqueous two-phase formation in
PEG-sodium sulfate system.
activity of Na2S04or the polymer is known at one point along the phase boundary, it should be possible to predict the entire phase diagram. In order to test whether or not the information obtained for a particular molecular weight polymer can be extended to predict the behavior of different molecular weight polymers of the same family, phase diagrams for three different molecular weight PEGS were generated (see Figure 9). Evidently the higher the molecular weight of PEG, the lower the salt required for two-phase formation. This is analogous to the fractional/ preferential precipitation of high molecular weight components upon changing the solvent characteristics of the medium. The results of Figure 9, expressed in a different way in Figure 10, show that less EO units are required at higher molecular weights to form a phase system. Even though those results may appear to be somewhat anomalous, considerations of changes in entropy with changes in the number of molecules for different molecular weight PEGS account for the systems' behavior.
Conclusions Aqueous solutions of polyethylene glycols and certain inorganic salts when mixed together under appropriate concentration conditions separate to form a two-phase aqueous system. A systematic study of such systems has been undertaken. The major conclusions from the study are the following: 1. Inorganic salts containing anions with low lyotropic number, high hydration, or high water-structure-making
Langmuir 1987, 3, 31-35 tendency form aqueous-two-phase systems with PEG. Lithium and multivalent cations, because of their interaction with the ether oxygens of PEG, reduce the tendency of the salt to form a two-phase system. 2. Among the various Na salts tested, the higher the valency of the anion, the lower the concentration required for phase formation. 3. The effectiveness of various salts to form two-phase systems with PEG is similar to their tendency to depress the cloud point of polyethoxylates. 4. Factors leading to incompatibility between PEG and inorganic salts in aqueous solution are suggested to be solvation, repulsive interaction between the ether oxygens and the anions, and image forces arising from the dielectric discontinuity at the polymer molecule-water interface. 5. The aqueous two-phase formation boundary can be characterized by the Setschenow equation for electrolyte-nonelectrolyte interaction. Also, along the phase boundary, the activity of Na2S04remains almost constant.
31
It is therefore possible to predict the entire phase boundary based on the information for a single point on the phase boundary. 6. The higher the temperature, the lower the amount of salt required for phase formation.
Acknowledgment. We thank E. Fu and R. Lopresti for their help with the experiments. Note Added in Proof. Further work has indicated that clouding and phase separation in the above systems in fact represent the same phenomenon. These results will be reported elsewhere. Registry No. PEG, 25322-68-3; NaOH, 1310-73-2; NaF, 7681-49-4;Na2C03,497-19-8; Na2SiOB,6834-92-0;Na2S04,7757o ~ , L ~ ~ s10377-487; o~, (NH~)~ 7783-20.2; so~, 82-6; N ~ ~ P7601-54-9; ZnS04,7733-02-0;CuS04,7758-98-7;FeS04,7720-78-7;MgSO,, 7487-88-9; (Al)2(S04)3,10043-01-3; sodium formate, 141-53-7; sodium succinate, 150-90-3; sodium tartrate, 868-18-8; sodium citrate, 68-04-2; alum, 10043-67-1.
Surface Light Scattering Study of Vinyl Stearate and Poly(viny1stearate) Monolayers at the Air/Water Interface Yen-Lane Chen,t Masami Kawaguchi,t Hyuk Yu,*i and George Zografi*t School of Pharmacy and Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received May 12, 1986. In Final Form: September 15, 1986 A surface light scattering technique for probing capillary waves in combination with the Wilhelmy plate method of surface tension measurement has been used to study the dynamic properties of vinyl stearate (VS) and poly(viny1 stearate) (PVS) monolayers spread at the air/water interface. Two molecular weight grades of PVS, 5300 and 3700, gave identical surface pressure-area isotherms and light scattering results. Estimation off,, the frequency shift of the power spectrum at maximum intensity, and the instrumenticorrected full width at half-height,allowed estimation of surface viscoelastic parameters of the monomer and its corresponding polymer. Whereas VS exhibited behavior in complete accord with that observed earlier for small amphiphiles such as pentadecanoic acid and dipalmitoylphosphatidylcholine,i.e., small surface viscosity relative to surface elasticity, above a surface concentration equivalent to 55 A2/monomer PVS exhibited significantly higher surface viscosities, i.e., surface poise, and a predominant contribution over surface elasticity to the overall dynamic modulus (e*). Thus both a quantitative and qualitative difference in surface viscoelasticity for “unconnected”and “connected”monomers in a monolayer are reported for the first time. N
Introduction The study of polymer monolayers spread at the air/ water interface provides a basis for understanding the interfacial properties of polymers adsorbed to any interface. To date, significant attention has been given to the static or thermodynamic properties of protein and synthetic polymer monolayers but much less attention has been given to their surface rheological properties. Of particular interest in the context of this paper are recent studies which have attempted to compare the monolayer properties of a surface-active monomer containing a polymerizable site and its corresponding polymer prepared in bulk or in situ from the spread monolayer. Such studies have been conducted on systems where the hydrophilic group contains the polymerizable site, e.g., odadecyl esters of acrylic acid,’ methacrylic a ~ i d , and ~ J acrylamide4 and vinyl stearate,“8 as well as on systems where the polymSchool of Pharmacy. t Department of Chemistry.
0743-7463f 87 f 2403-0031$01.50f 0
erizable group is in the aliphatic chain, such as heptadecenoic acidjgdiacetylenes,l0 and butadienes.” The intention of such work has been to understand the influence of molecular orientation and packing on polymerization. The methods used included measurements of surface ~
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(1) Beredjick, N.; Burlant, W. J. J. Polym. Sci. 1970, 3, 2807. (2) Hatada, M.; Nishii, J. J.Polym. Sci. 1977, 15, 927. (3) Hatada, M1; Nishii, J.; Hirota, K. J. Colloid Interface Sci. 1973, 45, 502. (4) Ackermann, R.; Inacker, 0.;Ringsdorf, H. Kolloid Z. Z. Polym. 1971, 299, 1118.
(5) Burlant, W. J.; Adicoff, A. J. Polym. Sci. 1968,27, 269. (6) Cemel, A.; Fort, T., Jr.; Lando, J. E.J.Polym. Sci. 1972,10,2061. (7) Letts, A.; Fort, T., Jr.; Lando, J. B. J.Colloid Interface Sci. 1976, 56, 64. (3) O’Brien, K.; Long, J.; Lando, J. B. Lungmuir 1985, 1, 514. (9) O’Brien, K.; Rogers, C. E.; Lando, 3. B. Thin Solid Film 1983,102, 131. (10) Hub, H. H.; Hupfer, B.; Koch, H.; Ringsdorf, H. J.Mucromol. Sci. Chem. 1981, A15 ( 5 ) , 701. (11) Ringsdorf, H.; Schupp, H. J.Mucromol. Sci. Chem. 1981, A15 (5), 1015.
0 1987 American Chemical Society
