J. Phys. Chem. 1992, 96, 2360-2364
2360
Apparent Volume and Heat Capacity of OxyethyOene(,,)-Oxypropylene(,,)-8xyethylene~,*), and Dispersions
Pluronic L-122, in Aqueous Solutions
Christian Camire, Luc Meilleur, and Franqois Quirion* ZNRS-Energie, 1650 Montde Ste-Julie, Varennes, Quebec, Canada J3X IS2 (Received: August 12, 1991)
The phase diagram of aqueous Pluronic L-122, oxyethylene(l2,-oxypropylene(67,-oxyethylene(l2,, was determined from -10 to +40 "C and up to 50% (w/w). Isotropic solution, liquid crystal, and gel phases are observed as well as an anomalous region consisting of slightly turbid blueish solutions. Two endothermic transitions border a region of high viscosity that leads to the gel phase. The apparent volume and heat capacity of Pluronic L-122 and of 2-butoxyethanol in the aqueous mixtures of Pluronic L-122 were determined at 10, 22, and 35 "C. At 10 "C, the isotropic solutions of Pluronic L-122 go through phase separation with a continuous increase in the apparent volume and a sharp increase in the apparent heat capacity. At 22 "C, a transition occurs at low concentrations into the biphasic region, with a sharp increase in the apparent volume and a large maximum in the apparent heat capacity. It is suggested that, from 22 to 35 "C, the transition is displaced to a concentration lower than that of the range covered in this investigation so that the trends of the thermodynamic properties correspond to the tail of the transition. This is consistent with an extrapolated volume higher than the one calculated by additivity of the homopolymers at infinite dilution at 35 "C.
Introduction A-B-A block copolymers of oxyethylene, A, and oxypropylene, B, also known as Pluronics or Poloxamers, are used widely as emulsifiers, foaming agents, and gelling agents. Their low toxicity has already made them very popular for applications in the pharmaceutical, agricultural, and food industry. Most studies deal with the micellization behavior of Pluronics using light and neutron scattering, viscosity, and interfacial tension measurements to elucidate the structure of the aggregates in aqueous solution. All these techniques and others are described in a very nice paper by Wanka et al.,I who investigated the behavior of three Pluronics in aqueous solutions. Almost all Pluronics can form thermoreversible gels in water, and the gelation is only slightly endothermic.2J Micellization of Pluronics is reported to be e n d ~ t h e r m i c l *with ~ - ~an increase in the apparent volumes,6 of the copolymer. The aggregation number increases with temperature while the size remains almost constant, suggesting a continuous dehydration of the aggregates as the temperature is r a i ~ e d . ~ ~ ~ ~ ~ ~ * Williams et aL5 observed that the cooperative transition of Pluronics can occur either in solution or in the biphasic dispersion, depending on their content in poly(oxypropy1ene). Because of the "unstable character" of biphasic systems, most studies are conducted with solutions so that little is known about the molar properties of the copolymers in the dispersed state. Recently, Desnoyers et aL9 successfully used thermodynamic techniques to determine the molar properties of the components of biphasic emulsions, and we believe that this approach can be applied to the aqueous dispersions of Pluronics. In the present study, we report an investigation of aqueous solutions and dispersions of Pluronic L- 122, a block copolymer containing 78% poly(oxypropy1ene). The phase diagram was studied from 1 to 50% Pluronic L-122 in terms of the temperature dependence of the opacity, viscosity, and heating rate of the samples while they were heated to 40 "C. The apparent specific volume and heat capacity of Pluronic L-122 in water were determined at 10,22, and 35 "C.Complementary information was obtained from the apparent molar volume and heat capacity of 2-butoxyethanol maintained at 0.1 m in the aqueous mixtures of Pluronic L- 122. Experimental Section Chemicals. 2-Butoxyethanol was purchased from Fluka, and it was used as received. All aqueous mixtures were prepared by
* Author
~
to whom
~~
correspondence should be addressed. 0022-3654/92/2096-2360$03.00/0
TABLE I: Properties of Pluronic L-122
from DroDertY BASF' Dresent work MW, g mol-' 5000 weight fraction of PO, % 80 78 k 2b cloud point at 1% in water, O C 19 16.7 f O S c cloud point at 10% in water, "C 13 12.8 f 0.Y water content, % 0 0.4d pour/melt temp, "C 20 23-35 (melting range)F 1750 viscosity at 25 "C,CP -66' glass temp, "C "From ref 13. *By proton NMR spectroscopy. CBy opacity measurements. d By Karl Fisher titration. eBy thermal analysis. weight with deionized and degassed water. The copolymer oxyethylene(12,-oxypropylene(,7,-oxyethylene~,2,, Pluronic L- 122, was obtained as a commercial sample from BASF (lot WPYG-5.31.B), and it was used as received. This commercial product is certainly polydisperse in molecular weight and in composition. Minor components such as homopolymers and/or diblocks have been reported1*12 for other commercial Pluronics, and they are often associated with an anomalous aggregation behavior. In the present study, we take the commercial Pluronic L-122 as an entity with the average properties listed in Table I. Opacity and Thermal Analysis. In this work, the opacity (Op) of a mixture is defined as (Io- o/Z0where Io and Z are the intensity of the light transmitted through a 1.2-cm test tube containing water and the aqueous mixture, respectively. Experimentally, the light beam from a light-emitting diode (Monsanto 5752) was ( 1 ) Wanka, G.: Hoffmann, H.; Ulbricht,
W. Colloid Polym. Sci. 1990,
268. 101-117.
(2) Vadnere, M.; Amidon, G.; Lindenbaum, S.; Haslam, J. L. I n t . J . Pharm. 1984, 22, 207-218. (3) Brown, W.; Schillen, K.; Almgren, M.; Hvidt, S.;Bahadur, P. J . Phys. Chem. 1991, 95, 1850-1858. (4) Leboeuf, D. 1990, MSc. Thesis, Departement de Chimie, Universite de
Sherbrooke.
(5) Williams, R. K.; Simard, M.-A.; Jolicoeur, C. J . Phys. Chem. 1985, 89, 178-182. (6) Dumont, J. M S c . Thesis, Departement de Chimie, Universite de Sherbrooke, PQ, Canada, 1984. ( 7 ) Zhou, 2.; Chu, B. J . Colloid Interface Sci. 1988, 126, 171-180. (8) Turro, N. J.; Kuo, P.-L. J . Phys. Chem. 1986, 90, 4205-4210. (9) Desnoyers, J. E.; Caron, G.; Perron, G. Colloids Surf. 1989, 38, 263-283. (IO) Zhou, 2.; Chu,8. Macromolecules 1987, 20, 3091-3094. ( I I ) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548-2554. (12) Almgren, M.; Alsins, J.; Bahadur, P. Langmuir 1991, 7, 446-450. ( I 3) Pluronics and Tetronics Surfactants: BASF: Parsippany, NJ, 1987.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2361
Block Copolymer Solutions and Dispersions detected by a photoconductive cell (Clairex CL907HL) with a resistance proportional to the reciprocal of the intensity of the light. Typically, a test tube of known Io containing about 5 g of sample and a magnetic bar was cooled to about 5 OC and placed in a reservoir maintained at a temperature (TR)around 56 OC. Stirring was started, and the temperature inside the sample (100 52 platinum resistance temperature device (RTD), Omega) as well as the intensity of the light transmitted, I, were determined at constant time intervals. For these experiments, the heating rate varies continuously from 3 K min-I around 10 O C to 1 K m i d around 40 OC. Mixtures with an opacity higher than 10% were considered as opaque, and the cloud temperatures were determined at the breaks of the Op vs T curves with a reproducibility within 0.5 OC for three independent determinations. The resulting “temperature versus time” curves were analyzed in terms of the heat leak modulus,14 R = (dT/dt)/(TR - 7‘). In the absence of transition, this parameter is almost constant while it goes through a minimum when an endothermic transition occurs in the sample. The temperature of transition, T,, was determined at the minimum of the R vs T curves, and the reproducibility was within 0.5 “ C over three runs. The same procedure was used to determine the solid-liquid phase diagram of the aqueous mixtures of Pluronic L-122. The temperature probe and about 0.2 g of mixtures were placed into a small polypropylene tip. The tip was rapidly cooled to -25 OC, and the temperature inside the tip was determined as the sample was heated. The melting and eutectic temperatures were determined graphically from the R vs T curves with a reproducibility of about 0.5 OC. Viscosity. The trend in the viscosity with temperature gives complementary information on the physical state of the mixtures. The viscosity was evaluated for samples contained in a Brookfield rotating cylinder immersed in a thermoregulated water bath that was heated from 5 to 40 OC with about the same heating rate as for the opacity experiments. The viscosities obtained with this technique are not equilibrium values, and only the trend and magnitude of the q vs T curves are of interest in the present study. In the text, a region of high viscosity refers to a range of temperatures over which a sample shows a positive deviation of the viscosity as compared to the usual decrease with temperature. Apparent Volume and Heat Capacity. The density, d, and the relative change of heat capacity per unit volume, dS/So, were obtained with a vibrating tube flow den~imeter’~ ( W e v 03D) and a differential flow microcalorimeter16 (Sodev CP-C and DT-C), respectively. From these parameters, the apparent specific volume and heat capacity of Pluronic L-122 in water were calculated with the following equations: up*+ =
-1 - (1 - WPNd - do) d
WPddO
(1)
1 - wp CPP,+ = CP + -(CP - CPO) (2) WP where Wp is the weight fraction of Pluronic L-122 in the mixture, cp is the specific heat capacity of the mixture given by cpo(1 dS/So)do/d, and doand cpo are the density and the specific heat capacity of water, respectively. By the assumption that the property of the solvent is not affected by the presence of the solute, all the nonideality of the mixture is accounted for in the apparent property of the solute. The apparent properties of 2-butoxyethanol, maintained at 0.1 m,can be used to emphasize” transitions of Pluronic L- 122 in water. In the presence of added 2-butoxyethanol, the transitions of Pluronic L-122 in water may be shifted in concentration and
+
(14) Quirion, F.; Lambert, D.; Perron, G. Can. J. Chem., submitted for publication. (15) Picker, P.; Tremblay, E.; Jolicoeur, C. J. Solution Chem. 1974, 3, 377. (16) Picker, P.; Leduc, P.-A.; Philip, P.; Desnoyers, J. E. J. Chem. Ther-
modyn. 1971, 3, 631-642. (17) HQu, D.; Roux, A. H.; Desnoyers, J. E. J. Solution Chem. 1987, 16, 529-553.
1
I
I
4 1
:x 3%
Jj ......
-
19%
41%
A
40
40 40 T/ “C Figure 1. Typical curves of the opacity, Op, heat leak modulus, R,and viscosity, 7, as a function of temperature at different weight fraction, %, of Pluronic L-122 in water. The magnitude of the maximum in the viscosity is given in centipoises. 0
in temperature, leading to significant contributions to the apparent properties of 2-butoxyethanol. Generally, the amplitude of the contribution is proportional to the ratio of the concentration of Pluronic L-122 at the transition to the concentration of 2-butoxyethanol in the ternary system. Thus, the apparent molar properties of 2-butoxyethanol will amplify transitions of Pluronic L-122 that occur at Wp > 1%. For transitions a t lower concentrations, the effect will be reversed. The corresponding apparent molar volume and heat capacity of 2-butoxyethanol were calculated with (3)
where MBEand mgE are the molecular weight and the molality of 2-butoxyethanol, respectively. d and cp are the density and the specific heat capacity of the ternary mixture, respectively. The subscript 0 now refers to the binary mixtures of water + Pluronic L-122. All the data presented in this paper are plotted with their respective experimental errors. In many cases, the error bar was smaller than the height of the symbols used in drawing the figures.
Results and Discussion Opacity, V i i t y , and Thermal Analysis. Figure 1 summarizes the typical trends observed for Op, R, and tl as the aqueous mixtures of Pluronic L-122 were warmed to 40 OC. As observed for Pluronic L-62’* a t 10% in water, most aqueous mixtures of Pluronic L- 122 exhibited more than one cloud temperature, corresponding to a sharp increase of the opacity. As shown in Figure 2, the cloud temperatures were used to obtain the phase diagram of Pluronic L-122 in water. Three monophasic regions are observed. It is important to realize that the Gibbs phase rule does not apply to such phase diagrams because Pluronic L- 122 is not a pure component. Thus, horizontal lines do not necessarily tie the composition of the phases in equilibrium. The first monophasic region, at low temperatures and concentrations, corresponds to isotropic aqueous solutions of Pluronic (18) Prasad, K. N.; Luong, T. T.; Florence, A. T.; Paris, J.; Vaution, C.; Seiller, M.; Puiseux, F. J. Colloid Interface Sei. 1979, 69, 225-232.
2362 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
Camire et al. TABLE U C o m p o h of Properties Extrapolated to Zero with Values Calculated by Additivity of the Homopolymers 10 O C 22 "C 35 OC upw+, cm3 g-l
exptl calcd"
T/"C , -10
0.8782 0.8935 0.8793 0.8918 cppw4, J K-I g-l exptl 3.55 calcdb 3.32 "From ref 21. bFrom ref 22 at 25 "C.
4
0
wp/ %
60
100
1
OP
i
3.13
I
I
1
10'C
I
0.9495 0.9048
i.
,
1 1
Figure 2. Liquid-liquid phase diagram of Pluronic L-122 in water.
Empty circles represent the break in Op vs T. The size of the solid circles represents the range of the opacity of the mixtures in that region of the phase diagram: no symbol, Op < 10%; ., 10% < Op < 30%; *, 30% < Op < 50%;*, 50% < Op < 70%; 0,70% < Op < 90%; 0,Op > 90%. S, LC, and G refer to isotropic solution, liquid crystalline, and gel phase, respectively.
0.90 VP,$
50
12
c
mi
t
-.
". i
F
c
m m-mmmmm
cp'p'90
T/ "C
WPI %
0
30
Figure 4. Comparison of the opacity, Op (%), apparent specific volume, vPA (cm3 g-I), and apparent specific heat capacity, cpp,+ (J K-' g-l), of
Pluronic L-122 in water at 10 OC. -10 0
Wp/%
60
Figure 3. Location of the thermal transitions, symbols, and regions of high viscosity, 1, into the phase diagram of Pluronic-L122 in wate:. Tm and Te refer to the melting and eutectic temperature of the mixture, respectively. The anomalous region is noted as A, and S,LC, and G stand for isotropic solutions, liquid crystalline,and gel phase, respectively.
L-122. The second and the third occur at higher concentrations, and they intersect the biphasic region of the phase diagram. Some nonionic 11101ec~les,~~J~ similar to Pluronic L-122, are known to have liquid crystalline phases, LC, that intersect the biphasic region. Pluronics are known to form thermoreversible gels, G. Gelation of Pluronics was studied by Vadnere et al.,z who came out with a group contribution for the enthalpy of gelation leading to 8.6 J g-l for the gelation of Pluronic L-122 in water. This is in fair accordance with 8 f 1 J g-I obtained experimentally from the slope of In W,vs l/TG Thus, phase G is the thermoreversible gel phase of Pluronic L-122 in water. The absolute value of the opacity of a mixture is proportional to the volume fraction of the dispersed phase within the biphasic region. This is shown in Figure 2, where the biphasic region is tilled with dots that have a size proportional to the opacity. One can identify an anomalous behavior of the opacity in the biphasic region correspondingto an enhanced solubility of Pluronic L- 122. This region consists of solutions slightly opalescent and blueish as reported by Lang19 for the anomalous phase of aqueous tetraethylene glycol monodecyl ether, CI,,EO,. Blueish solutions were also observed in the upper part of phase LC. The temperature dependence of the heat leak modulus, R, indicates the presence of at least two endothermic transitions which correspond to the beginning and the end of a region of high viscosity. In most cases, the temperature decreased while the (19) Lang, J. C. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsiom; Degiorgio, V., Corti, M., Eds.; Italian Physical Society: 1985;
336-375 (Course XC). (20) Hesp'e, H.; Crone, J.; Muller, E. H.; Schafer, E. E. Angew. Makromol. Chem. 1984, 1231124, 189-216. OD "
samples were warmed through the transitions. This fact cannot be accounted for solely by variation of the stirring rate due to changes in the viscosity of the mixtures. Thus, the transitions are real and their locations, in the phase diagram, are shown in Figure 3 together with the regions of high viscosity. The technique based on the heat leak modulus was also used to obtain the melting and eutectic temperatures of the aqueous mixtures of Pluronic L- 122, and the results are shown in Figure 3. One can see that phase LC intersects the solid-liquid phase diagram, in accordance with the phase behavior of similar nonionic m o l e c ~ l e s . ~ ~ ~ ~ ~ Up to 10% of Pluronic L-122, the region of high viscosity is the result of the superposition of two peaks that separate in two distinct regions of high viscosity at higher concentrations (see Figure 1). The first region is associated with the phase separation of phase S into S + LC with an amplitude that remains relatively low. The viscosity at the maximum of the second region, bordered by two endothermic transitions, increases steeply for concentrations higher than about 15% Pluronic L-122. This region of high viscosity ends with the endothermic formation of the anomalous phase. The purpose of this section was to evaluate the physical properties of aqueous mixtures of Pluronic L- 122 in relation to its phase behavior. In that sense, our methodology, based on the determination of the opacity, viscosity, and heat leak modulus of the samples as they are warmed, has proven to be very effective. Volume and Heat Capacity at Infinite Dilution. A partial quantity at infinite dilution is obtained by the extrapolation of the experimental apparent quantity. At infinite dilution, partial quantities have the advantage of being additives because they do not contain intermolecular contributions. Thus, it is possible to calculate the partial volume21*22 and heat capacityz2of Pluronic L-122 at infhite dilution from the additivity of the homopolymers. Table I1 compares these values with the experimental values obtained by extrapolation of the apparent volume and heat ca(21)
Harada, s.;Nakajima, T.; Komatsu, T.; Nakagawa, T. J . Solurion
Chem. 1978, 7 , 463-474. (22) Riedl, B. Ph.D. Thesis, Sherbrooke, PQ, Canada, 1984.
Departement de Chimie, Universite de
The Journal of Physical Chemistry, Vol. 96,No. 5, 1992 2363
Block Copolymer Solutions and Dispersions
136
1
116L
1
8oooi , .: .j
c p w
,
mm~mmmwm'
-2000 0
wp/ %
30
Figure 7. Apparent molar volume, VBEI(cm' mol']), and apparent molar heat capacity, CPB,,+(J K-' mol-'), of 2-butoxyethanolmaintained at 0.1 m in the aqueous mixtures of Pluronic L-122 at 10 OC. 0
wp/ %
30
F i 5. Comparison of the opacity, Op (%), apparent specific volume, up,* (cm3g-l), and apparent specific heat capacity, cpp,, (J K-I g-l), of Pluronic L-122 in water at 22 OC.
L
4
1201
22'C
8007
0
1
1
I
0.94k
i
4 7
7-
cp,p,!ii
~mlm-lm;m
I
2 0
Wp/%
~
1
pacity of Pluronic L-122 in water at 10,22, and 35 OC shown in Figures 4, 5 , and 6, respectively. At 10 and 22 O C , the extrapolated volumes are in good agreement with the values calculated by additivity. This means that at these temperatures the extrapolation leads to the infinite dilution value and that the interaction of the copolymer with water is the sum of the interactions of poly(oxyethy1ene) and poly(oxypropylene) with water. At 35 OC,the extrapolated volume is still much higher than the one calculated by additivity. This could be due either to an inaccurate extrapolation resulting from a lack of data at low concentrations or to a temperature-induced transition that would occur, at the molecular level, with an increase in the partial volume a t infinite dilution. At 22 "C, the initial slope of the apparent heat capacity of Pluronic L-122 in water is too steep to extrapolate a value at W, = 0. However, it is reasonable to say that, as for the apparent volume, the apparent heat capacity should decrease to the additivity value at infinite dilution. We were not able to compare our extrapolated values at 10 and 35 OC because there were no data available at these temperatures. However, at 10 OC the slope is small and the extrapolation probably leads to a good estimate of the heat capacity at infinite dilution. At 35 OC,the slope is also small, but since the extrapolated volume does not correspond to the value calculated by additivity, the same should also be true for the extrapolated heat capacity. Concentration Dependence of the Apparent Volume and Heat Capacity. The apparent volume and heat capacity of Pluronic L-122 in water shown in Figures 4,5, and 6 are compared with the opacity of the system, which reflects the solubility of Pluronic
30
Figure 8. Apparent molar volume, VB6+ (cm' mol-'), and apparent molar heat capacity, Cp,,,, (J K-I mol-'), of 2-butoxyethanol maintained at 0.1 m in the aqueous mixtures of Pluronic L-122 at 22 "C.
127 VBE,f
30
Figure 6. Comparison of the opacity, Op (a),apparent specific volume, up,# (cm' g-l), and apparent specific heat capacity, cpp,+(J K-I g-I), of Pluronic L-122 in water at 35 O C .
Wp/%
i
700
1
cp"'p~-'+-**+
1 1
m
,
300 0
wp/ %
30
Figure 9. Apparent molar volume, VBE+(cm' mol-]), and apparent molar heat capacity, CpBE,+(J K-I mol-'), of 2-butoxyethanol maintained at 0.1 m in the aqueous mixtures of Pluronic L-122 at 35 OC.
L-122 in water at 10, 22, and 35 OC, respectively. The apparent molar volume and heat capacity of 2-butoxyethanol, VBE,+and CpBE,+, maintained at 0.1 m in the aqueous mixtures of Pluronic L-122, were also determined in order to obtain additional information on the behavior of Pluronic L-122 in water. Adding 2-butoxyethanol at a concentration of 0.1 m does not affect much the phase diagram of Pluronic L-122 in water, so the opacity transitions are only shifted in concentration and temperature. Such shifts may contribute significantly to the apparent property of 2-butoxyethanol. The results a t 10, 22, and 35 O C are shown in Figures 7, 8, and 9, respectively. At 10 OC, the system is well below the region of endothermic transitions and below the region of very high viscosity. The opacity of the system indicates that the phase separation of phase S into phases S + LC starts around 17% Pluronic L-122 in water. The linear increase in the volume at the phase separation suggests that the apparent volume of Pluronic L-122 is higher in the LC phase than in phase S. In the two-phase region, S + LC, the heat capacity increases steeply to very high values, and it remains about constant up to 29%. In Figure 7, the trends of VBE,Iand CPBE,+ reflect a concentration shift of the phase separation of S in the presence of 2-butoxyethanol.
2364
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
At higher concentrations, the viscosity is too high to allow measurements with our flow microcalorimeter. Nevertheless, tentative measurements in phase LC indicate that the apparent heat capacity of Pluronic L-122 is much lower than that in the biphasic region. This is consistent with the work of Chernik and Filippo~?~ who observed a maximum in the apparent heat capacity of decyldimethylphosphine oxide in the biphasic region between the isotropic aqueous solutions and the lamellar liquid crystalline phase. In fact, they observed a maximum in the apparent heat capacity every time that a two-phase region was encountered. At 22 OC, the proximity of the endothermic transitions and the region of high viscosity add to the complexity of the phase diagram. The apparent volume and heat capacity were determined down to 0.015%, where the system was still in the biphasic region. The opacity goes through a maximum around 3%, and it decreases to zero around 20% Pluronic L-122, where the system reaches the LC phase. The maximum in the opacity indicates a gradual resolubilization of the phase rich in Pluronic L-122. This could be the consequence of a transition that occurs at low concentrations. This is consistent with the transition that occurs around 0.3% Pluronic L-122 with a sharp increase of the apparent volume and a large maximum in the apparent heat capacity. At higher concentrations, the volume remains high while the heat capacity gradually decreases to its corresponding value in water. In summary, the transition involves an enthalpy and it occurs in the biphasic region with an increase in the apparent volume and almost no changes in the apparent heat capacity of Pluronic L-122. RiedP also observed a maximum in the apparent heat capacity for the isotropic solutions of Pluronic F-127 at 25 OC. The transition of Pluronic F-127 occurs with a positive change of volume5*6and almost no changes in the heat ~ a p a c i t y . ~The temperature at the transition of Pluronic F-127 decreases with the c o n c e n t r a t i ~ n .Thus, ~ ~ ~ at 25 OC the transition is crossed as the concentration of Pluronic F-127 is increased. The same is probably true for the transition reflected in up
