Ind. Eng. Chem. Res. 1996, 35, 3415-3421
3415
Temperature-Induced Solubilization of Hydrocarbons in Aqueous Block Copolymer Solutions Paul J. M. Lebens and Jos T. F. Keurentjes* Akzo Nobel Central Research, Department RTA, P.O. Box 9300, 6800 SB Arnhem, The Netherlands
In this study, the use of aqueous solutions of amphiphilic block copolymers as an extractant for hydrocarbons has been examined. The solubilization of naphthalene in poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) block copolymer solutions is chosen as a model system. Special attention is paid to the aggregation mechanism and to the transformation of the micelle structure upon a variation in temperature. It is demonstrated that above a certain transition temperature the solubilization of naphthalene will increase exponentially as a result of the micelle growth, which is initiated at this temperature. The solubilization increase will take place over a temperature interval of about 15 °C after which the solubilization of naphthalene will stabilize. Values of the distribution ratio of naphthalene over the apolar micelle core and the surrounding aqueous phase in excess of 2000 were measured. Modelling results have shown that the solubilization is predominantly determined by an increase of the micelle radius, while the polarity change of the micelle core is of minor importance. Due to the reversibility of the process, the block copolymer solution seems a promising extractant with favorable regeneration properties. Introduction Amphiphilic block copolymers of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) type, commercially known as Pluronics and Synperonics, have found important applications in various fields. Since a broad variety of types is available with different molecular weights, different block lengths, and varying block ratios, it is possible to use a polymer with characteristic properties according to specific process requirements. Due to their aggregation behavior in water, the block copolymers can successfully be used for enhancing the solubility of hydrocarbons (Smith et al., 1986; Calvert et al., 1994; Noolandi and Hong, 1983; Nagarajan and Ganesh, 1989). Promising new applications of this type of block copolymers are found in extraction processes in which the extraction properties of the polymer aggregates are combined with the capability of membranes to retain these macromolecules while being permeable for small hydrocarbons. Several pathways have been suggested to make this combination of techniques feasible in practice. Scamehorn and co-workers (Christian and Scamehorn, 1989) developed a micellar enhanced ultrafiltration (MEUF) technique in which the micelle solution is directly added to the polluted stream. Subsequently, the micelle solution is led across an ultrafiltration membrane to separate the micellar aggregates from the purified water. In a recently published method, block copolymers are incorporated within the interstices of polymeric hydrogel beads, thus avoiding leakage of the micelles. When polluted water passes through a packed column containing these beads, organic contaminants will preferentially be solubilized in the immobilized micelles (Calvert et al., 1994). In a different approach, a hollow-fiber membrane is used to provide a semipermeable barrier between the block copolymer extraction solution and the contaminated water. This barrier is permeable to the compounds to be extracted but impermeable to the block copolymers (Hurter, 1992). This type of processes is often referred to as pertraction. The principle of a possible pertraction process is schematically given in Figure 1. At elevated tempera* Author to whom all correspondence should be addressed.
S0888-5885(96)00066-8 CCC: $12.00
tures, the amphiphilic block copolymers will form micelles with a poly(propylene oxide) (PPO) core stabilized by a hydrophilic poly(ethylene oxide) (PEO) corona. The nonpolar micelle core, which exhibits great selectivity for hydrophobic substances, can absorb significant amounts of pollutants that can pass through the membrane. Regeneration of the loaded block copolymer solution can be accomplished by decreasing the temperature. The micelle structure will be rearranged so that the pollutant will be forced to separate from the block copolymer solution, forming a second phase. Finally, this second phase can easily be separated from the block copolymer solution, which can be used again. Although this principle has been shown to work in previous investigations, little insight is available on the transformation taking place upon a variation in temperature. For large-scale applications, it is important to minimize regeneration costs, for which it is of great importance that the transformation of the micellar structure will take place at moderate temperatures and over a small temperature interval. In this study, the effect of temperature on the solubility of naphthalene in a micellar solution has been investigated. Besides these partitioning experiments, calorimetric experiments have been performed to gain insight in the structure changes of the micelle core that occur at the transformation. Finally, a description of the solubilization of naphthalene will be given using a thermodynamic model. Theory The aggregation behavior of poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solutions has extensively been studied (Zhou and Chu, 1988; Wanka et al., 1990; Brown et al., 1991). Basically three regions exist for the temperature-induced micellization behavior. A generalized effect of temperature and polymer concentration on the aggregation behavior is depicted in Figure 2. Below the critical micellization temperature (CMT), the so-called unimer region, only dispersed molecules occur. Upon an increase in temperature above the CMT, an intermediate transition region exists in which both © 1996 American Chemical Society
3416 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 1. Principle of pertraction and phase separation with aggregated block copolymer solutions.
Figure 2. Generalized representation of the aggregation behavior of an aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer solution.
unimers and micelles can be observed. In this region, the unimer-micelle equilibrium will shift toward the micelles with increasing temperature and polymer concentration. In the third regionsmicelle regionsthe micellar weight increases linearly with temperature. The hydrodynamic micelle radius remains nearly constant due to enhanced dehydration. Wanka and coworkers (Wanka et al., 1991) showed that the aggregation number of different PEO-PPO block copolymers (P-104, P-123, and P-127) strongly increases with increasing temperature while the hydrodynamic radius remains constant. The results of Brown et al. (1991) showed the coexistence of P-85 unimers, micelles, and micellar aggregates in relative proportions that depend critically on temperature and concentration. Increasing the temperature resulted in a reduced transition region that eventually disappears so that the unimers are transformed into micelles directly. Moreover, the critical micellization concentration (CMC) will decrease when the temperature is increased (Yu et al., 1992). Increasing the polymer concentration can lead to gelation at certain temperatures. From small angle neutron scattering data (Wanka et al., 1990) and DSC experiments (Yu et al., 1992), it can be concluded that the structure of the gel is a result of ordering of the
micelles (see Figure 2). Malmsten and Lindman (1992) showed that the stability of the gel strongly depends on the temperature. At low temperatures, decreased interaction between the micelles will prevent gelation while at high temperatures the gel will break down due to micelle shrinkage. A further increase of the temperature will eventually lead to phase separation of the polymer and the water at almost any concentration. The solubilization properties of a block copolymer solution can be calculated when a proper model of the micelle solution is defined. The model we use in this study has formerly been used by Noolandi and Hong (1983), Leibler et al. (1983), and Nagarajan and Ganesh (1989) and assumes that the poly(propylene oxide) and the poly(ethylene oxide) blocks are separately localized in the core and the corona of a spherical micelle. Due to the better PEO-water interaction as compared to the PPO-water interaction, the core will then be composed of a concentrated poly(propylene oxide) solution, while the corona will be a less concentrated poly(ethylene oxide) solution. The solution outside the micelle is composed of water containing some unimers. Moreover, it is assumed that the concentration profiles within the various regions are uniform and that the composition of the core is determined by an equilibrium with the
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3417
aqueous bulk phase. When a hydrophobic pollutant is added to the solution it will preferentially be solubilized in the core of the micelle. The partition coefficient between the micelle and the bulk can then be calculated assuming a three-component two-phase equilibrium with a PPO-rich and a water-rich phase, respectively. The phase equilibria between the core and the bulk phase (PPO and water phase) can be calculated with a modified Flory-Huggins theory developed for PEOwater and PPO-water systems (Karlstro¨m, 1985; Bjo¨rling et al., 1990). This theory suggests that, as a result of the rotational freedom, the PPO (or PEO) monomers can be in a polar or a nonpolar conformation. The polar and nonpolar states have the degeneration factors fp and fn, respectively (fn > fp), and are assumed to be in equilibrium with each other. The two conformations exhibit a different interaction with water, which is expressed in the polymer-water interaction parameters (χpw and χnw) that increase when the polymer becomes more hydrophobic. The derivation of the Gibbs free energy can be performed in the same way as the ordinary Flory-Huggins expression and will result in
G ) H - TS
(1)
H ) v1v2(Pχpw + (1 - P)χnw) + kTMo v22P(1 - P)χpn + v2(1 - P)R (2) v2 S ) v1 ln v1 + ln v2 + Mok m
(
v2 P ln P + (1 - P) ln
H ) v1v2(Pχpw + (1 - P)χnw) + v22P(1 - P)χpn + kTMo v2(1 - P)R + v1v3χw3 + v2v3χp3 (4) -
v2 S ) v1 ln v1 + ln v2 + v3 ln v3 + Mok m
(
v2 P ln P + (1 - P) ln
The additional enthalpy terms represent the interaction of the third component with water (χw3) and with the polymer (χw3), respectively. It is important to note, however, that it is assumed that the interaction between the extra component and the polar and nonpolar states of the polymer is equal. The calculation of the phase compositions, and thus of the partitioning of the third component, is similar to the calculation of the twocomponent system, except that now a tangent plane instead of a tangent has to be calculated. Since the micelle core is not a macroscopic phase, such as the bulk, an additional term accounting for the curvature of the surface has to be added to the partial Gibbs free energy of the solute. The same phenomenon has been observed in emulsion polymerization where polymer particles swell with monomer (Maxwell et al., 1992). The contribution of the surface tension γ and core radius to the partial Gibbs free energy is given by the Gibbs-Thomson equation:
µs )
)
(1 - P) (3) F
with P as the probability that the polymer segment will be in the polar state; F as the degeneration ratio (fp/fn); R as the energy difference between the polar and nonpolar conformations; v1 and v2 as the volume fraction water and polymer, respectively; m as the degree of polymerization; and Mo as the total number of segments. At low temperatures, most of the polymer segments will be in the polar state because this is favorable from an enthalpic point of view. However, when the temperature is increased, the entropic effect becomes more important. Consequently, the polymer segments will change to the nonpolar state with the highest degeneration factor. As a result from this change, the interaction between the polymer segments and water will decrease. At a certain temperature, the lower solution temperature, the increased enthalpy can no longer be compensated by the entropy and the initial homogeneous solution will separate into a polymer phase and a water phase, respectively. The equilibrium composition of the two phases can be determined by calculating the two points with a common tangent on the Gibbs free energy curve. In these points, the chemical potentials of both components in both phases are equal. When a third component is added to a two-phase system, this component will be distributed over the two phases. Hydrophilic components will preferentially solubilize in the water phase while hydrophobic components will prefer the polymer phase. The effect of the added component on the Gibbs free energy results in one additional term in the entropy equation and two additional terms added to the enthalpy equation, respectively:
)
(1 - P) (5) F
2Vmγ(1 - vm)1/3 Ro
(6)
with Vm as the molar volume of the solute, vm as the volume fraction of the solute, and Ro as the radius of the unswollen micelle core. Experimental Section In this study, three different block copolymers were used to study the effect of the block length and block ratio of the polymer on the solubilization of hydrocarbons. Initially P104 was used because it was known from literature that this polymer forms stable micelles (Wanka et al., 1990) and can absorb significant amounts of hydrocarbons at room temperature (Hurter, 1992). Furthermore, P103, a polymer with a different block ratio, and L64, a polymer with a different length, were used. The properties of the block copolymers, obtained from BASF (Ludwigshafen, Germany), are given in Table 1. As a model hydrophobic solute, the polyaromatic hydrocarbon naphthalene was used. The naphthalene was p.a. quality and was obtained from Aldrich. Saturation experiments were performed in a setup as depicted in Figure 3. Saturation was obtained by circulating 200 mL of the micelle solution over a column (diameter ) 2.5 cm, length ) 15 cm) packed with pure naphthalene crystals. The circulation velocity was varied between 10 and 20 cm/min. The temperature, Table 1. Physical Properties of Block Copolymers property molecular mass % PEO (units) γ (mN/m, 25 °C, 0.1%) cloud point (°C) 1 wt % polymer 10 wt % polymer HLB
L64
P104
P103
2900 40 43
5900 40 33
4950 30 34
58 60 12-18
81 78 12-18
86 52 12-18
3418 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 3. Schematic representation of the experimental setup used for the saturation experiments. Figure 5. Solubility of naphthalene in P104 solutions as a function of the temperature relative to the solubility in pure water: (+, 1 wt %; 4, 2 wt %; O, 3 wt %; +, 4 wt %; 2, 5 wt %; b, 10 wt % P104).
Figure 4. Solubility of naphthalene in the polymer solution relative to the solubility in pure water for different polymer solutions as a function of the polymer fraction: (+, P104; 4, L64; O, P103; T ) 30 °C).
measured in the top of the column using a Pt-100 thermocouple, was controlled by a Braun FrigoMix-S/ ThermoMix-S combination. To minimize heat losses, the column was isolated with glass wool and tin foil. Samples, taken in the top of the column, were diluted with a Hamilton Microlab 1000 diluter prior to analysis. The concentration of naphthalene was measured by monitoring light absorbance at a wavelength of 276 nm using a Beckman DU-500 UV/VIS spectrophotometer. The polymer concentration was measured by determination of the refractive index using an Index Instruments GPR 11-37 refractive index meter. Due to errors in dilution and analysis, the accuracy of the individual data points was within a 10% interval. DSC experiments on pure polymers were performed on a Seiko RDSC 220 apparatus. The measurements on the aqueous polymer solutions were performed on a Perkin-Elmer 7 Series thermal analysis system. The temperature program used depended on the sample type. Results and Discussion Saturation Experiments. The saturation concentration of naphthalene in the micelle solutions has been measured for solutions with varying polymer concentrations at different temperatures. The results are presented in Figures 4-6 in which the ratio between the maximum solubility of naphthalene in the micelle solution and the solubility in pure water (partition coefficient) is plotted as a function of the polymer concentration (Figure 4) or temperature (Figures 5 and
Figure 6. Solubility of naphthalene in solutions of different polymers as a function of the temperature relative to the solubility in pure water: (+, 10 wt % P104; 4, 10 wt % L64; O, 10 wt % P103).
6). The influence of the polymer concentration (in Figure 2 denoted as a change along the line a-b) is illustrated in Figure 4 for different polymers, from which it can be concluded that the saturation concentration increases approximately linearly upon an increase in polymer concentration. In the transition region, the number of micelles will obviously increase proportionally to the polymer concentration while the structure of the micelle will hardly be affected, so that it seems likely that all data of Figure 4 are located in the transition region. Based on the results of Leibler (Leibler et al., 1983), it has previously been suggested that the micelle properties of polymers with short PPO blocks exhibit increased concentration dependency (Hurter, 1992). This suggestion could be supported by Figure 4, from which it can be seen that the largest deviation from linearity is observed for L64. In Table 2, the normalized saturation ratios are given. This normalized saturation ratio KPPO is defined as
KPPO )
cpolymer/cwater wt %polymer × wt %PPO
(7)
where wt %polymer is the weight fraction polymer in the solution and wt %PPO is the weight fraction PPO in the polymer. It becomes obvious from Table 2 that the normalized saturation of L64 is the most dependent on the polymer concentration. It also appears from Table
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3419 Table 2. Normalized Partition Coefficient of L64, P104, and P103 Solutions at 30 °C and Varying Polymer Concentrations wt % polymer
L64
P104
P103
1 2 3 4 5 10
648 625 756 1080 953 1184
2064 1734 2091 2058 2176 2336
2165 1821 2038 2060 2112 2084
2 that the normalized partition coefficient of P103 and P104, which have equal length PPO blocks, are of the same order while the normalized partition coefficient of L64, a polymer with a shorter PPO block, is significantly smaller. The solubility behavior of naphthalene in the micelle solution is, as expected, probably determined by the PPO block. Hurter found that the normalized partition coefficient of polymers with the same PPO blocks but different PEO fraction increased moderately with increasing PEO fraction (Hurter, 1992). The suggestion was made that this might be the result of a minor change in the structure of the micelle. It might be expected that the effective fraction PPO in P103 is larger than in P104 because of the decreased interaction between the PPO and PEO blocks, the latter being smaller and therefore more hydrophilic in P103 than in P104. As can be seen from Figures 5 and 6, the solubility of naphthalene in a micelle solution strongly depends on the temperature (in Figure 2 denoted as a change along the line a-c). In contrast to the saturation of naphthalene in pure water, which increases almost linearly with temperature, the saturation concentration in a micelle solution increases exponentially above a certain transition temperature. A further increase of the temperature usually results in a moderate dependency only, comparable with pure water. As can be seen from Figure 5, at 30 °C a 10 wt % P104 solution can absorb as much as 135 times the saturation quantity of naphthalene in pure water while at low temperatures the solubility is of the same order of magnitude. This rapid increase takes place over a temperature interval of about 20 °C. Moreover, experiments have shown that this process is reversible. Cooling down a saturated P104 solution from 30 to 10 °C results in crystallization of the naphthalene. After filtration over a Whatman No. 5 filter, over 99% of the naphthalene was recovered from the polymer solution without any measurable polymer loss. Hence, in a continuous extraction process, the polymer solution can be recycled while the naphthalene can be separated from the extraction liquid effectively. It can be seen from Figure 6 that the transition temperature is a polymer characteristic that probably depends on the PPO-PEO block lengths and block ratios. The transition temperature appears to increase upon a decrease in polymer length. The same trend can be observed for micelle formation. From literature (McDonald and Wong, 1974; Al-Saden et al., 1982; Wanka et al., 1990), it is known that L64 will hardly form any micelles at 25 °C while P104 forms stable aggregates at this temperature. It is also known that when the temperature is increased up to about 35 °C, L64 will form stable aggregates with an aggregation number of 30. It has also been shown that at elevated temperatures decreased interaction between the polymer and the water molecules will result in a mechanism of micelle formation at which unimers will be trans-
Figure 7. Representation of DSC results of PPG and P104 (∆H ) 66 J/g).
formed into larger aggregates (Zhou and Chu, 1988). This implies that two important effects that can influence the solubilization of hydrocarbons take place simultaneously. Due to the increased interaction between the PPO blocks and the water molecules, the water will be driven out of the core. Consequently, the core will become more hydrophobic, which will result in an enhancement of the selectivity for nonpolar components. Another effect that will influence the solubilization of naphthalene is the growth of the micelles. Since the radius of the core increases while the micelles are growing, the surface partial Gibbs free enthalpy will decrease (eq 6) so that the solubility of naphthalene increases. Obviously, the transition in the solubilization of naphthalene is coupled to the formation and growth of the micelles. DSC Experiments. In order to gain some basic insight in the phase transition taking place in the micelles, DSC experiments have been performed. The results of the DSC experiments of the pure polymers, polypropylene glycol (3000 D) and P104 are shown in Figure 7. The curve of PPG shows a glass transition at about -70 °C. However, in the temperature range between 10 and 30 °C, no significant heat effects take place. Hence melting of the core does not seem a likely mechanism for the solubilization transition taking place in this temperature range. The DSC curve of P104, however, does show an endothermic heat effect at 35 °C. This peak is most likely the result of melting of the PEO blocks, as it exactly corresponds with the melting temperature of pure PEG of the same molecular weight (for P104 MWPEG ≈ 800; Tmelt ) 32-36 °C). It can also be seen from the P104 curve that the PPO blocks in this polymer show the same glass transition as in PPG. The heat effect at the glass transition temperature seems to be proportional to the mass fraction PPO in P104, so that the behavior of the PPO and PEO blocks seems to be independent from each other. The DSC curves of 10 wt % P104 and L64 solutions (Figure 8) show an endothermic peak, which is similar to the melting peak of the pure polymer (Figure 7), although it occurs at a lower temperature in the dissolved state (22 °C versus 35 °C for dissolved and pure P104, respectively). The heat effect of both experiments is of the same order of magnitude (66 J/g for the melting of P104 and 55 J/g for L64, respectively). Yu et al. (1992) found, however, that the standard enthalpy of micellization is 1 order of magnitude lower. The difference probably reflects the difference between the standard state and the real state of the P104 solution. Wanka et al. (1990) also reported endothermic peaks of similar block copolymer (F127, P104, and P123)
3420 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 9. Calculated and experimental normalized partition coefficient at different temperatures for different core radii (nm) (calculations are performed with mPPO ) 58; +, L64; b, P104). Figure 8. Transition of the saturation and the DSC experiments of P104 and L64. (∆Hboth polymer solutions ) 5.5 J/g; s 10 wt % P104; ‚‚‚ 10 wt % L64). Table 3. Parameters Used in a Modified Flory-Huggins Model To Calculate Saturation of Naphthalene in a Polymer Solution RTχpwa RTχnwa RTχpna RTRa RTχp3b RTχw3b Fa γc a
1.7 8.5 1.4 11.5 24.8 × 10-2 13.6 60 25.9 b
kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol mN/m c
Karlstro¨m (1985). Hurter (1992). Naragajan and Ganesh (1989).
solutions, all having the shape of a typical first-order phase transition. It was expected that the DSC peak of L64 would appear at a lower temperature due to the shorter PEO blocks. However, as can be seen in Figure 8, the results show that the transition temperature is higher for L64 than for P104. It can also be seen that the transition temperature agrees well with the transition in the saturation experiments. The heat effect, which is probably the result of a transformation of the PEO blocks in the micel corona from an ordered to a disordered state, seems to be dependent on the aggregation behavior of the polymer, which on its turn is reflected in the solubilization of naphthalene. Modeling. The saturation experiments have been modeled by calculating the three-phase equilibria between the PPO core and the water bulk phase as a function of the temperature. The energy parameter (R), interaction parameters (χ’s), degeneration ratio (F), and surface tension (γ) that were used are listed in Table 3. It might seem more appropriate to use different values for the interaction parameters for naphthalene with polar and apolar PO units, respectively. However, the fraction apolar PO blocks in the micelle core exceeds 0.99 over the entire temperature interval investigated here, so that only one single χp3 value suffices for an adequate description (Hurter, 1992). Without a correction for changes of the curvature of the micelle, the normalized distribution coefficient for naphthalene over the PPO core and water decreases upon an increase in temperature. When this is combined with the fact that the polarity of the PPO blocks does not change over the entire temperature interval, it seems mandatory to include the curvature correction. The calculated results are depicted in Figure 9, from which it can be concluded
that an increase of the core radius from 1 nm to approximately 4 nm is sufficient to explain the measured normalized partition coefficients. It is known from literature (Zhou and Chu, 1988; Brown et al., 1991) that PEO-PPO-PEO polymers (e.g., F68) form micelles with a radius of about 7 nm at elevated temperatures, so that a core radius of about 4 nm seems to be a realistic value. It can also be seen in Figure 9 that the solubility increase caused by the composition change of the micelle at a constant radius is of minor importance while the growth of the core has a predominant effect. This phenomenon supports the idea that the temperaturedependent micelle formation mechanism as proposed by Zhou and Chu (1988) describes the major effect in the sudden solubility increase of naphthalene in the micelle solution. From Figure 9, it can be concluded that the core radius of P104 doubles between 15 and 20 °C, while the growth of L64 micelles is initiated at 20 °C and lasts over a larger temperature range. This is in agreement with the DSC experiments, indicating that the ordering of the PEO blocks, and thus the growth of the micelles, is located at higher temperatures and occurs over a broader range as compared to P104. Although a shift in the ratio of polar and nonpolar PO segments in the micelle core is the driving force for phase separation, it is remarkable that it plays an inferior role in the solubilization process. The gradual change of the fraction nonpolar polymer segments will not result in the expected increase of the naphthalene concentration. It seems plausible, however, that the polarity change of the PPO blocks induces micelle growth due to the fact that this is determined by the interaction between water and the PO segments. Conclusions The foregoing has shown that the solubilization of polyaromatics, such as naphthalene, in an aqueous PEO-PPO block copolymer solution can be increased drastically when the temperature is increased only moderately. Saturation experiments have shown that above a certain transition temperature the solubilization of naphthalene in a 10 wt % polymer solution can increase up to 135 times the solubility at low temperature, which is comparable to the solubility in water. The temperature difference required for this transition is only 10-20 °C, dependent on the block lengths and block ratios of the polymer. Moreover, the saturation experiments have shown that the solubilization process
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3421
is reversible so that the naphthalene crystals can easily be separated from the polymer solution without polymer losses. Taking these effects into account, combined with the variety of commercially available polymers, it can be concluded that this type of block copolymer solutions are a promising extractant with favorable regeneration properties. The results of this study suggest that the transition in the micelle solution is a result of the growth of the micelles, which affects the surface partial Gibbs free enthalpy such that the naphthalene saturation concentration will be increased. DSC experiments have shown a characteristic endothermic peak at the transition temperature, which probably indicates that the enhancement of the naphthalene solubility is closely related with the formation and the growth of the micelles. Moreover, modeling results have shown that the solubilization of naphthalene is predominantly determined by the increase of the micelle radius, while the polarity change of the micelle core is only of minor importance. Acknowledgment The authors wish to thank A. Witteveen for his contribution to the DSC measurements. Nomenclature F ) degeneration ratio between polar and nonpolar state (-) fn ) degeneration factor of nonpolar state (-) fp ) degeneration factor of polar state (-) G ) Gibbs free enthalpy (J/mol) k ) Boltzmann constant (J/K) KPPO ) normalized partition coefficient (-) m ) degree of polymerization (-) Mo ) total number of segments (-) P ) fraction polar polymer segments (-) Ro ) radius of unswollen particle (m) S ) entropy (J/mol‚K) T ) temperature (°C) vi ) volume fraction of component i (-) Vm ) molar volume (m3/mol) Greek Letters R ) energy difference between polar and nonpolar polymer segments (J/mol) γ ) surface tension (N/m) µs ) surface partial Gibbs free enthalpy (J/mol) χw3 ) FH interaction parameter between water and naphthalene (-) χp3 ) FH interaction parameter between PO and naphthalene (-) χnw ) FH interaction parameter between nonpolar PO segments and water (-) χpn ) FH interaction parameter between polar and nonpolar PO segments (-) χpw ) FH interaction parameter between polar PO segments and water (-)
Literature Cited Bjo¨rling, M.; Linse, P.; Kalstro¨m, G. Distribution of segments for terminally attached poly(ethylene oxide) chains. J. Phys. Chem. 1990, 94, 471. Brown, W.; Schille´n, K.; Almgren, M.; Hvidt, S.; Bahadur, P. Micelle and gel formation in a poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymer in water solution. Dynamic and static light scattering and oscillatory shear measurements. J. Phys. Chem. 1991, 95, 1850. Calvert, T. L.; Philips, J. P.; Dungon, S. R. Extraction of naphthalene by block copolymer surfactant immobilized in polymeric hydrogels. AIChE J. 1994, 40, 1449. Christian, S. D.; Scamehorn, J. F. Surfactant-based separation processes; Marcel Dekker: New York, 1989. Hurter, P. N. Solubility of polyaromatic hydrocarbons in micelle solutions. Saturation experiments and molecular modelling. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, 1992. Karlstro¨m, G. A new model for upper and lower critical solution temperature in poly(ethylene oxide) solutions. J. Phys. Chem. 1985, 89, 4962. Leiber, L.; Orland, H.; Wheeler, J. C. Theory of critical micelle concentration for solutions of block copolymers. J. Chem. Phys. 1983, 79, 3550. Malmsten, M.; Lindman, B. Self-assembly in aqueous block copolymer solutions. Macromolecules 1992, 25, 5440. Maxwell, I. A.; Kurja, J.; Van Doremaele, G. H. J.; German, A. L. Partial swelling of latex perticles with monomers. Makromol. Chem. 1992, 193, 2049. McDonald, C.; Wong, C. Effect of temperature on the micellar properties of a polyoxypropylene-polyoxyethylene polymer in water. J. Pharm. Pharmacol. 1974, 26, 556. Nagarajan, R.; Ganesh, K. Block copolymer self-assembly in selective solvents: Spherical micelles with segregated cores. J. Chem. Phys. 1989, 90, 5843. Noolandi, J.; Hong, K. M. Theory of block copolymer micelles in sollution. Macromolecules 1983, 16, 1443. Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Equilibrium solubilization of benzene in micellar systems and micelle-enhanced ultrafiltration of aqueous solutions of benzene. Am. Chem. Soc. Symp. Ser. 1986, No. 342, 184. Wanka, G.; Hoffman, H.; Ulbricht, W. The aggregation behaviour of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene)-blockcopolymer in aqueous solution. Colloid Polym. Sci. 1990, 268, 101. Yu, G.; Deng, Y.; Dalton, S.; Wang, Q.; Attwood, D.; Price, C.; Booth, C. Micellisation and gellation of triblock copoly(oxyethylene/oxypropylene/oxyethylene), F127. J. Chem. Soc. Faraday Trans. 1992, 88, 2537. Zhou, Z.; Chu, B. Light-scattering study on the association behavior of triblock polymers of ethylene oxide and propylene oxide in aqueous solution. J. Colloid Interface Sci. 1988, 126, 171.
Received for review February 6, 1996 Revised manuscript received June 11, 1996 Accepted June 14, 1996X IE960066W
X Abstract published in Advance ACS Abstracts, September 1, 1996.
