Investigations of Naphthalene Solubilization in Aqueous Solutions of

University of Greenwich, Wellington Street, Woolwich, London SE18 6PF, U.K.. Received August 3, 1998. In Final Form: May 6, 1999. The enhanced apparen...
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Langmuir 1999, 15, 6187-6194

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Investigations of Naphthalene Solubilization in Aqueous Solutions of Ethylene Oxide-b-Propylene Oxide-b-Ethylene Oxide Copolymers Iain F. Paterson,† Babur Z. Chowdhry,‡ and Stephen A. Leharne*,† School of Earth and Environmental Sciences, University of Greenwich, Pembroke, Chatham Maritime, Kent ME4 4AW, U.K., and School of Chemical and Life Sciences, University of Greenwich, Wellington Street, Woolwich, London SE18 6PF, U.K. Received August 3, 1998. In Final Form: May 6, 1999 The enhanced apparent solubility of naphthalene in aqueous solutions of several ABA block copolymeric surfactants has been measured using HPLC. The surfactants investigated combine, within their structure, a block of propylene oxide (PO) (the hydrophobic B block) sandwiched between two blocks of ethylene oxide (EO) (the hydrophilic A blocks). This commercially produced family of surfactants encompass a variety of materials which differ from each other in terms of block sizes and thus in terms of the balance of hydrophobic and hydrophilic forces. It is this balance which controls the ability of the surfactants to effect particular solubility enhancements. Substantial increases in solubility arise from the incorporation of naphthalene into block copolymer micelles. However the experimental data also point to solubility enhancements arising from naphthalene-surfactant interactions at surfactant concentrations below the critical micelle concentration (cmc). The apparent equilibrium constantsdescribing the solute distribution between the aqueous phase and the surfactantsthat characterizes this interaction correlates well with the surfactant cmc. It is concluded that the more hydrophobic EO-PO-EO copolymers produce micellar environments that are more favorable for naphthalene incorporation compared to the hydrophilic members of the family and that surfactant-naphthalene interactions below the cmc can substantially increase apparent aqueous solubilities. Experimental determinations of cmc values were measured by reductions in surface tension and by high-sensitivity differential scanning calorimetry and compared with the values estimated by naphthalene solubilization. The values obtained by the three methods are comparable. The data clearly demonstrate that cmc values are strongly dependent upon molecular composition and molecular size and that the cmc values are largest for small molecules with large hydrophilic blocks.

Introduction In surfactant systems the molecular combination of hydrophobic and hydrophilic residues provides a strong driving force for the migration of these substances to aqueous interfaces where the hydrophobic residues are able to minimize their exposure to water. This surface activity also provides the principal impetus for the formation of colloidal sized aggregates or micelles in which the hydrophobic moieties reduce their water exposure by self-association and the hydrophilic molecular fragments occupy a position at the interface between this hydrophobic interior and water.1 The formation of micelles provides a hydrophobic environment dispersed throughout the aqueous phase into which hydrophobic organic compounds are able to transfer. This phenomenon thus increases their apparent aqueous solubility. The migration of surfactants to the solid/water interface may also have a profound effect upon wetting characteristics. For instance the binding of surfactants to water which wets solid-phase porous materials via their hydrophilic groups and which, as a consequence, directs the hydrophobic segments toward the fluid phase will tend to facilitate the spreading of a nonaqueous phase liquid. This occurs because of the modification of surface forces.1,2 * Author for all correspondence. Telephone: +44 (0)181 331 9565. Fax: +44 (0)181 331 9805. E-mail: [email protected]. † School of Earth and Environmental Sciences, University of Greenwich. ‡ School of Chemical and Life Sciences, University of Greenwich. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989. (2) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 3rd ed.; Butterworth: London, 1980.

The role that surfactants can play in the facilitated removal of organic compounds, of environmental concern, from contaminated soils and aquifers is thus an important area of remediation research. Surfactants can assist the removal process either through the enhanced solubilization of hydrophobic organic compounds or because of the decrease in interfacial tensions in miscible displacement schemes.3 A number of workers have examined both these aspects of facilitated remediation in some detail,4-21 and an important book dealing with the subject area has been (3) Adeel, Z.; Luthy, R. G. Environ. Sci. Technol. 1995, 29, 1032. (4) Edwards, D. A.; Luthy, R. G.; Liu, Z. Environ. Sci. Technol. 1991, 25, 127. (5) Edwards, D. A.; Liu, Z.; Luthy, R. G. Water Sci. Technol. 1991, 23, 475. (6) Edwards, D. A.; Liu, Z.; Luthy, R. G. Water Sci. Technol. 1992, 26, 147. (7) Edwards, D. A.; Liu, Z.; Luthy, R. G. J. Environ. Eng. 1994, 120, 5. (8) Edwards, D. A.; Liu, Z.; Luthy, R. G. J. Environ. Eng. 1994, 120, 23. (9) Sun, S.; Inskeep, W. P.; Boyd, S. A. Environ. Sci. Technol. 1995, 29, 903. (10) Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1989, 23, 832. (11) Brickell, J. L.; Keinath, T. M. Water Sci. Technol. 1991, 23, 455. (12) Pennell, K. D.; Abriola, L. M.; Weber, W. J., Jr. Environ. Sci. Technol. 1993, 27, 2332. (13) Jafvert, C. T.; Van Hoof, P. L.; Heath, J. K. Water Res. 1994, 28, 1009. (14) Roy, D.; Liu, M.; Wang, G. J. Environ. Sci. Health 1994, A21, 197. (15) Rajput, V. S.; Higgins, A. J.; Singley, M. E. Water Environ. Res. 1994, 66, 819. (16) Sobisch, T.; Ku¨hnemund, L.; Hu¨bner, H.; Reinisch, G.; Olesch, T. Contaminated Soil ’95; van den Brink, W. J., Bosman, R., Arendt, F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; pp 1357-1358. (17) Joshi, M. M.; Lee, S. Frensius Environ. Bull. 1995, 4, 617.

10.1021/la980964s CCC: $18.00 © 1999 American Chemical Society Published on Web 06/29/1999

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recently published.22 Luthy and co-workers4-8 have studied the removal of polycyclic aromatic hydrocarbons using a variety of nonionic surfactants. In all cases the surfactant systems tested increase the apparent aqueous solubilities of these compounds quite significantly. Interestingly, several of the surfactant systems have also shown a clear capability of producing more modest solubility enhancements when the surfactant concentrations are below the cmc.6 Naphthalene Solubilization. The ability of the surfactants to produce an enhancement in the apparent aqueous solubility of naphthalenesin this studyscan be formulated as follows:9

S/w ) 1 + Ksub-cmcCsub-cmc + KmicCmic Sw

(1)

S/w (g L-1) is the apparent aqueous solubility of naphthalene in the aqueous surfactant system, and Sw (g L-1) is the water solubility of naphthalene in the absence of surfactant. The ratio of the two quantities is a measure of the solubility enhancement. Ksub-cmc is the distribution coefficient for the interaction of naphthalene and surfactant at sub-cmc surfactant concentrations (L mol-1) and is formulated as the ratio

S/w - Sw Sw Csub-cmc is the sub-cmc concentration of surfactant (mol L-1). Kmic is the constant for the binding of naphthalene to surfactant micelles (L mol-1), and Cmic is the concentration of surfactant in micellar form (mol L-1). Equation 1 can be converted into the following model forms depending upon whether the surfactant concentration is greater or smaller than the cmc:

S/w ) Sw 1 + Ksub-cmcCsurf if Csurf < cmc 1 + Ksub-cmccmc + Kmic(Csurf - cmc) if Csurf g cmc (2)

{

These two equations can therefore be used to model the solubility enhancement of naphthalene as a function of surfactant concentration (Csurf). A useful way to evaluate the effectiveness of a surfactant in producing a particular solubility enhancement is to define the molar solubilization ratio (MSR).4 The MSR is formulated as

MSR )

S/w - S/cmc Csurf - cmc

(3)

where (18) Pennell, K. D.; Jin, M.; Abriola, L. M.; Pope, G. A. J. Contam. Hydrol. 1994, 16, 35. (19) Abdul, A, S.; Gibson, T. L.; Ang, C. C.; Smith, J. C.; Sobczynski, R. E. Groundwater 1992, 30, 219. (20) Martel, R.; Gelinas, P. J.; Desnoyers, J. E.; Masson, A. Groundwater 1993, 31, 789. (21) Inaba, K.; Hirata, T. Environ. Technol. 1992, 13, 259. (22) Sabatini, D. A., Knox, R. C., Harwell, J. H., Eds. SurfactantEnhanced Subsurface Remediation; ACS Symposium 594; American Chemical Society: Washington, DC, 1995; chapters within.

S/cmc ) (1 + Ksub-cmc × cmc)Sw In other words S/cmc is the solubility of the solute when the surfactant concentration is just equal to the cmc. Edwards et al.4 have also suggested the use of the micelle phaseaqueous phase partition coefficient (KM) as a convenient way of describing solubilization in aqueous surfactant systems. KM is defined as the ratio of the mole fraction of the compound in the micellar pseudophase to the mole fraction of the compound in the aqueous pseudophase. It can be shown4 that this definition allows the following mathematical expression to be formulated:

KM )

(S/w - S/cmc) (Csurf - cmc + S/w - S/cmc)(VwS/cmc)

(4)

where Vw is the molar volume of water. EO-PO-EO Copolymers. The EO-PO-EO family of block copolymeric surfactants is commercially prepared from propylene oxide and ethylene oxide. For a good comprehensive and recent review of association in these materials, the interested reader is directed to ref 23. These surfactants are ABA block copolymers in which the A blocks are synthesized from ethylene oxide monomer (and constitute the hydrophilic segments of the molecule) and the B block is constructed from propylene oxide (and represents the hydrophobic moiety). These copolymers show the unusual property of aggregating to form micelles upon heating. This phenomenon has attracted much research interest. In a number of publications we have examined micellization in dilute aqueous EO-PO-EO copolymer solutions, using high-sensitivity differential scanning calorimetry (HSDSC) (for example see ref 24) and NMR.25 Self-aggregation arises because the central PPO block, which together with the PEO blocks is reasonably watersoluble at low temperatures, becomes increasingly hydrophobic as the temperature is raised. Attempts to understand this phenomenon have normally proceeded from a consideration of the aqueous solubility of PEO and PPO homopolymers. Kjellander and Florin26 have suggested that PEO chains can be accommodated within an icelike structure. The formation of such a structure produces a favorable (exothermic) enthalpy change but also results in an entropy penalty associated with the enhanced structuring of water. At low temperatures this enthalpy contribution together with the combinatorial entropy contribution of the chains to the free energy of mixing outweighs the entropy penalty. However, an increase in temperature reverses this, giving rise to phase separation. The theory may also be used to account for the solubility of PPO. In this case, however, the pendant methyl group produces a strain in the icelike structure of water in the hydration sphere, which results in phase separation at lower temperatures. Karlstro¨m, on the other hand,27 has suggested that the origin of the increasing hydrophobicity of PEO is the result of changing conformations of ethylene oxide (EO) segments. For the segment O-C-C-O the preferred orientation about the bonds is (23) Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1 and references therein. (24) Paterson, I.; Armstrong, J.; Chowdhry, B.; Leharne, S. Langmuir 1997, 13, 2219. (25) Beezer, A. E.; Mitchell, J. C.; Rees, N. H.; Armstrong, J. K.; Chowdhry, B. Z.; Leharne S.; Buckton, G. J. Chem. Res. 1991, 254. (26) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2053. (27) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962.

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Table 1. Molecular Composition of the Poloxamers Used in This Study with cmc Data Obtained in the Investigation Combined with a Comparison of Data Obtained from Other Investigations and Solubilization Data

F38 L42 L62 F68 F77 P84 F87 L92 P103 P105 P108 F127 d

PEO mass

PPO mass

3850 300 440 6600 4550 1500 5450 700 1700 3250 11680 8000

950 1200 1750 1750 2050 2250 2250 2750 3250 3250 2920 4000

cmc,a mmol dm-3

cmc,b mmol dm-3

cmc,c mmol dm-3

1160 140

1.43 1.3 1.2 0.99 1.1

cmc,e mmol dm-3

log Ksub-cmc

21 7

470 460 2.0

cmc,d mmol dm-3

1.4 2.2

320f

4.1 59 1.0 0.1 0.2 0.2

3.5 0.8 1.3 0.7 0.4

0.7 0.8 0.51 0.8

log Kmic

2.0 1.8

0.1 0.5 3.1 0.6

2.4 2.5 3.0

3.7

3.7 3.9 3.8 3.7 3.9

4.1 4.1 4.0 3.8 4.0

a Obtained from solubilization measurements. b Obtained from HSDSC measurements. c Obtained by surface tension measurements. Data obtained from ref 36. e Data obtained from ref 31. f Estimated from cmt-concentration data in ref 31.

t-g-t.27,28 This polar conformation interacts favorably with water, there being some two water molecules per EO unit.28 It is of low energy but also of low statistical weight, there being only two of these conformations.27 At higher temperatures less polar orientations are favored. These are of higher energy but of higher statistical weight, there being some 23 of these conformations. The less polar conformations interact less favorably with water. The resulting loss of water at higher temperatures permits the chains to come together. This model has been used, with some success, to explain phase separation in aqueous PEO solutions.27 The changes in C-C from gauche to trans, thereby altering polarity, have been confirmed by 13C NMR.29,30 Micellization in PEO-PPO-PEO block copolymers is understood to arise for similar reasons.23,31 As the temperature of a block copolymer solution is raised, the PPO block progressively loses its hydration sphere, resulting in a greater interaction between the PPO blocks on different chains. On the other hand, the PEO blocks retain their strong interaction with water; thus, as is common for all amphiphilic molecules, the differing phase preferences of the blocks drive the copolymers to form micelles. The EO-PO-EO copolymer surfactant systems were chosen for study for two reasons. First, to the best of our knowledge no work has been reported in the literature on their direct use for contaminated land remediation and yet they appear to possess properties which make them suitable for this task.32 Second, some 26 members of the EO-PO-EO copolymer family are commercially manufactured. Since they differ from each other in terms of the size of the A and B blocks, this permits an evaluation of how changes in the balance between hydrophobic and hydrophilic segments affect the facilitated desorption of organic contaminants from soil. In this paper we report details on the solubilization of naphthalene, using an array of EO-PO-EO copolymers. Hurter and Hatton32 have also reported the use of micellizing EO-PO-EO copolymers for naphthalene solubility enhancements. However, in this investigation we extend their work by examining the enhanced solubilization of naphthalene in solutions where the EO-POEO copolymer concentration is both above and below the (28) Hergeth, W.-D.; Alig, I.; Lange, J.; Lochmann, J. R.; Scherzer, T.; Wartewig, S. Makromol. Chem., Macromol. Symp. 1991, 52, 289. (29) Bjo¨rling, M.; Karlstro¨m, G.; Linse, P. J. Phys. Chem. 1991, 95, 6706. (30) Rassing, J.; McKenna, W. P.; Bandyopadhyay, S.; Eyring, E. M. J. Mol. Liq. 1984, 27, 165. (31) Alexandridis, J. F.; Holzwarth; Hatton, T. A. Macromolecules 1994, 27, 2414. (32) Hurter, P. N.; Hatton, T. A. Langmuir 1992, 8, 1291.

cmc. In addition, we have examined the effects of EOPO-EO copolymers, that only micellize at high temperatures and/or concentrations, upon naphthalene solubility. Experimental Section Materials. The EO-PO-EO copolymers, used in this work, were kindly donated by ICI Chemicals Ltd (Cleveland, U.K.). These materials are polydisperse and probably contain diblock impurities.33,34 They were, however, used as received. Molecular composition details of the copolymers are outlined in Table 1. All polymer solutions were prepared using doubly distilled water. Naphthalene (99% grade) was obtained from Sigma Chemicals (Poole, Dorset, U.K.) and used as received. Measurements of Enhanced Solubility. The solubilization experiments were carried out by preparing saturated solutions of naphthalene. Saturation was inferred from the continuing presence of a separate solid naphthalene crystalline phase. The solutions were prepared as follows. 2 mL ambered glass vials were filled with aqueous surfactant solution in the range 0.16% w/v. A single crystal was placed in each solution, and the vials were then sealed with open top screw caps fitted with Teflonbacked septa. The vials were placed in a temperature-controlled orbital shaker and shaken at 100 rpm at a temperature of 25 °C for 24 h. The saturated solution was then directly injected into an HPLC instrument (Phillips PU4100) fitted with a UV detector (Phillips PU4020). The column used was a spherisorb ODS-5 column, and the mobile phase used was a 60/40 methanol/water mixture. HPLC was used for the determination of naphthalene in aqueous surfactant solution, since it conveniently provides a way of diluting the surfactant concentration below the surfactant cmc and thus obviating any problems which may result because of shifts in λmax which may occur as naphthalene becomes progressively incorporated into the hydrophobic micellar environment as Csurf increases. The maximum estimated uncertainty in the data is (2.0%. Measurement of cmc. A convenient and effective way to measure the cmc of the EO-PO-EO copolymers is to use highsensitivity differential scanning calorimetry (HSDSC).24 Significantly HSDSC can be shown to provide results which are comparable with cmc values derived using other techniques.23 The cmc values were determined in the following way. An EO-PO-EO copolymer solution of known concentration was scanned in the calorimeter and from the calorimetric output (see Figure 1) the temperature at which micellization begins was identified. This temperature is the critical micellization temperature (cmt). The concentration of surfactant in solution is the cmc value at this temperature. If several different concentrations of each individual EO-PO-EO copolymer are run, a matrix of cmc and cmt data is then obtained and a graphical technique can then be used to interpolate or extrapolate the cmc value at 25 °C. (33) Yu, G.; Deng, Y.; Dalton, S.; Wang, Q.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 2537. (34) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548.

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Figure 1. HSDSC output for F87 (50 g dm-3). The diagram indicates how the cmt is obtained. The scanning calorimetric data were obtained using a Microcal MC2 high-sensitivity differential scanning calorimeter (supplied by MicroCal, Northampton, MA). The instrument was interfaced to an IBM Model 30 personal computer. The DA2 software supplied by the manufacturers facilitated instrumental control and data acquisition. All calorimetric scans were normally obtained at a scan rate of 1 K min-1. In some cases samples were scanned at a variety of scan rates. The absence of any changes in the signal at different scan rates indicates that the micellization transitions examined are under strict thermodynamic control.35 Data uncertainty was estimated to be of the order (0.5%. cmc values were also obtained by surface tension measurements using the Du Nou¨y ring detachment method,2 employing an analogue torsion balance (White Electrical Instrument Co. Ltd., Worcestershire, U.K.). The measurements were performed at 25 °C using a platinum ring, which had previously been washed in propanone and heated in a flame until it glowed red. The platinum ring was treated in this way between each measurement. All glassware was cleaned between runs using chromic acid. Measurements were performed at each surfactant concentration until two consecutive readings were within about 0.3% of each other. Repeat determinations demonstrated good reliability, and cmc data uncertainty is on the order of (3%.

Results and Discussion cmc values. The interpretation of the solubilization data obtained in this investigation relies, in the main, upon an appreciation of the surfactant concentrations at which micellization occurs. The EO-PO-EO copolymers examined are readily divided into two groups: those that micellize at ambient temperatures and those that only micellize at elevated temperatures. Changes in surfactant concentration also affect the temperature at which micellization occurs. Micellization in aqueous EO-PO-EO copolymer systems is an endothermic process.24 If the aqueous surfactant concentration is increased, then this increases the thermodynamic driving force toward micelle formation, and as micellization requires heat, the temperature of the system will decrease. In practice, as the concentration of surfactant is increased, the temperature at which micellization occurs decreases. The set of EOPO-EO copolymers that micellize at ambient temperatures consists of block copolymers which possess large propylene oxide blocks and relatively small ethylene oxide segments. The collection of copolymers that do not readily micellize contains, in the main, short PO blocks sandwiched between large EO segments. (35) Sanchez-Ruiz, J. M.; Lopez-Lacomba, J. L.; Cortijo, M.; Mateo, P. L. Biochemistry 1988, 27, 1648.

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Figure 2. Plots of 1/cmt (the critical micellization temperature) as a function of ln(surfactant concentration) for various EOPO-EO copolymers. The plots were used to interpolate and in some cases extrapolate values for the cmc.

cmc values were obtained using a variety of techniques, and Table 1 provides a summary of these values using HSDSC, surface tension, and solubilization measurements. For comparison, the results obtained in two other investigations31,36 are also shown. For the micellizing copolymers with large PO blocks and/or large molecular masses which micellize at room temperature, the agreement between results is good. Figure 2 shows the data used to obtain surfactant cmc values using HSDSC. The graphs all reveal good linear plots between the reciprocal of the cmt and the log of the surfactant concentration. cmc values are readily estimated from these graphs by interpolation at 25 °C. For the “nonmicellizing” surfactants examined in this section, cmc values can be obtained by extrapolation back to 25 °C. Extrapolation can be misleading, and as a consequence, caution must be exercised in the use of the values obtained. However, in this study only the extrapolated cmc value obtained for F38 is far removed from the measured concentration-cmt data. The other data sets are much closer and importantly do not show any curvature which might invalidate the extrapolation procedure. Typically surface tension-log concentration plots for the micellizing copolymers show a break at surfactant concentrations in the range 10-4 to 10-3 mol dm-3 (Figure 3). This break is ascribed to the onset of micellization,23 whereas the nonmicellizing group shows no such feature even at concentrations as high as 10-2 and 10-1 mol dm-3 (see also Figure 3). By and large the cmc values obtained for the various micellizing copolymers are comparable. The HSDSC results for the P103, P105, and F108 groups are, however, somewhat lower than other estimates. For the nonmicellizing copolymers, however, the HSDSC results are some 2 orders of magnitude greater than those obtained by Lopes and Loh.36 As explained previously, the cmc estimates for these surfactants rely upon extrapolation of HSDSC data to 25 °C, which may result in making unacceptable assumptions about the nature of the relationship between the dependent and independent variables outside the region investigated. However, it is also possible that the discrepancy between the two data sets arises from the use made, by Lopes and Loh, of reporter molecules, which could reduce the measured cmc value through favorable interactions with micelles. (36) Lopes, J. R.; Loh, W. Langmuir 1998, 14, 750.

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Figure 3. (a) Surface tension versus log concentration plots for those EO-PO-EO copolymers identified in the text as nonmicellizing. (b) Surface tension versus log concentration plot for F127, which does micellize. Note the existence of two breaks. The higher concentration break occurs at the cmc.23

It is clear, however, that the data do demonstrate that small molecules and those which possess small PO blocks and/or large EO blocks show little surface activity and are consequently distinguished as having large cmc values. Bigger molecules with large PO blocks have much lower cmc values. Solubilization in Surfactant Micelles. Before discussing the results in detail, it should be noted that the distribution coefficients evaluated in this study were obtained under conditions in which the aqueous phase was saturated with solute. Previous work undertaken examining benzene, toluene, xylene, and chlorobenzene solubilization in aqueous EO-PO-EO copolymer systems has noted that partitioning is dependent upon the aqueous phase concentration of the solute.37 It has been proposed that this is due to differences in the free energy of solute binding at various locations in the EO-PO-EO copolymer micelles. The value of the data provided in our investigation resides in the fact that normally surfactant washing/ solubilization will only be used for contaminated soils where the levels of contamination will provide saturated conditions. P103 is a good example of the group of surfactants which micellize at 25 °C. A typical plot of solubility enhancement as a function of surfactant concentration is shown for P103 in Figure 4. The diagram indicates a clear break in the region of the cmc. Below the cmc there are solubility enhancements due to the interaction of the notionally unimeric surfactant with naphthalene (further discussion of these results appears later in the text). However above the cmc the slope of the line increases, demonstrating an increased affinity between the solute and the micellar aggregates. Figure 5 shows the impact of various micelle-forming EO-PO-EO copolymers upon enhanced solubility at surfactant concentrations in excess of the respective cmc values. Clear differences emerge between the compounds in terms of their effectiveness in increasing the apparent aqueous solubility of naphthalene. For example the surfactants in the group consisting of P103, P105, and P108 (shown in Figure 5) all possess the same size propylene oxide block but differ from each other in terms of their ethylene oxide composition. In this respect it must be anticipated that the order of decreasing hydrophobicity is P103 > P105 > P108, which is the order of decreasing (37) Gadelle, F.; Koros, W. J.; Schechter, R. S. Macromolecules 1995, 28, 4883.

Figure 4. Naphthalene solubility enhancement ratio (S/w/Sw) as a function of P103 surfactant concentration. b represents the experimental data. The solid line is the best fit line through the data using the model outlined in eqs 2 and 3.

Figure 5. Solubility enhancement as a function of micelleforming EO-PO-EO copolymer concentration.

solubilization enhancement effectiveness. We can therefore conclude that the more hydrophilic copolymers produce micellar environments that are slightly less favorable for naphthalene incorporation. Some effort has been devoted to attempting to understand this suppression of naphthalene incorporation into micelles comprising EO-PO-EO copolymer molecules of increasing hydrophile composition. Modeling studies have demonstratedsfor a series of copolymers in which the propylene oxide moiety is of constant sizesthat as the

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ethylene oxide composition increases, the micelles become smaller.23,38 This decrease in micellar size is demonstrated by a reduction in core size and smaller aggregation numbers. Naphthalene is confined to the hydrophobic core, indeed it will even avoid the core/corona interface; consequently, as the micellar core decreases increasingly smaller amounts of naphthalene can be accommodated in the micelles. Small micelles also possess large internal “Laplace” pressures.2 Increasing the pressure on the solute results in an increase in its fugacity, but since the solute fugacity in the aqueous pseudophase does not change, this process results in a decrease in the water micelle partition coefficient for the solute.32 The most effective polymeric surfactant for enhancing naphthalene solubility is L92. This compound possesses a fairly large propylene oxide block and small ethylene oxide segments. However, interpretation of its solubilization behavior is complicated by the fact that it appears to phase separate (the solutions were cloudy) over much of the concentration range examined. It would certainly be anticipated that solubility is dramatically enhanced in systems in which the surfactant undergoes phase separation.1 The data in Figure 4 were readily fitted to the solubilization model outlined in eqs 2 using the software package Scientist (MicroMath, Salt Lake City, UT). The data obtained from the model-fitting exercise are set out in Table 1. Kmic values were obtained only for those compounds which micellize at ambient temperatures and at reasonable concentration levels. Interpretation of the Kmic data is not straightforward. Multiple linear regression analysis of the data does not provide any statistically meaningful insights. However, it is clear that if we compare P103, P105, and P108, then Kmic does become smaller as the ethylene oxide composition increases, pointing to the conclusions arrived at previously that as the surfactants become more hydrophilic (namely as the ethylene oxide composition increases while the propylene oxide block length remains constant), their micelles become increasingly less attractive environments for hydrophobic compounds. This trend can be confirmed more dramatically if we normalize Kmic with respect to the propylene oxide composition to obtain Kmic′ (units: L/mol of PPO), which is then plotted against mass of ethylene oxide. This plot is shown in Figure 6 and indicates that as the ethylene oxide composition increases, the relative amount of naphthalene solubilized in the micellar environments compared to that solubilized in the aqueous pseudophase per mole of propylene oxide decreases. It should be noted that the data point for P84 appears to be an outlier. The plot for the rest of the data suggests that the impact of ethylene oxide is pronounced at low ethylene oxide compositions but seems to become less dramatic when the mass of ethylene oxide is large. An alternative way of viewing the data is to consider the compounds P103, P105, and P108, which have propylene oxide blocks of very nearly equal size but differ from each other in terms of the ethylene oxide block size. The inset in Figure 6 shows that Kmic′ increases as the proportion of the molecule composed of propylene oxide increases. It is therefore clear that as the surfactants become more hydrophobic, they provide an ever increasingly favorable environment for the solubilization of nonpolar solutes. Hurter and Hatton have reported similar results for the same materials.32 Table 2 presents the calculated MSR (molar solubili(38) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993, 26, 5592.

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Figure 6. Constant characterizing the distribution of naphthalene between water and micelles normalized for propylene oxide content Kmic′ as a function of ethylene oxide composition. The inset shows Kmic′ as a function of the fraction of propylene oxide in the surfactant molecule for P103, P105, and P108. Table 2. Molar Solubilization Ratio, Solubility at the cmc, and log Km Data Obtained for All EO-PO-EO Copolymers Capable of Micellization at 25 °C P84 L92 P103 P105 P108 F127

S/cmc, mg dm-3

MSR

log Km

0.38 1.08 1.33 1.01 0.70 1.37

1.3 3.4 2.9 2.5 1.7 2.5

4.6 4.3 4.3 4.6 4.5 4.4

zation ratio), S/w data, and KM (the dimensionless micelle water partition coefficient) data. The MSR values contained in the table are unsurprising in that L92 and P103, putatively the most hydrophobic copolymers, display the highest MSR values. For the P103, P105, and F108 group, the trend in S/w values demonstrates that for EO-POEO copolymers of equal PO block size, as the ethylene oxide composition is increased, the solubility at the cmc decreases. It is interesting, however, that the KM values do not seem to alter significantly. All the values are of the same order of magnitude and vary between 20 × 103 and 41 × 103. This probably arises from the definition of KM. The mole fraction of naphthalene in the aqueous pseudophase is defined as Vw × S/cmc, where Vw is the molar volume of water and S/cmc is the solubility of naphthalene in an aqueous surfactant solution when the surfactant concentration is equal to the cmc. S/cmc is the sum of the concentration of naphthalene that is dissolved in water and the concentration of naphthalene dispersed in solution through interaction with nonmicellar surfactant. For the surfactants investigated, the naphthalene-nonmicellar surfactant interaction will dominate the Scmc term. As the composition of the surfactants changes interactions with both nonmicellar surfactant and micelles will change in the same direction. Thus, it must be expected that KM will tend to remain constant because any compositional change that, for instance, increases the mole fraction of naphthalene in the micellar pseudophase will also increase the extent of naphthalene-nonmicellar surfactant interaction in the aqueous pseudophase. Under these circumstances KM is clearly not a useful parameter for expressing differences in the solubilization effectiveness of different EO-PO-EO copolymers. It does, however, provide a good guide to the ability of the solute to partition into the surfactant pseudophase.4 It is useful to note that the KM values estimated by Edwards et al.4 for naphthalene in four different nonionic

Naphthalene Solubilization in Solutions of Copolymers

Figure 7. Solubility enhancements as a function of surfactant concentration for a number of copolymers at concentrations below their respective cmc values.

surfactant systems are similar to those measured in this study and again are very nearly constant. This may suggest that the partitioning processes, as measured by KM, are similar for a variety of nonionic surfactants. Certainly the plots shown in ref 4 and the authors’ subsequent work6 demonstrate that surfactant-solute interaction produces solubility enhancements below the cmc. It is, therefore, likely that the constancy of KM is due to there being a constant free energy difference between naphthalene in the aqueous pseudophase (comprising water and nonmicellar surfactant) and in the micellar pseudophase. Their quoted MSR values, on the other hand,4 are an order of magnitude smaller than the ones measured in our study and may suggest that the EOPO-EO copolymers are more effective solubilizing agents. Solubility Enhancement at Sub-cmc Surfactant Concentrations. As well as the solubility enhancements noted above the cmc for a variety of EO-PO-EO copolymers, solubility enhancements have also been recorded for the same materials at sub-cmc concentrations. In addition, those surfactants which do not micellize at ambient temperatures also bring about increases in solubility. These enhancements are of interest because they point to a mechanism by which nonmicellar surfactant interacts with naphthalene to produce an increase in apparent aqueous solubility. And from the data reported in this article, this appears to be a general behavioral feature of all EO-PO-EO copolymers. Plots of solubility enhancement S*/S against surfactant concentration at sub-cmc concentrations, in the main, provide good linear relationshipssthe slope being described by the parameter Ksub-cmc. Figure 7 displays the data obtained for a number of surfactants that do not micellize at ambient temperatures. Again, the data indicate similar trends to those for the micellizing systemssnamely the largest enhancement in apparent solubility is associated with the surfactant (F87), which possesses the largest propylene oxide block and 70% ethylene oxide. The smallest enhancement is associated with the least hydrophobic material (F38), which has the smallest propylene oxide block and 80% ethylene oxide. The behavior of F87 is anomalous compared to that of the others in that the relationship between solubility enhancement and surfactant concentration is nonlinear. This may be due to the formation of a small number of micelles in the upper concentration range. In the upper concentration region, between 5 and 10 mmol dm-3, the cmt is close to but greater than 25 °C.24 As the concentration increases, the cmt decreases. The presence

Langmuir, Vol. 15, No. 19, 1999 6193

of the hydrophobic solute might be expected to reduce the cmt further. Thus, in the upper concentration region micellization probably begins to occur. However, the number of micelles in this region is small,39 and in the initial stages, the micelle concentration only slowly changes; thus, the solubilization regime comprises a relatively large number of nonmicellar surfactant molecules and a small number of micelles. It is this that will account for the curvature of the plot. It is clear that a suitable explanation must be provided to account for the solubilization enhancements at subcmc concentrations. The solubilization of nonpolar organic compounds in water involves the isolation of organic molecules from the bulk organic liquid phase and their subsequent insertion into water cavities.40 Thus, the extent to which naphthalene is soluble in aqueous solution is a function of the total hydrophobic surface area of the molecule and the interfacial tension at the solute-water cavity interface.40 The introduction of salts and cosolvents, which alter interfacial tension, can have an important impact upon solubility in the aqueous phase. Thus, it might be anticipated that the fundamental property of surfactants to adsorb at interfaces and consequently modify interfacial energies could play some role in these sub-cmc solubility enhancements. Indeed one can envisage that if dispersed naphthalene molecules have more than one surfactant molecule in the water cavities surrounding them, then the assemblies take on the appearance of premicellar aggregates. Marinov et al.41 have concluded from their fluorescence studies of pyrene in aqueous solutions of L64 that premicellar aggregates are formed. Wyn and co-workers42,43 have reported that, at low temperatures and concentrations, premicellar clusters involving large aggregation numbers are formed which disappear as concentration and/or temperature are raised. Incorporation of naphthalene into these clusters thus provides an additional proposition to explain the premicellization solubilization effects. Indeed, it seems perfectly feasible that a combination of both effects may play a role in these sub-cmc observations. From Tables 1 and 2 it is interesting to note the relationship which appears to exist between the tendency to micellize and Ksub-cmc for the entire range of surfactants. Indeed, a good linear free energy relationship exists between the cmc and Ksub-cmc as shown in Figure 8. The relationship should be treated with some caution, since some of the cmc values have been obtained by extrapolation. However, the data do seem to conform to a trend in which low cmc values are associated with more hydrophobic molecules. This implies that it is the scale of the hydrophobicity that gives rise to the value of Ksub-cmc. This would certainly tie in with the notion that increasing hydrophobicity provides a powerful driving force for interfacial adsorption1 and thus provides the necessary impetus for adsorption at the naphthalene-water cavity interface, thereby reducing the interfacial tension and increasing the apparent aqueous solubility of the solute. Multiple linear regressionsperformed using the data analysis tools supplied with the spreadsheet package Excelssuggests the following relationship between Ksub-cmc and molecular composition: (39) Patterson, I.; Chowdhry, B.; Leharne, S. Colloids Surf. A 1996, 111, 213. (40) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; John Wiley and Sons Inc.: New York, 1993; pp 98-102. (41) Marinov, G.; Michels, B.; Zana, R. Langmuir 1998, 14, 2639. (42) Brown, W.; Schille´n, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. (43) Brown, W.; Schille´n, K.; Hvidt, S. J. Phys. Chem. 1992, 96, 6038.

6194 Langmuir, Vol. 15, No. 19, 1999

Figure 8. Linear free energy plot of log cmc versus log Ksub-cmc, the nonmicellar surfactant-water partition coefficient. cmc values are in mmol dm-3.

Ksub-cmc ) -4430 - 0.038MEO + 3.1MPO The adjusted R2 value is 0.77. The contribution to Ksub-cmc made by the ethylene oxide segments is negative and small while the contribution made by the propylene oxide block is large and positive. In other words, ethylene oxide reduces nonmicellar surfactant-naphthalene interaction whereas propylene oxide significantly increases it. Examination of Solubilization with EO-PO-EO Copolymers Containing 10 and 20% Ethylene Oxide. The large increases in apparent aqueous solubility observed in L92 solutions prompted an examination of the use of EO-PO-EO copolymers that consist of between 10 and 20% ethylene oxide. The results obtained are shown in Figure 9. For the 20% ethylene oxide series the relationship between surfactant concentration and solubility enhancement is reasonably linear. It is clear that for L42 (EO4PO21EO4), the molecule containing the smallest propylene oxide block length and therefore the most hydrophilic one, the solubility enhancement produced is extremely modest. The solubility enhancements noted are due entirely to solute nonmicellar surfactant interaction. L42 only begins to aggregate at a concentration of 141 mmol dm-3 at 25 °C. Aggregation will only occur at higher temperatures if lower concentrations are used, as in this investigation. The highest concentration used in this investigation was just less than 40 mmol dm-3. It is unclear whether L42 actually forms micelles. It was noted that all the surfactants investigated in this section of the project produced cloudy solutions, thereby providing compelling evidence for phase separation. Propylene oxide oligomers of similar molecular masses to those of the copolymers observed here undergo phase separation, and yet the phase separation signal as observed by HSDSC is similar to the micellization signals noted for those copolymers which do not phase separate.44 It has been suggested for propylene oxide oligomers that phase separation may occur as a process in which nuclei of micellar dimensions are formed and subsequently interact to provide the growth stage of phase separation.44 It is likely that for the copolymers examined in this section a similar series of events occur. L62 (EO5PO30EO5) is more hydrophobic than L42, which is demonstrated by the higher solubility enhancements noted over the entire surfactant concentration range. No (44) Chowdhry, B. Z.; Snowden, M. J.; Leharne, S. A. J. Phys. Chem. 1997, 101, 10226.

Paterson et al.

Figure 9. Solubility enhancements as a function of surfactant concentration for a number of copolymers which contain either 10% ethylene oxide (L61, L81, and L121) or 20% ethylene oxide (L42 and L62).

datasdetailing aggregation temperature and aggregation concentration relationshipsswere obtained for this copolymer. However, a similar size propylene oxide oligomer has been shown to aggregate at 23 °C and at a concentration of 2.5 mmol dm-3.44 L62 contains ethylene oxide, which should counteract the commencement of aggregation. However, it is likely that aggregation still occurs in the upper concentration region of the plotsthe system is cloudy in this regionsand this may explain the superior solubility enhancements noted for the copolymer. Interestingly, L92 (EO8PO47EO8) is only slightly bigger than L42 and L62, and yet the solubility enhancements noted for this material are enormous. This points to the expectation of an even better solubilization performance obtainable with copolymers containing still bigger propylene oxide block lengths and 20% ethylene oxide. The 10% ethylene oxide series data are also displayed in Figure 9. The results demonstrate an excessive amount of scatter and clearly point to experimental problems with their use. The modest solubility enhancements noted probably preclude any future assessment of their utility in surfactant-facilitated desorption of naphthalene from contaminated soils. Evidently, other workers have also experienced problems with these copolymers.37 Conclusions The technical development of facilitated desorption of hydrophobic organic compounds from contaminated soils requires surfactants which are capable of producing significant aqueous solubility enhancements. The family of block copolymeric surfactants examined in this investigation have demonstrated that notable results are obtainable with materials which combine large propylene oxide (hydrophobic) block lengths with ethylene oxide compositions at least equal to 20% and preferably greater. Importantly, in many cases large solubility enhancements were also noted for systems below the cmc. This arises because of nonmicellar surfactant-naphthalene interaction. This aspect of solubility enhancement may provide an important perspective of the environmental importance of these materials, especially in waste water treatment facilities. Acknowledgment. S.A.L. and B.Z.C. wish to gratefully acknowledge the receipt of EPSRC Grant GR/H95174, which was used to partially support the work reported in this paper. LA980964S