Influence of the Hydrotrope Structure on the Physical Chemical

Feb 26, 2005 - The critical micelle concentration and the cloud point of the copolymer solutions were displaced to higher concentration values, indica...
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Langmuir 2005, 21, 2696-2703

Influence of the Hydrotrope Structure on the Physical Chemical Properties of Polyoxide Aqueous Solutions Claudia R. E. Mansur,†,‡ Renata V. Pires,† Gaspar Gonza´lez,§,| and Elizabete F. Lucas*,† Instituto de Macromole´ culas/Universidade Federal do Rio de Janeiro (IMA/UFRJ), C. T., Bloco J, P.O. Box 68525, 21945-970, Rio de Janeiro, Brazil, and PETROBRAS Research Center (CENPES/PETROBRAS), Ilha do Funda˜ o, Q.7, 21949-900 Rio de Janeiro, Brazil Received July 21, 2004. In Final Form: November 18, 2004 The physical chemical properties of block substituted poly(ethylene oxide-propylene oxide) (PEOPPO) block copolymer aqueous solutions were evaluated in the presence of two hydrotropes of different structures: sodium p-toluene sulfonate (NaPTS) and butyl monoglycol sodium sulfonate (NaBMGS). The critical micelle concentration and the cloud point of the copolymer solutions were displaced to higher concentration values, indicating that the solubility of the copolymer was increased in the presence of the hydrotropes. Temperature increased the micelle hydrodynamic radius, but concentration had a limited effect. Carbon-13 nuclear magnetic resonance (13C NMR) permitted the interaction between the surfaceactive agent and the hydrotrope to be evaluated: NaBMGS, which presented a more pronounced hydrotropic effect, interacts more effectively with the hydrophobic moiety of the surfactant, while NaPTS interacts rather mainly with the hydrophilic oxyethylenic groups. The results furnish experimental evidence to conclude that the hydrotropic phenomenon is specific in relation to both the hydrotrope and the solubilizate.

Introduction Aqueous solutions of nonionic block polymeric surfaceactive agents present phase separation with an increase in temperature. At temperatures below phase separation, such surface-active agents are dissolved as molecular aggregates known as micelles in equilibrium with free surfactant molecules, hereafter called unimers. The temperature at which micelle formation occurs is called the critical micelle temperature (cmt), and the concentration at which micelle formation starts, at a constant temperature, is known as the critical micelle concentration (cmc).1 Additives can change the solubility of these nonionic surfactants, and various mechanisms have been proposed to explain such modifications.2-7 A class of these additives is known as hydrotropes or hydrotropic agents. These compounds are amphiphilic molecules that, at high concentrations, present the ability of increasing the solubility of poorly soluble organic compounds.8-13 Mol* Corresponding author. E-mail: [email protected]. † IMA/UFRJ. ‡ E-mail: [email protected]. § PETROBRAS Research Center. | E-mail: [email protected]. (1) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, U.K., 1986. (2) Balasubramanian, D.; Srinivas, V.; Gaikar, V. G.; Sharma, M. M. J. Phys. Chem. 1989, 93, 3865-3870. (3) Ivanova, V. P.; Topchiyeva, I. N. Polym. Sci. U.S.S.R. 1989, 31, 2594-2599. (4) Schott, H. J. Colloid Interface Sci. 1995, 173, 265-277. (5) Ali, B. A.; Zughul, M. B.; Badwan, A. A. J. Dispersion Sci. Technol. 1995, 16, 451-468. (6) Schott, H. J. Colloid Interface Sci. 1998, 205, 496-502. (7) Gonza´lez, G.; Nassar, E. J.; Zaniquelli, M. E. D. J. Colloid Interface Sci. 2000, 230, 223-228. (8) Schott, H. J. Colloid Interface Sci. 1995, 173, 265-277. (9) Schott, H. J. Colloid Interface Sci. 1998, 205, 496-502. (10) Mansur, C. R. E.; Oliveira, C. M. F.; Gonza´lez, G.; Lucas, E. F. J. Appl. Polym. Sci. 1997, 66, 1767-1772. (11) Mansur, C. R. E.; Spinelli, L. S.; Oliveira, C. M. F.; Gonza´lez, G.; Lucas, E. F. J. Appl. Polym. Sci. 1998, 69 (2), 2459-2468. (12) Burns, R. L. J. Surfactants Deterg. 1999, 2, 13-16.

ecules containing a short hydrophobic segment and a polar group, such as benzoates, aromatic sulfonates, catechol, resorcinol, alkyl glycol sulfates, among others, are typical hydrotropic agents.14,15 Hydrotropes present a planar structure and self-association ability, and hydrotropy differs from the salting in effect, which occurs for high electrolyte concentrations and, for the particular case of nonionic surfactants, often is associated to the formation of complexes between metal ions and the ether linkages of the amphiphile. Besides, it has been shown that the hydrotropic effect also differs from micelle solubilization in which the amount dissolved increases monotonically with the micelle concentration. The molecular mechanism of hydrotropic solubilization is still not well-known,8,9 although there is some experimental evidence that links the phenomenon to the formation of aggregates by the hydrotrope at a particular concentration. From X-ray diffraction studies, Srinivas et al.15 concluded that these aggregates present alternated polar and nonpolar moieties. The hydrotrope concentration at which such structures form was called the hydrotropy minimum concentration (hmc) by the authors,8,15 and the increase in solubility observed at this concentration for certain molecules was ascribed to its incorporation into the hydrophobic regions of the aggregates. In previous work, we evaluated the properties of aqueous solutions containing poly(ethylene oxide-propylene oxide) (PEO-PPO) block copolymers of different structures and sodium p-toluene sulfonate (NaPTS).10,11,16-20 It was reported that the solubility of these copolymers was (13) Silva, R. C.; Spitzer, M.; Silva, L. H. M.; Loh, W. Thermochim. Acta 1999, 328, 161-167. (14) Srinivas, V.; Balasubramanian, D. Langmuir 1998, 14, 66586661. (15) Srinivas, V.; Rodley, G. A.; Ravikumar, K.; Robinson, W. T.; Turnbull, M. M.; Balasubramanian, D. Langmuir 1997, 13, 3235-3239. (16) Mansur, C. R. E.; Oliveira, C. M. F.; Gonza´lez, G.; Lucas, E. F. Colloids Surf., A 1999, 149, 291-300. (17) Mansur, C. R. E.; Benzi, M. R.; Lucas, E. F. J. Appl. Polym. Sci. 2001, 82 (7), 1668-1676. (18) Mansur, C. R. E.; Barboza, S. P.; Gonza´lez, G.; Lucas, E. F. J. Colloid Interface Sci. 2004, 271, 232-240.

10.1021/la048167j CCC: $30.25 © 2005 American Chemical Society Published on Web 02/26/2005

Influence of the Hydrotrope Structure

increased in the presence of the hydrotrope and that the extent of this effect was a function of the composition, structure, and kind of blocks present in the copolymers. Among the techniques used to assess the mechanism of the surfactant solubility increase caused by the hydrotrope, only nuclear magnetic resonance provided a detailed description of the interactions occurring in these systems. On the basis of these studies, it was inferred that nonaggregated NaPTS did not interact with any specific segment of the copolymer either in its unimeric form or in its micellar form, whereas NaPTS aggregates exhibited stronger interaction with the copolymer unimers, suggesting that only the free copolymer molecules in solution are able to penetrate the NaPTS organized network.19 The aim of the present work is to evaluate and compare the effect of two hydrotropes on the solution properties of three monofunctionalized PEO-PPO block copolymers. The hydrotropes selected for the studies were sodium p-toluene sulfonate (NaPTS) and butyl monoglycol sodium sulfonate (NaBMGS). These compounds present different structures. For NaPTS, the nonpolar group is a planar aromatic segment, typical of the hydrotropic agents as defined by Neuberg.21 NaBMGS, instead, contains a fourcarbon aliphatic chain and a glycol group, besides the sulfonate, as part of the polar group. Experimental Section Materials. Three diblock monofunctional poly(ethylene oxidepropylene oxide) copolymers (R-PEO-PPO-OH, where R ) linear C12-14) were purchased from Oxiteno do Nordeste S.A., Brazil. These copolymers are commercial samples obtained by anionic polymerization, using a 70/30 mixture of two linear alcohols containing 12 and 14 carbon atoms, respectively, as the reaction initiator. Sodium p-toluene sulfonate and butyl monoglycol sodium sulfonate hydrotropes were purchased from Resinac S.A., Brazil, and Hydrior AG, Switzerland, respectively. Methods. The structural characterization of PEO-PPO copolymers was carried out by size exclusion chromatography (SEC) and hydrogen nuclear magnetic resonance (1H NMR) and is detailed in previous publications.10,18 The cloud point (CP) and the surface tension measurements of the block copolymer aqueous solutions, as a function of concentration, containing or not containing the hydrotropes, were performed as described in previous works.10,18 The determination of the cmc and hmc values was made using the break in the surface tension log concentration plots by extrapolation of the straight lines describing both branches of the curve. Such straight lines were obtained by linear regression using the Excel software.10,18 The uncertainty in the cmc and hmc measurements is related to the correlation coefficient (r2) of these straight lines. For the cmc and hmc values, minimum r2 values of 0.9460 and 0.9700, respectively, have been observed. The hydrodynamic diameters were obtained using photon correlation spectroscopy (PCS), with the Malvern Zetasizer 3000 HSA particle size analyzer. The apparatus software uses the Stokes-Einstein equation22 to calculate the diameters from the measured diffusion coefficients. The software automatically calculates the viscosity for water from the temperature data. The aqueous copolymer solutions were prepared by dissolving the surface-active agent in deionized water, with the solutions being filtered through a Millipore filter (0.22 µm) before measurements. The data were collected below the solution cloud point at temperatures of 10, 20, 30, and 40 ( 0.1 °C and at an angle of 90 ( 0.1° using square quartz cells. The light source was a polarized 10 mW He-Ne laser operating at 633 nm. The analyses were done using a CONTIN program. (19) Mansur, C. R. E.; Pacheco, C. R. N.; Gonza´lez, G.; Lucas, E. F. Rev. Quim. Nova 2001, 24 (1), 47-54. (20) Mansur, C. R. E.; Gonza´lez, G.; Lucas, E. F. Polim.: Cienc. Tecnol. 1999, 9 (2), 45-53. (21) Neuber, C. Biochem Z. 1916, 76, 107-111. (22) McDonald, J. A.; Rennie, A. R. Langmuir 1995, 11, 1493-1499.

Langmuir, Vol. 21, No. 7, 2005 2697 Table 1. Characterization of PEO-PPO Block Copolymers copolymer structurea

M h nb

M h wb

M h w/M h nb

EO/PO ratioa

C12-14(EO)3-(PO)8-OH C12-14(EO)4-(PO)6-OH C12-14(EO)6-(PO)4-OH

750 700 700

800 750 750

1.07 1.07 1.07

0.4 0.7 1.5

a

By 1H NMR. b By SEC.

Carbon-13 nuclear magnetic resonance (13C NMR) analyses were carried out in a Varian INOVA-300 spectrometer, operating at 75.431 MHz in a 10 mm (o.d.) tube at 30 °C with a concentric 5 mm tube containing C6D6 and 5% tetramethylsilane (TMS). All 13C spectra were acquired with DEPT-45. An acquisition period of 0.911 s was used, with a spectral window of 122.8 kHz and a pulse interval of 2 s. Processing was carried out by filling with zero up to 65k points and multiplying the free induced decay (FID) by a Gaussian function (typical value of gf ) 0.067 s).

Results and Discussions Copolymer Characterization. The three copolymers used contain the polar EO group in an intermediate position between the C12-14 aliphatic group and the hydrophobic PO group. The molar masses are similar for the three samples, close to 700 g/mol. The polydispersities (M h w/M h n) are the same. For this type of copolymers, the length of the aliphatic chain and the EO/PO ratio determines the hydrophilic-lipophilic balance. Considering that, as shown in Table 1, the copolymers containing the higher number of EO groups also contain the lower number of PO groups; the solubility of these compounds must increase with the number of EO groups in the molecule. Micelle Formation. Surface tension measurements were carried out as a function of the logarithm of the solute concentration in aqueous solution in order to study the activity of the PEO-PPO block copolymers at the waterair interface and to determine the respective critical micelle concentration (cmc) values. The aqueous hydrotrope solutions were also analyzed to determine the hydrotrope minimum concentration (hmc). The surface tension against log concentration curves for the three copolymers and the two hydrotropes are presented in parts a and b of Figure 1, respectively. The data show that for the copolymers the water surface tension is lowered to an equilibrium value of around 32 mN/m. Besides, the initial values are significantly low even at low copolymer concentrations. This behavior is mainly observed for the case of C12-14-(EO)3-(PO)8-OH and may be explained by the low solubility of this copolymer in water that must favor its displacement toward the water-air interface. Analyses of the solution surface tension as a function of concentration for NaPTS and NaBMGS show that both hydrotropes have surface activity, although less important than that of the copolymers (Figure 1b). At a concentration close to 10 wt/vol %, surface saturation is observed, and from then on, the values of surface tension are kept practically constant. Among the hydrotropes, the surface activity of NaBMGS is higher than that of NaPTS. The effect of the three copolymers at a concentration of 1 wt/vol % on the surface tension against concentration diagrams of the aqueous solutions of NaPTS and NaBMGS was evaluated; this concentration is above the cmc of the copolymers (Figure 1a). The results presented in Figure 2 show that, as expected, the initial surface tension values in these curves are similar to the equilibrium values obtained above the cmc for the copolymer solutions (Figure 1a). Besides, it is observed that with the increase in the

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Figure 1. Surface tension versus concentration, at 25 °C, for (a) aqueous copolymer solutions and (b) NaPTS and NaBMGS.

hydrotrope concentration the surface tension is kept practically constant up to the aggregation point. Beyond this concentration, the solution surface tension rises. It may be inferred that the first part of these curves is associated to the adsorption of the copolymers at the water-air interface and that above the hydrotrope aggregation concentration (10 wt/vol %) copolymer molecules may migrate from the water-air interface toward the solution due to their enhanced solubility favored by the hydrotrope aggregates. The influence of the hydrotrope on the copolymer aqueous solution surface tension-concentration curves was evaluated for hydrotrope concentration at 5 and 15 wt/vol %, that is, below and above its aggregation concentrations. Table 2 lists the cmc data obtained from these curves. At the 5 wt/vol % hydrotrope concentration, the cmc, for the three copolymers, was shifted to higher concentration values. The effect was small and similar for both hydrotropes. Beyond the hydrotrope aggregation concentration (15 wt/vol %), the cmc values were shifted to even higher concentrations, and this effect was much more accentuated for NaBMGS, which is the more efficient hydrotrope for these systems. It seems that, below the aggregation concentration, the free hydrotrope molecules present a limited interaction with the copolymer micelles. In a previous work,19 it was suggested that the interactions between the anionic headgroups and the aromatic rings with the oxyethylenic chains of the nonionic surfactant would be predominant for the hydrotrope association with the nonionic micelles and it was concluded that this association would be limited

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Figure 2. Surface tension versus concentration, at 25 °C, for 1 wt/vol % aqueous copolymer solutions: (a) NaPTS; (b) NaBMGS. Table 2. Effect of NaPTS and NaBMGS on the cmc of Aqueous Copolymer Solutions cmc (wt %), at 25 °C NaPTS concn (wt %) copolymer

0

5

15

NaBMGS concn (wt %) 0

5

15

C12-14-(EO)3-(PO)8-OH 0.0001 0.004 0.006 0.0001 0.005 0.01 C12-14-(EO)4-(PO)6-OH 0.001 0.004 0.008 0.001 0.007 0.05 C12-14-(EO)6-(PO)4-OH 0.003 0.006 0.009 0.003 0.007 0.1

to the palisade rather than to the core of the micelle structure. Above the aggregation concentration, NaBMGS is more effective than NaPTS for copolymer solubilization, indicating that there is a stronger interaction between the copolymer chains with the NaBMGS aggregates. Besides, the solubilization effect comes to be more dependent on the copolymer structure, as is shown, for instance, in the case of C12-14-(EO)6-(PO)4-OH. These observations indicate that the type and structure of the hydrotrope aggregates are very important for the hydrotropic effect and illustrate an important feature in relation to hydrotropy; the phenomenon is specific in relation to both the hydrotrope and the solubilizate. Cloud Point Measurements. The cloud points of aqueous PEO-PPO copolymer solutions were obtained as a function of concentration, in the range 0.000 05-10 wt/vol %. No phase separation was observed below 0.005 wt/vol %, indicating that close to or below the copolymer cmc the systems remain completely soluble in the tem-

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Figure 3. Cloud points of PEO-PPO block copolymers in aqueous solutions as a function of copolymer concentration.

perature range investigated (from 0 to 80 °C). The curves in Figure 3 limit the phase separation for each copolymer solution. At temperatures above each curve, the corresponding copolymer solution shows two phases, corresponding to a cloudy solution, and at temperatures below the curves, a one-phase, transparent solution is observed. The profiles for all the copolymer curves were similar: the cloud points were kept practically constant in concentration range between 0.005 and 10 wt/vol %. From a comparison between the behavior of the three copolymers, it is observed, as expected, that the one having the higher EO/PO ratio, C12-14-(EO)6-(PO)4-OH, has the higher cloud point and the one having the lowest EO/ PO ratio, C12-14-(EO)3-(PO)8-OH, has the lower cloud point. The cloud points of the 1 wt/vol % aqueous solutions of PEO-PPO block copolymers for various concentrations of NaPTS or NaBMGS are shown in Figure 4. The solubility increases with an increase in the hydrotrope concentrations for all the copolymers, but as in the case of micelle formation, the effect is more pronounced for NaBMGS. Furthermore, other differences are observed in the effect of the two hydrotropes; the profiles of the CP-concentration curves for NaPTS nearly follow a straight line, whereas in the case of NaBMGS the cloud points increase exponentially with concentration, following a pattern closer to the solubility enhancement characteristic of the hydrotropic solubilization.2 For C12-14-(EO)4-(PO)6-OH and C12-14-(EO)3-(PO)8OH, it is clear that the effect is more evident for hydrotrope levels above 10 wt/vol %. These results agree with the previous surface tension results and indicate that the hydrotropic effect becomes effective above the hydrotrope aggregation concentration. Aqueous solutions of PEO-PPO copolymers were also analyzed by changing the copolymer concentration (from 0.005 to 1 wt/vol %) and keeping the hydrotropic concentration constant at 15 wt/vol %, above the aggregation concentration. The cloud points of the copolymer solutions are shown in Table 3. No cloud point was observed for the copolymer concentrations close and below the cmc for the three different systems (pure copolymer, copolymer/ NaPTS, and copolymer/NaBMGS). At higher concentrations, the addition of hydrotrope increases the copolymer solubility and the cloud point is detected only at higher concentrations. In all the cases, independent of the type of hydrotrope or the concentration range, the hydrotropic effect has been

Figure 4. Phase diagrams of 1 wt/vol % PEO-PPO block copolymers in aqueous solutions as a function of hydrotrope concentration: (a) NaPTS; (b) NaBMGS. Table 3. Cloud Point Values of the Copolymer Solutions

copolymer C12-14-(EO)3-(PO)8-OH

C12-14-(EO)4-(PO)6-OH

C12-14-(EO)6-(PO)4-OH

a

with hydrotrope aqueous ((1 °C) concn solutions (wt/vol %) ((1 °C) NaPTS NaBMGS 0.005 0.01 0.1 0.25 1.0 0.005 0.01 0.1 0.25 1.0 0.005 0.01 0.1 0.25 1.0

12 11 11 10 10 26 26 25 23 21 43 42 42 41 41

21 20 20 20 20 33 34 34 34 35 66 66 67 67 68

a a 28 27 29 40 43 43 44 47 a 81 83 82 80

No cloud point was observed in the temperature range (10-80

°C).

more effective for C12-14-(EO)6-(PO)4-OH. Such behavior may correlate with the higher EO/PO ratio contained in this copolymer, which may result in more efficient interactions with hydrotrope molecules either as unimers or as aggregates. Particle Size Analysis. The particle size analysis was carried out using PEO-PPO copolymer solutions containing or not containing hydrotropic agents at different concentrations and temperatures. The investigated temperatures (10, 20, 30, and 40 °C) for each system were

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Figure 5. Hydrodynamic diameter of aqueous copolymer solutions as a function of concentration: (a) C12-14-(EO)6(PO)4-OH (CP ≈ 41 °C); (b) C12-14-(EO)4-(PO)6-OH (CP ≈ 21 °C). (c) Size distribution of the C12-14-(EO)3-(PO)8-OH.

limited by the cloud temperatures of each copolymer solution. To calculate the particle hydrodynamic diameter, plots were drawn of the apparent diameter as a function of the free copolymer concentration (C-cmc); the hydrodynamic diameter was obtained by extrapolating the straight lines to copolymer concentrations equal to the cmc’s. A similar procedure was used in the presence of the hydrotrope at 5, 10, and 15 wt/vol % (that is, at concentrations below, equal, and above the hydrotrope aggregation concentration). The plots for the copolymer solutions in the absence of hydrotrope are shown in Figure 5. Table 4 summarizes all of the hydrodynamic diameter data of the copolymer

Mansur et al.

systems containing or not containing hydrotropes. The values presented in Figure 5 and Table 4, as aggregate diameter, are, in fact, average values of a range of different particle diameters, since the systems are polydisperse. As shown in Figure 5c, C12-14(EO)3-(PO)8-OH at 10 °C, for instance, presents an average diameter of around 10 nm in aqueous solutions of different concentrations but for each concentration there is a range of diameter values whose polydispersity increases with concentration. Figure 5 shows that, at temperatures below the cloud point, the copolymer solutions do not exhibit any variation in the hydrodynamic diameter with an increase in concentration. This indicates that the rise in concentration does not cause important modifications in the micellar size or shape but, rather, an increase in the number of such micelles. At temperatures close to, but still below, their respective cloud points, for the solutions of C12-14-(EO)6-(PO)4OH (CP ≈ 41 °C) and C12-14-(EO)4-(PO)6-OH (CP ≈ 21 °C) copolymers, an increase in the hydrodynamic diameter is observed as a function of the copolymer concentration. The reduction in solubility increases the micelle size. For each system, and independent of the copolymer concentration, the hydrodynamic diameter increases with temperature, reflecting an increase in the micelle size, caused by the solubility reduction under higher temperature conditions. This behavior is common to other nonionic surfactants that present a large increase in the aggregation number at temperatures close to the cloud point23 and in our case reflects the progressive dehydration of the PPO and PEO groups. The values of hydrodynamic diameter for copolymers at temperatures close to the cloud point are 13.5, 12.4, and 9.7 nm for C12-14-(EO)6-(PO)4-OH, C12-14-(EO)4(PO)6-OH, and C12-14-(EO)3-(PO)8-OH, respectively (Table 4). Considering that at these temperatures the copolymers are still in solution it may be inferred that the PO groups must already be dehydrated while most of the EO groups still maintain their hydration shells. In this context, the results reflect a more expanded conformation for the micelle, resulting from the copolymers containing a longer length water-soluble segment. The structure of the copolymers presenting the hydrophilic group intermediate between two hydrophobic groups may also contribute to the differences in particle size. In spherical aggregates, the two terminal hydrophobic groups must be oriented radially toward the interior of the micelle while the hydrophilic group must remain at the periphery. Longer EO groups require larger spaces to be accommodated at the surface, resulting in smaller curvatures and consequently in larger particle sizes. Table 4 presents the effect of the two hydrotropes under consideration on the micelle hydrodynamic diameter copolymers, at various temperatures. As a general trend, the results show that for temperatures below the solution cloud point (10, 20, and 30 °C for C12-14-(EO)6-(PO)4OH and 10 °C for C12-14-(EO)4-(PO)6-OH), the effect of the hydrotropes on the hydrodynamic diameter was rather limited. At temperatures closer to the solution cloud point, a systematic reduction in the particle hydrodynamic diameter with the hydrotrope concentration was observed for the three copolymers. A similar result was reported previously for n-dodecyl hexaoxyethylene glycol monoether in the presence of NaPTS.7 At these temperatures, it was also observed that, up to the hydrotrope aggregation concentration, NaPTS induced a more significant reduc(23) Rosen, J. M. Surfactant and Interfacial Phenomena, 2nd ed.; John Wiley & Sons, Inc.: New York, 1989.

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Table 4. Effect of Hydrotropes on the Micellar Hydrodynamic Radius of the Copolymers at Various Temperatures and Concentrations hydrodynamic diametera (nm) NaPTS concn (wt/vol %) temperatureb (°C)

0

5

10

15

5

10

15

C12-14-(EO)6-(PO)4-OH

10 20 30 40 10 20 10

7.8 7.8 9.4 13.5 8.2 12.4 9.7

7.5 7.6 7.7 9.0 7.4 8.4 7.6

7.0 7.3 7.5 7.5 8.1 9.2 8.2

6.5 7.1 7.7 7.8 7.5 7.8 6.9

7.8 8.2 8.5 9.6 8.9 10.8 10.3

8.5 9.0 8.3 8.6 9.8 9.7 8.8

6.9 6.5 5.8 5.5 7.4 8.0 6.2

C12-14-(EO)4-(PO)6-OH C12-14-(EO)3-(PO)8-OH b

NaBMGS concn (wt/vol %)

copolymer

a The hydrodynamic diameter was obtained by extrapolating the straight lines to zero concentration. Values with error of 10%. Temperatures of the analyses.

tion in the hydrodynamic diameter of the micelles than NaBMGS. Above the aggregation concentration, however, this latter hydrotrope seems to be more effective in some cases. The results indicate that, even considering that the aggregation number must be increased at temperatures close to the surfactant cloud point, more compact micelles are present in the solution when the hydrotropes are present. This particle size reduction must be related to the nature and type of interactions between the copolymers and the hydrotropes. As previously mentioned, for NaPTS, interactions involving the hydrophobic groups and interactions between the aromatic ring and the anionic headgroup of the hydrotrope with the oxyethylenic chains of the copolymer must be expected. For NaBMGS, the absence of the benzene ring limits the interactions to those arising from the butyl chain and to those of the sulfonate headgroup with the oxyethylenic chains, besides those resulting in the EO-EO and, eventually, EO-PO associations. These considerations indicate that the interactions between the copolymer and the hydrotrope are limited to particular regions of the molecules and the formation of mixed micelles appears to not be evident. Nuclear Magnetic Resonance. The use of nuclear magnetic resonance for determining the cmc and cmt of surface-active agents in solution by observing changes in the peaks corresponding to the surfactant CH3 groups when the molecule goes from the unimeric to the micellar form is a well established procedure.24 This technique can also be used to detect interactions between dissolved molecules through modifications of their spectra and through quantification of the chemical shifts found for the various atomic groups present in the molecules. The method represents a valuable tool for identifying the type of interaction occurring between the surface-active agent and other additives. In this section, the results of the carbon-13 nuclear magnetic resonance for assessing the interactions between the C12-14-(EO)6-(PO)4-OH copolymer and NaPTS and NaBMGS are presented. At first, the individual spectra of these molecules were obtained. Then surface-active agent solutions were tested in the presence of each of the hydrotropes. In view of limitations associated to the instrument sensitivity, all tests employed surface-active agent solutions at concentrations above their cmc. 13 C NMR Analysis of C12-14-(EO)6-(PO)4-OH, NaPTS, and NaBMGS. The assignment of the carbon-13 spectrum for the copolymer was carried out in another work, which also studied the interaction of a monofunctional PEOPPO block copolymer with a hydrotrope.19 These assignments obtained for the PEO-PPO have been employed in this work and are described below: (δ 73 ppm) for CH (PO), (δ 70 ppm) for CH2 (EO), (δ 73 ppm) for CH2 (PO), (24) Mikhalkin, A. P. Colloid J. 1994, 56 (3), 336-339.

Table 5. Chemical Shifts of C12-14-(EO)6-(PO)4-OH Copolymer Groups Obtained by 13C NMR chemical shift (Hz) group

0.1a (wt %)

10a (wt %)

∆δ

CH3 (PO) CH2 (PO) CH (PO) CH2 (EO) CH3 (hydrocarbon chain) CH2 (hydrocarbon chain)

1245 5451 5684 5290 1082 2284

1273 5422 5719 5319 1118 2322

28 29 35 29 36 38

a

Copolymer concentration.

Table 6. Chemical Shifts of NaPTS Groups Obtained by 13C NMR group

chemical shift (Hz)

CH3 H1 of ring H2 of ring H3 of ring H4 of ring

1574 10722 9778 9482 10557

(δ 17 ppm) for CH3 (PO), (δ 14 ppm) for CH3 (of the hydrocarbon chain), and (δ 31 ppm) for CH3-(CH2)9CH2-CH2-O-. Table 5 shows the signals present in the carbon-13 spectra for C12-14-(EO)6-(PO)4-OH at the two concentrations studied. At higher concentrations, the signals are shifted to higher frequencies (nearly 30-40 MHz), showing that a change in the chemical environment of the groups is occurring. This behavior can be explained considering that, as concluded previously by particle size analysis, the increase in copolymer concentration results in an increase in the number of micelles in relation to the number of free molecules that above the cmc remains constant. The assignments of the NaPTS spectra were also previously determined by means of unidimensional (using the nuclear Overhauser effect) as well as bidimensional correlation techniques, 1H f 13C, of straight link and long distance (Table 6).19 The assignments for the chemical shifts of the neat NaBMGS hydrotrope were obtained on the basis of the literature (Table 7).25 C12-14-(EO)6-(PO)4-OH Copolymer Carbon-13 NMR Analysis at 1 wt/vol % in the Presence of the NaPTS Hydrotrope. Figure 6 shows the variation of the chemical shifts related to the copolymer groups, at 1 wt/vol %, as a function of the concentration of the NaPTS hydrotrope. The presence of NaPTS causes a negative ∆δ for the CH3 and CH2 (10) groups of the hydrocarbon chain, CH3 of PO and CH2 of EO. The chemical shifts increase with an (25) Silverstein, R. M.; Bassler, C. G.; Morril, T. C. Spectrometric Identification of Organic Compounds, 3rd ed.; John Wiley & Sons: New York, 1974; pp 159-195.

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Figure 6. Variation of the chemical shifts related to the C12-14(EO)6-(PO)4-OH copolymer groups, at 1 wt/vol %, as a function of the concentration of NaPTS. Table 7. Chemical Shifts of NaBMGS Groups Obtained by 13C NMR group

chemical shift (Hz)

CH3 CH2 (1) CH2 (2) CH2 (3) O-CH2-CH2 CH2-O-SO3Na

1068 1472 2393 5140 5224 5386

increase in the NaPTS concentration in solution. It can also be observed that such an increase is more important at concentrations higher than the aggregation concentration (10 wt/vol %), although the increase is nearly linear in the concentration range considered. A larger variation in the chemical shift for the CH2 (EO) and CH2 (10) groups of the hydrocarbon chain (∼ -30 and ∼ -25 Hz, respectively) was observed. The shift corresponding to the CH3 of the hydrocarbon chain was only around -3 Hz. These results confirm that NaPTS interacts preferentially with the oxyethylenic groups of the surfactant. The small shifts observed for the CH3 group characteristic of the hydrophobic portion of the copolymer may indicate that NaPTS presents a limited interaction with these parts of the copolymer. Alternatively, the results may indicate that, if there are hydrophobic niches created by the hydrotrope aggregates in solution as reported in the literature,7 the surfactant does not penetrate these regions or, finally, that the surfactant molecules when transferred from the micellar environment to the hydrotrope aggregates do not perceive changes in their chemical environment. C12-14-(EO)6-(PO)4-OH Copolymer Carbon-13 NMR Analysis at 1 wt/vol % in the Presence of the NaBMGS Hydrotrope. Figure 7 shows the plots of the variation of the chemical shifts of the C12-14-(EO)6-(PO)4-OH copolymer, at 1 wt/vol %, in the presence of NaBMGS. As previously (Figure 6), the shifts of the groups present in the copolymer chains are shifted toward lower frequencies. In this case, however, the effect is significantly more important above the NaBMGS aggregation concentration, indicating that the surface-active agent-hydrotrope interactions are more effective when hydrotrope aggregates are present. The groups more affected by the presence in this case are CH3 and CH2 (10) of the hydrocarbon chain (∼ -40 and ∼ -55 Hz, respectively), indicating that important changes in the chemical environment are occurring in the region surrounding these groups. The CH2 (EO) group was also affected but to a lower extent than in the case of NaPTS. By comparison with the previous results, it may be inferred that the shifts for the

Figure 7. Variation of the chemical shifts related to the C12-14(EO)6-(PO)4-OH copolymer groups, at 1 wt/vol %, as a function of the concentration of NaBMGS.

hydrophobic groups of the copolymer are effectively modified in the case of NaBMGS and that for NaPTS these groups are only partially involved in the copolymerhydrotrope interaction. From the previous considerations, it is possible to conclude that there are important differences in the effects of NaPTS and NABMGS on the solution behavior of C12-14-(EO)6-(PO)4-OH. Clearly, in the case of NaPTS, the interactions between the benzene group and the sulfonate with the oxyethylenic chain of the copolymer predominate over the interactions resulting from the formation of the hydrotrope aggregates. This situation makes it difficult to incorporate the copolymer into the organized network of the hydrotrope aggregates and explains results from the literature indicating that NaPTS aggregates interacts less with the micellar form than with the unimeric form of copolymers.19 In the case of NaBMGS, the absence of the aromatic ring makes the interactions between the polar group of the hydrotrope and the copolymer less important. This situation facilitates the incorporation of the copolymer into the hydrophobic regions created by the aggregation of the hydrotrope molecules, as seems to be essential for the hydrotropic effect. These observations also explain the better performance of NaBMGS in increasing the cmc and the cloud point of the various copolymers. Conclusions The changes induced by NaPTS and NaBMGS on the critical micelle concentration and cloud point of substituted

Influence of the Hydrotrope Structure

poly(ethylene oxide-propylene oxide) (PEO-PPO) block copolymers confirm the important effect of hydrotropes on the solution behavior of these types of copolymers. At temperatures close to the cloud point, the two hydrotropes reduce the hydrodynamic radius of the copolymer micelles. These results confirm data reported previously for n-dodecyl hexaoxyethylene glycol monoether in the presence of NaPTS7 and may be ascribed to the hydrotrope-copolymer interactions that become predominant at these temperatures in which the EO groups are partially dehydrated. Important differences in the effect of NaPTS and NaBMGS on the solution behavior of C12-14-(EO)6(PO)4-OH were observed. In the case of NaPTS, the interactions between the benzene group and the sulfonate with the oxyethylenic chain of the copolymer predominate over the interactions resulting from the formation of the hydrotrope aggregates. For NaBMGS, the absence of the

Langmuir, Vol. 21, No. 7, 2005 2703

aromatic ring makes the interactions between the polar group of the hydrotrope and the copolymer less important and facilitates the incorporation of the copolymer unimer into the hydrophobic regions created by the aggregation of the hydrotrope. This type of association between the solubilizate and the hydrotrope aggregates seems to be an essential feature of the hydrotropic solubilization phenomenon. These observations also explain the better performance of NaBMGS in increasing the cmc and cloud point of the various copolymers. Acknowledgment. The authors are indebted to Hydrior AG and to Resinac S.A. for the NaBMGS and NaPTS hydrotrope free samples, respectively, as well as to Oxiteno do Nordeste S.A., Brazil, for the polyoxide-based copolymer free samples. LA048167J