Poloxamer association in aqueous solution

Poloxamer Association in Aqueous Solution A. A. AL-SADEN, T. L. WHATELEY, AND A. T. FLORENCE 1 Department of Pharmacy, University of Strathclyde, Glasgow Gl 1XW, United Kingdom Received September 14, 1981; accepted February 24, 1982 Aqueous solutions of the surface active poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers (poloxamers) were studied using photon correlation spectroscopy (quasi-elastic light scattering) and viscosity measurements. Poloxamers 184 and 237 showed detectable aggregates at 25 ° only at concentrations above about 6% with size increasing with concentration and with significant polydispersity, probably indicating a multiple association process. At 35 °, however, essentially invariant values for the hydrodynamic radius were found over a wide concentration range and the systems were essentially monodisperse: these systems are more fikely to be represented by a closed association model The more hydrophilic poloxamer 188, however, retained its concentration dependence of aggregate size up to 55 °. The variation with temperature of both the hydrodynamic radius of aggregates and the intrinsic viscosity of several poloxamers was rationalized by relating the temperature-dependency curves to the cloud point of the poloxamer. In some cases only certain sections of the curve are observable when the cloud point is high, e.g., >100 °, or low, e.g., <40 °.

Few reports have appeared regarding the physicochemical properties of aqueous solutions of the surface-active poly(oxyethylene)poly(oxypropylene)poly(oxyethylene) block copolymers (poloxamers). Schmolka (1) has reviewed the available literature in a paper published in 1977; however, the question of micelle formation by these poloxamers remains open for discussion and argument. Different values for the CMC of these block copolymers measured by surface tension and dye solubilization methods have been reported (2). A recent report (3) indicated two points of inflection in the surface tension vs concentration plots. The first inflection point was attributed to a conformational change in the copolymer molecule which gives rise to a close-packed monomolecular unit with the hydrophobic chain coiled in its interior, shielded by the poly(oxyethylene) units. The second point of inflection was attributed to the formation of multimolecular aggregates more akin to conTo whom correspondence should be addressed.

ventional micelles. The area per molecule at the air-water interface calculated at the first point of inflection was a function of the size of the poly(oxyethylene) chain; increasing the size of the hydrophobic group resulted in a marked decrease in the area occupied per molecule. These results suggested that the molecules were oriented in the surface in a coiled manner with the poly(oxypropylene) chain out of the aqueous phase and the hydrophilic group at the extremities of the molecules anchoring the polymer in the aqueous phase. Collett and Tobin (4) have studied the solubilization ofpara-substituted acetanilide in a range of poloxamers: they found that solubilization increased with polymer concentration. For the polar drug derivatives the amount solubilized increased with ethylene oxide content of the poloxamer, whilst with the more hydrophobic members of the series, solubilization decreased with increased ethylene oxide content. Published total intensity light-scattering results on poloxamer solutions are conflicting. While some authors have reported that

303 Journal of Colloid and Interface Science, Vol. 90, No. 2, December 1982

0021-9797/82/120303-07502.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.

304

AL-SADEN,WHATELEY,AND FLORENCE

poloxamers do not form aggregates (5, 6) others have reported that multimolecular aggregates form at moderately high temperatures, while at low temperatures, no aggregation occurs (poloxamer 184). For the hydrophilic members (poloxamer 188) of the series, aggregates did not form even at high temperatures. In this study more evidence is presented for the concept of micelle formation by poloxamers, obtained by dynamic light-scattering and viscosity studies.

EXPERIMENTAL

Materials and Methods Poloxamer (Pluronic ®) surfactants with the general formula HO(C2H40)a(C3H60)b(C2I-LO)cH were obtained from Union Carbide (Union Carbide, Rickmansworth, U. K.) and used as received. A description of the poloxamers used is given below in tabular form.

Poloxamer

Pluronic

Average MW

% EO

Value of a (= c)

MW of EO

Value of b

184 122 215 237 188

L64 L42 P75 F87 F68

2900 1630 4150 7700 8350

40 20 50 70 80

13 5 24 62 75

1167 430 2100 5250 7000

30 21 35 39 30

EO = ethylene oxide. Tween 80 (polysorbate 80 USP) and Triton X-100, scintillation grade, were from BDH, UK. Photon Correlation Spectroscopy A Malvern model 4300 photon correlation spectrometer with 48 channels was used with a Liconix He/Cd laser operating at 441.6 nm with a power of about 10 mW. The temperature was controlled within 0.1 °C and data were analyzed using the method of cumulants (8). The value In (g2(t) - 1) was routinely fitted to a linear function of time and the initial slope was used to determine the diffusion coefficient D. The equivalent spherical hydrodynamic radius, ru, is then calculated, assuming the applicability of the Stokes-Einstein equation, rh = kT/6~rT1D, where k is the Boltzmann constant, T, the absolute temperature, and n the viscosity of the solvent. With a polydisperse system a family of exponentials will be obtained and plots of In Journal of Colloid and Interface Science, Vol. 90, No. 2, December 1982

(g2(t) -- 1) will be nonlinear. Therefore In (ga(t) - 1) vs t plot is fitted to a polynomial of order up to 3. The coefficient of t gives the z-average diffusion coefficient from which an average hydrodynamic radius, rh, is obtained. The coefficient of t2 gives the width of the distribution expressed as the normalized variance of distribution (NVD) which is the normalized coefficient of t2, this being used as an indication of the degree of polydispersity. Values of NVD below 0.1 indicate an essentially monodisperse system (9, 10). Measurements were made at angles of 45, 90, and 135 ° and results were not significantly different. Viscosity Viscosity measurements were made using a suspended level dilution viscometer with a flow time for water of 193 sec. Temperature was controlled to +0.01 °C. The intrinsic viscosity [~], of the poloxamer was obtained from plots of the reduced specific viscosity 0/sp/C) vs concentration of polymer C, at C

305

POLOXAMER ASSOCIATION IN AQUEOUS SOLUTION

0. Huggins' constant KH was calculated from the equation, nsp/C = [rl] + [n]2Kn" C.

Solubilization The required amount of hydrocarbon was added to the poloxamer solution and the mixture was shaken mechanically for 24 hr in a water bath controlled to the required temperature. The saturation limit was identified by observing the conical flask placed in front of a black screen. The solubilized systems were filtered into the cell for photon correlation studies using a 0.1-/zm Millipore filter. RESULTS AND DISCUSSION

The Association Pattern of Pluronics The association process has been studied via the concentration dependence of the hydrodynamic radius of the aggregates derived from diffusion coefficients measured by photon correlation spectroscopy. The concentration dependence of the measured average hydrodynamic radius (Fh) for poloxamer 184 at 25°C is shown in Table Ia. Measurements were not possible below 8% (w/v) due to the low signal/noise ratio, above which measurements were possible although a long data collection time was required. The results indicate a marked increase in the hydrodynamic radius with increased concentration. Polydispersity was significant at all concentrations as indicated by the high values of the normalized variance of distribution (NVD), indicating a broad aggregate size distribution (monomer, dimer, trimer, etc). The detection of the signal at 8% w/v and its disappearance on further dilution suggests aggregate growth with increasing concentration, until a concentration region is reached where a sufficient average aggregate size is built up to give the scattering required for photon correlation measurements. This growth continues as long as the concentration increases and is indicated by the marked increase in the average hydrodynamic radius

TABLE I Hydrodynamic Radii (r~) and Diffusion Coetficients (D) of Poloxarner 184 Aqueous Solutions at 25 and 35 °C C (g/100 ml)

NVD

D × 106 (cm2 sec-I)

rh (rim)

(a) 25°C 8 10 15 20

0.29 0.27 0.27 0.27

0.251 0.235 0.218 0.197

9.76 10.4 11.3 12.5

0.479 0.457 0.482 0.482 0.493 0.474 0.519 0.536 0.596 0.707

6.55 6.86 6.50 6.50 6.37 6.61 6.04 5.84 5.26 4.44

(b) 35°C 2 4 6 8 10 12 14 16 20 30

0.09 0.07 0.08 0.04 0.04 0.06 0.03 0.06 0.09 0.15

values. This aggregate growth and high polydispersity index value indicates that a multiple association process is occurring in this system (11). Sodium cholate and sodium octanoate have been shown to undergo a multiple association process in aqueous solution, indicated by the decrease in the translational diffusion coefficient with increased concentration (12, 13). Protein molecules (14) and bile salts (15) undergo a multiple association process indicated by the increase in the hydrodynamic radius of the aggregates with increasing concentration. However, the situation was altered at 35 °C (Table Ib). Essentially invariant values for the hydrodynamic radius were found over the concentration range studied, including those values for concentrations lower than 8% w/v. A slight decrease was observed at moderately high concentration (over 14% w/v), which is difficult to account for at this stage. The systems were monodisperse as indicated by the nonsignificant values of the NVD (<0.1). A similar association behavior was Journal of Colloid and Interface Science, Vot. 90, No. 2, December 1982

306

AL-SADEN, W H A T E L E Y , A N D F L O R E N C E T A B L E II

Hydrodynamic Radii and Diffusion Coefficients for Triton X-100 and Tween 80 at 25 ° in Aqueous Solution c (g/100 ml)

NVD

DXl06 (era2 see-l)

rh (nm)

(a) Triton X-IO0 0.5 1 2 15 30

0.01 0.015 0.01 0.01 0.17

0.555 0.568 0.564 0.614 0.728

4.42 4.32 4.35 3.99 3.36

(b) Tween 80 2 4 6 8 10

0.05 0.07 0.07 0.05 0.05

0.531 0.545 0.571 0.582 0.548

4.61 4.49 4.29 4.20 4.47

dynamic radius with concentration persisted even at high temperatures, although a decrease in polydispersity was observed at high temperatures. This behavior may be due to the high solute-solvent interaction which is expected for hydrophilic molecules such as poloxamer 188; hence the system exists as a mixture of aggregates of different sizes even at high temperatures. However, light-scatter-

TABLE III

The Effect of Temperature and Concentration on the Hydrodynamic Radius and Diffusion Coefficient of Poloxamer 237 and Poloxamer 188 in Aqueous Solution C (g/100 ml)

NVD

D×10 ~ (era2 see-1)

r~ (nm)

Poloxamer 237 (i) 25°C

found for the conventional nonionic surfacrants, Triton X-100 and Tween 80 (Table II). The concentration independency of the hydrodynamic radius suggests that a single micellar species exists and that the association is more likely to be represented by a closed association model (11). A concentration-independent diffusion coefficient (i.e., constant hydrodynamic radius) has been explained by a closed association model for a hexa(oxyethylene) glycol monoether (16). The difference between the association pattern for poloxamer 184 at 25 and 3 5 ° C suggests that the poloxamer molecules exist in an aggregated form at 35°C, a conclusion arrived at by McDonald and Wong (7). They showed by total intensity light scattering that poloxamer 184 formed aggregates at 35, but not at 25°C. Poloxamer 237 showed the same pattern of association as poloxamer 184 (Table III); i.e., an increase in the average hydrodynamic radius with concentration is observed at low temperature with a constant hydrodynamic radius at high temperatures. However, poloxamer 188, the hydrophilic member of the series, showed very distinctive behavior (Table II). The increase in the average hydroJournal of Colloid and Interface Science, Vol. 90, No. 2, December 1982

6 8

10 12

(ii) 45°c 4 6

8 10 12

(iii) 55°C 4 6 8

10

12

0.49 0.45 0.50 0.61

0.313 0.278 0.251 0.170

7,87 9.14 9.58 14.4

0.09 0.08 0.08 0.12 0.14

0.536 0.521 0.557 0~616 0.560

7.28 7.50 7.0 6.4 6.95

0.09 0.09 0.09 0.11 0.10

0.676 0.674 0.702 0.760 0.710

7.04 7.08 6.8 6.27 6.71

Poloxamer 188 (i) 25°C

10 15 20 (ii) 35°c 10 15 20 (iii) 55°c 10 15

20

0.46 0.47 0.50

0.411 0.240 0.210

5.96 10.2 11.7

0.33 0.36 0.40

0.573 0.299 0.269

5.47 10.5 11.7

0.13 0.13 0.16

0.798 0.445 0.397

5.97 10.7 12.0

307

POLOXAMER ASSOCIATION IN AQUEOUS SOLUTION

ing results (7) showed that poloxamer 188 existed as monomers in aqueous solutions at high temperatures.

Solubilization of Hydrocarbons in Poloxamer Solutions The solubilization of n-hexane in poloxamer 188 solutions was studied at two temperatures: at 25°C hexane depressed the cloud point of the poloxamer: at a hexane/ poloxamer molar ratio of 1.24 the solution was cloudy and some unsolubilized oil droplets were observed, thus photon correlation measurements were not possible. However, at 35°C solubilization did occur, the saturation level for hexane being 8.43 mole hexane/mole poloxamer. The effect of hexane uptake on the hydrodynamic radius of the aggregates is shown in Fig. 1. There is an increase in the hydrodynamic radius of the aggregates with the increase in the solubili-

8-

•

7

TABLE IV The Effect of Temperature on the Hydrodynamic Properties of Poloxamer 184 and Poloxamer 188 Temp. (°C)

NVD

D × 106 (era 2 see -l)

rb (nm)

[~t] (ml g-l)

Kn

10.4 9.0

0.35 0.032

8% Poloxamer 184 25 35

4o 45 5o 55

0.28 0.04 0.03 0.04 0.05 0.17

0.251 0.482 0.598 0.615 0.450 0.301

9.76 6.50 5.87 6.35 9.62 15.8

7.6

0.2

18.65

0.41

19.5 18 12.5 11.4

0.39 0.46 1.32 1.49

10% Poloxamer 188

25 35 55 65 75

0.45 0.33 0.13 0.12 0.19

0.411 0.573 0.798 0.909 0.976

5.96 5.47 5.97 6.28 6.90

zate uptake. Nakagawa et al. (17), studied the uptake of n-decane and n-decanol by methoxy poly(oxyethylene)decyl ether; the increase in micellar weight on solubilization was found not to originate simply from the incorporation of solubilizate molecules into the existing micelles, but from the increased number of surfactant molecules present in the solubilizate laden micelles. The increase in the aggregate size with the uptake of oil at 35 °C adds weight to the concept of micelle formation in this system and illustrates the difference in the nature of aggregates at two temperatures.

Effect of Temperature on Poloxamer Aggregate Size

I

I

I

2

4

6

tool hexane/molpoloxamer FIG. 1. Variation of hydrodynamic radius of poloxamer 184 aggregates with hexane uptake at 35°C (saturation molar ratio is 8.43).

The influence of temperature has been studied using both photon correlation spectroscopy and viscosity measurements. The effect of temperature on the hydrodynamic radius and intrinsic viscosity for poloxamer 184 (cloud point 60°C) is shown in Table IV. There is an initial decrease in both hydrodynamic radius and intrinsic viscosity with temperature (between 25-50°C) followed by an increase (at 55°C) due to micellar growth Journal of Colloid and Interface Science, Vol. 90, No. 2, December 1982

308

AL-SADEN, WHATELEY, AND FLORENCE cloudpoinf

poinf

I

2O

I

4O

I

,oo

Ten~erafure(%) FIG. 2. Effect of temperature on the aggregate hydrodynamic radius of two poloxamers. II, 184 (cloud point 60°); O, 215 (cloud point 93°).

when the temperature approaches the cloud point (60°C). The system becomes monodisperse (i.e., exhibiting low values of NVD), in the range 25-50°C, which is more likely to be due to aggregation: polydispersity appears again at 55°C. Although Huggins constants should be considered with caution in interpreting an aggregating system, calculation of these values can perhaps indicate trends. The constant at 25°C is close to 0.35 suggesting that the molecules behave as flexible polymer chains (18). The decrease in this value between 25-50°C suggests that these molecules undergo considerable folding, and the increase again to a high value of 0.42 at 55 °C is an indication of a decrease in solutesolvent interaction and considerable aggregation (19). These aggregative transitions are shown by other members of the poloxamer series and are dependent on the proximity of the solution temperature to the cloud point. Poloxamer 215 (cloud point 93°C) also shows the complete picture of transitions with temperature (Fig. 2). As temperature increases and water becomes a poorer solvent, the polymer chain will contract, even when aggregation occurs. For a single nonaggregating polymer molecule, a contraction will mean a decrease in the hydrodynamic volume and hence a decrease in the hydrodynamic radius and visJournal o f Colloid and Interface Science, Vol. 90, No. 2, December 1982

cosity. When aggregation occurs, solute-solute interaction and contraction of the polymer chain will squeeze out much of the solvent and will again result in a decrease in hydrodynamic radius and viscosity, and thus a decrease in the Huggins' constant. It has been estimated that much of the effective volume of a flexible polymer chain may be solvent so that even in poloxamers where water may only be about 20% of the total volume there is a considerable opportunity for contraction of the polymer chain by exclusion of solvent. Conversely, a conventional nonionic surfactant molecule occupies a small hydrodynamic volume; when such molecules form a micelle, the molecule will be fairly closely packed. An increase in temperature would result in a further decrease in the solvent-solute interaction and cause a growth in miceUar size. When the temperature approaches the cloud point of the polymer, no more contraction is possible and the increased aggregation would result in an increase in hydrodynamic radius. This behavior has been shown by other copolymers in nonaqueous solvents when the selectivity of the solvent for one block is lowered (21, 22). The effect of temperature on the hydrodynamic radius and viscosity of poloxamer 188 is shown in Table IV. The average hydrodynamic radius increases with temperature. The polydispersity decreases with increase in temperature although the system does not become monodisperse; [~], however, decreases. The decrease in polydispersity associated with the increase in temperature may possibly be considered as an indication of a progressive increase in the degree of association, since for the other poloxamers (184 and 237) when aggregation occurred at high temperatures, the systems were found to be monodisperse. This conclusion is also supported by the increase in the Huggins' constant. If this is so then the hydrodynamic radius should increase due to the pronounced increase in the aggregation number. However, the decrease in the hydrodynamic ra-

POLOXAMER ASSOCIATION IN AQUEOUS SOLUTION

dius observed with poloxamer 184 and 237 can perhaps be seen more clearly in the case of poloxamer 188 as it is a more monodisperse system. The decrease in intrinsic viscosity with increasing temperature while the hydrodynamic radius increases, can possibly be explained by decrease in solvation while the aggregates remain in a near spherical shape. A micellar aggregate with a spherical shape could be envisaged for these poloxamers. At 10% w/v concentrations, where photon correlation measurements were made, the poloxamer molecules are more likely to exist as multimolecular aggregates. Poloxamer 122 Poloxamer 122 (cloud point 32°C) seems to behave in an anomalous manner: the measured hydrodynamic radius for this compound was 80 nm at 25°C, an extremely high value for a nonionic surfactant, with an intrinsic viscosity value of 7 ml g-l. It may well be that when the diffusion coefficient is determined close to the cloud point where the solution becomes optically inhomogeneous (23) the results may be artifactual. However, it may be that this hydrophobic poloxamer forms large spherical aggregates near the cloud point. ACKNOWLEDGMENT The SRC is thanked for financial contribution to purchase of the photon correlation spectrometer. REFERENCES 1. Schmolka, I. R., J. Amer. Oil Chem. Soc. 54, 110 (1977).

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2. Anderson, R. A., Pharm. ActaHelv. 47, 304 (1972). Schmolka, I. R., and Raymond, A. J., J. Amer. Oil Chem. Soc. 42, 1088 (1965). 3. Prasad, K. N., Luong, T: T., Florence, A. T., Paris, J., Vaution, C., Seiller, M., and Puisieux, F., J. Colloid lnterface Sci. 69, 225 (1979). 4. Collett, J. H., and Tobin, E. A., J. Pharm. Pharmacol. 31, 174 (1979). 5. Mankowich, A. M., J. Phys. Chem. 58, 1027 (1958). 6. Dwiggins, C. W., Bolen, R. J., and Dunning, H. N., J. Phys. Chem. 64, 1175 (1960). 7. McDonald, C., and Wong, C. K., Aust. J. Pharm. Sci. 6, 85 (1977). 8. Koppel, D. E., J. Chem. Phys. 57, 4814 (1972). 9. Pusey, P., in "Photon Correlation and Light Beating Spectroscopy" (H. Z. Cummins and E. R. Pike, Eds.), Plenum, pp. 387-429. London, 1974. 10. Rohde, A., and Sackmann, E., J, Colloid Interface Sci. 70, 494 (1979). 11. Elias, H. G., in "Light Scattering from Polymer Solutions" (M. B. Huglin, Ed.), pp. 397-448. Academic Press, New York/London, 1972. 12. Lindman, B., and Brun, B., J. Co[loidlnterface Sci. 42, 388 (1973). 13. Lindman, B., Kamenka, N., and Brun, B., J. Colloid Interface Sci. 56, 328 (1976). 14. Jullien, M., and Thusius, D., J. Mol. Biol. 101, 397 (1976). 15. Mazer, N. A., Carey, M. C., and Benedek, G. B., J. Phys. Chem. 80, 1075 (1976). 16, Corkill, J. M., and Walker, T., J. Colloid Interface Sci. 39, 620 (1972). 17. Nakagawa, T., Kuriyama, K., and Inone, H., J. Colloid Interface Sci. 75, 268 (1960). 18. Eirich, F., and Riseman, J., J. Polym. Sci. 4, 417 (1949). 19. Moore, W. R., Progr. Polym. Sci. 1, 1 (1967). 20. Tanford, C., in "Physical Chemistry of Macromolecules." Wiley, New York, 1967. 21. Price, C., and Wood, D., Polymer 15, 389 (1974). 22. Tuzar, Z., and Krathochvil, P., Makromol. Chem. 160, 301 (1972). 23. Herrrnan, K. W., Brushmiller, J. G., and Courchene, W. L., J. Phys. Chem. 70, 2909 (1966).

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