Solubilization of Alkylcyanobiphenyls in Aqueous Micellar Solutions of

School of Chemistry, University of Leeds, Leeds LS2 9JT, UK. Received November 20, 2000. In Final Form: January 18, 2001. A diblock copolymer of propy...
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Langmuir 2001, 17, 2106-2111

Solubilization of Alkylcyanobiphenyls in Aqueous Micellar Solutions of a Diblock Copolymer of Propylene Oxide and Ethylene Oxide Zhuo Yang,* Evangelia Pousia, Frank Heatley, Colin Price, and Colin Booth Department of Chemistry, University of Manchester, Manchester M13 9PL, UK

Valeria Castelletto and Ian W. Hamley School of Chemistry, University of Leeds, Leeds LS2 9JT, UK Received November 20, 2000. In Final Form: January 18, 2001 A diblock copolymer of propylene oxide and ethylene oxide, denoted P94E316, was prepared by sequential anionic polymerization of the two monomers. 13C NMR spectroscopy was used to obtain the absolute number-average molar mass and overall composition (whence the molecular formula), and gel permeation chromatography was used to confirm a narrow chain-length distribution. A number of techniques (light scattering, dye solubilization with DPH (1,6-diphenyl-1,3,5-hexatriene), surface tension) were used to confirm micellization in dilute aqueous solution and to determine the temperature dependence of the critical micelle concentration. Light scattering was also used to determine the temperature dependence of micellar association number and radius. Copolymer solutions of concentration 10 and 20 g dm-3 copolymer were used to solubilize a nematic liquid crystal mixture with a wide nematic range, coded BL002. The extent of solubilization in the temperature range 20-40 °C could be correlated with the extent of micellization of the copolymer, leading to an upper limit at 40 °C of ca. 50 mg (g of copolymer)-1, i.e., 180 mg (g of hydrophobe)-1. It was noted that BL002 solubilization could be adapted to provide a method for determining the critical micelle temperatures of aqueous solutions of the copolymer.

1. Introduction 1

Following early work by Collett and Tobin, many authors have described the solubilization of hydrophobic substances in aqueous micellar solutions of the commercially available linear triblock EmPnEm copolymers. We use E to denote an oxyethylene unit, OCH2CH2, and P an oxypropylene unit, OCH2CH(CH3), with m and n denoting block lengths in repeat units. Past experimental work in this area is included in a number of reviews, and considerable experimental activity is evident to the present time; see, for example, refs 2-5. Theory has been used to model the solubilization effect, e.g., by Nagarajan et al.6 and Hurter et al.7 However, to the best of our knowledge, in all this activity there has been no report of the micellar solubilization of hydrophobic molecules with nematic liquid-crystal properties. It is well-known that concentrated micellar solutions of block copolymers will self-assemble to form gel mesophases, including cubic structures of spherical micelles and hexagonal structures of cylindrical micelles.8-10 If (1) Collett, J. H.; Tobin, E. A. J. Pharm. Pharmacol. 1979, 31, 174. (2) Hurter, P. N.; Hatton, T. A. Langmuir 1992, 8, 1291. (3) Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1. (4) Kozlov, M. Y.; Melik-Nubarov, N. S.; Batrakova, E. V.; Kabanov, A. V. Macromolecules 2000, 33, 3305. (5) Patterson, I. F.; Chowdhry, B. Z.; Leharne, S. A. Langmuir 1999, 15, 6187. (6) Nagarajan, R.; Ganesh, K. J. Colloid Interface Sci. 1996, 184, 489. Nagarajan, R. Colloids Surf. B 1999, 16, 55. (7) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993, 26, 5595, 5530. (8) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998; Chapter 4. (9) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (10) Mingvanish, W.; Kelarakis, A.; Mai, S.-M.; Daniel, C.; Yang, Z.; Havredaki, V.; Hamley, I. W.; Ryan, A. J.; Booth, C. J. Phys. Chem. B 2000, 104, 9788.

liquid crystals can be solubilized in the cores of the micelles, then the possibility exists for patterning liquid crystals on the nanoscale, potentially leading to electrooptical devices. This paper reports a first step toward this end, i.e., an investigation of the solubilization of alkylcyanobiphenyls in a dilute micellar solution. Work is scheduled for the next 2 years to look in detail at the structures and properties of systems of this type, including studies of orientational order. Gel structure can be controlled by variation of concentration and temperature. At a given concentration, cubic gels (spherical micelles) are favored by low temperature and hexagonal gels (cylindrical micelles) by high temperature.10-12 With this in mind, we sought a liquid crystal solute with a wide nematic range, eventually settling on product marketed by Merck Plc as BL002, the nematic range of which is -20 to +72 °C. 13C NMR spectroscopy (slow pulse, NOE suppressed, including DEPT) showed this to be a mixture of cyanobiphenyls with alkyl and oxyalkyl substituents with a range of chain lengths from C2 upward. Besides ensuring a ready supply of a commercially available material, this choice gives us the possibility in the future of investigating a full range of copolymer concentrations, including the copolymer melt, which must necessarily be studied well above the melting point of the copolymer, in the present case above 65 °C. Efficient solubilization depends on the presence of micelles, and as a consequence, the cmc of the copolymer in water is an important variable. For given composition, chain length, and temperature, the cmc of a diblock EmPn copolymer is known to be much smaller than that of its (11) Glatter, O.; Scherf, G.; Schillen, K.; Brown, W. Macromolecules 1994, 27, 6046. (12) Yang, Y.-W.; Ali-Adib, Z.; McKeown, N. B.; Ryan, A. J.; Attwood, D.; Booth, C. Langmuir 1997, 13, 1860.

10.1021/la001610f CCC: $20.00 © 2001 American Chemical Society Published on Web 03/03/2001

Solubilization of Alkylcyanobiphenyls

triblock counterpart (Em/2PnEm/2).13-15 More important for solubilization is the concentration at which the copolymer is fully micellized, and this is sensitive to the block length distributions in the copolymer, not just the average lengths which are indicated by the overall composition and chain length. As discussed elsewhere,15,16 for reasons connected with the chemistry of their sequential anionic polymerization, diblock EmPn copolymers are considerably more uniform in chain length and composition than corresponding triblocks, which may even contain a proportion of diblock copolymer. Consequently, the concentration range between onset and completion of micellization is narrower for the diblock architecture. Even if the sequence of polymerization is reversed, i.e., in the preparation of PnEm copolymers, a considerable advantage remains with the diblock architecture. With this background in mind, the present work was based on a specially synthesized block copolymer P94E316, this sequence of anionic polymerization (propylene oxide followed by ethylene oxide) being adopted for convenience in the preparation. 2. Experimental Section 2.1. Preparation and Characterization. The copolymer was prepared by sequential anionic polymerization of propylene oxide followed by ethylene oxide. The methods used in preparation and characterization followed closely those described previously.13,14 Vacuum line and ampule techniques were used to eliminate moisture. The monofunctional initiator, 1-methoxy2-propanol, was activated by reaction with potassium metal (mole ratio OH/K ≈ 9). The precursor poly(oxypropylene) and the final copolymer were sampled, and the two samples were characterized by gel permeation chromatography (GPC, N,N-dimethylacetamide eluent at 60 °C) calibrated with poly(oxyethylene) standards in order to define the width of their chain length distributions, i.e., Mw/Mn where Mw and Mn are the mass-average and number-average molar mass, respectively. A small correction to Mw/Mn was made to account for instrumental spreading in the system. Both precursor and final copolymer had Mw/Mn ) 1.07. A small shoulder on the low elution volume side of the GPC peak of the copolymer indicated some 2 wt % impurity, assigned as triblock copolymer. Analysis of the 13C NMR spectra was based on the assignments of Heatley et al.17 Comparison of integrals from end group and backbone carbons gave absolute values of molar mass (Mn) and overall composition (mol % E units). A small proportion (0.3 mol %) of unsaturated ends originating from the transfer reaction in the anionic polymerization of propylene oxide18 was included in the calculation of Mn. The formula of the copolymer, P94E316, was obtained accurately from the molar mass of the precursor poly(oxypropylene), Mn ) 5450 g mol-1, and the overall composition (77.1 mol % E). The massaverage molar mass calculated from overall Mn and Mw/Mn was 20 700 g mol-1, and the mass fraction of E was 0.718. 2.2. Light Scattering from Micellar Solutions. Details of the methods used and their applicability to micellar solutions of the type under investigation have been discussed previously.13,14,19 A brief description is given below. Copolymer solutions were clarified by filtering through Millipore Millex filters (Triton free, 0.22 µm) directly into the cleaned scattering cell. Static light scattering (SLS) intensities were measured by means of a Brookhaven BI 200S instrument using (13) Altinok, H.; Yu, G.-E.; Nixon, S. K.; Gorry, P. A.; Attwood, D.; Booth, C. Langmuir 1997, 13, 5837. (14) Altinok, H.; Nixon, S. K.; Gorry, P. A.; Attwood, D.; Booth, C.; Kelarakis, A.; Havredaki, V. Colloids Surf., B 1999, 16, 73. (15) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (16) Nace, V. M.; Whitmarsh, R. H.; Edens, M. W. J. Am. Oil Chem. Soc. 1994, 71, 77. (17) Heatley, F.; Luo, Y.-Z.; Ding, J.-F.; Mobbs, R. H.; Booth, C. Macromolecules 1988, 21, 2713. (18) Yu, G.-E.; Masters, A. J.; Heatley, F.; Booth, C.; Blease, T. G. Macromol. Chem. Phys. 1994, 195, 1517. (19) Deng, N.-J.; Luo, Y.-Z.; Tanodekaew, S.; Bingham, N.; Attwood, D.; Booth, C. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1085.

Langmuir, Vol. 17, No. 7, 2001 2107 vertically polarized incident light of wavelength λ ) 488 nm supplied by an argon ion laser operated at 500 mW or less. The intensity scale was calibrated against benzene. Dynamic light scattering (DLS) measurements were made under similar conditions, using a Brookhaven BI 9000 AT digital correlator to acquire data. Experiment duration was in the range 5-20 min, and each experiment was repeated two or more times. Scattered light intensity was always measured at an angle θ ) 90° to the incident beam. Other measurements were made at 45° and 135°. The correlation functions from dynamic light scattering (DLS) were analyzed by the constrained regularized CONTIN method20 to obtain distributions of decay rates (Γ), hence distributions of apparent mutual diffusion coefficient [Dapp ) Γ/q2, q ) (4πn/λ) sin(θ/2), n ) refractive index of the solvent], and ultimately of apparent hydrodynamic radius (rh,app, radius of the hydrodynamically equivalent hard sphere corresponding to Dapp) via the Stokes-Einstein equation

rh,app ) kT/(6πηDapp)

(1)

where k is the Boltzmann constant and η is the viscosity of the solvent at temperature T. In section 3.1 the data are presented as normalized intensity-fraction distributions of log(rh,app). Average values of Γ, delivered by the CONTIN program by integration over the intensity distributions, were similarly converted to intensity-average values of rh,app. The basis for analysis of static light scattering (SLS) was the Debye equation

K*c/(I - Is) ) 1/Mw + 2A2c + ...

(2)

where I is the intensity of light scattering from solution relative to that from benzene, Is is the corresponding quantity for the solvent, c is the concentration (in g dm-3), Mw is the mass-average molar mass of the solute, A2 is the second virial coefficient (higher coefficients being neglected in eq 2), and K* is the appropriate optical constant. Values of the specific refractive index increment, dn/dc, its temperature increment, and other quantities necessary for the calculations have been given previously.14 Values of dn/dc are very similar for poly(oxyethylene) and poly(oxypropylene), making dn/dc insensitive to exact composition of the copolymers and making correction for refractive index difference within the copolymer unnecessary. 2.3. Critical Micelle Temperature by Light Scattering. Static light scattering was also used to detect the onset of micellization in solutions of known copolymer concentration. Scattering intensities relative to benzene were measured at intervals of 1-5 °C as the temperature was raised slowly, 0.11.0 °C min-1. The critical micelle temperature (cmt) was defined as that at which the scattering curve left the baseline established at low temperatures. The same general technique was used to investigate the effect of temperature on copolymer solutions containing solubilized BL002: see sections 3.4 and 3.5 for details. 2.4. Critical Micelle Temperature by Dye Solubilization. Measurement of the cmt by observation of the absorption spectrum of a hydrophobic dye is a well-established technique. The method depends on a change in spectrum caused by solubilization of the dye in the hydrophobic micellar core. DPH (1,6-diphenyl-1,3,5-hexatriene) has been used in this way by Alexandridis et al.21 to determine the cmts of a number of commercial EmPnEm copolymers. Essentially the same method was used in this work; i.e., for a P94E316 solution of given concentration, the absorption at 356 nm was determined at several temperatures by means of (in our case) a Cary 1E UV/vis spectrometer with temperature controller. The DPH concentration in the copolymer solution was 0.008 mmol dm-3. As discussed previously,22 the cmt was defined as the temperature at which the absorption curve left the baseline established at low temperatures. (20) Provencher, S. W. Makromol. Chem. 1979, 180, 201. (21) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (22) Yu, G.-E.; Altinok, H.; Nixon, S. K.; Booth, C.; Alexandridis, P.; Hatton, T. A. Eur. Polym. J., 1997, 33, 673.

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2.5. Critical Micelle Concentration by Surface Tension. The cmc was determined for the P94E316/water system by measuring the static surface tension γ as a function of the concentration of the solution using the Du Nou¨y ring method. An automatic processor tensiometer (Kru¨ss, model K12) which provides a temperature-controlled ((0.1 °C) environment for the sample was used to measure γ at 20 °C. A series of solutions of P94E316 in doubly distilled deionized water were obtained by sequential dilution of a concentrated stock solution. An equilibration period of 17 min was allowed before measurement of γ for each concentration, which was determined from the average of at least 12 ring detachment force curves. The cmc was defined as the concentration at which γ reached a steady value. 2.6. Solubilization of BL002. Copolymer solution (0.25 or 0.50 g in 25 cm3) and BL002 (0.2 g) were stirred together at a controlled temperature (22 or 40 °C). After several hours, three small samples of the solution were removed and filtered (Millipore, 0.22 µm) to remove excess BL002. It was ascertained in blank tests with water, and with aqueous copolymer solution below the cmc, that droplets of the liquid crystal did not penetrate the filter under the conditions used. The samples were then diluted with methanol to enable analysis by UV spectroscopy. Dilution with water was not an option, since that led to dissociation of the micelles and precipitation of the solute. More to the point, methanol, a good solvent for BL002, was used in calibrating the spectrometer. A Hewlett-Packard 8452A UV/vis spectrometer was used. It was calibrated by recording the absorbances (wavelength range 400-190 nm) of methanol solutions of BL002 (4-40 mg dm-3) against a solvent blank. The absorbance at 288 nm (strong peak) gave a satisfactory Beer’s law plot. The dilutions used in the experiments resulted in solutions containing no more than 0.5 wt % water, and the calibration for methanol solutions was used without correction.

3. Results and Discussion Copolymer solutions with concentrations up to 5 wt % were observed over the temperature range 5-60 °C and were found to remain optically clear. 3.1. Micellization and Hydrodynamic Radius. Dynamic light scattering (DLS) was used to confirm micellization of copolymer P94E316 in dilute aqueous solution. Intensity fraction distributions of log(rh,app) showed single peaks with maxima corresponding to rh,app ≈ 20 nm, consistent with micelles formed by closed association: the examples shown in Figure 1 were obtained for 10 g dm-3 solutions of copolymer P94E316 at 22 and 40 °C. The broad distribution found for the solution at 22 °C is typical of the result from CONTIN analysis of data for a solution containing a substantial proportion of unassociated molecules: these are not seen as a separate signal at low values of log(rh,app) because the intensity is z-weighted and so insensitive to molecules. 3.2. Critical Micelle Conditions and Enthalpy of Micellization. Figure 2 shows examples of the experimental results obtained by the three methods employed to investigate micellization conditions. In Figure 2c, the curvature in the semilogarithmic plot can be ascribed to the narrow but significant distribution of P-block lengths in the sample. The value of the surface tension at the cmc, γcmc ≈ 37 mN m-1, is similar to that previously obtained14 for solutions at 20 °C of E100Pn copolymers with n ) 3773. This indicates that γcmc for aqueous solutions of these diblock copolymers is rather insensitive to both the sequence of anionic polymerization and the P-block length. The critical micelle temperatures and concentrations obtained are collected in Table 1. The results are also plotted as log(c) against 1/T in Figure 3, using cmc or cmt data as appropriate. Agreement between the three methods is excellent. Given closed association (see section 3.1), the slope of this plot can be used in the usual way15,23 to obtain an apparent standard enthalpy of micellization,

Yang et al.

Figure 1. Apparent hydrodynamic radius (rh,app) from dynamic light scattering. Intensity fraction I(logrh,app) vs log(rh,app) for (as indicated) 10 g dm-3 aqueous solutions of copolymer P94E316 at 22 and 40 °C and an aqueous solution of copolymer P94E316 (10 g dm-3) containing 26 mg (g of copolymer)-1 of BL002 at 40 °C (see section 3.4).

i.e., ∆micHoapp ) 325 ( 55 kJ mol-1, from

∆micHoapp ) R d ln(c)/d(1/T)

(3)

The true value of ∆micHo can be obtained in this way only if the association number of the micelles is high (e.g., N > 50) and independent of temperature. For the present case, the first requirement is satisfied, but the second is not (see section 3.3). The consequences of variation of N with T have been explored,13,24 and the error in the present case will be small. Ignoring any contribution from the E-block, the contribution to ∆micHoapp per P unit is 3.5 kJ mol-1, in keeping with values found previously for EmPn copolymers (see Figure 15 of ref 14). 3.3. Association Number and Thermodynamic Radius. Static light scattering (SLS) experiments were performed on dilute copolymer solutions held at 25, 40, and 50 °C. The Debye equation (eq 2 of section 2.2) was used to analyze the data. Used for scattering at 90°, the equation assumes small particles relative to the wavelength of the light. Consistent with this, the dissymmetry, I45/I135, was observed to be 1.00 ( 0.01. Equation 2 truncated to the second term could not be used because micellar interaction (interparticle interference) caused significant curvature of the Debye plot even in the low concentration range. This feature is illustrated in Figure 4: the obvious upturn at low concentrations seen in the Debye curve for the solutions at 25 °C is consistent with a significant increase in the proportion of molecules in the molecules-micelle equilibrium. Rather (23) Price, C. Pure Appl. Chem. 1983, 55, 1563. (24) Kelarakis, A.; Havredaki, V.; Yu, G.-E.; Derici, L.; Booth, C. Macromolecules 1998, 31, 944.

Solubilization of Alkylcyanobiphenyls

Langmuir, Vol. 17, No. 7, 2001 2109 Table 1. Critical Conditions for Micellization of Copolymer P94E316 in Aqueous Solutiona light scattering c/g dm-3

cmt/°C

10 20

18 16.5

dye solubilization

LC solubilizationb

surface tension

c/g dm-3

cmt/°C

c/g dm-3

cmt/°C

T/°C

cmc/g dm-3

1 20 50

22 16 14

1 15

22 20

20

2.3

a Estimated uncertainties: cmt, (1 °C; cmc, (0.5 g dm-3. b See section 3.5 for details.

Figure 3. Log(concentration) versus reciprocal temperature for aqueous solutions of copolymer P94E316. Data points from determination of cmt by (b) light scattering intensity (relative to scattering from benzene), (1) dye solubilization, and (0) BL002 solubilization and from determination of cmc by (2) surface tension.

fraction occupied by the micelles acting as effective hard spheres. Specifically, δt is a thermodynamic expansion parameter defined by

δt ) vt/va

Figure 2. Determination of critical micelle temperature and concentration for aqueous solutions of copolymer P94E316. (a) Temperature dependence of light scattering intensity relative to that from benzene for a 10 g dm-3 solution. (b) Temperature dependence of the absorbance (356 nm) of DPH solubilized in a 20 g dm-3 copolymer solution. (c) Concentration dependence of surface tension for solutions at 20 °C.

than accommodate the curvature by use of a virial expansion, so introducing a number of adjustable coefficients, a method based on Percus-Yevick theory for hard spheres as adapted by Vrij to incorporate the CarnahanStarling equation was used to fit the data.25 This procedure is equivalent to using the virial expansion for the structure factor for hard spheres taken to its seventh term but requires only two adjustable parameters, Mw and δt. The new parameter, δt, relates to the volume excluded by one water-swollen micelle to another, i.e., to the volume

(4)

where vt is the thermodynamic volume (that is one-eighth of the excluded volume for a micelle acting as an effective hard sphere) and va is the anhydrous volume of the micelle, i.e., va ) Mw/NAFa (NA ) Avogadro’s constant). The density of the liquid copolymer (Fa) was calculated (assuming mass additivity) from the specific volumes of the homopolymers at the appropriate temperature.26 Details of the procedure have been described many times previously: for example, see refs 13, 19, and 27. The curves shown in Figure 4 were obtained in this way: the excellent fit is consistent with the micelles being spherical in shape. Strictly speaking, the extrapolation to obtain Mw should be to the cmc, but the low values found (illustrated in Figure 3) mean that no appreciable error is incurred in extrapolating to zero concentration. The values of Mw and δt obtained at the three temperatures are listed in Table 2. Given these parameters, values of the association (25) Percus, J. K.; Yevick, G. J. J. Phys. Rev. 1958, 110, 1. Vrij, A. J. Chem. Phys. 1978, 69, 1742. Carnahan, N. F.; Starling, K. E. J. Chem. Phys. 1969, 51, 635. (26) Mai, S.-M.; Booth, C.; Nace, V. M. Eur. Polym. J. 1997, 33, 991. Allen, G.; Booth, C.; Jones, M. N.; Marks, D. J.; Taylor, W. D. Polymer 1964, 5, 547. (27) Derici, L.; Ledger, S.; Mai, S.-M.; Booth, C.; Hamley, I. W.; Pedersen, J. S. Phys. Chem. Chem. Phys. 1999, 1, 2773.

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Yang et al. Table 3. Solubilization of Liquid Crystal BL002 in Aqueous Solutions of Copolymer P94E316a copolymer c/g dm-3 T/°C 10 10 10 20 20

22 40 22b 25 40

BL002 solubilized mg BL002 solubilized mg (g of hydrophobe)-1 (g of copolymer)-1 26 47 27 41 52

93 168 96 145 186

a Estimated uncertainty in mass BL002, (5%. b Solubilized at 40 °C then cooled and filtered at 22 °C.

Figure 4. Static light scattering. Debye plots for copolymer P94E316 in aqueous solution at the temperatures indicated. The curves were calculated using hard-sphere scattering theory. Table 2. Micellar Properties: Copolymer P94E316 in Aqueous Solutiona SLS

DLS

T/°C

Mw/106 g mol-1

N

δt

rt/nm

T/°C

rh,appb/nm

25 40 50

1.9 3.1 3.7

92 150 180

7.1 6.7 6.6

17 20 21

22 40

21 21

b

a Estimated uncertainties: M , N, δ , and r , (10%; r , (5%. w t h t At c ) 10 g dm-3.

number of the micelles were calculated from

Nw ) Mw,mic/Mw,mol

(5)

using the value of Mw,mol ) 20 700 g mol-1 given in section 2.1, and values of the thermodynamic radius were calculated from the thermodynamic volume via eq 4. As expected for copolymer solutions of this type,15,28 the hydrodynamic and thermodynamic radii are rather insensitive to increase in temperature, this being a result of compensation between the increase in association number and the contraction of the E-blocks in a poorer solvent. The high association numbers indicate large hydrophobic cores suitable for solubilization, particularly so at the higher temperatures. 3.4. Solubilization of BL002. The solubilization of the liquid crystal was investigated for 10 and 20 g dm-3 solutions of P94E316 at 22/25 and 40 °C. By extrapolation of the line drawn in Figure 3, the critical micelle concentration of the copolymer at 40 °C is ca. 0.001 g dm-3. Clearly, at 40 °C both 10 and 20 g dm-3 solutions can be considered completely micellized, and this is confirmed by the Debye plot of Figure 4, which shows no upturn characteristic of substantial micellar dissociation at low concentration (2 g dm-3). This is not the case for the 10 g dm-3 solutions at 22 °C: an equilibrium concentration of ca.1 g dm-3 of unassociated molecules would be expected on the basis of the cmc alone (see Table 1 and Figure 3). However, the distribution of P-block lengths in the sample (which is narrow but significant: see section 2.1) has the effect of spreading the micellization over a significant temperature range, as illustrated in Figure 2 by the light (28) Attwood, D.; Collett, J. H.; Tait, C. J. Int. J. Pharm. 1985, 26, 25.

scattering and dye solubilization curves. There the change in scattering intensity extends from 18 °C (cmt) to 30 °C where micellization is complete assuming that the gradual increase in scattering intensity above 30 °C is a consequence of micellar reorganization (i.e., an increase in association number). This variation in the extent of micellization with temperature affects the extent of solubilization of the liquid crystal. Pertinent results are shown in Table 3. The sensitivity to temperature can be ascribed to variation in the extent of micellization, as confirmed by the results presented in section 3.5. Given complete micellization, the extent of solubilization is significant, approaching 200 mg (g of hydrophobe)-1, i.e., approaching one-fifth by weight of the block which forms the micelle core. Indeed, given a micellar association number of 150 at 40 °C (see Table 2) and assuming an average molar mass of 250 g mol-1 for BL002, the average number of BL002 molecules per micelle core is ca. 600. At 22-25 °C, solubilization in the 20 g dm-3 copolymer solution is more efficient than that in the 10 g dm-3 solutions (see Table 3), which reflects the expected increase in extent of micellization with concentration at low temperature. Included in Figure 1 is the intensity distribution of hydrodynamic radius found by DLS for a 10 g dm-3 solution of copolymer P94E316 plus 26 mg (g of copolymer)-1 BL002 at 40 °C, i.e., an optically clear solution. The distribution is shifted to a higher average value of rh,app and remains narrow, consistent with the micelles retaining their spherical structure. While an increase in average rh,app is consistent with solubilization, we note that measuring the hydrodynamic radius is not an accurate way of determining the extent of solubilization, since this radius is determined largely by the width of the E-block corona. 3.5. Effect of Temperature on Solubilization Studied by Light Scattering. Figure 5 shows the scattering intensities measured for a 10 g dm-3 aqueous solution of copolymer P94E316 containing BL002 solubilized at 22 °C, i.e., 26 mg (g of copolymer)-1. The scattering curve for the copolymer alone (taken from Figure 2a) in included for purpose of comparison. In the experiment, the temperature of the P94E316/BL002 solution was lowered to 5 °C, causing dissociation of the micelles and consequent precipitation of the liquid crystal as a turbid suspension, followed by heating to 60 °C at 1 °C min-1 reduced to 0.1 °C min-1 in the region of the cmt. As can be seen in Figure 5, the scattering intensity fell sharply as micellization commenced at the cmt (18 °C, shown by the dotted line), reached a minimum at 20-22 °C at which point the solution was optically clear to the eye, and rose slowly thereafter. As noted for the copolymer alone (section 3.2), the slow rise in scattering intensity with temperature in the range 22-60 °C is ascribed to the increase in molar mass of the micelles. The enhanced scattering from the micelles of copolymer + BL002, compared with that from

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Langmuir, Vol. 17, No. 7, 2001 2111

(iii) Scattering intensities at 40-50 °C consistent with an increase in extent of solubilization as solubilization temperature is increased. The curves shown in Figure 6 indicate that the solubilization/desolubilization equilibrium can be a convenient method for determining the cmt. The method would be to equilibrate the copolymer solution with BL002 at a temperature above the expected cmt and then cool to obtain a turbid suspension which will clear on reheating through the cmt. This clearing temperature is easily seen by the eye. Results obtained in this way for 1 and 15 g dm-3 solutions, which are included in Table 1 and Figure 3, were consistent with those obtained by more conventional means.

Figure 5. Temperature dependence of light scattering intensity (relative to scattering from benzene) for (O) a 10 g dm-3 aqueous solution of copolymer P94E316, and (b) the same copolymer solution containing liquid crystal BL002 solubilized at 22 °C.

Figure 6. Temperature dependence of light scattering intensity (relative to scattering from benzene) for 10 g dm-3 aqueous solutions of copolymer P94E316 containing liquid crystal BL002 solubilized at (b) 20, (O) 30, and (2) 40 °C.

the micelles of copolymer alone, i.e., an increase of 1520% in scattering intensity, is consistent with the solubilization of the liquid crystal, the quantitative effect reflecting an increase in specific refractive index increment as well as (presumably) in molar mass. Figure 6 shows the results of a related experiment in which solubilization of BL002 in a 10 g dm-3 copolymer solution was effected at 20, 30, and 40 °C. Cooling these solutions to 5 °C followed by heating gave completely regular results, i.e.: (i) Scattering intensities at 5 °C (turbid solutions) which are consistent with the increase in extent of solubilization as solubilization temperature is increased: I40 > I30 > I20. (ii) Sharp reductions in scattering intensity starting at the cmt of the 10 g dm-3 copolymer solution (indicated by the dotted line), this being independent of extent of solubilization. It is clear that the cmt of the copolymer is essentially unaffected by the presence of the liquid crystal, which might be expected for a highly hydrophobic substance which forms a separate essentially pure phase in the absence of micelles.

4. Concluding Remarks The foregoing results allow a number of conclusions regarding the solubilization of liquid crystal BL002 in aqueous micellar solutions. (a) Liquid crystal BL002 can be solubilized in dilute aqueous micellar solutions of block copolymer P94E316. The extent of solubilization is as high as 186 mg (g of hydrophobe)-1, i.e., approaching one-fifth by weight of the block which forms the micelle core. (b) Light scattering is particularly useful for investigating micellar solubilization across a wide range of temperature. As a minimum, it gives a useful indication of the temperature at which micellization is complete. In the present work it was also used to show that the solubilization of BL002 in aqueous P94E316 micellar solutions was completely thermally reversible. (c) Given a highly hydrophobic solubilizate such as BL002, the critical micellization temperature (and presumably cmc) of the copolymer solution is unaffected by its presence in the system. This is because it resides in a separate, essentially pure, phase below the cmt (or cmc). This property makes BL002 solubilization a possible method for determination of the cmt of a copolymer solution. It also provides a useful simplification when considering the design of a system for solubilization. However, exceeding the cmt (or cmc) does not in itself ensure efficient solubilization of a hydrophobic solubilizate. That requires complete micellization of the copolymer, implying additional knowledge of the extent of micellization as a function of temperature. (d) The effect of temperature is a particularly important consideration for aqueous solutions of E/P block copolymers, since the extent of micellization may change rapidly with temperature, as quantified by the high standard enthalpies of micellization found for E/P copolymers in aqueous solution.15,29 The effect of temperature is less important in some related systems: for example, the enthalpies of micellization found for block copolymers of ethylene oxide and 1,2-butylene oxide in aqueous solution can be very low when the hydrophobe block is long.15,24 It is possible that copolymers of that family will prove easier to use for aqueous solubilization than E/P systems. Acknowledgment. We thank Miss C. Chaibundit, and Mr. S. K. Nixon for help with the characterization of the copolymer. The Engineering and Physical Science Research Council supported the work through Grant GR/ M51987. V.C. was supported by the EU-TMR network “Complex Architectures in Diblock Based Copolymer Systems”. LA001610F (29) Chu, B.; Zhou, Z.-K. In Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Surfactant Science Series Vol. 60; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; Chapter 3.