Langmuir 2003, 19, 10073-10076
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Aggregation Behavior of Short-Chain PDMS-b-PEO Diblock Copolymers in Aqueous Solutions Guido Kickelbick,*,† Josef Bauer,† Nicola Huesing,† Martin Andersson,‡ and Krister Holmberg‡ Institut fu¨ r Materialchemie, Technische Universita¨ t Wien, Getreidemarkt 9, A-1060 Wien, Austria, and Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden Received June 16, 2003. In Final Form: September 24, 2003 Short-chain poly(dimethylsiloxane)-poly(ethylene oxide) (PDMS-b-PEO) diblock copolymers with different ratios of the hydrophilic to hydrophobic segments were synthesized via coupling of endfunctionalized PDMS and PEO homopolymers. Their aggregation behavior was investigated by tensiometry, fluorescence, and cryogenic transmission electron microscopy. Interestingly, the critical aggregation concentration of the surfactants increased with increasing length of the hydrophobic chain. The electron microscopy studies showed that the surfactants preferentially aggregate into vesicles and lamellar structures.
1. Introduction Siloxane surfactants are widely used in many technical applications, such as in textile manufacture, in cosmetics formulations, as agricultural adjuvants, and as paint additives.1 One of the intriguing properties of siloxane surfactants, particularly well-known for aqueous trisiloxane surfactant solutions, is their spontaneous spreading, so-called superspreading, on hydrophobic solid surfaces.2 Two properties of the dimethylsiloxane chain, its flexibility and its low cohesive energy, are believed to be responsible for the unusual properties of siloxane surfactants.3 The most commonly used siloxane surfactants are low to medium molecular weight copolymers. The aggregation and phase behavior of siloxane surfactants with various structures, for example, ABA-type or combtype molecules, is described in the literature.4-9 In most of the systems studied, commercially available siloxane surfactants with different structures and compositions were compared with respect to their physicochemical properties in aqueous solutions. However, there is still a lack of systematic investigations on structure-property relationships of siloxane surfactants in aqueous solution. We have recently investigated the synthesis of diblock copolymer silicon-containing surfactants with tunable functionalities.10,11 Our interest is to use the self-assembly * To whom correspondence should be addressed. Fax: (+43)158801-15399. E-mail:
[email protected]. † Technische Universita ¨ t Wien. ‡ Chalmers University of Technology. (1) Hill, R. M. Silicone Surfactants; Hill, R. M., Ed.; Marcel Dekker: New York, 1999; Vol. 86, p 359. (2) Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247. (3) Owen, M. J. Ind. Eng. Chem., Prod. Res. Dev. 1980, 19, 97. (4) Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbricht, W.; Gru¨ning, B. Tenside, Surfactants, Deterg. 1990, 27, 366. (5) He, M.; Hill, R. M.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 8820. (6) Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir 1993, 9, 2789. (7) Stu¨rmer, A.; Thunig, C.; Hoffmann, H.; Gru¨ning, B. Tenside, Surfactants, Deterg. 1994, 31, 90. (8) Hill, R. M.; Li, X.; Davis, H. T. Silicone Surfactants; Surfactant Science Series Vol. 86; Marcel Dekker: New York, 1999; p 313. (9) Hoffmann, H.; Ulbricht, W. Silicone Surfactants; Surfactant Science Series Vol. 86; Marcel Dekker: New York, 1999; p 97. (10) Bauer, J.; Huesing, N.; Kickelbick, G. Chem. Commun. 2001, 137.
of such amphiphilic surfactants with siloxane segments for the templating of mesostructured inorganic materials.12 Within these studies, we synthesized poly(dimethylsiloxane)-poly(ethylene oxide) (PDMS-b-PEO) diblock copolymers and varied the ratio between the two blocks. Cryogenic transmission electron microscopy (cryo-TEM) images of a variety of these surfactants revealed a preferred formation of vesicles and multilamellar phases in aqueous solutions.13 In this report, we present a detailed study of the aggregation behavior of short-chain PDMSb-PEO diblock copolymers in aqueous solutions. 2. Experimental Section Materials. The short-chain PDMS-b-PEO diblock copolymers were prepared by the coupling of a Si-H end-functionalized PDMS segment prepared via anionic ring opening polymerization with an allyl ether end-capped PEO segment via hydrosilation.13 All polymers were characterized by NMR spectroscopy and size exclusion chromatography (SEC). Measurements. Equilibrium surface tension was measured with a SIGMA70 tensiometer (KSV) equipped with a Pt-Ir du Nouy ring. Double-distilled water was used for all sample preparations and for the calibration. All tensiometry measurements were carried out with freshly prepared solutions at (20 ( 0.1) °C. For the surfactants investigated, the variation of the surface tension with the surfactant concentration was determined. The sharp breaks in the curve (surface tension versus log concentration) marked the onset of the aggregation. The aggregates may be micelles or bilayer structures, or both may coexist. Steady-state fluorescence measurements were carried out on a Shimadzu RF-5000 spectrophotometer using pyrene as the fluorophore. The fluorescence spectra were measured between 350 and 500 nm with the excitation wavelength at 335 nm. The procedure for the measurement of the cryo-TEM images has been described previously.13
3. Results and Discussion The PDMS-b-PEO diblock copolymers were synthesized by coupling via hydrosilation of a Si-H end-capped (11) Bauer, J.; Huesing, N.; Kickelbick, G. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1539. (12) Huesing, N.; Launay, B.; Bauer, J.; Kickelbick, G.; Doshi, D. J. Sol.-Gel Sci. Technol. 2003, 76, 609. (13) Kickelbick, G.; Bauer, J.; Huesing, N.; Andersson, M.; Palmqvist, A. Langmuir 2003, 19, 3198.
10.1021/la035063k CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2003
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Table 1. PDMS-b-PEO Surfactants Investigated surfactant
Mn (Mw/Mn) (PDMS)
Mn (Mw/Mn) (PEO)
conv [%]
Mn (Mw/Mn) (diblock)
ratio by 1H NMR (theory) [PDMS/PEO]
DMS4-b-EO12 DMS6-b-EO12 DMS10-b-EO12 DMS14-b-EO12 DMS18-b-EO12 DMS24-b-EO8 DMS32-b-EO16
585 (1.13) 570 (1.17) 855 (1.21) 1145 (1.18) 1320 (1.14) 1730 (1.21) 2400 (1.30)
660 (1.15) 660 (1.15) 660 (1.15) 585 (1.16) 585 (1.16) 415 (1.13) 695 (1.17)
95 96 95 90 88 98 93
1235 (1.16) 1285 (1.21) 1460 (1.24) 1480 (1.43) 1780 (1.34) 1890 (1.24) 3380 (1.26)
0.49 (0.50) 0.71 (0.75) 1.18 (1.25) 1.91 (1.75) 2.45 (2.60) 4.97 (4.79) 3.04 (3.20)
Scheme 1
Table 2. Critical Aggregation Concentration of DMSn-b-EOm in Aqueous Solution and Other Parameters Derived from the Gibbs Plot at 20 °C
surfactant DMS4-b-EO12 DMS6-b-EO12 DMS10-b-EO12 DMS14-b-EO12 DMS18-b-EO12 DMS24-b-EO8 DMS32-b-EO16
CAC [mol/L]
γ at CAC [mN m-1]
Γmax [mol/m2]
area per molecule, a [Å2]
4.5 × 10-6 7.0 × 10-6 2.5 × 10-5 8.9 × 10-5 6.4 × 10-4
27.0 30.1 31.5 35.0 32.6
7.94 × 10-6 5.85 × 10-6 4.16 × 10-6 4.44 × 10-6 4.78 × 10-6 3.58 × 10-6 4.02 × 10-6
21 28 40 37 35 46 41
hexamethylcyclotrisiloxane with a poly(ethylene oxide) allyl ether (Scheme 1). Both segments, the PDMS and the PEO, were prepared via controlled polymerization techniques, which means that the degree of polymerization was thoroughly controlled in order to provide a rather narrow molecular weight distribution. Table 1 shows the various compositions of the short-chain diblock copolymers studied in this investigation and selected experimental data. Equilibrium Surface and Interfacial Tension. For the investigated surfactants, the variation of the surface tension with the surfactant concentration was determined. According to Gibbs’ law applied to equilibrium systems, the adsorption of the surfactant at the gas/liquid interface leads to a reduction of the surface tension of the solution. The surface excess concentration, Γ, and the surface area, a, per surfactant have been calculated using the Gibbs equation,
Γ)-
dγ 1 nRT d ln c
(
)
T
)
1 aNA
(1)
where R is the gas constant, T is the absolute temperature, and n is a constant which depends on the number of species constituting the surfactant that are adsorbed at the interface. The headgroup areas have been calculated using n ) 1. The obtained values of the critical aggregation concentration (CAC), the surface tension at the CAC, the maximum surface excess concentration, Γmax, and the surface area, a, of the surfactant are summarized in Table 2. Figure 1 shows the equilibrium surface tension, γ, as a function of surfactant concentration of aqueous solutions at 20 °C of the DMSn-b-EO12 copolymer series with
Figure 1. Equilibrium surface tension, γ, vs concentration in aqueous solutions of DMS4-b-EO12, DMS6-b-EO12, DMS10-bEO12, DMS14-b-EO12, and DMS18-b-EO12 at 20 °C.
different numbers of hydrophobic DMS units. The kink in the curves marks the onset of aggregation, that is, the CAC. By application of eq 1, the maximum surface excess concentration, Γmax, and the surface area, a, for each surfactant have been calculated from the linear slope of the reduction of the surface tension with increasing surfactant concentration at concentrations below the CAC (Table 2). The surface area per surfactant, a, was determined from the slope of the surface tension plots. The results obtained are given in Table 2. Two interesting observations can immediately be made: (i) the values increase with increasing size of the hydrophobic segment until a maximum is obtained at 10 DMS units, and (ii) the values for the surfactants with short hydrophobic chains are unrealistically low. Normal alcohol ethoxylates with 12 oxyethylene units in the polar headgroup typically have values for the surface area per molecule of 70-80 Å2. It is likely that the extremely hydrophobic PDMS segment will induce tight packing of the hydrophobic segments, which, in turn, may force the poly(ethylene oxide) chain to become more extended than normal, but a value of 21 Å2 for the shortest of the homologues still does not seem reasonable. It is interesting that the area per molecule for the DMS4-DMS6-DMS10 series increases (21 to 28 to 40) with increasing number of DMS units up to a value of 10. This seems to indicate that the area per molecule at the air-water interface is governed by the hydrophobic tail, that is, the PDMS segment, and that the polar headgroup, that is, the 12 oxyethylene units, has a smaller area demand. As mentioned above, this is surprising because 12 oxyethylene units in an alcohol ethoxylate normally occupy a considerably larger area. To date, we have no explanation for this peculiar behavior. The data for the CAC of the block copolymer series DMSn-b-EO12 from Table 2 were plotted and gave a straight line with an unexpected slope. Usually the CAC of amphiphilic block copolymer surfactants decreases with increasing length of the hydrophobic segment. For ex-
Aggregation Behavior of Diblock Copolymers
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Figure 2. Logarithmic plot of the CAC vs the number of DMS units in the DMSn-b-EO12 surfactants.
Figure 3. Equilibrium surface tension, γ, vs concentration in aqueous solutions of DMS24-b-EO8 and DMS32-b-EO16 at 20 °C.
ample, in the case of 1,2-butylene oxide-ethylene oxide diblock copolymers the logarithmic dependence of the aggregation concentration with increasing number of butylene oxide units has a negative slope.14,15 However, the logarithm of the CAC versus the number of DMS units for the investigated copolymers had a positive slope (Figure 2). The slope corresponds to an increment in ∆G°agg of approximately -850 J/mol DMS unit. The Gibbs energy of aggregation was calculated according to eq 2.
∆G°agg ) -RT ln K ≈ RT ln(CAC)
(2)
To investigate the influence of the length of the hydrophilic block, the tensiometry measurements were carried out on two larger surfactants with different ethylene oxide blocks. The results are shown in Figure 3. As can be seen from the figure, the surfactant solutions became turbid before the onset of aggregation. We interpret this as a clouding phenomenon; that is, at this concentration (at 20 °C) there is a transition from a onephase to a two-phase system. Determining the CAC in a two-phase system is not relevant. We, therefore, do not regard the break points on the curves to have any physical meaning. (14) Yang, Y.-W.; Deng, N.-J.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703. (15) Yang, Y.-W.; Deng, N.-J.; Zhou, Z.-K.; Attwood, D.; Booth, C. Macromolecules 1996, 29, 670.
Figure 4. Variation of the intensity ratio I/III of the fluorescence pyrene spectra in aqueous solutions of DMSn-b-EO12 surfactants at 20 °C. The dashed arrows indicate the onset of the decrease of the I/III ratio in the emission of pyrene (∼3.0 × 10-7 mol/L), and the dotted arrows indicate the inflection points of the curves for the three surfactants.
The areas per molecule were determined from the slope of the surface tension plots for the DMS24-b-EO8 and DMS32-b-EO16 surfactants. Interestingly, these values are close to the values of the theoretically calculated hydrophobic cross-sectional area of PDMS (approximately 40 Å2). They are, however, surprisingly small considering the size of the PEO segment. Fluorescence Studies. The aggregation of three surfactants with constant PEO chain length was also investigated by steady-state fluorescence using the emission spectrum of pyrene. The intensity ratio I/III of the first (I) and third (III) vibronic peaks is a good indicator of the polarity of the probe microenvironment.16 The micellization of low-molecular-weight surfactants is accompanied by an abrupt decrease in the ratio of I/III. In practice, the solvent polarity dependence of the pyrene emission is expressed in terms of the ratio I1/I3 of the intensities, I1 and I3, of the bands I and III corresponding to the S1ν)0 f S0ν)0 (0-0) and S1ν)0 f S0ν)1 transitions, respectively, where S1 and S0 are the first singlet excited state and the ground state of pyrene, respectively. The values typically range from ∼1.9 in polar solvents to ∼0.6 in hydrocarbons. Critical micelle concentration (CMC) and CAC of surfactants can be obtained from measurements of the changes in I/III as a function of surfactant concentration. The ratio decreases sharply at the onset of aggregate formation, reflecting the preferential solubilization of pyrene in a hydrophobic environment. Whereas the tensiometry gave an indication of aggregate formation by measurement at the air/water interface, the steady-state fluorescence directly determines the CAC in solution. The onset of the changes in the I/III ratio as a function of surfactant concentration (onset of aggregation) and the point of maximum decline, that is, the inflection point, have been determined for the three surfactants DMS4-b-EO12, DMS6-b-EO12, and DMS10-b-EO12 and are shown in Figure 4. Because the surfactants containing longer siloxane blocks form dispersions at the CAC, the fluorescence method was only applicable to surfactants containing a maximum of 10 DMS units. The dashed arrows in Figure 4 indicate the onset of the decrease of the I/III ratio in the emission of pyrene (∼3.0 (16) Zana, R. Steady-state fluorescence; Zana, R., Ed.; Dekker: New York, 1987; p 241.
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Table 3. Critical Aggregation Concentration of DMS4-b-EO12, DMS6-b-EO12, and DMS10-b-EO12 in Aqueous Solution Determined by Steady-State Fluorescence and Tensiometry at 20 °C
surfactant
onset of aggregation [mol/L]
inflection point [mol/L]
CAC, tensiometry [mol/L]
DMS4-b-EO12 DMS6-b-EO12 DMS10-b-EO12
3.0 × 10-6 4.9 × 10-6 5.5 × 10-5
2.2 × 10-5 2.6 × 10-5 1.3 × 10-4
4.5 × 10-6 7.0 × 10-6 2.5 × 10-5
× 10-7 mol/L), and the dotted arrows indicate the inflection points of the curves for the three surfactants. The data are summarized in Table 3 together with the CAC values obtained by the Gibbs plots of the tensiometry measurements. Addition of surfactant began to affect I/III at a concentration of approximately 3 × 10-6 mol/L for DMS4b-EO12, a concentration that is close to the CAC obtained by tensiometry measurements (indicated by the dashed arrows in Figure 4). The other surfactants showed a similar behavior. The inflection points of the curves appear at concentrations significantly higher than the CAC determined by tensiometry. An abrupt decrease in the intensity ratio I/III of the vibronic peaks indicates a cooperative process.17 In the case of DMSn-b-EO12, the decrease is more gradual, that is, the slope is not very steep. This shows that the association is a less cooperative process. This is probably due to a broad size distribution of the aggregates formed. Both the tensiometry and the fluorescence measurements give an order of CAC values for the series of copolymers with 4, 6, and 10 DMS units that is opposite to the expected order. The usual trend for surfactants is that the aggregation, which is usually micelle formation, starts at a lower concentration the longer the hydrophobic tail. With the dimethylsiloxane surfactants studied in this work, the trend is the opposite. We believe the reason for this is that what we are recording is not micelle formation but formation of vesicles. Thus, the kink of the curves of Figure 1 and the onset of the slope of the curves of Figure 4 represent the appearance of a lamellar phase; see the discussion in the next section. (Vesicles are dispersions of a lamellar phase in water.) The ease with which lamellar phases form depends strongly on the surfactant structure; in principle, the structure should be such that the critical packing parameter (CPP) of the surfactant is close to unity.18 Normal, micelle-forming surfactants have a relatively large headgroup compared to the tail; thus, they have CPP values considerably lower than 1. If the polar headgroup becomes small compared to the tail, micelles will not form; instead there may be a direct transition from a very dilute solution of surfactant unimers into a vesicular phase. One example of such behavior is tri(ethylene glycol)monododecyl ether, commonly abbreviated C12E3.19 The dimethylsiloxane tails of the surfactants used in this work are bulky, rendering micelle formation unfavorable and favoring formation of a lamellar phase already at low concentration, just as with C12E3. The reverse order of CAC values found in the measurements most likely reflects the ease with which these compounds pack into lamellar liquid crystals. Thus, the order of the CAC values reflects the surfactant geometry, not the (17) Alami, E.; Holmberg, K. J. Colloid Interface Sci. 2001, 239 (1), 230. (18) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; Wiley: Chichester, 2003; pp 90-92. (19) Hassan, S.; Rowe, W.; Tiddy, G. J. T. Surfactant Liquid Crystals; Holmberg, K., Ed.; Wiley: Chichester, 2002; Vol. 1.
Figure 5. Cryo-TEM of a 0.12 wt % solution (∼3.3 CACs) of DMS10-b-EO12 surfactants in water.
hydrophobicity, which is the factor that mainly governs the CMC values of ordinary surfactants. Cryo-TEM Studies. Cryo-TEM studies of aqueous solutions of the diblock copolymers at low concentrations were carried out to investigate the morphologies of the aggregates. At these low concentrations, spontaneous vesicle formation was observed.13 The thickness of the vesicle walls increased with increasing length of the PDMS segment. Increasing surfactant concentration in the aqueous phase led to a transition from vesicles to multilamellar aggregates. Figure 5 shows a cryo-TEM image of a 0.12 wt % solution (∼3.3 CACs) of DMS10-bEO12 diblock copolymer illustrating the dimensions of the vesicles obtained. Whereas the multilamellar vesicles in the image hardly exceed a size of 0.5 µm, the size of the unilamellar vesicle in the center of the image has a diameter of several micrometers. All vesicles, regardless of whether they are uni- or multilamellar, are polydisperse and globular in shape. Conclusions Short-chain PDMS-b-PEO diblock copolymer surfactants show a critical aggregation concentration that increases with increasing length of the hydrophobic chain. The origin of this behavior is not fully understood. It may be associated with steric constraints in the packing of the methyl-branched hydrophobic tails into closed aggregates, a problem that becomes more severe with increasing number of DMS units. The surfactants preferentially aggregate into vesicles and lamellar structures. Acknowledgment. We thank the Fonds zur Fo¨rderung der wissenschaftlichen Forschung, Austria, and the European Cooperation in the Field of Scientific and Technical Research (COST) Action D19 for their support of this work. M.A. is grateful for financial support from the Swedish Foundation for Strategic Research through its Colloids and Interface Technology Program. Gunnel Karlsson, from the Biomicroscopy Unit, Chemical Center, Lund University, Sweden, is acknowledged for performing the cryo-TEM studies. LA035063K
