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Characterization of Microemulsions Formed in a Water/ ABA Block Copolymer [Poly(hydroxystearic acid)-Poly(ethylene oxide)-Poly(hydroxystearic acid)]/ 1,2-Hexanediol/Isopropyl Myristate System M. Plaza,†,§ Th. F. Tadros,‡ C. Solans,† and R. Pons*,† Departament Tecnologia de Tensioactius, 11QAB-CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain, and UNIQEMA, Everslaan 45, B-3078 Everberg, Belgium Received October 25, 2001. In Final Form: March 25, 2002 Microemulsions formed with water, isopropyl myristate, PHS-PEO-PHS polymeric surfactant, and alkanediol have been investigated using small-angle X-ray scattering (SAXS) and conductivity at 25 °C. Isopropyl myristate has a low solubility in water; however, this molecule has some polar character because of its ester group. The conductivities of samples with some added NaCl (used here to increase the conductivity sensitivity) are low, but compared to the conductivity of water/alkanediol mixtures, the values are high for most of the samples. The X-ray scattering curves show a correlation peak if the water content is high enough. A two-phase model for the invariant calculation fits the experimental results reasonably well provided that the partition of the oil between the polar and nonpolar phases is taken into account. This partition can be obtained from the ternary water/isopropyl myristate/alkanediol system, which has some mutual-solubility regions. A hard-sphere model can be fitted to the SAXS scattering data. The volume fractions fitted from this model deviate from the volume fractions of the low-polarity phase, indicating that the structure of most of the samples is of the bicontinuous type.
Introduction Microemulsions are optically isotropic, transparent or translucent, and thermodynamically stable dispersions consisting of oil, water, and amphiphile(s).1 In microemulsions, microdomains with different polarities exist, with the separating oil/water interface having a strong surface concentration of the amphiphile. Thermodynamic stability arises from the balance between the low positive interfacial energy and the negative entropy of dispersion terms, so that the net free energy of formation of the system is zero or negative.2 The low interfacial energy is provided by amphiphile adsorption to the interfaces, and the large negative entropy term is provided by the small characteristic length. Two main general structures have been proposed and are accepted:3 discrete microemulsions and bicontinuous microemulsions. Discrete microemulsions consist of domains of water or oil dispersed in the other domain. In a bicontinuous microemulsion, both the aqueous and oil phases are continuous. It is possible to cause transitions from oil-in-water to water-in-oil emulsions via bicontinuous structure and vice versa. In nonionic surfactant systems, the solubility of the amphiphile is strongly affected by temperature (the aqueous solubility decreases as the temperature increases) in such a way that it is possible to change the structure of a system from water-continuous to oil-continuous via bicontinuous struc* To whom correspondence should be addressed. R. Pons, Dept. Tecnologia de Tensioactius, IIQAB-CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain. E-mail:
[email protected]. Tel.: (34-93) 400 61 50. Fax: (34-93) 204 59 04. † CID-CSIC. ‡ UNIQEMA. § Present address: Dermofarm, Can Sant Joan, 08191-Rubı´, Spain. (1) Lindman, B.; Danielsson, I. Colloids Surf. 1981, 3, 391. (2) Ruckenstein, E.; Chi, J. J. Chem. Soc., Faraday Trans. 2 1975, 71, 1690. (3) Zemb, Th. In Neutron, X-ray and Light Scattering; Linder, P., Zemb, Th., Eds.; North-Holland Delta Series: Amsterdam, 1991; p 177.
tures by simply increasing the temperature.4,5 For ionic surfactants, this transformation can be carried out by changes in the chain length and branching or, more practically, by changes in the salt concentration.6 Classical microemulsions are based on conventional ionic6-9 and nonionic4,5,10-12 surfactants and hydrocarbons. Studies on the structure of water-in-oil microemulsions have shown that, at a fixed surfactant concentration, the size of the microemulsion droplets increases as the volume fraction of water increases.13-18 Also, the droplet size increases for fixed surfactant/dispersed-phase volume fraction upon dilution.12 Although alkyl-poly(ethylene oxide) surfactants could be considered polymeric materials, they are usually low in molecular weight. Higher-molecular-weight polymeric (4) Buzier, M.; Ravey, J. J. Colloid Interface Sci. 1983, 91, 20. (5) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (6) Auvray, L.; Coton, J.; Ober, R.: Taupin C. In Physics of Complex and Supramolecular Fluids; Safran, S., Clark, N., Eds.; Wiley: New York, 1987; p 449. (7) Ekwall, P.; Danielsson, I.; Mandell, L. Kolloid Z. 1960, 169, 113. (8) Ekwall, P. In Advances in Liquid Crystals; Brown, G. M., Ed.; Academic Press: New York, 1975; Vol. 1. (9) Ekwall, P.; Danielsson, I.; Stenius, P. In Surface Chemistry and Colloids; Kerker, M., Ed.; MTP International Review of Science, Physical Chemistry Series; Butterworths: London, 1972; Vol. 7, 97. (10) Kahlweit, M.; Busse, G.; Winkler, J. J. Chem. Phys. 1993, 99, 5605. (11) Friberg, S.; Buraczewska, I.; Ravey, J. In Micellization, Solubilization and Microemulsions; Mittal, K., Ed.; Plenum Press: New York, 1977; p 901. (12) Gradzielski, M.; Langevin, D.; Farago, B. Phys. Rev. E 1996, 53, 3900. (13) Cebula, D.; Myers, D.; Ottewill, R. Colloid Polym. Sci. 1982, 260, 96. (14) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 24, 10. (15) Gillberg, G.; Lehtinen, H.; Friberg, S. J. Colloid Interface Sci. 1970, 33, 215. (16) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989, 5, 1210. (17) Merdas, A.; Gindre, M.; Ober, R.; Nicot, C.; Urbach, W.; Waks, M. J. Phys. Chem. 1996, 100, 15180. (18) Baker, R.; Florence, A.; Ottewill, R.; Tadros, Th. F. J. Colloid Interface Sci. 1984, 100, 332.
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surfactants have been in use for some time, although the use of polymeric surfactants in microemulsion stabilization is relatively new. Riess and co-workers19-21 used a block copolymer consisting of polystyrene (PS) and poly(ethylene oxide) (PEO) dispersed in a toluene/water mixture in combination with 2-propanol or butylamine. Boutillier and Candau22 studied the structure of this system and showed that microemulsions in this system form only when there is mutual solubility in the ternary solvent mixture. A model was proposed consisting of two concentric spheres having a PS part solvated by toluene and a PEO part solvated by water, the continuous medium being composed of the ternary solvent mixture at mutual solubility. Tadros and co-workers23,24 studied quaternary water/n-tetradecane/ n-butanol/ABA block copolymer systems forming waterin-oil microemulsions. They showed that these microemulsions behave as classical microemulsions in the sense that droplet size is constant for a constant water-tosurfactant concentration ratio, showing a noticeable increase close to the phase boundary.13,18 More recently, Mays et al.25 prepared microemulsions using triblock copolymers of PEO/PPO/PEO, water, and p-xylene and showed that the amphiphilic triblock copolymer in the water-in-oil (w/o) microemulsion phase behaves in many aspects as a typical nonionic surfactant. However, the amount of water solubilized was only slightly above two water molecules per ethylene oxide group. In this work, we will present data on the structure of water/isopropyl myristate microemulsions using an ABA block copolymer similar to that used by Tadros et al.23,24 The A portions are poly(12-hydroxystearic acid), whereas the B part is poly(ethylene oxide). To increase the solubilization power, a cosolvent, namely, 1,2-hexanediol, was added. It should be mentioned that isopropyl myristate has a low water solubility but has some polar nature because of its ester bond. The use of this alkanediol as a cosolvent was reported by Kahlweit et al., who prepared microemulsions using lecithin or glucoside surfactants.26,27 To study microemulsion formation, it is essential to establish first the phase diagram of the system under investigation. The various phases found were identified using polarizing microscopy.28 Conductivity is an easy-to-use technique to obtain information about microemulsion structure. In particular, many workers have used this technique to characterize the percolative behavior of microemulsions.29-34 Waterdiscontinuous systems present low conductivities, whereas water-continuous systems present high conductivities. The percolation theory establishes a model for the transition from low-conductivity systems to high-conductivity sys(19) Riess, G.; Nervo, J.; Rogez, D. Polym. Prepr. 1977, 18, 329. (20) Riess, G.; Nervo, J.; Rogez, D. Polym. Eng. Sci. 1977, 17, 634. (21) Riess, G.; Nervo, J. Ing. Chim. 1977, 170, 185. (22) Boutillier, J.; Candau, F. Colloid Polym. Sci. 1979, 257, 46 (23) Tadros, Th.; Luckham, P.; Yanaranop, C. ACS Symp. Ser. 1989, 384, 22. (24) Tadros, Th. F.; Dederen, C.; Taelman, M. C. Cosmet. Toiletries 1997, 112, 75. (25) Mays, H.; Almgren, M.; Brown, W.; Alexandridis, P. Langmuir 1998, 14, 723. (26) Kahlweit, M. Langmuir 1995, 11, 3382. (27) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 1576. (28) (a) Plaza, M.; Pons, R.; Tadros, Th. F.; Solans, C. Langmuir 2002, 18, 1077. (b) Plaza, M.; Solans, C.; Stickdorn, K.; Tadros, Th. F.; Pons, R. Prog. Colloid Polym. Sci. 1999, 112, 126. (29) Hilfiker, R.; Eicke, H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1621. (30) Boned, C.; Peyrelasse, J.; Saidi, Z. Phys. Rev. E 1993, 47, 468. (31) Mehta, S.; Dewan, R.; Bala, K. Phys. Rev. E 1994, 50, 4759. (32) Cametti, C.; Sciortino, F.; Tartaglia, P.; Rouch, J.; Chen, S. H., Phys. Rev. Lett. 1995, 75, 569. (33) Bordi, F.; Cametti, C. Colloid Polym. Sci. 1998, 276, 1044. (34) Paul, S.; Bisal, S.; Moulik, S. P. J. Phys. Chem. 1992, 96, 896.
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tems. The onset of percolation corresponds to conductivities that increase with volume fraction with an exponential behavior. Bicontinuous structures present conductivities intermediate between those of water-discrete and oildiscrete systems and higher than the conductivity at the onset of percolation. Mean-field theory predicts the conductivity of conducting spheres in a nonconducting medium as λ ) 3/2λd(φ - 1/3), where λd is the conductingphase conductivity and φ is its volume fraction.33-34 Small-angle scattering curves provide information about the microdomain distribution of molecules in a system and microdomain size and interactions. The microdomain composition can be established without recourse to particulate models by comparison of the experimental invariant with the calculated invariant for a particular molecular distribution.3 A second parameter that can be obtained directly from the scattering curves is the specific area. This can be obtained from the high-q behavior and the absolute scaling.3 The fit of form and structure factors allows for the determination of droplet sizes and interactions.35,36 On the other hand, other models are currently available to describe the transition from particulate to bicontinuous systems such as the DOC (disordered open connected) model.37 Within this framework, it is possible to relate the conductivity to the structure of the system. We have used conductivity measurements and SAXS to study the structure of water-in-oil microemulsions using a polymeric surfactant. To our knowledge, this kind of complex system has not been studied before, and our results fall far from some widely accepted observations for low-molecular-weight surfactants. In particular, we find a high nonaggregated surfactant solubility, a strong partition of the 1,2-hexanediol in favor of the hydrophobic phase, a strongly composition-dependent area per surfactant molecule, and a nonclassical percolation behavior. Materials and Methods Products. The surfactant is a polyester-polyether-polyester ABA block copolymer (Arlacel P135) was obtained from ICI Surfactants. The hydrophillic group B is a poly(ethylene oxide) chain (PEO), and the two tails A are poly(12-hydroxystearic acid) (PHS). The blocks are bonded by an ester bond, and the hydroxystearic chains terminate with a free hydroxyl group . This surfactant is an amber waxy solid and its density is 0.94 g/cm3. The weight-average molecular weight is 6809 g/mol, and the number-average molecular weight is 3500 g/mol. Gel permeation chromatography shows some traces of hydroxystearic and stearic acids.24 The area per molecule calculated from molecular models is about 2.35 nm2/molecule for perpendicular adsorption of the molecule to the interface.23 Poly(hydroxystearic acid) chains corresponding to the A block were also supplied by ICI Surfactants and used as received. PEO chains with 10 000 g/mol molecular chains were from Sigma. The isopropyl myristate (IPM) was supplied by ICI Surfactants and was used as received; 1,2-hexanediol (C6-diol) (purity > 97%) was purchased from Fluka, NaCl was obtained from Merck (p.a.), and water was deionized by milli-Q filtration. Small-Angle X-ray Scattering (SAXS) Measurements. SAXS measurements were carried out by using a Kratky Camera of small angle (M-Braun) and a Siemens KF 760 (3-kW) generator. The wavelength corresponding to the Cu KR line (1.542 Å) was used. The linear detector was a PSD-OED 50 M-Braun apparatus, and the temperature controller was a Peltier KPR AP PAAR model working at 25 °C ( 0.1. The sample was inserted between two Mylar sheets with a 1-mm separation. The SAXS scattering curves were smoothed by using a routine that fits a third-degree polynomial to an increasing number of points as the channel (35) Pedersen, J. Adv. Colloid Interface Sci. 1997, 70, 171. (36) Ashcroft, N.; Lekner, J. Phys. Rev. 1966, 145, 83. (37) Zemb, T.; Hyde, S.; Derian, P.; Barnes, I.; Ninham, B. J. Phys. Chem. 1987, 91, 3814.
Water/PHS-PEO-PHS/C6-diol/IPM Microemulsions
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number increases. This routine was set up to ensure no change in the slopes or in the peak position and sharpness. The smoothed curves were desmeared using the procedure of Singh and coworkers.38 These scattering curves were put on an absolute scale using the value of transmittance obtained with a moving slit device and the standard value of water (1.68 m-1).37 The SAXS scattering curves are shown as a function of the scattering vector modulus
q)
4π sin θ/2 λ
(1)
where q is the scattering angle and λ is the wavelength of the radiation. The q range obtained with our setup was from 0.2 to 6 nm-1. Background spectra of water/1,2-hexanediol mixtures and IPM have been subtracted, with scaling for the content of this mixture in the samples. This subtraction effectively removes most of the high-q signal due to 1,2-hexanediol self-aggregation. Electrical Conductivity. The conductivity measurements were carried out with a Crison 525 meter using a parallelplatinized-platinum-plate dipping cell with a cell constant of 0.998 cm-1. The cell constant was determined using standard KCl solutions. Measurements were carried out at a constant temperature of 25 °C. In this system, a small amount of an aqueous electrolyte must be added for electrical conduction. Thus, a solution of 0.01 M NaCl was used in the preparation of the samples instead of pure water. In principle, the substitution of a dilute aqueous solution of NaCl for water can induce variations in the sizes and shapes of the different regions of the phase diagram. However, if the amount of electrolyte added is small enough, these variations should not be significant.39 There were no observable changes in the phase behavior for the samples prepared with and without NaCl up to a 1 M concentration. Higher NaCl contents led to the formation of emulsions in the region of microemulsion formation. The conductivity of 0.01 M NaCl/1,2-hexanediol mixtures was measured under the same conditions.
Results and Discussion The system we studied has a polymeric surfactant of the ABA block copolymer as the main feature. The A block is a poly(hydroxystearic acid) with about 6 monomers, and the B block is a poly(ethylene oxide) chain of about 34 units. This surfactant has been the subject of a previous systematic phase behavior study to determine the water solubilization regions.28 In that study, the oil was isopropyl myristate (in the following, IPM), and the cosurfactant was 1,2-hexanediol (to be referred to as C6-diol). In Figure 1, a scheme of the pseudoternary surfactant (constant content, 20% w/w)/water/C6-diol/IPM phase diagram is shown (see ref 28 for details about the phase behavior). The solid curve shows the phase boundary between a clear isotropic solution and a multiphase region. The symbols show the compositions of the samples studied in this paper. At 25 °C and a total 20% surfactant content, the maximum water solubilization is 36%. At lower surfactant concentration, the isotropic phase region shrinks, and the maximum solubilized water reduces to 13% for a 10% surfactant content and to 2.5% for a 5% surfactant content. The three composition paths correspond to (1) constant IPM concentration, (2) constant water concentration, and (3) constant C6-diol concentration. To characterize the system we checked for the mutual solubility of the ternary system water/C6-diol/isopropyl miristate. We found that the water/C6-diol mixtures solubilize significant amounts of isopropyl myristate, with an almost linear phase boundary at a hexandiol/IPM ratio of 9 and also a near-linear phase boundary for water (38) Singh, M.; Ghosh, S.; Shannom, R. J. Appl. Crystallogr. 1993, 26, 787. (39) Ezrahi, S.; Watchtel, E.; Aserin, A.; Garti, N. J. Colloid Interface Sci. 1997, 191, 277.
Figure 1. Pseudoternary (constant surfactant content) water/ surfactant (constant)/cosurfactant/oil phase diagram showing the boundary of the L2 phase and the experimental compositions used for characterization. The solid curve shows the phase boundary between a clear isotropic solution and a multiphase region. The dotted line corresponds to the solubilization boundary of the ternary water/1,2-hexanediol/IPM system. The dashed lines correspond to isoconductivity lines. The symbols show the relative conductivities of the samples.
solubilization in C6-diol/IPM mixtures with a C6-diol/ water ratio of 5.7. Equal amounts of water and IPM are soluble in a ternary mixture that contains 60% w/w C6diol (this phase boundary is shown as the dotted line in Figure 1). The PHS chains are virtually insoluble in water/diol mixtures (
