J. Phys. Chem. B 1999, 103, 5347-5352
5347
Fluorinated Microemulsions: A Study of the Phase Behavior and Structure Pierandrea LoNostro,*,† Sung-Min Choi,‡ Chwen-Yuan Ku,§ and Sow-Hsin Chen‡ Department of Chemistry, UniVersity of Florence, Via Gino Capponi 9, 50121 Firenze, Italy, and Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and IPNS, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: June 22, 1998; In Final Form: April 21, 1999
Fluorinated surfactants have been studied for their peculiar property to form micellar aggregates in water and oils (hydrocarbons or fluorocarbons) and to produce stable microemulsions. Because of their capacity to dissolve large amounts of gases (such as oxygen and carbon dioxide) and for their characteristic physicochemical properties, fluorocarbons have been tested for specific medical purposes, and their microemulsions are among the most promising candidates for the production of suitable blood substitutes and other biocompatible fluids. We have synthesized a new partially fluorinated nonionic surfactant, namely, F(CF2)7-CO-(OCH2CH2)7.2OCH3 (I), that forms stable microemulsions with water and perfluorocarbons such as perfluorooctane (PFO). In this paper we describe for the first time the phase behaviors of perfluorooctanoic acid (PFOA) in water/ PFH and in water/PFO, and that of ester I in water/PFO. Small-angle neutron-scattering (SANS) experiments provide a detailed description of the microstructure of the H2O/PFO/PFOA ternary system.
I. Introduction Fluorinated molecules such as perfluorocarbons (CnF2n+2) and their derivatives represent a very interesting and stimulating class of chemicals in physical chemistry and polymer science because of their specific and unusual properties. Fluorocarbons (FC) are organic compounds in which hydrogens have been partially or totally replaced by fluorine atoms; they are water-insoluble, chemically and biochemically inert as a result of their strong C-F bonds (485 kJ/mol, that is 84 kJ/mol more than a regular C-H bond) and as a result of a dense coating of electron-rich, repellent fluorine atoms that protect the carbon skeleton. One of the most striking findings is that FC do not mix with their hydrogenated homologues (HC) because of the different conformations of the two chains.1 Many different theoretical models have been proposed in order to justify this behavior, but actually none of them has successfully and completely explained this unexpected phenomenon. The chemical structure and the weak intermolecular interactions in FC result in peculiar physicochemical properties in the liquid state. In fact fluorocarbons show much lower boiling points, surface tensions, refractive indexes, and dielectric constants than the corresponding hydrogenated homologues. On the other hand, FC possess higher viscosities and higher densities than hydrocarbons.1,2 FC dissolve large quantities of gases (CO2, O2, CO, N2, H2, He, etc.) more than hydrocarbons and water. For example, in the case of oxygen, they dissolve up to 25% (vol %) of gas more than H2O.3 Because of the chemical inertness of FC, no specific interaction such as dipolar attraction, coordination, or charge-transfer complexes can be invoked to justify this phenomenon.2 * To whom correspondence should be addressed. Fax: +39-055-240865. E-mail:
[email protected]. Internet: http://www.csgi.unifi.it. † University of Florence. ‡ Massachusetts Institute of Technology. § Argonne National Laboratory.
The possibility of dissolving large amounts of oxygen in fluorocarbons has produced several studies with the perspective of obtaining reliable blood substitutes and breathing liquids4,5 that may be used during situations of restricted blood flow, such as in myocardial infarction or stroke, for the prevention of ischemia, in cardioplegic solutions to protect the heart during cardiopulmonary bypass surgery, during extracorporeal circulation, to improve oxygen delivery to certain tumors, for organ and tissue preservation, and so forth.6 Fluorinated blood substitutes are also stable, sterizable, and carry no risks of infection. The oxygen dissolved in fluorinated fluids is not chemically bound to the carrier but is readily available to tissues, resulting in higher extraction rates and ratios than with hemoglobin. Not being metabolized, they do not present any metabolite-related toxicity; eventually, the fluorocarbon is excreted from the body through the lungs along with the expired air. The social relevance of using blood substitutes is particularly important in case of temporary shortage of blood, especially for rare groups, on-site rescue of trauma victims and support during transportation to hospitals, possibility of mass casualty situations, extension of the benefit of transfusion to those who refuse the transfusion of natural blood, and to the less privileged developing countries. Especially after the Creutzfeldt-Jacob disease (also known as “mad cow” syndrome), fluorinated microemulsions have become more popular and competitive than blood substitutes obtained from bovine hemoglobine derivatives. Perfluorooctyl bromide (perflubron, PFOB) is a versatile and general contrast agent for X-rays, NMR, and ultrasound tests.6 Because of their high densities,1,7 FC may also be used in ophthalmology, such as in retinal repair, replacement of vitreous liquid, and for improving oxygen delivery to the eye. Furthermore, fluorinated gels or microemulsions provide lubrication and cushioning for the treatment of articular disorders such as osteoarthritis and rheumatoid arthritis. Fluorinated vesicles have also been tested for drug delivery.6,8 FC and their derivatives can also be used as anticorrosive and antifriction components, as flame retardants, water repel-
10.1021/jp9827025 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/29/1999
5348 J. Phys. Chem. B, Vol. 103, No. 25, 1999
LoNostro et al.
Figure 1. Chemical structure of I (CPK model).
lents, or sliding agents, in paints, coatings, polymer technology, metal working, and uranium recovery procedures.9 Since FC are completely water-insoluble at all temperatures, they need to be converted to a water-based, biocompatible system before any administration to the body. Such aqueous medium can be obtained by preparing a microemulsion where the water and the fluorinated phases are stabilized by an emulsifying fluorinated surfactant.9 The introduction of a perfluoroalkylated chain in the hydrophobic tail of a surfactant greatly increases its amphiphilic character, resulting in considerably enhanced surface activity. Fluorinated amphiphiles organize at interfaces or self-associate into supramolecular structures. Likely fluorocarbons, fluorinated microemulsions are usually very stable, optically transparent, with low surface tension, and dissolve large amounts of gases, typically oxygen. For these reasons they can be exploited as lubricants, as oxidizing microcompartmentalized reaction media, as blood substitutes, as environments for aerobic fermentation processes in biotechnology, and for aerobic preservation of transplant organs.10,11 The stability and the microscopic structure of microemulsions produced by fluorinated surfactants with water and fluorocarbons are of great importance also for elucidating the physicochemical properties of these ternary mixtures. Along with electron microscopy,12 small-angle X-ray scattering (SAXS) and smallangle neutron scattering (SANS) are two of the best experimental techniques that one can use to investigate and describe the structural features and the physicochemical properties of supramolecular systems, such as micellar solutions and microemulsions. Because of the peculiar properties of the fluorine nucleus, fluorinated molecules produce better contrasted patterns in SAXS and SANS intensity distributions than their hydrogenated analogues. Since they behave similarly to their fully hydrogenated homologues, they can be usefully investigated with these techniques to deepen the understanding of the hydrophic effect.13,14 Schubert and Kaler9 reported the phase diagrams of some F(CF2)i(CH2CH2O)jH nonionic surfactants in microemulsion with water and PFOB, perfluorodecalin, or perfluorotetradecahydrophenanthrene. They described the microemulsion phase behavior and the existence of a liquid crystal phase region. Ravey and Ste´be´13 reported a detailed study of microemulsions systems produced by different fluorinated and hydrogenated surfactants and of their mutual miscibility, pointing out that the toxicity of a surfactant for biomembranes is strongly related to its capability of producing stable mixtures with the natural, hydrogenated lipids that constitute the membrane. In this paper we report the phase diagram of microemulsions obtained from perfluorooctanoic acid (PFOA), water, and perfluorohexane (PFH) or perfluorooctane (PFO). Small-angle neutron-scattering experiments were performed on some PFOA/ water/PFO samples and provided the first detailed description of the structure of such microemulsions. We also report the phase behavior of a novel surfactant, namely, F(CF2)7-CO(OCH2CH2)mOCH3 with m ≈ 7.2 (I) with water and perfluorooctane. I is constituted by a fluorinated tail linked to a hydrophilic ethylene oxide segment through an ester bond (see
Figure 1). Because of its chemical structure, this compound forms microemulsions with water and PFO. The study of the amount of dissolved oxygen in these fluorinated microemulsions is currently being carried out and will be the object of a future paper. II. Materials and Methods Pentadecafluorooctanoic acid (perfluorooctanoic acid), sulfuric acid, and diethyl ether were purchased from Fluka (Milan, Italy), polyethylene oxide monomethyl ether (average molecular weight Mw ≈ 350) was purchased from Aldrich (Milan, Italy), and perfluorooctane was provided by Flura Co. (Newport, TN). PFOA was recrystallized from water and kept in a desiccator under vacuum and in the presence of P4O10. Bidistilled water was purified with a Millipore apparatus to remove colloidal impurities. Ester I was synthesized in our laboratory from perfluorooctanoic acid and a poliethyleneoxide alcohol in concentrated sulfuric acid. In a 500 mL round-bottom flask, 25 mmol of perfluorooctanoic acid and 25 mmol of polyethylene oxid monomethyl ether (Mw ≈ 350) were mixed and heated at 140 °C for 6 h in the presence of a catalytic amount of sulfuric acid (typically 2%):
F(CF2)7COOH + H(OCH2CH2)7.2OCH3 f F(CF2)7-CO(OCH2CH2)7.2OCH3 + H2O The reaction mixture was poured over 400 mL of ice, and the fluorinated ester was extracted from the aqueous dispersion with diethyl ether. Should a thick solution be formed, some crystals of sodium chloride were added. The organic phase was then washed with distilled water and dried over anhydrous sodium sulfate. The filtered ether solution was then boiled with activated charcoal and filtered, and the solvent was evaporated, giving a viscous, pale-yellow liquid that was then distilled under reduced pressure to give a colorless product (yield ) 70%). Thin layer chromatography (ethyl acetate) showed the presence of one single spot. NMR (CDCl3, ppm): 3.35 (s, OCH3), 3.5-3.8 (m, CH2OCH2), 4.5 (m, OCH2CH2OCO). IR (KBR, cm-1): 2880 (C-H stretch), 1754 (CdO stretch), 1020-1237 (C-F stretch and C-O bend). MS (m/e): 617, [F3C(CF2)6-CO-O(CH2CH2O)4CH2CH2]•+; 573, [F3C(CF2)6-CO-O(CH2CH2O)3CH2CH2]•+; 441, [F3C(CF2)6-CO-OCH2CH2]•+; 169, [F3C-CF2CF2]•+; 147, [(CH2CH2O)3-CH3]•+; 103, [(CH2CH2O)2CH3]•+; 73, [CH2OCH2CH2O-CH3]•+; 59, [CH2CH2O-CH3]•+. Phase Diagram. The phase behavior and microstructure of fluorinated microemulsions is similar to those obtained from hydrogenated compounds. At constant pressure, a ternary system such as a mixture of water (W), oil (O), and a nonionic surfactant (S), is specified by three independent parameters; usually, the temperature T, the volume percentage of water VW/(VW + VO), and the overall weight percentage of the surfactant γ(%) ) S/(S + W + O).15 Each point of the three-dimensional phase diagram (see Figure 2a) is then defined by a set of T, R, and γ values. The phase behavior can be described using a phase prism where T is the ordinate and the Gibbs triangle W-O-S is the base. A vertical section plane corresponding to R ) 0.5 can be drawn
Fluorinated Microemulsions
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5349
(a)
(b)
(c)
Figure 3. T/γ curve for PFOA/water/PFH (R ) 0.5).
Figure 2. (a) Phase prism obtained from the phase behavior study of a surfactant/water/oil microemulsion. (b) T/γ curve obtained by cutting the phase prism at R ) 0.5. (c) T/R curve (“channel cut”) obtained by cutting the prism at γ > γX.
through the phase prism, and the obtained phase diagram is usually referred as a “fish” because of its characteristic shape. The extent of the fish and its location in temperature depend on the particular system of oil, water, and surfactant; for high γ values a liquid crystalline phase (LR) usually appears, whereas at intermediate surfactant concentrations and low temperatures the mixtures form oil-in-water microemulsions in equilibrium with an excess oil phase (see Figure 2b). Since hydrogenated oils have lower densities than water, the two-phase system will be denoted by 2 where the bar indicates the phase (the lower) in which the amphiphile is mainly dissolved. At moderate temperatures a single homogeneous monophasic microemulsion is formed, denoted as 1. Finally, at high temperatures, a twophase system is formed, with a water-in-oil microemulsion in equilibrium with an excess water phase and is denoted as 2h. When a fluorinated oil is used, the two notations 2 and 2h can be misleading and have the opposite meaning with respect to hydrogenated microemulsions. The phase boundaries between the upper and lower two-phase regions and the one-phase region define the so-called “fish tail”. At low surfactant concentrations, the phase sequence is 2 f 3 f 2 as T increases, where 3 indicates that both oil-rich and water-rich phases are in equilibrium with a microemulsion middle phase. As T increases, water and oil interchange as the best solvent for the amphiphile, and therefore, the distribution of the surfactant in the two immiscible solvents inverts, with the formation of three liquid phases in equilibrium. At lower surfactant concentrations, two phases are always found between the melting point and boiling point of the lowest boiling component, and the amphiphile smoothly moves from the waterrich to the oil-rich phase as T increases. The amphiphile concentration γX (the intersection between the fish body and its tail) represents the efficiency of the surfactant, meaning that this is the minimum amount of surfactant that is required to homogenize completely equal amounts of water and oil. The intersecting point is then defined by TX and γX, where the microemulsion middle phase in the three-phase portion of the phase diagram suddenly converts to one single phase. A
Figure 4. T/γ curve for PFOA/water/PFO (R ) 0.5).
common surfactant concentration is necessary to solubilize equal volumes of water and oil. Usually, mixed surfactants produce nonhorizontal fish bodies because the water-oil-surfactant mixture is not truly a ternary system. The phase behavior of multicomponent mixtures can be studied by erecting a vertical section through the phase prism, parallel to its water-oil-T side at a value of γ a little higher than γX for R ) 0.5. This new plane is perpendicular to the plane where the fish is located (see Figure 2a). On this plane there is a narrow channel of macroscopically homogeneous solutions that extends from the water-rich to the oil-phase side of the phase diagram (see Figure 2c). In the literature these T/R diagrams are usually referred to as “channel cuts”,17 where the one-phase region looks like a homogeneous channel between the two-phase regions. III. Results and Discussion A. Phase Diagrams. Figures 3 and 4 show the phase diagrams for the PFOA/water/PFH and the PFOA/water/PFO systems, respectively, with 5% < γ < 50% and 15° < T < 80 °C, and R ) 0.5. It is interesting to note that although PFH boils at about 58 °C, the PFOA/water/PFH mixed dispersions were stable up to 80 °C. Both diagrams show the usual fish body and fish tail regions. In the case of PFH, γX ≈ 8% and TX ≈ 40 °C, whereas for PFO γX ≈ 13% and TX ≈ 44 °C. The difference in the γX and TX values for PFH and PFO is to be related to the higher hydrophobicity of PFO, which requires a higher amount of surfactant to form a stable microemulsion with water and a higher temperature to increase the water solubility of PFO. Similarly, the comparison between the two solvents indicates that the amount of PFOA that is required to produce, at a given temperature, a stable single-phase system is higher in the case of perfluorooctane, at least below the boiling point of PFH (58-60 °C).
5350 J. Phys. Chem. B, Vol. 103, No. 25, 1999
LoNostro et al.
Figure 5. T/γ curve for I/water/PFO (R ) 0.5).
Figure 7. SANS intensities and their analyses using the CRW model.
Figure 6. T/R curve for PFOA/water/PFO (γ ) 20%).
Figure 5 reports the phase diagram (T versus γ at R ) 0.5) for the ternary I/water/PFO mixture. This plot shows an elongated and curved three-phase region with γX ≈ 35% and TX ≈ 45 °C, indicating that the ester I does not possess good emulsifying power for the mixture water/PFO, and for this reason this ternary mixture has not been further investigated by SANS. This result must be ascribed to the chemical structure of this nonionic amphiphile and particularly to the imbalance between the fluorinated and the polar chains. Figure 6 shows the channel cut plot for the PFOA/water/PFO ternary mixture, where T is plotted versus a, at γ ) 20%. B. SANS Experiment. For the reasons we already pointed out in the previous sections, it is important to detect the microstructure of fluorinated microemulsions. To the best of our knowledge, this is the first report on a PFOA/water/PFO system. A series of small angle neutron scattering (SANS) measurements were made with six different one-phase PFOA/water/ PFO microemulsions. While maintaining the volume fractions of water and PFO equal and at a constant temperature, T ) 50 °C, the surfactant weight fraction γ was varied (γ ) 17.5%, 19.7%, 24.9%, 30.1%, 34.4%, and 37.8%). These points are indicated as crosses in Figure 4. The SANS experiment is carried out using a small-angle neutron diffractometer (SAND) at the Intense Pulse Neutron Source (IPNS) at Argonne National Laboratory. SAND uses high-energy proton-induced spallation neutrons, which, after moderation, produce a pulse of a white neutron beam having wavelengths between 1 and 14 Å. All the neutrons are utilized by encoding individually their time-of-flights (wavelength λ selection) and scattering angles (θ) by a 40 × 40 cm2 2D position-sensitive detector. The sample-to-detector distance is
fixed at 2 m, and for this configuration the maximum scattering angle is about 9°. The reliable Q (magnitude of the scattering vector ) (4π/λ)sin[θ/2]) range covered in the measurements were from 0.006 to 0.3 Å-1. Sample liquids were contained in flat quartz cells with 1 mm path length. The temperature of the sample was set by a thermostated circulating water bath to an accuracy of 0.1 °C. Measured intensities were corrected for background and empty-cell contributions and normalized by a reference scattering intensity of a 1 mm water sample at room temperature. Since the scattering length density of the tail group of PFOA is essentially the same as that of PFO and since that of the headgroup is similar to that of water, from neutron scattering point of view, a three-component PFOA/water /PFO microemulsion is effectively a two-component system having a sharp oil-water contrast. The SANS intensity distribution of an isotropic, disordered two-component porous material can be computed as a Fourier transform of a Debye correlation function Γ(r),16
I(Q) ) 〈η2〉
∫0∞dr 4πr2 j0(Qr) Γ(Q)
(5)
where 〈η2〉 ) φ1φ2(F1 - F2) is the mean-square fluctuation of the local scattering length density, φ1 and φ2 the volume fractions of components 1 and 2, and F1 and F2 the corresponding scattering length densities. To analyze the SANS intensity distributions, we used a clipped random wave (CRW) model.17-19 The CRW model contains three parameters a, b, and c, which are related to the three basic length scales in a microemulsion as follows:
a ≈ 2π/d
b ≈ 1/ξ
c ≈ 1/ζ
(6)
where d is the interdomain distance (water to water or oil to oil), ξ the coherence length of the local order, and ζ the interfacial persistence length.
Fluorinated Microemulsions
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5351
TABLE 1: Fitted Parameters Extracted from Fits to the Clipped Random Wave Model φs
R
a (Å-1)
b (Å-1)
c (Å-1)
bgd (cm-1)
〈η2〉 (1020 cm-4)
(S/V) (10-2 Å-1)
0.1578 0.1782 0.2255 0.2747 0.3157 0.3503
0.1533 0.1789 0.2193 0.2648 0.3107 0.3689
0.029 74 0.034 03 0.044 00 0.057 79 0.068 72 0.081 45
0.006 725 0.007 679 0.009 198 0.011 69 0.013 22 0.015 66
0.1730 0.2888 0.3004 0.4672 0.5337 0.6017
0.275 0.255 0.257 0.234 0.224 0.215
4.200 4.117 4.000 3.879 3.870 3.700
1.631 2.092 2.445 3.314 3.782 4.328
Figure 8. 3D interfacial morphology of PFOA/water/PFO at γ ) 17.4%. The size of box is 250 × 250 × 250 Å3.
Figure 7 shows SANS scattering intensities and their analyses in terms of the CRW model (solid lines). It should be noted that the sacttering pattern of PFOA/water/PFO is very similar to that of other bicontinuous microemulsions such as AOT/ water/oil or CiEj/water/oil.20-22 The measured SANS intensities agree well with the CRW model that has been developed for the description of bicontinuous random porous material. The fitted parameters are given in Table 1. As the surfactant weight fraction γ increases, all three parameters a, b, and c increase. In another words, the three length scales d, ξ, and ζ decrease with γ. The decrease of d can be easily understood. As γ increases, meaning putting more surfactant molecules in a fixed sample volume, we create more interfaces in a sample, and thereby, the interdomain distance d decreases. The degree of the local order can be described by the ratio ξ/d. As shown in Table 1, the ξ/d increases from 0.74 to 0.83 with increasing γ. This means that the microstructure tends to become more ordered with an increase of surfactant concentration. This is reasonable because one knows that at sufficiently high surfactant concentrations, lyotropic liquid crystalline phases usually appear. Compared with other hydrogenated surfactant microemulsions such as AOT/D2O (0.6 wt % NaCl)/decane (ξ/d ) 0.40 at γ ) 19%)21 and C10E4/D2O/H-decane (ξ/d ) 0.50 at φs ) 12.5%),20 the PFOA/water/PFO microemulsion shows a higher degree of order. This may be due to the fact that the chain of the fluorinated surfactant is stiffer than that of the hydrogenated surfactants. The CRW model allows us to generate a 3D morphology of the disordered bicontinuous structure using the fitted parameters a, b, and c. Figure 8 shows a 3D interfacial morphology of PFOA/water/PFO at γ ) 17.5%. It can be clearly seen that the structure is truly bicontinuous. IV. Conclusions We studied the phase behavior and structure of microemulsions obtained from perfluorooctanoic acid (PFOA) with water
and two fluorocarbons, namely, perfluorohexane (PFH) and perfluorooctane (PFO), and from an ethylene oxide ester of PFOA, F(CF2)7-CO-(OCH2CH2)mOCH3 with m ≈ 7.2 (I), with water and PFO. Both molecules work as emulsifying agents for the water/fluorocarbon mixture, producing stable microemulsions in the temperature range between 5 and 80 °C. However, compound I does not possess good emulsifying power because of the imbalance between the hydrophilic and the fluorinated chains. SANS mesurements were performed on the PFOA/water/PFO samples in the one-phase region of the phase diagram as a function of surfactant concentration near the HLB temperature. Analysis of SANS intensity distributions using a CRW model provides a detailed description of the microstructure of such ternary microemulsion systems and reveals the presence of a stable bicontinuous structure that depends on the temperature and surfactant concentration. Further studies are being carried out at the present moment in order to investigate the oxygen solubilization capability of such fluorinated microemulsions. Acknowledgment. Research of S.M.C. and S.H.C. are supported by a grant from the Materials Research Division of the U.S. Department of Energy. P.L.N. acknowledges the Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST) and the Consorzio per lo Sviluppo dei Sistemi a Grande Interfase (CSGI) for partial financial support. References and Notes (1) LoNostro, P. AdV. Colloid Interface Sci. 1995, 56, 245-287. (2) Serratrice, G.; Delpuech, J.-J. NouV. J. Chim. 1982, 6 (10), 489493. (3) Delpuech, J.-J.; Hamza, M. A.; Serratrice, G.; Ste´be´, M.-J. J. Chem. Phys. 1979, 70 (6), 2680-2687. (4) Gross, U.; Palpke, G.; Ru¨diger, S. J. Fluorine Chem. 1993, 61, 11-16. (5) Riess, J. G.; Krafft, M. P. Artif. Cells, Blood Substitutes, Immobilization Biotechnol. 1997, 25 (1 & 2), 43-52.
5352 J. Phys. Chem. B, Vol. 103, No. 25, 1999 (6) Riess, J. G. Colloids Surf. A 1994, 84, 33-48. (7) Kloubk, J. Colloid Surf. 1991, 55, 191-203. (8) Zarif, L.; Gulik-Krzywicki, T.; Riess, J. G.; Pucci, B.; Guedj, C.; Pavia, A. A. Colloids Surf. A 1994, 84, 107-12. (9) Schubert, K. V.; Kaler, F. W. Colloids Surf. A 1994, 84, 97-106. (10) Cambon, A.; Delpuech, J.-J.; Matos, L.; Serratrice, G.; Szonyi, F. Bull. Soc. Chim. Fr. 1986, 6, 965-970. (11) Voiglio, E. J.; Zarif, L.; Gorry, F. C.; Krafft, M.-P.; Margonari, J.; Martin, X.; Riess, J.; Dubernard, J. M. J. Surg. Res. 1996, 63, 439-446. (12) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294-301. (13) Ravey, J. C.; Ste´be´, M. J. Colloids Surf. A 1994, 84, 11-31. (14) Ravey, J. C.; Ste´be´, M. J.; Sauvage, S. J. Chim. Phys. 1994, 91, 259-92. (15) Schubert, K.-V.; Kaler, E. W. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 190-205.
LoNostro et al. (16) Debye, P.; Anderson, H. R., Jr.; Brumberger, H. J. Appl. Phys 1957, 28, 679-683. (17) Chen, S. H.; Chang, S. L.; Strey, R. Prog. Colloid Polym. Sci. 1990, 81, 30-35. (18) Chen, S.-H.; Chang, Strey, R. J. Appl. Crystallogr. 1991, 24, 721731. (19) Chen, S.-H.; Lee, D.; Chang, S. L. J. Mol. Struct. 1993, 296, 259264. (20) Chen, S. H.; Choi, S. M. J. Appl. Crystallogr. 1997, 30, 755-760. (21) Choi, S. M.; Chen, S. H. Prog. Colloid Polym. Sci. 1997, 106, 14-23. (22) Choi, S. M.; Chen, S. H.; Sottmann, T.; Strey, R. Phy. ReV. Lett., submitted.
