Biocompatible Lipidic Formulations: Phase Behavior and

Langmuir 2004, 20, 5241-5246

5241

Biocompatible Lipidic Formulations: Phase Behavior and Microstructure S. Mele, S. Murgia, F. Caboi, and M. Monduzzi* Dipartimento di Scienze Chimiche, CSGI-University of Cagliari, Cittadella Universitaria Monserrato, S.S. 554 Bivio per Sestu, 09042 Monserrato-Cagliari, Italy Received January 20, 2004. In Final Form: April 1, 2004 Biocompatible systems formulated for use in the food, cosmetic, and pharmaceutical fields are characterized. Ternary phase diagrams of mixtures of natural lipids (glycerol trioleate, glycerol monooleate, diglycerol monooleate, and lecithin) and water were investigated by means of optical microscopy in polarized light and by multinuclear NMR spectroscopy. All systems showed a microemulsion region at high oil content and a large area of coexistence of two liquid crystalline (hexagonal and lamellar) phases. 1H and 13C NMR self-diffusion measurements were used to characterize microstructural features of the microemulsions. On water dilution, the two-phase liquid crystalline region transforms into a creamy emulsion area where the droplets of water are stabilized by both the lamellar and the hexagonal phases, as indicated by 2H NMR measurements. Due to the very effective dispersing action of the two liquid crystalline phases, these emulsions show a high stability toward phase separation.

1. Introduction Esterified polyalcohols such as monoglycerides and esters of polyglycerols have long been used to control the properties of aqueous dispersions in various cosmetic and food applications. In the pharmaceutical field, interest in the use of vesicles, microemulsions, micelles, and liquid crystalline (LC) phases formed from biocompatible surfactants is expanding. Consequently, the demand for systems with prescribed physicochemical properties, such as for example size of the aqueous cavities, curvature, and charge, has increased markedly. Glycerol monooleate (GMO), used as an emulsifier and food additive since 1950, has received much attention recently for applications in the pharmaceutical area (cf., e.g., reviews in refs 1-3). The phase diagram of the GMO/ water (W) system has been extensively explored along with a detailed characterization of its various LC phases.4,5 Many investigations have also focused on modifications of GMO/W system phase behavior induced by the presence of other components or due to the use of mixed solvents.1-3,6-14 The modifications depend on polarity, shape, and concentration of the additive significantly.11-14 Meth* Corresponding author. Phone: +39 070 675 4385. Fax: +39 070 675 4388. E-mail: [email protected]. (1) Lawrence, M. J. Chem. Soc. Rev. 1994, 23, 417. (2) Larsson, K. Curr. Opin. Colloid Interface Sci. 2000, 5, 64. (3) Ganem-Quintanar, A.; Quintanar-Guerrero, D.; Buri, P. Drug Dev. Ind. Pharm. 2000, 26, 809. (4) Larsson, K. Nature 1983, 304, 664. (5) Hyde, S. T.; Andersson, S.; Ericsson, B.; Larsson, K. Z. Kristallogr. 1984, 168, 213. (6) Pampel, A.; Michel, D.; Reszka, R. Chem. Phys. Lett. 2002, 357, 131. (7) Imberg, A.; Evertsson, H.; Stilbs, P.; Kriechbaum, M.; Engstrom, S. J. Phys. Chem. B 2003, 107, 2311. (8) Engstro¨m, S.; Alfons, K.; Rasmusson, M.; Ljusberg-Wahren, H. Prog. Colloid Polym. Sci. 1998, 108, 93. (9) Geraghty, P. B.; Attwood, D.; Collet, J. H. Pharm. Res. 1996, 13, 8. (10) Lading, P.; Lundsholm, Y.; Norling, T. Method and composition for controlled delivery of biologically active agents. U.S. Patent 5,143,934, 1992. (11) Caboi, F.; Nylander, T.; Razumas, V.; Talaikyte´, Z.; Monduzzi, M.; Larsson, K. Langmuir 1997, 13, 5476. (12) Pitzalis, P.; Monduzzi, M.; Kro¨g, N.; Larsson, K.; Ljusber-Wahren, H.; Nylander, T. Langmuir 2000, 16, 6358.

ods of preparation and storage conditions are also important factors that affect phase behavior and stability. Some recent studies on phase diagram modifications induced by different additives and performed over a long period (more than 2 years) have occasioned a focus on the role of additional phenomena such as GMO hydrolysis and acyl migration.12,13,15 These phenomena generally overlap with and are hard to disentangle from the macroscopic additive effects when short-term investigations are performed. Another problem is the eventual repartition of the additive among different locations within the LC matrix.14 In general, the most significant microstructural changes have been shown to be strongly influenced by the amount of water and by storage conditions. High water content and high temperatures favor GMO hydrolysis with the consequent production of oleic acid (OA). And oleic acid always promotes negative curvatures of GMO bilayers, either in the lamellar (LR) or in the bicontinuous cubic LC phases, thus inducing a transition toward hexagonal (H2) LC phases. This work focuses on the characterization of the ternary and pseudo-ternary phase diagrams formed by GMO, water, and other natural lipids with the aim of preparing new stable and biocompatible formulations. The apolar lipid used was glycerol trioleate (GTO), while GMO, diglycerol monooleate (DGMO), and technical lecithin (LCT) were used as surfactants. The GMO/GTO/W, GMO-LCT/GTO/W, and GMODGMO/GTO/W phase diagrams were investigated by visual inspection and optical microscopy in polarized light. The choice of GMO/DGMO mixtures, having a 85/15 mass ratio, and GMO/LCT mixtures, having a 95/5 mass ratio, had as its motivation the aim of improving the performance of GMO-based systems. Previous investigations showed that the long-term stability of GMO liquid crystalline phases is strongly affected by hydrolysis within 4-6 (13) Caboi, F.; Amico, G. S.; Pitzalis, P.; Monduzzi, M.; Nylander, T.; Larsson, K. Chem. Phys. Lipids 2001, 109, 47. (14) Caboi, F.; Murgia, S.; Monduzzi, M.; Lazzari, P. Langmuir 2002, 18, 7916. (15) Murgia, S.; Caboi, F.; Monduzzi, M.; Ljusberg-Wahren, H.; Nylander, T. Prog. Colloid Polym. Sci. 2002, 120, 41.

10.1021/la049822q CCC: $27.50 © 2004 American Chemical Society Published on Web 05/20/2004

5242

Langmuir, Vol. 20, No. 13, 2004

months from sample preparation.13,15 Both DGMO and LCT are known to favor microstructures having an average zero curvature.12,16,17 Particularly, the addition of 15 wt % of DGMO to a GMO/W system was shown to counteract hydrolysis effects.12 In a previous work,18 these systems were found to form very stable three-phase emulsions at relatively low oil content, provided that the water solute was added gradually to a previously formed LC phase. In that work, a characterization by optical microscopy only was reported. Here, 1H, 2H, and 13C NMR measurements were used to characterize the microstructural features of the whole phase diagram. The choice of material characterized by technical purity was dictated by the fact that these formulations are of applied interest. Generally, very pure materials are not used for industrial purposes. Moreover, the presence of different chain lengths, insaturation and mono- or diglycerides in GTO particularly, is important since these “impurities” play a synergic role in emulsion stabilization. 2. Experimental Section 2.1. Materials. Glycerol monooleate (GMO, 1-monooleoylglycerol, RYLO MG 90-glycerol monooleate; 98.1 wt % monoglyceride) and diglycerol monooleate (DGMO, 3-(2,3-dihydroxypropoxy)-2-hydroxypropyl oleate, 86 wt %) were kindly provided by Danisco Ingredients, Brabrand, Denmark. The diglycerol (DG, Solvay S. A., Brussels, Belgium) used to produce DGMO had a purity of at least 92% and a ratio between the linear 1,1-DG and the branched 1,2-DG isomer of 80/15 (wt/wt). The fatty acid composition of both esters was 92% oleic acid, 6% linoleic acid, and 2% saturated fatty acids, as here verified through 13C NMR spectra. L-R-Phosphatidyl-choline (LCT, L-R-lecithin) from soybeans with purity of 40 wt % (containing about 75 wt % of phosphatidyl-choline and 12 wt % of phosphatidyl-ethanolamine) and glycerol trioleate (GTO) with technical purity of 65 wt % (this reagent contains also monoglycerides and 1,3- and 1,2diglycerides; the fatty acid chain composition is 90 wt % as oleic acid and 10 wt % as linoleic acid) were from Sigma. Distilled water, passed through a Milli-Q water purification system (Millipore), was used to prepare the samples for the phase diagram determination. Emulsion samples for NMR experiments were prepared using 2H2O from Fluka. 2.2. Sample Preparation. Liquid crystalline samples were prepared by weighing the components into glass tubes (diameter, 5 mm) that were centrifuged, frozen for 12 h, and flame-sealed. The samples were homogenized by repeated cycles of heating, centrifuged back and forth at 3000 rpm, and stored at 25 °C for 3 weeks before any measurement was taken. The observation of macroscopic properties of the samples (phase number, physical state, homogeneity, and birefringence) at 25 °C allowed a preliminary phase diagram characterization. Emulsion samples for optical microscopy analysis were prepared by addition of water to the other components up to the desired composition into sample tubes that were accurately sealed by screw plugs. The tubes were stored in a thermostatic oven at 25 °C, observed daily, and centrifuged prior to optical microscopy analysis. Samples for 2H NMR measurements in the emulsion regions were prepared by addition of proper amounts of 2H2O (2.5 wt %) to GMO/GTO, (GMO/DGMO ) 85/15)/GTO, and (GMO/LCT ) 95/5)/GTO LC mixtures, at a surfactant/oil mass ratio of 15/75, up to the desired composition, followed by vigorous mixing by a Vortex mixer. 2.3. Optical Microscopy. The anisotropic LC phases were observed by optical microscopy (Zeiss Axioplan 2) in polarized light at 25 ( 1 °C. The obtained patterns were compared with the typical textures of other surfactants.19 2.4. Nuclear Magnetic Resonance (NMR). 1H, 2H, and 13C NMR measurements were performed by a Bruker Avance 7.05 (16) Kabalnov, A.; Tarara, T.; Arlauskas, R.; Weers, J. J. Colloid Interface Sci. 1996, 184, 227. (17) Engblom, J.; Miezis, Y.; Nylander, T.; Razumas, V.; Larsson, K. Prog. Colloid Polym. Sci. 2000, 116, 9. (18) Mele, S.; Murgia, S.; Monduzzi, M. Colloids Surf., A 2003, 223, 57. (19) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628.

Mele et al. T spectrometer at the operating frequencies of 300, 46.072, and 75.468 MHz, respectively, at 25 °C. A standard variable temperature control unit (with an accuracy of 0.5 °C) was used. The spectrometer is equipped with a Bruker field gradient probe DIFF30 that can reach field gradients of 3.00 T m-1. 2H NMR spectra were periodically recorded, without lock, to verify the achievement of equilibrium and to characterize the LC microstructure in mono- or multiphase samples. Nuclei with a spin quantum number I g 1, such as 14N or 2H, have an electric quadrupolar moment that can interact with nonzero net electric field gradients giving multiple resonance of 2I peaks, which in the case of oriented anisotropic LC phases are separated by the splitting:20

∆νq ) (3/m)PbχSb

(1)

where m ) 4 and m ) 8 apply for the lamellar and hexagonal phase, respectively, Pb is the fraction of the observed nucleus in the bound state, χ is the quadrupolar coupling constant, and Sb ) 1/2(3 cos2 ϑD - 1) is the order parameter related to the average time orientation (ϑD) of the nucleus with respect to the surfactant chain axis. For water molecules, Pb is linearly dependent on the surfactant/water (s/w) molar ratio and thus eq 2 can be rewritten as20

∆νq ) (3/m)nb(s/w)χSb

(2)

where nb is the number of bound water molecules per polar head. The inner and outer peaks correspond to the nuclei orientations at 90° and 0°. Each sample was left for 30 min in the NMR probe to ensure thermal equilibrium and alignment in the magnetic field before recording the spectra at 25 °C. The error in the quadrupolar splitting measurements was around 7%. Self-diffusion experiments were carried out using a DIFF30 Bruker probe equipped with a specific insert for each nucleus and varying the gradient strength (G) while keeping the gradient pulse length (δ) and the pulse intervals (∆) constant. Measurements were performed using the pulsed-gradient stimulated echo (PGSTE) sequence.21,22 The echo intensity (E) decay as the value of G is increased is given by

[ ( ) ( )] [(

E(δ,∆,G) ) exp -

2∆ τ T2 T1

(

exp -D(δγG)2 ∆ -

)]

δ 3

(3)

where D is the self-diffusion coefficient, τ is the storage period (the constant time between the second and the third 90° pulse), T1 and T2 are the longitudinal and transversal relaxation times, respectively, and γ is the magnetogyric ratio of the observed nucleus. Self-diffusion coefficients were calculated by means of a twoparameter nonlinear fit of the echo intensity decay measured at 18-20 different G values (Gmin ) 0.01 T m-1, Gmax ) 3.00 T m-1). The following experimental conditions were adopted for the different nuclei: (1H) δ ) 3 ms, ∆ ) 150 ms, and τ ) 4 ms; (2H) δ ) 8 ms, ∆ ) 50 and 200 ms, and τ ) 20 ms; (13C) δ ) 8 ms, ∆ ) 200 ms, and τ ) 10 ms. In addition, 1H-decoupling was applied in 13C diffusion experiments. The error on the fitting was always less than 1%, while the reproducibility (as judged by repeated measurements) was estimated to be within (7%. All samples were prepared in duplicate, and all measurements were carried out in triplicate.

3. Results and Discussion 3.1. Phase Behavior. Before introducing the ternary systems, we recall relevant data on the surfactant/water systems. The well-known GMO/W system5 at 25 °C shows, with increasing water content, the formation of an isotropic (20) Bleasdale, T. A.; Tiddy, G. J. T. In Organized Solutions; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker: New York, 1992; Vol. 44, p 125 and references therein. (21) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288. (22) Stilbs, P.; Moseley, M. E.; Lindman, B. J. Magn. Reson. 1980, 40, 401.

Biocompatible Lipidic Formulations

L2 phase in the range 2-4 wt % of water, a LR LC region in the range 5-12 wt %, and bicontinuous cubic phases in the range 14-40 wt %. The effect of 15 wt % of DGMO added to GMO is known from a previous investigation on the GMO/DGMO/W phase diagram.12 With increasing water content, a LR LC region is found in the range 0-20 wt % of water, whereas the bicontinuous cubic phases exist in the range 20-48 wt %. Substantially, the addition of DGMO enlarges the LR LC phase and extends the existence of the cubic phases at higher water content. The addition of 5 wt % of LCT to GMO induces the formation of a LR LC phase in the range 0-14 wt % of water and of cubic phases in the range 16-34 wt % at 25 °C. Differently from DGMO, LCT shrinks the cubic phase region. The phase diagrams, at 25 °C, of the ternary GMO/ GTO/W system and of the pseudo-ternary GMO-DGMO/ GTO/W and GMO-LCT/GTO/W systems are reported in Figure 1a-c, respectively. Figure 2a-d reports some optical micrographs, at 25 °C, in polarized light of 1-weekold samples, which clearly identify the main anisotropic LC phases observed in the three systems. The systems examined display very similar phase behavior with only small differences in the phase boundaries of the microemulsion (L2) and emulsion regions. A narrow area of microemulsion forms along the binary surfactant/oil axis with a maximum uptake of about 8 wt % of water. This occurs for 35-40 wt % of oil. The microemulsion region extends to a maximum oil content of about 90 wt % for the GMO system, 85 wt % for the GMO-DGMO system, and 77 wt % for the GMO-LCT system. With increasing water content, the L2 phase becomes a two-phase region in which the microemulsion coexists with an undefined milky dispersion. Then, in the proximity of the surfactant corner of the phase diagrams, two liquid crystalline phases, namely, LR and H2 phases, coexist in a quite large two-phase region. This was initially ascertained by means of optical microscopy.18 As already remarked in the previous paper,18 where the emulsions stabilized by the two H2 and LR phases were characterized only through optical microscopy, the occurrence of monophasic liquid crystalline regions was not ascertained. Certainly the monophasic H2 and LR should exist, but only in a very limited range of compositions, since among all examined samples none was monophasic as demonstrated also by 2H NMR spectra (see Figure 3a). The biphasic region extends up to 20-28 wt % of water. Further increasing the water content, the systems show a different phase behavior, depending on the manner of sample preparation. Along water dilution lines of samples having surfactant/oil weight ratios in the range 90/1075/25, a stable emulsion region forms in all systems. These emulsions are stable for several months if stored at 5 °C. However, when left at room temperature, they develop molds after a few weeks, since no preservatives were added in the formulations. As the water content increases, the extension of this emulsion decreases. A maximum water uptake of 80 wt % is observed for the GMO system. The maximum water content of GMO-DGMO and GMO-LCT emulsions is 48% and 68 wt %, respectively. When samples are prepared starting from binary or pseudo-binary surfactant/W compositions, by dilution with oil, an aqueous phase separates from a creamy phase. Conversely, in the emulsion prepared by adding water, phase separation is observed only after long centrifugation. Figure 2a shows a typical sample in the two-phase region of the GMO system. Only the dominant hexagonal texture is clearly identified. Figure 2b shows the optical micro-

Langmuir, Vol. 20, No. 13, 2004 5243

Figure 1. Phase diagrams: (a) Schematic diagram of the ternary system GMO/GTO/W at 25 °C, determined by using the procedure described in the text, in which small amounts of water are added to GMO/GTO mixtures. L2: microemulsion region. H2 + LR: two-phase region, where a reverse hexagonal (H2) and a lamellar (LR) LC phase coexist. The boundaries of the emulsion area are determined within (2.5%. (b) Schematic diagram of the system GMO-DGMO (85/15)/GTO/W at 25 °C. Notations are as in part a. (c) Schematic of the system GMOLCT (95/5)/GTO/W at 25 °C. Notations are as in part a.

graph obtained for a sample of the GMO system in the emulsion region. There, besides a dominant hexagonal texture, many water droplets, with diameters in the range 1-10 µm, can be identified.

5244

Langmuir, Vol. 20, No. 13, 2004

Mele et al.

Figure 3. 2H NMR spectra. (a) GMO/GTO/2H2O in the LR + H2 LC region. (b) GMO/GTO/2H2O in the LR + H2 + water emulsion region.

Figure 2. Optical microscopy: (a) Typical sample, containing 25 wt % of W, in the two-phase LC region (H2 + LR) of the GMO/GTO/W system. (b) Sample containing 37 wt % of W in the emulsion region of the GMO/GTO/W system. (c) Sample containing 40 wt % of W in the emulsion region of the GMODGMO (85/15)/GTO/W system. (d) Sample containing 60 wt % of W in the emulsion region of the GMO-LCT (95/5)/GTO/W system.

In the case of GMO/DGMO, Figure 2c shows the emulsion sample containing 30 wt % of water. A birefringent texture typical of a hexagonal phase can be seen. A very different aspect is found for the emulsions formed by the GMO-LCT system. Figure 2d shows the texture obtained for a sample containing 30 wt % of water. The LC LR or H2 textures cannot be clearly identified while

many rather small water droplets, almost monodisperse in size, dominate the texture of the sample. 3.1.1. Considerations on the Phase Behavior. Concerning the phase diagrams and the occurrence of an emulsion region, it is worth noticing similarities and differences with the GMO/glycerol dioleate (GDO)/W system.23 The presence of the less hydrophobic GDO instead of GTO allows H2 LC phase to form approximately in the same range of composition, but further addition of water is not incorporated in the LC matrix. A phase separation (H2 and excess of water) occurs. On the other hand, the presence of GTO, even in small amounts, prevents the formation of a pure cubic or lamellar phase. Finally, an emulsion with bimodal distribution of the water droplets but without any LC phase was shown to coexist with an oily phase and an aqueous phase at the composition GMO/GDO/W ) 30/51/19. The important differences can be certainly ascribed either to the preparation procedure or to the weak amphiphilic character of GDO. Indeed, the possibility of locating some GDO molecules at the polar-apolar interface should favor a stable negative curvature at relatively low GDO content. In the case of GTO, due to the presence of mono- and diglycerides (see the Experimental Section), a small amount of LR phase, which is generally more fluid than the H2 phase, can coexist with the preferred H2 phase. This confers on the mixture the necessary fluidity to obtain a suitable dispersing medium for water droplets. 3.2. L2 Microemulsion Regions: 1H and 13C NMR Self-Diffusion Measurements. The microemulsion region occurs in all systems at low water content and for oil weight fractions greater than 0.4. The water weight (23) Borne´, G.; Nylander, T.; Khan, A. Langmuir 2000, 16, 10044.

Biocompatible Lipidic Formulations

Langmuir, Vol. 20, No. 13, 2004 5245

Table 1. 1H and 13C NMR Self-Diffusion Coefficients Measured in the L2 Regions at 25 °C along Oil Dilution Lines at the Molar Ratios [W]/[S] ∼ 2.6a D (×10-12) m2/s, 13C, GMO

D (×10-12) m2/s, 13C, GTO

4.6 ( 0.4 4.6 ( 0.4 3.8 ( 0.3 4.6 ( 0.4

9.5 ( 0.8 9.9 ( 0.8 8.7 ( 0.7 8.7 ( 0.7

8.1/43.9/7/41 7.3/41.2/6.5/45 6.1/32.9/6/55 4.5/26/4.5/65

S ) GMO-DGMO 9.0 ( 0.7 5.4 ( 0.4 7.8 ( 0.6 4.0 ( 0.3 8.8 ( 0.7 4.2 ( 0.3 9.8 ( 0.8 4.5 ( 0.4

10.4 ( 0.8 9.6 ( 0.8 9.1 ( 0.7 10.2 ( 0.8

3.2/48.8/7/41 2.5/46.1/6.4/45 2/37/6/55 1.5/29/4.5/65

S ) GMO-LCT 12.0 ( 1.0 4.2 ( 0.3 10.3 ( 0.8 4.3 ( 0.3 8.7 ( 0.7 4.7 ( 0.4 12.0 ( 1.0 4.4 ( 0.4

9.8 ( 0.8 9.8 ( 0.8 9.7 ( 0.8 8.8 ( 0.7

sample composition, S/W/GTO, wt % 52/7/41 48.5/6.5/45 40/5/55 31/4/65

a

D (×10-12) m2/s, 1H, W S ) GMO 10.4 ( 0.8 10.1 ( 0.8 12.0 ( 1.0 9.5 ( 0.8

Reproducibility of the experimental values is within (8 %.

fraction is always lower than 0.2. This means that the L2 phase can form for a maximum water/surfactant (total surfactant) molar ratio [W]/[S] ≈ 6. The diffusion experiments were performed for samples along the oil dilution line at a water/surfactant ratio [W]/[S] ≈ 2.6. Water self-diffusion coefficients, and GMO, GTO selfdiffusion coefficients, were measured through the 1H NMR and 13C NMR PGSTE techniques, respectively. No timedependent self-diffusion coefficients were observed. Data are reported in Table 1. NMR signals belonging to DGMO and LCT were not clearly identified because of their low intensities, compared to those of GMO and GTO molecular species. In the samples of the L2 regions, almost constant values of the self-diffusion coefficients of the three main components are observed. GMO always has values around DGMO ) 4.5 × 10-12 m2/s, GTO around DGTO ) 9.0 × 10-12 m2/s, and for water Dw ) 1.0 × 10-11 m2/s. These values demonstrate that at [W]/[S] ∼ 2.6 in the three systems a similar microstructure occurs independently of the oil content. The low values of the self-diffusion coefficients of GMO are characteristic of species localized mainly at the interface. Moreover, the fact that the value does not change with changing composition confirms the constancy of the interfacial curvature in the microemulsion region. As expected along oil dilution lines, at low water content, these GMO self-diffusion coefficients indicate the occurrence of nanodroplets of similar size and shape. The decrease of 2 orders of magnitude of the water selfdiffusion coefficients suggests the existence of water-inoil droplets, as typically observed for such a low [W]/[S] ratio. The GTO self-diffusion coefficient is halved as a result of the obstruction effect caused by water droplets. Clearly GTO is always the continuum medium. 3.3. Emulsion Regions: 2H NMR Quadrupolar Splitting and Self-Diffusion Measurements. The H2 + LR two-phase and emulsion regions were investigated through 2H NMR. The 2H NMR spectra of samples prepared with 2H2O showed the quadrupolar splittings typical of anisotropic phases. Figure 3a shows the results obtained for the sample having the composition GMO/ GTO/2H2O ) 72.25/12.75/15. Three quadrupolar splittings can clearly be detected (∆νq1 ) 750 Hz, ∆νq2 ) 1500 Hz, and ∆νq3 ) 2000 Hz) as expected from the coexistence of LR and H2 phases which show both 90° and 0° alignments.

Table 2. 2H NMR Self-Diffusion Coefficients for Samples in the Emulsion Regions Belonging to the Three Different Ternary Diagramsa D (×10-11) m2/s samples

∆ ) 50 s

∆ ) 200 s

GMO/GTO/D2O (30 wt %) GMO-DGMO/GTO/D2O (30 wt %) GMO-LCT/GTO/D2O (30 wt %) GMO-LCT/GTO/D2O (55 wt %)

5.5 4.5 3.5 5.9

1.8 1.4 1.4 4.6

a Samples were prepared using 2H O. Two different ∆ times are 2 shown.

The higher intensity of the peaks defining the smallest ∆νq1, which belongs to the H2 phase, indicates the prevalence of the latter LC phase, as already inferred qualitatively by optical microscopy.18 Samples prepared with GMO-DGMO and GMO-LCT surfactant mixtures and having similar compositions showed very similar patterns and almost the same 2H NMR ∆νq values. Further addition of 2H2O induces the formation of the water droplets within a creamy emulsion. However, the LC matrix is not altered as demonstrated by the occurrence again of three quadrupolar splittings along with an isotropic signal in the 2H NMR spectra. Figure 3b shows a sample in the emulsion region with composition GMO/ GTO/2H2O ) 59.5/10.5/30. The largest ∆νq3 ) 2000 Hz can be seen only in the inset of Figure 3b where the vertical scale was enlarged 15 times. The three splittings are very similar to those measured in the previous sample located inside the region of two LC phases. This demonstrates the invariability of the amount of water involved in the thermodynamically stable LC phases. For the emulsions obtained in the presence of DGMO and LCT, again, very similar 2H NMR spectra were recorded. An exception is the case of the LCT system, for which the largest ∆νq3 splitting due to the LR phase is not detected in the emulsion system. From these data, it can be concluded that the isotropic 2H NMR signal belongs mainly to the water inside the droplets. Indeed, there is not fast exchange on the NMR chemical shift time scale. This fact was then used to investigate the water droplet mobility through 2H NMR self-diffusion measurements. The apparent self-diffusion coefficients of water, measured at two different ∆ times, in some selected emulsion samples prepared with 2H2O, are reported in Table 2 along with sample composition. It is clear that restricted diffusion occurs, since time-dependent (∆ time in eq 3) diffusion coefficients are measured. The modeling of these data to obtain size distributions is in progress. In the present context, the focus is on the phase behavior. Here, the most remarkable result is the use of two thermodynamically stable LC phases, characterized by different viscosities (a small amount of relatively fluid LR LC phase and a more abundant and sticky H2 LC phase), to obtain a very effective dispersing medium. The present results agree with the findings of another recent work on the emulsions formed by the surfactant didodecyldimethylammonium bromide (DDAB) with water and the perfluorooctane (PFO) oil.24 In that case, almost 80 wt % of PFO was incorporated in the two lamellar phases of the DDAB/W system.25 Due to incompatibility of PFO with both water and hydrocarbon chains, the dispersion of oil droplets resulted stabilized by rings of swollen LR1 LC phase, whereas the compact LR2 DDAB/W LC phase played a significant viscosifying role of the (24) Mele, S.; Khan, A.; Monduzzi, M. J. Surfactants Deterg. 2002, 5, 381. (25) Caboi, F.; Monduzzi, M. Langmuir 1996, 12, 3548.

5246

Langmuir, Vol. 20, No. 13, 2004

medium. Emulsions characterized by stability longer than 2 years were obtained provided that PFO oil was added gradually. 4. Concluding Remarks In this work, the GMO/GTO/W, GMO-LCT/GTO/W, and GMO-DGMO/GTO/W systems have been investigated. The ternary and pseudo-ternary phase diagrams are all characterized by a narrow microemulsion region along with a large area where H2 and LR LC phases coexist and, finally, by an emulsion region. The microemulsion regions occur only at relatively high weight fraction of oil, and a limited water uptake occurs due to the geometrical constraints imposed by the surfactant effective packing parameter. The microstructure within the L2 phase does not change significantly along the oil dilution lines and, moreover, is not altered by the presence of LCT or DGMO. As for the emulsion regions, it is remarkable that all systems dispersed in the liquid crystalline medium show

Mele et al.

a long-term shelf life. The stability toward phase separation is rather high also in the systems characterized by a high polydispersity. This is due to the presence of the LC phases as dispersing medium. The use of thermodynamically stable LC phases, preferentially with lamellar or hexagonal microstructure, allows the dispersion of many types of liquids. Obviously, no chemical reactions or strong intermolecular interactions should occur. In this case, that is, the LC phase is not chemically modified by the added liquid, the problems of depletion and osmotic effects due to the addition of thickeners should be avoided. In addition, the typical degradation processes such as creaming, flocculation, and Ostwald ripening occur with limited probability. The method of preparation seems to play a crucial role. Acknowledgment. Italian MIUR, PRIN 40% and Legge 488 funds (Italy), and Consorzio Sistemi Grande Interfase (CSGI-Firenze) are acknowledged for support. LA049822Q