Microemulsions Based on Soybean Phosphatidylcholine and

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Langmuir 1998, 14, 6864-6870

Microemulsions Based on Soybean Phosphatidylcholine and IsopropylmyristatesEffect of Addition of Hydrophilic Surfactants Christian von Corswant,*,† Camilla Olsson,‡ and Olle So¨derman§ Astra Ha¨ ssle AB, S-431 83 Mo¨ lndal, Sweden, SIK, Box 5401, S-402 29 Go¨ teborg, Sweden, and Physical Chemistry 1, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received May 13, 1998. In Final Form: September 16, 1998 This work reports the effect of the addition of n-alkyl β-D-maltosides (CnG2), sucrose monododecanoate, and sodium taurocholate on the phase behavior and microstructure of a microemulsion based on water, 1-propanol, soybean phosphatidylcholine (SbPC), and isopropylmyristate (IPM). The self-diffusion coefficients of the components were determined with the pulsed field gradient NMR technique. The spontaneous curvature (H0) of the surfactant monolayer separating the aqueous and oil domains was found to increase with an increasing proportion of hydrophilic surfactant for all the hydrophilic surfactants studied. The microstructure of the microemulsion, deduced from the self-diffusion coefficients and the conductivity measurements, changed from an oil-continuous structure to oil-swollen micelles through a bicontinuous structure. The general phase behavior could be described with a simple model where H0 was assumed to vary linearly with the changing weight fraction of hydrophilic surfactant. The molar ratio of CnG2/SbPC needed to obtain a balanced microemulsion bal was found to increase linearly with n for n g 10. The bal for C12G2 was 0.71. There was no observed difference in bal between sucrose monododecanoate and C12G2, but sucrose monododecanoate was found to increase the flexibility of the surfactant film to a greater extent than C12G2. Sodium taurocholate increased H0 more effectively than C12G2 (bal ) 0.22) but did not destabilize the surfactant film to the same degree.

Introduction It is well-known that water and oil do not mix, but by adding a surface-active compound, it is possible to obtain a thermodynamically stable, single-phase isotropic solution of large amounts of both water and an oil, generally referred to as a microemulsion. These unique properties make microemulsions an interesting alternative as delivery systems for water-insoluble drugs in the pharmaceutical industry. On a macroscopic scale, a microemulsion is a homogeneous phase, but microscopically, it possesses a structure with water and oil domains separated by a surfactant monolayer. The properties of a microemulsion can be understood in terms of the flexible surface model, first introduced by Helfrich.1 In this model, the surfactant layer is treated as a geometrical surface which is described by two principal curvatures c1 and c2, and it is customary to expand the local curvature free energy density to the second order in the curvatures

gc ) 2κ(H - H0)2 + κjK

(1)

In eq 1, H ) 1/2(c1 + c2) is the mean curvature and K ) c1c2 is the Gaussian curvature. κ and κj describe the elastic properties of the surfactant film and are often referred to as the bending rigidity modulus and the saddle splay modulus, respectively. H0, the spontaneous curvature that an unconstrained film would adopt, is an important parameter in describing the microstructure of the microemulsion. H0 depends both on the nature of the * To whom correspondence should be addressed. E-mail: [email protected]. † Astra Ha ¨ ssle AB. ‡ SIK. § University of Lund. (1) Helfrich, W. Z. Naturforsch. 1973, 28c, 693-703.

surfactant and on the composition of the polar and nonpolar phases. H0 is defined as positive when the film curves around the oil. For H0 close to zero, bicontinuous microemulsions, lamellar phases, or sponge phases (L3) are often formed. Which one of these three is formed depends on the values of κ and κj. A low κ value, comparable to kBT, characterizes systems such as microemulsions where thermal fluctuations are dominant. However, κj may be positive or negative: a negative κj promotes spheres or ellipsoids, and a positive κj promotes saddle splay structures, as in bicontinuous structures. Furthermore, for microemulsions where H0 > 0, oil-swollen micelles or aggregates dispersed in a continuous water phase are often formed, whereas when H0 < 0, the reverse structure is formed, with water droplets in the oil. It is an absolute necessity in pharmaceutical applications to use components that are nontoxic, and it has recently been shown that it is possible to form microemulsions based on the surface-active compound soybean phosphatidylcholine (SbPC) and the nontoxic polar oil isopropylmyristate (IPM).2-4 It should be noted that, in the present study as well as in the previous work,4 1-propanol is used as a cosolvent to change the curvature and increase the flexibility of the surfactant film. 1-Propanol is not acceptable for a final pharmaceutical formulation, and these microemulsion systems should be regarded as model systems in order to study in more detail the behavior of systems based on IPM and SbPC with a minimum of components. In the previous study of the microemulsion system water/1-propanol/SbPC/IPM,4 it was observed that the microemulsion formed is of a water-in-oil (w/o) type (that is, the spontaneous curvature of the SbPC film is negative), (2) Aboofazeli, R.; Lawrence, M. J. Int. J. Pharm. 1993, 93, 161-75. (3) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1997, 13, 52495251. (4) von Corswant, C.; So¨derman, O. Langmuir 1998, 14, 3506-3511.

10.1021/la980567h CCC: $15.00 © 1998 American Chemical Society Published on Web 11/06/1998

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and it was not possible to tune the system to a balanced state or an oil-in-water (o/w) type system using only the 1-propanol concentration as the tuning parameter. In the present study, we have investigated if a microemulsion still forms when nontoxic hydrophilic surfactants are added to the water/1-propanol/SbPC/IPM system and if it is possible to tune the spontaneous curvature of the surfactant to positive values in order to match H0 with a desired R to minimize the total amount of surfactant in the composition.5 The effect on H0 is demonstrated by phase behavior studies, determination of the self-diffusion coefficients, and conductivity measurements for some of the systems studied. Four n-alkyl β-D-maltosides (CnG2) with an alkyl chain length from C8 to C14, one sucrose ester (sucrose monododecanoate) and a bile salt (sodium taurocholate) were used in the study. Experimental Section Materials. Soybean phosphatidylcholine (Epicuron 200) was obtained from Lucas Meyer Co., Germany. The chain length distribution of the fatty acids in Epicuron 200 is as reported in ref 6. C16:0 ) 13.3%, C18:0 ) 3%, C18:1 ) 10.2%, C18:2 ) 66.9%, and C18:3 ) 6.6%. 1-Propanol and isopropylmyristate (98% purity) were supplied by Aldrich (Steinheim, Germany), n-alkyl β-Dmaltosides was supplied by Calbiochem, sucrose monododecanoate (g99% monoester according to the manufacturer) was supplied by Fluka (Buchs, Switzerland), and sodium taurocholate was supplied by Sigma (St. Louis, MO). All reagents were used as received. The water used was first purified by reverse osmosis, further treated in a primary and secondary ion-exchange pack with a photo-oxidation step in between, and finally passed through an ultra-microfiltration unit (Elgastat maxima-HPLC, ELGA Ltd, U.K.). The quality of the water was checked by measuring the conductance and the surface tension. Phase Diagrams. All phase diagrams were constructed and analyzed as described previously7 at the temperature 25 °C. The different phases observed in all the phase diagrams presented will be denoted as follows: W, excess aqueous phase; Lc, a liquid crystalline phase; O, excess oil phase; Me, microemulsion phase characterized by low viscosity, optical isotropic behavior, and a translucent appearance. No distinction between an oil-in-water (o/w) microemulsion, a water-in-oil (w/o) microemulsion, or a bicontinuous type of microemulsion was made. The phase diagrams in Figures 1 and 3-6 can be considered as pictures of the relative volumes of the different phases that appear in each vial in a series of samples, in which the total volume in each vial is normalized to one. Pulsed Field Gradient-Nuclear Magnetic Resonance Measurements. The PFG-NMR technique8,9 was used to determine the different self-diffusion coefficients of the components at 25 ( 0.5 °C by monitoring the 1H signal on a Varian 400 MHz spectrometer equipped with a gradient dual broad band probe. The self-diffusion coefficients of the hydrophilic surfactants could not be measured, owing to the low concentration of the compounds and a lack of well-defined peaks in the 1H spectrum. A stimulated echo with a longitudinal eddy current delay (LED) pulse sequence10 was used to minimize j-modulation effects and phase errors resulting from eddy currents induced by the gradients. In each experiment, the length of the gradient pulse was kept constant (3.0 ms or 4.5 ms), and the gradient strength was varied between 3 and 50 G/cm in at least 26 steps. Different values for the time between the leading edges of the gradient pulses (∆) and the time between the first two radio frequency pulses (τ) were used, depending on the value of the self-diffusion coefficient measured. The range of ∆ was between (5) Shinoda, K.; Kunieda, H.; Arai, T.; Saijo, H. J. Phys. Chem. 1984, 88, 5126-9. (6) Shinoda, K.; Araki, M.; Sadaghiani, A.; Khan, A.; Lindman, B. J. Phys. Chem. 1991, 95, 989-93. (7) von Corswant, C.; Engstro¨m, S.; So¨derman, O. Langmuir 1997, 13, 5061-5070. (8) Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 176. (9) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1-45. (10) Gibbs, S. J.; Johnsson, C. S., Jr. J. Magn. Reson. 1991, 93, 391.

Figure 1. (a) Variation in volume fractions of the different phases formed in the water/1-propanol/C12G2/SbPC/IPM system as a function of δ (the weight fraction of the hydrophilic surfactant in the mixture of SbPC and hydrophilic surfactant). The 1-propanol concentration was kept constant at 8.0 wt %, γ ) 0.05, R ) 0.5, and the temperature ) 25.0 °C. (O) and (b) represent two different experiments and show the reproducibility of the method. (b) Self-diffusion coefficients for water (b), 1-propanol (3), SbPC (2), and IPM (O) determined from samples of the microemulsion phase of part a. (c) Relative selfdiffusion coefficients for water (b) and IPM (O) determined from the microemulsion phase of part a, and the obstruction factor Aκ (×) obtained from the conductivity measurements. 100 ms and 410 ms, and τ was varied between 5 ms and 10 ms. Some samples were analyzed with different ∆, and the diffusion coefficients obtained were found to be independent of ∆. The accuracy of the method was tested by performing five consecutive experiments on a sample of a water/1-propanol solution. The standard deviation of the self-diffusion coefficients obtained was less than 0.6%. The gradient strength was calibrated by measuring the self-diffusion coefficient of small amounts of HDO in D2O. The PFG-NMR measurements were made on the same samples used in the phase behavior study. The microemulsion phase was sucked out from the vials with a syringe and placed in a 5 mm NMR tube together with a sealed glass capillary or a 3.3 mm insert (Wilmad) filled with deuterated DMSO. The DMSO capillary or insert was used to produce a lock signal. The self-diffusion coefficients were calculated by fitting the StejskalTanner equation11 to the peak heights obtained using a nonlinear, least-squares procedure based on the Levenberg-Marquart algorithm. The 1-propanol self-diffusion coefficient was measured through the peak heights of the 1-methylene protons, the SbPC self(11) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288-292.

6866 Langmuir, Vol. 14, No. 24, 1998 diffusion coefficient was measured through the peak heights of the choline group, and the IPM self-diffusion coefficient was measured through the peak heights of the R-methylene protons from the myristate group. Dw was measured from the OH peak. Since there is a fast exchange of protons between water and the hydroxyl group of 1-propanol, both in the aqueous medium and in the oil medium, and the exchange of water and 1-propanol between the aqueous medium and the oil medium is also fast, as inferred from the appearance of the NMR spectra, all these species contribute to the observed self-diffusion coefficient obtained from the OH peak. However, Dw was calculated according to a two-site model as described in ref 6, neglecting the contribution of the water and 1-propanol dissolved in the oil medium. This simplification is valid for the water-rich and bicontinuous microemulsions. However, for the oil-rich microemulsions, the water and 1-propanol dissolved in the oil medium contribute significantly to the observed diffusion coefficient, as will be further discussed below. The D° values for water (Dw°) and IPM (Do°) are defined as the self-diffusion coefficients of water and IPM in the aqueous phase and oil phase obtained from the ternary system water/1-propanol/ IPM with the same total 1-propanol concentrations as those in the corresponding microemulsions. In the studied 1-propanol concentration range, Do° was found to vary between 2.28 and 2.42 × 10-10 m2/s, and Dw° varied between 13.4 and 15.8 × 10-10 m2/s. The validity of the assumption that Dw° and Do° of the aqueous phase and the oil phase from the ternary system are the same as those in the microemulsion was verified by measuring Do and Dw in the excess oil and aqueous phases from the three samples with 10, 11.5, and 13 wt % 1-propanol shown in Figure 3a. The values obtained from the ternary system were found to differ by less than 3% from the values obtained from the excess oil and aqueous phases, and this was considered to be acceptable. Water Content. Karl-Fisher titration was used to determine the water content. Conductivity Measurements. The conductivity was measured with a Philips PW 9526 digital conductivity meter equipped with a platinum electrode with the cell constant 0.86 cm-1. NaCl was added to the water to the concentration 0.6 wt % to ensure a well-defined ionic strength. This small amount of NaCl did not influence the phase behavior to any significant degree. The compositions used for the conductivity measurements were the same as those for the PFG-NMR study. Oil/Water Partitioning of 1-Propanol. Using calibration curves from samples of known concentrations, the partitioning constant Ko/w for 1-propanol was calculated from the integrals obtained from the 1H NMR spectra from the oil phase and aqueous phase of samples containing equal amounts of water and IPM by weight and varying concentrations of 1-propanol.

Results and Discussion The influence of the hydrophilic surfactants on the phase behavior of the water/1-propanol/SbPC/IPM microemulsion system was studied at a constant R equal to 0.5, where R is defined as the weight fraction of IPM/(IPM + H2O). 1-Propanol is regarded as a cosolvent, and the concentration is expressed as weight percent of the total mixture. Within the 1-propanol concentrations used in this study, it was found that R ) 0.5 corresponds to approximately equal volume fractions of the aqueous (H2O + 1-propanol) and oil (IPM + 1-propanol) phases. The total concentration of hydrophilic surfactant and SbPC, expressed as weight fraction of the total mixture and denoted by γ, was kept constant and equal to 0.05 in all the figures presented. δ denotes the weight fraction of the hydrophilic surfactant in the mixture of SbPC and hydrophilic surfactant. The self-diffusion coefficients of water, 1-propanol, SbPC, and IPM were determined by the PFG-NMR technique to reveal the microstructure of three selected systems (C12G2, C8G2, and sodium taurocholate). The interpretation of the diffusion data in terms of the microstructure of the microemulsion is generally straight-

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forward.12,13 For an o/w microemulsion, the relative selfdiffusion coefficient for water Dw/Dw° is close to unity and the relative self-diffusion coefficient for IPM Do/Do°, is much less than 1 and, for a w/o structure, the opposite is true. In the bicontinuous case, both Dw/Dw° and Do/Do° are high and H0 ) 0 at the point where Dw/Dw° ) Do/Do°. This state is frequently referred to as a balanced microemulsion with the surfactant molecules forming a monolayer film, often described in terms of a minimal surface,14 between the water and the oil domains. n-Alkyl β-D-Maltosides. The general phase behavior of the four CnG2 (n ) 8, 10, 12, 14) compounds studied is very similar. It is exemplified with the C12G2 system in Figure 1a, which shows the phase behavior at constant 1-propanol concentration and varying δ. A progressive change from a microemulsion in equilibrium with an excess aqueous phase to a microemulsion in equilibrium with an excess oil phase via a one-phase region is observed as δ is increased. The phase behavior indicates that the CnG2 compounds are incorporated in the SbPC film separating the oil and the aqueous domains and that the curvature of the surfactant film is changed from a negative value to a positive value. Initial studies of the CnG2 systems showed that, to prevent the formation of lamellar liquid crystalline phases, a minimum of 8 wt % 1-propanol (in total) was needed. Figure 1b and c presents the absolute and relative selfdiffusion coefficients for the C12G2 system of Figure 1a. At δ > 0.4, Dw/Dw° is close to unity, while Do/Do° is ∼0.1 and Do ) DSbPC. These observations are strong indications of a microstructure consisting of oil-swollen micelles in a continuous aqueous phase. The mean hydrodynamic radius (RH) of the oil-swollen micelles at δ ) 0.6 can be estimated by means of the Stokes-Einstein equation to be ∼50 Å. As δ is decreased, Dw/Dw° decreases while Do/Do° increases. The two curves intersect at δ ≈ 0.32 with a value of Dw/Dw° ) Do/Do° ≈ 0.55. Since the NMR selfdiffusion method monitors translation over macroscopic distances (of the order of several micrometers), this implies a microstructure which is truly bicontinuous and which cannot be rationalized in terms of any closed aggregates that undergo dynamic processes such as droplet fusion or interdroplet exchange. The maximum in DSbPC at the balanced state corresponds to the lateral diffusion of the molecules in the bicontinuous surfactant monolayer. Furthermore, a minimum in DSbPC and Do at around δ ≈ 0.4 is observed, indicating that the oil-swollen micelles are transformed via a state with large, presumably polydisperse, nonspherical oil aggregates to a system with a bicontinuous structure. At low values of δ, Do/Do° is close to unity while Dw/Dw° , 1. This indicates an oil-continuous microstructure with discrete water domains, although the observed value of Dw is considerably larger than DSbPC. This is inconsistent with ordinary reversed spherical micelles, for which Dw should be equal to DSbPC in analogy with the situation at high δ. The interpretation of the OH peak in the NMR spectrum is more complex in the oil-rich system, however, since the contribution of water and 1-propanol dissolved in the continuous oil phase cannot be neglected. The solubility of “free” water in the oil phase was determined to be 0.4 wt % (using the oil phase from the ternary system (12) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 344-63. (13) Lindman, B.; Shinoda, K.; Olsson, U.; Anderson, D. M.; Karlstro¨m, G.; Wennerstro¨m, H. Colloids Surf. 1989, 38, 205-24. (14) Scriven, L. E. Nature 1976, 263, 123-125.

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Table 1. δBal, EBal, cmc, and CBal/cmc for All the Studied Surfactants hydrophilic surfactant

[1-propanol] (wt %)

δbala (wt fraction)

balb (mol/mol)

cmcc (mM)

Cbal/cmcd (mM/mM)

C8G2 C10G2 C12G2 C14G2 sucrose monododecanoate sodium taurocholate

8.0 8.0 8.0 8.0 6.5 10

0.23 0.25 0.32 0.38 0.32 0.14

0.51 0.53 0.71 0.88 0.69 0.22

23.4 1.6 0.15 0.012 0.34 10

2.2 32 410 5800 200 1.8

a δ bal is defined as the weight fraction of hydrophilic surfactant/(hydrophilic surfactant + SbPC) which forms a balanced microemulsion. It is calculated as the midpoint of the δ range of the one-phase region in the phase diagrams. b bal is the molar ratio of hydrophilic surfactant/SbPC for the balanced microemulsion. c The cmc values: for C8G2, from ref 21; for C10G2, from the supplier; for C12G2, from ref 22; for C14G2 from ref 23; for sucrose monododecanoate, from ref 24; for sodium taurocholate, from ref 25. d Cbal is the estimated concentration of the hydrophilic surfactant in the aqueous phase, assuming that all hydrophilic surfactant is dissolved in the aqueous phase and that the aqueous phase consists of all the added water and two thirds of the 1-propanol.

water/1-propanol/IPM at 8 wt % 1-propanol), and the total concentration of water in the microemulsion corresponding to δ ) 0 was found to be 13.3 wt %. Assuming reversed micelles, Dw and D1-prOH from the aqueous phase trapped in the reverse micelles should be equal to DSbPC (neglecting the monomeric solubility of SbPC in IPM), which was determined to be 3.0 × 10-12 m2/s. Using the values 4.6 × 10-10 and 7.6 × 10-10 m2/s for the self-diffusion coefficients of 1-propanol and water, respectively, in the oil phase (obtained from measurements of the oil phase from the ternary system water/1-propanol/IPM at 8 wt % 1-propanol) and a Ko/w of 0.31, the resulting self-diffusion coefficient for the OH peak can be calculated to be 3.5 × 10-10 m2/s. This should be compared with the observed value of 4.7 × 10-10 m2/s. Thus, the observed difference between Dw and DSbPC at δ ) 0 can be explained to a large extent by the contribution of “free” water and dissolved 1-propanol in the IPM phase, and the microstructure of the w/o microemulsion is most probably of a reversed micellar type. The small difference between the calculated and the observed values can be attributed to uncertainties in the input parameters used in the calculation. The conductivity, k, of a microemulsion also reflects the microstructure of the solution.15 An o/w microemulsion has a conductivity in the same range as that for the aqueous phase, and the conductivity in a w/o microemulsion is typically 4-5 orders of magnitude lower. The conductivity of the microemulsion phase at δ ) 0 was determined to be less than 0.02 µS/cm. The conductivity of the corresponding aqueous phase was 6.2 mS/cm. The low conductivity at δ ) 0 rules out any type of bicontinuous structure and strongly indicates a microstructure of the reversed micellar type. Figure 1c compares the relative self-diffusion coefficient of water with the obstruction factor Ak obtained from the conductivity measurements, assuming that the conductivity can be described by the following equation:

k ) k0AkΦaq where k0 is the conductivity in the neat aqueous phase (water + 1-propanol) and Φaq is the volume fraction of the conducting phase in the microemulsion (estimated from the phase diagram). Ak and Dw/Dw° should be equal for o/w microemulsions and bicontinuous microemulsions16 (for w/o structures Ak cannot be calculated, since the continuous phase is nonconductive), and as can be seen, this is true to a high degree for δ > 0.2. This supports the conclusions drawn from the PFG-NMR measurements and (15) Jonstro¨mer, M.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293. (16) von Corswant, C.; Thore´n, P.; Engstro¨m, S. J. Pharm. Sci. 1998, 87, 200-208.

Figure 2. Molar ratio of CnG2/SbPC at the balanced state bal as a function of the alkyl chain length of CnG2. The open circle shows the calculated value for C8G2; see text for details. The solid line is described by eq 2 and was obtained by linear regression.

furthermore implies that the assumptions made for the calculation of Dw are correct. In terms of curvature, the absolute values and the variation of the self-diffusion coefficients with δ are consistent with a negative curvature at δ ) 0. The curvature continuously increases without any discrete jumps with increasing δ and is roughly equal to zero when Dw/Dw° ) Do/Do°. We denote the value of δ where H0 ) 0 as δbal, and for the C12G2 system, δbal ) 0.32. An equivalent study of the self-diffusion coefficients of the C8G2 system was also undertaken. However, if the proportion of C8G2 is expressed in terms of the reduced weight fraction δ/δbal, the variation in the self-diffusion coefficients is almost identical to the data presented in Figure 1 for the C12G2 system. The data for the C8G2 system are thus not shown. It is evident from Figure 1a and c that the balanced microemulsion corresponds to the one-phase region, and δbal for the other surfactants studied can be estimated as the midpoint of the δ range for the one-phase region, assuming that the volumes of the aqueous phase and the oil phase are equal. In Table 1, these values are presented as both weight fraction (δbal) and molar ratio (bal) for all the systems studied. bal appears to increase linearly with n for n g 10 (see Figure 2), and linear regression gives the following relationship between bal and n

bal ) 0.086n - 0.329

(2)

The deviation of C8G2 from the straight line is most certainly caused by a high monomeric solubility of C8G2 in the aqueous phase, since the critical micellar concentration (cmc) of C8G2 is in the same area as the actual concentration used (see Table 1) (Cbal/cmc). Using the cmc as the monomeric solubility of CnG2 in the aqueous phase, the true amount of C8G2 in the surfactant film can be estimated by subtracting the amount of C8G2 dissolved

6868 Langmuir, Vol. 14, No. 24, 1998

in the aqueous phase from the total amount of C8G2. This gives an bal of 0.28 (represented as an open circle in Figure 2), which is close to the predicted value of 0.34 obtained from eq 2. The observed dependence of n could probably be explained by the geometrical shape of the CnG2 molecules. Since the polar head group is the same, a molecule with a shorter hydrocarbon chain is more efficient in increasing the curvature. Figure 3a shows the phase behavior as a function of the 1-propanol concentration at constant δ ) δbal for C12G2. Below 7.5 wt % 1-propanol, a lamellar liquid crystalline phase appears, and above 8 wt % 1-propanol, the microemulsion phase shrinks with increasing 1-propanol concentration but remains symmetric around the hatched horizontal line at the relative volume fraction 0.5. Furthermore, Dw/Dw° decreases slightly, and Do/Do° is, within experimental error, independent of the 1-propanol concentration in the concentration range studied (see Figure 3b). This indicates that the curvature of the surfactant film remains almost constant with increasing 1-propanol concentration, and the bicontinuous structure is retained but has a decreased solubilization capacity. The small decrease observed in Dw/Dw° is most probably caused by an increased hydration effect as the water/ surfactant ratio is decreased. Assuming that all surfactant remains in the shrunk microemulsion phase the relative volume of the surfactant (Φs) in the microemulsion phase must increase with increasing 1-propanol concentration. Daicic et al. have derived an expression for the minimum surfactant volume needed to obtain a balanced one-phase microemulsion17

Φs )

von Corswant et al.

Figure 3. (a) Variation in volume fractions of the different phases formed in the system water/1-propanol/C12G2/SbPC/IPM as a function of 1-propanol concentration. δ ) 0.32, γ ) 0.05, R ) 0.5, and the temperature ) 25.0 °C. (b) Relative self-diffusion coefficients for water (b) and IPM (O) determined from the microemulsion phase of part a.

( ) -a0

1/2

2b0/l2

where a0 and b0 are expansion coefficients and l is a molecular length. a0 and b0 are, according to Daicic et al., functions of the bending moduli and, although the exact relation between a0 and b0 and the bending moduli is unknown, a decrease in the bending moduli (increased flexibility) increases a0 (which is negative) and decreases b0. Consequently, the observed increase in Φs in the microemulsion phase is consistent with an increased flexibility of the surfactant film. Thus, an increased 1-propanol concentration seems to increase the flexibility of the surfactant film but not affect H0. In general, the addition of 1-propanol has a more pronounced effect on H0.6 The observed independence of H0 with increasing 1-propanol concentration is unexpected and may be the result of several counteracting effects. Increased amounts of 1-propanol decrease the polarity of the aqueous phase and thus increase H06 but also increase the monomeric solubility of C12G2 in the aqueous phase, which would decrease δ and therefore also H0. A third possible effect is that the partitioning of 1-propanol between the aqueous phase and the oil phase changes with an increasing total concentration of 1-propanol, causing a slight increase in R. This effect was examined in the ternary system water/1-propanol/IPM, where Ko/w was indeed found to increase from 0.2 at low 1-propanol concentrations (,1 wt %) to 0.6 at 14.5 wt % 1-propanol. Sucrose Monododecanoate. The phase behavior of the system water/1-propanol/sucrose monododecanoate/ SbPC/IPM was very similar to that of the C12G2 system, but the 1-propanol concentration needed to destabilize (17) Daicic, J.; Olsson, U.; Wennerstro¨m, H. Langmuir 1995, 11, 2451-8.

Figure 4. Variation in volume fractions of the different phases formed in the water/1-propanol/sucrose monododecanoate/ SbPC/IPM system as a function of 1-propanol concentration. δ ) 0.34, γ ) 0.05, R ) 0.5, and the temperature ) 25.0 °C.

the lamellar crystalline phase was 6.5 wt % as compared to 7.5 wt % for the C12G2 system. At 8 wt % 1-propanol, the microemulsion phase had shrunk considerably (see Figure 4). The determined value of bal for sucrose monododecanoate at 6.5 wt % 1-propanol is very close to the value of C12G2 obtained at 8 wt % 1-propanol (see Table 1). This observation suggests that sucrose monododecanoate has the same impact on H0 as does C12G2 but increases the flexibility of the surfactant film more effectively. Sodium Taurocholate. The bile acid sodium taurocholate represents a different type of amphiphilic compound than CnG2. It has a negative charge, and the hydrophobic part is composed of a rigid steroid nucleus with three hydroxyl groups substituted in such a way that the steroid nucleus has one hydrophilic side and one hydrophobic side. Bile acids do not show a proper cmc but rather a gradual association of low cooperativity.18,19 It is also well-known that bile acids interact with phos-

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Figure 6. Phase behavior of all systems studied plotted as a function of the reduced weight fraction δ/δbal. R ) 0.5, and the temperature ) 25.0 °C. The 1-propanol concentration was 8.0 wt % for CnG2, 6.5 wt % for sucrose monododecanoate, and 10.0 wt % for sodium taurocholate. (b) C8G2, (O) C10G2, (9) C12G2, (0) C14G2, (4) sucrose monododecanoate, and (×) sodium taurocholate. The solid lines represent the calculated volume fraction of the microemulsion phase using the theory presented in the text.

Figure 5. (a) Variation in volume fractions of the different phases formed in the water/1-propanol/sodium taurocholate/ SbPC/IPM system as a function of 1-propanol concentration. δ ) 0.14, γ ) 0.05, R ) 0.5, and the temperature ) 25.0 °C. (b) Relative self-diffusion coefficients for water (b) and IPM (O) determined from the microemulsion phase of part a.

pholipid membranes to form mixed micelles.20 Sodium taurocholate was found to have a strong influence on the phase behavior of the water/1-propanol/SbPC/IPM system at constant 1-propanol concentration, and it is evident that sodium taurocholate is incorporated in the SbPC film and increases H0 (see Table 1). The estimated value of δbal is 0.14. For the sodium taurocholate system, a minimum of 10 wt % 1-propanol was needed to destabilize the lamellar crystalline phase, which is 2 wt % more than that for CnG2. This is probably an effect of the fact that the hydrophobic part of sodium taurocholate is more rigid. The sodium taurocholate system also shows a different phase behavior than the C12G2 system when δ is kept constant and the 1-propanol concentration is varied (see Figure 5a). With increasing 1-propanol concentration, only the aqueous phase is expelled and there is a more pronounced decrease in Dw/Dw° and increase in Do/Do°, which undoubtedly imply a decrease in H0 (see Figure 5b). One possible explanation for the decrease in H0 is an increased monomeric solubility of sodium taurocholate in the aqueous phase, owing to an increased cmc, which would lead to a decrease in δ. Peaks from taurocholate were present in the 1H spectra of the expelled aqueous phase from the samples with 13.0 and 14.5 wt % 1-propanol, but the total concentration was too low to allow a quantitative determination of the concentration of the bile acid. Dependence of H0 on δ. In this section, attempts are made to rationalize the phase behavior of the studied (18) Mukerjee, P.; Cardinal, J. R. J. Pharm. Sci. 1976, 65, 882-886. (19) Lindman, B. Hepatology (Baltimore) 1984, 4, 103S-109S. (20) Hofmann, A. F.; Mysels, K. J. Colloids Surf. 1988, 30, 145-173. (21) Mechref, Y.; Rassi, Z. E. Electrophoresis 1997, 18, 912-918. (22) Warr, G. G.; Drummond, C. J.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1986, 90, 4581-4586. (23) Landauer, P. Biochem. Biophys. Res. Commun. 1982, 106. (24) Herrington, T. M.; Sarabjit, S. S. Colloids Surf. 1986, 17, 103113. (25) Gallarate, M.; Pattarino, E.; Marengo, E.; Gasco, M. R. Sci. Tech. Pharm. 1993, 3, 413-418.

systems by plotting the volume fraction of the microemulsion phases as a function of the reduced weight fraction, that is, δ/δbal (see Figure 6). There is an obvious resemblance in phase behavior among all the surfactants studied, and it is suggested that this can be understood by considering the spontaneous curvature of the surfactant film. A Taylor expansion to first order around the balanced state gives

H0(δ) ≈ (δ - δbal)

( ) ∂H0 ∂δ

δbal

where

( ) ∂H0 ∂δ

δbal

)

-H0(δ ) 0) δbal

and H0(δ ) 0) is the curvature of the SbPC film without hydrophilic surfactant. The volume of the microemulsion phase is given by

Vme ) Vcp + Vs + Vd

(3)

where Vcp is the volume of the continuous phase, Vs is the volume of the surfactants, and Vd is the volume of the dispersed phase. Vcp and Vs are known, and Vd can be calculated from the relation

Vd )

A 1 1 A ≈ ) 3 H0(δ) 3 [H0(δ ) 0)/δbal](δ - δbal) 1 A (4) 3 H0(δ ) 0)(δ/δbal - 1)

where A is the total area of the surfactant film. If A is assumed to be constant, Vme can be calculated from eqs 3 and 4 as a function of δ/δbal. The result for a system in which R ) 0.5, γ ) 0.05, A ) 600 m2/g of surfactant and H0(δ ) 0) ) -0.02 Å-1 is shown as a solid line in Figure 6. For convenience, the densities of all the components were assumed to be equal. Although the dispersed phase in the proposed model is assumed to remain as spherical droplets even when the curvature approaches zero, which is obviously not the case, the close resemblance between the calculated curve and the actual phase behavior is striking, and it seems reasonable to conclude that the

6870 Langmuir, Vol. 14, No. 24, 1998

general appearance of the phase behavior can be explained to a large extent with a constant change in H0 as a function of δ/δbal. Conclusions The main conclusion of this work is that the n-alkyl β-D-maltosides, as well as sucrose monododecanoate and sodium taurocholate, are incorporated in the SbPC film separating the oil and water domains in the water/1propanol/SbPC/IPM microemulsion and may readily be used to tune the spontaneous curvature of the surfactant monolayer from a negative value at zero or low δ to positive values at higher δ. The increase in H0 with increased δ is supported by the phase behavior, the self-diffusion NMR study, and the conductivity measurements. The general appearance of the phase behavior in the investigated

von Corswant et al.

systems can be explained to a large extent by a constant change in H0 as a function of the reduced weight fraction δ/δbal, and the self-diffusion coefficients demonstrate a continuous change in microstructure from an oil-continuous structure with reversed micelles to oil-swollen micelles in an aqueous phase over a bicontinuous structure. From a practical point of view, it is thus possible to tune the spontaneous curvature of the microemulsions using the pharmaceutically acceptable compounds IPM and SbPC with nontoxic hydrophilic surfactants in order to match H0 with the desired R to minimize the total amount of surfactant in the composition. Acknowledgment. P. Thore´n is acknowledged for technical assistance with the phase diagrams. LA980567H