Properties of microemulsions based on mixed nonionic surfactants and mixed oils

Properties of microemulsions based on mixed nonionic surfactants and mixed oils

Journal of Molecular Liquids 150 (2009) 25–32 Contents lists available at ScienceDirect Journal of Molecular Liquids j o u r n a l h o m e p a g e :...

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Journal of Molecular Liquids 150 (2009) 25–32

Contents lists available at ScienceDirect

Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q

Properties of microemulsions based on mixed nonionic surfactants and mixed oils Monzer Fanun ⁎ Colloids and Surfaces Research Laboratory, Faculty of Science and Technology, Al-Quds University, P.O. Box 51000 East Jerusalem, Palestine

a r t i c l e

i n f o

Article history: Received 21 June 2009 Received in revised form 4 September 2009 Accepted 17 September 2009 Available online 25 September 2009 Keywords: Percolation threshold Diffusion coefficients Scattering intensity Periodicity Correlation length

a b s t r a c t The systems studied were water/sucrose laurate/ethoxylated mono-di-glyceride/isopropylmyristate/peppermint oil. The solubilization capacity of water in the oils is dependent on the surfactants and oils mixing ratios (w/w). The transport properties (electrical conductivity and dynamic viscosity) were studied as function of water volume fraction. Electric percolation phenomenon was observed in these systems and the water volume fraction percolation thresholds were determined. The diffusion properties investigated by nuclear magnetic resonance confirm a progressive transformation of the water-in-oil to bicontinuous and inversion to oil-in-water microemulsions that occur upon dilution with water. The diffusion coefficients of surfactants increase with the increase in the water volume fraction. The structural parameters studied by small angle X-ray scattering that include the periodicity and correlation length were estimated. The periodicity increases linearly with the increase in the water volume fraction whereas the correlation length increases with the increase in the water volume fraction to a certain value then decreases. Cryogenic transmission electron microscopy images for diluted microemulsions revealed the presence of spheroidal droplets of up to 10 nm diameter. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Microemulsions are optically isotropic and thermodynamically stable colloidal assemblies having polar and nonpolar micro domains. Microemulsions are scientifically seeing a renaissance and applications are rising rapidly [1]. In many of these applications, the oils are mixtures of various components, and it is interesting to know how oil mixtures, rather than single component oils, are solubilized in microemulsions. It has been shown that different types of oil molecules can be solubilized at diverse places in the microemulsions [1–11]. Weakly hydrophobic or polar molecules can be located in the palisade layer or close to the surfactant headgroup while strongly hydrophobic molecules (e.g., saturated alkanes and alicyclic hydrocarbons) are solubilized preferentially in the hydrophobic core. It has been also suggested [1–11] that hydrophobicity is not the only factor affecting the placement of a particular solute in the surfactant aggregate. Other factors such as molecular size and shape, the free energy associated with molecular conformational constraints experienced by the solute in different solubilization placements, the surface activity of the solubilizate have also been thought to influence the placement. Accordingly, if oil mixtures are solubilized, it is possible that the division of the individual oil components between the different solubilization locations is not the same. A two-state solubilization model was postulated [6–9] for polar solubilizates involving a distribution of solute molecules between the adsorbed state close to the micelle/water interface and the dissolved

⁎ Tel.: +970 22 79 97 53; fax: +970 22 79 69 60. E-mail addresses: [email protected], [email protected] 0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2009.09.008

state in the hydrocarbon core. Solubilization in the adsorbed state is supposed to take place owing to the surface-active behavior of the solute at the interface. Based on the two-state solubilization theory, we assume that the total solubilization can be divided into two contributions: the interfacial and the core contributions. The two oil components have different core/interfacial site distributions characteristic because of their different molecular structures. Consequently, one component becomes enriched in the interfacial location. In this study, we aim to investigate for the first time the effect of adding peppermint oil to isopropylmyristate on the formation and properties of biocompatible microemulsions based on the mixed sucrose laurate and ethoxylated mono-di-glyceride. These microemulsions are intended for the use in cosmetic and pharmaceutical applications.

2. Experimental 2.1. Materials The sucrose monolaurate (L1695) was obtained from MitsubishiKasei Food Corp., (Mie, Japan). The purity of combined Lauric acid equals 95%, the ester compositions are 80% monoester and 20% di, tri and polyester, and HLB equals 16. Ethoxylated mono-di-glyceride (EMDG) (MAZOL 80 MG KOSHER), ethoxylated mono-di-glyceride was obtained from BASF Corporation (Gurnee, Illinois, USA). Ethoxylated mono-di-glyceride is a nonionic surfactant composed of a mixture of stearate and palmitate partial esters of glycerin ethoxylated with approximately 20 mol of ethylene oxide per mole of alphamono-glyceride reaction mixture, and HLB equals 13.5. Peppermint

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M. Fanun / Journal of Molecular Liquids 150 (2009) 25–32

oil (MNT), (98%) and isopropylmyristate (98%) (IPM) were purchased from Sigma Chemicals Co. (St. Louis, USA). Sodium chloride (NaCl) of analytical grade was purchased from J.T. Baker Inc. (Phillipsburg, USA). All of the components were used as supplied without further purification. Triple distilled water was used.

Where I is the measured signal intensity, Io is the signal intensity for G = 0, γ is the gyro magnetic ratio for the 1H nucleus, δ is the gradient pulse length, Δ is the time between the two gradients in the pulse sequence (and hence defines the diffusion time). Typically, we use Δ = 100 ms, δ = 8 ms, and vary G from 1.7 to 32.3 G cm− 1 in 32 steps.

2.2. Methods 2.2.1. Pseudoternary phase diagrams at constant temperature The phase behavior of a system consisting of water, mixed oil, surfactant (or a mixture of surfactants) may be described on a phase tetrahedron whose apexes respectively represent the pure components. However, it is more convenient to describe the phase behavior on a pseudo-ternary phase triangles. Obviously, a fixed (weight, volume or mole) ratio must be chosen for any two of the components and one of the triangle vertices represents 100% of this binary mixture. Mixtures at fixed weight ratios of mixed oil, surfactant (or mixed surfactants) were prepared in culture tubes sealed with Viton lined screw caps. Water was then added dropwise until its solubilization limit was reached. Vigorous stirring followed all of the aqueous phase additions on a vortex mixer. The time for equilibration between additions of successive aliquots was typically, from a few minutes up to 24 h. Phase transitions were detected visually by the appearance of cloudiness or sharply defined separated phases. The completion of this process was hastened by centrifuging the samples. The phase diagrams were determined at 25 ± 0.1 °C. 2.2.2. Electrical conductivity measurements Conductivity measurements were performed at temperatures ±0.1 °C on samples the compositions of which lie along the onephase channel, using a conductimeter, the conductivity cell used is Tetra Con® 325, the electrode material is graphite and the cell constant is 0.475 cm− 1 ± 1.5%. The range of application is between 1 μS/cm to 2 S/cm with an accuracy of ±0.5%, and the temperature range is from −5 to 100 °C. In the case of nonionic microemulsions, a small amount of an aqueous electrolyte must be added for electrical conduction [12]. Thus, a 0.01 M sodium chloride aqueous solution was used in the preparation of the microemulsion samples in place of pure water. The electrode was dipped in the microemulsion sample until equilibrium was reached and reading becomes stable. Reproducibility was checked for certain samples and no significant differences were observed. The constant of the conductivity cell was calibrated using standard KCl solutions and checked a minimum of three times during the course of the working shift.

2.2.5. Small angle X-ray scattering (SAXS) Scattering experiments were performed using Ni-filtered CuKα × 6 rotating X-ray generator that operated at a power rating up to 1.2 kW. X-radiation was further monochromated and collimated by a single Franks mirror and a series of slits and height limits and measured by a linear position-sensitive detector. The sample was inserted into 1–1.5 mm quartz or lithium glass capillaries, which were then flamesealed. Each sample was checked before and after the experiment to verify that, no fluid had been lost during the time of exposure, approximately 3 h. The temperature was maintained at 25 ± 1 °C. The sample-to-detector distance was 0.46 m, and the scattering patterns were measured using the Lake procedure [15]. 2.2.6. X-ray data analysis In this case, the scattering patterns after background subtraction were fit to the expression due to Teubner and Strey [16]: 2

4

IðqÞ = ð1 = a2 + c1 q + c2 q Þ + b

ð2Þ

with the constants a2, c1, c2 and b obtained by using the Levenburg– Marquardt procedure [17]. Such a functional form is simple and convenient for the fitting of spectra. Eq. (3) corresponds to a real space correlation function of the form −r =

γðrÞ = ðsin kr = krÞe

ξ

ð3Þ

The correlation function describes a structure with periodicity d = (2π/k) damped as a function of correlation length ξ. This formalism also predicts the surface to volume ratio, but because this ratio is inversely related to the correlation length and therefore must go to zero for a perfectly ordered system, calculated values are frequently found to be too low [18]. d and ξ are related to the constants in Eq. (2) by [16]: 1=2

−1 = 2

2.2.3. Viscosity measurements Viscosity was measured using a rotational viscometer, model DV1PL spindle from Anton Paar GmbH (Graz, Austria). “Double cylinder” geometry was used. Viscosities at 200 s− 1 shear rate were obtained at 25 ± 0.1 °C. Reproducibility (triplicate) was checked for the samples and no significant differences (±SD) where observed.

d = 2π½ð1=2Þðða2 =c2 ÞÞ

2.2.4. Pulsed gradient spin echo-nuclear magnetic resonance (PGSE-NMR) NMR measurements was performed on Bruker DRX-400 spectrometer with a BGU II [13,14] gradient amplifier unit and a 5-mm BBI probe equipped with a z-gradient coil, providing a z-gradient strength (G) of up to 55 G cm− 1. The self-diffusion coefficients were determined using bipolar-pulsed field gradient stimulated spin echo (BPFG-SSE). In this work, we used bipolar gradient pulses as described by Wu et al. [13] to reduce the eddy-current effects. Experiments were carried out by varying the gradient strength and keeping all other timing parameters constant. The self-diffusion coefficient (D) is given by

2.2.7. Cryo-transmission electron microscopy (Cryo-TEM) Samples were prepared in a controlled environment vitrification chamber (CEVS) [19], at 25 °C and controlled humidity (>95% relative humidity), by placing a ~ 5 μl drop of the microemulsion on a holey polymer film supported on a TEM grid. The drop was blotted by a filter paper creating a thin film of the liquid over the grid, which was then immediately vitrified in liquid ethane at its freezing temperature. The grid was transferred under liquid nitrogen to a cold-stage (Model 626, Gatan, Inc., Warrendle, PA) which was introduced into the electron microscope JEOL 200FX or a Philips CM12, operated at 100 kV in the conventional TEM mode with a nominal under focus of about 4 μm. The working temperature was below − 168 °C, and the images were recorded on Kodak SO-163 film, developed for maximum electron speed.

    δ 2 2 2 I = Io exp γ G δ Δ− D 3

ð1Þ

1=2

ξ = ½ð1 =2Þðða2 =c2 ÞÞ

−ðc1 =4c2 Þ

−1 = 2

+ ðc1 =4c2 Þ

ð4Þ

ð5Þ

M. Fanun / Journal of Molecular Liquids 150 (2009) 25–32

27

3. Results and discussion 3.1. Phase behavior Fig. 1 presents the pseudo-ternary phase diagrams of the water/ sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate systems studied at 25 °C. The mixing ratio (w/w) of sucrose laurate/ethoxylated mono-di-glyceride equals unity while that of peppermint oil/isopropylmyristate was varied from one half to one. The phase behavior indicate the presence of an isotropic and lowviscosity area that is a microemulsion one-phase region (1ϕ), the remainder of the phase diagram represents a multi phase region composed of water continuous micellar solution with excess oil designated by (Wm + O). Fig. 2 presents the pseudo-ternary phase diagrams of the water/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate systems studied at 25 °C. The mixing ratios (w/w) of sucrose laurate/ethoxylated mono-di-glyceride varied from one third to three while that of peppermint oil/ isopropylmyristate equals unity. From the study of the phase diagrams where some of them are presented in Fig. 1, it was evidently superfluous to use higher peppermint oil/isopropylmyristate weight ratios in order to get the same degree of water solubilization. It is considered that surfactant monolayers at the interface of water and mixed peppermint oil and isopropylmyristate domains inside the microemulsions are directly related to the solubilization of water and oils. In these systems where ethoxylated mono-di-glyceride was added to sucrose laurate at different mixing ratios, the total area of the

Fig. 2. Pseudo-ternary phase diagram of the water/sucrose laurate/ethoxylated monodi-glyceride/ peppermint oil/isopropylmyristate system. The mixing ratios (w/w) of sucrose laurate/ethoxylated mono-di-glyceride varied from [A] one third [B] to three. The mixing ratio (w/w) of peppermint oil /isopropylmyristate equals unity. The onephase region is designated by 1ϕ, and the multiple phase regions are designated by (Wm + O).

Fig. 1. Pseudo-ternary phase diagram of the water/sucrose laurate/ethoxylated monodi-glyceride/peppermint oil/isopropylmyristate system. The mixing ratio (w/w) of sucrose laurate/ethoxylated mono-di-glyceride equal unity. The mixing ratio (w/w) of peppermint oil/isopropylmyristate equals [A] one half and [B] unity. The one-phase region is designated by 1ϕ, and the multiple phase regions are designated by (Wm+O).

microemulsion phase region reaches a maximum value for equal amounts of ethoxylated mono-di-glyceride and sucrose laurate (i.e. ethoxylated mono-di-glyceride content = 50 wt.%) in the surfactants mixture. This result is explained according to the Huibers and Shah [20] model. This model demonstrated that the maximum water solubilization achieved using a mixture of nonionic surfactants could be due to two different synergism mechanisms. The first, the maximum water solubilization capacity could be related to simple additive contributions of the surfactant material in the HLB 9–13 region with no apparent additional benefit from synergism between the two surfactant. Secondly, synergistic effect must cause the majority of the two surfactants to preferentially partition at the interface that allows larger interfacial area and thus high levels of solubilization. When the microemulsions were composed of equal amounts of surfactants, the synergistic effect was found to be more pronounced. This important change in the water solubilization is achieved at the ethoxylated mono-di-glyceride to sucrose laurate molar ratio of 1 to 3. Shah [21] reported on the effect of the molecular ratio of mixed surfactants on the maximum water solubilization in microemulsions. He found that maximal water solubilization in microemulsions occurs when molecular ratio of mixed surfactants equals 1 to 3. In the 1 to 3 molecular associations, the component with the smaller area per molecule (in our case, the sucrose laurate) generally forms the larger fraction (0.75) in the mixed surfactant

28

M. Fanun / Journal of Molecular Liquids 150 (2009) 25–32

system. The molecular association depends upon the molecular areas and hence, on the inter-molecular spacing and, in turn, on geometrical factors or arrangements of surfactant molecules at interface. It is known that the hexagonal arrangement of molecules can provide the closest packing possible in two dimensions. This hexagonal packing would yield the minimal area per molecule due to close packing. The regular arrangement of molecules would require sufficient intermolecular interaction which is possible if the corners of hexagons are occupied by molecules having a smaller area per molecule (sucrose laurate) than those occupying the centers of hexagon (ethoxylated mono-di-glyceride). It is clear that the surfactants partitioning at the interface is the synergism mechanism that most probably allows larger interfacial area and thus high levels of water solubilization in this system. It seems that for the 1 to 3 mixed surfactants molar ratio, the spontaneous curvature and the bending elasticity modulus of the mixed surfactants film have values which results in this maximum water solubilization. The monodisperse solubilities of sucrose laurate and ethoxylated mono-di-glyceride in the mixed oils are very small [22]. This means that the mixed oil domains in the microemulsion phase are almost the same as bulk mixed oil phase when solubilization is large. It can be assumed that the monomeric solubilities of sucrose laurate (SL1695) and that of ethoxylated mono-di-glyceride (SEMDG) in the water phase forming the microemulsions are similar to their respective critical micelle concentrations (CMC) which equal to 3.4*10− 4 and 1.1*10− 5 M for sucrose laurate and ethoxylated monodi-glyceride, respectively. Since surfactant molecules at the water– mixed oil interface inside microemulsions are directly related to the solubilization, it is important to estimate the mixing fraction of each surfactant. The surfactant content at interface could be obtained by simple mass balance equations as follows: CEMDG = X

min

ð1−X min ÞSEMDG 2ð1−SL1695 −SEMDG Þ

min

XEMDG −

min

ð1−XEMDG Þ−

ð6Þ

and CL1695 = X

min

ð1−X min ÞSL1695 2ð1−SL1695 −SEMDG Þ

ð7Þ

where CL1695 and CEMDG indicate the weight of sucrose laurate and ethoxylated mono-di-glyceride at the water–mixed oils interface, Xmin is the minimum weight fraction of mixed surfactants capable of solubilizing equal amounts of water and mixed oils in the micromin emulsions, XEMDG is the minimum weight fraction of the lipophilic surfactant (in our case the ethoxylated mono-di-glyceride) corresponding to Xmin. CL1695 + CEMDG is the weight fraction of total surfactants in surfactants monolayer at the water–mixed oil interface inside the mixed surfactants microemulsion system and is directly related to the net maximum solubilizing power of the mixed surfactants. The value of CL1695 + CEMDG is much smaller than the CL1695 and CEMDG in single surfactant based system, which indicates that the mutual solubilization of water and mixed oils increases due to the mixing of surfactants. At equal amounts of water and mixed oils the values of CEMDG/(CL1695 + CEMDG) depend on the surfactants mixing Table 1 Mixing fractions of ethoxylated mono-di-glyceride at the water–mixed oils interfaces in the water/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate systems at 25 °C for different mixing ratios (w/w) of peppermint oil/ isopropylmyristate and for ethoxylated mono-di-glyceride/sucrose laurate mixing ratio (w/w) equals unity. MNT /IPM mixing ratio (w/w)

CEMDG/(CL1695 + CEMDG)

S SEMDG

1/2 1/1 2/1 3/1

0.150 0.240 0.243 0.243

0.148 0.240 0.243 0.243

ratio. Table 1 shows the values of the mixing fractions of ethoxylated mono-di-glyceride surfactant at the water–mixed oil interface. From Table 1 we can see that values of CEMDG/(CL1695 + CEMDG) increase with the increase of the peppermint oil/isopropylmyristate mixing ratio (w/w) from one half to one then stabilizes. From a molecular point of view, the ascending values of CEMDG/(CL1695 + CEMDG) reflect the gradual penetration of the peppermint oil molecules to the interface, thereby facilitating water solubilization within the microemulsion [23]. This indicates that increasing the peppermint oil content in the mixed oil has no effect on the maximum water solubilization since it is governed by the amount of surfactants at the interface. Another way to determine the surfactants content at the interface of water–mixed oils in the microemulsion systems is by calculating the mixing weight fraction of the surfactant at the interface using the equation: S

XEMDG = SEMDG +

  SEMDG SSL1695 −SL1695 SSEMDG 1 −1 Row X 1−SL1695 −SEMDG

ð8Þ

Where XEMDG represents the weight fraction of ethoxylated monodi-glyceride (the lipophilic surfactant) in the total mixed surfactants, S S SL1695 and SEMDG represents the mixing weight fraction at the water– mixed oil interface of sucrose laurate and ethoxylated mono-diglyceride respectively, Row is the weight fraction of mixed oils in water + mixed oils, and X is the weight fraction of mixed surfactants in the microemulsions. In order to do the calculations we estimate that the water and mixed oils are pure and do not dissolve in each other. By plotting XEMDG versus X1 −1 a straight line is obtained. S SEMDG is the intercept and should be equal to the value of CEMDG/ (CL1695 + CEMDG). The obtained values of SSEMDG are in good agreement with the values obtained for CEMDG/(CL1695 + CEMDG) at equal amounts of water and mixed oils in the microemulsions as shown in Table 1. Again we used Eqs. (6) to (8) to estimate the CEMDG/(CL1695 + CEMDG) and the SSEMDG for these systems with varying ethoxylated mono-diglyceride content in the mixed surfactants. The values of CEMDG/ S (CL1695 + CEMDG) and the SEMDG are presented in Table 2. From Table 2 one can see that the values of CEMDG/(CL1695 + CEMDG) increase with the increase of the sucrose laurate/ethoxylated mono-di-glyceride mixing ratio (w/w). In other words, it is assumed that the sucrose laurate molecules are present only at the surfactant layers inside the microemulsion phase. The ethoxylated mono-di-glyceride molecules are distributed between the micro-water domains and the interface inside the microemulsion phase in a one-phase microemulsion. Kuneida et al. [24–29] reported on similar results obtained with mixtures of sucrose monolaurate and polyethylene glycol alkyl ether systems in the presence of heptane, decane and hexadecane oils. The solubilization capability increases with mixing of surfactants in particular when surfactants with different hydrophilic–lipophilic balances are mixed [20]. The monomeric solubility of lipophilic surfactant (ethoxylated mono-di-glyceride) in mixed oils is low and its mixing with sucrose laurate enables us to obtain large solubilization capacity of water and mixed oils. Isopropylmyristate was widely used in the formulation of biocompatible microemulsions for biological applications [30–36]. The formation of a pharmaceutically useful microemulsion based on mixed nonionic biocompatible surfactants Table 2 Mixing fractions of ethoxylated mono-di-glyceride at the water-mixed oils interfaces in the water/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate systems at 25 °C for different mixing ratios (w/w) of ethoxylated mono-dihglyceride/sucrose and for peppermint oil/isopropylmyristate laurate mixing ratio (w/w) equals unity. L1695/EMDG mixing ratio (w/w)

CEMDG/(CL1695 + CEMDG)

S SEMDG

3/1 1/1 1/3

0.090 0.240 0.360

0.092 0.240 0.350

M. Fanun / Journal of Molecular Liquids 150 (2009) 25–32

and isopropylmyristate required the addition of cosurfactants and cosolvents [37–44]. In this study we show that it is possible to formulate microemulsions using mixed nonionic surfactants and isopropylmyristate without the addition of alcohols or glycols as cosurfactants or cosolvents but by the addition of peppermint oil. It is worth to mention here that peppermint oil contains a significant amount of menthol and menthone. These short chain alcohols behave much as 1-pentanol but are safer for the use in cosmetic or pharmaceutical applications [45,46]. 3.2. Transport properties 3.2.1. Electrical conductivity Fig. 3 presents the semi log plot of the variation of electrical conductivity of the water + sodium chloride/sucrose laurate/ ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate microemulsions as function of water volume fractions along the N60 dilution line. The mixing ratios (w/w) of sucrose laurate/ethoxylated mono-diglyceride and that of peppermint oil/isopropylmyristate equal unity. The concentration of sodium chloride in water equals 0.01 M. It should be mentioned here that the addition of this small amount of electrolyte to the water used in the formulation of the microemulsions has no effect on the area of the one-phase microemulsions region. The electrical conductivity increases exponentially with the increase in the water volume fraction. According to the percolation model, the conductivity remains low up to a certain volume fraction (ϕc) of water at constant temperature. It must be emphasized that these conducting water-in-oil droplets, below ϕc are isolated from each other embedded in nonconducting continuum oil phase and hence contribute very little to the conductance. However, as the volume fraction of water reaches the percolation threshold ϕc, some of these conductive droplets begin to contact each other and form clusters which are sufficiently close to each other. The number of such clusters increases very rapidly above the percolation threshold ϕc, giving rise to the observed changes of the electrical conductivity. The model introduced by Safran et al. [47], which is based on the dynamical picture of percolation, has been utilized to analyze the conductivity results of the systems. According to the theory −s

σ = Aðϕc −ϕÞ

t

if ϕ < ϕc

σ = Bðϕ−ϕc Þ if ϕ > ϕc

ð9Þ ð10Þ

29

regime. Thus, s < 1 identifies a “static percolation” regime, and s > 1 identifies a “dynamic percolation” regime. We have determined ϕc, s and t and the prefactors A, B by numerical analysis with adjustment by the least squares method using simultaneously Eqs. (9) and (10). The computed ϕc, s and t values are equal to 0.16, 0.56 and 1.05; respectively. These values indicate that the percolation process is static. This attributes to the formation of continuous oil and water structures showing the presence of bicontinuous microemulsions. The static percolation is related to the appearance of bicontinuous microemulsions, where a sharp increase in conductivity can be justified by a connected water path in the system. The conductivity transition is mainly caused due to the formation of a continuous connected disperse phase in the system. There is a reasonable agreement between calculated (by Eqs. (9) and (10)) and experimental values within prescribed range of composition with a mean deviation of 4%. The above equations are valid only near ϕc and cannot be extrapolated to infinite dilution and unit concentration. In addition, these are not applicable at the immediate vicinity of ϕc, where there is a continuous variation within a narrow interval around the percolation threshold. 3.2.2. Dynamic viscosity Fig. 4 presents the variation of the dynamic viscosity as function of water volume fraction on the water + sodium chloride/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate systems along the N60 dilution line at 25 °C. The concentration of sodium chloride in water equals 0.01 M. The viscosity measured in the presence of sodium chloride in order to have the same microemulsions measured by electrical conductivity. Newtonian behavior of the fluids was assumed since the systems were found to be low viscous under the studied conditions. The viscosity curve is bell-shaped. The increase in the viscosity in mixed oil-rich microemulsions is imitative from an increase in the dispersed droplet sizes and the enhanced attractive interactions between the droplets [48–50]. Yaghmur et al. [51] studied the dynamic viscosity of the microemulsions composed of R(+)−limonene, water, propylene glycol, ethanol, and polyoxyethylene sorbitan monostearate (Tween 60) and Fletcher et al. [52] studied the n-heptane/AOT/glycerol system. Both observed strong attractive interactions between the droplets, which led to the formation of clusters. It is well documented that increasing of volume fraction of the dispersed phase in microemulsion brings to increase in dynamic viscosity [53–55], and

The critical exponent t generally ranges between 1 and 2, whereas the exponent s allows assignment of the time-dependent percolation

Fig. 3. Semi log plot of electrical conductivity (log σ as function of water volume fraction) of the water + sodium chloride/sucrose laurate/ethoxylated mono-di-glyceride/ peppermint oil + isopropylmyristate system along the dilution line N60 at 25 °C. The mixing ratios (w/w) of peppermint oil/isopropylmyristate and that of sucrose laurate/ethoxylated mono-di-glyceride equal unity. The concentration of NaCl in water is 0.01 M. The phase diagram is presented in Fig. 1B.

Fig. 4. Plot of dynamic viscosity as function of water volume fraction of the water + sodium chloride/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil + isopropylmyristate system along the dilution line N60 at 25 °C. The mixing ratios (w/w) of peppermint oil/isopropylmyristate and that of sucrose laurate/ethoxylated mono-di-glyceride equal unity. The concentration of NaCl in water is 0.01 M. The phase diagram is presented in Fig. 1B.

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it would be expected that the dependence of dynamic viscosity on water volume fraction reflects structural transitions in the microemulsion system. The increase in dynamic viscosity for water contents below 0.20 water volume fractions indicates attractive interaction and aggregation of droplets of water phase including molecular reorganization on the interface where the water-in-oil microemulsions are present. The drastic increase in dynamic viscosity for water volume fractions between 0.25 and 0.4 indicates a structural transition from water-in-oil droplets to bicontinuous structure. The sharp decrease in dynamic viscosity for water contents above 0.5 water volume fractions indicates that the water which is the least viscous component of the microemulsion system becomes the outer phase and oilin-water microemulsions are formed. The reduction in the viscosity with the increase in water content seems to be attributed to the fact that the mixed surfactants move from the bulk to the interface to cover the water in the oil droplets. High water volume fractions cause a reduction in the inter-droplet interactions and reduction in the viscosity. It seems that the decrease in viscosity with the increase in water volume fraction should be correlated to compositional and structural effects derived from the interfacial packing. This phenomenon can also be explained in terms of a decrease in the hydrophobic interaction of the surfactant tails. The relatively low-viscosity values indicate that the microemulsions formulated are composed of individual spherical droplets or bicontinuous structures and no anisometric aggregates are present [53–55]. A number of authors [56–60] also reported on the effect of adding ethoxylated surfactants to the sucrose mono fatty acid ester micelles in water. The shape of the micelles change from spherical or very short rod micelles to long wormlike micelles due to the reduction of the average section area of each surfactant molecule at the interface. This change in the micelles shape causes the viscosity to increase. When mixed oils are solubilized in the micellar aggregate, they induce a change in the micellar shape depending on the oil type and structure, which is expected to be reflected in the change in viscosity. 3.3. Diffusion parameters Diffusion data obtained by nuclear magnetic resonance could be evaluated in terms of microstructure, this is done by the calculation of the relative diffusion coefficient, D/D0, of both oils and water [61–65]. Relative diffusion coefficients were obtained by dividing water (DWater) and oil (DOil) diffusion coefficients in the microemulsion by the diffusion coefficient of pure water (DWater ) and oil in the neat 0 phase (DOil 0 ). It is well documented [61–65] that if the D/D0 values of water and oil differ by more than 1 order of magnitude, discrete particles of the slowly diffusing solvent are implied, whereas if the D/ D0 values of water and oil are of the same order of magnitude, a bicontinuous structure is suggested. Fig. 5 shows the relative diffusion coefficients of water and both peppermint oil and isopropylmyristate in water/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil + isopropylmyristate microemulsion system as a function of the water volume fraction along the dilution line N60 at 25 ºC. The mixing ratios (w/w) of sucrose laurate/ethoxylated mono-di-glyceride and that of peppermint oil/isopropylmyristate equal unity. It is shown that the general diffusion coefficient behaviors of the microemulsion ingredients (peppermint oil, isopropylmyristate and water) at the two extremes of the water volume fractions (i.e., below 0.2 and above 0.70), are easily explained. As Fig. 5 indicates, microemulsions containing below 0.20 water volume fraction have a discrete waterin-oil microstructure, since the relative diffusion coefficients of water and both oils differ by more than 1 order of magnitude. Microemulsions containing 0.20–0.70 water volume fraction have a bicontinuous microstructure, as the diffusion coefficients of water and peppermint oil are of the same order of magnitude. Increasing the aqueous-phase concentration to above 0.70 water volume fraction induces the formation of discrete oil-in-water microstructure. The observed diffusion

Fig. 5. Relative diffusion coefficients of water (□), peppermint oil (Δ) and isopropylmyristate (○) for samples whose compositions lie along the N60 dilution line on the water + sodium chloride/sucrose laurate/ethoxylated mono-di-glyceride/ peppermint oil + isopropylmyristate system. The mixing ratios (w/w) of peppermint oil/isopropylmyristate and that of sucrose laurate/ethoxylated mono-di-glyceride equal unity. The concentration of NaCl in water is 0.01 M. The lines are presented as guides to the eye.

coefficients of sucrose laurate and ethoxylated mono-di-glyceride and are plotted in Fig. 6. It is shown that both surfactants diffusion coefficients increase with the increase in the water volume fraction indicating increasing mobility. The high molecular volume of ethoxylated mono-di-glyceride affects its mobility, which is lower than that of sucrose laurate. 3.4. Microstructure parameters We used the small angle X-ray scattering technique to investigate the microstructure of water+ sodium chloride/sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate microemulsion systems as a function of the water volume fraction (ϕ) along the dilution line N60 at 25 ºC. The mixing ratios (w/w) of ethoxylated mono-di-glyceride/sucrose laurate were varied from one third to three while mixing ratios (w/w) of peppermint oil/isopropylmyristate were varied from one half to unity (see phase diagrams in Figs. 1 and 2). The scattering profiles of all the samples exhibit a single intensity maximum at q ≠ 0, followed by a high-angle tail. With increasing water volume fraction, the position of the maximum moves to a lower angle. By fitting

Fig. 6. Diffusion coefficients of sucrose laurate (Δ), and ethoxylated mono-di-glyceride (○) for samples whose compositions lie along the N60 dilution line on the water + sodium chloride/sucrose laurate/ethoxylated mono-di-glyceride/ peppermint oil + isopropylmyristate system. The mixing ratios (w/w) of peppermint oil/isopropylmyristate and that of sucrose laurate/ethoxylated mono-di-glyceride equal unity. The concentration of NaCl in water is 0.01 M. The lines are presented as guides to the eye.

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all the scattering curves to the Teubner–Strey equation [16] (Eq. (2)) we were able to derive from values of the periodicity, d, and the correlation length, ξ, as described in the experimental section (Eqs. (4) and (5)). The dependence of the d parameter on the volume fraction of water, ϕ for the systems with variable mixing ratios (w/w) of sucrose laurate/ ethoxylated mono-di-glyceride and constant mixing ratio (w/w) of peppermint oil/isopropylmyristate equals to unity was plotted in Fig. 7. In Fig. 7, we find that the periodicity, d, increases linearly with the increase in the water volume fraction. Plots of d vs. ϕ can probe the dimensionality of swelling along the dilution lines. The linear relationship between the periodicity, d, and the water volume fraction indicates mono dimensional swelling of the microemulsions droplets. The values of d depend also on the mixing ratio (w/w) of sucrose laurate/ethoxylated mono-di-glyceride. Decreasing the ethoxylated mono-di-glyceride content in the mixed surfactants induces an increase in the d value. This behavior of d value as function of the ethoxylated mono-di-glyceride content is due to the high molecular volume of ethoxylated mono-di-glyceride compared to sucrose laurate. Eq. (4) was used to determine the values of the correlation length, ξ for the investigated systems. Fig. 8 presents the variation of the correlation length, ξ, as function of water volume fraction (ϕ). Initially, the growth of ξ parallels that of d, with the former being smaller than the latter. In these systems, ξ reaches a maximum at ϕ equals 0.21 then stabilizes over the whole range of water dilution. The values of d and ξ observed in this study approach the values of d and ξ reported in our previous studies of the microemulsion systems prepared using the single oils [7,23]. The behavior of the correlation length, ξ, as function of water volume fraction ϕ can be explained as follows: when the water is the dispersed phase, increasing the water volume fraction increases the size of the scattering units and the correlation length, ξ, whereas when water is in the bulk, increasing the water volume fraction dilutes the scattering units and ξ stabilizes. Again, the values of the correlation length, ξ, increases with the increase of the ethoxylated mono-diglyceride content indicating that ethoxylated mono-di-glyceride makes the systems more ordered. When the peppermint oil/isopropylmyristate mixing ratio (w/w) was reduced to one half for the sucrose laurate/ethoxylated mono-di-glyceride mixing ratio (w/w) equals unity the of d values smaller while ξ values are greater than those observed for the peppermint oil/isopropylmyristate mixing ratio (w/w) equals unity as shown in Fig. 9. The behavior of d value indicates that the peppermint oil due to its penetration at the palisade layer affects the dimension and the order of the aggregates.

Cryogenic transmission electron microscopy (Cryo-TEM) provides an important measure of the dimension of the microemulsion system. The water/sucrose laurate/ethoxylated mono-di-glyceride/ peppermint oil/isopropylmyristate microemulsion samples were imaged by the cryogenic transmission electron microscopy (Cryo-TEM). For low water contents, beam damage occurs for concentrated microemulsions and clear images were difficult to obtain [19,66,67]. The micrographs of microemulsions samples at 0.9 water volume fraction show spheroidal swollen micelles as shown in Fig. 10. In these preparations, suspended particles are pushed to the edge of the holes of the holey carbon film, giving rise to particle crowding and in essence changing the local concentration. In Fig. 10, away from the edge, in a thin area of the specimen, one sees individual micelles. The specimen in this study is extremely sensitive to the electron beam. To overcome electron beam damage, these micrographs were taken with minimal electron exposure [19,66,67].

Fig. 7. Microemulsions periodicity, d, for samples whose compositions lie along the N60 dilution line on the water + sodium chloride/sucrose laurate/ethoxylated mono-diglyceride/ peppermint oil + isopropylmyristate system. The mixing ratio (w/w) of peppermint oil/isopropylmyristate equals unity. The mixing ratios (w/w) of sucrose laurate/ethoxylated mono-di-glyceride are (□) 1/3, (○) 1/1 and (Δ) 3/1. The concentration of NaCl in water is 0.01 M. the lines are presented as guides to the eye.

Fig. 9. Microemulsions periodicity d (○) and correlation lengths ξ(Δ) for samples whose compositions lie along the N60 dilution line on the water + sodium chloride/ sucrose laurate/ethoxylated mono-di-glyceride/peppermint oil + isopropylmyristate system. The mixing ratio (w/w) of peppermint oil/isopropylmyristate equals one half and that of sucrose laurate/ethoxylated mono-di-glyceride equals unity. The concentration of NaCl in water is 0.01 M. The lines are presented as guides to the eye.

Fig. 8. Microemulsions correlation lengths, ξ, for samples whose compositions lie along the N60 dilution line on the water+ sodium chloride/sucrose laurate/ethoxylated monodi-glyceride/peppermint oil + isopropylmyristate system. The mixing ratio (w/w) of peppermint oil/isopropylmyristate equals unity. The mixing ratios (w/w) of sucrose laurate/ethoxylated mono-di-glyceride are (□) 1/3, (○) 1/1 and (Δ) 3/1. The concentration of NaCl in water is 0.01 M. The lines are presented as guides to the eye.

3.5. Cryogenic transmission electron microscopy imaging

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Fig. 10. Cryo-Tem image of vitrified microemulsion sample of water/sucrose laurate/ ethoxylated mono-di-glyceride/peppermint oil/isopropylmyristate at 0.9 water volume fraction. The mixing ratios (w/w) of peppermint oil/isopropylmyristate and that of ethoxylated mono-di-glyceride/sucrose laurate equal unity. The phase diagram is presented in Fig. 1B. Bar = 200 nm.

4. Conclusions Results are reported on the formulated biocompatible microemulsions based on mixed surfactants and the mixed peppermint oil and isopropylmyristate. It was found that increasing the weight ratio of peppermint oil in the mixed oils improved the water solubilization capacity in the microemulsions. The molar ratios of mixed surfactants play an important role in determining the maximum water solubilization. These microemulsions systems solubilizing sodium chloride were investigated using electrical conductivities, dynamic viscosity, small angle X-ray scattering, nuclear magnetic resonance and cryogenic transmission electron microscopy. This study reveals that the electrical conductivities increase with the increase water volume fraction. Static percolation was observed in these systems. The dynamic viscosities of these microemulsions depend also on the water volume fraction. The diffusion data suggest that the variations in the properties of the systems with the increase in the water volume fraction are correlated to structural transition from water-in-oil to bicontinuous to oil-in-water microemulsions. The periodicity of microemulsions increases linearly with the increase in the water volume fraction indicating mono dimensional swelling of the microemulsions droplets. The correlation length increases with the increase in the water volume fraction up to a certain value then decreases. The behavior of the correlation length indicates that when the water is the dispersed phase, increasing the water volume fraction increases the size of the scattering units and the correlation length, ξ, whereas when water is in the bulk, increasing the water volume fraction dilutes the scattering units and ξ stabilizes. References [1] M. Fanun (Ed.), Microemulsions: Properties and Applications, Taylor and Francis/ CRC Press, Boca Raton, 2009. [2] M. Bourrel, R.S. Schechter (Eds.), Microemulsions and Related Systems: Formulation, Solvency and Physical Properties, Marcel Dekker, New York, 1988. [3] R. Cash, J.L. Cayias, G. Fournier, D. McAllister, T. Shares, R.S. Schechter, W.H. Wade, J. Colloid Interface Sci. 59 (1977) 39.

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