Sugar-Based Microemulsions as Templates for Nanostructured

Article pubs.acs.org/jced

Sugar-Based Microemulsions as Templates for Nanostructured Materials: A Systematic Phase Behavior Study Regina Schwering,† David Ghosh,† Reinhard Strey,† and Thomas Sottmann*,‡ †

Institut für Physikalische Chemie, Universität zu Köln, Luxemburger Str. 116, 50939 Köln, Germany Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany

‡

ABSTRACT: Dynamic self-assembled structures found in complex fluids containing surfactant, water, and oil range from spherical and cylindrical aggregates to bicontinuous microemulsions and ordered liquid crystalline phases. These structures are extensively used as templates for the synthesis of nanomaterials. However, the topology of the initial structures and in particular their characteristic length scales often undergo significant changes during polymerization. Increasing the microemulsion viscosity should slow down its reorganization kinetics and, therewith, help to maintain the microemulsion nanostructure during the polymerization process. In this work, we report on systematic phase behavior studies of a new class of highly viscous microemulsions that comprise of surfactant, polymerizable oil, and concentrated water/(sucrose/trehalose) solutions. It is found that the substitution of H2O by sucrose/trehalose shifts the phase boundaries of nonionic microemulsions to lower temperatures, while the opposite trend holds for ionic microemulsions. Our systematic studies revealed that hydrophilic nonionic alkyl glycosides are the most suitable candidates for the preparation of highly viscous and polymerizable microemulsions.

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INTRODUCTION Microemulsions are a promising reaction media for a wide variety of applications due to their ability to dissolve large amounts of polar and nonpolar components within one phase. On a microscopic level, the surfactant monolayer separates the polar (usually water) and nonpolar (usually oil) components, resulting in large internal interfaces in the order of 100 m2/cm3. Depending on different factors such as temperature, pressure, overall composition, and the exact molecular structure of each component, a wide variety of nanostructures can be found. The shape and length scale of the structure is mainly determined by the so-called mean curvature of the amphiphilic film, i.e., the surfactant monolayer, which itself is a function of the preceding parameters.1 For nonionic surfactants, the surfactant monolayer curves toward the hydrophobic components at low temperatures, resulting in discrete aggregates of oil dispersed in water. As the temperature is increased, the mean curvature decreases. At the so-called phase inversion temperature (PIT), the mean curvature of the amphiphilic film is on average zero, which leads to the formation of sponge-like bicontinuous or lamellar structures. When increasing the temperature further, the surfactant monolayer curves toward the hydrophilic components, and discrete aggregates of water in oil appear.1−3 In general, the use of surfactants with long alkyl chains typically allows for a very efficient solubilization of oil in water (or vice versa). However, increasing the chain length of the surfactant promotes the formation of liquid crystalline phases, e.g., the lamellar phase.4 Thereby, the length scale of the nanostructure typically varies between (1 and 100) nm.5 © 2014 American Chemical Society

The fascinating properties of microemulsions are already used for many applications. For example, discrete waterdroplets in oil are utilized as nanosized reaction vessels for the synthesis of nanomaterials.6−8 Additionally, these complex media are used in chemical synthesis9 and preparative organic chemistry10 in order to overcome reagent-incompatibilities between water- and oil-soluble starting materials. Furthermore, polymerization reactions were performed in microemulsions to synthesize polymers with new properties.11,12 The concept of copying the fascinating nanostructure of microemulsions to the polymeric material on a one-to-one scale via polymerization was investigated already in the 1980s and developed rapidly since then to find novel polymeric materials with interesting morphologies.13,14 One goal is to use the nanostructural length scales of microemulsions to produce thermodynamically stable, monodisperse latexes in the nanometer range, which cannot be obtained by classical emulsion polymerization. These latex particles are desired for certain applications such as drug delivery systems or for microencapsulation. Of great interest isdue to its large surface to volume ratiothe polymerization of the bicontinuous structure. It leads to porous materials which can be used as microfilters, separation membranes, or solid polymers with a sponge-like structure.14 Gan et al. as well as Cheung et al. worked intensively on the polymerization of bicontinuous Received: September 18, 2014 Accepted: November 13, 2014 Published: December 5, 2014 124

dx.doi.org/10.1021/je500862w | J. Chem. Eng. Data 2015, 60, 124−136

Journal of Chemical & Engineering Data

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microemulsions.15−18 Gan et al. reproduced the pioneering experiment of Stoffer/Bone who studied the phase behavior of sodium dodecyl sulfate/pentanol/methyl methacrylate or methyl acrylate/water systems before and after polymerization.19−21 After confirming that transparent methyl methacrylate mixtures could not be fully polymerized in situ when using pentanol as cosurfactant, Gan et al. extended their studies to microemulsions in which all the components apart from water were polymerizable.15−18 The main conclusion obtained by Cheung et al. was that polymerization in bicontinuously structured microemulsions results in a polymer with an opencellular structure, i.e., an interconnected porous structure with water channels throughout the polymer.22−26 However, generally, the one-to-one copy of the nanostructure of microemulsions to the polymer material could not be obtained due to structural changes occurring during the polymerization process. These changes are mainly caused by the changing chemical composition of the microemulsion which leads to a considerable shift of the phase boundaries and often to a phase separation of the sample. Consequently, the length scale of the structure during the polymerization process increases by at least one order of magnitude. Also, the shape of the microstructure changes, in particular if either the hydrophilic or the hydrophobic domains of the bicontinuous microstructure are polymerized.27 Therefore, thorough research of the phase behavior of systems that contain not only the monomer but also the polymer must precede any study of microemulsion polymerization.14 Gelling the oil phase by forming an organogel to “arrest” the microemulsion to subsequently polymerize the aqueous phase was a promising approach reported by Stubenrauch et al. in 2007.28,29 But their results showed that the mesh-like gelator network has a length scale of more than 1 μm, giving therefore the underlying nanostructure of the microemulsion the ability for structural changes. Already in 2004, Co et al. reported for the first time a new class of water-free microemulsions that are composed of surfactant, oil, and glass-forming sugars, in particular equimolar mixtures of sucrose and trehalose, which are almost noncrystallizable.30−33 With the glass transition temperature for sucrose being just below 60 °C and for trehalose being 107 °C,34 these sugar-based microemulsions form amorphous solid and clear glasses that are surprisingly brittle considering that they contain equal masses of liquid oil and sugar.32 No visible phase separation was observed when samples containing a thermally stable UV-active initiator were polymerized at temperatures below the glass transition temperature of the sugar domains. Thus, the reorganization kinetics of the microemulsion system is slowed down to a degree where the system cannot kinetically adjust to the changing monomer/ polymer ratio. In 2006 Co et al. published the phase behavior of precursor microemulsions which are composed of sugar, divinylbenzene, and alkylglucoside surfactants.33 In contrast to the structures obtained on polymerizing systems containing isobutyl acrylate where the nanostructure collapsed upon dissolution of the sugar template,32 cross-linked divinylbenzene/sugar microemulsions yield robust structures with a pore size of the order of 25 nm.33 However, up to now the influence of sugar on the properties of microemulsions has not been studied systematically. With respect to industrial applications microemulsions containing highly concentrated sugar solutions instead of pure sugar are more appropriate due to their better processability. In this

study, we investigate the influence of sugar on the phase behavior of microemulsion systems formulated using pure and technical-grade nonionic surfactants as well as for nonionic/ ionic surfactant mixtures. The induced temperature shifts of the phase boundaries will be compensated by using appropriate surfactants. Aiming for the polymerization of these highly viscous microemulsions, the octane is replaced with the monomer methyl 2-methylpropenoate (C6MA) in the last step.

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EXPERIMENTAL SECTION Materials. The employed water was deionized and distilled twice. (2R,3S,4S,5R,6R)-2-(Hydroxymethyl)-6[(2R,3R,4S,5S,6R)-3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2yl]oxyoxane-3,4,5-triol (trehalose, C 1 2 H 2 2 O 1 1 ) and (2R,3R,4S,5S,6R)-2-[(2S,3S,4S,5R)-3,4-dihydroxy-2,5-bis(hydroxymethyl)oxolan-2-yl]oxy-6-(hydroxymethyl)oxane3,4,5-triol (sucrose, C12H22O11) were purchased from Merck (Darmstadt, Germany) with an analytical grade of > 0.99. All nonionic alkyl polyglycolether surfactants (CiEj; the chemical composition for C10E6 is C22H46O7, for C12E6 is C24H50O7, for C14E6 is C26H54O7, and for C18E8 is C34H70O9) were purchased from Fluka (Buchs SG, Switzerland) with an analytical grade of at least > 0.97 as well as the ionic surfactant dioctyl sulfosuccinate sodium salt (AOT; C20H37NaO7S) with a purity of > 0.98 and sodium chloride (NaCl) and octane (C8H18) both with a purity of > 0.995. The monomer methyl 2methylpropenoate (C6MA; C5H8O2) had a purity of > 0.98 and was used as purchased from Sigma-Aldrich (Munich, Germany). In order to avoid polymerization during phase investigation, C6MA was used without removal of the inhibitors. The technical grade n-alkylpolyglycolether surfactant Lutensol XL70, a C10-Guerbet-alcohol with an average number of seven ethylene oxide units, and the technical-grade sugar surfactants Agnique PG 264-G (C12−14G1.4) and Agnique PG 8105-G (C8−10G1.5) were provided by BASF (Ludwigshafen, Germany) and Cognis (Düsseldorf, Germany), respectively. Thereby Agnique PG 264-G, which contains (47 to 50) % water, and Agnique PG 8105-G containing (35 to 38) % water were dried up to weight constancy (approximately 1 week) in a desiccator under blue gel. n-Octyl-β-D-glucopyranoside (C8G1; C14H28O6) was purchased from Sigma-Aldrich with a fraction purity of ≥ 0.98. Methods. The determination of the phase diagrams was carried out in a thermostated water bath (Thermo-Haake DC30 with a temperature control up to ΔT = ± 0.01 K). The samples were weighted into test tubes with a precision of Δm = ± 0.001 g. Aiming for an overall mass of 1.000 g, the uncertainty of the mass fractions amounts to ± 0.001. After adding a stirring bar, the test tube was closed with a polyethylene stopper. For highly viscous microemulsions a neodymium stirring bar (ø 7 × 10 mm, AstroMedia Vertrieb, Essen) was used. Prior to the investigation of the phase behavior, the samples were homogenized by stirring under heat and carefully inspected to ensure that the solid components are completely dissolved. Afterward the temperature was regulated to the desired temperature while stirring the sample. Having reached the temperature equilibrium, the stirrer was turned off, and the number and kind of coexisting phases were determined by visual inspection of both transmitted and scattered light using crossed polarizers to recognize anisotropic phases. After recording all appearing phases and phase transition temperatures which are determined with a precision of ΔT = ± 0.1 K 125



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for a given composition, the sample was diluted with aqueous sugar solution and oil, and the process was repeated. The sample composition of microemulsions containing almost equal amounts of water/sugar and oil is given by the mass fraction of the oil (B) in the mixture of oil (B) and water/ sugar (A): mB α= mA + mB (1) and the mass fraction of surfactant (C) in the overall mixture mC γ= mA + mB + mC (2) Using a mixture of two surfactants, the mass fraction of the cosurfactant D in the mixture of the surfactants C and D is given by mD δ= mC + mD (3) Figure 1. T(γ)-sections of the system water/sucrose−octane−C10E6 at a constant mass fraction α = 0.413 of oil in the mixture of oil and water/sugar. The phase boundaries are shifted to lower temperatures if the sucrose mass fraction in the water/sucrose mixture is increased from Ψ = 0.00 up to Ψ = 0.50. Note that this shift is caused by a weaker hydration of the surfactant headgroups.

In this study, sucrose and trehalose are added to the water phase (A). The mass fraction of the sugar in the mixture of water and sugar is defined as msugar msugar Ψ= = msugar + m water mA (4) Thereby, the mass fraction of trehalose in the sucrose/ trehalose mixture is kept constant and given by mtrehalose ζ= = 0.38 msucrose + mtrehalose (5)

water-in-oil microemulsion coexisting with an excess water phase (2)̅ is found. At intermediate temperatures (between Tl and Tu) and intermediate surfactant concentrations, the threephase body (3) can be observed, meeting the one-phase region (1) at higher surfactant concentrations at the so-called X̃ -point. The X̃ -point defines the minimum amount of surfactant γ̃ needed to solubilize water and oil at the temperature T̃ , which corresponds to the phase inversion temperature (PIT). Adjusting the temperatures between Tl and Tu within the one-phase region, the nanostructure of the microemulsion is known to be bicontinuous.1 Bearing in mind to use these systems as templates for the production of nanoporous filters, separation membranes, or polymers, the location of the onephase region is of main interest, while the extension of the three-phase region has not been further investigated. Note that the parameters of the X̃ -points (γ̃ and T̃ ) of all studied systems are compiled in Table 1 together with their composition. To study the influence of sucrose on the phase behavior, water is substituted step-by-step by sucrose. All T(γ)-sections are recorded at the same mass fraction of oil in the water/ sucrose/octane mixture α = 0.413. Note that the volume fraction of octane in the water/sucrose/octane mixture increases due to the increasing density of the aqueous sugar solution. As one can see in Figure 1, increasing the sucrose mass fraction Ψ in the water/sugar mixture the phase inversion temperature T̃ shifts to lower values; i.e., for the pure water system a temperature of T̃ = 61.3 °C is found, while for the system containing 50% sucrose in the hydrophilic component (Ψ = 0.50) the phase inversion temperature decreases to T̃ = 27.7 °C. While T̃ decreases with increasing Ψ, the mass fraction γ̃ of surfactant at the X̃ -point stays almost constant. Plotting T̃ as a function of the sucrose mass fraction Ψ in the hydrophilic component, a parabolic dependence is found (not shown). In order to understand this trend, one may discuss the effect of a partial substitution of water by sugar on the binary side systems forming the Gibbs phase triangle. Because the properties of the binary octane−C10E6 mixture will, evidently,

If the oil octane is partially replaced with the monomer methyl 2-methylpropenoate (C6MA), the mass fraction of C6MA in the mixture of octane and C6MA is defined by mC6MA β= m C6MA+moctane (6) In case sodium chloride is added to the water phase, the mass fraction of sodium chloride in the hydrophilic component is given by mNaCl ε= mNaCl + m water + msugar (7)

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RESULTS AND DISCUSSION Influence of Sugar on Microemulsions Stabilized by Pure Nonionic Alkyl Polyglycolether Surfactants (CiEj). The starting point of these studies is the temperaturedependent phase behavior of the sugar-free ternary system H2O−octane−C10E6 (Ψ = 0.00).35 At constant pressure, the phase behavior of ternary or pseudoternary systems is best shown in an upright phase prism with the Gibbs triangle as base and a vertical temperature axis. To simplify this threedimensional diagram, a vertical 2-dimensional section through this phase prism at a constant mass fraction α of oil in the mixture of water and oil is performed as a function of temperature T and surfactant mass fraction γ. This so-called T(γ)-section is shown in Figure 1 for the systems H2O/ sucrose−octane−C10E6 at α = 0.413, which corresponds to equal volumes of water and oil. At low temperatures an oil-in-water microemulsion coexists with an excess oil phase (2), whereas at high temperatures a 126



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Table 1. Minimum Amount of Surfactant γ̃ Needed to Solubilize Water and Oil at the Temperature T̃ , Which Corresponds to the Phase Inversion Temperature (PIT) Together with the Composition of the Systemsa system composition

Ψ

β

δ

δPG

ε

water−octane−C10E6 water/sucrose−octane−C10E6 water/sucrose−octane−C10E6 water/sucrose−octane−C10E6 water/sucrose−octane−C10E6/C8G1 water/sucrose−octane−C10E6/C8G1 water/sucrose−octane−C10E6/C8G1 water/sucrose−octane−C12E6/C8G1 water/sucrose−octane−C14E6/C8G1 water/sucrose−octane−C14E6/C8G1 water/sucrose−octane−C18E8/C8G1 water/sucrose/trehalose−octane−C18E8/C8G1 H2O/NaCl−octane−C10E6 H2O/NaCl−octane−C10E6/AOT H2O/NaCl−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6 H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6 H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6 H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl/sucrose−octane−C10E6/AOT H2O/NaCl−octane−AOT H2O/NaCl/sucrose−octane−AOT H2O/NaCl/sucrose−octane−AOT H2O/NaCl− octane−C12E4/AOT H2O/NaCl/sucrose−octane−C12E4/AOT H2O/NaCl/sucrose−octane−C12E4/AOT H2O−octane−Lutensol XL70 H2O/sucrose−octane−Lutensol XL70 H2O/sucrose−octane−Lutensol XL70 H2O/sucrose−octane−Lutensol XL70 H2O/sucrose−octane−Lutensol XL70/Agnique PG 8105-G H2O/sucrose−octane−Lutensol XL70/Agnique PG 8105-G H2O/sucrose−octane−Lutensol XL70/Agnique PG 8105-G H2O/sucrose−octane−Lutensol XL 70/Agnique PG 8105-G H2O/sucrose−octane−Lutensol XL 70/Agnique PG 8105-G H2O/(sucrose/trehalose)−octane−Lutensol XL 70/Agnique PG 8105-G H2O/(sucrose/trehalose)−octane−C18E8/C8G1 H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1 H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1/Agnique PG 264-G H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1/Agnique PG 264-G H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1/Agnique PG 264-G H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1/Agnique PG 264-G H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1/Agnique PG 264-G H2O/(sucrose/trehalose)−C6MA−Agnique PG 264-G

0.000 0.300 0.400 0.500 0.500 0.500 0.500 0.500 0.500 0.650 0.650 0.750 0.000 0.000 0.000 0.100 0.100 0.100 0.100 0.200 0.200 0.200 0.200 0.500 0.500 0.500 0.500 0.000 0.100 0.200 0.000 0.100 0.200 0.000 0.300 0.400 0.500 0.500 0.500 0.500 0.650 0.650 0.750 0.750 0.750 0.750 0.750 0.750 0.750 0.750 0.750

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.150 0.150 0.400 0.400 0.800 0.800 1.000

0.000 0.000 0.000 0.000 0.200 0.400 0.500 0.500 0.500 0.500 0.500 0.700 0.000 0.100 0.200 0.000 0.100 0.200 0.300 0.000 0.100 0.200 0.300 0.000 0.100 0.200 0.300 1.000 1.000 1.000 0.900 0.900 0.900 0.000 0.000 0.000 0.000 0.200 0.300 0.400 0.400 0.550 0.550 0.700 0.700 0.700 0.700 0.700 0.700 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.400 0.400 0.700 0.700 1.000 1.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

T̃ /°C

γ̃

61.3 0.180 45.9 0.185 38.8 0.180 26.7 0.178 39.6 0.239 60.6 0.277 78.8 0.298 72.3 0.242 69.4 0.202 40.0 0.177 56.2 0.165 67.0 0.243 58.6 0.189 67.0 0.147 80.5 0.121 57.2 0.208 64.8 0.156 75.6 0.134 83.2 0.106 52.2 0.207 57.5 0.157 65.5 0.113 67.5 0.094 25.1 0.197 26.6 0.153 25.8 0.133 23.5 0.106 37.5 0.034 51.8 0.046 78.5 0.060 36.0 0.035 52.3 0.043 66.1 0.051 72.3 0.132 57.9 0.115 47.8 0.115 37.2 0.093 48.2 0.173 57.8 0.200 74.5 0.234 42.3 0.227 77.5 0.268 40.2 0.255 67.4 0.243 44.3 0.203 54.8 0.161 35.1 0.132 50.3 0.098 34.3 0.098 not accessible not accessible

The uncertainties of T̃ and γ̃ amount to ± 0.5 K and ± 0.005, respectively. Ψ specifies the mass fraction of sugar in the mixture of water and sugar, while β defines the mass fraction of C6MA in the mixture of octane and C6MA. Using a mixture of two surfactants, the mass fraction of the cosurfactant in the mixture of the two surfactants is given by δ. In the case that a mixture of three surfactants is used, δPG defines the mass fraction of Agnique PG 264-G in the surfactant mixture. Sometimes sodium chloride is added to the water/surfactant mixture, and the mass fraction of sodium chloride in the hydrophilic component is given by ε. The mass fraction of the oil in the mixture of oil and water/sugar (α = 0.431 = constant) and the mass fraction ζ of trehalose in the sucrose/trehalose mixture are not mentioned in the table. While for systems with Ψ ≤ 0.65 pure sucrose (ζ = 0.00) is used, ζ is set to (ζ = 0.38) for Ψ > 0.65. a

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not be affected, it suffices to study the effect of the sugar on the binary H2O−octane and, in particular, the H2O−C10E6 system. Concentrating on the latter system, replacing H2O partly by sucrose will, evidently, affect the interactions between the tails as well as the heads of the surfactant and the solvent. The effect of the interaction between the tails and the H2O/sucrose mixture (the hydrophobic effect) can be predicted qualitatively by comparing the solubility of hydrocarbons in H2O/sucrose solutions with that in H2O. Earlier studies indicate that the addition of sucrose has only a small effect on the solubility of hydrocarbons in H2O; e.g., for cyclic hydrocarbons only a slightly increase of their solubility is found.36 On the other hand, the interaction between the solvent molecules and the surfactant headgroup, which can be interpreted as being the result of a competition between the headgroups and the solvent for free water molecules, is strongly influenced by the addition of sucrose molecules. The added sucrose molecules are expected to bind water molecules, and therewith less water molecules are left to hydrate the headgroups. This decrease of the effective surfactant headgroup volume results in a decrease of the spontaneous curvature of the surfactant film and a shift of the phase boundaries to lower temperatures. The parabolic decrease suggests a cooperative effect: The more H2O molecules needed to hydrate the sucrose, the less are available for the hydration of the surfactant headgroups and the stronger is the effect on the curvature of the amphiphilic film. As the aim of this work was to prepare a highly viscous microemulsion with a sugar mass fraction of up to Ψ = 0.75 within the experimental temperature range, a more hydrophilic surfactant is needed to compensate for the decrease of T̃ with increasing mass fraction of sucrose in the water/sugar mixture. Currently there is a clear tendency to replace conventional surfactants with more environmentally benign “biosurfactants”. Sugar-based surfactants which can be produced from renewable resources and exhibit an excellent ecological behavior have become the object of increasing attention for many researchers.37−40 A prominent type of sugar-surfactant are alkylpolyglycosides (CmGn), which are strongly hydrophilic.41 In order to induce a temperature-driven phase inversion in aqueous systems usually the use of “hydrophilic oils”42,43 or the addition of a hydrophobic cosurfactant44−47 is necessary. In this work, we utilized the hydrophilicity of the alkylpolyglycosides to compensate for the parabolic decrease of the phase inversion temperature T̃ with increasing sucrose content in the water/ sugar mixture. Figure 2 shows the phase behavior of the system water/ sucrose−octane−C10E6/C8G1 for different mass fractions δ of C8G1 in the C10E6/C8G1 mixture at a mass fraction of sucrose in the water/sugar mixture of Ψ = 0.50 and a constant α = 0.413. With increasing δ and therefore increasing fraction of the hydrophilic C8G1 surfactant, the phase boundaries and therewith T̃ shift to higher temperatures. Starting from the T(γ)-section at δ = 0.00, the temperature T̃ increases from T̃ = 27.7 °C to T̃ = 78.8 °C at δ = 0.50. At the same time the surfactant mixture solubilizes octane in the water/sucrose mixture increasingly inefficient; i.e., γ̃ increases from γ̃ = 0.178 (δ = 0.00) up to γ̃ = 0.298 (δ = 0.50). Thereby, the short alkyl chain of the surfactant C8G1 is known to be the main reason for the increasing inefficiency of the C10E6/C8G1 mixture.35 As known from literature, alkyl polyglycolether surfactants (CiEj) become clearly more efficient and more hydrophobic in case the number i of alkyl groups is increased, keeping the number j of ethylene oxide groups constant.35 Therefore, in

Figure 2. T(γ)-sections of the system water/sucrose−octane−C10E6/ C8G1 at constant α = 0.413 and Ψ = 0.50. Increasing the mass fraction δ of the hydrophilic sugar surfactant in the surfactant mixture shifts the phase boundaries to higher temperatures. At the same time, the surfactant mixture becomes more inefficient.

order to increase the efficiency of the C10E6/C8G1 mixture, the surfactant C10E6 is replaced in a first step by C12E6 and later on by C14E6. Figure 3 shows the T(γ)-sections of the system

Figure 3. T(γ)-sections of the system water/sucrose−octane−CiE6/ C8G1 at α = 0.413, Ψ = 0.50, and δ = 0.50. Increasing the surfactant chain length from C10E6 over C12E6 to C14E6, the phase boundaries shift to lower surfactant mass fractions γ and lower temperatures. A similar trend is found for sugar-free-microemulsions. Note that a different temperature range (50 to 90 °C) has been chosen.

water/sucrose−octane−CiE6/C8G1 recorded at Ψ = 0.50, α = 0.413 and δ = 0.50. As can be seen, the minimal surfactant mass fraction γ̃ needed to solubilize the water/sugar mixture and octane decreases as expected strongly to γ̃ = 0.202 replacing C10E6 by C14E6. Simultaneously, the phase inversion temperature T̃ decreases to T̃ = 69.4 °C. Comparing this decrease of the phase inversion temperature T̃ (ΔT̃ = 9.4 °C) with the one observed for the nonsugar systems H2O−octane−C10E6 and H2O−octane−C14E6 (ΔT̃ = 21 °C, extrapolated from ref 35), the shift of T̃ is comparable, considering that the sugar128



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Furthermore, the surfactant mixture becomes slightly more efficient. In a next step, to compensate for the decrease in temperature and to increase the efficiency, the surfactant C14E6 is replaced by C18E8. The phase diagrams of the respective systems are shown in Figure 4, center. As can be seen, using the C18E8/ C8G1 mixture instead of the mixture C14E6/C8G1, the phase inversion temperature T̃ increases up to T̃ = 56.2 °C. Furthermore, the surfactant mixture C18E8/C8G1 solubilizes the water/sugar mixture and octane slightly more efficiently than the mixture C14E6/C8G1. Finally, the sugar mass fraction is increased up to Ψ = 0.75 using a sucrose/trehalose mixture (Figure 4, bottom) in order to avoid the crystallization of the sugar in water. The mass fraction of trehalose in the sucrose/trehalose mixture is kept constant at ζ = 0.38 (eq 5). Note that, in the following, for systems containing more than 65 wt % of sugar in the hydrophilic component this sucrose/trehalose mixture is used instead of only sucrose. It turned out, that the increase of the sugar mass fraction from Ψ = 0.65 to Ψ = 0.75 causes a strong shift of the phase boundaries toward very low temperatures (not shown). Therefore, the mass fraction of C8G1 in the surfactant mixture is increased at the same time to δ = 0.70. In agreement with the trends observed before, the increase of the sugar mass fraction to Ψ = 0.75 and the mass fraction of C8G1 to δ = 0.70 leads to a shift of the phase inversion temperature T̃ to T̃ = 67 °C, while the surfactant mixture becomes less efficient. Highly Viscous Sugar-Microemulsions Containing a Nonionic/Ionic Surfactant Mixture. An alternative route to compensate for the sugar-induced shift of the phase boundaries to lower temperatures is the partial replacement of the nonionic alkyl polyglycolether surfactants (CiEj) by rather hydrophilic ionic surfactants. As the phase boundaries of microemulsion systems shift to higher temperatures, if an ionic surfactant is added to nonionic surfactants at an appropriate salt concentration,48 the use of nonionic/ionic surfactant mixtures could enable the preparation of highly viscous sugar-microemulsions at the experimentally accessible temperature range. As an ionic surfactant, we chose the double chain ionic surfactant AOT. Figure 5 shows the T(γ)-sections of the system H2O/ sucrose/NaCl−octane−C10E6/AOT at α = 0.413 and a NaCl mass fraction in the mixture of water/sucrose/NaCl of ε = 0.004. Thereby, the mass fraction δ of AOT in the surfactant mixture has been varied as well as the mass fraction Ψ of sucrose in the hydrophilic component. In the upper left phase diagram, the AOT mass fraction δ is varied in the sucrose-free system. With increasing δ, T̃ shifts from T̃ = 58.6 °C for δ = 0.00 to T̃ = 80.5 °C for δ = 0.20. At the same time, the system becomes increasingly efficient.48 The same trend is seen in the upper right phase diagram for the system containing 10 wt % sucrose in the hydrophilic component (Ψ = 0.10) and in the lower left diagram for the Ψ = 0.20 system. In all cases, the phase inversion temperature T̃ increases with increasing δ. However, the effect becomes slightly less pronounced with increasing sugar mass fraction. The increase in efficiency, which is almost the same for all systems, is a result of the bulky hydrophobic part of the anionic surfactant AOT consisting of two branched C8-chains. Astonishingly, if the sucrose mass fraction in the hydrophilic component is further increased to Ψ = 0.50, the influence of the AOT on the phase behavior is significantly smaller (lower right

microemulsion contains only 50% CiEj surfactant in the surfactant mixture. To further increase the viscosity of the microemulsion, the mass fraction of sucrose in water is further increased. Figure 4 shows the journey toward a highly viscous microemulsion containing only pure nonionic surfactants. Starting from the system water/sucrose−octane−C14E6/C8G1 with α = 0.413, Ψ = 0.50, and δ = 0.50, the sucrose mass fraction is increased up to Ψ = 0.65, resulting in a decrease of the phase inversion temperature T̃ by approximately ΔT̃ = 30 °C (Figure 4, top).

Figure 4. T(γ)-sections of the system water/sucrose/trehalose− octane−CiEj/C8G1 at α = 0.413. The amount of sugar in the water phase is increased from Ψ = 0.50 to Ψ = 0.65 (top). To compensate for the decrease of the phase boundaries with respect to temperature, the surfactant mixture is changed; i.e., C14E6 is replaced by C18E8 (center). The shift of the phase boundaries associated with a further increase of Ψ = 0.65 to Ψ = 0.75 is compensated by an increase of mass fraction of C8G1 in the surfactant mixture to δ = 0.70 (bottom). Note that for the system with Ψ > 0.65 a sucrose/trehalose mixture (ζ = 0.38) is used. 129



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Figure 5. T(γ)-sections of the system H2O/NaCl/sucrose−octane−C10E6/AOT at α = 0.413 and ε = 0.004. Each diagram shows the influence of the mass fraction δ of AOT in the C10E6/AOT mixture on the phase behavior of systems containing different mass fractions Ψ of sucrose in the hydrophilic component. With increasing δ the surfactant mixture becomes more efficient as a result of the bulky hydrophobic part of the anionic surfactant AOT. The phase behavior shifts to higher temperatures with increasing δ for Ψ = 0.00 (upper left), Ψ = 0.10 (upper right), and Ψ = 0.20 (lower left). The trend changes at Ψ = 0.50 (lower right) where T̃ varies only slightly.

phase diagram). Increasing the AOT mass fraction δ to δ = 0.10 shifts the X̃ -point only slightly to higher temperatures (from T̃ = 25.0 °C to T̃ = 27.2 °C). A further increase of δ leads to a slight shift of the phase inversion temperature to even lower temperatures. A reasonable explanation for this weak temperature trend is the decreasing dissociation of counterions (Na+) with increasing sucrose mass fraction Ψ, by which the AOT becomes less hydrophilic. The minimal surfactant mass fraction γ̃ needed to solubilize the water/sugar mixture and octane decreases as found for the systems containing no or less sugar, i.e., Ψ = 0.00, Ψ = 0.10, and Ψ = 0.20. Switching to the ionic side, i.e., to high mass fractions δ of the ionic surfactant in the surfactant mixture, the system is modified using the more hydrophobic C12E4 as a nonionic surfactant. Figure 6 shows the phase behavior of the system H2O/sucrose/NaCl−octane−C12E4/AOT as a function of the sucrose mass fraction Ψ in the hydrophilic component for δ = 1.00 (top) and δ = 0.90 (bottom) at a constant α = 0.413 and ε = 0.004. Being on the ionic side, the temperature dependence of the phase behavior is dominated by the properties of the ionic surfactant. Therefore, compared to microemulsions dominated by the properties of nonionic surfactants, a temperature-wise inverse sequence of phases is found. At low temperatures, a water-in-oil microemulsion coexists with a water excess phase (2̅), while at high temperatures an oil-inwater microemulsion coexists with an excess oil phase (2̲). This trend is a consequence of the increasing dissociation of the counterions with increasing temperature.49

As can be seen in Figure 6, increasing the sucrose mass fraction Ψ leads to a shift of the phase boundaries to higher temperatures for both the systems containing only AOT, i.e., δ = 1 (upper diagram) and for the system containing 10 wt % of C12E4 in the surfactant mixture (lower diagram). This trend is attributable to the decreasing amount of water molecules being available for the hydration of the surfactant headgroups by which the surfactant mixture becomes less hydrophilic. Furthermore, for both δ = 1.00 and δ = 0.90, the surfactant solubilizes water/sucrose and octane less efficiently in case Ψ is increased. A reasonable explanation for the decreasing efficiency is the shift of the phase boundaries to higher temperatures at which the amphiphilic surfactant film is known to be less rigid. Consequently, a two-phase region, in which the microemulsion coexists with a lamellar phase, is only found in the systems without sucrose. Figure 7 summarizes the influence of sucrose and the surfactant composition on the phase behavior of a microemulsion system containing a mixture of nonionic and ionic surfactants. Considering at first the systems which contain mainly nonionic surfactant (δ ≤ 0.3), it is known from literature that T̃ increases with increasing mass fraction δ of the ionic surfactant in the surfactant mixture for microemulsions with low concentrations of salt.48 The same trend is found for the sucrose-free system, i.e., H2O/sucrose/NaCl−octane− C10E6/AOT at Ψ = 0.00. As one increases the sucrose mass fraction Ψ in the water/sucrose mixture, the increase of the phase inversion temperature T̃ with increasing δ becomes less 130



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pronounced. At a sucrose mass fraction of Ψ = 0.50, the phase inversion temperature T̃ varies only slightly with increasing fraction δ of AOT in the surfactant mixture. The shift of the phase inversion temperature with increasing Ψ is similar to the trend observed, adding a lyotropic salt. Both are a result of the hydration of the sucrose or lyotropic salt molecules as they dissolve in water. Thus, the surfactant headgroups are less hydrated and smaller, which results in a decrease of the spontaneous curvature of the surfactant film and a shift of the phase boundaries to lower temperatures. The influence of sucrose on the phase behavior of the surfactant mixture that contains mainly the ionic surfactant (δ ≥ 0.75) is studied by means of the system H2O/sucrose/NaCl−octane−C12E4/AOT. Note that not all T(γ)-sections have been shown in Figure 6 for clarity reasons. For Ψ = 0.00 the phase boundaries and therewith T̃ decreases with decreasing fraction δ of AOT in the surfactant mixture.48 Replacing only 10 wt % of brine by sucrose (Ψ = 0.10), this trend is inverted. As one can see, T̃ increases with decreasing δ. Thus, on the ionic side, smaller headgroups of the surfactants caused by the additional hydration of the sucrose shift the phase boundaries and therewith the phase inversion temperature T̃ to higher values. Highly Viscous Microemulsions Containing TechnicalGrade Nonionic Surfactants. In view of a technical application of highly viscous polymerizable microemulsions for the synthesis of monodisperse latexes in the nanometer range or highly porous polymeric materials, the used monodisperse nonionic surfactants are replaced with technical-grade nonionic surfactants. It has been shown that microemulsions formulated using technical-grade nonionic surfactants show qualitatively the same phase behavior as their monodisperse equivalents.50−52 However, the phase boundaries are distorted due to the polydisperse nature of technical grade surfactants. The influence of sucrose on the phase behavior of a microemulsion system containing a technical-grade surfactant is shown in Figure 8 (top). Starting with a mixture of H2O−octane−Lutensol XL70, a C10-Guerbet alcohol with an average number of 7 ethylene oxide units, the mass fraction Ψ of sugar in the water/sugar mixture is increased stepwise. Again, the T(γ)-sections are recorded at a mass fraction of oil in the water/oil mixture α = 0.413. As can be seen, the phase behavior follows the same trend found for the system H2O/sucrose−octane−C10E6. By increasing the sucrose mass fraction in the water/sugar mixture, the phase boundaries are shifted to lower temperatures; i.e., the phase inversion temperature T̃ shifts from T̃ = 72.8 °C for the sugar-free system to T̃ = 37.3 °C in case 50 wt % of water are replaced with sucrose. As expected for microemulsion systems containing technical-grade nonionic surfactants, the phase boundaries are distorted toward higher temperatures with decreasing surfactant concentration. Upon dilution, i.e., proceeding to lower γ, a stepwise extraction of the more (oil-)soluble homologues of the surfactants from the interface into the oil takes place. The less soluble homologuesthose with larger head groupsremain in the interface, so that the composition of the amphiphilic film changes gradually to a more and more hydrophilic one, which in consequence shifts the phase boundaries to higher temperatures. By comparing the phase inversion temperature T̃ of the technical-grade surfactant Lutensol XL70 system with that of the pure surfactant C10E6 system, a 10 °C higher value can be found for the Lutensol XL70 system attributable to the larger size of the hydrophilic headgroup. Note that the temperature

Figure 6. T(γ)-sections of the system H2O/sucrose/NaCl−octane− C12E4/AOT at α = 0.413 and ε = 0.004. The diagrams show the influence of the sucrose mass fraction Ψ in the hydrophilic component on the phase behavior for the system containing only AOT, i.e., δ = 1.00 (upper diagram) and for the system containing 10 wt % of C12E4 in the surfactant mixture (lower diagram). A two-phase coexistence of a microemulsion and a lamellar phase was only found in the systems without sucrose (circles with dot).

Figure 7. T̃ plotted as a function of δ for the system H2O/sucrose/ NaCl−octane−C10E6/AOT on the nonionic side (δ ≤ 0.3) and the system H2O/sucrose/NaCl−octane−C12E4/AOT on the ionic side (δ ≥ 0.75) at α = 0.413 and ε = 0.004 for different mass fractions Ψ of sucrose in the hydrophilic component. Note that not all T(γ)-sections are shown in the previous figures.

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temperatures in case the amount of Agnique PG 8105-G in the surfactant mixture is increased. The same trend is found for the system containing the pure surfactants C10E6 and C8G1 (see Figure 2). Because the headgroup size of Agnique PG 8105-G is slightly larger than that of C8G1, Agnique PG 8105-G is more hydrophilic. Accordingly, replacing 40% of C10E6 with C8G1 ΔT̃ increases by ΔT̃ = 33 °C, whereas replacing 40% of Lutensol XL70 with Agnique PG 8105-G leads to an increase of ΔT̃ = 38 °C. The decrease in efficiency from γ̃ = 0.095 (δ = 0.00) to γ̃ = 0.235 (δ = 0.40) is of the same range as the one found in the system H2O/sucrose−octane−C10E6/C8G1. As mentioned above, the short alkyl chain of the sugar surfactant is the main reason for the increasing inefficiency of the Lutensol XL70/Agnique PG 8105-G mixture. With the aim to slow-down the reorganization kinetics of microemulsions, the viscosity of the microemulsion is further increased by increasing the mass fraction Ψ of sucrose in the water/sucrose mixture. As can be seen in Figure 9, top, the phase inversion temperature T̃ decreases by ΔT̃ ≈ 30 °C in case Ψ is increased from Ψ = 0.50 to Ψ = 0.65 in the system H2O/(sucrose/trehalose)−octane−Lutensol XL70/Agnique PG 8105-G with α = 0.413, δ = 0.40, and ζ = 0 (no trehalose in the sucrose/trehalose mixture). Furthermore, the surfactant mixture becomes slightly more efficient. To compensate for the decrease in temperature, the amount of Agnique PG 8105-G is increased from δ = 0.40 to δ = 0.55 in a second step (Figure 9, center). As one can see, the phase boundaries and consequently the phase inversion temperature T̃ are shifted to higher temperatures. The surfactant mixture also becomes less efficient (γ̃ = 0.265) due to the increasing fraction of Agnique PG 8105G. Finally, the sugar mass fraction is increased to Ψ = 0.75, using a sucrose/trehalose mixture (ζ = 0.38) instead of pure sucrose. As expected from the trends observed so far, the increase of the sugar mass fraction from Ψ = 0.65 to Ψ = 0.75 leads to a decrease of the phase boundaries and consequently a decrease in phase inversion temperature T̃ (Figure 9, bottom). Furthermore, the surfactant mixture becomes slightly more efficient. Highly Viscous and Polymerizable Microemulsions. In order to enable a 1:1 copy of the nanostructure of the highly viscous microemulsion to a nanoporous material, octane is replaced with a polymerizable oil. Because of the low solubility in water, compared to short chain methacrylate oils, methyl 2methylpropenoate (C6MA) has been used.53 First studies of the influence of C6MA on the phase behavior of highly viscous microemulsions are performed using pure surfactants. As it is shown in Figure 10 for the system H2O/(sucrose/trehalose)− octane/C6MA−C18E8/C8G1 with α = 0.413, Ψ = 0.75, ζ = 0.38, and δ = 0.70, the replacement of octane by C6MA has a strong effect on the phase behavior. Replacing only 15 wt % of octane by C6MA (β = 0.15), the phase inversion temperature T̃ decreases by ΔT̃ = 22.5 °C. Simultaneously, the surfactant mass fraction γ̃ at the X̃ -point decreases by Δγ̃ = 0.038 to γ̃ = 0.205. The observed effect of C6MA on the phase behavior is considerably stronger as the one found in sugar-free microemulsions of the type H2O−octane/C6MA−C10E6,54,55 where the phase inversion temperature T̃ decreases only by ΔT̃ ≈ 30 °C if octane is completely replaced with C6MA. The explanations for this unexpected trend are 2-fold: First, the influence of C6MA on the central miscibility gap between the hydrophilic and hydrophobic component might be stronger in case a water/sugar mixture is used as a hydrophilic component. Second, in sugar microemulsion systems stabilized mainly by a

Figure 8. Top: T(γ)-sections of the system H2O/sucrose−octane− Lutensol XL70 at α = 0.413. The phase boundaries are shifted to lower temperatures and a slightly lower mass fraction γ of surfactant if the sucrose mass fraction Ψ in the water/sugar mixture is increased from Ψ = 0.00 to Ψ = 0.50. Bottom: T(γ)-sections of the system H2O/ sucrose−octane−Lutensol XL70/Agnique PG 8105-G at α = 0.413 and Ψ = 0.50. Increasing the mass fraction δ of sugar surfactant Agnique PG 8105-G in the surfactant mixture leads to a shift of the phase boundaries to higher temperatures. At the same time the surfactant mixture becomes more inefficient.

difference between the phase inversion temperature T̃ of the Lutensol XL70and the C10E6system of ΔT̃ ≈ 10 °C is also found for the sugar-containing microemulsion systems. Whereas the surfactant mass fraction γ̃ at the X̃ -point is nearly independent of the sugar mass fraction in the C10E6 system, γ̃ decreases considerably in the Lutensol XL70 system. As the surfactant is the only component, which is replaced, this increase in efficiency might be caused by the decreasing monomeric solubility of the hydrophobic homologues of Lutensol XL 70 in octane at lower temperatures. In order to compensate for the decrease in temperature, Lutensol XL 70 is partly replaced with the hydrophilic technical-grade sugar surfactant Agnique PG 8105-G. This technical-grade analogue to the pure surfactant C8G1 consists of an average number of 1.5 glucose units in the hydrophilic and 8−10 carbons in the hydrophobic part. In Figure 8 bottom the T(γ)-sections of the system H2O/sucrose−octane−Lutensol XL70/Agnique PG 8105-G at α = 0.413 and Ψ = 0.50 are shown increasing the fraction δ of Agnique PG 8105-G in the surfactant mixture. Starting from the system H2O/sucrose− octane−Lutensol XL70, the phase boundaries shift to higher 132



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Figure 10. T(γ)-sections of the system H2O/(sucrose/trehalose)− octane/C6MA−C18E8/C8G1 at α = 0.413, Ψ = 0.75, ζ = 0.38, and δ = 0.70. As one can see, T̃ decreases strongly if only 15 wt % (β = 0.15) of n-octane are replaced by C6MA. Furthermore, the surfactant mixture becomes more efficient.

hydrophilic methacrylate group partially as a hydrophobic cosurfactant. The strong decrease of the phase boundaries toward low temperatures observed in the system H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1 with Ψ = 0.75, ζ = 0.38, α = 0.413, δ = 0.70, and β = 0.15 does not allow for the substitution of more octane by C6MA, because the existence region of the one-phase microemulsion would shift out of the experimental accessible temperature range. Thus, in order to prepare highly viscous microemulsions with the polymerizable oil C6MA, an even more hydrophilic surfactant mixture has to be used. In the following the C18E8/C8G1 surfactant mixture is replaced with the technical-grade surfactant Agnique PG 264-G, a sugar surfactant with an alkyl chain of 12−14 carbons and a headgroup which consists of 1.4 sugar groups on average. Figure 11 shows the stepwise replacement of the surfactant mixture C18E8/C8G1 by the technical-grade surfactant Agnique PG 264-G in the system H2O/(sucrose/trehalose)−octane/ C6MA−C18E8/C8G1 with α = 0.413, Ψ = 0.75, and ζ = 0.38. At the same time, the oil octane is substituted by the polymerizable oil C6MA. Thereby, the mass fraction δC8G1 of C8G1 in the C18E8/C8G1 mixture is kept constant at δC8G1 = 0.70. The mass fraction of Agnique PG 264-G in the overall surfactant mixture is given by δPG. In a first step, the surfactant mixture C18E8/C8G1 is partially replaced with the technical-grade sugar surfactant Agnique PG 264-G (δPG = 0.40) in the system H2O/ (sucrose/trehalose)−octane/C6MA−C18E8/C8G1 with α = 0.413, Ψ = 0.75, ζ = 0.38, δC8G1 = 0.70, and β = 0.15. As can be seen in Figure 11, top left, replacing 40 wt % of the C18E8/ C8G1 mixture by the more hydrophilic surfactant Agnique PG 264-G, the phase boundaries are shifted to higher temperatures and smaller surfactant mass fractions; i.e., the surfactant mixture becomes more efficient. In a second step (Figure 11, left top to bottom), the mass fraction of C6MA in the octane/C6MA mixture is increased from β = 0.15 to β = 0.40. As expected, the phase boundaries are shifted to lower temperatures because C6MA is compared to octane, the less hydrophobic oil. Furthermore, as discussed above, C6MA also might partially act as hydrophobic cosurfactant explaining not only the shift of the phase boundaries to lower temperatures, but also to lower

Figure 9. T(γ)-sections of the system H2O/(sucrose/trehalose)− octane−Lutensol XL 70/Agnique PG 8105-G at α = 0.413. Top: The amount of sugar in the water/sugar-mixture is increased from Ψ = 0.50 to Ψ = 0.65. Center: To compensate for the shift of the phase boundaries to lower temperatures, the fraction of Agnique PG 8105-G in the surfactant mixture is increased from δ = 0.40 to δ = 0.55. Bottom: The fraction of sugar in the water/sugar-mixture is further increased from Ψ = 0.65 to Ψ = 0.75. Note that for the system with Ψ > 0.65 a sucrose/trehalose mixture (ζ = 0.38) is used.

sugar surfactant, C6MA might not only act as polymerizable oil but also due to its hydrophobic hexyl group and the more 133



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Figure 11. T(γ)-sections of the system H2O/(sucrose/trehalose)−octane/C6MA−C18E8/C8G1/Agnique PG 264-G at α = 0.413, Ψ = 0.75, ζ = 0.38, and δC8G1 = 0.70. The amount of methyl 2-methylpropenoate (C6MA) in the oil phase is increased gradually. To compensate for the shift of the phase boundaries to lower temperatures the surfactant mixture is changed, i.e., the C18E8/C8G1 surfactant mixture is gradually replaced with Agnique PG 264-G. Note that increasing the fraction δPG of Agnique PG 264-G, liquid crystalline phases become increasingly dominant. Thus, an extended multiphase region containing a lamellar phase (Lα present) can be observed temperature-wise below the regime of the one-phase microemulsion. The three-phase state consisting of a microemulsion, water, and oil excess phase, which is a typical feature of microemulsion systems was not found in the experimentally accessible temperature range. This situation is shown schematically in the inlet of the figure, bottom right.

multiphase region containing a lamellar phase (Lα present) can be observed temperature-wise below the regime of the onephase microemulsion. The three phase state consisting of a microemulsion, water, and oil excess phase and a “real” X̃ -point, which is a typical feature of microemulsion systems was not found in the experimentally accessible temperature range. This situation is already known from literature60 and shown schematically in the inlet of the Figure, bottom right. In Figure 11, bottom right, the remaining octane is replaced with C6MA. As can be seen, the system H2O/(sucrose/ trehalose)−C6MA−Agnique PG 264-G with α = 0.413, Ψ = 0.75, and ζ = 0.38 behaves very similarly to the systems containing the octane/C6MA mixture (β = 0.80). However, the extension of the multiphase region containing a lamellar phase becomes even larger which suggests that not only Agnique PG 264-G but also C6MA induces the formation of liquid crystalline phases.

surfactant mass fractions. In order to compensate for this shift of the phase boundaries, the mass fraction of the hydrophilic surfactant Agnique PG 264-G is increased to δPG = 0.70. Accordingly, the phase boundaries are shifted to higher temperatures and lower surfactant mass fractions (Figure 11, left bottom). Aiming at the formulation of a highly viscous polymerizable microemulsion, the mass fraction of C6MA is further increased to β = 0.80 (Figure 11, left bottom to right top). Again, the phase boundaries are shifted to lower temperatures and slightly to lower surfactant mass fractions. This shift of the phase boundaries is compensated by increasing the mass fraction of the hydrophilic surfactant Agnique PG 264-G to δPG = 1; i.e., the system contains only the Agnique PG 264-G surfactant (Figure 11, top right). Comparing the shape of the phase boundaries found for this system with the other microemulsion systems studied so far, it becomes obvious that the lower phase boundary does not increase in temperature with decreasing surfactant concentrations. It even slightly decreases. The reason for this behavior is the increasing dominance of liquid crystalline phases induced by the long-chain surfactant Agnique PG 264-G. Thus, instead of the coexistence between an oil-inwater microemulsion and an oil excess phase (2) found for ternary water−n-alkane−CiEj systems,49,56−59 an extended

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CONCLUSION Because of their multivarious complex fluid structures on the nanoscale microemulsions are recognized to be promising templates for the synthesis of nanomaterials. However, up to now, a one-to-one copy of the nanostructure to the polymeric material could not be obtained. Structural changes occurring 134



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during the polymerization process are mainly caused by the continuous increase of the polymer/monomer ratio, which results in a changing composition of the microemulsion and, therewith, its properties. To slow down the reorganization kinetics of the microemulsion system, water is systematically replaced with a sugar/water mixture. In this publication we show that by increasing the mass fraction of sugar in the water/ sugar mixture the phase boundaries are shifted systematically to lower temperatures when nonionic pure or technical-grade surfactants are used. This shift is primarily caused by the hydration of the sucrose molecules. Hence, less water molecules are left to hydrate the headgroups of the surfactant. This decrease of the effective surfactant headgroup volume results in a decrease of the spontaneous curvature of the surfactant film and a shift of the phase boundaries to lower temperatures. Aiming at the formulation of highly viscous microemulsions this shift is in a first approach compensated using the hydrophilic sugar surfactant C8G1 and the technicalgrade sugar surfactants Agnique PG 8105-G and Agnique PG 264-G. In an alternative approach the sugar-induced shift of the phase boundaries to lower temperatures is compensated by the partial replacement of the nonionic alkyl polyglycolether surfactants (CiEj) by rather hydrophilic ionic surfactants, as e.g. sodium-bis-ethylhexylsulfosuccinate (AOT). For the preparation of highly viscous, polymerizable microemulsions the oil octane has been replaced with the monomer methyl 2methylpropenoate (C6MA). These viscous, polymerizable microemulsions were stabilized using the long-chain sugar surfactant Agnique PG 264-G. Besides a one-phase microemulsion regime, the recorded phase diagrams are dominated by an extended multiphase region containing a lamellar phase. In first studies we could show that these microemulsions are indeed appropriate to synthesize highly porous polymeric materials and monodisperse latexes in the nanometer range.61

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 711 685 64494. Fax: +49 711 64443. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Michael Klostermann for his help in investigating the influence of sugar on the phase behavior for systems containing technical grade surfactants.

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REFERENCES

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Sugar-Based Microemulsions as Templates for Nanostructured

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