Microemulsions Supported by Octyl Monoglucoside and Geraniol. 2

cyclohexane-octyl monoglucoside (β-C8G1)-geraniol (trans-3,7-dimethyl-2 ... With increasing δ the ratio of geraniol to β-C8G1 molecules in the mixe...
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Langmuir 1998, 14, 6005-6012

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Microemulsions Supported by Octyl Monoglucoside and Geraniol. 2. An NMR Self-Diffusion Study of the Microstructure Cosima Stubenrauch and Gerhard H. Findenegg* Iwan-N.-Stranski-Institut fu¨ r Physikalische und Theoretische Chemie, Technische Universita¨ t Berlin, Strasse des 17. Juni 112, D-10623 Berlin, Germany Received April 23, 1998. In Final Form: July 8, 1998 The self-diffusion coefficients of all components of the quaternary microemulsion system watercyclohexane-octyl monoglucoside (β-C8G1)-geraniol (trans-3,7-dimethyl-2,6-octadien-1-ol) have been measured along a well-defined linear path in the composition space, using the NMR pulsed-gradient spin-echo technique. The results have been used to derive information about the microstructure of the microemulsion and to calculate the composition (proportion of alcohol to surfactant molecules) of the mixed interfacial film separating the microscopic oil and water domains in the microemulsion. For the present system a phase inversion 2-3-2 h occurs at a 1:1 oil-to-water volume ratio and constant overall mass fraction of surfactant plus alcohol (γ ) 0.05) by increasing δ, the relative mass fraction of alcohol in the amphiphile mixture (surfactant plus alcohol). For δ e 0.25 the relative self-diffusion coefficient Drel of water is about 2 orders of magnitude greater than Drel of oil, indicating an oil-in-water (o/w) droplet microemulsion; for δ g 0.50 an opposite behavior of Drel for oil and water is found, indicating a water-in-oil (w/o) droplet microemulsion. For δ values between 0.30 and 0.45, Drel of water and oil exhibit a pronounced opposite dependence on δ, with a crossover near the middle of the three-phase body (δ ≈ 0.40), where Drel is about 0.47 for both oil and water. This behavior implies that the transition from an o/w- to a w/o-droplet microemulsion proceeds via a bicontinuous microstructure with oil and water domains separated by a mixed interfacial amphiphile film of low mean curvature. The composition of this film has been calculated as a function of δ by two mutually independent methods for the droplet regime and the bicontinuous regime of the microemulsion, respectively. With increasing δ the ratio of geraniol to β-C8G1 molecules in the mixed film increases from about 1:5 at δ ) 0.15 (o/w domain) up to 1:1 at δ ) 0.60 (upper limit of w/o domain), amounting to about 1:2 for the state of the balanced microemulsion. These results demonstrate in a quantitative manner how the composition of the mixed interfacial film affects the structure of the oil and water domains in a quaternary microemulsion system.

1. Introduction In microemulsion systems based on alkyl glucoside (CnGm) surfactants, a phase inversion can be induced by adding a cosurfactant such as an alcohol,1-3 an alkyl glycerol ether,4 or a sorbitan ester5a as a fourth component. When the amount of added cosurfactant is increased, an oil-in-water droplet microemulsion in contact with an excess oil phase (two-phase region denoted as 2) can transform into a water-in-oil droplet microemulsion in contact with an excess water phase (two-phase region denoted as 2h . This transition commonly proceeds via a bicontinuous microemulsion in contact with both excess phases, oil and water (three-phase region denoted as 3). It is believed that the addition of the cosurfactant to the ternary system water-oil-surfactant changes the hydrophile-lipophile balance of the system, which in turn causes a change of the microstructure of the microemulsion. Recently, it has been shown that the addition of * To whom correspondence should be addressed. Fax: +493031426602. E-mail: [email protected]. (1) (a) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 3382. (b) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1996, 12, 861. (2) Stubenrauch, C.; Kutschmann, E.-M.; Paeplow, B.; Findenegg, G. H. Tenside Surfactants Deterg. 1996, 33, 237. (3) Stubenrauch, C.; Paeplow, B.; Findenegg, G. H. Langmuir 1997, 13, 3652. (4) Fukuda, K.; So¨derman, O.; Lindman, B.; Shinoda, K. Langmuir 1993, 9, 2921. (5) (a) Nickel, D.; Fo¨rster, T.; v. Rybinski, W.; (b) Tesman, H.; Kahre, J.; Hensen, H.; Salka, B. A. In Alkyl Polyglycosides; Hill, K., v. Rybinski, W., Stoll, G., Eds.; VCH: Weinheim, 1997; Chapters 3 and 4.

alkyl polyglycol ether (CnEm) surfactants as the fourth component also tunes the hydrophile-lipophile balance in the desired way.6 A well-established method to detect changes of the microstructure of microemulsions is the measurement of self-diffusion coefficients by NMR techniques.7 On the time scale of NMR experiments one is concerned with molecular self-diffusion over macroscopic distances of the order of micrometers (i.e., distances much larger than the size of microemulsion droplets). Therefore, the experiment is not sensitive to molecular displacements within the droplets. Accordingly, in the case of an oil-in-water (o/w) microemulsion the monitored translation of the oil molecules in the droplet phase corresponds to the translation of the entire droplet. Droplet diffusion is much slower, typically 2 orders of magnitude, than the diffusion of the water molecules of the continuous aqueous phase. For this reason in droplet-type microemulsions the diffusion coefficients of oil and water will differ by typically 2 orders of magnitude. On the other hand, in a bicontinuous microemulsion the diffusion coefficients of oil and water are of the same order of magnitude. In this way the selfdiffusion coefficients of the solvent molecules (oil and water) give direct information concerning connectivity of the phase (i.e., unicontinuous and bicontinuous microstructures (discrete droplets or spongelike structures) can be distinguished). In addition, for droplet microemulsions (6) Ryan, L. D.; Schubert, K.-V.; Kaler, E. W. Langmuir 1997, 13, 1510. (7) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 344 and references given there.

S0743-7463(98)00463-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/15/1998

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Figure 1. Phase diagram of the quaternary system watercyclohexane-β-C8G1-geraniol at an 1:1 oil-to-water volume ratio (R ) 0.44) as a function of γ and δ at 25 °C. The composition variable γ denotes the sum of the mass fractions of the surfactant + alcohol in the quaternary system and δ represents the mass fraction of the alcohol in the surfactant + alcohol mixture. The full symbols (b) indicate the composition of the samples for which the self-diffusion measurements have been made. (In the NMR measurements a mixture of C6H12 and C6D12 was used, corresponding to R ) 0.46 for a 1:1 oil-to-water volume ratio; see section 2.2.)

information about the composition of the mixed interfacial film between water and oil domains can be derived from the self-diffusion coefficients. To understand this point, note again that the value of the self-diffusion coefficient of a molecule in the dispersed phase reflects the slow diffusion of the aggregate itself whereas the self-diffusion of a molecule in the continuous phase will be fast. If the molecules of one component are partitioned between the dispersed phase and the continuous phase, and if the exchange between these two states is fast compared to the observation time, the experimental self-diffusion coefficient will represent a weighted average of the different environments which the molecules experience during the observation time. If the self-diffusion coefficients of the molecules in the droplet phase and in the continuous phase are known, the partition of the molecules between the two states can be calculated. Thus, for a four-component system like the present microemulsion system, the composition of the mixed interfacial film can be calculated by analyzing the partition of both the surfactant and the alcohol between the film and the continuous phase. This paper forms part of a projet to study the structure and properties of the quaternary microemulsion system water-cyclohexane-β-C8G1-geraniol. Systems containing alkyl glucosides or alkyl polyglucosides as the surfactant are of interest, for instance, for formulations in cosmetic products in view of their favorable dermatological properties and low toxicity.5b Geraniol, a monoterpene alcohol (C10H17OH) represents one of the most frequently used perfume oils. This alcohol was chosen as a cosurfactant in this study in view of the growing interest in nontoxic microemulsions.1,5 In a recent paper3 (to which we shall refer as Part 1 of this series) the contour of the three-phase body of this four-component microemulsion system was mapped in terms of the variables δ (mass fraction of alcohol in the binary amphiphile mixture of surfactant plus alcohol) and γ (overall mass fraction of the binary amphiphile mixture in the quaternary system). Figure 1 shows the region of three-phase coexistence (3) and the two-phase regions 2 and 2 h at a 1:1 oil-to-water volume ratio plotted in terms of the variables γ and δ. As was discussed in Part 1, the shift of the three-phase body toward higher δ for decreasing γ is a consequence of the high solubility of the alcohol in the excess oil phase.

Stubenrauch and Findenegg

Moreover, the composition of the mixed interfacial film separating oil and water domains in the microemulsion phase was calculated for the balanced microemulsion (i.e., the state of zero mean curvature H of the mixed interfacial film), using the formalism developed by Kunieda et al.,8 taking into account the different solubilities of surfactant and alcohol in the excess phases. It was found that the mole fraction of alcohol in the mixed interfacial film of the balanced middle-phase microemulsion amounts to ca. 0.28, corresponding to a ratio of geraniol-to-β-C8G1 molecules of about 1:2.5. In the present paper we report results of a NMR selfdiffusion study of this system. The self-diffusion coefficients of all components of the four-component system were measured in the respective microemulsion phase along a path corresponding to the phase inversion 2-32 h . From the variation of the self-diffusion coefficients of oil and water we can monitor the structural changes in the microemulsion along this path. In addition, from the self-diffusion coefficients of all four components we have determined the composition of the mixed interfacial film in the domain of the droplet microemulsions. Together with the results of Part 1 it was thus possible for the first time to quantify the alcohol-to-surfactant ratio in the mixed interfacial film along the entire phase inversion of a four-component microemulsion system. 2. Experimental Section 2.1. Materials. β-Octyl monoglucoside (denoted as β-C8G1; molar mass M ) 292.37; 99.3% purity by GC), a white crystalline powder, was purchased from Calbiochem-Novabiochem GmbH (Bad Soden, Germany); geraniol (trans-3,7-dimethyl-2,6-octadien1-ol; M ) 154.25; 98% purity) was purchased from Aldrich-Chemie (Steinheim, Germany), cyclohexane (>99% purity) from Riedelde Hae¨n (Seelze, Germany), and cyclohexane-d12 (isotopic purity 99.7%, purity >98%) from Cambridge Isotope Laboratory (Andover, MA). These materials were used without further purification. The water used in this study was distilled and passed through a Milli-Q pure-water system. 2.2. Preparation and Composition of the Microemulsions. The samples were prepared by weight in Teflon-sealed graduated glass tubes and allowed to equilibrate at 25 °C in a water bath for at least 1 week. To characterize the overall composition of the samples, we use the notation introduced by Kahlweit and co-workers.9 Thus, the mass fraction of oil in the binary system water(A)-oil(B) is denoted by the symbol R ) B/(A + B), the mass fraction of the amphiphile mixture, viz., surfactant (C) plus alcohol (D), in the quaternary system by the symbol γ ) (C + D)/(A + B + C + D), and the mass fraction of alcohol in the amphiphile mixture by δ ) D/(C + D). In the NMR measurements a mixture of protonated and deuterated cyclohexane (20 wt % C6H12, 80 wt % C6D12) was used to reduce the signal intensity in an appropriate way. At 25 °C the density of this isotope mixture is 0.862 × 103 kg m-3. NMR measurements were made for a 1:1 cyclohexane-to-water volume ratio, corresponding to a mass ratio R ) 0.46 for this isotope mixture. The influence of the isotope substitution on the phase diagram along the chosen path (Figure 1) was negligible. Parts of the equilibrated phases were removed from the sample tubes for NMR measurements and analysis. The microemulsion phase was transferred to NMR glass tubes (L ) 5 mm), which were then evacuated, refilled with N2, and flame-sealed. The compositions of the microemulsions (see Table 1) were determined indirectly from the phase volumes of the coexisting phases and from the densities and compositions of the excess phases. The compositions of the excess phases were determined by HPLC. (8) (a) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (b) Kunieda, H.; Yamagata, M. Colloid Polym. Sci. 1993, 271, 997. (c) Kunieda, H.; Yamagata, M. Langmuir 1993, 9, 3345. (d) Kunieda, H.; Nakano, A.; Pes, M. A. Langmuir 1995, 11, 3302. (e) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78. (9) Kahlweit, M.; Strey, R. J. Phys. Chem. 1987, 91, 1553.

NMR Self-Diffusion Study of the Microstructure

Langmuir, Vol. 14, No. 21, 1998 6007

Table 1. Composition (wt %) of the Equilibrated Microemulsions for r ) 0.46 and γ ) 0.05, T ) 25 °Ca δ

H 2O

C6D12/C6H12

β-C8G1

geraniol

0.15 0.20 0.25 0.30b 0.35 0.40 0.45 0.50 0.55 0.60c

83.6 82.7 81.8 / 49.7 39.7 18.4 7.9 4.0 /

8.9 9.8 11.2 / 33.1 41.5 63.4 83.1 86.6 90.2

6.9 6.5 6.0 / 13.9 13.8 12.0 4.4 4.0 3.7

0.7 0.9 1.0 / 3.3 5.0 6.3 4.7 5.4 6.1

δ

a The composition of the microemulsions was determined indirectly from the phase volumes of the coexisting phases and from the densities and compositions of the excess phases. b As two microemulsion phases coexist with an excess oil phase, the compositions of the microemulsions could not be calculated with the available data. c No water is solubilized in the oil phase; the aggregates consist of β-C8G1 and geraniol.

The concentration of surfactant in the excess water phases was measured using a Nucleosil 300 C-8 (125 × 4 mm) column and a mixed eluent consisting of 60% methanol and 40% water.10a The concentration of geraniol and cyclohexane in the excess water phase was below the limit of detectability (differential refractive index detector). The alcohol concentration in the excess oil phase was determined using an Eurospher 100 C-18 column (250 × 4 mm) and a mixed eluent consisting of 85% acetonitrile and 15% water10b (detection: UV at 268 nm). The solubility of β-C8G1 and water in cyclohexane was neglected. Furthermore, it was assumed that the solubility of geraniol in protonated cyclohexane (which was used in the HPLC analysis samples) is equal to its solubility in the C6H12/C6D12 mixture. 2.3. NMR Self-Diffusion Measurements. The self-diffusion measurements were carried out on an AM360 NMR spectrometer (Bruker) operating at 360 MHz for protons, which was equipped with a field gradient unit (BGU gradient unit Z, H 3856, Bruker) capable of performing gradients up to 0.5 T m-1 at a current of 10 A. The regular spin-echo technique and the stimulated echo with two pulsed magnetic field gradients,11 separated by a constant time interval ∆, were employed to measure self-diffusion coefficients. The echo amplitude recorded at 2τ is given by

A2τ ) A0 exp(-γ2Gz2δ2D(∆ - δ/3))

(1)

In eq 1, A0 is the echo amplitude at 2τ in the absence of the gradient pulses, γ the magnetogyric ratio for protons, Gz the gradient strength, δ the duration of the gradients, and D the self-diffusion coefficient. A2τ was measured as a function of Gz in the range 0.05-0.5 T m-1. The strength of the applied field gradients was calibrated by reference measurements with pure cyclohexane (D0(C6H12) ) 14.3 × 10-10 m2 s-1 12). The accuracy of the self-diffusion coefficients was within (4%. The evaluation of the self-diffusion coefficients of water D(H2O) requires special consideration as the surfactant and the alcohol contain exchangeable protons.13 The observed self-diffusion coefficient of the OH protons represents a sum of three terms:

Dexp(OH) ) 2

Table 2. Experimental Self-Diffusion Coefficients Dexp(OH), Dexp(C6H12), Dexp(C8G1), and Dexp(ger) and Self-Diffusion Coefficients of Water D(H2O) As Derived from Equation 2 (for δ ) 0.15-0.45), or Using D(H2O) ) Dexp(C8G1) (for δ ) 0.50 and 0.55)a

X(H2O) X(ger) D(H2O) + Dexp(ger) + N N X(C8G1) 4 Dexp(C8G1) (2) N

(10) (a) Lafosse, M.; Marinier, P.; Joseph, B.; Dreux, M. J. Chromatogr. 1992, 623, 277. (b) Morin, P.; Caude, M.; Richard, H.; Rosset, R. Analysis 1985, 13, 196. (11) Stilbs, P. Prog. NMR-Spectrosc. 1987, 19, 1. (12) Holz, M.; Weinga¨rtner, H. J. Magnet. Reson. 1991, 92, 115. (13) (a) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1983, 87, 4756. (b) Cheever, E.; Blum, F. D.; Foster, K. R.; Mackay, R. A. J. Colloid Interface Sci. 1985, 104, 121. (c) Gue´ring, P.; Lindman, B. Langmuir 1985, 1, 464.

0.15 0.20 0.25 0.30, mb 0.30, ub 0.35 0.40 0.45 0.50 0.55 0.60c

Dexp(OH) D(H2O) Dexp(C6H12) Dexp(C8G1) Dexp(ger) 19.0 18.8 18.8 15.5 21.2 11.0 8.79 5.22 0.700 0.518 0.670

19.2 19.0 19.0 15.8 21.0 11.5 9.20 5.68 0.102 0.141

0.242 0.193 0.148 1.25 0.174 3.37 5.17 7.97 11.5 12.1 11.8

0.566 0.476 0.510 0.303 / 0.324 0.341 0.277 0.102 0.141 0.206

0.323 0.295 0.243 0.427 / 0.906 1.39 2.12 4.44 4.55 4.86

a D in units of 10-10 m2 s-1 (25 °C). b D values of middle (m) and lower (l) microemulsion phase; concentrations of β-C8G1 and geraniol in the lower phase were too small to measure Dexp(C8G1) and Dexp(ger). c No water is solubilized in the oil phase; the aggregates consist of β-C8G1 and geraniol.

with N ) 2X(H2O) + X(ger) + 4X(C8G1) where X is the mole fraction of the corresponding component in the mixture. D(H2O) was calculated according to eq 2 from the experimental values Dexp(OH), Dexp(C8G1), and Dexp(ger), and from the known mole fractions of the components in the microemulsion phase. This procedure was applied in a range of relative concentrations of geraniol from δ ) 0.15 to δ ) 0.45, δ denoting the mass fraction of alcohol in the amphiphile mixture. For the samples with δ ) 0.50 and 0.55 we found that the OH signal is mainly due to geraniol and thus we did not use eq 2 to calculate D(H2O). Instead, D(H2O) was equated with Dexp(C8G1), assuming noninteracting w/o droplet microemulsions. Strictly, this will hold only if water and the surfactant both were insoluble in the oil phase. In reality, the solubilities of the two components are low but nonzero, but even small concentrations of water or monomeric surfactant in the oil phase could affect the self-diffusion coefficient in a strong way (see section 3.1). For the sample with δ ) 0.60 no contribution of water to the OH signal was found, applying eq 2. Thus, at this high alcohol concentration the aggregates appear to consist of surfactant and alcohol only (i.e., no significant amount of water is solubilized in the oil phase at δ ) 0.60). The experimental self-diffusion coefficients, Dexp(OH), Dexp(C6H12), Dexp(C8G1), and Dexp(ger), and the resulting values for D(H2O) are summarized in Table 2. The self-diffusion coefficients of monomeric β-C8G1 and nonaggregated geraniol in water, Dfree(C8G1) and Dfree(ger), were obtained from experiments in D2O. In a solution of 0.5 wt % β-C8G1 in D2O (cmc: 0.73 wt % in H2O), Dfree(C8G1) is 3.8 × 10-10 m2 s-1, in agreement with the value obtained by Nilsson et al. (3.64 × 10-10 m2 s-1 14). For geraniol in a D2O solution containing 0.4 wt % β-C8G1 and 0.4 wt % geraniol, Dfree(ger) ) 4.5 × 10-10 m2 s-1 was obtained. (Note that the solubility of geraniol in water was neglected in section 2.2. However, the fact that the self-diffusion coefficient of geraniol in the o/w microemulsions is somewhat higher than the corresponding value for cyclohexane implies that a small amount of geraniol must be dissolved in water in the o/w microemulsion. This amount does not affect the calculated compositions of the microemlusions but has an appreciable effect on the self-diffusion data which are very sensitive to fast monomer diffusion.) The self-diffusion coefficient of nonaggregated geraniol in oil, Dfree(ger) ) 7.3 × 10-10 m2 s-1, was obtained from experiments on a 3 wt % geraniol solution in C6D12/C6H12. This concentration has been chosen as it represents the upper limit of the geraniol concentration in the excess oil phase (HPLC measurements), assuming that the geraniol concentration in the oil microdomains does not exceed that in the excess oil phase. For most of the self-diffusion coefficients reported in this work the experimental uncertainty is within 4%. The values of Dexp(C8G1) were obtained by analyzing the signal at 0.9 ppm (14) Nilsson, F.; So¨derman, O. Langmuir 1996, 12, 902.

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Stubenrauch and Findenegg

(methyl group of the alkyl chain). As a mixture of protonated and deuterated cyclohexane was used (see section 2.2), the signal intensities of the chosen surfactant protons were sufficient in the entire experimental concentration range. The values of Dexp(ger) were obtained by analyzing the signal at 4.15 ppm (methylene group next to the -OH group). However, due to the low intensity of this signal, the methyl group signals at 1.601.68 ppm had to be taken for all samples with δ < 0.30. As these methyl group signals of geraniol are overlapping somewhat with the methylene groups of the surfactant, the accuracy of the Dexp(ger) values was only within 10% for samples with δ < 0.30. The signals of the two solvents are at 4.8 ppm (OH protons) and 1.44 ppm (cyclohexane), respectively.

3. Results 3.1. General Considerations. The following general remarks pertain to the subsequent data analysis. (i) One of the aims of self-diffusion measurements for microemulsions is to determine the nature of its microstructure (i.e., to distinguish between discrete (unicontinuous) and bicontinuous structures). For this purpose it is appropriate to express self-diffusion data of its main components, oil and water, in terms of the relative self-diffusion coefficients:

Drel )

D D0

(3)

where D0 denotes the self-diffusion coefficient in the pure solvent and D the corresponding value of the solvent in the microemulsion. If Drel of water and oil differ by more than 1 order of magnitude, this result implies discrete particles (droplets) of the slowly diffusing solvent. On the other hand, if the values of Drel of the two solvents are of the same order of magnitude, a bicontinuous structure is suggested. Thus, the self-diffusion coefficients of water and oil are related directly to the microstructure of the microemulsion and allow one to distinguish between droplet and bicontinuous phases. (ii) The experimental self-diffusion coefficients Dexp represent a weighted average of the different environments which the molecules experience during the observation time. Thus, if the molecules are partitioned between the interfacial film (aggregated molecules) and the continuous phase (nonaggregated molecules), Dexp is related to the self-diffusion coefficients of the aggregated and the nonaggregated molecules, Dagg and Dfree, respectively, by13a

Dexp ) pfreeDfree + paggDagg

(4)

where pfree and pagg denote the fractions of molecules in the nonaggregated and aggregated state. If Dexp, Dfree, and Dagg are known, eq 4 can be used to calculate pfree and pagg ) 1 - pfree as well as the respective distribution coefficient K ) pfree/pagg. Knowing these fractions for the surfactant and the alcohol, the composition of the mixed interfacial film can be calculated. (iii) Self-diffusion studies can also be made to investigate the shape and size of the microemulsion droplets. For this purpose, the self-diffusion coefficients of the solvents have to be measured as a function of the volume fraction Φ of the droplets without changing their size and shape. However, due to the different monomeric solubilities of surfactant and alcohol in the two solvents, it is difficult to determine the corresponding dilution lines in quaternary systems of the present type so that we did not determine the self-diffusion coefficients along these dilution lines yet. 3.2. Discrete Droplets or Bicontinuous Structures? The self-diffusion coefficients of water, cyclohex-

Figure 2. Relative self-diffusion coefficients, Drel ) D/D0, of water and oil (cyclohexane) as a function of the composition variable δ at R ) 0.46 and γ ) 0.05, T ) 25 °C (the symbols R, γ, and δ are explained in Figure 1). The vertical dotted lines indicate the limits of the three-phase region at the chosen values of R and γ. The dashed lines connecting the Drel values of water and oil, respectively, are drawn to guide the eye.

ane, β-octyl monoglucoside (β-C8G1), and geraniol were measured as a function of δ, the relative concentration of alcohol in the binary amphiphile mixture, for a constant overall mass fraction of the amphiphile mixture in the system (γ ) 0.05) and a constant oil-to-water ratio (R ) 0.46) at T ) 25 °C. The overall sample compositions along the chosen path are indicated by the full symbols in Figure 1. The compositions of the corresponding microemulsion phases along this trajectory are summarized in Table 1; numerical values of the self-diffusion coefficients are given in Table 2. (Note that all data refer to the microemulsion phase and not to the coexisting excess phases.) To calculate the appropriate relative self-diffusion coefficients Drel, corrections have to be made to account for effects on D other than the microstructure. To account for the hydration effect,13a it was assumed that each glucose unit is hydrated by six water molecules.14 Thus, neglecting the hydration of the alcohol and using eq 4, the water diffusion hindrance due to the hydration effect can be deduced from the relation

D(H2O) ) pfreeDfree(H2O) + phydrDhydr

(4a)

where pfree and phydr, the fractions of free water and hydration water, can be expressed by the relative amounts of β-C8G1 and water in the microemulsion phase (phydr ) 6n(C8G1)/n(H2O) ) 1 - pfree). D(H2O) is the average selfdiffusion coefficient of water in the microemulsion (calculated according to eq 2), Dfree(H2O) is the self-diffusion coefficient of free water in the microemulsion, and Dhydr refers to the molecules to which the water is bound (i.e., Dhydr ) Dexp(C8G1)). The resulting values Dfree(H2O) are expected to differ from D0(H2O), the self-diffusion coefficient of pure water, only due to the microstructure of the microemulsion. Accordingly, Drel ) Dfree(H2O)/D0(H2O), with D0(H2O) ) 22.99 × 10-10 m2 s-1 at 25 °C.15 The relative self-diffusion coefficients Drel of water and cyclohexane as a function of the alcohol content δ are plotted in Figure 2. Note that the corrections due to hydration (15) Mills, R. J. Phys. Chem. 1973, 77, 685.

NMR Self-Diffusion Study of the Microstructure

have to be taken into account only for the samples with δ between 0.15 and 0.45. In the region of the w/o microemulsions (i.e., for the samples with δ ) 0.50 and 0.55) water forms the droplet phase. Thus, assuming that neither H2O nor β-C8G1 are soluble in oil, we have D(H2O) ) Dexp(C8G1); division by D0(H2O) leads to the values Drel(H2O) shown in Figure 2. For δ ) 0.60 there is no solubilized water in the oil phase (see section 2.3.) (i.e., no stable microemulsions exist at such a high alcohol content). We return to this point later. Besides the hydration effect of the polar headgroup of the surfactant discussed above one also has to consider effects due to the solvation of the alkyl chain of the surfactant by the oil. Although an analysis of this effect is still lacking, it is commonly neglected on the argument that the interaction of the hydrocarbon tails of surfactant molecules with the oil molecules is weak compared to the effect of the polar headgroups on the self-diffusion of water. Therefore, the experimental self-diffusion coefficients of oil have not been corrected in the present work. The Drel values of oil shown in Figure 2 were calculated with the experimental Dexp(C6H12) values presented in Table 2 using D0(C6H12) ) 12.8 × 10-10 m2 s-1 for 25 °C, as determined experimentally for the self-diffusion coefficient of C6H12 in the C6H12/C6D12 mixture. As seen in Figure 2, the Drel values of water and cyclohexane both exhibit a pronounced dependence on δ: at low alcohol content (δ < 0.30) the water diffusion is faster than the oil diffusion by 2 orders of magnitude whereas at high alcohol content (δ > 0.45) the situation is reversed. This result implies structures consisting of discrete particles of the slowly diffusing solvent (i.e., o/w droplet microemulsions at low δ and w/o droplet microemulsions at high δ). At the intersection point of the two Drel curves at an intermediate alcohol content of δ ≈ 0.40, high Drel values of about 0.47 are observed for both oil and water. Around this δ, the diffusion behavior appears to be symmetric with respect to δ. Such a behavior is expected for the occurrence of a bicontinuous structure.7 The comparatively high Drel values observed in this regime reflect a structure for which the diffusion of water and oil molecules over macroscopic distances is not strongly hindered. Moreover, at the intersection point of the two Drel curves the structure of the water and oil domains must be equivalent. These findings strongly indicate that the microemulsion phase at this composition has a bicontinuous structure and thus represents a balanced microemulsion. Models for bicontinuous microemulsions invoke the concept of constant mean curvature surfaces (so-called H surfaces).7,16 In terms of these models a theoretical upper limit for Drel in a bicontinuous phase is 2/3 for both water and oil, corresponding to a minimal surface of zero mean curvature (H ) 0). In fact, for a number of systems Drel values of approximately 2/3 at the intersection point have been found,4,17,18 but somewhat lower values (about 0.5) have also been reported.19-21 Deviations of Drel from the theoretical upper limit of 2/3 have been attributed to defects in the structure (i.e., deviations from zero mean curvature16,18,19). In particular, (16) Anderson, D. M.; Wennerstro¨m, H. J. Phys. Chem. 1990, 94, 8683. (17) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90, 4083. (18) Lindman, B.; Shinoda, K.; Jonstro¨mer, M.; Shinohara, A. J. Phys. Chem. 1988, 92, 4702. (19) Olsson, U.; Nagai, K.; Wennerstro¨m, H. J. Phys. Chem. 1988, 92, 6675. (20) Shinoda, K.; Araki, M.; Sadaghiani, A.; Khan, A.; Lindman, B. J. Phys. Chem. 1991, 95, 989. (21) Carnali, J. O.; Ceglie, A.; Lindman, B.; Shinoda, K. Langmuir 1986, 2, 417.

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local tubular structures are discussed as defects significantly reducing Drel. For the present system the intersection point of the two Drel curves is at Drel ≈ 0.47 (i.e., significantly lower than 2/3). Accordingly, it is concluded that the bicontinuous structure of the present system represents a structure of low mean curvature (nonideal structure of zero mean curvature). As any irregularities in the structure, such as restricting necks, cause a significant reduction of the self-diffusion coefficients, the proposed description of the microstructure is reasonable. At the boundaries of the three-phase coexistence the values Drel of water and oil indicate neither discrete droplets nor bicontinuous structures. In these transition regions the diffusion processes and thus the values Drel cannot be interpreted unambiguously. Moreover, the presented measurements of Drel yield no quantitative information about the size and shape of the aggregates. However, a comparison of the present findings with results for ternary microemulsion systems of the type water-oil-CnEm yields a consistent picture of the behavior of our system. In Part 13 it was shown that the role of temperature in these ternary systems corresponds to the role of the alcohol content in quaternary systems of the present type. By varying either the temperature or the alcohol content amount, one can tune the curvature of the mixed interfacial film which is the dominating parameter for the phase diagram and the microstructure. Ideally, in a ternary system at the so-called HLB temperature T h, the microemulsion phase contains equal volumes of water and oil and its structure is bicontinuous. For the present system the corresponding balanced state is reached at δ ≈ 0.40, as discussed above. For the systems wateroctane-C12E522 and water - decane-C12E5,23 it has been shown that the transition from microemulsion droplets to bicontinuous structures occurs via tubular, locally cylindrical structures (i.e., via an anisometric growth of the droplets). In view of the similarities between these threecomponent systems and the present system, we conjecture the existence of anisometric aggregates in the transition regions 2-3 and 3-2 h . Incidentally, NMR self-diffusion measurements for the system water-cyclohexaneC12G1.8-octylglycerolmonoether published by Fukuda et al.4 are in qualitative agreement with the present results. Although no detailed structural investigations have been performed in that study, these authors also presume an anisometric growth of the microemulsion droplets as the region of bicontinuous structures is approached. To close this section, we briefly discuss the findings for the sample with δ ) 0.30. This sample represents a state of three-phase coexistence (3) close to the border 2-3, and thus the water-rich phase may still be considered as a microemulsion. In Figure 2 we show values of Drel for both the middle phase and the water-rich phase of this sample. The Drel values of water and oil in the middlephase microemulsion differ by ca. 1 order of magnitude as expected for a transition from discrete droplets to a bicontinuous structure; on the other hand, the values of Drel for water and oil in the water-rich microemulsion of this sample differ by 2 orders of magnitude, implying discrete droplets of oil. Specifically, the high value of Drel for water (as compared with that in samples with δ between 0.15 and 0.25) can be rationalized by inferring that the volume fraction of oil (i.e., the concentration of oil droplets) is quite small in that phase. From the coexistence of two microemulsion phases, it can be deduced (22) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (23) (a) Olsson, U.; Schurtenberger, P. Langmuir 1993, 9, 3389. (b) Leaver, M.; Olsson, U.; Wennerstro¨m, H.; Strey, R. J. Phys. II 1994, 4, 515. (c) Leaver, M.; Furo´, I.; Olsson, U. Langmuir 1995, 11, 1524.

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Figure 3. Composition of the mixed interfacial film in terms of the ratio of alcohol-to-surfactant molecules, n(ger)/n(C8G1), as a function of δ along the path indicated in Figure 1. These results are derived on the basis of (i) the self-diffusion coefficients (9) and (ii) mass balances based on the composition of the excess phases (b). The vertical dotted lines indicate the limits of the three-phase region at the chosen values of R and γ. The dashed line connecting the n(ger)/n(C8G1) values is drawn to guide the eye.

that for the chosen R and γ the lower critical end point of the three-phase region must be located near δ ) 0.30. (At a critical end point at which the lower and middle phase become identical in the presence of a third phase, the self-diffusion coefficients of the components in the two critical phases must coincide.) However, we did not attempt to map out the critical end points of the present system. 3.3. Composition of the Mixed Interfacial Film. The composition of the mixed interfacial amphiphile film separating the oil and water domains was determined from the partition of surfactant and alcohol between the interface and the microdomains of the microemulsion. Assuming that the microdomains have the same composition as the respective excess water and oil phases (see section 2.2.), the composition of the mixed interfacial film within the middle-phase microemulsion can be calculated.8 However, in the case of the o/w and w/o droplet microemulsions, the amount of surfactant and alcohol contained in the continuous microemulsion phase cannot be measured directly or derived from the concentration of the excess phases. For this droplet regime the composition of the mixed interfacial film can be calculated from the measured NMR self-diffusion coefficients.24 In this section we discuss the procedure to obtain this information for the o/w and w/o droplet microemulsions. The compositions of the excess phases as well as those of the microemulsions, which are needed for this analysis, were determined by HPLC measurements, as described in section 2.2. (a) Oil-in-Water Droplet Microemulsions (0.15 e δ e 0.25). According to eq 4 the fractions of aggregated and nonaggregated surfactant and alcohol, pagg and pfree, can be calculated from the experimental self-diffusion coefficient Dexp if Dagg and Dfree are known. Assuming that the solubility of cyclohexane in water can be neglected, one has Dagg ) Dexp(C6H12). With pagg ) 1 - pfree and with the values for the nonaggregated species in water, Dfree(C8G1) ) 3.8 × 10-10 m2 s-1 and Dfree(ger) ) 4.5 × 10-10 m2 s-1 (see section 2.3.), the amounts of surfactant and alcohol in the mixed interfacial film can be calculated. The results are shown in Figure 3. (Note that even a small amount of oil dissolved in the continuous aqueous phase would lead to Dagg < Dexp(C6H12). For the present (24) Giustini, M.; Palazzo, G.; Colafemmina, G.; Della Monica, M.; Giomini, M.; Ceglie, A. J. Phys. Chem. 1996, 100, 3190.

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system, if the calculation of the amount of nonaggregated surfactant pagg in the continuous water phase were based on a value Dagg < Dexp(C6H12), this assumption would lead to pagg > cmc which is physically unreasonable. This result supports the assumption of a negligible solubility of the oil in the aqueous phase.) In the calculation of pagg for the alcohol one has to keep in mind that pagg represents the overall amount of alcohol connected with the aggregate, without discriminating between the mixed interfacial film and the interior of the oil microdomain. Strictly, the part of alcohol which is dissolved within the oil microdomain has to be subtracted from the overall amount of aggregated alcohol, to obtain that part which is incorporated in the mixed interfacial film. As the amount dissolved in the microdomains is not measurable, two alternative assumptions for the concentration of alcohol in the oil microdomains were made: (i) equal to the concentration in the excess oil phase; (ii) zero. These two model assumptions yield results differing by less than 10%, which is less than the error bars indicated in Figure 3. (b) Water-in-Oil Droplet Microemulsions (δ g 0.50). In the analysis of this regime we assume that the solubilities of water and β-C8G1 in cyclohexane are negligible, leading to Dagg ) D(H2O) ) Dexp(C8G1). Thus, for the surfactant one has pagg ) 1. Strictly, as discussed for the alcohol in the preceding paragraph (a), that part of the surfactant which is dissolved in the interior of the water microdomains should be subtracted from the overall amount of aggregated surfactant to obtain the part which is incorporated in the mixed interfacial film. However, for the present system this amount is negligible because the solubility of the surfactant in water and the volume fraction of the dispersed phase are both small. According to eq 4 the amount of alcohol incorporated in the film has been calculated using Dfree(ger) ) 7.3 × 10-10 m2 s-1 and pagg ) 1 - pfree. The resulting amount of nonaggregated alcohol in the continuous oil phase corresponds to 3.3 wt % (δ ) 0.50), 3.7 wt % (δ ) 0.55), and 4.3 wt % (δ ) 0.60). The results for the composition of the mixed interfacial film are shown in Figure 3. As already mentioned in section 2.3., the microemulsion is no longer stable at δ ) 0.60. The results for the self-diffusion coefficients of the surfactant and alcohol for this sample indicate that the aggregates consist almost solely of β-C8G1 and geraniol (possibly swollen with traces of water). Nevertheless, the composition of these aggregates can be calculated following the described procedure. In this way we can estimate the upper limit of alcohol content in the mixed interfacial film up to which the formation of microemulsions is possible. For the present system, the microemulsion appears to be stable up to a ratio of geraniol to β-C8G1 molecules of about 1:1. Increasing the amount of alcohol beyond this limit leads to inverse mixed micelles of β-C8G1 and geraniol in alcohol-saturated cyclohexane. (c) Middle-Phase Microemulsions (0.30 e δ e 0.45). We have shown that the composition of the mixed interfacial film can be calculated if Dfree and Dagg are known. In the case of droplet microemulsions the term due to the diffusion of the monomers can be separated from the term due to the diffusion of the aggregates, provided that a further component is present which is insoluble in the continuous phase and thus represents a probe for the aggregate diffusion. Although there exist no discrete aggregates in bicontinuous microemulsions, one has to distinguish between aggregated and nonaggregated molecules within the domains, the former being incorporated in the interfacial film between oil and water domains. The problem is that in the present system there are no probe molecules for the diffusion in the interface, and

NMR Self-Diffusion Study of the Microstructure

thus the composition of the mixed interfacial film cannot be calculated by the procedure described above. However, as mentioned in the Introduction, the composition of the mixed interfacial film in the three-phase region can be derived from the known compositions of the excess phases, assuming that the composition of the microdomains is equal to that of the respective excess phases. The results of these calculations are also shown in Figure 3. 4. Discussion The results of the present analysis yield a consistent picture for the composition of the mixed interfacial film and its variation as a function of the overall alcohol-tosurfactant ratio in the system. As shown in Figure 3, the molar ratio n(ger)/n(β-C8G1) in the mixed film increases with δ from about 1:5 (at δ ) 0.15) to 1:1 (at δ ) 0.60). Two independent methods have been used to derive this composition in the regime of droplet microemulsions and bicontinuous microemulsions, respectively, each relying on several simplifying assumptions. In view of this fact the mutual consistency of the two sets of data is remarkable. It appears that the composition of the mixed interfacial film in the microemulsion phase varies smothly with the content of alcohol in the amphiphile mixture. Recalling the results of section 3.2, concerning the structural changes as a function of δ, we may conclude that the increasing content of geraniol in the mixed interfacial film causes a decrease of the mean curvature of the mixed film from H > 0 in the o/w droplet microemulsion to H < 0 in the w/o droplet microemulsion. Intuitively, such a change in H may be attributed to the small headgroup area of geraniol. As shown in section 3.2., at the chosen 1:1 oil-to-water volume ratio and at the given overall concentration of the amphiphile mixture (γ ) 0.05), the balanced microemulsion is reached near δ ) 0.40. At this composition the molar ratio n(ger)/ n(β-C8G1) in the balanced mixed film is found to be ca. 1:2, in agreement with the rough estimate in Part 13, which had been obtained without corrections for the composition of the excess phases. As the alcohol content is further increased up to δ ) 0.60, the ratio of alcohol-to-surfactant in the mixed film becomes equimolar. However, at this point the oil phase does no longer represent a true microemulsion, as outlined in section 3.2. As mentioned above, a major source of error in determining the composition of the mixed interfacial film results from the partition of alcohol and surfactant between the mixed interfacial film and the interior of the domains in the microemulsion. As this partition cannot be measured directly, not much work has been devoted to this aspect so far. Giustini et al.24 determined the composition of the mixed interfacial film for w/o droplet microemulsions of the system water-hexane-CTAB-pentanol by means of NMR self-diffusion measurements, similar to the procedure described above. SANS measurements have also been performed for the determination of the mixed film composition, using film contrast techniques with deuterated components to enhance or vary the contrast appropriately.25,26 In accordance with the present work, Caponetti et al.25 showed for w/o microemulsions that an increase in δ leads to an increase in the alcohol content of the mixed film. Furthermore, these authors observed a decrease in the aggregate size with increasing δ. If these findings also apply to the w/o microemulsion regime of the present system (δ g 0.50), the aggregate size should (25) Caponetti, E.; Lizzio, A.; Triolo, R.; Griffith, W. L.; Johnson, J. S. Langmuir 1992, 8, 4. (26) Strey, R.; Jonstro¨mer, M. J. Phys. Chem. 1992, 96, 4537.

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increase as δ approaches the balanced state from the region 2 h . Such a behavior is consistent with the conjectured anisometric growth of the domains discussed in section 3.2. However, to our knowledge, it has not yet been possible to directly show a correlation between the structural changes of the microdomains and the composition of the mixed interfacial film. In the present work, for the first time such a correlation is presented for the entire range of the phase inversion (i.e., from the o/w to the w/o droplet regime). It has to be stressed that the present results form just a first step toward this goal. What is yet missing is a direct determination of the size of the microdomains and hence of the mixed film curvature H, and a quantitative relation between the mixed film composition and the curvature, similar to the wellestablished relation between the temperature and the curvature in ternary microemulsion systems.22 In both cases the spontaneous curvature determines both the microstructure and the phase behavior of microemulsionss the main difference between ternary and quaternary systems being the nature of tuning parameter. In ternary systems with CnEm amphiphiles the tuning parameter for the curvature is temperature. It has been shown by Strey22 that in a limited temperature range the curvature H is a linear function of temperature. A further important result is that the curvature, the domain size, and the interfacial tension are directly interrelated: A mean curvature H ) 0 at T ) T h corresponds to a maximum domain size and a minimum interfacial tension. In the present system, the tuning parameter of the curvature is the composition of the mixed interfacial film. Thus, it is appropriate to correlate the curvature H, the domain size ξ, and the interfacial tension σ with the composition of the mixed interfacial film or, if the composition is not known, with the parameter δ. In Part 13 we have shown that σ exhibits a minimum as a function of δ;27 accordingly, we expect a maximum for the domain size ξ as a function of δ. SANS measurements are planned to determine the domain size and the curvature H ) 1/ξ of the mixed film. 5. Conclusion The results of the present paper and those of Part 13 can be summarized as follows: (1) In the present quaternary microemulsion system with β-C8G1 as the surfactant and the alcohol geraniol as the cosurfactant, an increase of the composition variable δ (mass fraction of alcohol in the amphiphile mixture) at constant R (oilto-water mass ratio) and constant γ (overall mass fraction of the amphiphile mixture) causes a phase inversion of the type 2-3-2 h . On decreasing γ the three-phase body becomes distorted toward larger δ due to the different solubilities of β-C8G1 and geraniol in oil and water. (2) The macroscopic phase inversion 2-3-2 h reflects the transition of the microstructure from an oil-in-water to a water-in-oil droplet microemulsion via a bicontinuous structure. We monitor this transition by NMR selfdiffusion measurements of oil and water in the respective microemulsion phase. The relative self-diffusion coefficients Drel of oil and water exhibit a pronounced and opposite dependence on the variable δ: for δ e 0.25 the relative self-diffusion coefficient Drel of water is about 2 orders of magnitude greater than Drel of oil, indicating an oil-in-water (o/w) droplet microemulsion whereas for δ g (27) In Figure 2b of Part 13 the experimental data for the interfacial tension σ vs δ were connected by a line to guide the eye. The apparent discontinuity of σ at the border of the three-phase region at δ ) 0.5 is to be attributed to experimental errors, not to the proximity to a critical point as argued in ref 3. Such a discontinuity in σ(δ) is physically unreasonable.

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0.50 an opposite behavior of Drel for oil and water is found, indicating a water-in-oil (w/o) droplet microemulsion. At an intermediate alcohol content δ of about 0.40 the two Drel curves intersect at Drel ≈ 0.47. The comparatively high Drel value of the intersection point reflects a structure for which the diffusion of water and oil molecules over macroscopic distances is not strongly hindered. Moreover, at the intersection point of the two Drel curves the structure of the water and oil domains must be equivalent. These findings strongly indicate that the microemulsion phase at this composition has a bicontinuous structure and therefore represents a balanced microemulsion. Thus, the transition from an o/w- to a w/o-droplet microemulsion proceeds via a bicontinuous microstructure with the oil and the water domains separated by a mixed interfacial amphiphile film of low mean curvature. (3) The reason for the observed change of the microstructure is the change in the composition (i.e., in the ratio of alcohol-to-surfactant molecules) of the mixed interfacial film. To calculate this film composition for the droplet regimes, we measured the NMR self-diffusion coefficients of all four components of the system. For the regime of the bicontinuous microemulsion where the NMR data are not sufficient to determine the composition of

Stubenrauch and Findenegg

the interfacial film, the film composition was calculated from the partition of surfactant and alcohol between the film and the microdomains of the microemulsion, assuming that the microdomains have the same composition as the respective excess water and oil phases. These two independent methods for deriving the composition of the interfacial film lead to a consistent picture for the entire range of microemulsions: with increasing δ the ratio of geraniol to β-C8G1 molecules in the mixed film increases from 1:5 to 1:1. For the balanced state we obtain an interfacial composition of about 1:2. To our knowledge, these results for the first time demonstrate in a quantitative manner the way in which the composition of the mixed interfacial film affects the structure of the oil and water domains in a quaternary microemulsion system. Acknowledgment. The authors wish to thank M. Kowall and D. Ziessow for cooperation and advice in the NMR measurements and G. Hedicke and O. Dietsch for help with the HPLC measurements. Financial support by the Fonds der Chemischen Industrie is also gratefully acknowledged. LA9804637