Letter pubs.acs.org/Langmuir
Structural Evolution in the Isotropic Channel of a Water−Nonionic Surfactant System That Has a Disconnected Lamellar Phase: A 1H NMR Self-Diffusion Study Cosima Stubenrauch,*,† Felix Kleinschmidt,‡,§ and Claudia Schmidt∥ †
Universität Stuttgart, Institut für Physikalische Chemie, Pfaffenwaldring 55, 70569 Stuttgart, Germany Laboratoire de Physique des Solides, Batiment 510, Université Paris Sud, 91405 Orsay, France § Albert-Ludwigs-Universität Freiburg, Institut für Makromolekulare Chemie, Stefan-Meier-Str. 31, 79104 Freiburg, Germany ∥ Universität Paderborn, Department Chemie, Warburger Str. 100, 33098 Paderborn, Germany ‡
ABSTRACT: We showed in a previous study that a water− nonionic surfactant system, where the surfactant is a 9:1 mixture of tetraethylene glycol monodecyl ether (C10E4) and pentaethylene glycol monodecyl ether (C10E5), forms a disconnected lamellar (Lα) phase. Thus, the isotropic phase spans the whole concentration range from the water-rich L1 region to the surfactant-rich L2 region of the phase diagram. The L1 and L2 regions are connected via an isotropic channel that separates the two regions of the Lα phase. In this letter, we monitored the structural evolution of the isotropic phase along a path through this isotropic channel via 1H NMR selfdiffusion measurements. We used this technique because it enables us to distinguish between discrete and bicontinuous structures by comparing the relative self-diffusion coefficients (obstruction factors) D/D0 of the solvents (i.e. of water and surfactant in the present case). We found that the obstruction factor of water decreases whereas the obstruction factor of the surfactant increases with increasing surfactant concentration and increasing temperature. This trend is interpreted as the transition from a water-continuous L1 region, which contains discrete micelles, to a bicontinuous structure, which may extend to very high surfactant concentrations. Although there is good evidence of bicontinuity over a broad concentration range, there is no evidence of inverse micelles or any other microstructure at the highest concentration studied in the surfactant-rich L2 phase. two phases before the highly dilute Lα phase finally disappeared. An example is shown in Figure 1, where the phase diagram of the H2O−C10E4/C10E5 system is plotted as a function of the temperature T and the total surfactant concentration γ for a fixed surfactant mass ratio of 9:1 (δ = 0.10). From phase diagrams, 2H NMR, and SAXS measurements, we concluded (a) that the disconnected Lα phase is a general feature of nonionic surfactants, (b) that the disconnection, if it occurs, takes place only if the distance between two bilayers equals the thickness of the bilayers, and (c) that the disconnection is tunable by the rigidity of the monolayer. A topic that still has to be addressed is the structure of the isotropic region that separates the two Lα regions. We labeled it as 1 in our previous work, and it will be referred to as the intermediate isotropic region in the following text. In other words, we are talking about the structural evolution from the dilute (L1) to the concentrated (L2) region of the isotropic phase via the intermediate isotropic region between the two Lα
1. INTRODUCTION The formation of a disconnected lamellar (Lα) phase in water− nonionic surfactant systems was not discovered until 13 years ago.1−3 Wagner and Strey reported that the (CH3)3Si(CH2)6(OCH2CH2)5OCH3 surfactant stabilizes a lamellar phase from 15 up to 80 wt %. Surprisingly, two separate Lα regions were observed, one at concentrations of between 15 and 35 wt % and a second one between 60 and 80 wt %.1 Similar features were observed for a trisiloxane surfactant whose hydrophilic part consists of 10 polyoxyethylene units.2,3 Because of the lack of experimental data, the influence of the surfactant structure on the formation of disconnected Lα phases could not be clarified. In our previous studies,4,5 we demonstrated that the highly dilute and the disconnected Lα phase are not two distinct phenomena but belong together. We could show and explain the transition from systems with a highly dilute Lα phase (C10E4 and C12E5) to systems where the Lα phase is restricted to high concentrations (C10E5 and C12E6). Surprisingly, the addition of C10E5 to C10E4 and C12E6 to C12E5 did not lead to a continuous shrinkage of the Lα phase from the dilute side (thus remaining connected to the concentrated Lα phase); instead, it first led to a separation of the Lα phase into © 2012 American Chemical Society
Received: May 12, 2012 Revised: June 1, 2012 Published: June 6, 2012 9206
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ments to monitor the structural evolution because it enables us to distinguish between discrete and bicontinuous structures unambiguously.7−9 On the time scale of the NMR self-diffusion experiment, one observes molecular self-diffusion over macroscopic distances on the order of micrometers (i.e., distances much larger than the size of micelles). Thus, in the case of normal (inverse) micelles the monitored translation of the surfactant (water) molecules corresponds to the translation of the entire micelle. Because micelle diffusion is typically 2 orders of magnitude slower than the diffusion of the molecules of the continuous phase, the diffusion coefficients of surfactant and water also differ by 2 orders of magnitude if small micelles are dispersed in a continuous medium. However, in a bicontinuous structure the diffusion coefficients of surfactant and water are of the same order of magnitude. Hence, the self-diffusion coefficients of surfactant and water can provide direct information about the connectivity of the phase. In other words, discrete micelles and bicontinuous structures can be distinguished.
Figure 1. Phase diagram of the H2O−C10E4/C10E5 system as a function of the temperature T and the total surfactant concentration γ at δ = 0.10. Shown are the lamellar phase (Lα), the isotropic sponge phase L3, and the upper miscibility gap. Labels L1 and L2 are used to denote different regions of the single isotropic phase. The compositions and temperatures within the isotropic channel at which self-diffusion measurements were carried out are represented by square symbols. Data for the phase diagram are taken from ref 4.
2. EXPERIMENTAL PART Materials and Sample Preparation. Nonionic surfactants tetraethylene glycol monodecyl ether (C10E4) and pentaethylene glycol monodecyl ether (C10E5) with a purity >98% were purchased from Bachem Biochemica GmbH, Heidelberg, Germany. The surfactants were used without further purification. H2O was ultrapure Millipore water, type Milli-Q RG. The composition of the samples is given by the total mass fraction of surfactant in the mixture
phases (Figure 1). It should be noted that L1 and L2 do not represent distinct phases but simply different regions of the isotropic phase. The transition from the concentrated Lα phase to the intermediate isotropic region can certainly be compared to the transition from the dilute Lα to the adjacent isotropic L3 phase. The L3 phase is an example of a bicontinuous structure where two aqueous phases are separated by a random network of surfactant bilayers. It was shown that the transition from the Lα to the L3 phase occurs via passage formation between the two aqueous subvolumes for which bilayers need to be bent.6 To compare this process with the transition in question, one has to be aware of the fact that the L3 phase and the intermediate isotropic region are located on two different sides of the dilute Lα phase, namely, at lower (L3) and higher (intermediate isotropic region) concentrations, respectively. Taking into account that the distance between the lamellae decreases with increasing surfactant concentration, we argued in our previous work that at low concentrations passages are formed between two bilayers whereas at high concentrations the passage-forming entities are monolayers. In other words, it is a question of the repeat distance d whether the lamellar phase has to be considered to be an arrangement of either stacked bilayers or stacked monolayers. On the basis of these arguments, we proposed that the structure of the intermediate isotropic region between the dilute and the concentrated Lα phases is also bicontinuous. In contrast to the bicontinuous L3 phase, where the two subvolumes are water, in the bicontinuous region located between the two lamellar phases it is the surfactant itself that takes over the role of the second subvolume. In this letter, we studied the structure of the intermediate isotropic region with 1H NMR self-diffusion measurements. We carried out measurements along a path through the isotropic channel of the phase diagram, as is indicated in Figure 1. The starting point is normal micelles (L1) at low concentrations and temperatures, and the final point is in the L2 region of the isotropic phase, where the formation of inverse micelles is often suggested (but has seldom been proven). If inverse micelles formed and if the curvature of the surfactant monolayer changed continuously, one would expect a bicontinuous structure between L1 and L2. We chose self-diffusion measure-
γ=
mC10E4 + mC10E5 mH2O + mC10E4 + mC10E5
(1)
and by the mass fraction of C10E5 in the binary surfactant mixture mC10E5 δ= mC10E4 + mC10E5 (2) which was δ = 0.1 throughout the whole study. Each sample was prepared in a small test tube and homogenized with a little magnetic stirrer in a vibrating mill. The samples at γ = 0.6, 0.7, and 0.8 were heated to the isotropic phase to ensure homogeneous mixing. Each sample was then transferred to a 4 mm NMR tube and sealed to prevent changes in composition due to water evaporation. 1 H NMR FTPGSE. Self-diffusion coefficients were measured via the FTPGSE (Fourier transform pulsed gradient spin echo) method10,11 with a Bruker Avance 500 solid-state NMR spectrometer. Pulsed-field gradient measurements of the self-diffusion coefficient of the solvent D were carried out in a Bruker high-resolution-type probe with a singly tuned saddle coil (5 μs corresponds to a 90° pulse) equipped with the Bruker micro5 imaging system. The stimulated echo-type variant of the FTPGSE experiment was chosen. The diffusion delay Δ between the two gradient pulses was kept constant at 20 ms. Trapezoidal gradient shapes from δp = 0.7 to 1.5 ms length were employed for encoding and decoding (the length was adjusted to achieve a complete echo attenuation for the strongest gradient in each experiment). An additional burst gradient was applied during the diffusion delay Δ. Each acquisition was preceded by a train of gradient pulses of matched amplitude and timing in order to ensure reliable gradient power. Signal intensities were analyzed using the well-known relation12
⎛ ⎛ δp ⎞ ⎞ I = exp⎜⎜− γ 2δp2g 2⎜Δ − ⎟D⎟⎟ I0 3⎠ ⎠ ⎝ ⎝
(3)
where a correction for the finite duration δp of the gradient pulses was included. The echo intensity further depends on the magnetogyric ratio γ and the gradient magnitude g. The diffusion coefficients were obtained by fitting the signal decay as a function of the applied gradient strength g with a single Gaussian function according to eq 3. 9207
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Table 1. Self-Diffusion Coefficients D of Surfactant Mixture C10E4/C10E5 at δ = 0.1 (Dsurf) and Water (Dw) for Each Composition and Temperature That Was Measured as Well as Their Ratio (Dw/Dsurf)a
A small offset was included for water diffusion whenever the signals from the surfactant and water could not be clearly separated. Sample heating was performed with the standard setup of a hot air stream heating the sample from below. The true sample temperature was calibrated with the temperature-dependent peak shifts of pure ethylene glycol.13 In Figure 2, the self-diffusion coefficients D0 of water (top) and the surfactant mixture (bottom) are shown as a function of temperature.
γ 0.10 0.20 0.33 0.40 0.50 0.60 0.60 0.70 0.80 a
T/K 287 287 298 308 318 330 333 338 343
Dsurf/m2 s−1 1.02 1.13 2.27 4.27 7.17 1.04 1.47 2.40 3.57
× × × × × × × × ×
−11
10 10−11 10−11 10−11 10−11 10−10 10−10 10−10 10−10
Dw/m2 s−1 1.44 1.14 1.02 1.30 1.13 1.30 1.50 1.28 9.39
× × × × × × × × ×
10−9 10−9 10−9 10−9 10−9 10−9 10−9 10−9 10−10
Dw/Dsurf 140 100 45 30 16 13 10 5.3 2.6
The error in the self-diffusion coefficients is about ±5%.
divided by the respective values D0 of the neat components at the given temperature. The resulting relative self-diffusion coefficients, which are called obstruction factors D/D0, allow us to distinguish between different structures. For example, an obstruction factor of 2/3 indicates free diffusion in two spatial directions (e.g., diffusion in lamellar phases or bicontinuous microemulsions) and a value of 1/3 is indicative of free diffusion in one spatial direction as in wormlike micelles, hexagonal phases, or any other tubular structures.7 The obstruction factors of our system are presented in Figure 3.
Figure 2. Temperature-dependent self-diffusion coefficients D0 of pure water (top) and of the pure surfactant mixture (bottom). The solid lines are fourth-order polynomial fits. From these fits, we took the D0 values to calculate the obstruction factors. The error is always about the symbol size. For water, both measured and literature14 values are shown. The comparison of the measured diffusion coefficients of water with values from the literature14 shows a deviation at higher temperatures. This is caused by convection in the sample due to a temperature gradient. A similar effect is expected to occur for the surfactant. Thus, systematic errors of about a factor of 2 are introduced by the convection at higher temperatures. However, these errors cancel to a large extent in the following analysis in which we always discuss ratios of diffusion coefficients, namely, D/D0 and Dw/Dsurf. Here, D is the self-diffusion coefficient of water (Dw) and surfactant (Dsurf) in the water−surfactant mixture and D0 is the self-diffusion coefficient of the corresponding pure compound. We fitted the experimental D0 values with a fourth-order polynomial and used the D0 values obtained by these fits to calculate the ratio D/D0. From repeated measurements, we estimated an error of ±5% for all D values.
Figure 3. Obstruction factors D/D0 for water (black symbols) and surfactant (white symbols) along the path shown in Figure 1 as a function of the sample composition γ. Note that the temperature T and surfactant concentration γ change simultaneously. The gray symbols are data measured at a temperature slightly higher than the one indicated in Figure 1, namely, at 333 K instead of at 330 K. See the text for further details. The error in the obstruction factors is ±5%.
As can be seen in Figure 3, the obstruction factor of water decreases continuously from 0.84 at γ = 0.1 to 0.08 at γ = 0.8 whereas that of the surfactant increases from 0.36 to 1.05. At a surfactant concentration of γ = 0.33, the obstruction factors of both components were found to be nearly equal, which we will discuss below. Taking a closer look at Figure 3, one sees that the D/D0 values change monotonically with increasing surfactant concentration γ except at one point, namely, at γ = 0.6. An explanation of this deviation is the fact that the path through the phase diagram was chosen such that it passes the region between the two lamellar phases in the “middle”. This path, however, is not necessarily the most direct path in terms
3. RESULTS AND DISCUSSION We measured the self-diffusion coefficients of surfactant and water along a path through the isotropic channel in the phase diagram, as indicated in Figure 1. The results are summarized in Table 1, which also includes the ratio of the diffusion coefficients of water and surfactant (Dw/Dsurf). To obtain structural information, the self-diffusion coefficients D are 9208
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understand in depth how the bicontinuous structure changes, we can certainly assume that the changes are very similar to those described by He et al. (i.e., that bicontinuous, spongelike structures are formed up to very high surfactant concentrations). This is not unusual: in microemulsions bicontinuous structures were also observed over an enormous range of waterto-oil ratios, namely, from around 10 to 90 vol % water.20 Here, coexisting discrete droplets of the minor compound were observed in the limits of large water and large oil contents, respectively, which is in line with the very small value observed for the D/D0 values of water in the study at hand. Another difference between our study and He’s is the microstructure of the solution at concentrations above 80 wt %, as will be discussed in the following text. Whereas the crossover of the obstruction factor curves at intermediate surfactant concentrations (γ = 0.33) may be taken as evidence of a bicontinuous structure, it is not clear whether the isotropic L2 solution at higher surfactant concentrations can be considered to be an inverse micellar solution. Further insight into this topic can be obtained from a comparison of the diffusion coefficients of the two components. Table 1 shows that the ratio Dw/Dsurf decreases from 140 to 2.6 along the chosen path from L1 to L2. In the L1 region, water diffuses about 100 times faster than does the surfactant. This is due to the fact that the surfactant forms discrete micelles and that the observed self-diffusion coefficient is that of the entire micelle, which is slow. As γ increases and bicontinuous structures form, the ratio Dw/Dsurf decreases and the obstruction factors become very similar (and in one case even equal). The question arising in this connection is what can be expected for inverse micelles. He et al. argued that water restricted to small droplets should diffuse only with the entire droplet, that is, much more slowly than the surfactant. This is due to the same reason that holds for the slow surfactant diffusion in normal micellar solutions.16 Hence, if Dw > Dsurf as observed in earlier investigations on surfactant-rich isotropic solutions, then a structure of inverse micelles can be excluded. Following He’s arguments, there is also no evidence of inverse micelles in our system. Let us assume now that there is no structure at all at high surfactant concentrations and that the system can be regarded as a simple homogeneous mixture of components. In this case, the ratio of the self-diffusion coefficients of both components can be estimated on the basis of the Stokes−Einstein equation
of structural changes. The data points represented by gray symbols in Figure 3 correspond to an additional point in the phase diagram, which lies somewhat above the path shown in Figure 1, namely, at γ = 0.6 and T = 333 K instead of at γ = 0.6 and T = 330 K. These additional points indicate first that the structure is very sensitive to changes in temperature and second that the most direct path for a continuous change in the microstructure lies closer to the diluted Lα phase. The obstruction factors shown in Figure 3 show trends that are very similar to those observed for water and oil in microemulsions.7,15 In the latter case, the crossover of the two diffusion curves is taken as evidence of the change from a discrete oil-in-water to a discrete water-in-oil structure via a bicontinuous one. Please note that in microemulsions water and oil are the main compounds that are separated by a surfactant monolayer. In this case, the self-diffusion coefficient of the surfactant has a maximum at the crossover. However, in the water−surfactant system investigated here, the surfactant plays the role of the oil. To stay with this analogy, one is tempted to describe the structural changes from the L1 to the L2 phase as a continuous change from surfactant droplets (micelles) dispersed in a continuous water phase via a bicontinuous structure in which both the water and the surfactant form continuous networks to water droplets (inverse micelles) dispersed in a continuous surfactant phase. We will discuss in the following text whether such a structural evolution is in line with our experimental data. A change from L1 to L2 via bicontinuous structures has been discussed for isothermal cuts through phase diagrams of other water−nonionic surfactant systems.16−18 Degiorgio et al. carried out SANS measurements across the isotropic singlephase region of the water−C12E8 system from 0 to 100 wt % surfactant and suggested that at intermediate surfactant concentrations a bicontinuous structure is formed.16 A much more detailed study was carried out a couple of years later by He et al., where a trisiloxane surfactant solution was investigated via rheology, SANS, SAXS, cryo-TEM, and 1H NMR self-diffusion measurements.17,18 They found that the structure changes continuously with increasing surfactant concentration: starting from spherical micelles (
