LETTER pubs.acs.org/Langmuir
Effect of Shear Rates on the MLV Formation and MLV Stability Region in the C12E5/D2O System: Rheology and Rheo-NMR and Rheo-SANS Experiments Luigi Gentile,*,† Cesare Oliviero Rossi,† Ulf Olsson,‡ and Giuseppe Antonio Ranieri† † ‡
Department of Chemistry, University of Calabria, P. Bucci 14C, 87036 Rende, Italy Physical Chemistry 1, Kemicentrum, Lund University, Box 124, 221 00 Lund, Sweden ABSTRACT: At high temperatures, pentaethylene glycol monododecyl ether (C12E5) in D2O forms a swollen lamellar phase. This letter reports the shear-induced multilamellar vesicle (MLV) formation in a sample that contains 40 wt % C12E5 dissolved in D2O at 55 °C. This transition has been investigated by timeresolved rheo-nuclear magnetic resonance, rheo small-angle neutron scattering, and rheometry. The typical transient viscosity behavior of MLV formation has been discovered at 1 s-1. For the first time, it has been found that MLVs are not stable over time when subjected to high shear rates. Our results show that the MLV stability is confined in a narrow region in the range 1-10 s-1 shear rates. This is not observed for other CnEm surfactants.
hear-induced transformations and ordering in complex fluids are interesting and important topics in science. The influence of shear on the structure of complex fluids has attracted a lot of interest for several reasons.1,2 Knowledge about such effects is important for various industrial applications, which often depend on the viscosity of a lyotropic phase. The rheological properties of lyotropic liquid crystals are often affected by their mechanical history because materials with a lamellar structure show a rich variety of shear-induced orientation states. The effects of the shear rate on the lamellar phase are, normally, widely investigated.3-6 Multilamellar vesicles (MLVs) often occur when a defective lamellar phase is subjected to shear flow,7 and this shear-induced transition has received great attention.3-6 MLVs are multilayered closed structures, and for this reason, they are also called onions or spherulites. Shear-induced onions may be stable for a long time, but they do not correspond to the thermodynamic equilibrium structure of the multilayered system.8 Closed bilayer structures are important in biological processes and in pharmaceutical applications because liposomes encapsulating drugs are made from natural lipid bilayers.9 The development of rheotools (X-ray, neutron, and light scattering and NMR) allowed experimentalists to investigate fluid structures under flow, such as MLV formation.10-16 In many nonionic surfactant aqueous systems of the CnEm type (CnH2nþ1(OC2H4)mOH), a lamellar phase occurs over a wide temperature and concentration range. Within this region, a relatively limited area exists where MLVs can be formed by shear.
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Shearing can also cause an MLV-to-lamellar transition if the sample is brought to the upper temperature boundary, neighbored by the L3 (sponge phase), of the phase diagram. It is worth mentioning that, for some of the CnEm surfactants, MLVs can also be achieved by shearing the L3 through an intermediate transition to the lamellar phase.17-20 In this letter, for the first time, we show that a sample made of up to 40 wt % C12E5 dissolved in D2O forms MLVs upon shearing at 55 °C. Lu et al. investigated the same binary system, but they did not find any MLV formation at 35 °C.20 We instead observed that after a long time at a certain shear rate MLVs appears. Different techniques have been used for MLVs determinations: rheology, rheo-NMR, and rheo-SANS. The C12E5/ D2O system is very sensitive to the applied mechanical history, and for this reason, we have performed rheological measurements in triplicate and used strain- and stress-controlled rheometers. Considering the high temperature of the experiments (55 °C) and the fact that the measurement takes a long time, we paid particular attention to the control of solvent evaporation. In the rheo-SANS and rheo-NMR experiments, evaporation was avoided by using a sealed cuvette. In addition, the rheometers have been equipped with evaporation traps. Time-resolved neutron scattering experiments have been carried out over a q Received: October 11, 2010 Revised: January 12, 2011 Published: January 25, 2011 2088
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Langmuir range of 0.02-0.15 Å-1, and the sample has been subjected to shear. The data have been corrected for background and empty cell scattering. Figure 1 shows the typical transient viscosity behavior of MLV formation at 1 s-1. The MLVs are formed after 3000 strain units. SANS experiments have been performed in a q range where the Bragg peak corresponding to the layer spacing is observed and the first-order peak is observed at q ≈ 0.1 Å-1. SANS spectra in the radial direction, perpendicular to the direction of flow, have been recorded at intervals of 120 s, with an acquisition time of 1 s. At low strain, a pronounced anisotropic scattering is observed for measurements through the cell side with distinct peaks in the q direction (first SANS pattern, Figure 1). Therefore, it is possible to affirm that an oriented lamellar phase is present. The lamellae are now ordered parallel to flow-shear gradient plane orientation c. SANS patterns change from the anisotropic to the isotropic ring. These changes in the scattering patterns clearly demon-
Figure 1. Time evolution of the transient viscosity at a shear rate of 1 s-1 (strain-controlled instrument) and relative SANS patterns at 55 °C. The transition from lamellar (0 s) to MLVs (3000 s) appears after a small plateau region that it is attributed to the elongated objects oriented along the flow in the steady state.
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strate the formation of the isotropic onions. The first viscosity plateau (after 600 s) has been ascribed to elongated objects oriented along the flow in the steady state (typical oily streaks) as reported by Lu et al.20 for a 45 wt % C12E5/water system. The “oily streaks” are characteristic defects of lamellar phases with melted chains. The defect structure can be described in terms of two opposite edge dislocations.21 It is plausible that the increase in viscosity over the region of 0-600 s is due to the formation process of the oily streaks. These defects can be considered to be promoters of MLVs. The defects play a crucial role in shear-induced structural transformations because a perfectly ordered and oriented lamellar phase should not be able to store elastic energy when sheared parallel to the bilayers.17,22 In fact, the pronounced increase in viscosity due to the mechanism of MLV formation occurs within 1500-3000 s. Moreover, it is believable that MLVs and oily streaks are coexistent in the steady state, as cited in the literature for other similar systems.21 It is worthwhile to note the oscillations in the viscosity (Figure 1). The nature of these oscillations has not been understood yet. Future investigations are needed to examine the cause of this experimental data. The MLV formation is confirmed by rheo-NMR experiments. In fact, the doublet of the planar lamellae turns into a broad single peak as a consequence of the MLVs’ presence (Figure 2).12 Up to a strain of about 6000, the viscosity evolution of the shear rates from 1 to 10 s-1 is very similar with time (Figures 1 and 4). After approximately 3000 strain units, the rate at which the viscosity increases speeds up and then it slows down, and at γ = 6000, a steady state is reached. This result has been observed for the applied shear rate values of 1 and 10 s-1 by SANS scattering. Studies on the pathway of MLV formation revealed a characteristic sequence of lamellae morphology changes. A parallelorientated prealigned lamellar phase first undergoes changes into multilamellar cylinders (MLCs) or coherent undulations (buckling modulation) followed by MLVs.13,15 The intensity in the neutral direction reveals a maximum at around 3000 strain units (Figure 3). This maximum may be due to multilamellar cylinder (MLC) formation as observed in C10E3 and C12E4.15 The narrow peak indicates the existence of a small region of MLCs presence;
Figure 2. Time evolution of 2H NMR spectra during MLV formation from planar lamellae at 55 °C and a constant shear rate of 1 s-1. 2089
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Langmuir consequently, it does not allow us to observe the MLC plateau clearly (because it is too narrow) in the transient viscosity experiment. In this study, it is not possible to assess the presence of MLCs unambiguously. For this reason, further investigations are required to determine the presence of this intermediate state. At 6000 strain units, the intensity in both neutral and flow directions reaches a steady-state value. By comparing our results with those of Nettesheim et al.,15 we could conclude that for
Figure 3. Evolution of SANS intensity at the lamellar Bragg peak in neutral and flow direction in the radial beam for two different shear rates, 1 and 10 s-1.
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C12E5, C10E3, and C12E4 the transition from planar lamellae to MLVs follows a similar path. Additionally, C12E5 and C12E4 require nearly the same strain at 10 s-1, but a higher strain is required for C10E3. This comparison is made at different temperatures: 25 °C for C12E4 and C10E3 and 55 °C for C12E5. This different experimental condition does not affect the transition from planar lamellae to MLVs. Temperature and the surfactant chain length are the two parameters that can control the transition from planar lamellae to MLVs. According to the literature,15 one can see that the temperature does not influence the strain dependence of the process but affects only the transition. Upon increasing the temperature (changing the spontaneous curvature of the bilayer) at constant shear rate, the system enters a coexistence region of MLVs and planar lamellae. One possible explanation for the shift of the transition to lower deformation is the increased membrane stiffness. The viscosity of the C12E4 sample is higher; consequently, the transition requires less strain because a higher stress has to be applied to obtain the desired shear rate. In general, the shear rate promotes the formation of densely packed aggregates whose size is affected by the applied shear rate value. High shear induces small MLV aggregate formation and consequently good stability of the structure phase.23,24 It is worthwhile to note that the MLV phase in 40 wt % C12E5/D2O at 55 °C covers a narrow stability region. Only with shear rates from 1 to 10 s-1 do the MLVs show a steady state (Figure 4). After 10 h at 20 s-1, a significant decrease in viscosity appears, which is due to the reverse transition from MLVs to planar lamellae as
Figure 4. Viscosity as a function of time at 55 °C for two solutions at different shear rate values: (A) 2, 5, and 10 s-1 for 5 h and 20 s-1 for 10 h; (B) 2, 5, 10, and 20 s-1 for 5 h and 40 s-1 for 10 h. All measurement have been executed with a stress-controlled instrument. 2090
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Figure 5. Time evolution of 2H NMR spectra during the transition from MLVs to planar lamellae at 55 °C and at a constant shear rate of 40 s-1.
Figure 6. Isothermal dynamic phase diagram of a 40 wt % C12E5/D2O system at 55 °C.
evidenced by rheo-2H NMR in Figure 5. By shearing the system at 20 s-1 for 5 h, an apparent plateau in the viscosity is recorded (Figure 4B). A sharp decrease in viscosity (after 10 h) has been measured at shear rate values of 20 and 40 s-1 by increasing the time frame of the experiment. On the contrary, no significant viscosity changes have been observed for 20 h under shearing of 10 s-1. Figure 5 shows 2H NMR spectra under a shear rate of 40 s-1 as a function of time. The spectrum of the initial state reveals two coexisting structures: MLVs and planar layers. In general, the area under each NMR spectrum is proportional to the concentration of molecules in a particular state. The decay of the central peak (MLVs) and the growing intensity of the Pake doublet peak (planar layers) can be recognized. A broad single peak typical of MLVs turns into the Pake doublet of the planar lamellae after 10 h (1 440 000 strain units) according to the rheological results. Rheo-NMR experiments evidence a lamellae-onion coexistence region at 40 s-1. This coexistence has been observed in other systems.10 Additionally, the low viscosity values at high deformation indicate a prevalence of the classical lamellar phase over the MLVs. Low-viscosity oriented lamellar structures are present when the layer is normal to the direction of the flow with a low defect density (Figure 6). The relative stability of multilamellar vesicles and lamellae has been shown to have a dependence on the shear rate.25 Furthermore, the relative stability is expected to be crucially dependent on the saddle-splay modulus (eq 1) kB ðTÞ ¼ 2k-kB lb H 0 ðTÞ
ð1Þ
where lb is the bilayer thickness, hk is the monolayer counterpart of hkb(T) and kb is the bilayer bending. The predominant temperature dependence resides in the monolayer spontaneous curvature, H0. This makes the weak temperature dependence in the monolayer elastic modules negligible.26 The shear-induced transition from MLV to lamellar phase can be qualitatively described by a balance between the shear rate and H0 because they have competitive effects. The shear effect lightly overcomes the spontaneous curvature,27 leading to MLV formation. It is hypothesized that closed bilayer structures cannot become smaller, with the MLV aggregate reaching a critical size high-shear-rate value. Consequently, the effect of temperature on H0 overcomes the shear effect, leading to a metastable structure, which means that the system is in equilibrium (not changing with time) as it falls into a lower-energy state (planar layers). The shear-induced lamellar formation from MLVs has not been found in other systems and it is not yet fully understood. In this letter, we focused on the presence of the MLV-tolamellar transition in nonionic surfactant C12E5 at 40 wt % in D2O. The shear-induced lamellar-to-MLV transition will be the subject of extensive research in order to clarify not only the mechanism and driving force of the transition but also to evaluate future practical applications. In conclusion, we have found MLVs in C12E5 40 wt % in D2O at 55 °C. For the first time in the CnEm systems, we have observed a circumscribed stability region of the MLV phase with time (Figure 6).
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Prof. Kell Mortensen and Dr. Bruno Silva for theoretical and practical assistance and the Paul Scherrer Institute (PSI) for allowing us to use their SINQ facility (SANSII 2091
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Langmuir instrument). We are also grateful to Dr. Luigi Filippelli for the constructive scientific discussions.
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