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J. Phys. Chem. B 2007, 111, 9496-9503
Flow-Induced Effects in Mixed Surfactant Mesophases J. Penfold*,† and I. Tucker‡ ISIS, CCLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, U.K., and Port Sunlight Laboratory, UnileVer Research and DeVelopment, Quarry Road East, Bebington, Wirral, CH63 3JW U.K. ReceiVed: April 11, 2007; In Final Form: June 8, 2007
Spatially resolved small-angle neutron scattering, SANS, has been used to investigate the response of the mixed microstructure of the dialkyl chain cationic and nonionic surfactant mixtures of (2,3-diheptadecyl ester ethoxy-n-propyl-1), 1,1,1-trimethyl ammonium chloride/octadecyl monododecyl ether, DHTAC/C18EO10, and DHTAC/dodecyl monododecadecyl ether, Coco20, over the velocity flow pattern of a crossed-slot elongational flow cell. The two different surfactant mixtures have different relative amounts of lamellar and micellar components, and this results in some differences in the flow-induced response. For the DHTAC/C18EO10, which is predominantly in the form of lamellar fragments, a complex pattern of orientational ordering is observed which reflects the competition between or demixing of the two principal flow directions in the cell.
Introduction The structure and behavior of mixed surfactant solutions are of widespread interest because of their extensive use in many of the industrial, technological, and domestic applications of surfactants.1 The dialkyl chain cationic/nonionic surfactant mixtures, such as DHTAC/C18EO10 (DHTAC, 2,3-di-heptadecyl ester ethoxy-n-propyl-1,1,1-trimethyl ammonium chloride; C18EO10, octadecyl monododecyl ether) and DHTAC/Coco20 (Coco20, dodecyl monododecadecyl ether), which are the subject of the paper, are of particular relevance to many home and personal care formulations, such as fabric and hair conditioners, shampoos, and shower gels, and in more diverse areas such as membrane solubilization studies. The general phase behavior in such mixtures is now well-established2,3 and consists predominantly of lamellar fragments or multilamellar vesicles for solutions rich in the dialkyl chain cationic surfactant and globular micelles for solutions rich in the nonionic surfactant. At intermediate solution compositions there is commonly a region of coexistence of the principal microstructures. In the processing and delivery of the many formulations that are comprised of these complex mixtures of surfactant phases, flow-induced structures, transformations, and reorganizations, and their attendant rheological responses, are important considerations. As a consequence, in recent years this has developed into a rich area of research, and many aspects of the recent developments in this area are extensively discussed in a number of recent review articles.4-7 Predominantly these recent studies have involved shear flow, in a Couette, or closely related flow geometry, and a wide range of different phenomena have been reported. In the micellar region of the surfactant mesophase behavior, flow can simplify the analysis of scattering data,4-8 but more often or not it results in the deformation of the microstructure, the induction of new structures, or the breakup of existing structures.4 In viscoelastic elongated micellar phases, which exhibit either shear thinning or shear thickening rheological properties, in addition to increasing the orientational order, shear † ‡
Rutherford Appleton Laboratory. Unilever Research and Development.
flow can result in stretching, disentanglement, and “banding” associated with flow instabilities within the system.6 Of particular relevance to the focus of this paper are the rich patterns of behavior of surfactant lamellar phases when subjected to shear flow. In Couette flow, lamellar ordering in the flowvorticity plane (“c” orientation), and alignment in the orthogonal flow-shear gradient direction (“a” orientation) have been reported in a variety of systems.9,10 Flow regimes where the coexistence of both the c and a orientations exist have been identified,9,10 and their relationship to the complex flow profiles across the Couette flow cell gap have been established.3,11 Following the initial work of Diat et al.,12 an increasing number of lamellar systems have been shown to exhibit a lamellar to multilamellar vesicle transition at relatively low shear rates,10,13-16 where the lamellar phase is stabilized by both undulation forces and by charge. Subsequent detailed experimental studies17-19 and theoretical treatments20 have shed light upon the mechanisms involved in this important structural transformation. Micellar to vesicle and sponge phase to vesicle transformations are also evident and reported in the recent literature.21-23 The particular focus of this paper is on the effects of flow on mixedphase systems, and there have been very few studies reported in the recent literature. A notable exception is the work of Chirvoulu et al.24 on the flow response of a mixture of liposomes and entangled tubular vesicles, induced in the phospholipid DMPC (dimyristoylphosphatidyl choline) by the addition of a perfume molecule like Geranoil. Small-angle X-ray and neutron scattering (SAXS and SANS) are important probes of surfactant mesophase structure and have been extensively exploited in combination with flow. In these studies, predominantly shear flow, in Couette, cone and plate, or Poiseuille flow geometries, have been used, and relatively little attention has focused on extensional or elongational flow. However, extensional flow is commonly found in the flow geometries associated with product processing, such as in extruders and pipe flow. Indeed, in most practically encountered flow geometries associated with processing there is an element of both shear and extensional flow. Hence it is important to understand the impact of both shear and extensional flow.
10.1021/jp072808c CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007
Flow-Induced Effects in Surfactant Mesophases The four-roll mill25 and opposing jet26 geometries have been exploited in optical birefringence measurements to obtain elongational flow profiles. Idziak et al.27,28 demonstrated that the crossed-slot geometry29 was particularly attractive for combining X-ray and neutron scattering studies with elongational flow, and the simplicity and adaptability of the geometry for scattering techniques is particularly appealing. In particular they demonstrated how a fine X-ray beam could be used to map out the variation in the microstructure over the flow distribution within the cell.27,28 Bent el al30 used spatially resolved SANS measurements to map out the flow profile of a polymer blend in an extruder. SAXS, using a microfocus X-ray beam, has been used to investigate the variation in ordering of a lamellar phase across the gap of a Couette flow cell.3,11 More recently Penfold et al.31 have used spatially resolved SANS to investigate the flow response of a dilute surfactant elongated rod phase and a concentrated surfactant lamellar phase to elongational flow, using a “crossed-slot” geometry similar to that pioneered by Idziak et al.29 The focus of this paper is to use such spatially resolved SANS measurements to investigate the response of a complex mixture of surfactant mesophases to elongational flow, and in particular to map out the response of the microstructure over the flow profile of the cell. To that end we have studied two different systems, 15 wt % DHTAC/0.5 wt % C18EO10 and 15 wt % DHTAC/2 wt % Coco20, which are typical of systems that we have previously studied in static conditions and under Couette shear flow.2 These are systems which, dependent upon concentration and composition, exist as lamellar fragments, vesicles, or micelles and also have extensive mixed-phase (lamellar/ micellar) regions. The 15 wt % DHTAC/0.5 wt % C18EO10 mixture is predominantly comprised of lamellar fragments, whereas the 15 wt % DHTAC/2 wt % Coco20 mixture has coexisting lamellae and micelles. In addition to the elongational flow measurements in the crossed-slot cell, comparison will be made with some complementary Couette flow measurements.
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Figure 1. (a, top) Schematic of elongational flow cell, showing the incident beam direction. (b, bottom) Schematic of Couette flow cell and scattering geometry, for “cell-side” measurements.
Experimental Details The SANS measurements were made on the LOQ diffractometer32 of the ISIS pulsed neutron source. The measurements were made using the “white beam” time-of-flight method in the scattering vector, q, range of 0.008-0.25 Å-1. The measurements were made in D2O, for a 5 mm path length and a transmission ∼ 50% in the elongational flow cell, and a variable path length over the beam cross-section for the Couette flow cell with a mean transmission ∼ 80%. The elongational flow cell measurements were made using a finely collimated neutron beam, using a 2 mm diameter Cadmium aperture immediately before the sample cell window, with the neutron beam incident orthogonal to the flow (x-y) plane, as shown in Figure 1a. The Couette flow cell measurements were made with a 8 mm diameter aperture, with the neutron beam incident orthogonal to the shear gradient-vorticity plane, “cell side” of tangential measurements (see Figure 1b). The data were corrected for detector response, the spectral distribution of the incident neutron beam, and converted to absolute scattering cross-section, I(q) in cm-1, using standard procedures. Typical measurement times were ∼10-20 min. The elongational flow cell (see Figure 1a) is a crossed-slot flow cell, based on the original design for SAXS by Idziak et al.,28 and described in more detail elsewhere.31 A 20 mm diameter quartz window provides access to the central region. The flow channels have dimensions ∼10 × 2.5 mm, with the input and output ports well separated from the central region.
Figure 2. Flow pattern, calculated for elongational flow cell, for a Newtonian fluid at 4.4 cm3/min. Reproduced with permission from ref 2. Copyright 2005 American Chemical Society.
The maximum extensional flow rate, ′, ′ ) dVy/dy, is ∼2J s-1, where J is the volume flow rate. The calculated flow pattern in the central region, for a Newtonian fluid, is shown in Figure 2.31 SANS measurements were made at different positions over the central 20 mm window of the cell, defined by Cartesian coordinates such that (10,10) is the cell center and (0,0) the bottom left-hand corner. The Couette flow cell, with a fixed inner stator and outer cup, constructed of quartz for maximum neutron transparency, has been described in detail elsewhere.33 The SANS measurements were made through the cell side, with neutron beam incident orthogonal to the shear gradient-vorticity plane (see Figure 1b). The 0.5 mm gap and the 10 cm cell diameter provide conditions for a constant shear gradient across the gap. The cationic surfactant DHTAC was custom-synthesized and purified at Unilever Research. The C18EO10 (Brij97) was obtained from Sigma and recrystallized before use. The Coco20
9498 J. Phys. Chem. B, Vol. 111, No. 32, 2007 (∼C12EO20) was obtained from Genopol as Genopol C200 and used as supplied. Coco20 is polydisperse in both alkyl and ethylene oxide chain lengths, but this is not important in the context of these studies and the analysis made here. The deuterium oxide, D2O, was obtained from Sigma-Aldrich, and high-purity water (Elga Ultrpure) was used throughout. All glassware and sample cells were cleaned using alkali detergent (Decon 90), followed by copious washing in high-purity water. All the measurements were made at 30 °C in the Couette flow cell and at 35 °C in the elongational flow cell, but within this range of temperatures the scattering is not strongly temperaturedependent. Measurements were made for two different surfactant mixtures, 15 wt % DHTAC/0.2 wt % C18EO10 and for 15 wt % DHTAC/2 wt % Coco20, and all solutions were made in D2O to optimize the “contrast” for neutron scattering. The Couette flow measurements were made for the shear gradient range of 0-5000 s-1, and the elongational flow measurements were made in the flow rate range of 0-40 s-1. Results and Discussion (a) 15% DHTAC/0.5% C18EO10. Measurements were made for the surfactant mixture of 15 wt % DHTAC/0.5 wt % C18EO10 in the Couette shear flow cell, in the shear rate range from 0 to 5000 s-1, and in the “cell-side” or tangential scattering geometry, that is, with the neutron beam incident orthogonal to the shear gradent-vorticity plane. The evolution of the scattering in the Qx-Qy plane with increasing shear rate is shown in Figure 3a. At low shear rates the scattering is from lamellar fragments aligned in the flow-vorticity plane and consistent with previous observations on related systems.9,10 The scattering is predominantly lamellar with little evidence of a coexisting micellar component. The forms of the scattering in the shear rate range up to 50 s-1 are broadly similar; see also Figure 3b, which shows the Qy (Qperp) and Qx (Qpar) cuts through the two-dimensional scattering patterns for 0 and 50 s-1. The differences between the 0 and 50 s-1 data are associated with some modest shearinduced disorder, but without any major disruption of the aligned lamellar structure. At these lower shear rates the scattering is consistent with highly aligned lamellar fragments and a highly ordered lamellar structure. The scattering data at the higher shear rates (500, 5000 s-1) show an increasingly larger isotropic component in the Qy (Qperp) direction (see Figure 3c). The Qperp and Qpar cuts also now show a different form, and in particular the lamellar oscillations are less well pronounced, but the scattering still has a Q-2 dependence expected for planar structures. The scattering is now attributed to that from multilamellar vesicles at these higher shear rates, consistent with a shear-induced lamellar-vesicle transition as observed in related systems.13 This decrease in the anisotropy in the scattering should not here be confused with any increasing micellar component, where the scattering power would be shifted the higher Q values and have a different Q dependence. SANS measurements were also made for the same DHTAC/ C18EO10 mixture in the crossed-slots elongational flow cell, and the scattering data as a function of flow rate (for 0-20 s-1) at the cell stagnation point or cell center (coordinates 10,10). With increasing flow rate the anisotropy in the scattering I(Qperp)/ I(Qpar), measured at a Q value of 0.21 Å-1 at the first Bragg peak, increases, as shown in Figure 4a. This ratio provides a simple way of quantifying the variation in the anisotropy in the scattering. For flow rates > 12 s-1 the data are consistent with highly aligned lamellar fragments, aligned in the flow direction. This
Penfold and Tucker is very similar to what is observed at low shear rates in the Couette flow cell (see Figure 3). At lower flow rates (100 s-1) the scattering patterns are further complicated (not shown here) as there is an additional
isotropic contribution from the lamellar to vesicle transition, which now largely masks the micellar component. In elongational flow a pattern of behavior similar to that observed at low shear rates is observed. The scattering is consistent with the alignment of lamellar fragments which increases with increasing flow, and with an isotropic (micellar) component superimposed; see Figure 3 in the Supporting Information. The variation in the anisotropy in the scattering, I(Qperp)/I(Qpar), measured at a Q value of 0.19 Å-1 at the first diffraction peak, is shown in Figure 5, along with the equivalent data for the 15 wt % DHTAC/0.5 wt % C18EO10 mixture. The behavior is broadly similar, except that the onset of the lamellar alignment is a lower flow rate, and the value of the anisotropy is lower once the lamellae are aligned. This is attributed largely to the contribution of the micellar component in the 15 wt % DHTAC/2 wt % Coco20 mixture. The lower threshold for alignment, compared to that for the 15 wt % DHTAC/0.5 wt % C18EO10, would normally be associated with larger lamellar fragments, but this is not consistent with other observations, and this will be discussed more fully later in the discussion. Figure 4 in the Supporting Information shows the intensity contour maps (Qperp, Qpar) for 15 wt % DHTAC/2 wt % Coco20 at a flow rate of 20 s-1, measured at 2 mm intervals over the elongational flow cell aperture. Behavior similar to that measured for 15 wt % DHTAC/0.5 wt % C18EO10 is observed, except as also shown in Figure 3 in the Supporting Information there is an isotropic component to the scattering from the coexisting micellar component evident. (c) General Discussion. The major difference between the two DHTAC/nonionic surfactant mixtures studied here is that the higher intrinsic curvature of the Coco20 compared to the C18EO10 and the higher amount of nonionic surfactant added results in a more significant coexisting micellar component. This is evident from the more significant isotropic component to the scattering that is observed for DHTAC/Coco20 compared to DHTAC/C18EO10. It has been previously reported that such a coexisting micellar component can exert a depletion force on the major lamellar component,2 but this is not observed here.
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Figure 6. Intensity contour maps (Qperp (Qy), Qpar (Qx)) for 15 wt % DHTAC/0.5 wt % C18EO10 at a flow rate of 16 s-1, measured at 2 mm intervals over the elongational flow cell aperture.
Figure 7. Intensity contour map for 15 wt % DHTAC/0.5 wt % C18EO10 at a flow rate of 18 s-1 at position (16,10).
However the different types of nonionic cosurfactant and the cosurfactant concentration do have an impact upon the microstructure and its response to the flow field. Comparing I(Qperp)/ I(Qpar) for the two mixtures in Figure 5 shows some differences. In both cases there is a threshold for the onset of the major alignment of the fragments, which occurs at a lower flow rate for DHTAC/Coco20 than for DHTAC/C18EO10. Furthermore, once aligned the magnitude of the anisotropy is greater for DHTAC/C18EO10 than for DHTAC/Coco20. The difference in magnitude of the anisotropy is partially accounted for by the isotropic micellar component to the scattering, which is not present for the DHTAC/C18EO10 mixture. The different threshold for the onset of alignment between the two mixtures is interesting and can be rationalized in terms
of the relative sizes of the associated fragments. Given that these are relatively concentrated dispersions, the total weight percent of 15.5 and 17%, and that the lamellar fragments are somewhere between 0.2 and 10 µm in diameter,34 there is a significant hindering in the rotational diffusion of the individual fragments due to its neighbors. In such circumstances a threshold for the onset of alignment is to be expected. Initially it might be expected that the lower threshold would be associated with a larger fragment size, and indeed in dilute solution lower flow rates would be expected to align larger objects more readily. However, from previous related studies34 the higher nonionic content and the use of Coco20 instead of C18EO10 is expected to give rise to smaller fragments for the DHTAC/Coco 20 mixture. Hence here the lower flow threshold for the onset of significant alignment for the DHATC/Coco20 mixture is attributed to there being less overlap and a less hindered rotational diffusion. The comparison between Couette shear flow and elongational flow for these surfactant mixtures shows that at low shear rates and at low flow rates both flow regimes provide efficient orientational ordering of the lamellar fragments. A notable difference between the two flow regimes is that at higher shear rates in Couette flow sufficient work can be done on the system to induce fluctuations which drive the transition from lamellar fragments to liposomes. At the relatively low flow rates used in the elongational flow cell this transition is not observed. Indeed for the DHTAC/Coco20 mixture the anisotropy remains relatively constant once the initial alignment threshold is exceeded (see Figure 5). The situation for the DHTAC/C18EO10 mixture is slightly different, and at the higher flow rates the anisotropy starts to decrease. Whether this is due to the onset of the fragment to liposome transition, the onset of chaotic flow in the cell, or the breaking of the fragments (without liposome
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Figure 8. Intensity contour maps (Qperp (Qy), Qpar (Qx)) for 15 wt % DHTAC/0.5 wt % C18EO10 at a flow rate of 18 s-1, measured at 1 mm intervals over one sector of the elongational flow cell aperture
Figure 9. Intensity contour map for 15 wt % DHTAC/2 wt % Coco20/D2O at shear rates of 0, 10, 20, 50, and 100 s-1, from measurements in the tangential scattering geometry.
formation) is difficult to distinquish from these measurements alone, and this needs to be explored in more detail. The occurrence of the more complex form of the orientational ordering observed principally in the “off-axis” flow directions for both DHTAC/nonionic mixtures, and illustrated in detail in Figures 7 and 8, is different from what has been previously observed under shear flow. In shear flow orientational ordering in the flow-vorticity plane (c orientation), in the orthogonal direction (a orientation), and the coexistence of both orientations has been observed and discussed in a variety of lamellar phase systems.9,10 Berghausen et al.11 and Penfold et al.3 have demonstrated how that coexistence of orientational ordering can arise from different spatial distributions across the gap of the Couette flow cell. The patterns observed here (see Figures 7 and 8) are different and are neither in the principle flow direction nor orthogonal to it. However, in general, the orientational ordering of the lamellar fragments follows the flow profile expected for a Newtonian fluid in the crossed-slot flow cell.
The bimodal distribution observed in the off-axis regions of the flow cell implies a more complex flow pattern. The Newtonian flow profile (see calculation in Figure 2) is associated with a complete mixing of the two principle flow directions. The general trends observed in the orientational ordering suggest that this indeed occurs to some degree within the cell. However, there are regions where the more complex orientational ordering, which is shown in Figure 7b, although reproducible, is at a particular point within the cell sensitive to the actual flow rate and evolves with time. These observations suggest that there are distinct regions where the mixing is not complete. That is, there is a partial demixing of the principle flow directions and a more complex flow pattern emerges, and this is reflected in the more complex scattering pattern. It is suggested that this could be considered as a form of “banding”. Whether there are regions that are spatially separated within the depth of the cell (analogous to the banding observed across the shear gradient-
Flow-Induced Effects in Surfactant Mesophases vorticity plane in the Couette shear flow geometry) is difficult to deduce from measurements in this scattering geometry alone. Recent theoretical36,37 and experimental studies38-40 have reported the phenomena of shear-induced banding in a range of different surfactant systems. Olmsted et al.36,37 have shown how shear banding occurs primarily in the proximity of a discontinuity in the stress-strain relationship and can be associated with a phase transition, different coexisting microstructures, regions of different degrees of orientational ordering, or simply demixing (phase separation). In Couette flow geometry shear banding can exist for common stress (in shear gradient direction) and for common strain (vorticity direction) for either shear thickening or shear thinning systems. Experimental evidence for shear banding has been reported for wormlike39 or elongated micelles,38,40 elongated micelles undergoing an isotropic to nematic transition,11 in lamellar systems,3 and in colloidal solutions of rodlike macromolecules.40 We propose here that the demixing observed here in the elongational flow geometry is analagous to that attributed to banding in Couette flow geometry. Summary We have used spatially resolved SANS measurements to map out the response of the mixed microstructure of a dialkyl chain cationic surfactant (DHTAC) with different nonionic cosurfactants (C18EO10, Coco20). In both mixtures the predominant microstructure is lamellar fragments, and for the DHTAC/ Coco20 mixture there is a significant coexisting micellar component. The lamellar fragments align in the flow direction and broadly follow the flow pattern expected for a Newtonian fluid. However, for both mixtures, in the off-flow axis regions a more complex distribution of orientations is observed. This is attributed to an incomplete mixing of the two principle flow directions or regimes, resulting in a more complex flow pattern. The demixing can be considered as a form of banding. These more highly aggregated systems are showing an enhanced sensitivity to the effect of flow Supporting Information Available: Graph of scattered intensity as a function of scattering vector, Q, and intensity contour maps. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scamehorn, J. F. Phenomena in mixed surfactant systems. Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, D.C., 1986; p 324.
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