Morphology Transitions in Nonionic Surfactant Adsorbed Layers near

Oct 22, 2005 - structure in solutions of nonionic surfactants near their lower consolute temperature or cloud point. Although the clouding of nonionic...
0 downloads 0 Views 936KB Size
Download PDF
11850

Langmuir 2005, 21, 11850-11855

Morphology Transitions in Nonionic Surfactant Adsorbed Layers near Their Cloud Points Annabelle Blom and Gregory G. Warr* School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia

Erica J. Wanless School of Environmental & Life Sciences, University of Newcastle, Newcastle, Australia Received July 26, 2005. In Final Form: September 14, 2005 The structure of adsorbed layers of several polyoxyethylene alkyl ether (CnEm) nonionic surfactants on silica and graphite surfaces has been imaged using atomic force microscopy as a function of temperature up to their cloud points. For all surfactants with a cloud point within the experimentally accessible range, the adsorbed layer morphology on silica evolved from globules at low temperatures first into rods and then a mesh with increasing temperature. This mesh structure was retained even when the solutions were heated above their cloud points into the two-phase coexistence region. Only C12E3 was observed to form a laterally unstructured bilayer. On graphite, all surfactants formed straight, parallel hemicylinders at all temperatures examined.

Introduction Controversy persists regarding the origin and microstructure in solutions of nonionic surfactants near their lower consolute temperature or cloud point. Although the clouding of nonionic solutions has been investigated theoretically and experimentally over many years by many authors,1-11 questions about its origin and the influence of temperature on micellar size remain. Early light scattering experiments on dilute aqueous solutions of polyoxyethylene oxide surfactants showed a considerable increase in scattered light intensity with increasing temperature,12,13 interpreted as micelle growth via a sphere to rod transition driven by the reduction in wateraccessible headgroup area.14 This interpretation neglects any effect of intermicellar interactions on scattered intensity, as the critical concentration for many nonionic surfactants is below 1 wt %.15 However, as it is known that the approach to the critical point in binary mixtures is accompanied by long-range * To whom correspondence should be addressed. E-mail: g.warr@ chem.usyd.edu.au. (1) Rupert, L. A. M. J. Colloid Interface Sci 1992, 153, 92-105. (2) Zulauf, M.; Rosenbusch, J. P. J. Phys. Chem. 1983, 87, 856-862. (3) Corti, M.; Degiorgio, V. Phys. Rev. Lett. 1980, 45, 1045-1048. (4) Tasaki, K. J. Am. Chem. Soc. 1996, 118, 8459-8469. (5) Hey, M. J.; Ilett, S. M.; Davidson, G. J. Chem. Soc., Faraday Trans. 1995, 91, 3897-3900. (6) Thurston, G. M.; Blankschtein, D.; Fisch, M. R.; Benedek, G. B. J. Chem. Phys. 1986, 84, 4558-4562. (7) Blankschtein, D.; Thurston, G. M.; Benedek, G. B. Phys. Rev. Lett. 1985, 54, 955-958. (8) Blankschtein, D.; Thurston, G. M.; Benedek, G. B. J. Chem. Phys. 1986, 85, 7268-7288. (9) Schick, M. J. Nonionic Surfactants; Marcel Dekker Inc.: New York, 1967; Vol. 1. (10) Kuriyama, K. Kolloid-Z. 1962, 180, 55-63. (11) Dwiggins, C. W., Jr.; Bolen, R. J. J. Phys. Chem. 1961, 65, 17871788. (12) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman, B. J. Phys. Chem. 1983, 87, 4548-4553. (13) Strey, R.; Pakusch, A. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum Press: New York, 1986; Vol. 4, Part II; pp 465-472. (14) Lum Wan, J. A.; Warr, G. G.; White, L. R.; Grieser, F. Colloid Polym. Sci. 1988, 265, 528-534. (15) Wan Os, N. M. Physico-chemical properties of selected anionic, cationic and nonionic surfactants; Elsevier: New York, 1993.

concentration fluctuations, and hence by a large scattering cross-section, cloud points can also be interpreted in terms of attractions between micelles.16,17 This approach has been used to interpret light12,13 and neutron scattering18,19 as well as transient fluorescence quenching experiments,20 invoking minimal changes in micelle size with temperature. Both micellar growth and intermicellar interactions may be important, and a variety of theoretical frameworks encompassing both effects have been devised.6-8,21,22 The classical explanation of clouding in this interpretation is that nonionic surfactant micelles interact via an attractive potential whose well depth increases with temperature.23 As the cloud point is approached, these micelles attract each other and form clusters.3 Again, micellar hydration is implicated. At low temperatures, the ethylene oxide units are well hydrated, and this bound water prevents interpenetration of headgroups of neighboring micelles. At higher temperatures, dehydration may induce intermicellar hydrogen bonding through shared water molecules associated with the headgroups.2 Glatter et al.24-26 have recently interpreted small-angle neutron scattering (SANS) and viscosity measurements of various nonionic surfactants as interacting, rodlike micelles, finding a sphere-to-rod transition with increasing tem(16) Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984, 88, 309-317. (17) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 1442-1445. (18) Triolo, R.; Magid, L. J.; Johnson, S. J.; Child, H. R. J. Phys Chem. 1982, 86, 3689-3695. (19) Zulauf, M.; Weckstrom, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J. Phys. Chem. 1985, 89, 3411-3417. (20) Zana, R.; Weill, C. J. Phys. Lett. 1985, 46, L-953-L-960. (21) Goldstein, R. E. J. Chem. Phys. 1986, 84, 3367-3378. (22) Kjellander, R. J. Chem. Soc., Faraday Trans. 2 1982, 78, 20252042. (23) Manohar, C. Mechanism of the Clouding Phenomenon in Surfactant Solutions. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Marcel Dekker Inc.: New York, 2003; Vol. 109, p 211. (24) Glatter, O.; Fritz, G.; Llindner, H.; Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Egelhaaf, S. U. Langmuir 2000, 16, 8692-8701. (25) Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1997, 30, 431-442. (26) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197-209.

10.1021/la0520334 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/22/2005

Morphology Transitions in Surfactant Layers

perature. They found that surfactants with small headgroups and low cloud points, such as C8E4 and C12E5, formed rodlike micelles even at low temperatures. An alternate mechanism for clouding was proposed involving branching of long, flexible, rodlike micelles of ionic surfactants in electrolyte solutions. At a critical concentration and temperature, excess solvent is expelled as the network of branched, interconnected cylinders spontaneously shrinks.27 This has been extended recently to explain criticality in binary nonionic surfactant and water systems and nonionic surfactant microemulsions.28 With increasing temperature a sphere to rod transition is driven by a reduction in the water-accessible headgroup area which is favored by the hydrophobic effect, reducing the interfacial area per molecule.29 At higher temperatures, micelles begin to branch by the formation of junctions which are lower in free energy than end caps. As the number of these thermally generated junctions increases, a larger network of branched rods forms.30 Upon reaching a critical density of junctions, a connected or saturated network spanning the entire volume of the system is formed. The induced attraction for 3-fold junctions is strong enough to drive a first-order phase transition between a densely connected network and a dilute phase,30,31 triggered by the increase in configurational entropy of the large number of junctions. The observation of dense networks of branched micelles exhibiting 3-fold junctions in nonionic surfactant solutions by cryogenic transmission electron microscopy (cryo-TEM)32,33 provides strong visual support for this mechanism of phase separation. A number of recent studies using atomic force microscopy (AFM) have shown that there are strong parallels between the aggregate structures in surfactant adsorbed layers at solid/solution interfaces and those that occur in the bulk.34-38 Surfactant adsorbed layer structures on silica surfaces in particular have been shown to differ little from bulk aggregate structures.39 For instance, DTAB, TTAB,40 and CTAB41 form globular aggregates on silica, as they do in solution, whereas they form cylinders or bilayers on mica. Polyoxyethylene surfactants adsorb onto silica through hydrogen bonding with surface silanol groups, and adsorbed micelles of Triton X-100 were identified on silica by fluorescence quenching.42 Nonionic (27) Porte, G.; Gomati, R.; El Haitamy, O.; Appell, J.; Marignan, J. J. Phys. Chem. 1986, 90, 5746-5751. (28) Tlusty, T.; Safran, S. A.; Menes, R.; Strey, R. Phys. Rev. Lett. 1997, 78, 2616-2619. (29) Heerklotz, H.; Tsamaloukas, A.; Kita-Tokarczyk, K. K.; Strunz, P.; Gutberlet, T. J. Am. Chem. Soc. 2004, 126, 16544-16552. (30) Zilman, A.; Safran, S. A.; Sottmann, T.; Strey, R. Langmuir 2004, 20, 2199-2207. (31) Zilman, A.; Tlusty, T.; Safran, S. A. J. Phys.: Condens. Matter 2003, 15, S57-S64. (32) Bernheim-Groswasser, A.; Wachtel, E.; Talmon, Y. Langmuir 2000, 16, 4131-4140. (33) Bernheim-Groswasser, A.; Tlusty, T.; Safran, S. A.; Talmon, Y. Langmuir 1999, 15, 5448-5453. (34) Binnig, G.; Quate, C. H.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-933. (35) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692. (36) Manne, S.; Cleveland, J. P.; Gaub, B. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (37) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (38) Blom, A.; Duval, F. P.; Kovacs, L.; Warr, G. G.; Almgren, M.; Kadi, M.; Zana, R. Langmuir 2004, 20, 1291-1297. (39) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548-2556. (40) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (41) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 85588567. (42) Levitz, P.; van Damme, H.; Kervais, D. J. Phys. Chem. 1984, 88, 2228-2235.

Langmuir, Vol. 21, No. 25, 2005 11851

surfactants that aggregate into spherical micelles in solution also form globules on silica.43 In contrast with this, graphite strongly directs the self-assembly of adsorbed nonionic surfactants into hemicylinders, templated by a strongly bound horizontal monolayer.44 One might reasonably expect, therefore, that the morphology evolution observed on warming a bulk nonionic surfactant solution toward its cloud point would also occur at the solid/solution interface. In this paper atomic force microscopy imaging is used to examine the evolution of the adsorbed layer morphology of several nonionic surfactants with differing alkyl and ethoxy chain lengths on both silica and graphite as a function of temperature. Materials and Methods Pentaethylene glycol monododecyl ether (C12E5), hexaethylene glycol monotetradecyl ether (C14E6), hexaethylene glycol monohexadecyl ether (C16E6), triethylene glycol monododecyl ether (C12E3), octaethylene glycol monododecyl ether (C12E8), and octaethylene glycol monotetradecyl ether (C14E8) were purchased from Fluka and used as received. Polyoxyethylene 23 lauryl ether (Teric G12A23, analogous to Brij-35) was provided by Orica Australia, contains a distribution of ethylene oxide chains about a mean of 23, and was used as received. All solutions were prepared in Milli-Q water. AFM experiments were performed on both hydrophilic silica (Silicon Valley Microelectronics Inc., California) and graphite (Advanced Ceramics Corp.). Immediately prior to each graphite experiment, a fresh surface was produced by stripping the top layers of graphite with adhesive tape. The silica surface was cleaned by rinsing with copious quantities of deionized water and ethanol, irradiated under UV for 30 min, and then soaked in a 1 wt % NaOH solution for 1 min to reestablish the hydrophilicity of the surface. The silica was cleaned in exactly the same manner for all experiments performed on this surface. The surfactant film was imaged using a Digital Instruments NanoScope IIIa MultiMode instrument in contact mode in a standard fluid cell. Standard Si3N4 cantilevers with sharpened tips (Digital Instruments, California) were used and cleaned by irradiation with ultraviolet light for 30 min prior to use. The solution was held in a fluid cell and sealed by a silicone O-ring. Both were cleaned by sonication for 10 min in the surfactant solution to be studied, rinsed copiously in ethanol and deionized water, and dried using filtered nitrogen. The method of soft contact imaging45 was employed with the tip held above the adsorbed film primarily by steric repulsion. This method enables generation of a force map of the adsorbed layer without the tip physically contacting the sample. All force curves within a temperature series on a surfactant film were performed on the one tip; however, different tips were used between surfactant systems. For temperature control, the AFM instrument was placed inside a thermal chamber (Sable systems) during the experiment. Using a thermal sensor placed at the base of the sample cell (Digital Instruments), temperatures inside the cell once the laser was illuminated and scanning commenced were found to be 4 °C greater than the ambient temperature of the chamber. All temperatures reported in this paper are thus cell temperatures, which were varied between 15 and 40 °C.

Results and Discussion Adsorbed Morphologies on Silica. Figure 1 shows AFM deflection images of an adsorbed layer of 0.12 mM C16E6 on silica at three temperatures. At 20 °C this surfactant forms adsorbed globules (Figure 1a) with a nearest-neighbor periodicity of 9.2 nm, which agrees with previously results for this surfactant.43 Increasing the cell (43) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys Chem. B 1998, 102, 4288-4294. (44) Kiraly, Z.; Findenegg, G. H. J. Phys. Chem. B 1998, 102, 12031211. (45) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. E.; Hansma, P. K. Langmuir 1994, 10, 4409-4413.

11852

Langmuir, Vol. 21, No. 25, 2005

Blom et al.

Figure 1. AFM deflection images of 0.12 mM [50(cmc)] C16E6 on silica showing an adsorbed morphology of (a) globules at 20 °C, (b) rods at 25 °C, and (c) a mesh at 30 °C. Inset: Fourier transforms of the AFM images show their evolution from isotropic through anisotropic and back to an isotropic lateral structure on warming.

Figure 2. AFM deflection images of 0.05 mM [5(cmc)] C14E6 on silica at various temperatures showing an adsorbed layer morphology of (a) globules at 22 °C, (b) rods at 26 °C, and (c) a mesh at 30 °C. Inset: Fourier transforms show the morphology evolution with temperature.

temperature to 25 °C caused a change in aggregate shape to flexible cylinders (Figure 1b), and further warming to 30 °C resulted in the recovery of an isotropic structure (Figure 1c). The progression in observed textures is consistent with decreasing aggregate curvature on warming and the formation of an adsorbed mesh38 consisting of a network of densely branched, cylindrical micelles near the cloud point. We have previously identified mesh structures in a variety of adsorbed surfactant layers on mica where the aggregate curvature is intermediate between those of cylinders and a bilayer. Meshes can be difficult to distinguish from adsorbed globules in an individual AFM image but are identified primarily by their place in a curvature progression, as they are here. The alternative is an aggregate morphology that evolves from globules into cylinders and then back to globules on warming, which would be more difficult to understand. On mica the Fourier transform of a mesh exhibits a “filled circle” pattern reflecting a distribution of correlation lengths which differs from the ring that defines the nearest-neighbor distance in adsorbed globular micelles. On silica the roughness of the substrate masks this difference, and both globule and mesh adsorbed films can exhibit filled circle Fourier transforms. The final characteristic previously noted for adsorbed meshes on mica is that the smallest correlation length or apparent nearest-neighbor spacing determined from the Fourier transform is significantly larger than that for an array of globules. This is again observed for C16E6, where the apparent periodicity increases from 9.2 nm for globules at 20 °C to 10.9 nm for the mesh at 30 °C. For C16E6 the adsorbed mesh structure was observed from 30 °C to temperatures slightly above the cloud point at 38 °C. At higher temperatures the solution turbidity limited the resolution of the AFM instrument.

A similar progression of adsorbed layer textures was obtained for adsorbed layers of C14E6 on silica (Figure 2). At 22 °C, globules were again observed but with a slightly smaller periodicity of 8.8 nm. Increasing temperature again causes a reduction in curvature, leading first to the formation of rods at 26 °C and then to a mesh for temperatures of 30 °C and above. C12E5 also showed the same progression (Figure 3). At 21 °C, globules with a periodicity of 7.3 nm were observed. Increasing temperatures similarly resulted in a transformation to rods at 28 °C and a mesh at 30 °C. The solution remained clear at this temperature but very rapidly turned cloudy on warming to its cloud point of 31 °C. Measurements at intermediate temperatures showed that the changes in morphology were gradual and reversible. Elongation of spheres into rods proceeded by formation of short rods and their subsequent growth. The evolution of the mesh was preceded by a region of branched rods. A similar but incomplete sequence of adsorbed layer morphologies were observed for C14E8 and C12E8, whose cloud points lie well above the accessible temperature range at 71 and 78 °C, respectively. Results for C14E8 on warming from 22 to 32 °C are shown in Figure 4. At 22 °C, the adsorbed layer of C14E8 consists of globular aggregates, again in line with previous results.43 Increasing the temperature to 32 °C resulted in a reduction in curvature of the adsorbed aggregates as they transformed continuously from globules into rods. C12E8 behaved similarly but only transformed from globules into short rods over the temperature range investigated. Adsorbed layer morphologies on silica for all the surfactants studied are summarized in Table 1. The three surfactants with accessible cloud points all exhibit a complete progression from globules into cylinders and then

Morphology Transitions in Surfactant Layers

Langmuir, Vol. 21, No. 25, 2005 11853

Figure 3. AFM deflection images of 0.12 mM [2(cmc)] C12E5 on silica showing an adsorbed morphology of (a) globules at 21 °C, (b) rods at 28 °C, and (c) a mesh at 30 °C.

Figure 4. AFM deflection images of C14E8 adsorbed layers on silica showing (a) a globular morphology at 22 °C, (b) elongated globules or short rods at 28 °C, and (c) longer rods at 32 °C. Table 1. Summary of Structures Formed by CnEm Surfactants on Silica at 20 and 30 °Ca surfactant C16E6 C14E8 C14E6 C12E8 C12E5 C12E3

cloud point15 structure spacing (°C) at 20 °C (nm) 38 71 40 75 31

globules globules globules globules globules bilayer

9.2(0.2 8.8(0.4 8.8(0.2 7.0(0.6 7.3(0.3

structure at 30 °C

“spacing” (nm)

mesh 10.9 ( 0.9 cylinders 10.1 ( 0.5 mesh 10.3 ( 0.8 short rods 9.2 ( 0.5 mesh 8.5 ( 0.3 bilayer

a The “spacing” or center-to-center distance between neighboring aggregates was determined from the maximum intensity in the two-dimensional Fourier transform of each image.

a mesh structure, and their Fourier transforms all show a greater periodicity or apparent nearest-neighbor spacing for the mesh at 30 °C than for the array of adsorbed globules at 20 °C. These changes in the adsorbed layer morphology are also consistent with the previously reported increase in the saturation surface coverage of nonionic surfactants adsorbed at the silica/aqueous solution interface.46 As noted previously,38,47,48 the maximum surface coverage increases as adsorbed micelles transform from spheres (60%) into rods (79%), a branched network, and ultimately a bilayer (100%). For these more soluble surfactants studied, two isotropic phases coexist in dilute aqueous solution. In contrast, C12E3 is biphasic in quite dilute solution, forming only a lamellar phase that coexists with a dilute aqueous solution. Similar to its behavior in the bulk, this surfactant forms only (46) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloids Surf. 1984, 12, 255-270. (47) Schulz, J. C.; Warr, G. G.; Hamilton, W. A.; Butler, P. D. J. Phys. Chem. B 1999, 103, 11057-11063. (48) Schulz, J. C.; Warr, G. G.; Butler, P. D.; Hamilton, W. A. Phys. Rev. E 2001, 63, 0416041-0416045.

planar aggregates and adsorbs onto silica as a laterally unstructured bilayer over the entire temperature range examined (Figure 5). This contrasts markedly with the behavior of C16E6, C14E6, and C12E5, none of which formed bilayers even above their cloud point. Force curves for these surfactants show only minor changes to the interaction between the imaging tip and silica surface with temperature (Figure 6). For C12E5 and C14E6, a single jump into contact from approximately 4.0 and 4.3 nm, respectively, was observed for each surfactant at all temperatures (Figure 6a,b). In C12E5 both the height and thickness of the repulsion generated by the adsorbed layer are independent of temperature; for C14E6 the barrier height increases as the temperature is raised, but the thickness is unchanged. Unusually, two repulsive walls were observed for C16E6, at tip-surface separations around 7 and 2 nm (Figure 6c). A similar double repulsive barrier in the AFM force profile has been reported previously for nonionic surfactants adsorbed on silica.43 Some adsorbed layer images in that work were also sensitive to whether imaging was carried out on the nearer or farther wall. In that study it was C12E5 and C14E6 that exhibited two repulsive walls, whereas C16E6 did not. Here, however, only one morphology was observed for adsorbed C16E6, independent of the imaging pressure or distance. A similar double repulsive barrier was also observed in a colloid probe AFM study of adsorbed C12E5 films adsorbed on silica.49 In that study an adsorbed bilayer was assumed, and the outer force wall was interpreted as arising from steric interaction between the bilayers on the colloid probe and substrate. The inner force wall was concluded to result from bilayer hemifusion, where the two surfaces are in monolayer-monolayer contact. However, hemifusion is (49) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412-418.

11854

Langmuir, Vol. 21, No. 25, 2005

Blom et al.

Figure 5. AFM deflection images of 0.13 mM C12E3 adsorbed layers on silica at (a) 22 °C and (b) 30 °C showing a laterally unstructured (bilayer) film. Fourier transforms (not shown) reveal no lateral periodicity.

unlikely to occur in these systems, where the adsorbed layer morphology is not a bilayer.50,51 We speculate that the second repulsive wall observed here for C16E6 is due to nonionic surfactant adsorbed onto the AFM tip. The strength of this adsorption will be sensitive to the shape and chemistry of the tip, as well as to solution conditions such as the surfactant concentration. The double repulsion we observe for C16E6 may simply be due to the high surfactant concentration (relative to the cmc), and further work will be required to resolve this. The observed changes in the adsorbed layer morphology of nonionic surfactants on silica as they are warmed toward their cloud points (Figures 1-4) are consistent with the formation of a saturated network of branched adsorbed micelles and thus support the proposal that demixing in the bulk is driven by network formation. Polyoxyethylene surfactants are soluble in water due to hydrogen bonding of the solvent with the ether oxygens of the headgroup. Raising the temperature effectively reduces the solvent quality for the polyoxyethylene chain and hence the headgroup size. This lowers the optimal aggregate curvature and increases the surfactant packing parameter.52 The energy cost of maintaining end caps leads first to elongation of adsorbed globular micelles into rods, and then further lowering of the mean curvature leads to the formation of branch points and a saturated network.32,33 This curvature progression is all evident in the adsorbed layer at the silica/solution interface. Adsorbed Morphologies on Graphite. Most conventional surfactants, both ionic and nonionic, adsorb onto (50) Drummond, C.; Israelachvili, J.; Richetti, P. Phys. Rev. E 2003, 67, 066110-066116. (51) Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. Biochemistry 1992, 31, 1794-1805. (52) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1526-1568.

Figure 6. Force curves for nonionic surfactants with accessible cloud points adsorbed onto silica above their cmc’s at various temperatures: (a) 0.12 mM C12E5, (b) 0.05 mM C14E6, and (c) 0.12 mM C16E6.

graphite as hemicylinders.36,53-55 The basal plane of graphite consists of sheets of hexagonal rings of carbon atoms with a 3-fold axis of rotation. Strong anisotropic attractions between the substrate and alkyl chain make them lie down on the surface along the crystallographic axes, propagating “bilayer” tail-to-tail stripes that act as a templating layer for long, straight hemicylinders. The same or very similar structures were observed for C16E6, C14E6, and C12E5 at all temperatures examined up to and in some cases beyond their cloud points. The periodicity of the observed stripes did not vary with temperature within experimental error. C12E23 is unusual in that it has been reported to form a featureless monolayer on graphite (Figure 7a). This was attributed to the large headgroup inhibiting the formation of hemicylinders.54,56 Increasing the temperature to 28 °C transformed the C12E23 adsorbed film into hemicylinders (53) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 1150711511. (54) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349-4356. (55) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214. (56) Patrick, H. N.; Warr, G. G. Colloids Surf., A 2000, 162, 149157.

Morphology Transitions in Surfactant Layers

Langmuir, Vol. 21, No. 25, 2005 11855

unstructured film at 22 °C, a C12E23 solution was monitored for approximately 40 h. However, no rearrangement into hemimicelles was observed in this time. We conclude from this that an increase in temperature is necessary to reduce the headgroup size and allow registry between the surface and the alkyl chain to be established, allowing the formation of hemicylinders. However, the first layer of highly oriented surfactant, once formed, is so strongly bound44 that the assembled hemimicelle layer is retained. This also suggests that adsorbed layers of other surfactants on graphite will not in general reorganize once the templated hemicylinder morphology has been established, consistent with our observations on other nonionic surfactants at elevated temperature. Conclusions

Figure 7. AFM deflection images showing the change in the adsorbed layer morphology with temperature of 0.2 mM [2(cmc)] C12E23 on graphite. At 22 °C a laterally unstructured monolayer was observed (a) and at 28 °C hemicylinders (b). Inset: The Fourier transform in (b) shows the high degree of order of the adsorbed hemicylinders. The hemicylinders remained for several hours after temperature was lowered back to below the initial starting temperature of 22 °C.

identical to surfactants with shorter polyoxyethylene chains (Figure 7). That is, the aggregate curvature apparently increased with temperature contrary to expectations and to our observations on silica. However, upon cooling back to 22 °C, the adsorbed C12E23 film did not return to a featureless monolayer after a further 5 h. Instead hemicylinders persisted. To ascertain whether the hemicylinder morphology would eventually develop from a possibly metastable

AFM soft contact imaging shows that the adsorbed layer morphologies of several nonionic surfactants on silica decrease in curvature upon warming. Three surfactants with cloud points within the experimentally accessible range, T < 40 °C, all exhibited the same progression from adsorbed globular micelles into cylinders and then a mesh at temperatures immediately below their cloud points. None of these surfactant adsorbed layers proceeded to form bilayers even above their cloud points. This was only observed for the lamellar phase forming C12E3. This curvature/morphology progression at the silica/solution interface supports the proposal of saturated network formation in bulk aqueous solution as the mechanism for the lower consolute boundary in nonionic surfactants. In contrast, no effect of temperature was observed for adsorbed nonionic surfactant morphologies on graphite, except for C12E23, which was found to reorganize on warming from a laterally unstructured film into hemicylinders, which then persisted on the surface after cooling. Acknowledgment. This work was supported by the Australian Research Council Linkage (International) Program and the NEDO International Joint Research Grant Program. A.B. acknowledges receipt of a Henry Bertie and Florence Mabel Gritton Postgraduate Scholarship from The University of Sydney. LA0520334