Structure and Composition of Cationic− Nonionic Surfactant Mixed

Structure and Composition of Cationic−Nonionic Surfactant Mixed Adsorbed Layers on Mica. Annabelle Blom, and Gregory G. Warr*. School of Chemistry, ...
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Langmuir 2006, 22, 6787-6795

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Structure and Composition of Cationic-Nonionic Surfactant Mixed Adsorbed Layers on Mica Annabelle Blom and Gregory G. Warr* School of Chemistry, The UniVersity of Sydney, NSW 2006 Australia ReceiVed February 15, 2006. In Final Form: May 22, 2006 The composition and morphology of mixed adsorbed layers comprising one of several poly(oxyethylene) alkyl ether nonionic surfactants, CiEj, and two cationic surfactantssdodecyltrimethylammonium bromide (DTAB) and tetradecyltriethylammonium bromide (TTeAB)sat the mica/solution interface have been studied using depletion adsorption and atomic force microscopy. The nonionic surfactants do not themselves adsorb onto mica, but can coadsorb with a cationic surfactant. The extent of their hydrophobic association with the adsorbed cationic surfactant depends on alkyl chain length, while the adsorbed layer morphologies are sensitive to the number of ethoxy groups. Nonionic surfactants with headgroups containing less than eight ethylene oxide units decrease the adsorbed aggregate curvature, gradually transforming globular TTeAB or cylindrical DTAB adsorbed aggregates into a rod, mesh, or bilayer structure. Those with larger headgroups favor globular aggregates. The mechanism by which the nonionic surfactant modifies the adsorbed morphology is the formation of defects in the form of cylinder end-caps or branchpoints, leading to adsorbed layer compositions that differ from ideal mixing predictions. All mixed adsorbed films become saturated with the nonionic component when the capacity of the aqueous side of the adsorbed layer is reached.

Introduction Mixed surfactant systems are frequently used to provide optimal performance and effect in a wide range of applications. Despite their extensive use, however, there is a lack of microscopic understanding of the behavior of mixed surfactants at interfaces. Adsorption isotherms have been used most extensively to study mixtures of nonionic and ionic surfactants at the alumina1 and quartz2 interfaces, as well as on poly(styrene) and poly(tetrafluoroethylene). For both cationic and anionic surfactants, the addition of nonionic surfactant displaced the ionic surfactant from the interface, reducing its adsorption density. This was attributed to competition by the nonionic surfactant reducing the amount of ionic surfactant able to pack on the surface.1 Adsorption isotherms as well as other probes of mixed adsorbed surfactants on various solid surfaces, including microcalorimetry and spectroscopy, have recently been reviewed.3 Most of this work has been interpreted by regarding the adsorbed layer as a laterally unstructured film or adsorbed bilayer at sufficiently high concentrations above the critical micelle concentration (cmc) of the mixture.3 Adsorbed mixtures are most often treated using the ideal or regular solution models, although this overlooks differences in surfactant-substrate interactions.4 Atomic force microscopy (AFM) has been used over the past decade to examine adsorbed layer structures of surfactants and surfactant mixtures on a wide range of solid substrates.5-12 * Author for correspondence. E-mail: [email protected]. (1) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177, 222-228. (2) Dixit, S. G.; Vanjara, A. K.; Nagarkar, J.; Nikoorazm, M.; Desai, T. Colloids Surf., A 2002, 205, 39-46. (3) Somasundaran, P.; Huang, L. AdV. Colloid Interface Sci. 2000, 88, 179208. (4) Penfold, J.; Staples, E. J.; Tucker, I.; Thomas, R. K. Langmuir 2000, 16, 8879-8883. (5) Manne, S.; Cleveland, J. P.; Gaub, B. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (6) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (7) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207-3214. (8) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (9) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349-4356.

Together with neutron reflectometry13,14 and ellipsometry,15,16 it has shown that rarely are adsorbed films simply bilayers. Recent AFM investigations have shown strong correlations between hydrophobic chain and headgroup structure and miscibility, and the equilibrium morphology of mixed adsorbed layers.17,18 Neutron reflectometry has recently been used to show that pH affects the relative affinities of cetyltrimethylammonium bromide (CTAB) and C12E6 for oxidized silicon and, hence, adsorbed layer compositions at pH 2.4 and 7.4,19 The addition of diblock hydrophobe-poly(ethylene oxide) copolymers to adsorbed cationic surfactants on mica has been shown to induce a morphological transition from cylinders to globules with sequential addition of copolymer.20 Both octadecyl chains and polystyrene oligomers were used as hydrophobes, with ethylene oxide chains of between (on average) 50 and 1300 units, lending a steric stabilizing coating to the adsorbed film. Like poly(oxyethylene) nonionic surfactants, these amphiphilic copolymers do not adsorb to mica on their own,21 but they can coadsorb to the mica surface by hydrophobic association with an adsorbed surfactant layer. In this paper, we examine the effect of nonadsorbing poly(oxyethylene) alkyl ether nonionic surfactants, CnEm, on the composition and morphology of adsorbed surfactant layers on (10) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692. (11) Blom, A.; Duval, F. P.; Kovacs, L.; Warr, G. G.; Almgren, M.; Kadi, M.; Zana, R. Langmuir 2004, 20, 1291-1297. (12) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5, 88-94. (13) Schulz, J. C.; Warr, G. G.; Butler, P. D.; Hamilton, W. A. Phys. ReV. E 2001, 63, 0416041-0416045. (14) Schulz, J. C.; Warr, G. G.; Hamilton, W. A.; Butler, P. D. J. Phys. Chem. B 1999, 103, 11057-11063. (15) Tiberg, F. J. J. Chem. Faraday Trans. 1996, 92, 531-538. (16) Tiberg, F.; Jonsson, B.; Lindman, B. Langmuir 1994, 10, 3714-3722. (17) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283-5287. (18) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915-5920. (19) Penfold, J.; Staples, E.; Tucker, I.; Thompson, J.; Thomas, R. K. Int. J. Thermophys. 1999, 20, 19-34. (20) Robelin, C.; Duval, F. P.; Richetti, P.; Warr, G. G. Langmuir 2002, 18, 1634-1640. (21) Blom, A.; Drummond, C.; Wanless, E. J.; Richetti, P.; Warr, G. G. Langmuir 2005, 21, 2779-2788.

10.1021/la0604477 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/22/2006

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mica by combining AFM with depletion adsorption isotherms. The roles of the alkyl and ethylene oxide chain lengths of the nonionic surfactant are assessed as these are independently varied and added to cationic surfactants with both globular and cylindrical adsorbed layer geometries. Experimental Section Dodecyltrimethylammonium bromide (DTAB; Aldrich), diethylene glycol monododecyl ether (C12E2; Fluka), pentaethylene glycol monododecyl ether (C12E5; Fluka), hexaethylene glycol monododecyl ether (C12E6; Fluka), heptaethylene glycol monododecyl ether (C12E7; Fluka), octaethylene glycol monododecyl ether (C12E8; Fluka), octaethylene glycol monotetradecyl ether (C14E8; Aldrich), tetraethylene glycol hexadecyl ether (C16E4; Aldrich), diethylene glycol monooctadecyl ether (C18E2; Fluka), and octaethylene glycol monooctadecyl ether (C18E8; Fluka) were all used as received. Purity was confirmed by high-performance liquid chromatography (HPLC) in the course of the depletion adsorption experiments (see below). Polyoxyethylene 23 lauryl ether, (Teric G12A23) was a sample provided by Orica Australia containing a distribution of ethylene oxide chains about a mean of 23, and was used as received. Tetradecyltriethylammonium bromide (TTeAB) was synthesized and purified using a method described previously.22 The solid substrate used in all AFM experiments was muscovite mica (Probing and Structure, Queensland), which was cleaved using adhesive tape immediately before use. All experiments were performed at room temperature (approximately 23 °C) in Millipore water with a conductivity of 18 MΩ cm-1. Solutions were prepared with the concentration of the quaternary ammonium surfactant kept constant and with the sequential addition of nonionic surfactant. Component Adsorption Isotherms. Adsorbed amounts of nonionic and cationic surfactant components in each mixture were determined using solution depletion. Solutions of surfactant mixtures (18.5 mL) were combined with 1.5 g of mica powder (Brown C D Mica Co, Sydney) and mixed for 3 days on a roller-mixer (Ratek BTR5, Ratek Instruments). The mica powder has a low specific surface area around 1 m2 g-1, so we have disregarded adsorption onto the edges. The solutions were centrifuged (Hettich Universal centrifuge) at 8000 rpm for 5 min, and concentrations of each component in the supernatant were analyzed by HPLC. A Waters HPLC system was used with a differential refractometer detector (Waters 2410) and a Symmetry C18 (5 µm, 3.9 × 150 mm) column operating with Empower Pro software. The eluent system was an 85:15 methanol/water mixture with 0.2 M NaCl, and the flow rate used was 0.7 mL/min. Under these operating conditions, the retention time was approximately 5 min for DTAB, 7 min for TTeAB, and the nonionic surfactants ranged from 10 to 15 min, depending on the alkyl tail length. Sample injection volumes were 200 µL, and there were suitable differences in the retention time between the two surfactants to ensure good peak separation, despite these peaks being broad. The area of the peak was used for calculation of the concentration. AFM Experiments. Adsorbed layer structures at the mica/aqueous solution interface were examined using a Digital Instruments NanoScope IIIa Multimode in contact mode in a standard fluid cell. Standard Si3N4 cantilevers with sharpened tips (Digital Instruments, Santa Barbara, CA) were irradiated with ultraviolet light for 30 min prior to use. The solution was held in a fluid cell and sealed by a silicone O-ring, which 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. Images display deflection data captured using a soft-contact method5 in which surfactant adsorption on (the tip and) the substrate allows for imaging using electrical double-layer and steric forces, with the imaging set-point force kept below that necessary to push the tip through the adsorbed layer and into contact with the substrate. This method enables the generation of a force map of the adsorbed (22) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236-10244.

Blom and Warr layer without the tip physically contacting the sample. Typical imaging scan rates varied between 8 and 15 Hz, and low integral gains (∼0.8-1) were used. Variation of frequency, scan angle, imaging pressure, and gains had no effect on the observed morphology. All images are unmodified except for flattening along the scan lines. The adsorbed layer morphology of the surfactant mixtures was initially monitored over a period of 24 h, but no change was observed after the first few minutes. All images shown are for solutions that were allowed to equilibrate for approximately 2 h after injection into the cell.

Results We determined the adsorbed layer morphologies on mica for C12H25N(CH3)3Br (DTAB), which forms long, cylindrical aggregates,10,11,18 and C14H29N(C2H5)3Br (TTeAB), which forms globular aggregates,10,11 as a function of added nonadsorbing nonionic surfactant CiEj. The concentration of the cationic surfactant is held constant at 26mM for DTAB and 6.3mM for TTeAB, which is about twice their respective cmc’s, ensuring that all the mixed surfactant solutions are also well above their mixed cmc’s. All nonionic surfactant concentrations are also above their cmc’s, which are no greater than 0.1 mM.23 As detailed below, the adsorbed layer morphology often changes with the concentration of nonionic surfactant, sometimes through a progression of several structures. For selected systems representing each morphology progression, the adsorption isotherms of each component of the mixed system has also been determined by depletion. Here also the experiment was designed to yield an equilibrium concentration of the cationic surfactant after adsorption close to twice its cmc, so that conditions are as similar as possible between the AFM and depletion experiments. Results for various mixed systems are clustered by their common morphology change with composition. 1. DTAB: Uptake of Nonionic Surfactants with Large Headgroups. Figure 1 illustrates the effect of C12E23 on the morphology of an adsorbed DTAB layer. In pure DTAB (Figure 1a), the adsorbed film consists of long, meandering adsorbed cylinders with a periodicity of approximately 5.5 nm.10 As C12E23 is added progressively, the adsorbed cylinders shorten along their primary axis (Figure 1b) until they become globular at a bulk concentration around 10 mM (Figure 1c). The change in morphology toward an isotropic arrangement of adsorbed aggregates is clearly seen in the Fourier transforms (shown as insets), where the arcs increase in length with uptake of the nonionic surfactant. The periodicity (nearest-neighbor spacing) in these images remains constant at approximately 5.2 nm, despite a structure change from rods to globules. This behavior closely parallels the observed change in DTAB adsorbed layers on mica with the uptake of block copolymers C18E40 and C18E100, as well as styrene-b-E1300,20 and is consistent with the uptake of the nonionic surfactant by hydrophobic association with adsorbed DTAB. For these block copolymers, the mechanism of the morphology change was not clear, and might have arisen from steric repulsions between polyoxyethylene chains as well as from mismatches in the packing of alkyl chains of different lengths. The behavior of DTAB films containing C12E23 implies that steric repulsions between polyoxyethylene chains is responsible for the observed transformation from rods into more highly curved globules. The same sequence of morphologies is observed when nonionic surfactants with headgroups as small as CiE8 are added into a DTAB adsorbed layer (Figure 2). For each of C12E8, C14E8, and (23) Van Os, N. M. Physicochemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: New York, 1993.

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Figure 1. 200 × 200 nm2 AFM deflection images of mixed adsorbed layers of (a) 26 mM DTAB, with (b) 3.9 mM C12E23 and (c) 10.4 mM C12E23. Insets show Fourier transforms of the images with a full range of 0.781 nm-1.

Figure 2. 200 × 200 nm2 AFM deflection images of mixed adsorbed layers of (a) 26 mM DTAB with (b) 4.11 mM C12E8; (c) 10.3 mM C12E8; (d) 2.6 mM C14E8; (e) 5.2 mM C14E8; (f) 0.3 mM C18E8; and (g) 2.6 mM C18E8. Insets show Fourier transforms of the images with a full range of 0.781 nm-1.

C18E8, the transition from rods to globules occurs similarly by cylinders (Figure 2a) gradually shortening along one axis as nonionic surfactant is added (Figure 2b) to yield globules (Figure 2c), which continue to be observed up to the highest concentrations studied. Although the morphology progression is the same, the change from rods into globules is complete at lower concentrations (or solute mole fractions) of added nonionic surfactants with longer alkyl chain length: 2.6 mM, 5.2 mM, and 10.3 mM for C18E8, C14E8, and C12E8, respectively. Both C12E8 and C12E23 require approximately the same minimum amount to yield a layer of globular aggregates. This is most likely simply due to the greater uptake of surfactants with longer alkyl chains by the DTAB film and the comparatively minor effect of polyoxyethylene chain

length on hydrophobicity, as indicated by the relative effects of alkyl and ethoxy chain lengths on the cmc.23 The observed shape transformations are consistent with nonionic surfactants introducing end-cap “defects” into the adsorbed cylinders. The equilibrium distribution of the lengths of cylindrical micelles in bulk solution is determined by the free-energy difference between the end caps and the cylinder body.24 A smaller end-cap energy cost gives rise to shorter rods. This could readily be achieved here by preferential uptake of the nonionic surfactants into the rod ends, where the local aggregate curvature is highest. Along the body of the cylinders, steric (24) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1526-1568.

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Figure 4. 200 × 200 nm2 AFM deflection images of adsorbed layers of 6.3 mM TTeAB (a) alone, and (b) with 6.3 mM C12E8. Insets show Fourier transforms of the images with a full range of 0.781 nm-1. Figure 3. (a) Adsorbed amounts of DTAB ()), C12E8 (0), and their total (4) on ground mica as a function of C12E8 concentration, and (b) measured surface composition versus bulk solute composition. The vertical dashed line denotes the composition at which the transformation to globular aggregates is complete, and the solid line denotes equal bulk and surface compositions.

interactions would be reduced by a sparser population of nonionic headgroups. One could alternatively postulate a constant density of nonionic surfactant but different conformations in the rod body and end caps. We discuss this further below. AFM imaging reveals only adsorbed layer morphology and not composition. Component adsorption isotherms of C12E8 and DTAB onto mica, measured by solution depletion, are shown in Figure 3. Figure 3a shows the adsorbed amounts of both surfactants onto the surface of powdered mica as the concentration of C12E8 is increased and the equilibrium bulk concentration of DTAB is held constant at 26mM, to closely approximate the conditions used for AFM imaging. All compositions shown in Figure 3 are expected to be well above the mixed cmc for this system. In contrast with previous studies of mixed surfactant systems3 and the results reported below, the adsorbed amount of DTAB on the surface remains constant within experimental error as C12E8 is taken up into the film. As a result, the total amount of adsorbed surfactant increases substantially with the addition of nonionic surfactant. Figure 3b shows the same data presented as mixed film composition versus bulk solution composition. As all the solutions are far above the mixed cmc, the solution and mixed micelle compositions are equivalent in the ideal mixing case, which should be a good description of cationic/nonionic surfactant mixtures in bulk.25 Ideal mixing in the film is thus represented by a line of unit slope, as shown in Figure 3b. The uptake of C12E8 into the DTAB film is well described by ideal mixing at compositions up to nearly 40 mol % (10 mM) in solution, corresponding to the bulk composition at which the adsorbed film is fully transformed from cylinders into globules. Further increasing C12E8 content causes the adsorbed amount to increase more gradually and then plateau at a surface composition near 50 mol % C12E8. (25) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204-5211.

Figure 5. (a) Adsorbed amount of TTeAB (]), C12E8 (0), and their total (4) on ground mica, and (b) measured surface composition against solution composition. The solid line denotes equal bulk and surface compositions.

The same behavior is reflected in the component adsorption isotherm (Figure 3a). At low C12E8 concentrations, the uptake increases linearly, reminiscent of adsolubilization (the incorporation of insoluble solutes into an adsorbed layer) behavior in similar films.26 Above 10.3 mM, the adsorbed amount increases more gradually and appears to plateau at higher concentrations when the adsorbed amounts of DTAB and C12E8 are nearly equal. The AFM images make clear, however, that the adsorbed film is not acting as an inert solvent. Its morphology is changing continuously in the dilute C12E8 range, as steric repulsions between polyoxyethylene groups create more end caps and the rods shorten toward globules. It is remarkable that the total adsorption density increases as the film transforms from rods into globules, which should not (26) Kovacs, L.; Warr, G. G. Langmuir 2002, 18, 4790-4794.

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Figure 6. 200 × 200 nm2 AFM deflection images of mixed adsorbed layers of (a) 26 mM DTAB, with (b) 0.26 mM C18E2; (c) 1.3mM C18E2; (d) 1.3mM C12E2; and (e) 2.6 mM C12E2. Insets show Fourier transforms of the images with a full range of 0.781 nm-1.

pack onto the surface as well as cylinders. There must be sufficient space available between the DTAB rods that the adsorbed layer can approximately double its volume by incorporating nonionic surfactant. 2. TTeAB: Uptake of Nonionic Surfactants with Large Headgroups. TTeAB on its own forms globular adsorbed micelles on mica, by virtue of the steric repulsions between triethylammonium headgroups.10 As Figure 4a shows, the adsorbed film appears as tightly packed globules with an average periodicity of 5.7 nm, and Fourier transforms often show a degree of hexagonal order in the layer.10 Figure 4b shows that the addition of equimolar C12E8 causes no change in either the globular morphology or periodicity. Although imaging is more difficult at higher C12E8 concentrations, as Figure 4b shows, the globule morphology and periodicity persisted up to at least 50 mol % nonionic surfactant. Mixed adsorbed films of TTeAB and C18E8 behaved identically. Adsorbed globules were present at all compositions studied (up to 10 mol % C18E8), supporting the interpretation that the alkyl chain length does not strongly affect adsorbed layer morphology in these systems. Component adsorption isotherms for mixtures of TTeAB (at an approximately constant 6.3 mM) and C12E8 show (Figure 5a) that C12E8 displaces TTeAB from the adsorbed layer. These differ qualitatively from mixed C12E8/DTAB adsorbed layers, but are similar to results reported elsewhere for mixtures of cationic and nonionic surfactants on Al2O3 and quartz.1,2 In C12E8/ TTeAB systems, the total adsorbed amount is constant as a function of composition within experimental error. The adsorbed layer and bulk solution (micelle) compositions are almost equal at low C12E8 content, but, at higher concentrations, the film composition tends toward a plateau near, but below, 50 mol %. This is also clear from the variation in surface and bulk compositions shown in Figure 5b. Adsorbed layer and bulk compositions are nearly identical up to around 30 mol % C12E8, then they deviate toward a plateau composition near 40 mol % C12E8. As with C12E8/DTAB and all adsorbed layers in this study, this deviation from ideality is a necessary consequence of the fact that nonionic surfactants do not adsorb onto mica. We imagine that solvation and steric repulsions preferentially locate them on the water side of the adsorbed layer like a tethered copolymer,21

Figure 7. (a) Adsorbed amount of DTAB (]), C12E2 (0), and their total (4) on ground mica as a function of bulk concentration of C12E2, and (b) measured surface composition versus solution composition. Vertical dashed lines denote compositions where surface aggregate shape transitions occur, and the solid line denotes equal bulk and surface compositions. The gray area denotes the solubility limit of C12E2.

and that each aggregate is “pinned” by electrostatically bound surfactant cations.27 3. DTAB: Uptake of Nonionic Surfactants with Small Headgroups. Nonionic surfactants with small poly(oxyethylene) (27) Stiernstedt, J.; Froberg, J. C.; Tiberg, F.; Rutland, M. W. Langmuir 2005, 21, 1875-1883.

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Figure 8. 200 × 200 nm2 AFM deflection images of mixed adsorbed layers of 6.3 mM TTeAB, with (a) 0.32 mM C12E2; (b) 1.3 mM C12E2; and (c) 1.9 mM C12E2. Insets show Fourier transforms of the images with a full range of 0.781 nm-1.

headgroups, exemplified here by C12E2 and C18E2, do not contribute steric repulsions with sufficient range to increase aggregate curvature from rods to globules. As shown in Figure 6, the addition of both C12E2 and C18E2 decreases the aggregate curvature of the adsorbed film. This is evident as a gradual transformation from rods into branched rods (Figure 6b), then a mesh (Figure 6c,d) and finally, for C12E2, a laterally unstructured bilayer (Figure 6e). As discussed extensively elsewhere,11,28 the adsorbed mesh structure can be distinguished by its position in a curvature progression between cylinders and bilayers, by intermediate structures such as branched rods, and by the larger than expected nearest-neighbor spacing from the Fourier transform. The periodicities observed for the meshes in Figure 6c,d are 6.6 nm, which is much larger than the 5.7 nm observed for comparable globular micelles of DTAB + C12E8 (Figure 2). A very low concentration of C18E2 added to adsorbed DTAB rods (Figure 6a) first induces micelle branching (Figure 6b) and then induces the formation of a mesh structure (Figure 6c). No bilayer was observed for C18E2 mixed films because of its low solubility in water and DTAB solutions. C12E2 thus represents the opposite extreme in the modification of aggregate curvature to the sphere-favoring surfactants with polyoxyethylene chains of eight or more units. Although the bilayer endpoint is the same as that for the adsolubilization of 2-naphthol by TTAB,26 the abrupt change from rods to bilayers observed there is quite distinct from this gradual morphology change. A mesh requires the formation of branch points with lower curvature than the cylinder body and, like end caps on rods, a free-energy difference.29 As with the rod-globule transition in, for example, DTAB+C12E8, this can occur by the preferential uptake of CiE2 into the low curvature region. (With such small hydrophilic groups, it is unlikely that different conformations could give rise to inhomogeneous curvature.) The branch-point density would increase with the concentration of nonionic surfactant until all the rods were connected as a saturated network. Figure 7 shows the component and total adsorption isotherms of DTAB onto mica (constant at approximately 26 mM) as the concentration of C12E2 is varied. The DTAB film has been completely transformed into a bilayer when the C12E2 concentration has reached 2.6 mM (Figure 6d) and remains a bilayer up to the highest concentration studied. Like the DTAB + C12E8 system, the amount of DTAB on the surface initially remains constant as C12E2 is added. Above a bulk C12E2 concentration of around 10 mM, the DTAB surface excess decreases, and that of C12E2 increases more gradually, indicating the bilayer can (28) Blom, A.; Warr, G. G.; Wanless, E. J. Langmuir 2005, 21, 11850-11855. (29) Zilman, A.; Tlusty, T.; Safran, S. A. J. Phys.: Condens. Matter 2003, 15, S57-S64.

Figure 9. (a) Adsorbed amounts of TTeAB (]), C12E2 (0), and their total (4) on ground mica as a function of C12E2 concentration, and (b) measured surface composition versus bulk solute composition. The solid line denotes equal bulk and surface compositions. The vertical dashed lines denote compositions at which the transformations of the adsorbed layer structure occurs.

only be further enriched in the nonionic surfactant by displacing DTAB. As with the other systems examined, the surface excess of the nonionic surfactant plateaus near equimolar composition with DTAB. Figure 7b shows the surface compositions as a function of bulk composition. Uptake of this nonionic surfactant into the DTAB film is elevated above that expected from ideal solution theory at all compositions (gradually returning to it only at the highest concentrations examined), well above the bilayer formation composition and corresponding to the plateau in C12E2 adsorption density. This is consistent with the preferential uptake of C12E2 into low curvature regions of the adsorbed film as it transforms from rods through a mesh into a bilayer. The outer leaf of the bilayer eventually becomes saturated with nonionic surfactant, and the surface composition tends toward a plateau composition near the solubility limit of C12E2. 4. TTeAB: Uptake of Nonionic Surfactants with Small Headgroups. Figure 8 shows the effect of added C12E2 on the

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Table 1. Effect of Nonionic Surfactant Coadsorption on the Morphology of a DTAB Adsorbed Layer on Mica DTAB (rods) additive

shape change

solution composition (mol %)

C12E8 C12E23 C14E8 C18E2 C12E2

spheres spheres spheres mesh mesh bilayer bilayer mesh spheres

28 28 16 5 5 9 40 40 10

C12E5 C12E6 C18E8

Table 2. Effect of Nonionic Surfactant Coadsorption on the Morphology of a TTeAB Adsorbed Layer on Mica TTeAB (spheres) additive

shape change

C12E2

rods mesh bilayer rods rods rods no change rods rods no change

C12E5 C12E6 C12E7 C12E8 C16E4 C18E2 C18E8

solution composition (mol %) 5 17 23 33 33 33 1 5

globular adsorbed TTeAB film morphology. At low C12E2 insertions, these globules were observed to lengthen gradually along one axis into meandering cylinders (Figure 8a). With increasing C12E2 addition, the adsorbed cylinders displayed evidence of branching, leading to a mesh (Figure 8b). At even higher C12E2 loading, the adsorbed film shows no remnants of lateral structure (Figure 8c), indicating a bilayer.6,13 The effect of C18E2 on TTeAB adsorbed layers closely parallels that on DTAB films. C18E2 lowers the aggregate curvature leading to the formation of adsorbed rods, but here its solubility limit is reached and the transformation does not progress to a mesh or bilayer as is observed in C12E2. The addition of C12E2 to TTeAB causes a continuous lowering of the aggregate curvature from globules, through cylinders and a mesh, to a laterally unstructured bilayer, and the component adsorption isotherms (Figure 9) display many features of the transitions already discussed. Not unexpectedly, the total amount of adsorbed surfactant increases as C12E2 is added and is taken up by the adsorbed film, and as the adsorbed film transforms from TTeAB globules into a bilayer. The amount of adsorbed TTeAB decreases as the surface aggregate morphology is changed, then decreases more gradually once a bilayer is realized and C12E2 continues to be taken up by the film. The uptake of C12E2 likewise becomes more gradual once the transformation to a bilayer is completed, tending once again to a plateau, in this case above equimolar composition. The surface compositions of C12E2 with TTeAB (Figure 9b) behave similarly to those with DTAB (Figure 7b). As with DTAB/ C12E2 mixtures, the mixed TTeAB/C12E2 adsorbed film is slightly richer in C12E2 than expected from ideal mixing considerations at low concentrations. At higher concentrations, the compositions return to their ideal mixing values as the C12E2 surface excess approaches its plateau value. 5. DTAB and TTeAB: Uptake of Nonionic Surfactants with Intermediate Headgroups. The uptake of C12E5, C12E6, C12E7, and C16E4, all with headgroups containing between four

Figure 10. (a) Adsorbed amounts of DTAB (]), C12E6 (0), and their total (4) onto ground mica as a function of C12E6 concentration, and (b) measured surface composition versus bulk solute composition. The vertical dashed line denotes the composition at which the transformation to a mesh is complete, and the solid line denotes equal bulk and surface compositions.

Figure 11. (a) Adsorbed amounts of TTeAB (]), C12E6 (0), and their total (4) onto ground mica, and (b) measured surface composition against bulk solute composition. The vertical line denotes the composition at which the film transformation to rods is complete, and the solid line denotes equal bulk and surface compositions.

and seven ethylene oxide units, by DTAB and TTeAB adsorbed films all resulted in a morphology change to lower adsorbed aggregate curvature, but only C12E5 added to DTAB realized a bilayer. The effects of nonionic surfactants on the morphology of adsorbed DTAB and TTeAB films are summarized in Tables 1 and 2. The addition of C12E6 to adsorbed DTAB films causes a gradual transformation from rods into a mesh that is complete at bulk

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Figure 13. Deflection of the tip at film rupture for (a) TTeAB and C12E2, (b) DTAB and C18E2, and (c) DTAB and C12E23. All AFM experiments were performed on the same tip.

Figure 12. Force profiles for mixed surfactant films of (a) TTeAB and C12E2, (b) DTAB and C18E2, and (c) DTAB and C12E23 on mica. All experiments were performed on the one tip so information is comparable both within and between all three data sets.

compositions near 40 mol %. Since the mesh describes a broad range of structures between branched rods and a bilayer, this transition composition is only approximate. Figure 10 shows component adsorption isotherms for mixed C12E6 and DTAB films. As observed with the addition of C12E2 to DTAB, there is a slight decrease in the amount of DTAB on the surface as C12E6 is taken up. Also like C12E2 and C12E8 in DTAB films, the total amount of adsorbed surfactant increases quite substantially as C12E6 is incorporated into the adsorbed film. This system alone shows no evidence of a plateau in the surface excess at high concentrations of the nonionic surfactant. We speculate that this is because the adsorbed layer morphology continually changes to accommodate more C12E6 over the composition range examined. Like C12E2 uptake, the surface compositions in this system always exceed expectations from ideal mixing (Figure 10b). Component adsorption isotherms for TTeAB and C12E6 (Figure 11) at low concentrations are similar to those for TTeAB + C12E8 mixed systems (Figure 5). The surface excess of TTeAB decreases as C12E6 is taken into the adsorbed layer such that the total amount of adsorbed surfactant is constant up to approximately 5 mM C12E6, slightly beyond the point of the shape transition

from globules into (short) rods. At higher concentrations of C12E6, there is a qualitative change in the isotherms. The TTeAB surface excess decreases more slowly (it is almost constant), and there is an increase in the uptake of C12E6 so that the total amount of adsorbed surfactant increases. This behavior is qualitatively similar to that of adsorbed DTAB rods because C12E8 (Figure 3) and C12E6 (Figure 10) are incorporated at concentrations below their respective shape transitions. 6. Force Curves. Representative force profiles for the three mixed systemssTTeAB/C12E2, DTAB/C18E2, and DTAB/ C12E23sare shown in Figure 12. There is no significant change in the adsorbed layer thickness as nonionic surfactant is added for any of these systems. The maximum tip deflection (Figure 13) observed at breakthrough almost doubles as C18E2 is added to DTAB, and the curvature is reduced from rods to a bilayer. Such an increase is consistent with the formation of a more tightly packed layer resisting displacement by the AFM tip. The maximum deflection likewise almost doubles upon uptake of C12E2 by TTeAB and the accompanying transformation from globules into a bilayer. The globular TTeAB film itself is less robust than DTAB rods (Figures 12a,b and 13a,b), and the magnitude of the increase is smaller. This behavior noticeably contrasts that of DTAB/C12E23 mixtures, in which the deflection at breakthrough first decreases up to a solution composition of 15 mol % C12E23, and then increases at higher loadings. The reduction in film stiffness is a reasonable consequence of the induced shape change from rods to globules (cf. TTeAB versus DTAB). The subsequent increase in stiffness is expected to arise from the increased resistance to compression as the polyoxyethylene headgroups become more closely packed, and is consistent with trends observed for tethered polyoxyethylene block copolymers in bilayer films.21 The shape of the force-separation curves between the tip and mica surface is independent of composition for TTeAB/C12E2 and DTAB/C18E2 systems. This is consistent with purely

Mixed Cationic-Nonionic Surfactant Adsorbed Layers

electrostatic interactions and is expected for such short hydrophilic groups. In DTAB/C12E23 mixtures, however, an additional shortrange repulsion was observed at higher C12E23 concentrations. This occurred at separations of around 5 nm beyond the breakthrough distance. This is again consistent with expectations based on the thickness of the polyoxyethylene headgroup layer on each surface, which should be well described by its end-toend distance in a good solvent (2.3 nm).30,31

Discussion and Conclusions The morphology changes observed in mixed cationic and nonionic surfactants on mica can be understood in terms of interactions between neighboring headgroups, the length and volume of the hydrocarbon chains, as described by the surfactant packing parameter,24 or aggregate curvature. The strong correlations between surface composition and adsorbed layer morphology reinforce the idea that adsorbed surfactant films cannot be understood by treating them as laterally unstructured layers.4,19 The uptake of nonionic surfactants with eight or more ethoxy groups favors the formation (or retention) of globular aggregates. Steric interaction between the nonionic headgroups favors such high-curvature aggregates (with a small packing parameter). The solution composition at which this increase in film curvature was realized is sensitive to the alkyl chain length of the nonionic surfactant, but depends little on headgroup size. We attribute this to the greater uptake of the more hydrophobic surfactant, so that the adsorbed layer is richer in a longer-chain surfactant at the same bulk composition. Nonionic surfactants with a headgroup size of seven or fewer ethoxy groups favored lower curvature adsorbed aggregates. The adsorbed layer morphologies are insensitive to hydrocarbon chain length and the structure formed by the cationic surfactant, but the transition compositions do depend on carbon number. The ultimate morphology and solution composition at which shape transitions occur also depends on headgroup size. For example, C12E7 transforms a TTeAB globular adsorbed layer to rods at a bulk composition of 33 mol %, whereas C12E2 does so at 5 mol %, on the way to a bilayer at 23 mol %. Adsorbed layer compositions of all mixtures examined showed a plateau in uptake above a particular bulk nonionic surfactant concentration. This reflects the maximum capacity of the adsorbed film for the nonadsorbing additive and depends on mixed film morphology and, hence, both components. Nonionic surfactants with large headgroups, exemplified by C12E8, have film (30) Flory, P. J. Statistical Mechanics of Chain Molecules; Wiley-Interscience: New York, 1969. (31) Devanand, K.; Selser, J. C. Macromolecules 1991, 24, 5943-5947.

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compositions that are well-described by ideal mixing at low concentrations, but deviate to a plateau film composition at or below 50 mol % at higher nonionic surfactant loading. This uptake plateau is due to a combination of the nonadsorbing nature of the surfactant and steric repulsions between neighboring ethylene oxide headgroups. Systems in which film curvatures were reduced by the addition of nonionic surfactant showed nonionic surfactant uptake to be elevated above ideal mixing expectations at low concentrations. At higher nonionic surfactant concentrations, these also deviated toward a plateau in film composition, in some cases crossing the ideal mixing line. For these systems, the plateau in uptake is higher because of the smaller polyoxyethylene headgroups. In the limiting case of C12E2, we can neglect steric repulsions between these much smaller neighboring headgroups, and the plateau uptake is determined by the capacity of the adsorbed bilayer film to incorporate additional alkyl tails. Several studies examining the mixing behavior of quaternary ammonium and polyoxyethylene surfactants in solution have shown that they mix ideally25 or slightly synergistically (attractive interactions),32,33 particularly at high nonionic surfactant contents.34 Neutron reflectometry studies on CTAB and C12E6 mixtures at the hydrophilic silica interface have revealed identical solution and surface compositions up to 50 mol % C12E64 and CTAB-enriched layers at higher loadings. Although the situation is somewhat different, because CTAB35 and C12E636 both adsorb onto silica (as globular aggregates), this is generally consistent with our observations of a plateau in the component isotherms. The adsorbed layer may be mixed, but the cationic surfactant is more strongly bound by electrostatics to the silica substrate and cannot be easily displaced from the surface at high loading of nonionic surfactant. Acknowledgment. This work was supported by 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. The authors thank Christine Allen for her assistance with the depletion adsorption isotherms. LA0604477 (32) McDermott, D. C.; Kanelleas, D.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Majkrzak, C. F. Langmuir 1993, 9, 2404-2407. (33) Hassan, P. A.; Bhagwat, S. S.; Manohar, C. Langmuir 1995, 11, 470473. (34) Carnero Ruiz, C.; Aguiar, J. Colloids Surf., A 2003, 224, 221-230. (35) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447-4454. (36) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294.