Langmuir 2006, 22, 1143-1149
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Adsorbed Layer Structure of Cationic Gemini and Corresponding Monomeric Surfactants on Mica Franck P. Duval,† Raoul Zana,‡ and Gregory G. Warr*,† School of Chemistry, The UniVersity of Sydney, Sydney, NSW 2006, Australia, and Institut Charles Sadron (CNRS-ULP), 6 rue Boussingault, 67000 Strasbourg, France ReceiVed July 26, 2005. In Final Form: NoVember 29, 2005 We report a comprehensive study of the adsorbed layer morphologies of cationic gemini surfactants of the type dodecanediyl-R,ω-bis(dimethylalkylammonium bromide) and their corresponding monomers, dimethyldodecylalkylammonium bromide, on mica using atomic force microscopy soft-contact imaging. As in the bulk, aggregate curvature of the adsorbed geminis is found to increase with increasing spacer length, but the adsorbed aggregate curvature also increases in the presence of CsCl and CsBr. The monomeric surfactants exhibit an unexpected transition from globular adsorbed aggregates to a bilayer when the alkyl side chain reaches butyl, and this transition is also sensitive to added electrolyte.
Introduction Surfactants can self-assemble in water to give rise to micelles of various shapes. This shape is determined by the value of the surfactant packing parameter V/a0lc, where V and lc are the volume and length of the surfactant alkyl chain, respectively, and a0 is the optimal surface area occupied by one surfactant at the aggregate surface.1 The concept of the surfactant packing parameter has proven very successful in explaining the sequence of micelle shapes encountered in, for example, homologous series of surfactants, and the changes in micelle shape that can be induced by changing one of the parameters of the system. For instance, the addition of salt to a micellar solution of ionic surfactant may bring about a change in micelle shape from globular to elongated owing to the screening of the electrostatic repulsion between surfactant headgroups at the micelle surface by the added salt. This results in a decrease in the curvature of the surfactant aggregate.2-4 Atomic force microscopy (AFM) has been instrumental in showing that surfactants adsorb on solid surfaces in the form of micelle-like aggregates of various shapes.5-7 Like bulk structures, adsorbed surfactant aggregates on hydrophilic surfaces can be globular, cylindrical, or planar (bilayer), and a mesh structure has also recently been identified.8 Changes in the shape of surface aggregates induced by various means have been observed, just as in the case of bulk surfactant micelles. However, unlike bulk micelles the surface micelles become more curved as the electrolyte concentration is increased. The sequence bilayer f cylinders f globular micelles has been observed upon increasing CsCl concentration in a surfactant solution of hexadecyltrim* Corresponding author. E-mail:
[email protected]. † The University of Sydney. ‡ Institut Charles Sadron (CNRS-ULP). (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans 2 1976, 72, 1575. (2) Vinson, P. K.; Bellare, J. R.; Davis, H. T.; Miller, W. G.; Scriven, L. E. J. Colloid Interface Sci. 1991, 142, 74-91. (3) Imae, T.; Abe, A.; Ikeda, S. J. Phys. Chem. 1988, 92, 1548-1553. (4) Magid, L. J.; Han, Z.; Warr, G. G.; Cassidy, M. A.; Butler, P. D.; Hamilton, W. A. J. Phys. Chem. B 1997, 101, 7919-7927. (5) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (6) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (7) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5, 88-94. (8) Blom, A.; Duval, F. P.; Kovacs, L.; Warr, G. G.; Almgren, M.; Kadi, M.; Zana, R. Langmuir 2004, 20, 1291-1297.
ethylammonium chloride or bromide (CTAC or CTAB) adsorbed on mica.9,10 This sequence runs opposite to the effect of added electrolyte on the shape of bulk micelles. It arises because of the competition between the surfactant ion (CTA+) and the cation (Cs+) from the added salt for adsorption onto the negatively charged sites on the mica surface (one charge per 0.5 nm2). Lamont et al. have shown that the change in shape of the CTA+ aggregates on the mica surface in the presence of CsCl is determined by the molar concentration ratio [CTA+]/[Cs+] and not by the CsCl concentration in the solution.9 By comparing the results obtained using salts with a common anion and various cations, Ducker et al.10 also showed that the change in aggregate shape depends on the nature of the competing cation. By comparing the effect of salts with a common cation, they were able to show that Br- leads to aggregates of lower curvature than Cl-. This is expected because Br- binds more strongly than Clto cationic surfactant headgroups and thus lowers the curvature of the aggregates, whether they are in the bulk phase or adsorbed at a solid surface. A new class of surfactants, the so-called dimeric (or gemini) and oligomeric surfactants, has recently attracted considerable interest.11-13 In gemini surfactants, two conventional or monomeric surfactant moieties are connected together close to, or at the level of, the headgroups by a spacer group. Most studies on dimeric surfactants have involved the alkanediyl-R,ω-bis(alkyldimethylammonium bromide) that are usually referred to as m-s-m (m and s are the carbon numbers of the alkanediyl spacer and of the surfactant alkyl chain, respectively; Figure 1). The shape of bulk micelles of 12-s-12 dimeric surfactants changes from elongated rods (s e 3) to spheroids (4 e s e 12) and then to vesicles (s > 12). These changes in shape can be explained in terms of changes in the surfactant packing parameter with s.14-19 As s increases, the surface area of a 12-s-12 molecule, (9) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (10) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (11) See, for instance, Zana, R., Xia, J., Eds. Gemini Surfactants: Synthesis, Interfacial and Solution Phase BehaVior, and Applications; Marcel Dekker: New York, 2004. (12) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205-253. (13) FitzGerald, P.; Carr, M. W.; Davey, T. W.; Serelis, A. K.; Such, C. H.; Warr, G. G. J. Colloid Interface Sci. 2004, 275, 649-658. (14) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072-1075. (15) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 14651467. (16) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Langmuir 1994, 10, 2177-2187.
10.1021/la052032b CCC: $33.50 © 2006 American Chemical Society Published on Web 01/07/2006
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DuVal et al. Table 1. Critical Micelle Concentrations and Aggregate Shapes of the 12-s-12 Gemini and Corresponding 12-s/2 Monomeric Surfactants in Bulk Solution and at the Mica Surface gemini 12-s-12 14
Figure 1. Structures of quaternary ammonium 12-s-12 gemini and 12-s/2 monomeric surfactants used in this study.
a0, also increases at constant lc and (approximately) V up to s ) 12; consequently, V/a0lc decreases.15 Manne et al.20 have investigated the micelle shape of three 12-s-12 surfactants adsorbed on mica and graphite using AFM. They showed that, as for many other surfactants, adsorption onto the mica surface yields aggregates of lower curvature than in the bulk phase. Thus, on mica 12-2-12 adsorbs as a bilayer but forms elongated micelles in the bulk, whereas 12-4-12 and 12-6-12 adsorb as long cylindrical micelles but form globular (spheroidal) micelles in the bulk. From these results, it was suggested that all 12-s-12 surfactants with s g 4 would form cylindrical micelles when adsorbed on mica. This article reports a more comprehensive AFM study of 12-s-12 surfactant adsorbed layer structures in mica. We have investigated 12-s-12 surfactants with s ) 2, 3, 4, 6, 8, 10, and 12. These are compared with several cationic surfactants of the dodecylalkyldimethylammonium bromide type (referred to as 12-s/2) that can be regarded as the corresponding monomers of the 12-s-12 dimeric surfactants (Figure 1). The selfassociation behavior of these surfactants in the bulk is well characterized,18,21-23 but their behavior in the adsorbed state on the mica surface has not previously been investigated. Last, we have investigated the behavior of two conventional surfactants, dodecyl- and tetradecyltrimethylammonium bromide (DTAB and TTAB). These surfactants have been extensively investigated on mica, and they were used in this study to elucidate the effect of additions of CsCl and CsBr on the shape of the surfactant aggregates. Materials and Methods Sample Preparation. DTAB, TTAB, CsBr (Aldrich), and CsCl (Sigma) were used as received. The samples of dimeric surfactants 12-s-12 and of the corresponding monomeric surfactants 12-s/2 were the same as in previous studies or were prepared and purified as described previously.14,21 The critical micelle concentrations (cmc) of all of the surfactants investigated have been reported and are listed in Table 1. For imaging experiments, the surfactant solutions were prepared at a concentration of about twice the cmc in the absence of salt. Because the cmc decreases upon addition of salt, this ensured that the investigated solutions were always at a (17) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Karthauser, J.; Vanos, N. M.; Zana, R. Science 1994, 266, 254-256. (18) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448-1456. (19) Zana, R.; Talmon, Y. Nature 1993, 362, 228-230. (20) Manne, S.; Schaffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382-6387. (21) Zana, R. J. Colloid Interface Sci. 1980, 78, 330-337. (22) Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983, 91, 276-279. (23) Schosseler, F.; Anthony, O.; Beinert, G.; Zana, R. Langmuir 1995, 11, 3347-3350.
shape on mica
monomeric 12-s/2 cmc21 s/2 (mM)
bulk shape22
shape on mica
s
bulk cmc (mM) shape18,19
2 3 4 6
0.84 0.90 1.09 1.01
cylinders cylinders globular globular
bilayera bilayer cylindersa cylindersa
1
15.3
globular
cylinders
2 3
14.3 11
globular globular
8
0.83
globular
4
7.5
globular
10 12
0.63 0.37
globular globular
short cylinders globular globular
cylinders short cylinders bilayer
5 6
5.2 3.06
bilayer globular bilayer f cylinders
12
0.08
bilayer
bilayer
a
These results agree with those previously been reported by Manne et al.20
concentration above the cmc. All solutions were prepared using Milli-Q water with a conductivity of 18 MΩ cm-1. AFM Imaging. The experiments were performed using a Nanoscope III (Digital Instruments) in contact mode with an E-scanner. The imaging method was to use the double layer (or steric) repulsion between the tip and the surface layer and fly the tip over the adsorbed film.5,6,24 The mica substrate (Probing and Structure) was freshly cleaved before use with adhesive tape. Silicon nitride N-P cantilevers (Digital Instruments) with nominal spring constants of 0.58 and 0.12 N m-1 were cleaned by UV irradiation for 30 min prior to use. The solution was injected into the cell, sealed with an O-ring, and thermally equilibrated for 15 min to 2 h before imaging. The scan rate, integral gain, and z-deflection range were varied from 8 to 15 Hz, 0.4 to 0.6, and 0.4 to 0.6 nm, respectively. (All other gains were set to 0.) All experiments were performed at room temperature. All images shown are raw deflection images that have been flattened along the scan lines to remove any tilt from the sample. No other image processing was used. Only 200 × 200 nm2 images are shown below, but similar results were observed in 300 × 300 and 100 × 100 nm2 images.
Results and Discussion Gemini 12-s-12 Surfactants. Figure 2 shows representative adsorbed-layer morphologies of 12-s-12 surfactants with different spacers as determined from AFM imaging, together with their Fourier transforms. Table 1 summarizes the adsorbed aggregate shapes of all 12-s-12 surfactants studied. Also listed for comparison are the corresponding micelle shapes in the bulk phase. The shapes of the aggregates on mica change systematically with spacer length from bilayers (s ) 2, 3; Figure 2a) into cylinders (s ) 4, 6; Figure 2b) and finally globular aggregates (s ) 10, 12). Both short cylinders and globular surface aggregates were observed in 12-8-12. This is shown in Figure 2c, where some short rods are visible in the direct image, and the inset Fourier transform shows some anisotropic intensity rather than a completely isotropic ring of nearest-neighbor correlations, suggesting that there is some preferential orientation of aggregates over the area scanned. The trend with increasing spacer length is toward more highly curved aggregates, corresponding to a smaller packing parameter, or a larger area per molecule, a0. This parallels the structural trend in bulk micelles over the same range of spacer lengths. If bulk and surface aggregate shapes are compared, then the general observation is that adsorption onto mica results in (24) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692.
Layer Structure of Surfactants on Mica
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Figure 2. AFM deflection images (200 × 200 nm2) showing adsorbed layer morphologies of 12-s-12 gemini surfactants at the mica/aqueous solution interface. Insets show Fourier transforms of the direct images. (a) Bilayers of 12-2-12; (b) cylinders formed by 12-6-12; and (c) short cylinders and globules formed by 12-8-12.
Figure 3. Deflection of the AFM tip versus the separation between the tip and a mica surface with adsorbed 12-8-12 gemini surfactant (2 cmc).
aggregates with lower curvature. This is consistent with previous observations for a large number of monomeric cationic surfactants adsorbed on mica.5,20,24 Thus for s ) 2, 3, cylindrical micelles in the bulk form an adsorbed bilayer; for s ) 4, 6, bulk spherical micelles form adsorbed cylinders; and for s ) 8, there is a borderline increase in curvature from bulk spheres to short rods (Figure 2). Globular aggregates formed in the bulk are retained on the mica surface only for 12-10-12 and 12-12-12. Force versus separation curves for gemini adsorbed layers exhibit a long-range electrostatic repulsion followed by a shortrange repulsion ascribed to steric and solvation effects and then a “push-through” instability into adhesive contact between the tip and underlying mica. The magnitude of the short-range interaction between the tip and the surface decreases as the spacer length increases. 12-2-12 and 12-3-12 exhibited a single, smooth repulsion with a steep “wall” and an instability in the range of 2 to 4 nm. Longer spacers had much weaker walls and often exhibited two instabilities, as illustrated for 12-8-12 in Figure 3, similar to those reported for CTAC bilayers on mica by Lamont et al.9 The detailed structures of these force curves are tip-dependent, and the first, weaker push-through sometimes appeared as an inflection point rather than a discontinuity in the force curve. We interpret this as arising from adsorbed surfactant on both the mica and the negatively charged, hydrophilic tip.25 Because imaging of adsorbed layers on mica was possible only after the first breakthrough, this is ascribed to the collapse of the tip layer. The different micelle shapes in the bulk phase have been explained in terms of the variation of the surfactant packing parameter V/a0lc with the spacer length.18 The same explanation (25) Senden, T. J.; Drummond, C. J. Colloids Surf., A 1995, 94, 29-51.
probably holds for the shape of the aggregates formed at the mica surface,20 but the packing parameter would be now an effectiVe packing parameter that is different from that in the bulk phase. This larger effective packing parameter has been attributed to the fact that the mica surface acts as a highly charged, laterally extended “counterion” that allows a closer packing of the headgroups at the surface than is found in free micelles.20 That is, the molecular area is lower at the mica surface, thus V/a0lc is higher. Although this explanation captures a useful idea, it is not completely satisfactory, as exemplified by 12-3-12. Surface tension and neutron reflectivity studies both show that each molecule (two headgroups) occupies 0.66-0.70 nm2 at the air/ water interface.15,26,27 However, a mica surface with fully dissociated potassium ions offers only one singly charged site per 0.5 nm2.28 Thus, the area per molecule at the mica surface should be higher than at the bulk micelle surface and the packing parameter lower, and simple application of the packing parameter suggests an adsorbed layer structure that is more curved than in the bulk. In fact, it is only when s g 6 that the molecular area of 12-s-12 at the air/water interface is large enough to span two mica charged sites. The formation of an adsorbed bilayer by 12-2-12 and 12-3-12, or even the formation of cylinders by 12-4-12, must therefore involve some other effect on surfactant packing at the interface. The importance of the fact that the mica surface confines the adsorbed layer into a plane may have been underestimated. AFM images reveal nothing about the “bottom” of the adsorbed layer, adjacent to the mica, but a cartoon view of adsorbed spherical or cylindrical micelles with circular cross sections must be an oversimplification. The values of the packing parameters for 12-2-12 and 12-3-12 have been calculated to be around 0.61. This is too high for even cylinders. Didodecyldimethylammonium bromide (DDAB or 1212 in the present notation) has a packing parameter only marginally higher, and it forms a vesicle dispersion in aqueous solution and a bilayer on solid substrates.6,29,30 This suggests that the arrangement of 12-2-12 on a planar mica surface would strongly favor the organization of the adsorbed layer into a bilayer. Intercalation of the surfactant tails decreases the effective chain length and hence increases V/a0lc toward its optimal value of 1 (26) Espert, A.; von Klitzing, R.; Poulin, P.; Colin, A.; Zana, R.; Langevin, D. Langmuir 1998, 14, 4251-4260. (27) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392-4396. (28) Manne, S. Prog. Colloid Polymer Sci. 1997, 103, 226-233. (29) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774-783. (30) Schulz, J. C.; Warr, G. G.; Butler, P. D.; Hamilton, W. A. Phys. ReV. E 2001, 63, 041604.
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for a planar structure. This is consistent with film thicknesses of 12-2-12 above its cmc measured by Fielden et al. on mica using the surface forces apparatus.31 They found the total thickness of two bilayers in contact to be 4.0 ( 0.5 nm, which is appreciably less than four fully extended dodecyl chains (6.7 nm) expected as a minimum for two nonintercalated bilayers and found for DDAB.32,33 Using this result yields a monolayer half-thickness of 1 nm and an effective packing parameter for 12-2-12 of almost exactly 1. Our AFM force curves also indicate an adsorbed layer thickness of about 2 nm for 12-2-12 on mica, which is less than is typically observed for conventional cationic surfactants. Although the situation is somewhat different, Atkin et al.34 also concluded that 12-s-12 geminis form some intercalated structures adsorbed onto amorphous silica, based on calculated adsorbed layer thicknesses. Aggregate geometry in gemini surfactants is further complicated by questions of counterion binding, which affects the charge on an adsorbing surfactant. Molecular areas derived from neutron reflectometry and surface tension results15,27 are consistent with each other only if 12-s-12 surfactants with short alkyl spacers adsorb at the air/solution interface as a 1:1 complex with a bromide counterion. That is, they may be effectively [12-s-12.Br]+. However recent conductivity results by one of us make it clear that there is no evidence for such ion pairing in bulk solution in 12-s-12 systems,35 and the Debye length in dilute gemini surfactant solutions is consistent with it being a 2:1 electrolyte.36 Studies of competitive ion binding to gemini surfactants at the air/solution interface reveal nothing remarkable,37 so the situation still seems to be unresolved. 12-6-12 has a molecular area of 0.95 nm,2,15,27 which equates almost exactly to two charged mica sites, and this yields a packing parameter (neglecting any effect of the spacer on V) of 0.44. Here, cylindrical micelles are expected on the basis of packing considerations, but globular micelles are observed in the bulk. At the mica surface, the molecular area is greater, even assuming full dissociation of K+, so the packing parameter is marginally lower but still in the expected range for rods. 12-4-12 has a molecular area of 0.76-0.82 nm2,15,27 giving a packing parameter of 0.51-0.57. It forms globular micelles in the bulk but cylinders on the mica surface. As with the bilayer-forming geminis, 124-12 occupies less area than two mica charged sites, so the mica cannot be inducing rods by packing the headgroups closer together. This may be simply an effect of the high concentration in the adsorbed layer allowing the aggregates to adopt their preferred shape, like 12-6-12. In both of these cases, we might expect globular or spherical micelles in dilute bulk solution because of their greater configurational entropy, overcoming a modest cost to the increased curvature energy. 12-8-12 has an experimental molecular area of 1.17 nm2,15,27 so its predicted packing parameter is 0.36 (i.e., just outside the range allowed for spherical micelles). However, the interfacial packing parameter, using 1 nm2 as the area of two charged mica sites, is 0.42 and in the predicted range for rods. This is the first surfactant in this series where the charge density of the mica can reduce the interfacial area to achieve a shape transformation.20 (31) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 39243934. (32) Blom, A.; Drummond, C.; Wanless, E. J.; Richetti, P.; Warr, G. G. Langmuir 2005, 21, 2779-2788. (33) Boschkova, K.; Feiler, A.; Kronberg, B.; Stalgren, J. J. R. Langmuir 2002, 18, 7930-7935. (34) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Phys. Chem. B 2003, 107, 2978-2985. (35) Zana, R. J. Colloid Interface Sci. 2002, 246, 182-190. (36) Richetti, P. Personal communication. (37) Thalody, B.; Warr, G. G. Aust. J. Chem. 2004, 57, 193-196.
DuVal et al.
Figure 4. Deflection image (200 × 200 nm2) of 12-3 micelles at the mica/solution interface.
The estimated packing parameters for 12-10-12 and 12-12-12 are both very close to 0.30, corresponding to about 0.70 nm2 per quaternary nitrogen group. It has been suggested that such long spacers are not fully extended, so the incorporation of some of these methylenes into the micelle interior would increase V somewhat. Even so, the aggregates formed in the bulk and at the mica/solution interface are both globular, as expected from packing considerations. Monomeric 12-s/2 Surfactants. Table 1 also summarizes the shapes of the aggregates of 12-s/2 surfactants in water and at the mica surface. DTAB (s/2 ) 1) and 12-2 form rodlike aggregates on mica, 12-3 adsorbs as a mixture of short rodlike and globular aggregates (like 12-8-12 and showing similar features in its direct images and Fourier transforms, see Figure 4), and 12-4, 12-5, and 12-6 are all adsorbed as a bilayer. The transition toward short rods or globules as the side arm length is increased from 2 to 3 is consistent with increasing curvature, as observed for the corresponding geminis and for monomeric surfactants with trialkylammonium headgroups.24 However, further increasing the side-arm length abruptly and unexpectedly lowers the curvature of the adsorbed aggregates. For the 12-s/2 surfactants, force-distance curves show a typical electrostatic repulsion before the tip breaks through the surfactant layer adsorbed on the mica surface. This instability occurs at a separation distance between the tip and the mica surface of 3-3.5 nm, approximately twice the length of the surfactant. In bulk solution, aggregates of 12-s/2 are globular micelles up to s/2 ) 4. For s/2 ) 6, globular micelles grow into rods with increasing concentration, and vesicles are observed for s/2 g 8.18,22 12-12 (DDAB) also forms vesicles in water and adsorbs on mica as a bilayer.6,30 As observed for the 12-s-12 dimeric surfactants, 12-s/2 aggregates at the mica surface are in every case less curved than bulk solution aggregates. The trend in adsorbed layer morphologies for s/2 ) 1-3 toward higher curvature is consistent with a modest decrease in the micelle aggregation number22 and an increase in molecular area at the air/water interface15,26,27,38 observed in bulk up to s/2 ) 6. However, the adsorbed layer exhibits an abrupt transition to a laterally unstructured bilayer at s/2 ) 4. It has been suggested from measured critical micelle concentrations that s/2 ) 4 is the minimum length necessary to allow incorporation of the side group into the hydrophobic core of the micelle.21 For surfactants electrostatically bound to a substrate (in the bottom half of the adsorbed film), the side arm is constrained to be included in the (38) Hiramatsu, K.; Kameyama, K.; Ishiguro, R.; Mori, M.; Hayase, H. Bull. Chem. Soc. Jpn. 2003, 76, 1903-1910.
Layer Structure of Surfactants on Mica
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Table 2. Effect of Added CsCl on the Shape of Surface Aggregates of DTAB, TTAB, and CTAC on the Mica Surface
surfactant
bulk micelles (no CsCl)
0
adsorbed layer morphology CsCl concentration (mM) 3 5 25
12 (DTAB) globules rods 14 (TTAB)
globules rods
16 (CTAC)
globules bilayer rods
100
short globules rods rods short rods/globules globules
adsorbed filmsit cannot adopt many conformations and extend into solution. This effectively increases the surfactant packing parameter by increasing the hydrophobic chain volume from 0.350 nm3 for a single dodecyl chain to 0.490 nm3 for dodecyl + butyl chains. Taken together with the decrease in molecular area from approximately 0.64 nm2 in bulk micelles to 0.49 nm2 on a single mica charged site, this is sufficient to increase the packing parameter from 0.33 to 0.6, which is comparable to that of DDAB and 12-2-12 and in the stability region of bilayers. This bilayer, like those formed by 12-5 and 12-6, should be intercalated much like 12-2-12 and 12-3-12. For these surfactants, we observe an adsorbed layer thickness of 2-2.4 nm in the force curve, which is less than two surfactants and consistent with an intercalated bilayer. Effects of Electrolyte. In bulk solution, specific counterion effects are known to change micelle morphology. A strongly binding counterion such as bromide39 lowers micelle curvature more efficiently than chloride.4,40 However, on the mica surface Ducker et al. have shown that competitive co-ion uptake by the mica lattice can, for example, transform a bilayer of CTAB into rodlike or even globular surface aggregates.9 Cesium is particularly effective in this regard. DTAB and TTAB. Both TTAB and DTAB form cylindrical micelles upon adsorption onto mica from aqueous solution above their cmc’s.6,24 The addition of CsCl results in the formation of adsorbed globular micelles (Table 2). The shape transformation from cylinders to spheres is achieved with less CsCl in the case of the shorter DTAB and at much a lower ratio of cesium to surfactant. Consistent with this, TTAB forms cylindrical micelles in the bulk at lower salt concentrations than DTAB does, indicating a somewhat larger packing parameter. It therefore produces surface aggregates that are more resistant to CsCl than DTAB. These observations are in agreement with Ducker’s observations for CTAC9 (Table 2), which has the longest alkyl chain of all of these surfactants. 12-s-12 Gemini Surfactants. The effects of CsCl and CsBr addition on aggregate shapes of 12-s-12 surfactants with various spacer lengths are compared in Figure 5. As with conventional surfactants, the addition of Cs+ increases the aggregate curvature of these systems, favoring globular aggregates over cylinders or bilayers. Gemini surfactants with long spacers (s ) 10-12) that form globular surface aggregates do not have their morphology affected by Cs+ addition, but at high concentration, they frequently gave rise to a structure that appeared “cloudy” and in which adsorbed layer structures could not be resolved. Force curves in these systems showed only a weak repulsion due to adsorbed layers, and in some cases, there was no repulsion at all. This cloudy aspect of the imaged structure thus may reflect a weak binding of the micelles to the mica surface that prevents proper imaging by the AFM tip. Strong competition between the surfactant ions and Cs+ for the mica surface sites makes fewer (39) Morgan, J. D.; Napper, D. H.; Warr, G. G.; Nicol, S. K. Langmuir 1994, 10, 797-801. (40) Zana, R. Langmuir 1996, 12, 1208-1211.
Figure 5. Summary of aggregate shapes of 12-s-12 gemini surfactants at the mica/aqueous solution interface as a function of added CsCl (top) and CsBr (bottom). R denotes rods, G denotes globules, RG denotes a mixture of rods and globules, M denotes a mesh, and B denotes a bilayer. Dashed lines indicate approximate boundaries between different adsorbed layer morphologies, and shading indicates precipitation.
surface sites available for surfactant ion binding at higher CsCl concentration. This of course results in weaker binding of the aggregates to the surface that may result in this image, analogous to the effects observed near the isoelectric point of mineral oxides.41 Gemini surfactants that form rods or short rods (4 e s e 8) are transformed into globules by the addition of Cs+, and more salt is required to achieve this as the spacer length and hence the aggregate curvature is decreased. A typical example of adsorbed globules formed in this way is shown in Figure 6c for 12-6-12 in 25 mM CsCl. For the shortest of these spacers, s ) 4, the gradual transformation into globules is incomplete even at 100 mM CsCl, where short rods or a mixture of short rods and globules remains. Imaging difficulties were again encountered at very high concentrations of added electrolyte in 12-6-12. The addition of 5 mM CsCl to 12-3-12 causes a shape transformation into adsorbed cylinders. These persist at higher CsCl concentrations (Figure 6b) until at 100 mM added electrolyte a mixture of rods and globules is observed. Adding 5 mM CsCl to 12-2-12 causes no change in the observed bilayer, but at 25 and 100 mM, an isotropic lateral structure has developed in the adsorbed layer. As we have described in detail elsewhere, this lateral structure is consistent with a mesh composed of branched, cylindrical micelles.8 As noted elsewhere, an AFM soft-contact image of an adsorbed mesh is qualitatively similar to an adsorbed layer of globular (41) Schulz, J. C.; Warr, G. G. Langmuir 2002, 18, 3191-3197.
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Figure 6. Deflection image (200 × 200 nm2) of adsorbed gemini surfactants at the mica/solution interface in CsCl solutions: (a) 12-2-12 + 100 mM CsCl, (b) 12-3-12 + 25 mM CsCl, and (c) 12-6-12 + 25 mM CsCl.
micelles.8 The mesh can be distinguished in a variety of ways. First, in this system we expect a monotonic increase in aggregate curvature with spacer length at each CsCl concentration, just as is observed in water. For example, at 25 mM CsCl the aggregate structures increase in curvature from a mesh of 12-2-12 to rods of 12-3-12, mixed rods and globules of 12-4-12, and globules for 12-6-12 and higher spacers (Figures 4 and 5). A transition from globules to rods and back to globules would be anomalous. Likewise, an increasing concentration of CsCl should yield a monotonic increase in curvature as the Cs+ displaces the surfactant ion from the mica. Together these strongly suggest that the adsorbed layer structure of 12-2-12 in 25 and 100 mM CsCl solution is a mesh. Second, two-dimensional Fourier transforms of mesh films formed by a wide variety of systems differ from those of an array of globular micelles.8 Although both show an isotropic ring, meshes frequently display a “filled ring” due to connectivity between adsorbed aggregates. These transforms also typically yield larger repeat spacings than the expected nearest-neighbor distance between discrete aggregates. The relevant Fourier transforms are shown as insets to Figure 6, where the filled ring for 12-2-12 is clearly visible and contrasts noticeably with 12-8-12 (Figure 2c) and 12-6-12 (Figure 6c) globular adsorbed layers. In this case, an adsorbed mesh is also supported by the ease with which the layer could be imaged at 100 mM CsCl compared with the globular aggregates with longer spacers. Atkin et al. recently reported AFM images of globular “flattened ellipsoid” aggregates for 12-2-12 adsorbed onto silica with a thickness of 3.5 nm and a peak-to-peak separation of 8.0 ( 0.5 nm.34 Our results raise the possibility that this structure is actually an adsorbed mesh. This would be consistent with the large observed periodicity and the surface excess of 12-2-12 on silica, which was much higher than for geminis with longer spacers. In the absence of adsorbed layer images for longer spacers, it is not possible to make any conclusion based on trends in aggregate curvature. Fourier transforms were not shown, but the surface roughness of silica would limit their usefulness in identifying meshes, as noted elsewhere.42 12-s/2 Monomeric Surfactants. The shape of the surface aggregates of 12-s/2 surfactants, summarized in Figure 7, is similarly modified by the introduction of cesium salts into the surfactant solution. The trend toward higher curvature with added salt is consistent with both our observations on the gemini surfactants discussed above and the work of Lamont and Ducker.9 (42) Blom, A.; Warr, G. G.; Wanless, E. J. Langmuir 2005, 21, 11850-11855.
Figure 7. Aggregate shapes of 12-s/2 monomeric surfactants at the mica/aqueous solution interface as a function of added CsCl (top) and CsBr (bottom). B denotes bilayer, R denotes rods, RG denotes either mixed rods and globules or short rods, and G denotes globular aggregates. Dashed lines indicate approximate boundaries between different morphologies.
As the side-arm length increases from methyl- to propyl-, so does the surface aggregate curvature, and rodlike micelles are transformed into globules more readily at lower Cs+ concentration. However, adsorbed bilayers form in the absence of salt beyond s/2 ) 3, and these become increasingly resistant to the effect of Cs+ as the side-arm length is further increased up to 12-6, which can withstand over 100 mM CsCl while remaining a bilayer. (The force curve shows a much weaker short-range repulsion and easier push-through at 100 mM Cs+, suggesting that the structure is near its stability limit.)
Layer Structure of Surfactants on Mica
12-4 and 12-5 are both transformed into isotropic, laterally structured adsorbed layers by a small amount of CsCl or CsBr. Neither surfactant formed rodlike (anisotropic) adsorbed aggregates under any of the salt concentrations examined. As with the 12-s-12 series, most adsorbed layers were impossible to image at 100 mM CsCl. This leads us to conclude that the adsorbed layer directly transforms from bilayers into adsorbed globules. We infer that rods are bypassed in this case because of the specific mechanism by which the side arm alters aggregate morphology. As Cs+ reduces the density of available sites for surfactant adsorption, the side arm is “released” from the bilayer and can adopt other conformations that increase the molecular area and favor higher curvature. Counterion Effects. The relatively minor role of surfactant counterions is evident by comparing the effects of CsCl and CsBr on adsorbed layer morphology in Figures 5 and 7. At a particular concentration of added cesium salt, CsBr increases the curvature of the surface aggregate somewhat less than does CsCl. This is most clearly seen in Figure 5 along the boundary between rodlike and globular surface aggregates, where CsBr is less effective at increasing the aggregate curvature. No comparison was possible in the case of 12-2-12 because this surfactant precipitated from solution in the presence of CsBr.
Conclusions The structures of the adsorbed layers formed by quaternary ammonium gemini surfactants (12-s-12) on mica depend sensitively on spacer length. These are generally lower in curvature than their bulk structures, as previously observed for cationic surfactants on mica. Whereas 12-2-12 and 12-3-12 both form bilayers, increasing spacer length leads to cylinders (s ) 4-6), short cylinders (s ) 8), and then globular aggregates (s ) 10-12). This extends but is consistent with previous observations.
Langmuir, Vol. 22, No. 3, 2006 1149
We have also shown that the observed adsorbed layer morphologies cannot be explained simply as a “counterion effect” due to the high surface charge density of mica because this is lower than the native surface charge density of both 12-2-12 and 12-3-12 monolayers. Bilayers are formed by 12-2-12 and 123-12 because of the high density of available adsorption sites, leading to a hydrophobic monolayer that then supports the formation of an intercalated bilayer. For spacers of 6 and longer, the mica surface charge density with added cesium is higher than that of the surfactant. Here the aggregate curvature may be lowered by the charged sites reducing the molecular area of the surfactant and hence increasing the packing parameter. Added cesium ions compete effectively for mica charged sites, lowering the density of adsorbed surfactant and increasing aggregate curvature. In the case of 12-2-12, the addition of CsCl leads to the formation of an adsorbed mesh. For s g 6, globular aggregates are favored on the surface, suggesting that these are like bulk micelles “pinned” to the mica by a few adsorption sites. These become difficult to image at high electrolyte concentrations as the number of available sites is decreased. The corresponding monomeric surfactants with side arms follow the same trends of aggregate curvature. Increasing the side-arm length first increases the adsorbed aggregate curvature from rods toward short rods and globules (up to s/2 ) 3), but there is an abrupt transition to a laterally unstructured, intercalated bilayer for s/2 g 4 as the longer side arms are incorporated into the hydrophobic domains. As with gemini surfactants, the addition of cesium ions decreases aggregate curvature, favoring an adsorbed layer of pinned globules for s/2 < 5. Acknowledgment. This work was funded by the Australian Research Council. LA052032B
