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Langmuir 1998, 14, 2790-2795
Alkyl Group Dependence of the Surface-Induced Assembly of Nonionic Disaccharide Surfactants Nolan B. Holland,† Mark Ruegsegger,‡ and Roger E. Marchant*,†,‡ Departments of Macromolecular Science and Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106 Received October 20, 1997. In Final Form: February 18, 1998 N-Alkylmaltonamide nonionic diblock surfactants of varying hydrophobic segment lengths were synthesized, and their surface active properties on highly oriented pyrolytic graphite were characterized by atomic force microscopy (AFM). The N-alkylmaltonamide surfactants are composed of maltose, a hydrophilic disaccharide, with amide-linked alkyl groups of varying length, from octyl to octadecyl. All the surfactants readily adsorb uniformly onto the hydrophobic graphite surface from solution. Surfactants with alkyl lengths of 10 or more carbons exhibit epitaxial adsorption on graphite, forming ordered hemicylinders with a diameter that increases with increasing surfactant length. The solution concentration necessary for inducing surface ordering decreases with increasing alkyl chain length. N-Octadecylmaltonamide, which is insoluble in water, adsorbed from methanol solution without ordering and upon solvent replacement with water, assembled into hemicylinders. Once formed, the surfactant structures appeared stable in pure water and under high scanning forces. The effects of alkyl chain length on surfaceinduced assembly and molecular packing are discussed from the viewpoint of surfactant composition and adsorbed configurations.
Introduction Surface activity including self-assembly is a central distinguishing feature of surfactant molecules.1-3 In aqueous solution, surfactants will undergo spontaneous assembly at a solid/water interface. The driving force is analogous to adsorption at an air/water interface, but with a two-dimensional geometric constraint presented by the solid surface. Surface-induced assembly of surfactants and aggregation in aqueous solution is driven by entropic hydrophobic attraction of the nonpolar component of the surfactants. This process maximizes intermolecular contact between surfactant molecules and increases the configurational entropy of water. The hydrophilic properties of the polar headgroup oppose assembly by remaining solvated by water. Attractive van der Waals, hydrophobic, hydrogen bonding, and electrostatic intermolecular forces can stabilize an assembly, while steric and/or ionic repulsion between headgroups inhibit assembly. The attractive and repulsive forces compete to decrease or increase the interfacial area of the polar headgroup exposed to the aqueous phase. Thus, assembly is largely determined by the nonpolar component of the surfactant, its composition, length, flexibility, and geometry; while the molecular packing arrangement is largely determined by the repulsive interactions of the headgroup: its size (steric exclusion), hydration, and charge.4 Recently, investigators have been able to utilize high resolution scanning probe microscopies to directly visual-
ize surfactant assembly on solid surfaces under aqueous conditions.5-13 Several recent papers have described surface induced assembly of nonionic,5 cationic,6-9 anionic,10-12 and zwitterionic surfactants13 on highly oriented pyrolytic graphite, mica, silica, and/or gold, as observed by atomic force microscopy (AFM), although no work has been reported on saccharide surfactants under aqueous conditions. Surface assembly of ionic surfactants is dependent on the relative strength of the interactions of the ionic and hydrophobic components of the surfactant with the substrate. It was demonstrated that the structure of the ionic portion of surfactants,8 the ionic strength of the solution,11 and the addition of alcohols to the solution12 can all affect surfactant aggregation on surfaces. For the cationic surfactant cetyltrimethylammonium bromide adsorbed on hydrophilic mica and hydrophobic graphite, surface-adsorbed cylinders and hemicylinders are observed, respectively, while hemicylinder structures are believed to form on gold.6,9 Surface-directed ordering of surfactant aggregation was observed on graphite, although the extent to which the substrate dictates the assembled structure was not resolved.7 The interfacial self-assembled structures for a series of n-(ethylene oxide) dodecyl ether (C12En) nonionic surfactants on graphite were determined using AFM.5 With the dodecyl alkyl group kept constant, the nature of the ordered structures was found to depend on the ethylene oxide chain length. Hemicylinders were formed from C12E5 to C12E10, while
* To whom correspondence should be addressed at the Department of Biomedical Engineering, Wickenden Building, Case Western Reserve University, Cleveland, OH 44106. Telephone: (216) 368-3005. Fax: (216) 368-4969. E-mail:
[email protected]. † Department of Macromolecular Science. ‡ Department of Biomedical Engineering.
(4) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992. (5) Patrick, H.N; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349. (6) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (7) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (8) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915. (9) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381. (10) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207. (11) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463. (12) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4223-4228. (13) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 11507.
(1) Rosen, M. Surfactants and Interfacial Phenomena; John Wiley and Sons: New York, 1978. (2) Schick, M. J., Ed. Nonionic Surfactants: Physical Chemistry; Marcel Dekker: New York, 1987. (3) Marchant, R. E.; Ruegsegger, M.; Qiu, Y. Polysaccharide surfactants: structure, synthesis and surface active properties. In Structural Diversity and Functional Versatility of Polysaccharides; Marcel Dekker: New York, in press.
S0743-7463(97)01141-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
Surface Assembly of Saccharide Surfactants
Figure 1. Molecular model and chemical composition of N-alkylmaltonamides surfactants: (a) molecular model of dodecylmaltonamide (MAL-C12); (b) chemical composition of N-alkylmaltonamides, where n ) 7, 9, 11, and 17, and the surfactants are defined as MAL-C8, MAL-C10, MAL-C12 and MAL-C18. Notice that the planar amide linkage causes a kink in the molecular conformation. The model was generated and energy-minimized using BIOSYM software on a Silicon Graphics workstation.
C12E3 formed anchored lamellae and C12E23 did not form ordered structures. In this report, we describe results obtained by AFM on the solid surface adsorption for a well-defined series of nonionic N-alkylmaltonamide surfactants. The surfactants consist of a constant amide-linked maltose disaccharide headgroup, but with increasing alkyl chain length, from octyl to octadecyl. Using both tapping and contact AFM imaging modes, we examine the effect of increasing alkyl chain length on the adsorption, ordering and assembly, and stability of the four surfactants on graphite. The results demonstrate the respective roles of the saccharide headgroup and alkyl chain length on surface assembly and molecular packing. The effects of alkyl chain length on the mechanism of surface-induced assembly and molecular packing are discussed from the viewpoint of surfactant composition and the adsorbed molecular configurations. Experimental Section Surfactants. N-Alkylmaltonamides with octyl, decyl, dodecyl, and octadecyl segments were synthesized by reacting excess alkylamine with the reducing end group (aldehyde) of the disaccharide maltose, so that well-defined surfactants were prepared. The aldehyde group was selectively oxidized to carboxylic acid, cyclized to produce maltonolactone, and then reacted with an alkylamine to form an amide linkage as previously reported.14,15 The synthesized surfactants all have the same polar maltose headgroup, but increasing alkyl chain length (Figure 1): N-octylmaltonamide (MAL-C8), N-decylmaltonamide (MALC10), N-dodecylmaltonamide (MAL-C12), and N-octadecylmaltonamide (MAL-C18). Surfactants were purified by flash chromatography and by passing through weak anionic and cationic exchange columns. Fourier transform infrared (FTIR) and 1H-NMR spectroscopies were used to confirm surfactant structure and purity (>98%) as previously described.14,15 Stock solutions of the water soluble surfactants were prepared and then diluted to the desired concentrations. For the MALC18 surfactant, which is insoluble in water, a methanol solution (0.50 mg/mL) was prepared. The critical micelle concentrations (cmc) for MAL-C8, MAL-C10, and MAL-C12, as determined from (14) Zhang, T.; Marchant, R. E. Macromolecules 1994, 27, 7302. (15) Zhang, T.; Marchant, R. E. J. Colloid Interface Sci. 1996, 177, 419.
Langmuir, Vol. 14, No. 10, 1998 2791 surface tension measurements, are 5.7, 1.3, and 0.3 mM respectively15 (Table 1). The water used in all experiments was highly purified using reverse osmosis, followed by ultraviolet treatment, to a total organic carbon level lower than 5 parts per billion, and resistivity of 18 MΩ‚cm. AFM Imaging. Surfactant samples were imaged using a Nanoscope III Multimode AFM equipped with a fluid cell attachment and triangular silicon nitride cantilevers with a nominal spring constant of 0.58 N/m (Digital Instruments, Santa Barbara, CA). Graphite substrate (ZYA grade, highly oriented pyrolytic graphite) was freshly cleaved and imaged under water prior to the addition of surfactant solution in each experiment. Aqueous solutions of surfactant were flowed into the fluid cell and imaged in the presence of the surfactant solutions. The effect of surfactant concentration was studied by introducing higher concentration solutions into the fluid cell and subsequently imaging the adsorbate. Aqueous surfactant solutions were replaced by pure water and the adsorbed surfactants imaged again by AFM. In the case of MAL-C18 which is water insoluble, AFM imaging was carried out after surface adsorption from methanol solution (0.50 mg/mL). The methanol solution was replaced by pure water and the adsorbed surfactant imaged again. AFM images were collected at ambient temperature in both tapping and contact modes. In tapping mode, both topographic and phase data were collected. The imaging setpoint was set so that the decrease in the RMS amplitude during imaging was between 5 and 15 nm corresponding to a static normal force between 3 and 9 nN. The integral and proportional gains and the scan rate were optimized for providing the best line trace and retrace; typical values were 0.5, 0.3, and 2-5 Hz, respectively. In contact mode, the setpoint was set to image with an imaging force of less than 3 nN. In some cases however, imaging forces became as high as 12 nN during imaging, due to thermal drift in the microscope. AFM data were not filtered, although the topographic image data were flattened using a first- or secondorder line fit to eliminate sample tilt and/or piezo bow. To accurately characterize the observed periodicity of the surfactant assemblies on graphite, the scan angle was adjusted to align the bands in the slow scan direction,8 and the assemblies were analyzed using both one- and two-dimensional Fourier transforms.
Results The results obtained by AFM for the adsorption on graphite of the N-alkylmaltonamide surfactants with increasing alkyl chain lengths are shown in Figure 2. The adsorption of all surfactants was observed by AFM by a change in the roughness and appearance of the graphite image after the introduction of a surfactant solution. The adsorption behavior of the surfactants was noticeably different with increasing alkyl chain length. MAL-C8, with the shortest alkyl segment, adsorbed uniformly over the graphite surface without assembling into ordered structures (Figure 2a). Both MAL-C10 and MAL-C12 assembled into ordered structures that extended over several hundred nanometer regions of the graphite with nearly no observable defects (Figure 2b,c). Surfactant adsorption was rapid with respect to the imaging time, consistent in up, down, and 90° scans and reproducible in repeated experiments. The bands are evident in the topographic mode AFM images with height difference of nearly 1 Å from the peak to valley; however, phase mode images typically provided superior contrast of the banding periodicity. The ordered structures appear as well-organized linear bands with a periodicity of 4.7 ( 0.3 nm for MAL-C10 and 5.7 ( 0.4 nm for MALC12 (Table 1). The result obtained for MAL-C12 is consistent with the 5.3-5.8 nm periodicity range obtained for C12En nonionic surfactants.5 The observed surface ordering of MAL-C10 and MAL-C12 was highly dependent upon the concentration of surfactant in the bulk solution. Ordered bands of adsorbed MAL-C10 did not appear until
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Table 1. Surface Active Properties of N-Alkylmaltonamide Surfactants air-watera surfactant Mal-C8 Mal-C10 Mal-C12 Mal-C18 a
graphite-water
cmc (mmol/L)
Γ× (mol/cm2)
area/ molecule (Å2)
observed periodicity (nm)
2 × molecular length (nm)
herringbone model (nm)
5.7 1.3 0.3
4.3 4.6 3.8
39 36 43
NA 4.7 ( 0.3 5.7 ( 0.4 6.1 ( 0.3
4.4 4.9 5.4 6.9
3.8 4.2 4.7 6.0
1010
From ref 15.
Figure 2. AFM tapping mode images of N-alkylmaltonamide surfactants with increasing alkyl chain lengths adsorbed on graphite: (a) MAL-C8; (b) MAL-C10; (c) MAL-C12; (d) MALC18. Notice the increasing width of the periodic bands as the alkyl chain length increases, from decyl to octadecyl. No banding was observed with MAL-C8.
the solution concentration was 3 × cmc, whereas full surface coverage of ordered bands was first observed for MAL-C12 at cmc/20. Increasing the solution concentration of the MAL-C12 surfactant up to the cmc provided no discernible change in the surface structure. The MAL-C18 surfactant is insoluble in water but was successfully imaged following its adsorption from methanol solution. In methanol solution, the surface adsorption of MAL-C18 was similar to MAL-C8, with an increased surface roughness, but with no apparent ordering. Replacing the methanol surfactant solution with pure water initiated a transition from a disordered to an ordered structure. The MAL-C18 undergoes surface-induced rearrangement into a distinctive banding pattern similar to MAL-C10 and MAL-C12, but with a wider band spacing of 6.1 ( 0.3 nm (Figure 2d). In this case however, there were frequent defects in the banding pattern, in which a single band would terminate abruptly at the junction of two adjacent bands or bands would curve and change direction of orientation. For the surfactants exhibiting surface assembly, banding patterns were observed in domains with varying directional orientations. Changes in orientation were observed in the middle of large planes (Figure 3a) and at graphite step edges (Figure 3c). As shown in Figure 3a,b, three major orientations of the bands were observed, at 120° to each other, suggesting that the adsorption was epitaxial to the graphite substrate. The stability of the adsorbed MAL-C12 surfactant
Figure 3. AFM images of banding orientations for MAL-C12. (a) Tapping mode phase AFM image (300 nm) of MAL-C12 that shows a change in band orientation at a graphite plane step. The phase image has high contrast by keeping the two planes of graphite on the same image plane. The MAL-C12 bands have a periodicity of 5.5 nm. (b) Contact mode images (150 and 50 nm) of MAL-C12 showing boundary where three differently oriented surfactant domains meet without the perturbation of a graphite plane step. (c) Higher resolution image showing the hemispherical arrangement where the bands merge.
structure was assessed by two methods. First, the surfactant structures exhibited stability under conditions of high shear forces deliberately applied in contact mode imaging. Normal forces in excess of 250 nN at scan rates up to 60 Hz were applied to 256 nm regions for several minutes, and upon returning to normal imaging conditions the banding structure appeared unaffected, with no observed change in orientation or position. Second, structural stability of adsorbed surfactant was examined by replacing the surfactant solution with pure water, to create a thermodynamic driving force for desorption. After 30 min, the banding structure remained unperturbed, showing no evidence of desorption. Thus, once ordered surfactant structures are formed on graphite, the structures appeared stable even under media conditions of pure water. Discussion At an air/water interface, N-alkylmaltonamide surfactants assemble in a close packed arrangement whereby the hydrophobic alkyl tails are shielded from the aqueous solution by the hydrophilic maltose headgroups. Under dilute conditions, surfactant molecules assemble at the air/water interface with the nonpolar component exposed to air, and the polar headgroup in contact with water.
Surface Assembly of Saccharide Surfactants
Since the nonpolar component is now surrounded by hydrophobic air and the polar component is surrounded by water, the interfacial free energy is minimized. Thus, surface tension of water decreases with increasing surfactant concentration, until the cmc is reached. The information obtained from air/water surface tension measurements provides a useful basis from which to appreciate the properties of surfactants at a solid/water interface. At the air/water interface, increasing the alkyl chain length in the N-alkylmaltonamide surfactants from hexyl to dodecyl, increases surface activity as reflected in decreasing cmc values and in parameters that can be calculated from the Gibbs adsorption equation, such as surface excess concentration (Γ), which is related to packing density at the air/water interface and the surface area occupied by each surfactant molecule (Table 1). At an air/water interface, the assembly of surfactant molecules depends largely on the properties of the hydrophobic segment, while molecular packing at the interface depends largely on the size and hydration of the polar headgroup.15 Unlike surfactant packing at an air/water interface, our AFM results demonstrate that both assembly and packing of the surfactants at the solid graphite/liquid interface are dependent on the hydrophobic alkyl chain length. This is directly attributable to the geometric constraint presented by the two-dimensional solid graphite surface, compared with the three-dimensional air/water interface in which alkyl chains can protrude into the air phase. Adsorption of the N-alkylmaltonamide surfactants on graphite was rapid with respect to imaging time, and the band domains observed for MAL-C10 and MAL-C12 were large (>500 nm). Banding structures were not observed for MAL-C8. Banding structures were also not observed for MAL-C18 adsorbed from methanol solution but appeared spontaneously after replacing the methanol with water. MAL-C18 molecules adsorbed on the graphite from methanol, but the driving force for ordering was insufficient, since methanol solvates both the maltose and octadecyl segments. Only after methanol was replaced by water did the resident adsorbed MAL-C18 molecules rearrange to form banding structures. The ordered banding structures observed for the N-alkylmaltonamides are similar to those reported for C12En and ionic surfactants on graphite, with the bandwidths approximately twice the molecular length of the surfactants (Table 1). Several models have been proposed for the adsorbed surfactant structures. Interpretation of the banding structure observed for surfactants with a hydrocarbon segment g10 carbons include monolayer adsorption in alkyl group tail to tail configuration, hemicylinder formation, and a stacked structure.11 It is probable that the surfactant molecules adsorb initially as a monolayer in registry with the graphite surface in a tail to tail configuration, as observed for the deposition of long hydrocarbon alcohols and carboxylic acids using scanning tunneling microscopy16 and of N-(n-alkyl)-D-gluconamides (AGA) using atomic force microscopy.17 AGAs, which have a similar chemical structure to our N-alkylmaltonamides, were shown to form stacked double-layer structures in deposition experiments.17 However, geometric arguments indicate that it is unlikely that the hydrated N-alkylmaltonamides will stack due to larger headgroup size. Using the model of Israelachvili for solution aggregates18 (16) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (17) Tuzov, I.; Cra¨mer, K.; Pfannemu¨ller, B.; Magonov, S. N.; Whangbo, M.-H. New J. Chem. 1996, 20, 23. (18) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
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Figure 4. Schematic drawing of surfactant adsorbed on a solid support: (a) a monolayer adsorption pattern of diblock surfactant in a tail to tail configuration; (b) a hemicylinder structure where the hydrocarbon chain of the surfactant favors contact with the hydrophobic graphite surface, while the hydrophilic maltose favors exposure to the aqueous phase. This structure is comparable to a cylindrical micelle in solution.
and the hydrated headgroup area obtained from surface tension data for the N-alkylmaltonamides,15 the packing parameter, defined as a ratio of the molecular volume (v) to the product of the headgroup area (ao) and tail length (l), is
v/aol ≈ 0.5
(1)
suggesting the micellar structure of these surfactants is cylindrical. By a similar argument, the packing of additional surfactant molecules beyond monolayer coverage should form a hemicylindrical structure at the hydrophobic graphite-water interface (Figure 4b). Following theory, the hemicylinders should have a radius
r ) 2l(v/aol) + lp
(2)
where lp is the length of the polar headgroup. Therefore, the hemicylinder radius is simply the total length of the surfactant (l + lp) for a packing parameter of 0.5. The observed periodicity of the surface assemblies for MALC10 and MAL-C12 surfactants fit well to this model, but there are deviations for both the short (MAL-C8) and long (MAL-C18) surfactants. The behavior of MAL-C8 can be understood by examining how the saccharide headgroup area is related to the curvature of structures formed at the interface. The MALC8 headgroup area was determined from surface tension data, which corresponds to a planar interface between polar and nonpolar domains. If one imagines imposing a finite curvature, as in hemicyclinder formation, polar headgroups will tend to separate and the nonpolar tails will compress. Therefore, as the hemicylinder radius decreases with decreasing alkyl chain length, the effective headgroup area decreases. This introduces strain in forming hemicylinders that will tend to maximize the radius of curvature. For MAL-C8, with a small hemicylinder radius of curvature, this would decrease the effective headgroup area, so that the packing parameter has a value greater than 0.5, signifying a transition from a cylindrical geometry to a planar bilayer. This is observed as a confluent layer of MAL-C8 by AFM. The behavior of the MAL-C18 can be interpreted by understanding the interaction of the surfactant with the surface. The observation of bands in preferred alignment at 120° to each other suggests epitaxy of the adsorbed surfactant molecules to the graphite substrate. Due to organization of ionic surfactants on crystalline surfaces other than graphite, it has been suggested that crystalline anisotropy is the mechanism of ordering.7 Either way, it is probable that alkyl tails lay down along graphite atoms, as was observed by scanning tunneling microscopy of low
2794 Langmuir, Vol. 14, No. 10, 1998
Figure 5. Models of N-alkylmaltonamide surfactants on the basal plane of graphite. (a) Alkyl tails of the surfactants adsorb on the substrate epitaxially along the graphitic carbon. The alkyl chain lies along the graphite so the hydrogen atoms lie toward the center of the graphite hexagons. This energetically favorable state limits the molecule to three symmetric orientations on the surface. The maltose headgroup, attached to the tail via an amide bond (nitrogen labeled with N), can extend into the aqueous solution by rotational freedom of the nitrogen-R carbon bond. (b) MAL-C12 is shown here in a perpendicular configuration, which maximizes the radius of the hemicylinders formed. The alkyl chains align parallel to each other and in a tail to tail configuration with the tails aligned along the graphite carbons as in part a. This gives a band spacing of twice the molecular length. (c) The MAL-C18 assumes a herringbone configuration where the chains still pack parallel to each other and in a tail to tail configuration, but alternating rows of the surfactant are tilted with respect to each other. Notice that the surfactants still align along the graphite carbons as in part a, but the band spacing is only 1.73 times the surfactant length in this configuration. This organization allows the molecules to pack together efficiently, even when the orientation changes direction.
Holland et al.
molecular weight alkanes,19 with the carbons slightly offset from the graphite carbons (Figure 5a). The enthalpic gain from epitaxial association of the alkyl chain binds the surfactant tightly, overcoming the tendency for conformational disorder in adsorbed alkyl chains, and increasing the stability of the surface adsorbate. This ordered monolayer of surfactant acts as a template for subsequent hemicylinder formation and the periodic banding structures (Figure 5b,c). Constrained by epitaxial adsorption, the first layer of surfactant can align only in limited configurations. It was reported that adsorption on graphite for long chain alkanes and fatty acids results in perpendicular orientation of chains to the bands and a tilted orientation for alkyl alcohols and dialkylbenzenes.16 This suggests that these two orientations are low energy states for alkyl chain adsorption on graphite. For the N-alkylmaltonamides, one would expect the perpendicular configuration to be favored, which increases the radius of curvature and broadens the bands. This fits the structure predicted by the packing parameter and is consistent with the observed results for the middle length surfactants, MAL-C10 and MAL-C12 (Table 1 and Figure 5b). The MAL-C18 data are not consistent with the perpendicular configuration. The band spacing is significantly less than twice the molecular length, suggesting that the first layer of adsorbate is in a tilted, herringbone orientation (Figure 5c). We propose that the stronger interaction between the long octadecyl chain and the graphite drives the adsorption to its favored herringbone monolayer configuration. Since MAL-C18 was adsorbed from methanol solution, it could be argued that the surface concentration of surfactant is not high enough to form large hemicylinders, so the structure defaults to the narrower herringbone configuration. If this were the case, we would expect to see a transition for the MAL-C12 as we increased the concentration from narrow bands (herringbone configuration) to broader bands (perpendicular configuration). This was not observed. Further evidence for the perpendicular and herringbone structures is observed in the degree of order in the banding patterns. As shown in Figure 5, the herringbone structure can change its direction of orientation with relatively little energy loss from overlapping polar and nonpolar structures, compared with the perpendicular structures. This leads directly to the higher number of defects and band direction changes evident for MAL-C18 (Figure 2d). In addition, small hemispherical structures are observed at the boundaries of the domains of MAL-C12 (Figure 3c). These are unlikely to form from the herringbone configuration. The ability to image the adsorbed structures using contact mode AFM provides evidence of their renitence. Images of adsorbed surfactant structures by other researchers5-13 were obtained by a pseudo-non-contact mode which uses either an electrical double layer or steric repulsion as the imaging force,20 whereas the contact mode uses the Born repulsion force. The former has been shown to reduce tip interaction with the surface, which can improve image resolution; but since the tip is not in direct contact, some surface detail potentially can be smeared out.20 Using the Born repulsion as the imaging force can generate large lateral friction forces which may affect images of delicate surfaces, as was illustrated by the loss (19) McGonigal, G. C.; Bernhardt, R. H.; Thompson, D. J. Appl. Phys. Lett. 1990, 57, 28. (20) Senden, T. J.; Drummond, C. J.; Ke´kicheff, P. Langmuir 1994, 10, 358.
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those contained in micelles, so it seems unlikely they would persist in the pure water rinse. Furthermore, the images of the surfactant monolayer would produce the same height data as a hemicylinder, so would be indistinguishable. The expected AFM output using a 30 nm radius of curvature probe imaging a hemicylinder and an adsorbed monolayer with a spacing of 6 nm, is illustrated in Figure 6. The peak to valley dip for both is expected to be about 0.7 Å, which matches the data. Figure 6. Schematic of the expected height data for AFM images for surfactant hemicylinder and monolayer structures. A tip of 30 nm radius of curvature was used to predict the expected height data for the hemicylinders (upper line) and monolayer of surfactant (lower line). The height difference from the peak to trough for both structures is about 0.7 Å. This indicates that AFM height data cannot be used to distinguish between monolayer and hemicylindrical structures.
of apparent surface structure for ionic surfactant when true contact mode was used.6,10,11 The structures formed by the N-alkylmaltonamide surfactants were imaged in contact mode demonstrating the relative stability of the surface structures. Since the structures are periodic and spaced close compared to the size of the probe, detail is limited by the probe size. MAL-C18, which is insoluble in water, was expected to exhibit irreversible adsorption. However, the apparent irreversible adsorption of MAL-C12 surfactant was unexpected, considering its solubility in water. This can be understood if the association of the adsorbate and the surface has created an energetic barrier to desorption. This supports the model of the epitaxial adsorption of the alkyl chain, which provides an additional 6.28 kJ/mol per adsorbed methylene group.21 For the MAL-C12, it is not clear as to whether the hemicylinder structures were desorbed, leaving only the epitaxially bound monolayer of surfactant. The molecules that fill a hemicylinder should have energetics similar to (21) Groszek, A. J. Proc. R. Soc. London A 1970, 314, 473.
Conclusions The alkyl chain length of disaccharide surfactants plays a key role in the structures formed following adsorption on graphite. N-Octylmaltonamide adsorbs from aqueous solution uniformly on graphite, but does not demonstrate ordering. N-Decylmaltonamide and N-dodecylmaltonamide form hemicylindrical structures that assemble as ordered bands on the graphite surface. N-Octadecylmaltonamide, which is water insoluble, adsorbs uniformly on graphite from methanol solution. Replacement of methanol solution by water induces surface rearrangement of the surfactant into ordered banding structures. Banding periodicity for adsorbed surfactants was shown to depend on alkyl chain length. We also suggest that banding periodicity depends on the configuration (perpendicular or herringbone) of the initial monolayer of adsorbate. Once formed, ordered surfactant assemblies appeared stable under pure water as observed after 30 min. This surface stability of adsorbed surfactants is attributed to epitaxial association of the alkyl chains with the graphite, which creates a high energy barrier for desorption. Acknowledgment. We gratefully acknowledge the financial support of this work provided by NIH Grant HL-40047 and Whitaker Foundation, and use of facilities at the Center for Cardiovascular Biomaterials. LA971141Q
