VOLUME 100, NUMBER 28, JULY 11, 1996
© Copyright 1996 by the American Chemical Society
LETTERS Effect of Substrate Hydrophobicity on Surfactant Surface-Aggregate Geometry William A. Ducker* and Lachlan M. Grant Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed: March 6, 1996; In Final Form: May 8, 1996X
Surface aggregates of the zwitterionic surfactant dodecyldimethylammoniopropanesulfonate (DDAPS) have greater curvature on mica and silicon nitride than on graphite. This supports the hypothesis that surface aggregates will be less curved on hydrophobic solids. Lower aggregate curvature reduces the area of interaction between the hydrophobic solid and water for a given surfactant surface concentration. A zwitterionic surfactant was examined on neutral surfaces to reduce the influence of charge-charge interactions.
Introduction The assembly of surfactant molecules into aggregates in water is driven by the lack of a strong interaction between the surfactant hydrocarbon tails and water molecules. The curvature of the aggregate is then influenced by head-group interactions with water and with each other.1 The study of surfactant aggregation in bulk solution is relatively mature, with extensive literature on phase diagrams and on theoretical aspects. In contrast, investigation of surface aggregation is relatively immature. On the theoretical side, difficulties are introduced by the extra phase: the imposed geometry, the surface groups, and the dielectric discontinuity. On the experimental side, the problem is the very small amount of material at the surface. There have been a number of investigations by indirect techniques, for example, by fluorescence probes2 and electron spin resonance,3 and from adsorption isotherms4,5 but the recent development of the atomic force microscope (AFM)6 has provided a technique which is now being used to obtain direct images of surface aggregates.7 The advantages of the AFM technique are (1) nanometer resolution to allow interrogation of a single aggregate, (2) the capability to operate in liquids, thus reducing the intrusion of preparative and fixing artifacts, * E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(96)00702-2 CCC: $12.00
Figure 1. DDAPS: dodecyldimethylammoniopropanesulfonate.
and (3) control, or at least adjustment of, the interrogating force. The AFM has now been used to determine surface-aggregate shapes under some conditions for cationic surfactants on graphite,7 mica,8 and gold9 and for an anionic surfactant on graphite.10 For either hexadecyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate (SDS) on graphite, the aggregates are very long and probably hemicylindrical in shape. These surface aggregates exist in equilibrium with micelles in bulk.11 In contrast, tetradecyltrimethylammonium bromide surface aggregates were found to be spherical on silica and cylindrical8 (or perhaps hemicylindrical) on mica above the bulk critical micelle concentration (cmc). The difference in curvature between bulk and surface aggregates highlights the influence of the solid substrate on the surface-aggregate structure. Specifically, this raises the question: why does it appear that micelle-forming surfactants form lower curvature aggregates on (hydrophobic) graphite? Previously, it was proposed that one contribution to lowering the curvature is the arrangement of the surfactant so as to reduce the contact between the hydrophobic graphite and water.10 Here © 1996 American Chemical Society
11508 J. Phys. Chem., Vol. 100, No. 28, 1996
Letters
we investigate this proposition by comparing the shape of aggregates on mica (water contact angle ∼5°) to those on graphite (water contact angle ∼90°). We find that the aggregate curvature is lower on graphite. A factor sometimes overlooked when considering the shape of surface aggregates is the effect of the surface in preconcentrating the surfactant at the surface. It is important to note that when a surface structure is compared to a bulk structure, it is not always the simplest approach to compare surface and bulk structures which are in equilibrium with each other. Often the surface attracts the surfactant, so that the concentration of surfactant is much higher at the surface than in bulk (e.g., the concentration of tetradecylpyridinium chloride at the surface of silica is 120 times the bulk concentration at the cmc5). Thus the surface structure may be more similar to a bulk structure at much higher concentration than the solution with which it is in equilibrium. For example, hexadecyltrimethylammonium bromide (CTAB) in bulk undergoes a transition from spherical to cylindrical micelles at 0.14 M.12 CTAB is electrostatically attracted to mica (by the negative surface potential) and is concentrated at the surface by mass-action ion-exchange so it is preconcentrated at the surface. This helps us to rationalize the observation that a single-chain quaternary ammonium surfactant will form cylindrical aggregates on mica8 at only twice the cmc, 0.0035 M.13 The high surface concentration of ions screens the electrostatic forces, decreasing the separation between head-groups and lowering the curvature. Here we report on the surface aggregation of dodecyldimethylammoniopropanesulfonate (DDAPS, see Figure 1), a zwitterionic surfactant. By examining a net-uncharged surfactant, we reduce the effect of preconcentration, and the ionicstrength dependent electrostatic interactions between headgroups, allowing us to focus on the hydrophobicity of the substrate. Repulsive electrostatic interactions between surface aggregates are particularly significant because of the high density of surface aggregates. The surface structures were determined at 10 mM DDAPS, above the cmc of 2.2 mM.14 Surfaces covered in zwitterionic surfactant have previously been shown to exhibit short-range forces which are ideal for AFM imaging.15 Here we have studied the surface aggregation on three substrates: graphite, mica, and silicon nitride. A comparison of adsorption on graphite and mica allows us to observe that the surface-aggregate curvature is lower on the more hydrophobic substrate. It is also possible for a solid substrate to influence the geometry of adsorbed aggregates through the topography of the solid surface. We hold this parameter close to constant when comparing mica and graphite (because they are both molecularly smooth), then investigate the effect of surface roughness through comparison to silicon nitride (which has nanometer scale roughness). Experimental Section Sample Preparation. Water was prepared by distillation and then passage through a Milli-Q RG system consisting of charcoal filters, ion-exchange media, and a 0.2 µm filter. The resulting water has a conductivity of 18 MΩ cm-1, and a surface tension of 72.4 mJ m-2 at 22.0 °C. Dodecyldimethylammoniopropanesulfonate (DDAPS) was obtained from Aldrich Chemicals, 98% pure, and used as received. (A small amount of surface active contaminant is present in some batches of DDAPS. This can be removed by recrystallization from distilled 2-propanol). Muscovite mica or pyrolytic graphite (monochromator grade ZYH manufactured by Union Carbide) were freshly cleaved before each experiment. Silicon nitride substrates, prepared by
Figure 2. Forces between tip and surface for the images shown. The zero of distance is where a constant high gradient of force was measured and is assumed to be where the tip is touching the sample. The steep repulsive force between 3 and 5 nm separation shows the depth of film between the tip and solid substrate as a function of applied load. The force scale is approximate and was calculated using the nominal spring constants of 0.07 N m-1 in each case. The solid arrows show the approximate force at which the images were captured, and the arrow heads marked J each show the onset of mechanical instabilities, presumably caused when the applied load is sufficient to remove the surfactant from between the tip and sample. Note that the force on the silicon nitride surface was measured with a silicon nitride tip of much larger radius than the silicon tips, so we would expect all forces to be much larger for this case. It is interesting that the surfactant coating produces a similar force in all three cases, despite the difference in solid substrate and the difference in surfactant arrangement.
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J. Phys. Chem., Vol. 100, No. 28, 1996 11509
low-pressure chemical vapor deposition on silicon, were purchased from Silica Source Technology Corp., Tempe, AZ. The silicon nitride substrates and the AFM tips were irradiated for 40 min (∼9 mW/cm2 at 253.7 nm) in a laminar flow cabinet before use. Microscopy. Images were captured using a Nanoscope III AFM (Digital Instruments, CA) using silicon ultralevers (Park Scientific, CA) for the mica and graphite images, and silicon nitride cantilevers (Digital Instruments, CA) to image the silicon nitride substrate. All the cantilevers had nominal spring constants of 0.07 N m-1. All images presented are deflection images (showing the error in the feedback signal) with integral and proportional gains of between 1 and 2 and scan rates of 10 Hz. No filtering of images was performed other than that inherent in the feedback loop. Distances parallel to a surface were calibrated by imaging a 1 µm standard grid. Distances normal to a surface were calibrated by imaging etch pits and have an error of (3%. All measurements were performed in the range 23 ( 3 °C, which is above the Krafft temperature of DDAPS solutions (
