Rsc_cc_c0cc05762b 1..3

Surfactant molecules containing long hydrophobic alkyl chains and hydrophilic groups can organize into various self-assemblies, both in bulk solution and at a solid interface. Atomic force microscopy (AFM) has been employed to demonstrate a strong correlation between the bulk self-assembly structure and the morphology of adsorbed surfactant layers. Adsorbed layers comprised of globular and cylindrical aggregates on hydrophilic substrates, of which mica is the most widely studied, have been well-characterized. Micellar structures of common cationic surfactants, such as hexadecyltrimethylammonium bromide (CTAB) and hexadecyltrimethylammonium chloride (CTAC), adsorbed at the mica/liquid interface have been directly observed by AFM. These ordered interfacial aggregates have well-defined sizes and shapes that can be controlled by the addition of metal halide salts. When a higher concentration of competing cations is introduced, the density of available lattice anions on the mica surface is reduced, so that the spacings between surfactant headgroups increase. In other words, the influence of the mica interface is reduced by the introduction of the salt, and hence the surface aggregated structure becomes more similar to the micellar structure found in bulk solution. Lamont and Ducker have shown that CsCl tends to increase the curvature of CTAC aggregates on a mica surface due to the transformation from cylinders to globules with variation of the salt concentration. However, these measurements were performed at equilibrium conditions and no information was gained regarding the initial aggregation dynamics, due to the lack of available techniques for observing the dynamic behavior of surfactant aggregates. Rapid acquisition of successive highresolution images of surfactant aggregates is required. The scanning speeds of commercially-available AFMs are usually several minutes per frame, and are therefore too slow to monitor many physicochemical processes in liquids that occur at much faster rates. Recently, this limitation has been challenged by the use of high-speed AFM, which can capture images at a frame rate of more than 30 frames s . Various imaging studies have attempted to capture biological processes on video at high imaging rates. In this study, high-speed AFM was used to observe the initial aggregate behavior of CTAC at the mica–water interface, and provide new insights regarding surfactant aggregation dynamics at a mica surface. The muscovite mica used in this study was cleaved immediately prior to use. Water was prepared using a Milli-Q system and the resulting water had a conductivity of 18 M O cm . CTAC (495%, TCI, Tokyo, Japan) was recrystallized three times from ethanol. Analytical grade CsCl (99.9%, Wako, Tokyo, Japan) was used as received. Observations of the CTAC aggregates were performed in soft tapping mode using a laboratory-built high-speed AFM. The cantilever (Olympus, Tokyo, Japan) is 6–7 mm long, 2 mm wide, and 90 nm thick with a spring constant of 0.1–0.2 N m . Its resonant frequency in an aqueous solution is B1 MHz. To achieve a small tip–sample loading force, the free-oscillation peak-topeak amplitude of the cantilever (A0) was set to B1 nm and the amplitude set point was set at more than 0.9 A0. The probe tip was grown on the tip of a cantilever by electron beam deposition and was further sharpened by argon plasma etching. The tapping force estimated was less than 30 pN. All observations were performed at room temperature (25 1C) and at a surfactant concentration equal to twice the critical micelle concentration (CMC) in the absence of salt. The scan area was 200 200 nm with 200 200 pixels. The imaging rate was 1–2 frames s . The high-speed AFM used in this study can be used to observe at a video rate of ca. 15–30 frames s 1 for a scan range of ca. 250 nm under feedback operation that is capable of preventing weak interactions from being disturbed by the scanning tip. However, high-resolution observations were necessary in this study, so we measured at 1–2 frames s . High-speed AFM measurements were recorded immediately after injection of CsCl solutions including the CTAC. The diameters of the observed globular or cylindrical micelles are approximately 9–11 nm. Taking the tip radius into account, the true sizes of the micelles are actually smaller than the observed diameters. Their heights are in fact approximately 5–6 nm. a Kao Corporation, Wakayama 640-8580, Japan. E-mail: [email protected]; Fax: +81-73-426-8626; Tel: +81-73-426-8514 Kanazawa University, Kanazawa 920-1192, Japan c CREST, Japan Science and Technology Agency (JST), Japan ChemComm Dynamic Article Links