Surfactants are used in many important industrial processes. These include stabilizing colloidal dispersions, preventing corrosion or lubrication. 1 Surfactants lead to ordered colloidal structures called micelles on the surface of the liquid, which means they can be used to produce templates when nanostructures are to be built.2,3 Thus it is vital to understand how surfactants act at solid-liquid interfaces.
One such surfactant is Cetyl trimethylammonium bromide (CTAB) which not only forms micelles in liquids but also undergoes self-assembly on its own to form semicylindrical structures called hemimicelles, in rows, when it is adsorbed at the interface of the liquid with a solid called highly ordered pyrolytic graphite (HOPG).4 The model showing the assembly of molecules of surfactant exposing the polar groups or heads to the solution with the nonpolar or hydrophobic tails packed together towards the inside of the hemimicelles is depicted in Figure 1.
These hemimicelles show dynamic processes while they are forming as well as when they are exposed to mechanical or chemical disturbances.1, 5 Knowing more about how these changes occur is important in putting them to practical use. Newer atomic force microscopes or AFMs can help accomplish imaging to generate a wealth of spatial and time-related data compared to earlier times. This article discusses the use of high-resolution imaging at video rate using AFMs in order to understand the changes occurring to CTAB hemimicelles produced at the interface of HOPG with the solution.
AFM Imaging of Surfactant Structures
AFMs are sophisticated devices which help to understand how things work at solid-liquid interfaces. They are superior to scanning electron microscopes in being able to provide images with high spatial resolution even when they are in liquid. It was in this way that, over a couple of decades before, hemimicelles were first successfully imaged using AFMs.6 This research led to direct visualization of the actual structure of a surfactant, which could then be compared to the predicted form.
The issue with conventional AFMs is that they require several minutes to acquire one image, and this does not capture the dynamic changes. It was only a comparison of side-by-side scan lines that allowed investigators a glimpse into the millisecond order rate of healing of defects induced by the tip.1
Fast-scanning AFMs were launched in 2008, when the first Asylum Research Cypher AFM came out. It speeded up image acquisition by 10-20 times, but dynamic changes such as aggregation of hemimicelles still occurred on timescales that were greater than the temporal resolution achievable. Some scientists therefore went to the drawing board to build faster AFMs for their own use7. These have been used to bring out remarkable results8 but are an expensive and often unattainable method for those researchers who are not involved in developing better AFMs.
This obstacle went down with the emergence of the Asylum Cypher VRS, in 2017, the first AFM with video rate imaging in the world and still the only one with full features. The Cypher VRS can acquire images at line rates up to 625 Hz, at high resolution, allowing monitoring of dynamic events at frame rates well over 10 frames per second (fps). This rate has been matched by other AFMs since then, 7 but the Cypher VRS retains the distinction of being the first to build an intuitive AFM with a complete array of controls for environmental settings and different modes. Thus CTAB hemimicelles can now be imaged at high speed and with high spatial resolution, as shown in Figure 2.
Figure 1: Schematic diagram of CTAB hemimicelles on HOPG
Figure 2: Single frame taken from a video captured at 5.8 fps (173 ms per frame), clearly showing the rows of CTAB hemimicelles on the HOPG surface and the orientation of the domain boundaries. Tapping mode phase data is shown here for optimum contrast. Watch the full video: http://AFM.oxinst.com/CTAB
Materials and Methods
The samples for all the experiments described here were prepared by placing 5 mM CTAB solution onto freshly cleaved substrate of HOPG. After imaging began, the micelles were encouraged to rearrange themselves dynamically, by injecting 10% isopropyl alcohol through a syringe pump at 2-6 mL/h. Imaging was continued throughout.
A Cypher VRS AFM (Oxford Instruments Asylum Research) was used to acquire all images at room temperature in tapping mode, with Olympus AC 10DS probes. The probes had cantilevers which were much smaller than usual, at just 2 microns across and 9 microns long. Compared to traditional cantilevers, these have high resonance frequencies of about 450 kHz in liquid, and a low quality factor (Q) which allow them to have a higher system bandwidth, and hence imaging at video rate.
The images shown below are selected frames from movies that were produced by performing continuous scans at between 0.5 and 5.8 fps. The images were acquired using tapping mode, since this created more contrast for the micelle structure to be seen compared to the topography mapping.
In order to excite the cantilever to resonance frequency, photothermal excitation using blueDriveTM was employed so that tapping mode could be used in place of piezoacoustic excitation as in the conventional instruments. The advantage of using this technology is the lack of clutter in the drive response which simplifies the detection and tuning of the resonance. It is still more important to note that the response to the photothermal excitation is preserved over time, and is unrelated to the fluid volume. Thus, continuous imaging can be carried out over the full duration of the experiments, even allowing for continuous fluid perfusion.
Figure 3: Sequence of selected frames from a 0.48 fps (250 Hz line rate) video showing two CTAB grains of opposite row orientation (upper-left and lower-right) bounded on the left and right by two grains with parallel orientation. Over time, the two grains narrow at their boundary, eventually separating and drifting apart while the adjacent grains on the left and right merge together. Note the three different row orientations, corresponding to the three directions perpendicular to the HOPG symmetry axes. Though one can appreciate the overall evolution of the grain structure from these four images, the full video shows the process in far greater detail. See it at: http://AFM.oxinst.com/CTAB
Figure 4: Sequence of selected frames from a different video captured at 0.95 fps (250 Hz line rate) showing the spontaneous formation of a narrow CTAB grain along the boundary of two orientation domains (upper left). The grain continues to grow for about 20 s until a broader grain in the same orientation begins to emerge behind it. The full video shows that this new grain continues to grow and push the smaller grain out of the field of view. See the full video at: http://AFM.oxinst.com/CTAB
Results and Discussion
The images shown in Figures 3 to 5 were acquired over a slightly larger scan size than those shown in Figure 2. This is because multiple grains, referring to regions where uniform orientation is seen, are better picked up on broader scan fields. These grains are formed by CTAB, and they as well as the boundaries that separate them are seen clearly with larger scan sizes. Each video lasts for several minutes and contains hundreds of frames, so only a few frames are selected for this article, though the links to the full movies are given.
Asylum Research focuses on providing cutting-edge development in AFM instruments and related technology, but the subject of the images, namely, self-assembly of surfactant molecules, is not their field of specialization. These images are illustrative of the type of research modalities that are made possible with the Cypher VRS.
Figure 3 shows four grains which vary from each other in the angular orientations of the micelles. As time passes, as shown by the frames, the boundaries of the grains show noticeable shifts, however, the micelles in each grain keep to the same orientation.
In Figure 4, the images show a somewhat different change. Here the boundary marking the limits of two already existing grains is the site of emergence of a new grain. On the other hand, Figure 5 shows the absorption and dissipation of smaller structures into the larger grains around them. These dynamic events occur along with others evident in the full videos, at a much faster rate than could be captured by a typical AFM given its longer image acquisition time. In other words, the video-rate AFM is needed to show these dynamic processes with clarity.
These images are meant to show how these AFMs can provide images of the way the micelle grains develop, and to monitor the event over time. For example, the grain’s edge velocity could be easily examined, as could the rate of change of grain size in terms of the area, as well as the angular orientation. Since the phase and the topography are both monitored, intriguing events may be observed, such as when the micelle grains seem to be trapped at the step edges in the HOPG substrate, as well as other moments when the grains flow across the steps quite regardless of their presence.
These dynamic changes are just a few of the events that could be visualized using this type of AFM. Researchers in this field would be benefited by coming together with Asylum Research to discuss these potentially exciting developments, and to see how the Cypher VRS could be of help in this investigation.
Many dynamic events are now capable of being visualized thanks to the use of video-rate AFM. These encompass several biochemical reactions, processes involving self-assembly, and phase and grain structure evolution in a non-equilibrium manner. The combination of high speed with excellent resolution in the Cypher VRS video-rate AFM allows the behavior of surfactant micelles to be visualized as well as assessed quantitatively, at the interface of solid and liquid. It is hoped that this advance will make it possible to understand better how surfactant self-assembly occurs and how these molecules can be of practical use.
Figure 5: These two sequences of frames (top and bottom) were taken from a video acquired 0.95 fps (250 Hz line rate). Both examples show small features that seem to absorb into surrounding grains. The process appears analogous to Ostwald ripening, except in this two-dimensional system instead of a bulk dispersion. In both cases, the feature appears with higher phase contrast and appears less structured than the surrounding hemimicelles. The full video also shows how the scan size and offset can be adjusted during the video acquisition to survey a wider area. See it at: http://AFM.oxinst.com/CTAB
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This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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