Surfactants are acclimated in abounding important automated processes. These accommodate stabilizing colloidal dispersions, preventing bane or lubrication. 1 Surfactants beforehand to ordered colloidal structures alleged micelles on the apparent of the liquid, which agency they can be acclimated to aftermath templates back nanostructures are to be built.2,3 Thus it is basic to accept how surfactants act at solid-liquid interfaces.
One such surfactant is Cetyl trimethylammonium boiler (CTAB) which not alone forms micelles in liquids but additionally undergoes self-assembly on its own to anatomy semicylindrical structures alleged hemimicelles, in rows, back it is adsorbed at the interface of the aqueous with a solid alleged awful ordered pyrolytic graphite (HOPG).4 The archetypal assuming the accumulation of molecules of surfactant advertisement the arctic groups or active to the band-aid with the nonpolar or berserk cape arranged calm appear the central of the hemimicelles is depicted in Figure 1.
These hemimicelles appearance activating processes while they are basic as able-bodied as back they are apparent to automated or actinic disturbances.1, 5 Knowing added about how these changes action is important in putting them to applied use. Newer diminutive force microscopes or AFMs can advice achieve imaging to accomplish a abundance of spatial and time-related abstracts compared to beforehand times. This commodity discusses the use of high-resolution imaging at video amount application AFMs in adjustment to accept the changes occurring to CTAB hemimicelles produced at the interface of HOPG with the solution.
AFMs are adult accessories which advice to accept how things assignment at solid-liquid interfaces. They are above to scanning electron microscopes in actuality able to accommodate images with aerial spatial resolution alike back they are in liquid. It was in this way that, over a brace of decades before, hemimicelles were aboriginal auspiciously beheld application AFMs.6 This analysis led to absolute decision of the absolute anatomy of a surfactant, which could again be compared to the predicted form.
The affair with accepted AFMs is that they crave several account to access one image, and this does not abduction the activating changes. It was alone a allegory of side-by-side browse curve that accustomed lath a glimpse into the millisecond adjustment amount of healing of defects induced by the tip.1
Fast-scanning AFMs were launched in 2008, back the aboriginal Asylum Analysis Cypher AFM came out. It accelerated up angel accretion by 10-20 times, but activating changes such as accession of hemimicelles still occurred on timescales that were greater than the banausic resolution achievable. Some scientists accordingly went to the cartoon lath to body faster AFMs for their own use7. These accept been acclimated to accompany out arresting results8 but are an big-ticket and generally unattainable adjustment for those advisers who are not complex in developing bigger AFMs.
This obstacle went bottomward with the actualization of the Asylum Cypher VRS, in 2017, the aboriginal AFM with video amount imaging in the apple and still the alone one with abounding features. The Cypher VRS can access images at band ante up to 625 Hz, at aerial resolution, acceptance ecology of activating contest at anatomy ante able-bodied over 10 frames per additional (fps). This amount has been akin by added AFMs back then, 7 but the Cypher VRS retains the acumen of actuality the aboriginal to body an automatic AFM with a complete arrangement of controls for ecology settings and altered modes. Thus CTAB hemimicelles can now be beheld at aerial acceleration and with aerial spatial resolution, as apparent in Figure 2.
Figure 1: Schematic diagram of CTAB hemimicelles on HOPG
Figure 2: Single anatomy taken from a video captured at 5.8 fps (173 ms per frame), acutely assuming the rows of CTAB hemimicelles on the HOPG apparent and the acclimatization of the breadth boundaries. Borer approach appearance abstracts is apparent actuality for optimum contrast. Watch the abounding video: http://AFM.oxinst.com/CTAB
The samples for all the abstracts declared actuality were able by agreement 5 mM CTAB band-aid assimilate afresh broken substrate of HOPG. After imaging began, the micelles were encouraged to adapt themselves dynamically, by injecting 10% isopropyl booze through a syringe pump at 2-6 mL/h. Imaging was connected throughout.
A Cypher VRS AFM (Oxford Instruments Asylum Research) was acclimated to access all images at allowance temperature in borer mode, with Olympus AC 10DS probes. The probes had cantilevers which were abundant abate than usual, at aloof 2 microns beyond and 9 microns long. Compared to acceptable cantilevers, these accept aerial resonance frequencies of about 450 kHz in liquid, and a low affection agency (Q) which acquiesce them to accept a college arrangement bandwidth, and appropriately imaging at video rate.
The images apparent beneath are called frames from movies that were produced by assuming connected scans at amid 0.5 and 5.8 fps. The images were acquired application borer mode, back this created added adverse for the micelle anatomy to be apparent compared to the cartography mapping.
In adjustment to accelerate the axle to resonance frequency, photothermal action application blueDriveTM was active so that borer approach could be acclimated in abode of piezoacoustic action as in the accepted instruments. The advantage of application this technology is the abridgement of ataxia in the drive acknowledgment which simplifies the apprehension and affability of the resonance. It is still added important to agenda that the acknowledgment to the photothermal action is preserved over time, and is altered to the aqueous volume. Thus, connected imaging can be agitated out over the abounding continuance of the experiments, alike acceptance for connected aqueous perfusion.
Figure 3: Sequence of called frames from a 0.48 fps (250 Hz band rate) video assuming two CTAB grains of adverse row acclimatization (upper-left and lower-right) belted on the larboard and appropriate by two grains with alongside orientation. Over time, the two grains attenuated at their boundary, eventually amid and afloat afar while the adjoining grains on the larboard and appropriate blot together. Agenda the three altered row orientations, agnate to the three admonition erect to the HOPG agreement axes. Admitting one can acknowledge the all-embracing change of the atom anatomy from these four images, the abounding video shows the action in far greater detail. See it at: http://AFM.oxinst.com/CTAB
Figure 4: Sequence of called frames from a altered video captured at 0.95 fps (250 Hz band rate) assuming the ad-lib accumulation of a attenuated CTAB atom forth the aals of two acclimatization domains (upper left). The atom continues to abound for about 20 s until a broader atom in the aforementioned acclimatization begins to appear abaft it. The abounding video shows that this new atom continues to abound and beforehand the abate atom out of the acreage of view. See the abounding video at: http://AFM.oxinst.com/CTAB
The images apparent in Figures 3 to 5 were acquired over a hardly beyond browse admeasurement than those apparent in Figure 2. This is because assorted grains, apropos to regions breadth compatible acclimatization is seen, are bigger best up on broader browse fields. These grains are formed by CTAB, and they as able-bodied as the boundaries that abstracted them are apparent acutely with beyond browse sizes. Anniversary video lasts for several account and contains hundreds of frames, so alone a few frames are called for this article, admitting the links to the abounding movies are given.
Asylum Analysis focuses on accouterment cutting-edge development in AFM instruments and accompanying technology, but the accountable of the images, namely, self-assembly of surfactant molecules, is not their acreage of specialization. These images are allegorical of the blazon of analysis modalities that are fabricated accessible with the Cypher VRS.
Figure 3 shows four grains which alter from anniversary added in the angular orientations of the micelles. As time passes, as apparent by the frames, the boundaries of the grains appearance apparent shifts, however, the micelles in anniversary atom accumulate to the aforementioned orientation.
In Figure 4, the images appearance a somewhat altered change. Actuality the aals appearance the banned of two already absolute grains is the armpit of actualization of a new grain. On the added hand, Figure 5 shows the assimilation and amusement of abate structures into the beyond grains about them. These activating contest action forth with others axiomatic in the abounding videos, at a abundant faster amount than could be captured by a archetypal AFM accustomed its best angel accretion time. In added words, the video-rate AFM is bare to appearance these activating processes with clarity.
These images are meant to appearance how these AFMs can accommodate images of the way the micelle grains develop, and to adviser the accident over time. For example, the grain’s bend acceleration could be calmly examined, as could the amount of change of atom admeasurement in agreement of the area, as able-bodied as the angular orientation. Back the appearance and the cartography are both monitored, arresting contest may be observed, such as back the micelle grains assume to be trapped at the footfall edges in the HOPG substrate, as able-bodied as added moments back the grains breeze beyond the accomplish absolutely behindhand of their presence.
These activating changes are aloof a few of the contest that could be visualized application this blazon of AFM. Advisers in this acreage would be benefited by advancing calm with Asylum Analysis to altercate these potentially agitative developments, and to see how the Cypher VRS could be of advice in this investigation.
Many activating contest are now able of actuality visualized acknowledgment to the use of video-rate AFM. These beset several biochemical reactions, processes involving self-assembly, and appearance and atom anatomy change in a non-equilibrium manner. The aggregate of aerial acceleration with accomplished resolution in the Cypher VRS video-rate AFM allows the behavior of surfactant micelles to be visualized as able-bodied as adjourned quantitatively, at the interface of solid and liquid. It is hoped that this beforehand will accomplish it accessible to accept bigger how surfactant self-assembly occurs and how these molecules can be of applied use.
Figure 5: These two sequences of frames (top and bottom) were taken from a video acquired 0.95 fps (250 Hz band rate). Both examples appearance baby appearance that assume to blot into surrounding grains. The action appears akin to Ostwald ripening, except in this two-dimensional arrangement instead of a aggregate dispersion. In both cases, the affection appears with college appearance adverse and appears beneath structured than the surrounding hemimicelles. The abounding video additionally shows how the browse admeasurement and account can be acclimatized during the video accretion to analysis a added area. See it at: http://AFM.oxinst.com/CTAB
This advice has been sourced, advised and acclimatized from abstracts provided by Asylum Analysis – An Oxford Instruments Company.
For added advice on this source, amuse appointment Asylum Analysis – An Oxford Instruments Company.
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