Structure and Phase Transitions of Alkyl Chains on Mica

We use molecular dynamics as a tool to understand the structure and phase transitions [Osman et. al. J. Phys. Chem. B 2000, 104, 4433; 2002, 106, 653] in alkylammonium micas. The consistent force field 91 is extended for accurate simulation of mica and related minerals. We investigate mica sheets with 12 octadecyltrimethylammonium (C18) ions or 12 dioctadecyldimethylammonium (2C18) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary. The nitrogen atoms are 380 to 390 pm distant to the superficial silicon-aluminum plane. With increasing temperature, rearrangements of C18 ions on the mica surface are found, while 2C18 ions remain tethered due to geometric restraints. We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche-trans ratios to analyze the chain conformation. Also, the individual phase transitions of the two systems on heating are explained. Where experimental data are available, the agreement is very good. We propose a geometric parameter lamba for the saturation of the surface with alkyl chains, which determines the preferred self-assembly pattern, i.e., islands, intermediate, or continuous. Lambda also determines the tilt angles in continuous layers on mica or other surfaces. The thermal decomposition appears to be a Hofmann elimination with mica as a base-template.Comment: 45 pages with 6 tables and 5 figure

Bubble formation during the collision of a sessile drop with a meniscus

The impact of a sessile droplet with a moving meniscus, as encountered in processes such as dip-coating, generically leads to the entrapment of small air bubbles. Here we experimentally study this process of bubble formation by looking through the liquid using high-speed imaging. Our central finding is that the size of the entrapped bubble crucially depends on the location where coalescence between the drop and the moving meniscus is initiated: (i) at a finite height above the substrate, or (ii) exactly at the contact line. In the first case, we typically find bubble sizes of the order of a few microns, independent of the size and speed of the impacting drop. By contrast, the bubbles that are formed when coalescence starts at the contact line become increasingly large, as the size or the velocity of the impacting drop is increased. We show how these observations can be explained from a balance between the lubrication pressure in the air layer and the capillary pressure of the drop