5 research outputs found
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
Development of hypointense lesions on T1-weighted spin-echo magnetic resonance images in multiple sclerosis: Relation to inflammatory activity
Objective: To evaluate whether degree of inflammatory activity in multiple sclerosis, expressed by frequency of gadolinium enhancement, has prognostic value for development of hypointense lesions on T1-weighted spin- echo magnetic resonance images, a putative marker of tissue destruction. Design: Cohort design with long-term follow-up. Thirty-eight patients with multiple sclerosis who in the past had been monitored with monthly gadolinium-enhanced magnetic resonance imaging for a median period of 10 months (range, 6-12 months) were reexamined after a median period of 40.5 months (range, 33-80 months). Setting: Magnetic Resonance Center for Multiple Sclerosis Research, Amsterdam, the Netherlands, referral center. Main Outcome Measures: The new enhancing lesion rate (median number of gadolinium- enhancing lesions per monthly scan) during initial monthly follow-up; hypointense T1 and hyperintense T2 lesion load at first and last visit. Results: The number of enhancing lesions on entry scan correlated with the new enhancing lesions rate (r = 0.64; P<.001, Spearman rank correlation coefficient). The new enhancing lesion rate correlated with yearly increase in T1 (r = 0.42; P<.01, Spearman rank correlation coefficient) and T2 (r= 0.47; P<.01, Spearman rank correlation coefficient) lesion load. Initial T1 lesion load correlated more strongly with yearly increase in T1 lesion load (r = 0.68; P<.01, Spearman rank correlation coefficient). Conclusions: Degree of inflammatory activity only partially predicted increase in T1 (and T2) lesion load at long-term follow-up. Initial T1 lesion load strongly contributed to subsequent increase in hypointense T1 lesion load, suggesting that there is a subpopulation of patients with multiple sclerosis who are prone to develop destructive lesions