ABSTRACT:The rapid expansion from a supercritical solution with a nonsolvent (RESS-N) was applied to the formation of polymeric microcapsules containing medicines such as p-acetamidophenol, acetylsalicylic acid, 1,3-dimethylxanthine, flavone, and 3-hydroxyflavone. A suspension of medicine in carbon dioxide (CO 2 ) containing a cosolvent and dissolved polymer was sprayed through a nozzle to atmospheric pressure. The pre-expansion pressure was 10 -25 MPa, and the temperature was 308 -333 K. The polymers were poly(l-lactic acid) (molecular weight ϭ 5000), poly(ethylene glycol) (PEG; PEG4000, molecular weight ϭ 3000; PEG6000, molecular weight ϭ 7500; and PEG20000, molecular weight ϭ 20,000), poly(methyl methacrylate) (molecular weight ϭ 15,000), ethyl cellulose (molecular weight ϭ 5000), and PEG-poly(propylene glycol)-PEG triblock copolymer (molecular weight ϭ 13,000). The solubilities of the polymers as coating materials and these medicines as core substance were very low in CO 2 . However, the solubilities of these polymers in CO 2 significantly increased with the addition of low molecular weight alcohols as cosolvents. After RESS-N, polymeric microcapsules were formed according to the precipitation of the polymer caused by a decrease in the solvent power of CO 2 . This method offered three advantages: (1) enough of the coating polymers, which were insoluble in pure CO 2 , dissolved; (2) the microparticles of the medicine were encapsulated without adhesion between the particles because a nonsolvent was used as a cosolvent and the cosolvent remaining in the mixture was removed by the gasification of CO 2 ; and (3) the polymer-coating thickness was controlled with changes in the feed composition of the polymer for drug delivery.
Using scanning tunneling microscopy (STM),
the behavior of Mg submonolayers on a Si(100)2×1 surface
has been studied during deposition at room temperature (RT) and upon
annealing at 250°C and 400°C. RT-deposited Mg forms
meandering chains of features that run roughly perpendicular to the
substrate Si dimer rows and, at saturation, tend to form the arrays of
the 2×2 reconstruction.
Annealing at 250°C transforms the chains to
random groups of Mg clusters. Subsequent annealing at 400°C
induces Si redistribution at the surface and results in the
formation of straight chains of features that are again aligned perpendicular
to the Si dimer rows. These high-temperature (HT) features are plausibly composed
of 1 Si atom and 1–2 Mg atoms. The spacing of the HT features within
the chain is 2a (a=3.84 Å) and stacking of the chains produces
the domains of 2×2, 2×3 and other 2×n reconstructions.
At saturation, almost the entire surface is occupied by the 2×2
reconstruction. At higher Mg coverages, the growth of a silicide occurs
both at RT deposition and upon annealing.
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