Supercritical fluid deposition

Crystallization by reaction to form metals, semiconductors (e.g.. Si), and metal oxides including nanocrystals Supercritical fluid deposition... 

Bleidiessel R et al (2008) Supercritical fluid deposition of metals for micro electromechanical systems. In Proceedings of 11th European meeting on supercritical fluids, Barcelona, 4-7 May, Ppl-6... 

Medical and surgical devices (such as stents) can be coated with compositions containing supercritical fluids. Thin polymer film, containing a therapeutic agent uses a supercritical fluid deposition process. ... 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

The solvent elimination appro2K h is quite straightforward for supercritical fluids lAich are often gases at atmospheric pressure. Each chromatographic peedc is deposited fron the end of a restrictor, connected to the end of the column by a heated transfer line, onto a small area of infrared-transparent support [110,128,129,134]. The support can be moved manually to collect each peak at a n osition or stetq>ed continuously to record the... 

Figure 7.16 Schematic representation of off-line SFC-FTIR. After deposition of the eluites on to a moving ZnSe substrate the window is moved to the focus of a stand-alone FTIR microscope, where the spectmm of each spot is measured with the plate stationary. After Griffiths et al. [374]. Reprinted from P.R. Griffiths et al., in Hyphenated Techniques in Supercritical Fluid Chromatography and Extraction (K. Jinno, ed.), pp. 83-101, Copyright (1992), with permission from Elsevier...

SFC-TLC is largely unexplored. Stahl [927] developed a device for supercritical fluid extraction with deposition of the fluid extracts on a moving TLC plate. Wunsche et al. [928] have described an automated apparatus for direct pSFC-TLC coupling. Compared to collecting the effluent from the SFC in decompression vessels, the direct deposition of the effluent on the TLC plate leads to significant losses of analytes. Multidimensional SFC has been reviewed [929]. 

Adhesives. Supercritical fluids might also be used to deposit adhesive films without the use of solvents. They have even been suggested for ungluing at the time of final disposal/recycling of the bonded product. 

Microemulsions. Systems comprising microwater droplets suspended in an scCO T oil phase can be achieved with the use of appropriate surfactants, of which the best appear to be fluorinated. Microemulsions in supercritical hydrofluoro carbons are also possible. Potential may also exist for speciality coatings via low concentration solutions of fluorinated products in supercritical fluid for, e.g., thin-fitm deposition, conformal coatings, and release coatings. Supercritical CO2 will dissolve in formulated systems to improve flow and plasticize melt-processable materials to improve melt-flow characteristics and lower the glass transition temperature. 

The interest in mass transfer in high-pressure systems is related to the extraction of a valuable solute with a compressed gas. This is either a volatile liquid or solid deposited within a porous matrix. The compressed fluid is usually a high-pressure gas, often a supercritical fluid, that is, a gas above its critical state. In this condition the gas density approaches a liquid—like value, so the solubility of the solute in the fluid can be substantially enhanced over its value at low pressure. The retention mechanism of the solute in the solid matrix is only physical (that is, unbound, as with the free moisture), or strongly bound to the solid by some kind of link (as with the so-called bound moisture). Crushed vegetable seeds, for example, have a fraction of free, unbound oil that is readily extracted by the gas, while the rest of the oil is strongly bound to cell walls and structures. This bound solute requires a larger effort to be transferred to the solvent phase. 

This method has a number of positive features it may be applied to most supercritical fluids with critical temperatures close to ambient deposition of the solid product occurs in a controlled manner, if necessary under an inert atmosphere and the high pressure "stabilizing" conditions are maintained right up to the point of precipitation. The precipitated solid product may then be analysed and characterised by other off-line spectroscopic techniques. In our example, the 13C-NMR spectrum of the solid material, redissolved in d8-toluene, shows the same resonances as those observed with a genuine sample of Cr(CO)4(C2H4)2. 

K.L. Norton, A.M. Haefner, H. Makishima, G. Jalsovsky and P.R. Griffiths, Comparison of direct-deposition supercritical fluid and gas chro-matography/Fourier transform infrared spectra to condensed-phase library spectra, Appl. Spectrosc., 50, 1125-1133 (1996). 

Three different zeolites (USY-zeolite, H-ZSM-5 and H-mordenite) were investigated in a computer controlled experimental equipment under supercritical conditions using the disproportionation of ethylbenzene as test reaction and butane or pentane as an inert gas. Experiments were carried out at a pressure of 50 bar, a flow rate of 450 ml/min (at standard temperature and pressure), a range of temperatures (573 - 673 K) and 0.8 as molar fraction of ethylbenzene (EB) in the feed. The results showed that an extraction of coke deposited on the catalysts strongly depends on the physico-chemical properties of the catalysts. Coke deposited on Lewis centres can be more easily dissolved by supercritical fluid than that on Brnsted centres. 

Figure 7 shows the relative areas of different acid centres on ZSM-5 in dependences on temperature and pressure. It can be found that the acid centres were less reduced at 623 K under supercritical conditions than those at normal pressure. Especially on Lewis centres the coking tendency is weak. This implies that the coke deposited on Lewis centres may be loosely built and can be easily removed by supercritical fluid. At 673 K the acid centres of ZSM-5 disappeared almost totally. This indicates that coking tendency increases more quickly with increasing temperature than the ability of coke extraction. 

The first experiments reported here lead us to think that the impregnation of porous supports by drugs can be achieved by means of supercritical fluids. This one-step method yields a final product exempt from any residual trace of toxic solvent. The kinetics of the mass transfer is faster, besides the thermodynamics of the adsorption seems more favourable here. The main problem encountered up to now is the weak solubility of many active molecules in pure C02, which induces a limitation of the percentage of deposited product. However, this difficulty can be overcome by the use of few amount of an entrainer. In particular, ethanol which does not show any toxicity, would greatly extend the range of active substances which could be used. 

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