Introduction

Carbon dioxide (CO2), when heated and pressurized above its critical temperature (31 °C) and pressure (7.377 MPa), is identified as being in a fourth, supercritical state with properties intermediate between liquid and gas (Table 1). At the critical point, the interface of CO2 liquid and vapor starts to vanish (Fig. 1). Supercritical CO2 (scCO2) has liquid-like densities providing good solvent capability, gas-like viscosities, and diffusivities to benefit mass transport, and a non-hazardous nature for the environment. Because of these properties and other advantages of CO2 over organic solvents such as low surface tension and low cost,[1] scCO2 has received increasing industrial[2] and research[1] attention in a variety of processes such as extraction,[3'4] cleaning,[5'6] polymer synthesis,[7'8] and more recently microelectronics processing.[9'10] CO2 has been proposed to serve as an energy-efficient and environmentally benign solvent platform.[11] The real-time monitoring of the time-dependent mass change is crucial to understanding, characterizing, designing, and controlling the aforementioned processes. However, this monitoring presents a number of challenges because of the difficulty in applying various detection methods under high-pressure conditions.

In this entry, two microweighing techniques in scCO2, gravimetric and piezoelectric, are reviewed. A comparison is made between two representatives of these techniques: the magnetic suspension balance (gravimetric) and the quartz crystal microbalance (QCM, piezoelectric). The QCM theory in high-pressure fluids is briefly introduced, followed by a summary of recent research which includes 1) the experimental verification of QCM theory; 2) the scCO2 adsorption on metal surfaces; and 3) the dissolution of polymer films for applications in scCO2-based lithography processes. Finally, the application of QCM in absorption, solubility, and other surface-specific processes in scCO2 will be reviewed to highlight the versatility of the QCM technique.

MICROWEIGHING TECHNIQUES: GRAVIMETRIC AND PIEZOELECTRIC

Many important physical and chemical processes can be monitored by observing the associated mass changes. Currently, two microweighing techniques, gravimetric and piezoelectric, have been widely used as an analytical tool for research and applied applications. Gravimetric microbalances are the most direct technique to measure mass variations at the microgram level. However, in a number of cases, most conventional gravimetric microbalances such as spring, beam, and torsional balances are not designed for the operation under extreme conditions. The extreme conditions are usually defined as conditions of high pressures (>1450 psi), viscous fluids, and/or high temperatures. In order to adapt the conventional gravimetric balances to work under extreme pressures, some researchers[12,13] have designed a high-pressure container to include both the microbalance and the sample. This not only greatly increases the system expenses, structural complexity, and operational difficulties, but also sacrifices the capability under high temperatures (>125°C) because of the microbalance working conditions. Jwayyed, Humayun, and Tomaska[13] modified a commercial high-pressure microbalance (Cahn C-1000 balance) by redesigning the container (nipple shape with 30 cm o.d. and 25 cm height) for use in scCO2. They were only able to extend the pressure limit from 1600 to 3000 psi.

In recent years, some researchers[14,15] have been able to design a unique balance, a magnetic suspension balance (MSB), for mass measurements under extreme conditions (>3000 psi). The MSB takes advantage of the principle of magnetic coupling to isolate the balance from the sample that is contained in a small pressure cell (Fig. 2), while maintaining most features of the traditional microbalances. Using this magnetic suspension coupling, the measured force is transmitted from the pressure cell to the microbalance that is located in the ambient environment. Such novel design eliminates the large container, enables the capability of mass determinations under extreme conditions, and

Table 1 Physical properties of gas, liquid, and supercritical fluid (order of magnitude)
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