© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.3.3 Pressure-Altered Physical Properties

It is only possible here to briefly mention the many pressure-mediated changes of physical properties that may be relevant in nanomedical systems. Perhaps the most familiar is phase change, as illustrated by Figure 10.11 in the case of water.^2039,2040 Note that as pressure rises from 1 atm to 2054 atm, the temperature at which water remains liquid dips from 273.2 K to 251.2 K (the Ice I/III/water triple point,²⁹⁶²), but then the liquidity threshold returns to higher temperatures as pressure rises further and a different ice phase forms; D₂O shows similar behavior.²⁹⁶⁰ At a constant 310 K (37°C), isothermal static compression of pure liquid water causes crystallization from the liquid to solid Ice VI near ~11,500 atm. Unlike water, most materials contract upon freezing, thus freezing point is normally raised by higher pressure. For example, the melting point of paraffin wax is 319.8 K at 1 atm but rises to 323.1 K at 100 atm.¹⁶⁹⁷

At 273 K, gaseous carbon dioxide compressed to 35 atm suddenly liquefies; at 300 K, pressurized CO₂ remains liquid up to ~4500 atm, whereupon it solidifies.²⁰³⁵ However, T_crit = 304.2 K for CO₂, so if the gas is stored in pressure vessels at 310 K (human body temperature) > T_crit, the physical properties change continuously, showing no sign that the gas has condensed to a liquid, although over ~200 atm the fluid behaves like carbon dioxide liquid.²⁰³⁶ CO₂ compressed to 400,000 atm at 1800 K forms a translucent quartzlike extended covalent solid that is metastable down to ~room temperature at pressures >10,000 atm.³¹⁸¹ Other gases of nanomedical interest such as oxygen (T_crit = 154.4 K), nitrogen (T_crit = 126.1 K), hydrogen (T_crit = 33.3 K), and helium (T_crit = 5.3 K) show a similar continuous progression of gas/liquid properties at 310 K with rising pressure. Helium solidifies to ~1000 kg/m³ crystals at 115,000 atm at 297 K.²⁰³⁷ Room temperature isopropyl alcohol solidifies into a glassy state at ~44,000 atm, while a 4:1 methanol/ethanol mixture remains hydrostatic up to 104,000 atm.²⁰⁵¹ Hydrogen solidifies at 57,000 atm at 298 K, making 600 kg/m³ crystals;⁵⁶⁸ solid deuterium at 300 K compacts from ~620 kg/m³ at 100,000 atm to ~1400 kg/m³ at 1 million atm.²⁰³⁸

Vessel wall materials also are subject to pressure changes. Diamond crystal plastically deforms at a pressure of only ~1 atm at ~1773 K, but at ~1300 K, ductile flow in diamond requires pressures >60,000 atm.²⁰⁴¹ At room temperature the diamond {111} surface compressed in the {110} direction is indented by pressures >~880,000 atm; compression of the {001} surface requires 560,000-1 million atm for indentation, depending upon the direction of the applied load.¹⁵⁹⁷ Colorless diamond takes on a light brown coloration between 1.0-1.7 million atm, a probable crystallographic phase transition.²⁰⁴³ The equilibrium phase diagram for the carbon system suggests that graphite could begin to be converted to diamond at room temperature as low as 20,000 atm, although pressures of 50,000-65,000 atm at 1600-2000 K are employed for commercial diamond production in combination with Group VIII metal catalysts,^537,2042 and experiments show that graphite transforms to a transparent (but nondiamond) phase at ~180,000 atm at room temperature.²⁸⁵⁶ Fullerenes are also susceptible to allotropic change at high pressure -- nonhydrostatic compression of C₆₀ molecules induces a transformation to diamond above ~150,000 atm,²⁰⁴⁴ and bulk C₆₀ converts into two different metastable structures (e.g., face-centered cubic at 600-700 K and rhombohedral at 800-1100 K) at 50,000 atm.²⁰⁴⁵ Computer simulations suggest that an "armchair" carbon nanotube can tolerate ~10⁶ atm for ~10 years before failing.²⁹⁵⁷ As for sapphire and ruby (primarily corundum or Al₂O₃), no structural transformations were observed in an X-ray diffraction study of ruby compressed to 1.75 million atm at room temperature,²⁰⁴⁶ a similar study found a polymorphic phase transformation at pressures as low as 850,000 atm when the ruby was heated above ~1000 K.²⁰⁴⁷ Finally, while negative volume compressibility is thermodynamically forbidden, lanthanum niobate crystals and several other materials display negative linear or areal compressibilities -- that is, they expand in one or two directions (while maintaining constant volume) when hydrostatic pressure is applied.¹²⁹⁷

The solubility of gases in liquids strongly increases with pressure (Section 9.2.6), but the effect of pressure on solubility of crystalline or liquid substances in liquids is usually very small and may be predicted by Le Chatelier's principle since it depends on the relative volumes of the solution and the component substances.²⁰⁴⁸ For example, raising the pressure from 1 atm to 1000 atm only increases the 298 K aqueous solubility of sodium chloride from 359 gm/liter to 370 gm/liter.²⁰³⁶ Room temperature liquid carbon dioxide at ~200 atm pressure (~753 kg/m³ at 310 K) has been used for decades as a nontoxic solvent in chemical processing, such as the process for extraction of caffeine from coffee pioneered by General Foods; a microemulsion of aqueous micelles in liquid carbon dioxide extends solvation to proteins which are normally insoluble in pure CO₂.²⁰⁴⁹

There are many other effects of elevated pressures. For example, the speed of sound generally increases with higher pressure (e.g., by ~0.25 (m/sec)/atm in water near 1 atm; Section 4.5.1). As another example, very highly compressed materials may enter a metallic (electrically conducting) state (e.g., room temperature cesium iodide becomes metallic at ~1.15 million atm²⁰⁵²).

There are also significant effects of elevated pressures on biology.³²¹⁸ The summary in Table 10.3 suggests that the largest biological cells are harmed by pressures under 1000 atm; small cells and viruses may be inactivated at a few thousand atm; antigens, toxins, enzymes, and other proteins are deactivated or denatured near ~10,000 atm of pressure.⁵⁸⁵ (See also Section 10.4.2.3.) Microbial death at >2000 atm is considered to be due to changes in the permeability of cell membranes.³¹⁰⁶ Protein denaturation occurs at higher pressures because noncovalent bonds are destroyed or formed as system volume declines.³¹⁰⁷ Proteins, carbohydrates and nucleic acids, whose tertiary structures are composed of noncovalent bonds, change their structures at pressures between 2000-10,000 atm, leading to denaturation, coagulation, or gelatinization; covalent bonds in pure substances usually do not undergo changes at these pressures.³¹⁰⁶ Irreversible effects on biological materials are generally observed at pressures >1000-2000 atm.³¹⁰⁶ Most of the challenges presented by very high pressures as described in this Section can be avoided by conservatively restricting nanomedical storage vessels to a maximum operating pressure of ~1000 atm.

Last updated on 24 February 2003