Structure of a classical monatomic liquid. Atoms have many nearest neighbors in contact, yet no long-range order is present.
Liquid is one of the three classical states of matter. Like a gas, a liquid is able to flow and take the shape of a container, but, like a solid, it resists compression. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena.
The density of a liquid is usually close to that of a solid, and much higher than in a gas. Therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, they are both called fluids.
Types of liquids
Only two elements are liquid at room temperature and pressure: mercury and bromine. Four more elements have melting points slightly above room temperature: francium, caesium, gallium and rubidium.
Pure substances that are liquid under normal conditions include water, ethanol and many other organic solvents. Liquid water is of primordial importance in chemistry and biology; it is believed to be a necessity for the existence of life.
Important everyday liquids include aquous solutions like household bleach, other solutions (homogeneous mixtures, multiphasic liquids) like mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like milk and blood, and colloids like paint.
Liquid crystals, used in LCD displays, cannot be classified within the classical three states of matter; they possess solid-like and liquid-like properties. The same holds for biological membranes.
Properties
Main article:
Fluid dynamicsQuantities of liquids are commonly measured in units of volume. These include the SI unit cubic metre (m³) and its divisions, in particular the cubic decimetre, more commonly called litre (dm³=l), and the cubic centimetre, also called millilitre (cm³=ccm=ml).
The volume of a quantity of liquid is fixed by its temperature and pressure. Unless this volume exactly matches the volume of the container, (one or more) surfaces are observed. Liquids in a gravitational field, like all fluids, exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth.
Liquids have little compressibility: water, for example, does not change its density appreciably unless subjected to pressures on the order of 100 bars. In the study of fluid dynamics, liquids are often treated as incompressible, especially when studying incompressible flow.
If a liquid is at rest in a uniform gravitational field, the pressure 
at any point is given by
where:
= the density of the liquid (assumed constant)
= gravity
= the depth of the point below the surface.
Note that this formula assumes that the pressure at the free surface is zero, and that surface tension effects may be neglected.
Objects immersed in liquids are subject to the phenomenon of buoyancy, which is also observed in other fluids, but is especially strong in liquids due to their high density.
The surface of a liquid behaves like an elastic membrane in which surface tension appears, allowing the formation of drops and bubbles. Capillarity, wetting, and ripples are another consequence of surface tension.
Viscosity measures the resistance of a liquid which is being deformed by either shear stress or extensional stress.
Phase equilibria
At a temperature below the boiling point, a liquid will evaporate until, if in a closed container, the concentration of the vapors belonging to the liquid reach an equilibrium partial pressure in the gas. Therefore no liquid can exist permanently in a complete vacuum. Liquids at their respective boiling point change to gases (except when superheating occurs), and at their freezing points, change to solids (except when supercooling occurs). Even below the boiling point liquid evaporates on the surface.
Liquids can display immiscibility. The most familiar mixture of two immiscible liquids in everyday life is the vegetable oil and water in Italian salad dressing. A familiar set of miscible liquids is water and alcohol. Liquid components in a mixture can often be separated from one another via fractional distillation.
Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception.
Structure of liquids
Unlike crystalline solids, liquids exhibit a significant degree of atomic and/or molecular mobility. Strong forces of interaction (both repulsive and attractive) compete to bind the atoms of any solid object together firmly, while the bonds of the corresponding liquid will remain temporary in nature. This is what distinguishes the mechanical properties (e.g. rigidity and shear strength) in condensed matter between the liquid and solid state.
In order to visualize the arrangement of atoms or molecules in a typical liquid, it is helpful to recognize the relationship between the long-range order present in the crystalline solid state and the short-range order present in a typical liquid, both exhibiting similar densities. The molecular mobility of liquids is what makes them deformable and subject to flow on various spatial scales, while a solid remains rigid.
Radial distribution
A description of the radial distribution of atoms or molecules about a central point is a generalized way of describing the average positions of atoms or molecules within a liquid. The radial distribution function g(r) is a pairwise correlation function, which describes how -- on average -- the particles in a system are arranged (or 'packed') around each other. This is an excellent way of describing the average structure of disordered molecular systems such as liquids and glasses. In systems like liquids, where the particles are in constant motion, a single snapshot of the system is limited to a description of the instantaneous microstructure (or spatial correlations). In this context, it becomes very useful in the description of the macroscopic (or average) structure over a longer time interval (or temporal scale). [1]
The radial distribution function g(r) can be deduced experimentally from x-ray or neutron diffraction studies, and provides a direct comparison between laboratory experiment and computer simulation. It can also be used in conjunction with the interatomic pair potential function in order to calculate such macrospopic thermodynamic parameters as the internal energy, Gibbs free energy, entropy and enthalpy of the disordered system. g(r) is determined by a relatively simple calculation of the average number of particles found within a given volume of shell located at a distance r from the center. The average density of atoms at a given radial distance from the center is given by the formula:
g( r ) = n( r ) / ( ρ 4π r 2Δr )
where n(r) is the mean number of atoms in a shell of width Δr at distance r, and ρ is the mean atom density. Thus, g(r) is plotted as a function of the interatomic separation r. A typical plot shows a number of important features.
1) At short separations (small r), g(r) = 0. This indicates the effective width of the atoms, which ultimately limits their distance of approach.
2) A number of obvious peaks appear, at increasingly reduced intensities. The peaks indicate that the atoms pack around each other in 'shells' of nearest neighbors. At very long range, g(r) approaches a limiting value of 1 (or unity), which describes the average density at this range.
3) The attenuation of the peaks at increasing radial distances from the center indicates the decreasing degree of order from the center particle. This illustrates vividly the origin of the term "short-range order" in classical liquids and glasses.
Experimental verification of the radial distribution in simple liquids has been obtained by methods relying on the scattering of X-rays, where constructive interference is limited to peaks found within a limited radial distance r. The result is the characteristic periodic arrangement of light and dark bands of local intensity maxima and minima, analogous to those seen from the scattering off crystals. Thus, peaks of decreasing amplitude in the radial distribution function appear only where the conditions for the constructive interference of X-rays are satisfied.[2]
Hidden structure
A number of authors have identified a static "hidden structure" and explored the dynamics of structural transitions in liquids. Utilizing molecular dynamics methods, they have separated the study of the liquid state into two parts:
1) Mechanically stable packings of molecules via potential minima;
2) Vibrational motion (generally anharmonic) about those mechanically stable points.
All configurations are "quenched" by a steepest-descent construction into a nearby potential minimum. The systems exhibit a "defect softening" phenomenon, or mean attraction between defects, which influences the spectrum of normal mode vibrational frequencies at the local potential minima for liquids that solidify into body centered cubic crystals. Attempts to reconstitute the equilibrium pair correlations functions by thermally broadening the quenched versions, using Einstein or Debye approximations, were clear failures. Evidently, the true phenomena in such systems entail substantial anharmonicity. [3]
The presence of "hidden structure" in supercooled liquids has been supported by the electron microscopic studies, indicating a well-defined "micellar" structure of glass which is interpreted as being the result of a superlattice of paracrystalline domains. The geometrical disorder of glass is therefore only exhibited at length scales above 10 nanometers (approximately the size of the elementary domain). Various degrees of interdomain ordering can therefore be realized. [4]
This conceptualization of the paracrystalline nature of glass was further substantiated by authors who observed domains as large as 100 nanometers in thin films of chalcogenide alloys. These observations led Phillips to interpret glass formation as the aggregation of molecular clusters upon a reduction of the amplitude of thermal motion, yielding a nearly polycrystalline microstructure. [5]
References
- ^ McQuarrie, D.A., Statistical Mechanics (Harper Collins, 1976)
- ^ Berry, R.S. and Rice, S.A., Physical Chemistry, App.23A: X-Ray Scattering in Liquids: Determination of the Structure of a Liquid (Oxford University Press, 2000)
- ^ Stillinger, F.H. and Weber, T.A., Phase transitions in the Gaussian core system, J. Chem. Phys., Vol. 65, p. 3968 (1976) Hidden structure in liquids, Phys. Rev. A, Vol. 25, p. 978 (1982); Inherent structures and distribution functions for liquids that freeze into bcc crystals, J. Chem. Phys., Vol.81, p. 5089 (1984); Point defects in bcc crystals: Structures, transition kinetics, and melting implications, J. Chem. Phys., Vol.81, p. 5095 (1984); Packing Structures and Transitions in Liquids and Solids, Science, Vol. 225, p. 983 (1984)
- ^ J. Zarzycki, R. Mezard (1962). "A direct electron microscope study of the structure of glass". Physics and Chemistry of Glasses 3: 163.
- ^ C.H. Chen et al. (1981). "Domain microscopy in chalcogenide alloy glass thin films". Solid State Communications 38: 657. doi:10.1016/0038-1098(81)90962-5.
