Page 31 - Instant notes

Liquids 17

In the absence of other forces, the free energy of a liquid is
minimized when it adopts the minimum surface area possible.
The free energy change in a surface of area A, depends on the
surface tension of the liquid, γ, and is given by dG=γ dA. For a
gas cavity of radius r within a liquid, the pressure difference
between the inside and the outside of the cavity is given by
∆p=p gas −p liquid =2γ/r.

For bubbles, the presence of two surfaces doubles the pressure
differential between the inside and outside of the bubble for a
given radius: ∆p=p inside −p outside =4γ/r.
Surfactants are chemical species with a tendency to accumulate at
surfaces, and tend to lower the surface tension of a liquid. Most
surfactants are composed of a hydrophilic head and a
hydrophobic tail. Assuming the solvent to be water or another
polar solvent, the conflicting requirements of the two groups are
met at the surface, with the head remaining in the solvent and the
tail pointing out of the solvent. Above a critical concentration and
above the Krafft temperature, surfactant molecules may not only
accumulate at the surface, but may also form micelles. Micelles
are clusters of between some tens and some thousands of
surfactant molecules whose tails cluster within the micelle so as
to maximize interactions between the tails, leaving a surface of
solvated hydrophilic heads.
Materials in a superficially liquid state which retain most of their
short-range order, and some of their long-range order are no
longer solid nor are they truly liquid, and are termed liquid
crystals. Liquid crystals tend to be formed from molecules which
are highly anisotropic, with rod, disk, or other similar shapes. In
the smectic phase, molecules are aligned parallel to one another
in regular layers. In the nematic phase, the molecules are aligned
parallel to one another, but are no longer arranged in layers, and
in the cholosteric phase ordered layers of molecules are aligned
with respect to one another within each layer, but the layers are
no longer ordered with respect to one another.
Related topics Molecular behavior in perfect Molecular aspects of ionic
gases (A2) motion (E7)

Structure of liquids

The structure of a liquid is intermediate between that of a solid (see Topic A5) and a gas
(see Topic A3). The molecules in a liquid have sufficient energy to allow relative motion
of its constituent molecules, but insufficient to enable the truly random motion of a gas.
Liquids have a limited degree of short-range order, but virtually no long-range order, and
in contrast to a solid, a liquid cannot be adequately described in terms of atomic