Page 162 - Multidimensional Chromatography

154 Multidimensional Chromatography

our experiences of living our entire lives at ambient pressure. However, in actuality,
there is a continuum of fluid behavior linking the normal liquid and gas states, shown
by the shaded area in the figure. In order to avoid discontinuities, we simply need to
place the phase transitions off limits.
By traveling within the continuum and avoiding the boiling line, we can convert
the ordinary liquid to its gaseous form without undergoing a phase change. One path
accomplishing this is path b in the figure. Of course, an unlimited number of such
paths is possible. The only requirement for continuous change is that we never cross
the boiling line. Instead, we will go around the critical point, the point at which liq-
uid and vapor states merge into a single fluid with shared properties.
Now, we should ask ourselves about the properties of water in this continuum of
behavior mapped with temperature and pressure coordinates. First, let us look at
temperature influence. The viscosity of the liquid water and its dielectric constant
both drop when the temperature is raised (19). The balance between hydrogen bond-
ing and other interactions changes. The diffusion rates increase with temperature.
These dependencies on temperature provide us with an opportunity to tune the solva-
tion properties of the liquid and change the relative solubilities of dissolved solutes
without invoking a chemical composition change on the water.
If the property changes that occur when we raise the temperature are useful to us,
and if we would like to continue moving continuously in the direction of these
improvements, we are not limited by the normal boiling point. We simply need to
apply enough pressure to prevent boiling, and then continue going on with our tem-
perature exploration. Pressure does have a measurable effect on the solvent properties
of liquids and on the relative retention of solutes in LC (20), but this effect is small in
liquids at ordinary pressures and is usually ignored. However, the compressibility of
the liquid and the effect of pressure both increase with increasing temperature.
Thus, there is not necessarily a boundary at the normal boiling point when we
control the pressure. Why would we not want to take full advantage of the full range
of properties of water, or of any other solvent, whenever advantages discovered away
from ambient conditions improve our ability to separate solutes of interest?
Of course, LC is not often carried out with neat mobile-phase fluids. As we blend
solvents we must pay attention to the phase behavior of the mixtures we produce.
This adds complexity to the picture, but the same basic concepts still hold: we need
to define the region in the phase diagram where we have continuous behavior and
only one fluid state. For a two-component mixture, the complete phase diagram
requires three dimensions, as shown in Figure 7.2. This figure represents a Type I
mixture, meaning the two components are miscible as liquids. There are numerous
other mixture types (21), many with miscibility gaps between the components, but
for our purposes the Type I mixture is sufficient.
The shaded region is that part of the phase diagram where liquid and vapor phases
coexist in equilibrium, somewhat in analogy to the boiling line for a pure fluid. The
ordinary liquid state exists on the high-pressure, low-temperature side of the two-
phase region, and the ordinary gas state exists on the other side at low pressure and
high temperature. As with our earlier example, we can transform any Type I mixture