Organic Physical Chemistry Laboratory

Phase Change

Phase change involves the collective movement of bulk molecules. Such bulk molecular re-organisation can therefore be a relatively slow process compared to the time scales that can be monitored with modern lasers. Despite this the dynamics of phase change are not entirely understood in terms of molecular motion and changes in molecular contacts such as H-bonds.

We have attempted to examine the dynamics of phase change using probes of both morphology and molecular interactions. The main trick that we use is to generate a very rapid (8ns) temperature jump (T-jump) in aqueous solutions by Raman shifting 1064nm light from an Nd-YAG laser to 1900nm in a high pressure cell containing 35 atmospheres of hydrogen. 1900nm light can be absorbed by water combination bands vibrationally heating the solution without needing to dissolve an extra species in the system to absorb light.

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In this way we can rapidly induce phase change in a neat water solution.
For example we can boil water in under a microsecond (as shown below ). 1900nm light is absorbed by a liquid jet raising its temperature far above its boiling point and after 25ns we see a dark patch form on the liquid surface. This dark patch swells and grows and eventually explodes into a plume of ejected material re-condensing into droplets by about 20 microseconds after the initiation of the explosive boiling.
These images were taken using the method of shadowgraphy, in which we use a fast strobe light produced by photo-excited fluorescent dyes to capture images of a fast process on a CCD camera. The timing of the strobe is regulated by timing the T-jump laser pulse with a second laser that excites the strobe producing dye solution.

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Phase Separation In Binary Liquid Mixtures.

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Some binary liquid mixtures have a lower critical consolute temperature (an LCST) meaning that they mix at low temperature BUT de-mix at higher temperatures. We have used our T-jump to very rapidly send such mixtures (a)triethylamine/water (J. Phys.Chem. B, (2003) 107, (41), 11411 -11418.) and b) 2-butoxyethanol/water (Phys. Rev. E, rapid commun. (2003), 68, 020501-1-4) from the mixed state to the un-mixed region of the phase diagram and monitored the changes in these systems using shadowgraphy and Raman spectroscopic probes.

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In this way we have shown that molecular level changes finish after about 1-2 microseconds (left) at a time when phase domains are themselves extremely small (~50-100nm). Phase domain growth was shown to be self similar and isotropic forming bi-continuous phases that grew with persistent growth exponents for several hundred microseconds.
This kind of domain growth is characteristic of spinodal phase change.

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From our work it seems that the early and intermediate stages of phase separation are over within 2 microseconds and after this time we simply observe the later stages of spinodal phase change when the domains merge and coarsen to reduce surface energy.