Many of us evaporate some of the naphtha in order to improve freeze precipitation.

However, as the naphtha is a mixture of compounds with different boiling points, could it be that lower BP components evaporate sooner and the naphtha becomes "heavier" over time, losing some of its solvent power?

This is probably not a real issue, but more of an academic question. It could mean that the recycled naphtha will eventually behave more like heptane or octane, becoming a more selective solvent.

If anything it seems like you have it backwards. Earlier/lighter boiling fractions should contain the aliphatics & the later/heavier boiling fractions should contain any aromatics, which are much more powerful solvents & dont work well for freeze precipitation because they tend to hold more DMT at freezing temperatures.

I have considered an aliphatic naphtha, without aromatics, like Zippo. At least I assume the Zippo lighter fluid has no aromatics.

Most of the naphtha i find locally lists only light aliphatic or hydrotreated light aliphatic hydrocarbons in the MSDS. I have not yet found any that lists aromatics but have noticed that some brands do just list aliphatic naphtha which i assume is potentially a heavier naphtha than light aliphatic.

It is really hard to say what the exact composition of technical grade naphthas are. Regulations are fairly loose & most msds list ingredients that cover a wide range of hydrocarbons which may differ from batch to batch.

Yes. Since I reuse and recycle (distill) my naphtha many times, I am just pondering whether, after many evaporations, the naphtha will be just say heptane/octane/decane... containing no lighter aliphatics, possibly losing some of its solvent power.