I have always been unsure of the behaviour of mixtures of acetonitrile and water. In some places it is lauded for the usefulness of liquid-liquid partitioning between the two of them while I also see plenty of recrystallizations from homogeneous mixtures of acetonitrile and water.
Recently in an old US Pat. 4,954,260 filed in 1989 I found the linking piece of data. Water and acetonitrile have an upper critical solution temperature (UCST) of -0.4 C. That is to say below -0.4 C they are generally immiscible That is to say the two phases each contain both ingredients but they do give two liquid phases. Above this temperature, they constitute a single homogeneous phase. Thus, if you try a recrystallization by heating a mixture of acetonitrile and water you will be working with a homogeneous liquid and even if you cool the solution in ice there will still be one liquid phase.
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