Put your compound (18-crown-6) in a vaccumdesiccator under vaccum for certain time it will be dry and close it, when you open it use He or N2 gas in a glovebox.Good Luck
You may want to check the purifying strategy that concludes the following protocol, as it implicitly addresses the dehydration of 18-crown-6 (Org. Synth. 1977, 57, 30; DOI: 10.15227/orgsyn.057.0030):
Hello Dr Salim Y Hanna and Dr Carlos Araujo Queiroz . I noticed that the 18-crown-6 was sublimed partially if only operated under high vacuum. I have to proceed to the procedure mentioned in the article suggested by Dr. Carlos Araujo Queiroz.
I will mention the result once I complete the procedure.