# Lithium compounds with good solubility in organic solvents?

I want to dissolve enriched $\ce{^6Li}$ into a liquid scintillator (made almost purely of di-isopropyl-naphthalene, mixed isomers, with a tiny bit of fluors and wavelength shifters) for enhanced neutron detection and background rejection. My group's previous attempts at this have been to use Lithium-6 Chloride with surfactants, but this negatively affects the attenuation length of the scintillation light passing through the scintillator (since the droplets of the microemulsion are essentially giant scattering surfaces).

Having had some O-Chem and P-Chem as an undergrad I know some organolithium compounds are soluble in organic solvents, but those are eat the container walls nasty. I know that lithium bromide has some limited solubility in organic solvents, but I need to get to at least 0.1%, preferably 0.5% atom doping/fraction, so I need something that dissolves well. I know that lithium perchlorate is soluble in many organic solvents, but the explosive nature of the mixtures it makes with those solvents kills that idea.

Are there any lithium compounds that are not caustic, nor explosive, but are soluble in greasy organic liquids (like di-isopropyl-naphthalene)?

• LSC cocktails for the detection of neutrons usually contain boron-10 instead of lithium-6. Did you consider using such a cocktail? – Loong Dec 14 '15 at 17:18
• We are looking at that as well, and 10B is nice because of the much larger neutron capture cross-sections and the greater ease of finding compounds that will dissolve. However, 10B, when it breaks up into a 7Li and 4He on capture, releases much less light than 6Li breaking up into a 3H and 4He. Therefore it is harder to separate the capture light from background. 10B is being examined, but since this is for a neutrino measurement, we need to make sure that we can separate the signal from noise well to give better precision. If you have a 10B compound suggestion it would be welcome. – James Matta Dec 14 '15 at 18:25

I would suggest lithium iodide as it is very soluble in many polar organic solvents or a lithium alkyl carboxylate like lithium stearate if a non-polar solvent is required. Cyclopentadienyllithium would also be a good option for your specific solvent though more expensive.

• How reactive is Cyclopentadienyllithium? Would it attack acryllic (what the container walls are made from), 2,5-Diphenyloxazole (fluor), or 1,4-bis(5-phenyloxazol-2-yl) benzene (wavelength shifter)? – James Matta Dec 14 '15 at 18:09
• I do not forsee any reaction of cyclopentadienyllithium with the aforementioned materials. – A.K. Dec 14 '15 at 18:12
• Also, the lithium stearate might be interesting, though with that atom fractions above 1.7% are impossible, but it is worth a look. Assuming it is not so large that when dissolved it acts as a scattering center. – James Matta Dec 14 '15 at 18:16
• Lithium Benzoate may be a viable option – A.K. Dec 14 '15 at 18:17
• Sodium and lithium Cp (cyclopentadienide) salts are very sensitive to oxygen, so I think that LiCp is a poor choice. Also I worry about it reacting with the chemicals in the LSC cocktail. – Nuclear Chemist Apr 18 '18 at 18:22

Under inert conditions you may try dissolving pure metallic lithium. Lithium forms naphtalenide salt , I don't see why di-isopropyl-naphthalene should be any different.

Another option would be use of 12-crawn-4, which make a lot of salts lipo-soluble. Presence of TMEDA (tetramethylethilendiamine) should affect solubility of many lithium salts as well, though I would focus on salts like benzoate or perchlorate.

• How reactive is that naphtalenide salt? Also, I had read that lithium perchlorate is can form explosive mixtures with organic solvents, which would be a big problem in our lab. – James Matta Dec 14 '15 at 17:24
• It does react with air and water. However, I beleave, it may be prepared in a sealed vial by careful heating of lithium and a freshly distilled DIPN (beware, some gases may develop). It does not explode, but may self-ignite on air. It is a very strong reducing agent| Yes, it can. But it is a lithium salt with very weak crystal lattice. Though I think, nitrate should work just as well. But none of Li salts are likely to dissolve in DIPN without a helper , like TMEDA or DME – permeakra Dec 14 '15 at 17:33
• The napthelenide salt seems a bit too scary for this (we have certain conditions on what is in the detector cell because of where we want to put it) same for the perchlorate. However, the crown ether for solubility might be attractive, and the TMEDA may also be worth a look. Thanks for your suggestions. – James Matta Dec 14 '15 at 18:11
• Could you elaborate about temperature conditions? – permeakra Dec 14 '15 at 18:18
• The LS will sit at room temperature. So anything in it needs to be stable at that temperature, including not attacking the fluors (2,5-Diphenyloxazole), wavelength shifter (1,4-bis(5-phenyloxazol-2-yl) benzene) or walls of the container (acryllic). – James Matta Dec 14 '15 at 18:28