Is there a commercial liquid with refractive index greater than two?

Question

I need a commercially available liquid with the following properties:

• refractive index $n > 2$ at room temperature $(T \approx\pu{23 °C});$
• transparent in visible region $(380$ to $\pu{700 nm});$
• suitable for gap fillers in refractometers (gap between prism and stones);
• not poisonous.

What is the chemical composition of such liquid? The RI liquids with related properties I found online usually only have $n = 1.81.$

Answer 1

Taking the definition of refractive index $n = c/v$ into account, $n > 2$ means the velocity of light in the material $v$ is less than half than in vacuum $c,$ so there must be a strong interaction with the material.

One of the highest refractory index in liquids is $1.79~(\pu{20 °C})$ possessed by a solution of sulfur in methylene iodide $(\ce{CH2I2}).$ Liquids are less dense than solids and this explains why it's difficult to have a refractory index greater than 2. According to Anderson and Payne [1]:

Selenium monobromide, $\ce{Se2Br2}$, has a higher refractive index than that of any pure liquid hitherto recorded. Prepared by direct combination, the value for $\nu$ is $1.96±0.01$ rising to $2.02$ on exposure to the atmosphere, owing to decomposition of the bromide, with separation and reabsorption of selenium.

From a more recent paper [2], selenium monobromide is confirmed as one of the liquid with the highest refraction index $(n > 2).$ However, diiodomethane is more easily accessible commercially.

A promising candidate solvent is the organic liquid diiodomethane $(\ce{CH2I2}),$ which is one of the liquids with the highest known refractive index values $(n = 1.74).$ While other high refractive index liquids exist (phenyldiiodoarsine $(\ce{C6H5AsI2})$ with $n = 1.85$ and selenium monobromide $(\ce{Se2Br2})$ with $n = 2.1),$ diiodomethane has the key advantage of being commercially available. In addition, diiodomethane is an excellent solvent, and many liquid formulations using salts dissolved in diiodomethane are reported to increase the refractive index and are even available commercially.

References

1. Anderson, B. W.; Payne, C. J. Liquids of High Refractive Index. Nature 1934, 133 (3350), 66–67. DOI: 10.1038/133066b0.
2. Laskar, J. M.; Kumar, P. S.; Herminghaus, S.; Daniels, K. E.; Schröter, M. High Refractive Index Immersion Liquid for Superresolution 3D Imaging Using Sapphire-Based Aplanatic Numerical Aperture Increasing Lens Optics. Appl. Opt., 2016, 55 (12), 3165–3169. DOI: 10.1364/AO.55.003165.

Answer 2

To complement @blu_potatos's answer, there are some other contenders having refractive index greater than 2

1. Arsenic di/trisulfide and sulfur and/or selenium and/or mercuric sulfide in arsenic tribromide solvent having refractive index ranging from 2.0 to 2.07 (arsenic tribromide seem to attack the lead-glass prisms of refractometers, corroding metal, poisonous in nature and reactive toward some minerals)
2. phosphorus and/or sulfur in carbon sulfide solvent having R.I= 2.01/2.07 (carbon disulfide is highly volatile, very flammable and poisonous. Solutions containing white phosphorus should be kept in the dark as light causes the conversion of the white phosphorus to the red form, which is more poisonous).
3. Selenium and/or sulfur in phosphorus solvent having R.I =2.2-2.5 (Solutions of selenium in phosphorus cannot be stored for long under water as they decompose).
4. Selenium in selenium monobromide solvent having R.I>2.02 (Selenium monobromide is unstable decomposing to selenium)
5. A ternary system of phosphorus-sulfur-methylene iodide having R.I upto 2.06 has been observed

\begin{array}{c|c} \mathbf{component} & \mathbf{R.I} \\\hline \text{P(solid) at 29°C } & \text{2.15}\\ \text{P(liq.) at 44°C } & \text{2.10}\\ \text{P-S-CH2I2 at 25°C } & \text{2.06} \\ \text{P-CH2I2(satd.) at 18°C} & \text{1.94}\\ \text{CH2I2 at 15°C } & \text{1.74}\end{array}

There are loads of other liquids having R.I value in between 1.5-2.0 which are stable and can be used commercially. See references

References

1. http://www.minsocam.org/ammin/AM40/AM40_398.pdf (1-4)
2. http://www.minsocam.org/ammin/AM21/AM21_245.pdf (5)
3. https://www.osapublishing.org/DirectPDFAccess/71A8545D-AFC6-6350-82F044C815273272_338928/ao-55-12-3165.pdf?da=1&id=338928&seq=0&mobile=no

Answer 3

Solutions of phosphorous in carbon disulfide are not only smelly and toxic, but they're hideously prone to catching fire. Methylene iodide is carcinogenic. Selenium and its compounds are toxic and notoriously evil-smelling. Because we're playing around in the periodic table's bad neighborhood, high refractive index (RI) liquids are uniformly extremely unpleasant.

The RI of liquid iodine is 1.934 at 114 °C [1]. This might be the best candidate for conducting experiments. Transparency in the visible range (above 450 nm) is very limited, but at least the stuff won't incinerate you, kill you, or induce your neighbors to kill you.

Reference

1. Donaldson, A.; Caplin, A. D. The Optical Properties of Liquid Bromine and Iodine. Philos. mag. B, 1986, 54 (3), 231–239. DOI: 10.1080/13642818608239022.

Answer 4

In gemological testing of the refractive indices of gemstones, the upper limit of refractive index is $1.81,$ if a contact liquid is used in a conventional gemological refractometer. See, for example, The International Gem Society — How to Measure an OTL Gemstone Refractive Index. As noted in The Gemology Project — Refractometer, the two high refractive index contact liquids are

1. a saturated solution of sulfur in diiodomethane $(n = 1.79);$
2. a saturated solution of sulfur, diiodomethane and tetraiodoethylene $(n = 1.81).$

Not the most pleasant solutions, but much better than the exotic and far more toxic possibilities that

are so toxic that they are only used in specially equipped laboratories.

There is no such RI liquid that would satisfy all parameters requested by OP.

Answer 5

Besides the already mentioned negatives of 'Highly Toxic', 'Noxious', 'Flammable', 'Carcinogenic', 'Corrosive', it needs to be understood that in order to use such high RI materials, a suitably high RI Prism or Hemicylinder and appropriate scale are also required ( there were only ever a few such instruments ever produced - using, Sphene, Strontium Titanate and Diamond - none of which were made commercially ).

My first response to your question was - what is it that you are looking to test ?

If it is a natural ( or even synthetic ) OTL stone I would suggest a 'Digital' Refractometer ( ie. Reflectometer ), which although somewhat inaccurate, will get you close - then confirm it with an SG. NM Aug, '22

Answer 6

Sucrose or glucose in H2O = approx 1.4-1.5 depending on composition and concentration

Glycerol = 1.473 Toluene = 1.5 Chlorobenzene = 1.524

Getting higher than that without very high flammability/toxicity/reactivity is hard