# Is there a commercial liquid with refractive index greater than n=2?

For an application I need a commercial, transparent liquid with refractive index $$n>2$$.

1. Is this kind of liquid available?

2. What's the chemical composition of this?

On the web, only $$n=1.81$$ is routine.

• Transparent over what wavelength range? This type of requirement needs more specifications. Can you tolerate some absorption at some wavelengths? How much? What about dispersion? Do you need n=2 at 450nm? At 700nm? – J... Aug 19 at 13:49
• The temperature ranges over which it should be liquid may also be relevant. – Roger Aug 19 at 15:40
• @Roger at visible region (380-700 nm) – Persian_Gulf Aug 20 at 8:12
• @Roger at room temperature (T=23 C) – Persian_Gulf Aug 20 at 8:13
• By any chance, does this question have to deal with gemology and OTL ("over the limit") refractive index measurements of gemstones? If so, please see this URL for how to modify a microscope to measure refractive indices above 1.81: gemsociety.org/article/measuring-gemstone-ri-1-81 . Given the stated requirements (room temperature; 380-700 nm; transparent), the standard sulfur in methylene iodide is the practical solution for gemological and mineralogical testing with the standard methodology. – Ed V Aug 20 at 14:10

Definition of refractive index:

$$n=\frac {c}{v}$$

If n>2 then this means that the velocity of light in the material is less than half than in vacuum so there must be a strong interaction with the material.

One of the highest refractory index in liquids is 1.79(20°C) possessed by a solution of sulfur in methylene iodide($$CH_2I_2$$). Liquids are less dense than solids and this explains why it's difficult to have a refractory index greater than 2. I found a nature article that says:

Selenium monobromide, 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.

https://www.nature.com/articles/133066b0

EDIT: From a more recent paper(2016), selenium monobromide is confirmed as one of the liquid with the highest refraction index(>2). However, diiodomethane is more easily accessible commercially.

A promising candidate solvent is the organic liquid diiodomethane (CH2I2), which is one of the liquids with the highest known refractive index values (n = 1.74). While other high refractive index liquids exist (phenyldi-iodoarsine (C6H5AsI2) with n = 1.85 and selenium monobromide (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.

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-55-12-3165

• Ah, and why you'd think sulfur in any solvent would be transparent? – Mithoron Aug 20 at 14:36
• Important note, "hitherto recorded" refers to 1934, the year of the paper. – Davidmh Aug 20 at 14:54
• @Mithoron it is sulphur in CH2I2 not in any solvent! – blu potatos Aug 20 at 17:18

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

• Are you just listing the real part of a complex index of refraction of opaque liquids? Or is a solution of selenium in phosphorous somehow transparent at some wavelengths? (cf. Born & Wolf; Alkali metals transparent to UV? Cesium transparent to blue?) – uhoh Aug 18 at 15:37
• Hmmm, none of these seem especially attractive to use in a classroom demonstration ... – davidbak Aug 18 at 17:21
• @davidbak yeah, I won't recommend using those chemicals. They are nasty. – Nilay Ghosh Aug 18 at 17:24
• -1 Seriously, what does selenium dissolved in phosphorous actually look like? Are you sure it's not metallic and opaque, and you've simply dropped the imaginary part of the index of refraction? Which materials in your list can actually transmit light to any appreciable degree? – uhoh Aug 20 at 1:49
• @NilayGhosh which one of these chemicals are transparent, not poisonous and can be used as gap fillers in refractometers (gap between prism and stones)? – Persian_Gulf Aug 20 at 8:39

Solutions of phosphorous in carbon disulfide are not only smelly and toxic, but they're hideously prone to catching fire. Methyene 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 RI liquids are uniformly, extremely unpleasant.

The refractive index of liquid iodine is 1.934 at 114°C https://www.tandfonline.com/doi/abs/10.1080/13642818608239022 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.

Based on the OP’s two questions and the OP’s several comments, the OP wants to know if there is a commercially available liquid which, at 23°C, is transparent in the range 380 to 700 nm. As the OP stated in a comment, the liquid should be “not poisonous and can be used as gap fillers in refractometers (gap between prism and stones).”

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, https://www.gemsociety.org/article/measuring-gemstone-ri-1-81/ . As noted in this substantive article, at http://gemologyproject.com/wiki/index.php?title=Refractometer , the two high refractive index contact liquids are

1. a saturated solution of sulfur in di-iodomethane (n = 1.79) and
2. a saturated solution of sulfur, di-iodomethane 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”. (Quote from http://gemologyproject.com/wiki/index.php?title=Refractometer).

So the answers to the OP's two questions are "no" and "does not apply", respectively.

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

• See en.wikipedia.org/wiki/List_of_refractive_indices for some approaching 2 – DrMoishe Pippik Aug 19 at 6:05
• If someone's looking for a refractive index greater than 2, a list of substances with refractive indices around 1.5 isn't very useful. – Mark Aug 19 at 23:41
• @Mark I have a feeling that answer telling no, it's a bad idea, even indirectly, is only appropriate here. Much better then then listing hideous and mostly opaque things, just because they're on some list. – Mithoron Aug 20 at 14:32
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