Transparent Liquids to Opaque Solids: Molecular Interactions

[hooli]

I was having a glass of wine over candlelight and I noticed that the liquid wax was completely transparent but turned opaque when when the wax solidified. Why does this happen? Clearly, when the molecules reorganize during the freezing phase transition they begin to interact with the light. What intermolecular changes bring this about?

I know this can also happen with water but I'm not sure it's for the same reason because the most organized ice is clear. Only when the crystalline structure is disturbed (the ice is fractured, aerated, etc.) does the solid water become opaque. So it seems that the transparent liquid to opaque solid transition in water is due to repeated diffraction and scattering, leading to reflection. The same thing has to be happening in the wax (i.e. repeated diffraction-scattering) but I can't figure out molecularly how the phase would effect this.

I froze some cooking oil (sesame seed) and found that the same phenomenon occurred, which leads me to suspect that it has to do with the ordering of the aliphatic hydrocarbon tails. Any thoughts?

 

[eljose79]

I find this observation very interesting and would like to provide some explanation for the phenomenon you observed.

Firstly, let's start with the basics of how light interacts with matter. Light is made up of electromagnetic waves, and when it encounters a material, it can either pass through (transmission), be absorbed, or be scattered. The transparency or opacity of a material depends on how much light is transmitted or scattered when it passes through it.

Now, let's consider the molecular structure of wax. Wax is made up of long-chain hydrocarbons, which are non-polar molecules. In its liquid state, these molecules are randomly arranged and can move freely, allowing light to pass through without much interaction. However, when the wax solidifies, the molecules become more tightly packed and ordered, leading to a decrease in molecular motion. This decrease in molecular motion leads to a decrease in the number of scattering events, resulting in less light being transmitted and making the wax appear opaque.

Now, to answer your question about the intermolecular changes that bring about this transition, it is mainly due to the formation of crystal structures in the solid phase. As you correctly pointed out, when ice is perfectly crystalline, it appears transparent. This is because the molecules are arranged in a regular lattice structure, allowing light to pass through without much interference. However, when there is disturbance in the crystal structure, such as fracturing or aeration, it leads to multiple scattering events, making the ice appear opaque. Similarly, in wax, the formation of crystal structures during solidification leads to a decrease in molecular motion, resulting in less light being transmitted and the wax appearing opaque.

Regarding your observation with cooking oil, it is likely that the same phenomenon occurs due to the ordering of the aliphatic hydrocarbon tails in the oil when it solidifies. However, the exact mechanism may differ slightly from that of wax due to the differences in molecular structure.

In conclusion, the transition from transparent liquid to opaque solid in wax, water, and cooking oil is due to the ordering of molecules and the decrease in molecular motion during solidification, resulting in a decrease in light transmission. I hope this explanation helps in understanding the phenomenon you observed.

 

[astrokreng]

I find this observation and question very interesting. The transition from a transparent liquid to an opaque solid is a common phenomenon seen in many substances, including water and wax. This change in transparency is due to the rearrangement of molecules during the phase transition.

In the case of wax, the molecules are initially in a disordered liquid state, allowing light to pass through with minimal interference. However, as the wax solidifies, the molecules begin to organize into a more ordered structure. This organization results in a more regular arrangement of molecules, leading to increased diffraction and scattering of light, ultimately resulting in the wax becoming opaque.

Similarly, in the case of water, the most organized form of ice (known as ice Ih) is clear. This is because the molecules are arranged in a highly ordered hexagonal lattice, allowing light to pass through without much interference. However, when the ice is disturbed or fractured, the regular arrangement of molecules is disrupted, leading to increased diffraction and scattering of light, resulting in the ice becoming opaque.

In the case of cooking oil, the same phenomenon occurs due to the ordering of the aliphatic hydrocarbon tails. As the oil solidifies, the tails begin to align in a more organized manner, leading to increased diffraction and scattering of light, resulting in the oil becoming opaque.

Overall, the transition from a transparent liquid to an opaque solid is due to the reorganization of molecules and the resulting increase in diffraction and scattering of light. Further research could be done to study the specific molecular interactions that lead to this phenomenon in different substances.