I have been working on a body wash made from a combination of non-ionic and amphoteric surfactants.  I added an essential oil fragrance made from palmarosa, geranium, lemon, rose, however, upon adding them, the viscosity of the gel decreased so that it went from a thick gel to a runny liquid. 

In order to find out why, I decided to try and isolate the offending essential oil by doing some tests of each one; then decided that while I was at it, I might as well test other common essential oils to see what effect they have on my particular combination of detergents.  This, I hope, will save me a lot of time in the future.

The Experiment

For this experiment, I made a large batch of the bodywash and separated it into 20 x 20g lots, to which I added 0.2 (roughly 1%) of a single essential oil and left one as a control (something to compare them to.) I documented the initial reaction and observed what happened to them a few weeks later. I focused on two things: whether the essential oil changed the viscosity, and whether it turned the formula cloudy.

Please note, I do not have an equipment to fully analyse viscosity, so I did this by looking at each one in relation to the benchmark.

I created a code for each which is as follows:  

+++ = major increase in viscosity

++   = moderate increase in viscosity

+     = minor increase in viscosity

0 = no change

-     = minor loss of viscosity

- -   = moderate loss of viscosity

- - - = major loss of viscosity

General Observations

So to answer the question: “Do essential oils influence the viscosity of surfactants?’

In this particular surfactant blend they do indeed! But not in ways you might think.

All the citrus oils seemed to increase viscosity with Lemon being highly viscous. Of the non-citrus oils, Frankincense and chamomile also increased the viscosity. Rose, vanilla, thyme and rosemary showed virtually no changes.  Lavender, ylang ylang, geranium, petitgrain and peppermint were slightly thinner than the benchmark, whereas lemongrass and palmarosa showed moderate/high loss of viscosity. The results do not explain why the blend I used caused the shower gel to turn quite so runny. I can only guess that the combination of the geranium and palmarosa, both at a high percentage of the essential oil blend, must have had a compound effect. I am not sure what chemicals in the essential oils had an affect on the overall changes but I can see that all the citrus oils contain limonene in high amounts and this could be a reason for their similar results. Similarly, palmarosa is rich in geraniol which could have caused the issue.

Most of the essential oils caused a degree of cloudiness which indicates that they didn’t solubilise completely; this might be because they were added at the end and there might be a different result if they are added at the beginning, when the surfactants are blended, and before additional water is added. Rose and vanilla were the only essential oils that didn’t cause the solution to go cloudy.

Conclusion

I had some interesting results and I certainly didn’t expect the citrus oils to have a thickening effect. I think that, although some of the oils thinned my formulation slightly, it was possibly a combination of oils that created such a sharp decrease in viscosity. It is possible that with your particular formulation, you don’t have such effects as each formula is different but if you have issues with essential oils causing problems, taking time out to do your own experiments might help in isolating the problem.

References:

Main constituents of essential oils (excluding lavender) came from Tisserand and Young (2014) Essential Oil Safety, A guide for Health Care Professionals, Churchill Livingstone

Note: With regard to the essential oils in the chart, in some cases I have entered ranges that cover various chemotypes.