Do Essential Oils Influence the Viscosity of Surfactants?

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

ESSENTIAL OIL 1% DILUTION	MAIN CONSTITUENTS *	WEEK 1 OBSERVATIONS	WEEK 2 OBSERVATIONS
THYME ESSENTIAL OIL	85% carvacrol, 42% thymol, 59% borneol 23% p-cymene	- Slight loss in viscosity Cloudy	No change to the first week, still cloudy
SWEET ORANGE	83-96% Limonene 1.7-3.1% β-myrcene	+ Increase in viscosity	++ Viscosity Increase Still Cloudy but 1/2 the jar is clear.
GRAPEFRUIT Citrus x paradisi	84-95% Limonene 0.2-1.6% α-Pinene 0.4-1.0% Sabinene	+ Increase in viscosity Cloudy and opaque	++ Increase in viscosity. Still cloudy but 1/5th of the jar is clear.
FRANKINCENSE Boswellia Carterii	10-51% alpha-pinene 0-41.8% alpha-Phellandrene 6.0-21.9% Limonene 0-20.7% B-Myrcene	+ Initially got thicker in consistency.  Cloudy	+ viscosity increase. Still Cloudy.
VANILLA	77.2% Vanillin 5.6% 4-Hydroxybenzaldehyde	0 No change to viscosity Colour clear but dark golden brown.	0 Change to viscosity. Clear.
ROSEMARY Rosmarinus officinalis L	0-57.7% 1,8-Cineole 2.5-35.8% alpha-Pinene 2.1-27.3% Camphor 0.5-52.1%b-Myrcene	- Initially slightly thinner than benchmark.	0/+ change to viscosity/very slightly thicker, clear solution.
CHAMOMILE ROMAN Anthemis nobilis L	0-37.4% Isobutyl angelate 0-34.9% Butyl angelate 0-22.7% 3-Methylpentyl angelate 0-20.5% Isobutyl butyrate	+ Initially got thicker in viscosity Cloudy	++ increase in viscosity, cloudy with 1/5 of blend still cloudy.
PEPPERMINT Mentha piperita L. (average over different chemotypes)	19-54% Menthol 8-31% Menthone 22.1-31.6% Menthyl acetate 2.6-10.6% Neomenthol	-Viscosity visibly decreased instantly.	-- Decrease in viscosity. Still cloudy.
PALMAROSA Cymbopogon martinii	74.5-81% Geraniol 0.5-10% Geranyl acetate 0.5-6.1% Farnesol 1.3-3.1% Linalool	---Viscosity very thin Cloudy	-- quite runny texture. Cloudy
PETITGRAIN Citrus x aurantium	48-71% Linalyl acetate 12.3-42.5% Linalool 0.4-8% (+)-Limonene 2.1-5.2% a-terpineol 1.9-3.4% Geranyl acetaTE	- Loss of viscosity Cloudy	- Loss of viscosity cloudy, but 1/5th clear
GERANIUM Pelargonium graveolens	35.5-39.1% Citronellol 9.2-10.1% Citronellyl formate 8.7-8.9% Geraniol 6.5-6.8% Guaia-6,9-diene 5.4-5.7% Isomenthone 3.6-3.9% Linalool 1.4-2.4% Menthone	-1 Loss viscosity and very cloudy	- 1 loss viscosity, maybe slightly thinner than week one. Still cloudy.
CEDARWOOD Juniperus virginiana	21.1-38.0% a-Cedrene 21.3-23.4% Thujopsene 12.3-22.2% Cedrol 8.2-9.2% b-Cedrene	- Slightly thinner than benchmark. Cloudy.	- viscosity same as week 1, 1/3 jar is still cloudy.
LEMONGRASS Cymbopogon flexuosus	65-90% Citral 30-36% Neral 36-55% Geranial 0.1-4% Geranyl acetate 0.2-7% Geraniol	- Initially thinned when added to surfactant blend.	--Visible decrease in viscosity. Cloudy with a sunny yellow colour
LEMON (expressed) Citrus x Limon L.	64-70.5% Limonene 8.2-14% B-Pinene 8.4-10-7% y-Terpinene 0.7-2.2% Geranial	+ initially got thicker in consistency and cloudy (almost opaque)	+++ significant increase in viscosity. Although still slightly cloudy, most has become clear.
BERGAMOT (expressed) Citrus bergamia	27.4-52% Limonene 1.1-40.4% Linalyl acetate 1.7-20.6% Linalool 0.8-12.8% Sabinene 5.0-11.4% y-Terpinene	0/- Very slightly thinner than benchmark and cloudy	++ Thicker than benchmark. Cloudy
EUCALYPTUS Eucalyptus globulus	46.9-83.7% 1,8-Cineole 1.3-14.7% a-Pinene 0-11.2% Limonene 0-8.4% a-Terpineol 0-7.9% B-Pinene	- Slightly thinner in viscosity.	-- Viscosity decreased from week one, still cloudy.
YLANG YLANG Canaga odorata	17.1-21.7% Germacrene D 8.2-23.8% alpha-Farnesene 5.1-12.4% B-Caryophyllene 0.7-12.4% Benzyl acetate 0.8-8.6% Linalool	- slight loss in viscosity. Slightly cloudy.	-- Viscosity was thinner than the benchmark. Slightly cloudy
ROSE ABSOLUTE Rosa centifolia	64.8-73.0% 2-Phenylethanol 8.9-12.0% Citronellol 4.9-6.4% Geraniol 0-3.0% Nerol 0.7-2.2% Eugenol	0 Change in viscosity. Clear solution.	0 change in viscosity. Clear solution
LAVENDER lavandula angustifolia	linalool (20.60%-35.99%) linalyl acetate (12.58%-19.65%) lavandulyl acetate (3.74%-10.48%) t-p3-ocimene (1.26%-9.23%) a-terpineol (3.67%-6.73%) nerol (0.81%-3.32%) neryl acetate (0.95%-3.64%) beta-caryophyllene (0.93%-2.43%).	- Slightly thinner.  Cloudy.	-- moderately thinner than benchmark. Starting to lose cloudiness.

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.