Do Essential Oils Influence the Viscosity of Surfactants?

Essential oils in surfactants.jpg

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% DILUTIONMAIN 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 ORANGE83-96% 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.
VANILLA77.2% 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.

Rebecca Wright