A Practical Look At How Essential Oils Affect A...

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A Practical Look At How Essential Oils Affect A Surfactant Solution

We've all been there, created a beautiful bubble bath, body wash or shampoo only to have it fall apart the moment we pop in our Essential oils. While this sort of thing doesn't exactly come as a huge surprise - essential oils are volatile mixtures and are prone to wreaking havoc in a formula - we thought it was time to take a closer look at the situation and so we did.

We decided to look at a couple of things, firstly we know that building viscosity and nice flow in a surfactant based product is key to it looking good in terms of its commercial viability - nobody wants to buy a shampoo or bubble bath that squirts or dribbles out everywhere as soon as you invert the pack although thin surfactant products do work well in a foaming bottle- but we weren't focused on that this time. We decided to create a surfactant-based product that could be used as a basic every-day shampoo or a body wash and thicken it in two different ways, firstly with citric acid and salt - a cost effective, natural and simple way of building viscosity and secondly by using a commercial and purpose-designed surfactant thickener - PEG-120 Methyl Glucose Dioleate. While the PEG isn't a completely natural solution it has a long history of safe use in cosmetic formulations and has the added bonus of being a skin conditioning agent and it also increases the overall mildness score for a surfactant mix which we liked the sound of! The two different thickening methods gave us two different starting viscosities - salt and citric thickened reached a maximum viscosity of 3000 RPM while the PEG-120 thickened product reached a viscosity of 25,200. That is a huge difference and shows the viscosity that can be achieved with a purpose-made thickener but to put it into perspective a viscosity of 3000 RPM in a properly gelled surfactant system is about right for your average shampoo or body wash - 25,200 is super thick and goopy, more like hair gel. We made just over 1Kg of each base and coloured the PEG formula light blue so that we could quickly identify it during the experimental stage.

The second thing that we were keen to look at was how different Essential Oils affected the look and perceived viscosity of a surfactant base. We know that most Essential Oils are dosed into a surfactant base at anything from 0.2% up to around 1.5% depending of course on the oil and application. We decided that to keep things simple we would work with single oils only at this point, rather than create blends and that we would add the oil into the surfactant based product step by step from 0.2% >0.4%>0.8%. 1.6%, mixing and reviewing the result after each addition before leaving the blends for a week to settle and then reading their actual viscosities. We felt that this most accurately reflected what the market does in terms of dose and in only adding single essential oils rather than blends we could more easily relate the physical formula changes to individual oil.

The formulation is shown below:





Phase A Water




Demineralised Water






Chelating Agent


Decyl Glucoside


Key Non-ionic Surfactant


Sodium Lauryl Sarcosinate


Key Anionic Surfactant


Cocamidopropyl Betaine


Secondary Surfactant. Foam booster

Phase B Thickener and Active






Skin conditioner




Thickener and conditioner

Phase C Preservative & Aroma




Plantaserve E (Phenoxyethanol, Ethylhexylglycerin)




Essential oils/ perfume




pH Adjustment to 5.5


Citric acid/ sodium hydroxide


Salt as required








So to recap, we made two batches of the formula above, one thickened with the PEG-120 Methyl Glucose Dioleate and one thickened with Sodium Chloride (approx. 0.6%) and Citric Acid (approx. 0.4% to pH 5), measured 100g into clear cups then added our essential oils in the increments stated.


  • The average drop in Viscosity for the salt and citric thickened product was 43%
  • The average drop in viscosity for the PEG-120 thickened product was 55%
  • 6/9 of the salt and citric thickened products retained a usable viscosity of over 1000 CPS at the highest level of Essential Oil.
  • 6/9 of the salt and citric thickened products retained a usable clarity at the highest level of Essential Oil.
  • 6/9 of the salt and citric thickened products passed in terms of viscosity and clarity after a week of testing.
  • 8/9 of the PEG thickened products retained a usable viscosity of over 1000 CPS at the highest level of essential oil.
  • 7/9 of the PEG thickened products retained a usable clarity at the highest level of Essential Oil.
  • 7/9 of the PEG thickened products passed in terms of viscosity and clarity after a week of testing.


Our testing showed that it takes a while for the viscosity and clarity of the mixture to settle down enough to be fully reviewed. Many of the essential oils added turned the mixture cloudy initially but on standing cleared totally although the oils that did break the product down - the Peppermint and the Geranium got progressively worse over the week. In terms of viscosity it was surprising to us all that the citrus oils did not thin down the surfactant mixture as we had anticipated and indeed the oils actually thickened the surfactant mix in the case of the lemon with orange only losing a small percentage of initial viscosity. Of course we don't know if these results would be replicated with an alternative surfactant base but as the base we have chosen is pretty standard we feel the results we have achieved can be viewed as a good starting point to further investigations.

The overall loss of viscosity of the PEG thickened blends was larger at 55% than for the salt thickened (43%) but we are not sure that any solid conclusions can be made from that at this stage as the sample size was relatively small - 9 variants and we only ran one test cup per essential oil variant. Basically that means that we can't be sure that the difference between the two figures is statistically significant enough to draw a conclusion. That said it does look, from the results we found that the PEG thickener is clearly susceptible to loss of viscosity caused by essential oils.

In terms of comparing the same essential oil in the two different bases we see that some oils - such as Peppermint and Geranium - performed badly in both bases once we got to the highest dose of essential oil although the PEG thickened product seemed to hold up much better than the salt and citric thickened base with the smaller doses of Essential Oil. For example it looks possible to add 0.8% of either Peppermint or Geranium into the PEG thickened base and maintain a decent viscosity although product clarity is still impacted at that level. With the salt and citric thickened base it looks likely that viscosity is lost at 0.4% - half the dose. Bergamot also drops the viscosity of the salt and citric thickened base instantly while it has no effect on the PEG base until the highest dose of 1.6% is reached.

With regards to how the experiment was conducted we eliminated batch-to-batch viscosity changes between the different surfactant samples by using the same bulk batch of product for each set of nine samples. In terms of essential oil dosing we did this in drops rather than weighing out and then calculated the dose as a percentage. We acknowledge that this is a rough-and-ready way to run the testing but it suited out time frame and experimental conditions. It would be better to accurately weigh out the dose to ensure it was exactly the same for each oil. We divided the oil dosing up between each member of the team and so it is also possible that different operators had different perceptions of 'drops'.


It is clear from this experiment that even taking into account the margin for error present in how this data was collated essential oils do affect the viscosity and stability of a surfactant based product. It is clear that some (such as Lemon and Orange in our experiment) either stabilize or slightly increase the viscosity of the system we tested while others (such as Rosemary) had very little effect on viscosity. Further oils such as peppermint and geranium had a dramatic effect on both viscosity and clarity, reducing both to the point of making the product un-usable.

In terms of the thickening method used and which one is 'best' for Essential-oil containing products we conclude that both the salt and citric thickened and the PEG thickened bases were affected in similar ways to the essential oils although the PEG thickened system did tend to hold up for longer and could also achieve a much higher starting viscosity than the alternative. That said when the surfactant mix has had enough viscosity drops dramatically in both systems.

We would recommend using this data as the starting point for your investigations and as a template for how you might carry out your own experimenting. One important thing we learned from this exercise was to leave the products to settle for up to a week before reviewing to give everything time to settle.

Have fun experimenting and do remember to keep your variables (things that you change) to a minimum to avoid confusion.

Amanda Foxon-Hill

28 May 2015

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