How to use the Turbiscan Stability Index (TSI) to determine surfactant efficiency on emulsions stability
In this post, we discuss how the Turbiscan Stability Index (TSI) from Formulaction can help you establish surfactant efficiency and emulsion stability.
The objective of this study was to analyse the influence of various parameters on the stability of emulsions including surfactant choice, temperature and concentration, using the Turbiscan AGS and Tower as well as the TSI.
What are emulsions?
Emulsions are defined as unstable colloidal systems that may experience many destabilisation phenomena, such as flocculation, creaming, and coalescence, that can be caused by a variety of reasons, including lack of surfactant to stabilise the interfaces, and attractive forces, etc.
This means that if you are formulating an emulsion, it is important to know the origins of these processes and how to overcome them, to achieve a stable final product.
Due to the presence of small and big droplets, as well as chemical interactions - droplets tend to flocculate, although the addition of surfactants may minimise this problem.
This study investigated how different surfactants influenced the flocculation of the droplets, as well as the effect of surfactant concentration and the storage temperature.
What was the measurement protocol?
An O/W emulsion was formulated using oil (triglyceride) , 1% xanthan, and one of two additional non-ionic surfactants with different lipophilic chains. These were Tween 80 (HLB 150 and Tween 65 (HLB 10.5).
A reference emulsion was also studied, with no additional surfactant present.
How were the destabilisation phenomena detected?
Using the Turbiscan technology, destabilisation phenomena were detected in these formulations, showing evidence of pure coalescence in the samples without any droplet migration.
How were the phenomena quantified?
The TSI can monitor the coalescence kinetics in the samples versus ageing time. It does this by adding up all the variations detected in the sample (size and/or concentration). At any given ageing time, it was found that the higher the TSI, the more inferior the stability of the sample.
The effect of both surfactants can be compared to the reference, which showed that Tween 65 has no influence on the stabilisation of the coalescence in this sample, as its HLB is not high enough to play a stabilising role. This is compared to Tween 80, which significantly decreases the kinetics of coalescence.
This means that to increase the stability of the emulsion, surfactant Tween 80 must be added to this formulation. However, it’s important to remember that the surfactant is an expensive part of the final product, and the global stability is not proportional to the amount of surfactant.
As a next step, we then need to identify the optimal amount of surfactant to achieve the best stability for the lowest price.
How was this determined?
An analysis was made with a 0%, 0.5%, 1% and 2% of surfactant, and the TSI was computed for each formulation after 1 day of ageing.
The results showed that the optimum amount of surfactant for this particular formulation is 1%. The TSI was similar for the 2% of surfactant, which shows that increasing the upper limit beyond 1% is unhelpful for stability and increases the cost for no reason.
The Turbiscan allows the qualification of the best surfactant for a given emulsion to minimize the coalescence rate. This is possible in all types of concentrated dispersions without dilution, by comparing different formulations and optimising the best one.
The Turbiscan also provides a unique tool for monitoring the kinetics of system destabilisation, and quantifying it accurately after identifying the phenomena.
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