Application example: How a picolitre dosing system can help to develop volatile surfactants
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How a picolitre dosing system can help develop volatile surfactants

Volatile Surfactants - Studying the surface active behaviour of fragrances

The fragrance and cosmetics industry intensely promoted the development of aroma molecules and studies on their structure-odour relationship leading to a good overall understanding. An aroma compound, also known as fragrance, is a chemical compound that has a smell. It can be found in various places, such as food, wine, spices, floral scent, perfumes, fragrance oils, and essential oils.

One subclass of aroma molecules are the so called volatile surfactants which possess a higher volatility and dynamic interfacial activity at a time scale of milliseconds compared to conventional surfactants. However, these unique and powerful features of volatile surfactants have not been studied in depth so far.

To shed light on the influence volatile surfactants have on the interfacial behaviour, Soboleva at al. recently presented different measurements to evaluate the structure-function relationship of volatile surfactants. In this work, aroma molecules as dynamic volatile surfactants were shown to possess functionality, like high dynamic activity and volatility, beyond the scent.

First of all, dynamic and static surface tension measurements were made by the maximum bubble pressure method and the pendant drop method to study the interfacial behaviour of volatile surfactants. The data demonstrated that volatile surfactants like linalool could decrease the surface tension of aqueous solutions on a time scale of milliseconds, much faster than conventional surfactants like sodium dodecyl sulfate (SDS), which typically takes seconds.

Schematic evaporation with constant base diameter or constant contact angle

Schematic evaporation with constant base diameter or constant contact angle

To further study the dynamic wetting behaviour, picolitre small droplets (30 pL) of different solutions (such as conventional surfactant DX4005N, volatile surfactant linalool, the mixture DX4005N/linalool as well as Milli-Q® water) were ejected onto a fresh polymer substrate from the picolitre dosing system PDDS.

The process of drop deposition and spreading was recorded using fast video imaging due to the fairly rapid evaporation. Different behaviours during the wetting and evaporation process with either a constant diameter d or a constant contact angle θ can be observed during evaporation.

Picolitre dosing system PDDS
The picolitre dosing system PDDS can reproducibly dispense droplets down to 30 pL. Depending on the contact angle, drop base diameters considerably below 100 μm can be achieved. The picolitre dosing system PDDS is capable of dosing up to 1000 droplets per second. Disposable cartridges are used as test liquid containers. Their volume is only 100 μl being economic regarding both material cost and waste. Moreover, time-consuming cleaning procedures are not necessary and cross-contamination is impossible rendering the PDDS an ideal system for the dosing of small but precise liquid amounts.

While droplets with the conventional surfactant DX4005N primarily evaporated with a constant diameter, droplets with the volatile surfactant linalool evaporated primarily with a constant contact angle. Additionally, the contact angle, evaporation time and drop base diameters of various solutions were recorded.

Dynamics of receding contact angle of droplets with solutions containing Milli-Q water (MQ-water), linalool (LO), the mixture linalool/DX4005N (LO+DX) and DX4005N (DX).

Dynamics of receding contact angle of droplets with solutions containing Milli-Q® water (MQ-water), linalool (LO), the mixture linalool/DX4005N (LO+DX) and DX4005N (DX).

The conventional surfactant DX4005N (DX) displayed a good wetting behaviour due to the strong adsorption onto the substrate surface. Accordingly, the contact angle continuously declined as a result of the reduction of droplet volume. Meanwhile, the evaporation process of linalool (LO) droplets changed from constant area to constant contact angle within 0.5 s. This phenomenon implies a weaker adsorption of LO onto the polymer surface.

Moreover, the dynamic contact angle curve of the mixture linalool/DX4005N (LO+DX) combined both modes of DX and LO. This is because LO desorbs into the gas phase and thus reduced the interfacial potential barrier. Also, the interactions of components inside the mixture might change the mechanism of the droplet evaporation. Hence, it is possible to adjust the dynamic surface tension by simply mixing various surfactants to further control the contact mode. Considering surface-emerging technologies like ink jet printing, this method holds considerable promise for conveniently fabricating printed structures of better quality and resolution.

In summary, the authors describe how volatile surfactants, such as linalool, show characteristics of a high dynamic activity and volatility beyond the odour trigger. These special properties allow volatile surfactants like linalool to lower the surface tension of aqueous solutions much faster than conventional surfactants like DX4005N. In addition, the mixture of conventional and volatile surfactants displayed a synergetic effect due to the reduction of the interfacial potential barrier. This research illustrates how the dynamic surface tension and surface modification properties of a formulation can be easily tuned providing considerable prospects for applications in surface-emerging technologies.

A picolitre dosing system PDDS was used in this research. For more information please refer to the original publication: