*3.1. Materials*

For all the experiments, ultrapure water—produced by a MilliRO plus MilliQ (Millipore, Burlington, MA, USA) system and characterized by resistivity larger than 18 MΩ·m—was used. Surface tension measurement provides a value of 72.5 ± 0.2 mN/m, stable for at least two hours at 20 ◦C meaning negligible amount of surface active impurities.

The Medium Chain Triglyceride (MCT) oil was Miglyol 812N, obtained from IOI OLEO (Hamburg, Germany) and utilized without further purification. Miglyol 812N contains about 95% of triglycerides of the fractionated C8 (50%–65%) and C10 (30%–45%) vegetable fatty acids. Lighter or heavier triglycerides compose the 5% left. The absence of surface active impurities in the used MCT oil was checked by interfacial tension measurements against water, which did not reveal any appreciable kinetics over some hours. At 20 ◦C the interfacial tension of 25.4 ± 0.2 mN/m was found, which is in agreement with the values available in the literature [17,26–28].

Saponin was reagent grade purchased from VWR (catalogue n. VWRV0163, Radnor, PA, USA) and utilized as supplied. This product has been used also in Reference [11], where its composition has been analyzed and discussed in detail. It is an extract from *Quillaia saponaria* containing a mixture of components with a rather broad distribution of molecular weights (from 1070 to 1700 g/mol). However, in order to analyze the adsorption properties of saponin, in comparison with the other surfactants investigated, we assume, like in many studies of saponin or mixtures of saponins [29–31], the value of 1650 g/mol as the average molecular weight. Moreover, from the results reported in Reference [11] for neutral pH, like in this work, saponin is expected to be predominantly dissociated and to behave as an ionic surfactant.

Tween 80 was purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received. It is a non-ionic surfactant whose molecular weight and cmc are 1310 g/mol and 0.012 mM at 20–25 ◦C, respectively (as provided by the supplier).

Citronellol-β-d-glucoside, CG, was kindly supplied by the producer (4GENE GmbH, Freising, Germany), which also provided a value of the molecular weight equal to 318.39 g/mol and an estimation of the HLB and cmc, that are 11.3 and 0.01 M, respectively.

The chemical formulas of saponin and CG are given in Figure 1.

**Figure 1.** Natural surfactants investigated in the study.
