## 2 Experimental
### Preparation of Oil-in-Water Emulsions  
In this work, rapeseed oil in water was chosen as the extraction medium. Rapeseed oil is a low-cost, local product, which is also biodegradable and a proper solvent for various mineral hydrocarbons. Other plant oils such as sunflower oil of local origin could also have been used. The main idea was to use a completely biodegradable, natural solvent, of which residues in the ground would not pose an environmental problem.  
For the lab experiments during process development, deionized water and tap water were used. For the field trials, groundwater was taken. An amount of 5% plant oil in water (by weight) was used as the default emulsion, with different emulsifiers. A higher oil content resulted in higher viscosity and slower wetting/permeation of the soil.  
The emulsions were found to be stable for a maximum of 50 h thanks to systematic screening with surfactants and co-surfactants. The approach of increasing the density of the emulsions by adding salt did not work because the salts greatly reduced the stability of the emulsions. Lab trials were performed with ultrasonic mixers. For the field trials, a 300 L vessel with a disk stirrer was used. A surfactant/emulsifier is required to stabilize an emulsion of oil in water (O/W emulsion). This addition prevents coalescence of the disperse phase (here: oil). Surfactants have a hydrophilic and a hydrophobic/lipophilic part; a co-surfactant may also be necessary. The so-called HLB value (hydrophilic-lipophilic balance) is decisive. It indicates the mass ratio between the polar and non-polar parts on a scale of 0-20, where the range of 0-8 describes lipophilic materials and 9-20 hydrophilic ones. For water-in-oil emulsions, emulsifiers with an HLB value of 3-6 are recommended, and for oil-in-water emulsions, 8-18 has proven advantageous. Trials were performed with the following surfactants: Sodium laurethyl sulfate SDS), sodium dodecyl sulfate (SDS), polysorbates (Tween(tm) 20 to Tween(tm) 80), various vegetable lecithins, polyethylene glycole (PEG 400), glycerol stearate and sucrose stearate. As co-tensides, \(n\)-pentanol and n-propanol were tested.  
Whole milk, condensed milk and coconut milk were tested as ready-made emulsions with 3.5 to 10% oil-in-water content, but showed slow permeability. These are the most stable emulsions (Table 2):  
The last three formulations from Table 2 were used in the field trials. The lecithine was purchased from Lasenor (Barcelona, Spain; Giralce HE-60). This is a highly enzymatically hydrolyzed sunflower lecithin (E-322). The advantages of lecithines are their availability, biodegradability and good separability, since the emulsions have a stability that hardly exceeds 50 h. The hydrolysis of the lecithin leads to a better water solubility so that less energy is required for mixing. In Figure 4, the setup of the field trials is shown.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Rapeseed** & **Deionized** & \multirow{2}{*}{**Tenside [g]**} & \multirow{2}{*}{**Cotenside [g]**} \\
**Oil [g]** & **Water [g]** & & & \\ \hline
25 (5\%) & 465 & SSL & 10 (2\%) & - & - \\
25 (5\%) & 450 & SSL & 25 (5\%) & - & - \\
17.5 (5\%) & 297.5 & Lutensol TO 5 & 35 (10\%) & - & - \\
17.5 (5\%) & 262.5 & Emulan A & 35 (10\%) & Lutensol & 35 (10\%) \\
25 (5\%) & 465 & SSL & 5 (1\%) & Sucrose stearate & 5 (1\%) \\
25 (5\%) & 455 & Sucrose stearate & 15 (3\%) & Glycerine stearate & 5 (1\%) \\
25 (5\%) & 460 & Lecithine & 15 (3\%) & & \\
25 (5\%) & 455 & Lecithine & 20 (4\%) & & \\
25 (5\%) & 450 & Lecithine & 25 (5\%) & & \\ \hline \end{tabular}
\end{table}
Table 2: Emulsions with stabilities in excess of 50 h.