# Storage Stability of Bimodal Emulsions vs. Monomodal Emulsions
## 2 Materials and Method
### Emulsion Preparation  
The method used for emulsion preparation was the high internal phase ratio (HIPR), described in the introduction. This method facilitates obtaining emulsions with very narrow droplet size and distribution [25,26]. The criteria governing the HIPR method are:  
* The emulsion is produced with an initial volume fraction, \(\phi\), between 0.75 and 0.95.
* The dispersed phase has high viscosity, 1-5000 Pa\(\cdot\)s at 100 \({}^{\circ}\)C.
* A low mixing rate is used, between 800 and 1200 rpm (laminar regime).
* The addition of the aqueous phase in the oily phase is done altogether in one step.
* The emulsion produced in is an inversion emulsion which changes very quickly to a direct emulsion with contact of the aqueous phase.
* The emulsion is obtained within a few seconds.
* Once the emulsion is formed we obtain a viscoelastic paste that could be diluted in water to achieve the required bitumen/water relation.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Characteristics** & **Standard** & **Units** & **15/25** & **35/50** \\ \hline Penetration & UNE-EN 1426:2007 & 0.1 mm & 25 & 45 \\ Softening temperature & UNE-EN 1427:2007 & \({}^{\circ}\)C & 61 & 52 \\ Viscosity @ 100 \({}^{\circ}\)C & UNE-EN 13302 & mPa\(\cdot\)s & 18,220 & 5450 \\ Saturated & ASTM 4124 & \% & 3.00 & 4.40 \\ Aromatics & ASTM 4124 & \% & 48.30 & 47.63 \\ Resins & ASTM 4124 & \% & 30.00 & 34.82 \\ Asphaltenes & ASTM 4124 & \% & 17.00 & 16.53 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physicochemical characteristics of used bitumens.  
Figure 3: Schematic representation of a monomodal emulsion (**left**) and a bimodal emulsion (**right**).  
* The emulsions should be stored at temperature between 40-60 \({}^{\circ}\)C.  
To prepare bimodal emulsions, it is necessary to first manufacture two separately monomodal emulsions with very controlled droplet size. This will require producing the emulsion separately to be mixed before, with different ratios of initial dispersed phase volume fraction to control the final size of droplet [24; 25]. To prepare bitumen emulsions with the same base materials but different particle size, we vary the volume ratio of bitumen/soap phase at the moment of emulsification. The ratio of bitumen/soap phase used was: 95/5 for small droplet size, and 85/5 for large droplet size. The monomodal emulsions were prepared mixing the necessary quantity of bitumen 35/50 or 15/25 penetration grade bitumen heated up at 100 \(\pm\) 2 \({}^{\circ}\)C with an aqueous solution of long chain alkyl propylenediamine surfactant at a concentration of 10,000 ppm on bitumen, heated up to 40-50 \({}^{\circ}\)C adjusted at a pH of 2 \(\pm\) 0.5, by the addition of 6-10 gr HCl in the soap phase, to obtain the emulsion with small average droplet size, hereinafter d(0.5)s, and 6750 ppm for the large size emulsion, onwards d(0.5)l. Once emulsions were obtained after mixing between 1-15 min at 800-1300 rpm, they were diluted with water to give concentrations studied in this paper, 60, 70% wt./wt. The conditions used are summarized in Table 2. We evaluated these two different concentrations because as the concentration of bitumen increases, the expected viscosity will also increase, and the storage stability should improve. To obtain the best bimodal emulsion formulation, and as result of the lowest viscosity, the large droplet size should be mixed with the small droplet size at a proportion of (1:2) [24].  
In industrial processes, we think it could be possible to manufacture bimodal emulsions with the same emulsion plant used to produce monomodal emulsion. It will be so easy to alternate the manufacture of a large particle size with the small particle size and make a soft agitation in the storage tank in order to obtain the bimodal emulsion.