16
Le Floch - What are Dispersants?
cinogenic potential and their high toxicity and, which could explain the high mortality ob
served in marine organisms. Scientific investigations conducted after the accident showed
that the observed toxicity was essentially induced by the solvents and not the surfactants.
The conclusions of these investigations led the British government to establish an authorisa
tion process for the use of dispersants. As a consequence, research was initiated in order to
optimise dispersant formulation to ensure high efficacy and low harmful effects on the envi
ronment.
Solvents
The initiated research on dispersants has led to the generation of new formulations suitable
for use on drifting oil slicks in the open sea. The research mainly focused on two topics: iden
tification of solvents with low toxicity and optimisation of solvent/surfactant ratios.
Hence, 2 nd generation dispersants were developed, characterised by solvents with few (or
no) aromatic compounds (BTX - benzene, toluene, xylene). The necessary doses remained
unchanged, i.e. 1 volume of dispersant for 2-3 equivalent volumes of oil.
Finally, 3 rd generation dispersants are now available with the following characteristics: toxici
ty lower than that of the dispersed oil, and more importantly with LC 50 values well above the
recommended use concentrations. Moreover, they are more concentrated, i.e. 1 volume of
dispersant can be applied to 50 equivalent volumes of oil.
Surfactants
The suitability of surfactants to interact with oil in the water phase is often described using
the Hydrophilic-Lipophilic Balance (HLB), the balance between the hydrophilic head and hy
drophobic tail. Commonly, marine dispersants have an HLB value between 6 and 14 (Figure
4.2).
HLB = Hydrophilic/Lipophilic Balance
HLB SCALE
0 5 10 15 20
I ..I I
More
I
Hydrophilic
^
Oil
In Water
Emulsion
Water
In Oil
/
Typical
Emulsion
/ Dispersant
Zone
Figure 4.2: Typical HLB value for marine dispersant (Clark, 2004).
However, it has to be noted that commercial formulations typically contain surfactant mix
tures containing 2 to 4 compounds with different HLB values in variable ratios in order to ob
tain optimal efficacy.
Moreover, due to the chemical structure of surfactants, their efficacy depends on the salinity
of the water. Hence, the majority of surfactants developed for the dispersion of oil slicks at
sea are optimized for salinities of 30 psu. Their efficacy decreases continuously until 10 psu,
and most products become ineffective below this value. Therefore, in France distinct disper
