Chemistry Reference
In-Depth Information
broadly be classed into two categories: depletion and bridging floccu-
lation. Depletion flocculation is driven by an osmotic pressure gradient
caused by the exclusion of a non-adsorbing 'depleting' agent from the
space between two (or more) approaching droplets. Depletion floccula-
tion is considered as a weak, reversible type of flocculation, since the
formed flocs can be easily broken down by mild shearing. Bridging
flocculation on the other hand is the linking of two or more droplets at
the droplet interface to form a bridge, for example due to electrostatic
interactions, and can effectively be an irreversible process. Bridging
flocculation can also take place during emulsification when the emul-
sifier concentration is not sufficient enough to completely cover the
available oil-water interfaces in the system. Under such conditions,
emulsifier molecules may be 'shared' by more than one droplet, form-
ing a 'bridge' between them.
9
.
2
.
1
.
3
Creaming
Creaming is the motion of droplets under the influence of gravitational
forces to form a concentrated layer at the top of an oil-in-water emulsion;
the same process is known as sedimentation in the case of water-in-oil
emulsions, where the concentrated layer of (water) droplets is formed
at the bottom of the system. The creaming rate is affected by the density
difference between the oil and the water phases, the size of the emulsion
droplets and the rheological properties of the continuous phase.
The creaming rate can be calculated by the Stokes-Einstein equation:
R
2
(
2
g
·
ρ
−
ρ
)
0
v
s
=
(9.1)
9
η
0
where
v
s
is the creaming speed,
g
is the acceleration due to gravity,
R
is the radius of a spherical droplet,
0
are the density and
Newtonian shear viscosity of the continuous phase, respectively, and
ρ
0
and
η
ρ
is the density of the dispersed phase (Binks, 1998).
What should be stressed at this point is that all the types of emulsion
instabilities briefly described above are interrelated. For example floc-
culation will result in an increased creaming rate. Also, creaming can
potentially accelerate coalescence events in the system due to the close
proximity of the emulsion droplets in the cream layer. It is therefore
of great importance, when considering potential routes of eliminating/
reducing the occurrence of a specific instability, to also take into account
the effect of such action on the other types of emulsion instabilities. For
instance, addition of biopolymers to the continuous phase of an oil-in-
water emulsion, in order to increase its viscosity and thus reduce the
rate of creaming, can result in depletion flocculation.
