Hydroxyapatite-Biodegradable Polymer Nanocomposite Microspheres Toward Injectable Cell Scaffold - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 221

Hydrophobic particle
( q aw > 90º)

Hydrophilic particle
( q aw < 90º)

Oil-in-Water
emulsion

Water-in-Oil
emulsion

Oil

Oil

q

Oil

q

Water

Oil

Water

Water Water

Oil Oil

Oil

Water

Water Oil

Water Water

Figure 9.1 Dispersed systems comprising of oil and water stabilized by particles.
In mixtures of oil and water, oil-in-water emulsions are formed by using
relatively hydrophilic particles and water-in-oil emulsions are formed by using
relatively hydrophobic particles.

index to the hydrophile-lipophile balance (HLB) value and the critical
packing parameter in an emulsion system stabilized by a molecular-level
emulsifi er.
The energy change ( G ) due to the adsorption of spherical solid par-

ticles onto the oil/water interface from the phase where the particles are
dispersed is expressed by the following equation if the gravity effect can
be ignored [27],

Δ

Δ G = − g ow pa 2 (1 ± cos q ) 2 (9.1)

where g ow is an interfacial tension between oil and water, a is a particle
radius, and q is a contact angle. When the sign inside the parentheses
is negative, it represents the case in which the particle adsorption takes
place from the water phase onto the oil/water interface. When the sign
is positive, it represents the case in which the particle adsorption takes
place from the oil phase. From Equation 9.1, we can discern that the larger
the particle size and oil-water interfacial tension and the closer the con-
tact angle to 90 , the greater the adsorption energy. For example, if a par-

ticle with a radius of 10 nm is adsorbed onto a water-toluene interface
(interfacial tension: 36 mN/m) with a contact angle of 90

°

, the adsorp-
tion energy is calculated to be 2750 kT . Since the adsorption energy of
a general low-molecular-weight emulsifi er is 10-20 kT [28], the adsorp-
tion energy of the particulate emulsifi er is clearly very high. Thus, once a
particle with a suitable wettability is adsorbed onto the interface, desorp-
tion from the interface is diffi cult due to the high adsorption energy. In
many cases, the particulate emulsifi er densely covers the droplet interface
to stabilize the liquid droplets. Because of the above reasons, the stabil-
ity of a Pickering emulsion against coalescence between liquid droplets is
higher than that of an emulsion stabilized with a normal molecular-level
emulsifi er [22-25].

°