Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 76–81
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Colloids and Surfaces A: Physicochemical and
Engineering Aspects
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Effect of mixed costabilizers on Ostwald ripening of monomer miniemulsions
Yenny Meliana, Lilik Suprianti, Yi Chia Huang, Chun Ta Lin, Chorng-Shyan Chern
∗
Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
a r t i c l e i n f o
Article history:
Received 7 April 2011
Received in revised form 22 July 2011
Accepted 28 August 2011
Available online 3 September 2011
Keywords:
Miniemulsion
Ostwald ripening
Mixed costabilizers
a b s t r a c t
The role of the mixed costabilizers of cetyl alcohol (CA) and hexadecane (HD) in retarding Ostwald ripen-
ing occurring in the three-component disperse phase monomer miniemulsions at 25
◦
C was investigated.
The modified Kabalnov equation failed to describe the Ostwald ripening behavior. Thus, the miniemul-
sion disperse phase comprising monomer (methyl methacrylate (MMA) or styrene (ST)) and the mixed
costabilizers of CA and HD was simply treated as a pseudo-two-component disperse phase miniemul-
sion system. All the results that are self-consistent with one another justify the validity of the proposed
pseudo-two component disperse phase miniemulsion model. A unique feature observed in this study is
the synergistic effect of the mixed costabilizers of CA and HD in retarding the Ostwald ripening process.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Miniemulsion polymerization involves the free radical poly-
merization of monomer in discrete droplets (ca. 50–500 nm in
diameter), prepared by homogenizing a mixture of oily monomer,
water, surfactant and costabilizer [1–4]. The polymer reactions are
generally initiated by water- or oil-soluble thermal initiators. The
initial size and size distribution of monomer droplets, presum-
ably the major reaction loci, and the stability of the unnucleated
droplets during the polymerization show a significant influence on
the particle nucleation and growth mechanisms, the reaction kinet-
ics and the performance properties of latex products. Thus, to gain
a fundamental insight into the miniemulsion stability upon aging
is a must for attaining a better understanding of the miniemulsion
polymerization mechanisms and kinetics and the successful prod-
uct development in the laboratory and quality control in the plant
production.
Thermodynamically unstable emulsions (greater than 100 nm in
diameter) undergo Ostwald ripening and/or coalescence processes
in order to reduce the extremely large total oil–water interfa-
cial area (i.e., the total interfacial free energy). Emulsion droplets
continue to grow in size and, ultimately, macrophase separation
of the colloidal system into oil and water phases occurs due to
the gravitational force. If emulsions were adequately stabilized by
surfactants against coalescence, the former would be the predom-
inant mechanism responsible for the substantial degradation of
droplets. The Ostwald ripening process occurring in emulsion with
a relatively broad droplet size distribution involves the growth of
∗
Corresponding author. Tel.: +886 27376649; fax: +886 27376644.
E-mail address: cschern@mail.ntust.edu.tw (C.-S. Chern).
larger droplets with lower chemical potential at the expense of
smaller ones with higher chemical potential via the molecular dif-
fusion of monomer molecules from smaller droplets to larger ones
[4–7].
Higuchi and Misra [8] were the first researchers to show that
oil-in-water emulsions can be stabilized against Ostwald ripening
by the incorporation of an extremely hydrophobic compound (i.e.,
costabilizer) via the osmotic pressure effect. The presence of costa-
bilizer in the disperse phase results in the compensation of the
Ostwald ripening effect by the osmotic pressure effect. As a result of
such a diffusional degradation process, the average droplet size first
gradually increases (Ostwald ripening effect > osmotic pressure
effect) and then levels off (Ostwald ripening effect ∼ osmotic pres-
sure effect) for the miniemulsion sample upon aging. Ultimately, a
relatively stationary average droplet size of the quite stable emul-
sion (termed the miniemulsion) is achieved when the Ostwald
ripening effect is completely counterbalanced by the osmotic pres-
sure effect. Some representative studies were carried out to gain a
better understanding of the Ostwald ripening phenomenon asso-
ciated with miniemulsions in the presence of costabilizers [9,10].
Based on the modified Lifshitz–Slyozov–Wagner (LSW) theory [7],
the Ostwald ripening rate (R
O
) of two-component disperse phase
miniemulsion systems can be expressed as
R
O
=
1
8
d(d
3
m
)
dt
=
8D
c
V
m
C
∞,c
9RTϕ
c
(1)
where d
m
is the average oil droplet diameter, D
c
the diffusion coef-
ficient of costabilizer in water, C
∞,c
the water solubility of the bulk
costabilizer, the interfacial tension at the oil–water interface, V
m
the molar volume of monomer in the droplet, ϕ
c
the volume frac-
tion of costabilizer in the droplet, T the absolute temperature, and R
the gas constant [7]. According to Eq. (1), the Ostwald ripening rate
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doi:10.1016/j.colsurfa.2011.08.047