shell parecle

Cli$on Suspension Bridge
Bristol
Enrico Fermi School of Physics: “Physics of Complex Fluids”
July 2012, Varenna, Italy
The Synthesis of Func0onal Colloids
Brian Vincent
School of Chemistry, University of Bristol, U.K.

Five Lectures
1) An Overview of Par0cle Structure/Proper0es and
Synthesis
2) Inorganic Par0cles (including metals)
3) Polymer (latex) Par0cles
4) Composite (two phase) Par0cles
5) Porous and Swellable Par0cles (including microgels)

Lecture 4 : Composite (two-­‐phase)Par0cles

composite par0cles: some types
Red = S or L
Blue = S
core-­‐shell par0cle “acorn” or bulk Janus par0cle
Red = S
Blue = S or L
“currant bun” par0cle “knobbly” par0cle
Red = S or L
Blue = S
Red = S or L
Blue = S

Some Applica0ons
Liquid core-­‐solid shell par0cles (i.e. microcapsules) are mainly used for protec0on and
controlled release of “ac0ve” molecules contained in the core. The release may be either
sustained or triggered .
We shall describe in detail in this lecture methods for preparing such microcapsules.
Acorns have no major applica0ons; they are more of a curiosity, since they are some0mes
formed, inadvertently, instead of core-­‐shell par0cles, in certain prepara0on methods – see
later.
“Knobbly” par0cles may be used to form patchy surfaces (see lecture 3).
Nanopar0cles absorbed on liquid droplets form the basis of Ramsden-­‐Pickering emulsion
systems, which are highly stable to coalescence and Ostwald ripening.
Such systems may also be used as precursors to forming liquid core / solid shell par0cles –
again see later.
Currant-­‐bun par0cles, with liquid inclusions, may also be used for sustained release.
Current-­‐bun par0cles with solid inclusions have been used in surface coa0ngs, e.g. In
electro-­‐coa0ng of paints onto automobiles (or other metal structures); the solid inclusion
is then a pigment or dye.
Also used in filled “rubber” (i.e. elas0c) materials, formed, e.g. from silica-­‐filled polymer
par0cles, to give “tougher” products, less likely to crack or tear.

currant-­‐bun par0cles
The inclusions could be solid or liquid (or even gas – see the next lecture: porous
par0cles), within a solid matrix., i.e. S/S, L/S or G/S
Principle method of prepara0on of S/S par0cles: dispersion of (small, insoluble)
solid par0cles in monomer droplets, followed by polymerisa0on of the monomer
(c.f. suspension polymerisa0on: lect. 3)
10 µm poly(methylmethacrylate) par0cle filled (25 %
by vol) with 90 nm silica par0cles
Cooper and Vincent, J. Colloid Interface Sci. 1989 132 592

“knobbly” par0cles
Formed by the adsorp0on of small solid par0cles onto:
1) Larger solid par0cles [S/S]
2) Larger liquid droplets (or indeed air bubbles) to give Ramsden-­‐Pickering
emulsions (or foams) [S/L or S/G]

S/S par0cles (1)
small (115 to 690 nm) ca8onic
PS par0cles adsorbed onto large
(2.1 µm) anionic PS
par0cles.
Harley and Vincent Colloids Surfaces,
1992 62 163

S/S par0cles (2)
Courtesy: S.P. Armes research group (Sheffield)
PMMA par0cles coated with
conduc0ng PPy par0cles

S/L par0cles:
Ramsden-­‐Pickering Emulsions

Four basic types:
Core-­‐Shell Par0cles
solid shell / liquid core [S/L]
solid shell / solid core [S/S]
liquid shell / liquid core [L/L]
liquid shell / solid core [L/S]
NB the core could also be a gas (i.e. hollow par0cles);
we saw an example of this in lect. 3: hollow graphene par0cles.
We shall consider porous par0cles, per se, in lect. 5

Vincent, unpublished
An example of a L/S system
PS latex encapsulated by PDMS droplets
N.B. it is essen0al to get the balance of the 3 interfacial tensions involved correct
I will come back to this point later.

S/L systems: microcapsules
The rest of this lecture will be concerned with the various
methods for producing microcapsules.
The most important (poten0al) applica0on is in controlled
release, e.g.:
• agrochemical (pes0cides, herbicides, fungicides, fer0lizers,
plant growth promoters, insect pheromones).
• pharmaceu0cal (targeted drugs)
• food addi0ves (e.g. flavourings)
• laundry products (perfumes, bleaches, enzymes,)
• dyes and pigments
• floccula0ng / gelling agents
• lubricants (e.g. oil-­‐drilling bits)
N.B. The liquid core may be oil or water-­‐based.

Sustained release of ac0ve [X]
c x o
δ
C x i
Permeability [P] of the shell depends on:
(1) porosity of the solid shell
(2) solubility of X in the shell
(3) diffusion coefficient of X in the shell
R o
R i
δ = R o -­‐ R i

Sustained Release Profile
A = zeroth order : constant release rate (X is solid or in saturated solution)
B = first order:
c x o
A
dc
dt
B
0
X
4πR
=
c x o, eq
0me
0 i ( c − c )
For sustained release the solid shell is usually a polymer
o
R
i
P
δ
X
X

Triggered Release
For triggered release the shell is usually ruptured at some point in
the process (so release is “instantaneous). Hence the solid shell
needs to be briole. Inorganic solids are usually invoked,
(e.g. silica, calcium carbonate).
Methods of rupture:
1) Mechanical stress
2) Osmo0c (if the core liquid and exterior liquid are miscible , e.g.
both aqueous, (so that diffusion across the solid shell can occur)
N.B Double-­‐shelled microcapsules can offer both triggered and
sustained release: inner shell polymeric, outer shell briole.
(see later)

Principle Methods of Prepara0on of S/L microcapsules
1) From “knobbly“ par0cles : fusion or growth of the solid “knobs”
2) Template methods, e.g. using surfactant liposomes
3) Interfacial polymerisa0on
4) Layer-­‐by-­‐layer adsorp0on of polymers / polyelectrolytes
5) Induced phase separa0on of polymers in the con0nuous phase.
6) Induced phase separa0on of polymers within preformed droplets
(the preferred route in my group!)
I will consider each of these 6 methods in turn

Yow & Routh, Langmuir, 2009 25 159
Prepara0on of S/L Microcapsules
1) From “knobbly“ par0cles: fusion or growth of the “knobs”
Rate of release of the “ac0ve” molecules (e.g. a dye) is reduced with increase in
temp. above the T g of the poly(styrene-­‐co-­‐butylacrylate) latex par0cles “knobs”, and
with increasing hea0ng 0me, but release cannot be completely retarded.

2) Template method: surfactant liposomes
The basic concept is to swell the surfactant (e.g. a phospholipid) bilayer with a
suitable monomer (+ a light-­‐sensi0ve ini0ator ) and to ini0ate polymerisa0on
with u.v. light .
NB the above is a mono-­‐bilayer liposome. Mul0-­‐bilayer liposomes are also
frequently formed.

3) Interfacial polymerisa0on
In this method, a polycondensa0on reac0on between two
monomers occurs at the droplet interfaces in an O/W or W/O
emulsion.
Monomer 1 is oil-­‐soluble, and monomer 2 is water-­‐soluble, so that
the interface is the only locus of polymerisa0on.
e.g. a diamine in the aqueous phase + a diacid chloride in the oil
phase would give a nylon shell at the oil / water interface.
Note that, in this case, since neither monomer can easily diffuse
across the polymer shell, the process is self-­‐limi0ng, so only
rela0vely thin solid nylon shells may be built-­‐up (NB the nylon
“rope trick”!)

The nylon rope trick

Silica Shells formed by interfacial Polymerisa0on
Two types of system will be described:
1) oil core / silica shell systems
2) water core / silica shell systems
With these systems rela0vely thick shells may be produced.

OIL CORE /
SILICA SHELL
PARTICLES

Fresh PDMS Emulsion
undialysed
(unreacted monomer present)
EtO OEt
Si
EtO OEt
Zoldesi in situ method
Zoldesi and Imhof, Adv. Mater. 17, 924-928, 2005
NB Recall last lecture: preparation of silicone oil (PDMS) droplets
reaction time x =
reaction time x
TEOS/DMDES product adsorbed
onto droplet surface
Centrifuge
Wash with EtOH
24 hours 48 hours 72 hours
Hollow-shells

Bristol Modifica0on of the Zoldesi Method:
post-­‐addi8on of a mixture of DEODMS + TEOS
AIM: beoer predic8ve control over the shell thickness.
1) Let the emulsion growth reac0on go to comple0on first (5days)
i.e. no un-­‐reacted TEOS les in the PDMS droplets
2) Add a mixture of DEODMS + TEOS (rather than just TEOS) in various ra0os.
3) For the present studies the TEOS conc. was fixed at 0.018 mol dm -­‐3
The DEODMS conc. was varied between 0.005 and 0.035 mol dm -­‐3 .
O’Sullivan, Zhang & Vincent, Langmuir 2009 25 7962

Core-­‐shell par0cles prepared by the post–addi8on method
bar = 20 µm
Op0cal micrograph of par0cles dried on a microscope slide

Shell thickness as a func0on of DEODMS conc.
TEOS conc. fixed at 0.018 mol dm -­‐3

SEM: DEODMS conc. = 0.023 mol dm -­‐3

SEM: DEODMS conc. = 0.035 mol dm -­‐3

Mechanical Strength Studies
• Micromanipulator (+ Z. Zhang)
• Need par0cles large enough to be
viewed under an op0cal
microscope
Sun and Zhang, International Journal
of Pharmaceutics, 2002 242, 307
Force
transducer

The Equipment

Force / µm
40
35
30
25
20
15
10
5
0
Breaking Force/Displacement
0 0.5 1 1.5 2 2.5 3
Displacement / µ m

WATER-­‐CORE / SILICA SHELL
PARTICLES

Shells grown around water droplets in hexadecane, stabilised with “Tegopren
8700”, by adding DEODMS + TEOS mixtures.
NB the par>cles may be transferred into water by centrifuga>on
O’Sullivan & Vincent 2010 343 31

4) Layer-­‐by-­‐layer adsorp0on of oppositely charged polyelectrolytes
e.g. Deposi0on of alterna0ng layers of anionic polystyrene sulfonate (PSS) and
the ca0onic protein β-­‐glucosidase (β-­‐GLS) .
Caruso et al, Colloid Surfaces 2000 169 287
Easier to do with flat surfaces (e.g. plates) where one can wash out excess
polyelectrolyte in solu0on between each addi0on step (can be automated).
With par0cles, need to centrifuge / re-­‐disperse between steps. Time-­‐consuming!

5) induced Phase Separa0on in the Con0nuous Phase
In this method , rather than building-­‐up the polymer shell by sequen0al
monolayer adsorp0on, a polymer mul8layer is formed in situ at a solid par0cle (or
liquid droplet) interface , by precipita0on from solu0on, followed by migra0on of
the polymer molecules to, and adsorp0on at, the interface.
This can be achieved by either inducing phase-­‐separa0on of pre-­‐formed polymer
in solu0on (e.g. through a change in temperature or solvency) or by
polymerisa0on in the con0nuous phase, to form an insoluble polymer.
A common example of this method of forming capsules is the produc0on of a
(low permeability) highly cross-­‐linked melamine–formaldehyde [MF] shell
around an oil droplet in water. A polycondensa0on reac0on is induced in the
aqueous con0nuous phase by mixing low MW MF pre-­‐condensate with a polyacid
and raising the temperature to ca. 65°C to effect the condensa0on reac0on.
Control of the shell thickness is rather poor in this method.

6) Induced phase separa0on within preformed droplets
This method was devised in my group in Bristol, and is our preferred method
of forming microcapsules.
First paper: Loxley & Vincent, J. Colloid Int. Sci, 1998 208 49-­‐62
It leads to microcapsules where both the shell thickness and the core size are
readily controlled.
We shall consider the following systems:
1) oil core / polymer shell par0cles in water
2) water core / polymer shell par0cles in water

Oil core / polymer shell systems: the basic process
polymer
+
non volatile non
solvent : hexadecane
+
good solvent :
dichloromethane
the core
the shell
water
+
surfactant :
EMULSIFICATION
EVAPORATION OF THE
GOOD SOLVENT

Interfacial Tensions
Consider the three immiscible phases in mutual contact
w
p
o
γ po
γ pw
γ ow
keep γ ow as large as possible, with respect to γ po and γ pw , to
reduce forma0on of OW interface.

predicted and observed morphologies
Oil
(o)
HD
HD
HD
HD
Decane
Octanol
θ op θ pw γ ow
γ op
aqueous
emulsifier
(w) degrees mN m –1
PMAA
PVA
SDS
CTAB
PMAA
PMAA
17.3
17.3
17.3
17.3
< 5
< 5
71.4
65.6
50.0
38.3
71.4
71.4
35.4
21.6
6.7
< 5 ∗
34.4
< 5 ∗
14.6
14.6
14.6
14.6
17.1
13.7
γ
pw S1 S2 S3 Morphology
18.9
18.8
11.5
13.1
18.9
18.9
< 0
< 0
< 0
< 0
< 0
< 0
< 0
< 0
< 0
< 0
< 0
< 0
> 0
< 0
< 0
< 0
> 0
< 0
predicted Observed
Core-shell
Acorn
Acorn
Acorn
Core-shell
Acorn
Core-shell
Core-shell
Acorn
Acorn
Core-shell
Acorn

poly(methyl methacrylate) capsules
with hexadecane cores
NB the shell has been deliberately broken (with a spatula) to reveal the core.
Loxley & Vincent, J. Colloid Int. Sci, 1998 208 49-­‐62

WATER
CORE /
POLYMER
SHELL
PARTICLES

Mineral Oil + Span 80
Water
Acetone
P(THF)
rotary evaporate at
room temperature
Mineral Oil + Span 80
Water
The process
evaporation of
some good solvent
(acetone)
Evaporation of all
good solvent
P(THF) precipitates
at interface
Polymer Shell
Atkin, Davies, Hardy & Vincent, Macromol., 2004 37 7979
Mineral Oil + Span 80
Mineral Oil + Span 80
Water
P(THF) and
acetone
( coacervate
phase)
Water
If the spreading
conditions are
correct, the
coacervate phase
migrates to interface,
fuses and engulfs
the core.
Polymer rich
phase at
interface

final form
↕
Phase Separa0on
OM SEM ↓
applied pressure

a
c
Poly(methylmethcrylate) Capsules
10 µm
10 µm
d
10 µm
b

double-­‐shell par0cles: release rates
Model “perfume oil” droplets, surrounded by an inner shell of melamine
formaldehyde [MF] and an outer shell of calcium carbonate.
Leakage (%)
2.5
2.0
1.5
1.0
0.5
0.0
Ripened CaCO3 Microcpaulses
MF Microcapsules
Double Shell Microcapsules
0 5 10 15 20 25
Time (hours)
Long, Preece, York & Zhang, private communica]on