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14.5 New Intercalation Mechanism Other than the Ionic-Exchange Reaction 469
Figure 14.6 Interfacial tensions of the hybrids prepared from
MMT intercalation with POP - or POE - grafted PP copolymers
in toluene/water [34] .
14.4.2
Colloidal Properties
The silicate – copolymer hybrids, intercalated with hydrophobic organics, are dis-
persible in toluene and exhibit a surfactant property [34, 35, 101] . In Figure 14.6 ,
the hybrids can be seen to exhibit an ability to reduce the toluene/water interfacial
tension, from 28 mN m
− 1
to 5 mN m
− 1
. The critical micelle concentration ( CMC ) or
critical aggregation concentration ( CAC ) was estimated as 0.01 wt%, by extrapolat-
ing from the interfacial tension versus concentration curve.
14.5
New Intercalation Mechanism Other than the Ionic - Exchange Reaction
14.5.1
Amidoacid and Carboxylic Acid Chelating [65, 103]
The conventional method of modifying Na
+
- MMT requires the use of organic
onium salts, such as ammonium and phosphonium salts, for the ionic - exchanging
intercalation. The driving force for the organic incorporation is via the ionic
exchange reaction, the replacement of ammonium salts ( −
−
NH Cl
+
3
) for sodium
ions ( ≡ Si – O
−
Na
+
). In the case of using poly(oxyalkylene) - amidoacids in the form
of zwitterions as the intercalating agents, the intercalation of Na
+
- MMT occurred
[103] . The mechanism via a chelating intermediate between the amidoacid and the
470 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays
Figure 14.7 Schematic representation of sodium ion chelation
via an oxypropylene - amidoacid [103] .
counter sodium ion in the clay gallery was proposed. The chelating intermediate
is supposedly stable due to the formation of a cyclic, seven - membered ring
structure (Figure 14.7 ).
Another mechanism [65] was reported for metal chelating in the clay gallery.
Different alkyl lengths of alkyl - carboxylic acids, CH
3
(CH
2
)
n
COOH, were allowed
to intercalate into the clays that contained divalent metal counter ions. Convention-
ally, it was believed that the C
12 − 18
carboxylic acids [e.g., lauric acid ( n = 10) and
stearic acid ( n = 16)] would be unable to incorporate into the silicate clays, such
as the naturally occurring MMT. However, it was reported that the sodium or
potassium salts of alkyl carboxylates could be incorporated into Na
+
- MMT to afford
a low organic embedment of 10 − 15 wt% organics, and an XRD basal spacing of
15 Å . However, the same intercalating carboxylate salts in fact intercalated the
divalent M
2+
- MMT (XRD = 10.1 Å ) to achieve a wide spacing of up to 30 or 43 Å ,
in the example of the naturally occurring smectite silicates (K10), which comprised
layered aluminosilicates and mostly divalent counter cations. The difference in
intercalation between the M
2+
- MMT and Na
+
- MMT was explained by the com-
plexes of divalent M
2+
with carboxylate ( – COO
−
) and SiO
−
anions (Figure 14.8 ).
Formation of the thermally stable complex served as the driving force for the
organics to incorporate into the gallery.
14.5.2
Intercalation Involving Intermolecular Hydrogen Bonding [104]
Besides the chelating mechanism, hydrogen bonding of the intercalated organics
(structures shown in Scheme 14.1 ) may serve as additional forces for the interac-
tion, in similar fashion to the hydrophobic interaction described above. With the
structure consisting of – (CH
2
)
8
– and amide functionalities, the intercalating agents
may be further accumulated in the clay gallery. In the examples of the layered
aluminosilicates of Na
+
synthetic mica and MMT, in both cases intercalation with
the POP - amidoamine quaternary salts afforded an enlarged basal spacing of up to
78 Å , with an organic embedment up to 70 wt% in the hybrids. The driving forces
applied by the hydrogen bonding interaction were evidenced by using a stepwise
intercalating method. Continuing addition of the POP - amidoamine salts after the
14.6 Self-Assembling Properties of Organoclays 471
Figure 14.8 Diagrammatic representation of a carboxylic acid
salt complex through divalent cations on silicate K10 [65] .
Scheme 14.1 Representative structure of POP - amidoamines
with hydrogen - bonding functionalities [104] .
CEC equivalent amount led to the generation of hybrids, while the basal spacing
was increased from 58 Å to 70 – 78 Å .
14.6
Self - Assembling Properties of Organoclays [36, 37]
With the incorporation of hydrophobic POP, the MMT hybrids were amphiphilic
in nature. For example, MMT/POP2000 at 58.0 Å basal spacing and 63 wt%
472 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays
Figure 14.9 (a, b) Scanning electron microscopy images of
the self - assembly of a POP - modifi ed clay [36] .
organics was dispersible in toluene, and exhibited an ability to lower the toluene/
water interfacial surface tension. The surfactant property was shown to depend on
the hydrophobic – hydrophilic balance between the hydrophobic POP portion and
the ionic character of the platelet. With varied amounts of organics in the clay, the
amphiphilic nature of the hybrids can be tailored. Those hybrids with an organic
content in the range of 22 to 63 wt%, and a corresponding d spacing of 19 – 58 Å ,
can easily be prepared. Among these hybrids, the POP/MMT hybrid with a bal-
anced hydrophobic – hydrophilic ratio (19 Å d spacing, 26 wt% organics) enabled
self - assembly into rod - like microstructures which were up to 40 μ m in length [36]
(Figure 14.9 ). These unique self - assemblies were observed using scanning elec-
tron microscopy ( SEM ). The POP - modifi ed MMT hybrid was fi rst dispersed in a
water/toluene mixture at a concentration of 1.0 wt%, and then subjected to evapo-
ration at 80 ° C before the SEM observations were made. The formation of rod - like
arrays [37] was uniform, with individual rod dimensions ranging from 100 to
800 nm in width, and from 2 to 10 μ m as the average length. The mechanism for
the formation of lengthy rods was proposed to be a two - directional growth of
horizontal and vertical self - alignment, as illustrated conceptually in Figure 14.10 .
The correct balance between the POP hydrophobic aggregation and the silicate
ionic charge attraction is important for forming orderly microstructures.
14.7
Exfoliation Mechanism and the Isolation of Random Silicate Platelets
14.7.1
Thermodynamically Favored Exfoliation of Na
+
- MMT by the
PP - POP Copolymers [40]
The hybrids prepared from the intercalation of PP - POP2000 copolymers with Na
+
-
MMT at 19.5 Å [34] could be further exfoliated under the conditions of 120 ° C and
500 p.s.i. N
2
in a closed system, such that the layered structure of Na
+
- MMT was
transformed into random platelets. The process required the copolymers to form
14.8 Isolation of the Randomized Silicate Platelets in Water 473
Figure 14.10 Schematic representation of the self - assembling mechanism [36] .
a fi ne emulsion before interacting with the hydrophilic clays. Two types of inter-
calating hybrid may be involved, as shown conceptually in Figure 14.11 . Transfor-
mation of the initial copolymer/layered silicate hybrid into the exfoliation could
be explained by the relative stability of the water - in - oil ( W/O ) and oil - in - water
( O/W ) hybrid structures. An inversion from the W/O to the O/W hybrid was
favored thermodynamically such that, ultimately, exfoliation of the clay - layered
structure would occur.
14.7.2
Zigzag Mechanism for Exfoliating Na
+
- MMT
An alternative mechanism of exfoliation was reported which used Mannich
polyamines as the intercalating agents [39] ; these were prepared from the reaction
of p - cresol, formaldehyde, and POA - diamines. After being converted into the
corresponding amine salts, the copolymers proved to be effective for exfoliating
the layered silicates. Due to the presence of multiple amine sites in the polymer
backbone for anchoring onto the silicate surface, a zigzag conformation of the
copolymers within the confi nement was proposed as the exfoliation mechanism.
This process is illustrated schematically in Figure 14.12 .
14.8
Isolation of the Randomized Silicate Platelets in Water [41]
The process of exfoliation led to the next step of isolating random silicate platelets.
As shown in Scheme 14.2 ), the layered Na
+
- MMTs were exfoliated through
474 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays
Figure 14.11 Emulsion of the copolymers affecting Na
+
- MMT
intercalation and exfoliation [40] .
ionic - exchange reactions, while further Na
+
exchange permitted recovery of the
organic amines.
The nature of the material of the random silicate platelets was confi rmed by
using XRD analysis, transmission electron microscopy ( TEM ), and atomic force
microscopy ( AFM ). In particular, the material was shown to have a unique ionic
character in water. Based on parameters of 1.2 mEq g
− 1
CEC for Na
+
- MMT and a
surface area of 720 m
2
g
− 1
, the platelets were calculated to contain 20 000 ions per
platelet, with an area per ion of 0.9 nm
2
, and with 4 × 1 0
16
platelets per gram. The
intensive charges rendered the silicate plates with a high affi nity for polar organics
such as poly(ethylene glycol) ( PEG ) and water molecules. During measurement of
the zeta potential (see Figure 14.13 ), a Na
+
- MMT suspension in water demon-
strated a relatively constant charge character at − 18 mV, over a pH range from 2
to 11. However, the zeta potential of the exfoliated platelets showed an isoelectric
point of pH 6.4, and varied from 21 mV to − 25 mV over the pH range from 5 to
8. It appeared that the platelet surface possessed different ionic potentials that
were originally buried within the MMT layered structure.
14.9 Emerging Applications in Biomedical Research 475
Figure 14.12 Schematic representation of silicate stack and
exfoliation by the presence of polyamine salts [39] .
Scheme 14.2 The scheme of stepwise intercalation,
exfoliation, and isolation of random silicate platelets [41] .
14.9
Emerging Applications in Biomedical Research
Both types of ionic clay, namely the anionic LDH and the cationic Na
+
- MMT, have
been recognized for their abilities to adsorb biomaterials [105] , the key properties
for this application being the high surface area, the ionic character, and the layered
structure [106 – 108] . Previously reported examples have used clays as the support
476 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays
Figure 14.13 Zeta potentials of montmorillonite (
䊐
) and
exfoliated silicate platelets (
䊉
) at different pH values [41] .
for biomaterial adsorption [109] , and DNA encapsulation for gene therapy [26] . In
general, the key elements in this area of research are to specify the functions of
encapsulating large molecules within the layered confi nement. Earlier studies
have revealed several examples of proteins complexing with naturally occurring
clays, simply by their adsorption directly onto the clay surface [110, 111] . Such
adsorption between proteins and silicates is affected by the properties of the clay,
the ionic charges and the type of species, as well as the size of the protein and its
isoelectric properties [111] .
Recent improvements in this area have focused on the stability of the biomate-
rial, the controlled release profi le, and targeting. Indeed, it has been reported
that the incorporation of bovine serum albumin ( BSA ) into the layered spacing of
MMT and synthetic fl uorinated mica is possible via a step - wise ionic exchange
reaction [42, 43] , as shown in Figure 14.14 . Specifi cally, the layer spacing of
the synthetic mica was fi rst expanded with a POA - amine salt of 2000 or 4000 M
w
,
from the pristine 12 Å to the fi nal 93 Å , and this was followed by BSA incorpora-
tion. By comparison, the direct incorporation of BSA into a clay interlayer was
found to be relatively diffi cult, particularly for those micas with a larger platelet
size. This stepwise process was performed at a suitable pH (less than the isoelec-
tric point of 4.8 for BSA), at which point the protein structure was cationic
(
−−
+
NH COOH
3
); this proved to be ideal for the ionic - exchange intercalation with
the counter ions in the silicate galleries. The stepwise intercalation can be general-
ized for embedding large biomolecules into the clay layer confi nement. This
synthetic methodology is considered to have practical applications, including bio-
sensing for target molecular detection [112] , as well as interface modifi cation for
drug delivery [113, 114] and tissue engineering [62, 115] .
14.10 Conclusions 477
Figure 14.14 Schematic representation of BSA intercalation
into mica, via direct or step - wise processes [42] .
14.10
Conclusions
The naturally abundant clays have been long recognized for their wide applications
as catalysts, absorbents, metal - chelating agents, colloid stabilizers, and fi llers for
polymer composites. Recent developments in nanocomposite designs have
required the preparation of organophilic clays, which may then become compat-
ible with the ionic smectite clays. Since 2001, the present authors ’ group has
explored tailoring of the basal spacing of the layered silicates, as well as the reac-
tion profi le and amphiphilic properties of these hybrids. By using a series of
hydrophobic and hydrophilic POA - salts, it was possible to control the XRD basal
spacing within the range of 20 – 92 Å and, for the fi rst time, to observe a unique
self - assembled morphology. During this time, the use of anionic LDH clays for a
variety of applications, including gene therapy, has been suggested. With these
spatially enlarged silicates, biomolecules such as proteins could be embedded in
the clay gallery, which would retain its original size. It was also found that several
comb - branched polyamines could randomize the layered structure into individual
platelets. More recently, innovative families of polymer – clay hybrids have been
identifi ed with new properties of interfacial tension activity, compatibility to hydro-
phobic materials, and intensive affi nity for biomacromolecules. It appears that the
fundamental exploration of clay surface chemistry has in fact directed us towards
a new horizon of biomedical research opportunities, including enzyme/drug
encapsulation, wound surface anti - adhesives, and biosensors.
478 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays
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