interfacial engg lecture 4
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iNTERFACIAL ENGG LECTURE 4TRANSCRIPT
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NPTEL Chemical Engineering Interfacial Engineering Module 8: Lecture 4
Joint Initiative of IITs and IISc Funded by MHRD 1/22
Interfacial Forces
&
Biomineralization
Dr. Pallab Ghosh
Associate Professor
Department of Chemical Engineering
IIT Guwahati, Guwahati781039
India
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Table of Contents
Section/Subsection Page No. 8.4.1 Introduction 3
8.4.2 van der Waals force 3
8.4.3 Hydration force 4
8.4.4 Steric forces 711
8.4.4.1 Undulation force 8
8.4.4.2 Peristaltic force 9
8.4.4.3 Protrusion force 9
8.4.4.4 Head-group overlap force 11
8.4.5 Electrostatic double layer force 12
8.4.6 Hydrophobic force 14
8.4.7 Biomineralization 16
Exercise 20
Suggested reading 22
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8.4.1 Introduction The van der Waals, electrostatic double layer, hydrophobic interaction, hydration
and steric forces (such as undulation, peristaltic, protrusion and head-group
overlap forces) play important roles in biological systems. We have discussed the
origin and various aspects of some of these forces in the Lectures 15 of Module
3.
There are two major aspects regarding the role of these forces. The first aspect involves their role in the formation of stable assemblies in biological systems
(such as membranes, cells and cell organelles). The second aspect is the
interactions between the separate assemblies which determine their stability as the
building blocks of biological tissues.
Cell adhesion and cell fusion are direct consequences of membrane interactions. The hydrophobic interaction is a strong attractive interaction between the
hydrocarbon molecules in water, and it is believed to be much stronger than the
van der Waals attraction. It plays a very important role in the formation of
assemblies.
The interaction between separate assemblies is generated by a complex interplay of the various forces mentioned above. The hydrophilic head groups of the
molecules, which constitute the assemblies, face the water phase and determine to
a large extent the two-body interactions.
The hydrophobic moieties are shielded by these hydrated (hydrophilic) head groups from the water phase so that no long-range hydrophobic attraction is
expected, and only attractions through the van der Waals interaction remain.
Therefore, van der Waals interaction is either partly or totally responsible for
phenomena like cell adhesion, membrane stacking and cell recognition in
immunological processes (Marra, 1986a).
8.4.2 van der Waals force The van der Waals interaction energy between two planar surfaces (per unit area)
is given by (see Lecture 1 of Module 3),
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212HAD
(8.4.1)
where HA is the non-retarded Hamaker constant and D is the separation
between the surfaces. The value of Hamaker constant for biological systems is
21~ 5 10 J. Due to the retardation effect in the bilayers, the value of HA diminishes with D .
Electrolytes present in the biological systems provide additional reduction in the
van der Waals interaction. The zero-frequency contribution to the Hamaker
constant is reduced due to the ionic screening. It is given by the following
equation.
0 0 0 2 exp 2H HA D A D D , 1D (8.4.2) where is the DebyeHckel parameter.
From Eq. (8.4.2), it can be observed that this term becomes significantly reduced is salt solution with increasing separation. If the salt concentration is 0.15
mol/dm3, for which 1 0.8 nm, it can be easily shown that 0HA D becomes 10% of 0 0HA at 1.5D nm.
8.4.3 Hydration force The repulsive hydration force plays a very important role in lipid bilayers. This
force is responsible for the lack of strong adhesion or aggregation of bilayers and
vesicles composed of uncharged lipids (e.g., lecithin). It is believed that hydration
forces arise when water molecules bind strongly to hydrophilic surface groups
because of the energy needed to dehydrate these groups as two surfaces approach
each other (Israelachvili, 1997). Its origin has been subject to a large amount of
debate.
Repulsive short-range forces have been measured between bilayer and other amphiphilic surfaces in water. The typical range of these forces is 13 nm, and
below this separation, they can dominate over the van der Waals and electrostatic
double layer forces. These forces do not have a simple electrostatic origin since
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they can be observed between uncharged bilayers. These forces have been
measured accurately between lecithin and other uncharged bilayers in aqueous
solutions. The repulsion per unit area is found to decay exponentially with
distance, D , according to the following equation (Israelachvili and Wennerstrm,
1990).
exph C D (8.4.3) where is the decay length whose value is ~0.2 nm, close to the size of a water molecule. Earlier it was believed that this repulsive force is due to water structure.
Apart from the term hydration force, some scientists call it structural force.
Marra (1985) used the LangmuirBlodgett technique (see Lecture 1 of Module 4) for depositing lipid bilayers on molecularly smooth mica surfaces. He deposited
phospholipids such as dipalmitoyl phosphatidylethanolamine (DPPE) and
dilanroyl phosphatidylcholine (DLPC), and the galactolipids, monogalactosyl
diglyceride (MGDG) and digalactosyl diglyceride (DGDG). The van der Waals
and hydration forces between two opposing galactolipid bilayers were measured
using the surface force apparatus (SFA).
As galactolipids are uncharged, contribution from electrostatic double layer force was absent. The experimental results indicated the presence strong short range
repulsive hydration force, as shown in Fig. 8.4.1.
Fig. 8.4.1 Short-range interactions between the DGDG and MGDG bilayers
(Marra, 1985) (adapted by permission from Elsevier Ltd., 1985).
The DGPG bilayers have an energy minimum at 1.3D nm, and the MGDG bilayers have a deeper energy minimum at 0.6D nm. Below these separations,
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the bilayer interaction is strongly repulsive due to the short-range hydration
interaction.
Below 4 nm separation, the distance dependence of the long-range van der Waals interaction between DGDG bilayers in aqueous solutions obeyed the nonretarded
Hamaker equation. The experimental Hamaker constant in pure water was found
to be 217.5 1 10 J, which decreased to 213.1 0.6 10 J upon addition of 0.2 mol/dm3 NaCl.
Early theoretical work on hydration force appeared to confirm the existence of an exponential repulsive force arising from the electric polarization of water
molecules by the surfaces (Marcelja and Radic, 1976). The decay length was
attributed to a length characteristic of water. However, the decay length was not
easy to derive theoretically, and had to be assumed, or fitted, although it seemed
conceivable that it could be close to the size of a water molecule.
However, the experimental data obtained in 1980s presented quite a complex scenario. Experiments with different surfactant and lipid bilayers in water yielded
values of in Eq. (8.4.3), which varied between 0.1 nm and 0.6 nm. With such a large variation, does not appear to correlate with the size of water molecule or with some obvious characteristic property of water.
Molecular dynamic simulations did not predict the monotonically decaying force. Instead, with surfaces modelled on lecithin and mica, decaying oscillatory forces
were obtained (Kjellander and Marcelja, 1985). Some of the important
observations which do not support the modified water-structure origin of
hydration force are given below.
(i) Helm et al. (1989) measured forces between partially hydrophobic bilayers.
They found attractive hydrophobic forces and repulsive hydration forces
existing simultaneously, each one dominating over a different distance
regime. If both of these forces arise from the water-structure effect, it is
unlikely that they would exist simultaneously.
(ii) Much weaker or no hydration force was observed between highly charged
bilayer surfaces even though these are expected to have equally strong or even
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stronger effects on water molecules than that for the uncharged surfaces
(Marra, 1986b).
(iii) The parameter, C , in Eq. (8.4.3) varies strongly (by one or two orders of
magnitude) for surfaces that are chemically very similar (e.g., lecithins and
phosphatidylethanolamines in gel and liquid-crystalline states) (Rand and
Parsegian, 1989). The NMR measurements of hydration and water structure
reveal only minor differences.
(iv) Many surfactant bilayers in water do not swell at all when they are in the solid
state. However, they swell in the liquid crystalline (LC) state. There has been
no satisfactory explanation to why more solvent structure develops in the less
ordered liquid crystalline state than in the more ordered solid crystalline state.
(v) Another important observation is that the repulsion between bilayers in water
usually increases with increasing temperature (Marra and Israelachvili, 1985).
If the hydration model is applied, this trend would suggest that the water
structure is enhanced with increasing temperature. However, this is very
unlikely because the amphiphilic molecules become less ordered with
increasing temperature, and thus one would expect less order in the adjoining
water molecules.
It has been suggested that the short range repulsive force between amphiphilic surfaces originate from the entropic (osmotic) repulsion of molecular groups that
are thermally excited to protrude from these fluid-like surfaces (Israelachvili and
Wennerstrm, 1990, 1992). The genuine hydration effects play a rather minor
role. They mainly determine the hydrated size of the protruding groups.
8.4.4 Steric forces The structures such as bilayers and biological membranes are aggregates of
weakly held amphiphilic molecules. These structures are thermally mobile. Their
shape changes continuously as their molecules twist, turn and bob in and out of
the surfaces (Israelachvili, 1997).
Four types of entropic forces between amphiphilic surfaces arise when they approach each other. These are the undulation force, the peristaltic force, the
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protrusion force and the head-group overlap force. The molecular origin of these
forces is explained in Fig. 8.4.2.
Fig. 8.4.2 Four types of thermal fluctuation forces between amphiphilic surfaces such as surfactant and lipid bilayers (Israelachvili, 1997) (adapted by permission
from Elsevier Ltd., 1992).
None of these forces should exist between hard surfaces such as solid colloid particles. Of the four types of osmotic forces, the undulation force has longer
range than the others. It can extend well beyond 3 nm separation between the
surfaces. At separations smaller than 2 nm, the protrusion and head group overlap
forces can dominate the undulation repulsion.
8.4.4.1 Undulation force The lipid bilayers have thermal undulations whose amplitude increases with
increasing temperature and decreasing bilayer bending modulus. For two bilayers
at a mean distance, D , apart under no external tension, the repulsive force per
unit area is given by (Helfrich, W., 1978; Israelachvili and Wennerstrm, 1992),
22u 3
3
64 b
kT
D
(8.4.4)
The undulation force is essentially an entropic force, which arises from the confinement of thermally excited undulation waves into a smaller region when
two surfaces approach each other.
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The undulation force has been measured and the dependence of u on 3D has been experimentally verified (Safinya et al., 1986). The repulsive undulation
force has the similar form as the non-retarded van der Waals force (i.e., 3D ). However, the undulation force can be drastically reduced or even eliminated
when a membrane is in tension since it suppresses the undulations.
Mechanical or osmotic stresses can bring about tension, and it can enhance the adhesion between stressed membranes or bilayers. The van der Waals force, on
the other hand, does not change when the surfaces are subjected to a tensile or
compressive stress.
8.4.4.2 Peristaltic force In addition to the bending fluctuations, bilayers or membranes also undergo
peristaltic (or squeezing) fluctuations. The thickness of the membrane fluctuates
about the mean thickness as shown in Figure (B). The peristaltic pressure
between two membranes is given by (Israelachvili and Wennerstrm, 1992),
2p 2 5
2
a
kT
D (8.4.5)
where a is the area expansion or compressibility modulus, which is associated with the elastic energy of the membrane. The two elastic properties of the
membrane, a and b , are different and have different dimensions though they are not necessarily independent of each other.
8.4.4.3 Protrusion force The surfaces of amphiphilic aggregates are molecularly rough. This idea is
supported by the quasi-elastic neutron scattering studies of liquid crystalline
dipalmitoyl phosphatidylcholine (DPPC) bilayers (Pfeiffer et al., 1989).
Computer simulations of micelles and bilayers have shown that the interfaces are very rough or diffuse. When two amphiphilic surfaces come close enough that
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their molecular-scale protrusions overlap, a repulsive pressure develops [see
Figure (C)].
This force is analogous to the steric force between surfaces with adsorbed polymer layers (see Lecture 5 of Module 3). In this case, the protruding segments
of the approaching surfaces are forced back into the surfaces whereas, for the
polymers, the molecules are compressed but remain between the surfaces.
The protrusion forces are very important between amphiphilic surfaces interacting in aqueous and highly polar liquids. The protrusion force per unit area
(i.e., protrusion pressure) is given by (Israelachvili and Wennerstrm, 1990),
pr
exp,
1 1 expn D D
kTD D
(8.4.6)
where n is the number of protrusion sites per unit area, is the protrusion decay length and is an interaction parameter. The value of for the single-chained and double chained amphiphiles in water range between 111.5 10 and 115 10 J/m at 298 K, which corresponds to decay lengths between 0.08 and 0.3 nm
(Israelachvili, 1997). The value of n is 18~ 2 10 m2. When the separation between the surfaces lies in the range from to 10 , the
protrusion pressure varies exponentially with the decay length, similar to that
given by Eq. (8.4.3),
pr 2.7 exp , 10n D D (8.4.7) Equations (8.4.6) and (8.4.7) were derived considering only one type of
protruding mode. In reality, surface groups generally have several conformational
degrees of freedom which leads to the development of additional protrusion
modes. Therefore, an exponentially decaying entropic repulsion always exists
between fluid amphiphilic surfaces whose decay length depends upon the
amphiphilesolvent interaction.
The role of hydration is to determine the size of the hydrated protruding head groups and the interaction between them (see Fig. 8.4.2). Therefore, the hydration
effects modulate the thermal forces.
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8.4.4.4 Head-group overlap force The head groups of many lipids are longer than the chains. They extend into the
aqueous phase and repel each other. A flexible head-group is similar to the end-
grafted polymer as described in Lecture 5 of Module 3.
The mean separation between the head groups is typically 0.60.9 nm, and they extend into the solution by this much distance. Thus, the head group overlap
interactions can be described by the interaction between polymer brushes (i.e.,
polymer molecules grafted at one end to the surface) which is given by the de
Gennes equation [viz. Eq. (3.5.5)].
For 2D L in Eq. (3.5.5) in the range of 0.2 to 0.9, the repulsive pressure s is roughly exponential and given by,
3 23100 exp 100 exps kT D L n kT Ds , L (8.4.8)
where s is the mean distance between the head-groups 1s n .
Example 8.4.1: The variation of repulsive force with separation between adsorbed
monolayers of C18EO40 in water at 298 K is given below (Homola and Robertson, 1976).
D (nm) 20.9 17.1 12.1 11.1 8.8 7.2 5.9 5.5
s (Pa) 30.5 1377.8 4338.7 7196.9 9426.7 20000 32000 39000 Fit the de Gennes equation to the data taking 10.5L nm and obtain the value of s . Present your results graphically.
Solution: The de Gennes equation is given by,
9 4 3 4
32 , 2
2skT L D D L
D Ls
Given: 10.5L nm. The variation of s with D as per the given data is shown in Fig. 8.4.3.
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Fig. 8.4.3 Variation of s with separation.
After substituting 231.38 10k J/K and 298T K in the above equation, s was obtained by fitting the equation to the experimental data (using the Solver of Microsoft
Excel). The fitted value of s is 12.96 nm.
8.4.5 Electrostatic double layer force The electrostatic double layer force is important in charged lipid bilayers. For
example, phosphatidyl glycerol is the major negatively charged lipid in bacterial
and plant membranes. It carries a single charged phosphate group.
The contribution of electrostatic forces in intrinsically uncharged lipids such as phosphatidylcholine, phosphatidylethanolamine and galactolipids becomes
important in electrolyte solutions. The electrostatic double layer force arises from
the adsorption of the cations (such as Ca+2 and Mg+2 to the bilayer surface), which
gives the bilayers a net surface charge. The electrostatic double layer force is
sensitive on the electrolyte concentration, pH of the solution and the surface
charge density (see Lectures 24 of Module 3).
Marra (1986b) measured the electrostatic double layer repulsive force between the negatively charged bilayers of distearoylphosphatidyl glycerol (DSPG) and
dimyristoylphosphatidyl glycerol (DMPG) using the Surface Force Apparatus
(SFA).
The experimental procedure was as follows. Two thin molecularly smooth mica surfaces were silvered on one side with a 50 nm-thick highly reflecting silver
coating and glued on two cylindrically curved silica-glass disks (radius of
curvature = 1 cm), with the silvered sides down. A bilayer was deposited on each
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mica surface applying a surface pressure in the Langmuir trough that was found
to give the desired transfer ratios of lipids before these glass disks were mounted
in the force-measurement apparatus. The bilayers were kept immersed in water
throughout since they lose their outer monolayer upon being retracted from water.
In order to maintain equilibrium between the lipid molecules in the bilayers and the free lipid monomers in solution, the water in the apparatus was pre-saturated
with lipid monomers by adding a lipid crystal to the water on the previous day.
This precaution is particularly important for fluid state DMPG bilayers because
the outer DMPG monolayers desorbed within a few hours unless the pre-
saturation was done. Pre-saturation for a time period of 18 hours was sufficient to
carry out reproducible experiments.
For the gel state DSPG bilayers, these precautions were less important, probably because of the much lower solubility of DSPG monomers. In the force-
measurement apparatus, one of the glass disks was mounted on a rigid support,
and the other (which faced the first one) was positioned on a spring with a known
spring constant. Now, when the bilayer surfaces were brought to close separation,
the surfaces experienced an interaction from each other which could be measured
through the deflection of the spring.
The separation between the two surfaces was measured by employing an optical technique using fringes-of-equal-chromatic-order (FECO) interferometry. From
the position and the shape of the FECO fringes observed in the spectrometer, the
distance between the two bilayers could be measured to an accuracy of 0.10.2
nm. By measuring the surface force F as a function of the surface separation D , the force profile was obtained.
The measured force between two DSPG bilayers at 293 K and pH = 9 at various concentrations of sodium chloride are shown in Fig. 8.4.4.
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Fig. 8.4.4 Measured forces between two DSPG bilayers. The lines indicate
DLVO profiles assuming fully charged bilayers and a Hamaker constant of 6 1021 J (Marra, 1986b) (adapted by permission from The Biophysical Society,
1986).
The inter-bilayer force can be accounted for by the electrostatic repulsion down to a bilayer separation of 2 nm, below which the force measurement was not
possible. At surface separations below 2 nm, the double layer repulsion became
so strong that the supporting curved mica surfaces began to flatten. In calcium
chloride solutions, the surface charge reduced due to the binding of Ca+2 ions to
the bilayer.
8.4.6 Hydrophobic force The attractive hydrophobic interaction between hydrocarbon molecules and
surfaces in water is of long range. It is much stronger than the van der Waals
attraction (see Lecture 5 of Module 3).
In unstressed bilayers, the hydrophilic head groups shield the underlying hydrocarbon groups from the aqueous phase. This effectively masks the
hydrophobic interaction between them. However, when the bilayers are stretched,
they expand laterally. The increased hydrophobic area exposed to the aqueous
phase allows hydrophobic interaction to occur between the bilayers. The direct
fusion of bilayers takes place by the hydrophobic interaction (Helm et al., 1989).
The bilayers do not have to overcome the repulsive force barrier (e.g., the barrier due to the hydration force) before they can fuse. Once the bilayer surfaces come
within ~1 nm of each other, local deformations and molecular rearrangements
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cause parting of the head-groups on opposite sides. In this way, hydrophobic
hydrocarbon regions are exposed or opened up. Since the hydrophobic
interaction is of long range, these hydrophobic regions facing each other become
unstable and jump together spontaneously, or break through across the gap and
fuse.
The force profiles between two dilauroyl phosphatidylcholine (DLPC) bilayers deposited by the LangmuirBlodgett method on solid dipalmitoyl
phosphatidylethanolamine (DPPE) monolayers are shown in Fig. 8.4.5.
Fig. 8.4.5 Force versus distance profiles between two LB-deposited DLPC monolayers (each on a solid DPPE monolayer) (Helm et al., 1989) (adapted by
permission from The American Association for the Advancement of Science and Professor Jacob N. Israelachvili, 1989).
The force profile for the DLPC surfaces in water saturated with DLPC shows van
der Waals attraction beyond 2.5 nm separation and hydration repulsion below 2.5
nm. There was no fusion even up to force/radius value of 1000 mN/m. The van
der Waals attraction caused the bilayers jump into adhesive contact from the point
A at 4.2D nm. The force profile between partially depleted DLPC monolayers in which the bilayers had thinned to about 85% of the original thickness showed
the effect of hydrophobic interaction. The depletion of bilayers caused more
exposure of the hydrophobic groups and as a consequence, a long range strong
hydrophobic attraction emerged that caused the two surfaces to jump into contact
from a greater distance, i.e., 4.2D nm, from the point B in Fig. 8.4.5.
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The bilayers spontaneously fused into one bilayer when the pressure between them reached about 0.3 MPa. For thinner bilayers, the attractive force was even
greater in range as well as in magnitude.
Fusion, therefore, is caused by the hydrophobic attraction between the internal hydrocarbon chains that become exposed to each other across the aqueous phase.
The attractive van der Waals force plays a negligible role in fusion, but it
enhances bilayer adhesion.
As we have discussed before, fusion can occur spontaneously between repelling bilayers without requiring to overcome the repulsive hydration force barrier.
Highly localized molecular rearrangements allow this to happen through
spontaneous instabilities or breakthrough mechanisms.
The hydrophobic interaction acts between the interiors of membranes. The attractive van der Waals forces between the exterior surfaces of membranes
should only lead to adhesion. The lipids in free bilayers can undergo
deformations more easily than the supported bilayers. In vesicles and membranes,
the exposed areas could emerge from inhomogeneous ionic or osmotic stresses, or
local packing stresses induced by integral membrane proteins.
8.4.7 Biomineralization
An important area where biological interfaces play a crucial role is biomineralization. Understanding the structure and dynamics at the interface
between biological templates and minerals is one of the most challenging
problems in molecular biology.
In the last few decades, scientists have been exploring various avenues for the synthesis of biomaterials by directly using available biological constructions in
synthetic systems. An important example of this is the use of DNA in materials
science and technology. Before the discovery of its structure and role in heredity,
DNA was considered as exotic but not very useful. However, DNA is now at the
center stage of biotechnology (Willner, 2002).
The strong and selective bonds between complementary DNA sequences permit one to tag a material with a unique code. These selective attractions can be used
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to link colloid particles and assemble particles on surfaces. The small size and
information-content of DNA can make it the ultimate assembly module. The
DNA-strand can be converted into a metal by selectively coating the strand with a
metal sheath by an electrochemical technique (Braun et al., 1998). Such materials
can be useful in nano-scale electronic devices. The usefulness of DNA is due to
the small length scale, which is still not accessible to photolithography. The DNA
can be assembled into cubic and other shapes with varied topology (Seeman,
1998).
Proteins can have an almost limitless variation of their sequence. Therefore, once folded, they are imparted with a unique three dimensional shape. They are central
in biological catalysis (enzymes), transport (motor proteins, transmembrane
pores) and structural functions (actin filaments). Several common proteins are
routinely used in materials applications, e.g., the use of strong biotinstreptavidin interaction for the immobilization of a desired tag onto the surface of a material.
The inside of a virus capsid can act as a near-perfect chemical reactor for the synthesis of materials. The protein residues on the interior and exterior of a self-
assembled virus shell are highly organized. They are amenable to precise and
predetermined modifications. The protein coats of virus particles (i.e., virions)
commonly comprise hundreds of subunits that self-assemble into a cage for
transporting viral nucleic acids.
Many virions, moreover, can undergo reversible structural changes that open or close gated pores to allow switchable access to their interior. Douglas and Young
(1998) showed how a virion of the cowpea chlorotic mottle virus can be used as a
host for the synthesis of materials. They mineralized paratungstate and
decavanadate clusters inside this virion, controlled by pH-dependent gating of the
virions pores. The viral RNA was removed and ultracentrifugation was
employed to purify the virus shell. This was followed by selective encapsulation
of 24WO inside the cationic cage of the virus at pH > 6.5. The virus capsid
swells at pH > 6.5 allowing it to be loaded by the inorganic ions (see Fig. 8.4.6).
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Fig. 8.4.6 Cryo electron microscopy and image reconstruction of the cowpea
chlorotic mottle virus (CCMV): (a) in an unswollen condition induced by low pH, and (b) in a swollen condition induced by high pH (Douglas and Young, 1998)
(reproduced by permission from Macmillan Publishers Ltd., 1998).
When the pH was reduced below 6.5, the pores of the virus capsid closed and the confined tungstate anions underwent oligomerization and mineralization to form
102 12 42H W O
within the viral shell. The composite matter was purified subsequently by centrifugation. The TEM images of paratungstate-mineralized
virus particles are shown in Fig. 8.4.7.
Fig. 8.4.7 TEM images of paratungstate-mineralized virus particles after isolation by centrifugation on a sucrose gradient: (a) an unstained sample showing discrete
electron dense cores, and (b) a negatively stained sample of a showing the mineral core surrounded by the intact virus protein cage (Douglas and Young, 1998) (reproduced by permission from Macmillan Publishers Ltd., 1998).
The mineral core was surrounded by the protein cage and the internal diameter of the particle was 15 nm. The diversity in size and shape of such virus particles can
make this procedure a versatile strategy for materials synthesis and molecular
entrapment.
Apart from viruses, bacteria have also been used to synthesize inorganic materials. They can be used as templates for inorganic colloid particles to prepare
semiconductor and magnetic fiber composites. The bacterial cultures are drawn at
the airwater interface to produce macroscopic bacterial thread with organized
internal superstructure that can be reversibly swollen in aqueous inorganic
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colloidal suspensions. This allows for the infiltration of particles to form
inorganicorganic fibrous composite materials. Semiconducting CdS nanoparticles have been incorporated into the organized
superstructure of B. subtilis (Davis et al., 1998). The air-dried materials consisted
of a close-packed array of multi-cellular bacterial filaments of 0.5 m diameter coated with 3070 nm thick layers of the aggregated colloid particles. The surface
charge on the bacterial filaments was negative which allowed negatively charged
colloid particles to infiltrate into the swollen inter-filament spaces.
In a similar manner, silicate micelles have been imbibed into the voids between the close-packed bacterial filaments and polymerized to form siliceous fiber
composites. The macropores were filled with the bacterial filament and the silica
channel walls were permeated with ordered mesopores (i.e., pores that have
diameter between 2 nm and 50 nm).
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Exercise
Exercise 8.4.1: Answer the following questions clearly.
1) Explain the role of interfacial forces in biological systems. What are the
major interfacial forces that operate at the biological interfaces?
2) How does the van der Waals force depend upon the concentration of
electrolyte in the medium?
3) What is the main role of hydration in biological interfaces?
4) What is undulation force? How does the disjoining pressure due to the
undulation force depend upon the separation between the surfaces?
5) What is peristaltic force? How does the disjoining pressure due to the
peristaltic force depend upon the separation between the surfaces?
6) What is protrusion force? How does the disjoining pressure due to the
protrusion force depend upon the separation between the surfaces?
7) What is head-group overlap force? How does it compare with the polymeric
steric force?
8) In what biological systems is the electrostatic double layer force important?
Explain why.
9) Explain how the hydrophobic interaction force causes the fusion of bilayers.
10) What is biomineralization?
11) Explain how DNA is being used in biomineralization?
12) Explain how virus particles can be used for mineralizing inorganic salts?
Exercise 8.4.2: The variation of repulsive pressure between fluid state
dipalmitoylphosphatidylcholine bilayers in water is given below (Israelachvili and
Wennerstrm, 1992).
D (nm) (Pa) D (nm) (Pa) 0.181 2.15 108 0.521 2.89 106 0.195 1.54 108 0.554 2.23 106 0.214 5.73 107 0.602 1.72 106
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NPTEL Chemical Engineering Interfacial Engineering Module 8: Lecture 4
Joint Initiative of IITs and IISc Funded by MHRD 21/22
0.295 6.15 107 0.661 1.07 106 0.343 1.96 107 0.669 7.13 105 0.402 5.62 106 0.735 4.41 105 0.417 9.75 106 0.953 2.26 105 0.439 7.82 106 1.245 4.29 104 0.491 3.00 106
Assuming that the force law: expC D is valid, fit the given data to this force law, and determine C and .
Exercise 8.4.3: Calculate the repulsive force per unit area between two bilayers
generated by the undulation force at 300 K for separation in the range of 1 nm to 3 nm.
Given: bilayer bending modulus = 190.5 10 J. Present your results graphically.
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NPTEL Chemical Engineering Interfacial Engineering Module 8: Lecture 4
Joint Initiative of IITs and IISc Funded by MHRD 22/22
Suggested reading
Textbook
P. Ghosh, Colloid and Interface Science, PHI Learning, New Delhi, 2009, Chapter 10.
Reference books
J. N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1997, Chapters 14 & 18.
Journal articles
C. A. Helm, J. N. Israelachvili, and P. M. McGuiggan, Science, 246, 919 (1989). E. Braun, Y. Eichen, U. Sivan, and G. Ben-Yoseph, Nature, 391, 775 (1998). I. Willner, Science, 298, 2407 (2002). J. Marra and J. N. Israelachvili, Biochemistry, 24, 4608 (1985). J. Marra, Biophys. J., 50, 815 (1986b). J. Marra, J. Colloid Interface Sci., 107, 446 (1985). J. Marra, J. Colloid Interface Sci., 109, 11 (1986a). J. N. Israelachvili and H. Wennerstrm, J. Phys. Chem., 96, 520 (1992). J. N. Israelachvili and H. Wennerstrm, Langmuir, 6, 873 (1990). N. C. Seeman, Angew. Chem. Int. Ed., 37, 3220 (1998). R. Kjellander and S. Marcelja, Chem. Phys. Lett., 120, 393 (1985). R. P. Rand and V. A. Parsegian, Biochim. Biophys. Acta, 988, 351 (1989). S. A. Davis, H. M. Patel, E. L. Mayes, N. H. Mendelson, G. Franco, and S. Mann,
Chem. Mater., 10, 2516 (1998).
S. Marcelja and N. Radic, Chem. Phys. Lett., 42, 129 (1976). T. Douglas and M. Young, Nature, 393, 152 (1998).