interfacial engg lecture 4

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



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



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),



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



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,



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



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



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.



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



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.



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.



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



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.



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



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.



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



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).



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



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).



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



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.



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

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Chem. Mater., 10, 2516 (1998).

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interfacial engg lecture 4

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