06 Introduction To Colloid Chemistry And Sol-gel Process.pdf

CHAPTER 1 Introduction to Colloid Chemistry and Sol-gel Process 1.1 Colloid Chemistry: Introduction A colloid is defined as a dispersion of finely divided particles in a homogeneous medium. By convention, colloidal particles are considered to range from 1 to 1000 nm in size and consist of 103 to 109 atoms. Due to their small size, these particles are small enough that they remain suspended indefinitely due to Brownian motion, a random walk resulting from momentum imparted by collision with molecules of the suspending medium. A dispersion of solid particles in a liquid medium is termed a sol. Sols can be prepared by two techniques, condensation and dispersion. Condensation proceeds by nucleation and growth of particles to the appropriate size, whereas dispersion involves the reduction of large particles down to the colloidal dimensions. Dispersion of a precipitate by chemical means, such as the introduction of an electrolyte or washing with a solvent, to form a sol is referred to as peptization. While this approach has been used in sol-gel chemistry, this method is not as convenient as condensation and will not be described at length in this report. Condensation proceeds in two stages: (1) nucleation or the formation of crystallization centers and (2) the growth of the crystals. The size and properties of the resulting particles depends on the relative rates of these two processes. Sol formation is favored when the rate of nucleation is high and the rate of crystal growth is low. Depending on the degree of crosslinking and the growth process by which they are formed, the inorganic clusters can be either colloidal or polymeric in nature and can range from 10 to 200 A⁰in diameters. Gelation is the process whereby a free flowing sol is converted into a 3D solid network enclosing the solvent medium. The point of gelation is typically identified by an abrupt rise in viscosity and an elastic response to stress. For preparation of aerogels, the gelation is most conveniently induced through a change in the pH of the reaction solution. Under controlled conditions, the pH change reduces the electrostatic barrier to agglomeration and promotes inter-cluster crosslinking, leading to the formation of the 3D network. A colloidal system is made up of two phases. The substance distributed as a colloidal particle is called the dispersed phase (analogous to solute) and the phase 1

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where the colloidal particles are dispersed is called the dispersion medium (analogous to solvent). A colloidal solution can form eight different types based upon the physical state (solid, liquid, gas) of dispersed phase / dispersion medium. The common examples of colloids are milk, curd, cheese, clouds, paint etc. The properties of these colloidal solution are in many ways different from that of true solution.The Scottish chemist Thomas Graham [1,2] found that certain materials, which are dispersed through the solvent, are very much larger than the molecules of the solvent, such systems are called as colloidal dispersion. The following two conditions are essential to form a colloidal dispersion of a solid in a given liquid;

True Solution

Colloidal Sol

(a)

(b)

Fig. 1.1 shows scattering of light by a (a) true solution and (b) colloidal dispersion (Tyndall Effect). (a) The solid must be insoluble in the liquid. (b) The solid must have certain definite size of the particles. Hence, depending upon the particle sizes the solutions are classified into three types as below and their properties are listed in table 1.1. (1) True solution (common salt dissolved into water) (2) Colloidal solution (3) Suspension Fig. 1.1 shows scattering of light by true solution and colloidal sol. In a true solution the size of the particles of dissolved substance and that of molecules of the liquid are comparable i.e. smaller than the colloidal particles. Whereas in suspension, 2

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the particles are much larger (>1000 nm) than the colloidal particles. Colloidal particles are usually of the order of 1 to 1000 nm in size [3-5]. Properties

Colloidal solution

True solution

Suspension

Size

1nm-1 m

Less than 1nm

More than 1 m

Diffusion

Diffuses slowely

Diffuses rapidly

Will not diffuse

Settling of particles

Will not settle by

Will not settle at all

Settles by just

gravity but settles

gravity

by centrifugation Filterability

Cannot be filtered

Cannot be filtered

Can be filtered

Appearance

Transluscent

Very clear

Opaque

Nature

Heterogeneous

Homogeneous

Heterogeneous

Tyndall effect

Will show tyndall

Does not show

Will show tyndall

effect

effect

Number of

Particle is the

Particle is single

Particle is the

molecules are in

aggregate of few

molecule or ion

aggregate of

one particle

hundreds of

millions of

molecules

molecules

Table1.1: Difference between Colloidal solution, true solution and Suspension In the case of substances like glue, gum and gelatin the molecules are very big in size compared to the colloidal particles. Therefore, in the dissolved state these substances are in molecular form and they exhibit the properties of colloids. One way of classifying the colloids is to group them according to the phase (solid, liquid or gas) of the dispersed substance and that of the dispersion medium. There are two basic methods of forming a colloid: (i) reduction of larger particles to colloidal size and (ii) condensation of smaller particles (e.g. molecules) into colloidal particles. Some substances like gelatin or glue are easily dispersed (in a proper solvent) to form a colloid. This spontaneous dispersion is called peptization. A metal can be dispersed by evaporating it in an electric arc, if the electrodes are immersed in water, colloidal particles of the metal form as the metal vapour gets cooled. A solid like paint pigment can be reduced to colloidal particles in a colloid mill. This is a mechanical 3

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device, which uses a shearing force to break the larger particles in to smaller ones. Table 1.3 gives the various types of colloidal systems with a common example [6]. 1.2 Dispersed phase and dispersing medium If a substance A, is insoluble in substance B then it will be usually possible to break the substance A into very small particles that can be distributed more or less uniformly throughout the substance B. Substance A is then called the dispersed phase and the substance B is called the dispersing medium. The colloidal solution is not homogeneous but it is an heterogeneous system, since the colloidal solution has two phases, one phase is continuous (liquid) and the other phase is discontinuous (particles) for example, colloidal solution of sulfur in water, sulfur is discontinuous and water is a continuous phase. In a colloidal solution, the continuous phase corresponds to solvent and is called the dispersing medium. 1.3 Lyophilic and lyophobic colloidal systems Depending on the extent to which the dispersion medium (solvent) is able to interact with the atoms of the suspended particles, the colloidal systems are classified as lyophilic and lyophobic sols. The lyophilic and lyophobic colloids have different characteristics, which is given in Table. 1.3.1 Lyophilic colloidal system The particles in a lyophilic colloidal system have a great affinity for the solvent, and are readily solvated (combined, chemically or physically) with the solvent and dispersed even at high concentrations. Lyophilic sol can be prepared from the substances containing large molecules simply by mixing them with the dispersing medium under a suitable catalytic condition [7]. If the dispersing medium is water then the term hydrophilic is used. The lyophilic colloid solution is thermodynamically stable since there is reduction in the Gibbs free energy when the solute is dispersed. The strong interaction between solute and solvent usually supplies sufficient energy to break up the disperse phase and there is often increase in the entropy as well. 1.3.2 Lyophobic colloidal system In a lyophobic colloidal system the particles resist solvation and dispersion in the solvent, and the concentration of particles is usually relatively low. For the lyophobic colloid, the Gibbs free energy increases when the disperse phase is 4

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distributed throughout the dispersion medium. The lyophobic dispersion can be prepared by grinding the solid with the dispersing medium in a colloidal mill, which over a prolonged period of time reduces the substance to a size in the colloidal range (i.e. between 102 to 103 nm). In general, the lyophobic sol is produced by precipitation reactions under catalytic conditions in which a large number of nuclei are produced while limiting their growth for example: - by oxidation, reduction, hydrolysis, double decomposition etc. Particles in a lyophobic system are readily coagulated and precipitated, and the system cannot easily be restored to its colloidal state. When the dispersion medium of a system is water, the term hydrophobic system is used. 1.4 Forces of interaction between the colloidal particles The stability of suspensions and emulsions against coagulation is governed by the forces between the particles. Dispersion is said to be stable when the particles are permanently free. In dispersions of fine particles in a liquid, frequent encounters between the particles occur due to Brownian movement. Such encounters may result in permanent contact or the particles become free depend upon the forces between them. The different types of forces between the particles are:a) Van der Waals forces: - Van der Waals forces are always attractive between particles of the same nature. Hamaker [8] derived equations for these forces on the basis of additivity of Vander Waals energies between pairs of atoms or molecules, and assuming these energies to be proportional to the inverse sixth power of the distance. b) Electrostatic forces:- Electrostatic forces, due to the interaction of the electrical double layers surrounding the particles, always lead to a repulsion between particles if they are of the same chemical nature and have surface charges and surface potentials of the same sign and magnitude. When the surface charge is generated by the adsorption of potential determining ions, the surface potential,

o, is

determined by the activity of these ions and remains

constant during interaction at least if complete adsorption equilibrium is maintained. In that case, interaction occurs at constant surface potential. The Van der Waals forces fall off as an inverse power of the separation between the particles and have a range comparable to the particle size, whereas the 5

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electrostatic repulsion falls off as an exponential function of the distance and has a range of the order of the thickness of the electric double layer. Sr.No.

Property

Lyophilic colloids

Lyphobic colloids

1.

Preparation

Easily be prepared

Need some special methods to prepare

2.

Affinity

Solvent attracting

Solvent heating

3.

Coagulation

Coagulation requires

A small quantity of

large quantity of

electrolyte is sufficient

electrolytes 4. 5.

Detection through

Cannot be easily

Can be easily detected

ultramicroscope

detected

Viscosity

Very much different

Almost the same as that of

from that of the

the dispersion medium

dispersion medium 6.

Surface tension

Very much different

Almost the same as that of

from that of the

dispersion medium

dispersion medium 7.

Density

Very much different as

Almost the same as that of

that of the dispersion

the dispersion medium

medium 8.

Electrophoresis

9.

Reversibility

Particles migrate in

Migrate in particular

either direction

direction

The reaction is

Irreversible

reversible 10.

Example

Starch solution, Soap

Colloidal gold, colloidal

solution

silver

Table1.2: Distinction between lyophilic and lyophobic colloids For molecules of a simple monatomic liquid the interaction energy between a pair of molecules is given by equation (1.1). The interaction only depends on the separation between the centers of the molecules. At large separations, r > 6

the

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interaction is attractive, u(r) <0. At smaller separations, overlap of the electron orbital of the two molecules is giving rise to a repulsion that increases very rapidly as the separation decreases. u(r) = 4 [ ( /r) 12 Where

and

( /r)6 ]

(1.1)

give the measure of size and the strength of interaction between the

molecules respectively. Fig (1.2) shows the curve of potential energy of interaction against distance r. Dispersed

Dispersing

phase

medium

Solid

Gas

Technical

Notation

Examples

name

S/G

Smoke

Aerosol

Hairspray, mist, fog Liquid

Gas

L/G

Solid

Liquid

S/L

Aerosol

etc.

Sol or

Printing ink, paint

dispersion Milk, Lubricants,

Liquid

Liquid

L/L

Emulsion

Gas

Liquid

G/L

Foam

Solid

Solid

Solid

S/S

Liquid

Solid

L/S

Gas

Solid

G/S

dispersion Solid

crude petroleum Foam Ruby glass (Au in glass), some alloys Ice-cream, moist soil, adsorbents

emulsion Solid foam

Insulating foam

Table 1.3: Types of colloidal systems

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At short distances, a deep potential energy minimum occurs, the position of which decides the distance of closest approach, r0. At intermediate distances the electrostatic repulsion makes the largest contribution and hence a maximum occurs in the potential energy curve, Vm. At larger distances, the exponential decay of the electrical double layer term causes it to fall off more rapidly than the power law of the attractive term and a second minimum appears in the curve Vsm.

Potential energy U (r)

Repulsive Energy U2= B / r12

6

12

U (r) = ( A / r ) + (B / r )

ro Umin

Attractive Energy U1= A / r

6

nnn

Interparticle distance (r) Fig. 1.2 Potential energy (P. E.) U(r) of two particle system as a function of distance (r) between them. 'ro' is equilibrium distance where the P. E. is minimum (Umin). 1.5 Charge on the colloidal particles It is well known that most of the colloidal particles are electrically charged and will migrate in one or other direction when subjected to an electric field. It has been observed that there is a tendency for charges to accumulate at any interface between two phases, because, the relative affinities of the cations and anions for the 8

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two phases are, in general, different, one phase acquires a positive and the other a negative charge. Therefore, the potential curve drops steeper at places where there are more compensating counter ions. The potential at the outer surface of the double layer (plane AB) is still higher than that of the solution which accounts for the electrokinetic properties of the sol and is called as electrokinetic or zeta potential. The state of the charge on the electrolyte molecules is also determined by the pH (concentration of H+ and OH– ions) of the sol. Thus a molecule having positive charge at low pH may acquire a negative charge at higher pH values and there is an intermediate pH called the isoelectric point at which the molecule will be electrically neutral [9] H+ 2

(SiO3)

H+

H+ 2

(SiO3)

2

(SiO3) H+

H+ 2

SiO2

(SiO3)

2

(SiO3)

H+

+

H

2

2

(SiO3)

(SiO3) 2

(SiO3)

H+

H+

H+ H+ Fig. 1.3 Formation of electric double layer on a particle of an aqueous sol of silicon dioxide due to ionization 1.6 Shape of colloidal particles It is a general observation that the colloidal particles acquire spherical shape. This is explained by the excess of free energy. It is known that among the bodies of different geometrical shapes, a sphere has the smallest surface energy, and the process of sphere formation occurs spontaneously in accordance with the second law 9

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of thermodynamics. In a spherical drop, all the surface molecules are indistinguishable from one another and also differ from the bulk ones in their orientation.

Potential

Fixed layer

+ + + + + + + + + + + + + + + + +

Particle surface

B

Mobile layer +

+ + + + + +

Zeta Potential

+

+

+ A

+

+

+

Distance

Fig. 1.4 Variation of attractive potential due to the charge on the particle surface and Zeta potential ( ).

The excess free energy makes the disperse systems thermodynamically unstable. The process of lowering the excess free energy and reducing the dispersity is the fundamental characteristics of all dispersed systems. If a dispersed system remains unchanged in its chemical composition, but changes its free energy characteristic then this will result in a change in colloidal properties.

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1.7 Molecular-kinetic properties of colloidal systems 1.7.1 Brownian motion The first macroscopic observation of the rapid and erratic motion of small particles was made by Robert Brown, an English Botanist, who was examining the pollen grains. The motion was thought to be caused by the bombardment of the particles by the surrounding molecules of the solvent. The intensity of the bombardment would vary from moment to moment on one side or other of the particle and this was the cause of erratic motion. When viewed under microscope, by putting few such grains on a drop of water, he found that they are continuously moving in random directions but along straight-line paths as shown in Fig (1.5). The zig-zag motion of colloidal particles along straight-line path in random directions is called the Brownian motion [10].

Fig. 1.5 Brownian motion of colloidal particles. When the particles are very small, they can be viewed only under ultra microscope. Because of the Brownian motion the particle remains suspended or dispersed in the medium and do not settle down as precipitate. The motion is characterized by the mean displacement x of a particle during the time t. Therefore, , the root mean square value of projection of displacement of a particle on X-axis parallel to the selected direction is given by 2

(

Where

1,

2,

1

2 2

n

....

)1 / 2

(1.2)

…..... = Projections of displacements of a particle on X- axis and n=

number of such projections taken for calculation. 11

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1.7.2 Diffusion In the absence of any external fields, the chemical potential,

i,

of the substance

within a phase at equilibrium is constant. If for some reason the value of

i

varies

from place to place, then the substance 'i' will tend to diffuse in such a way as to equalize the value of

through out the phase. The driving force for the diffusion

process is the gradient of

i

– larger it is, the faster is the diffusion process. For one

dimensional process: Fd = d i / dx

(1.3)

Where Fd is the driving force for the diffusion process. Diffusion is a spontaneous equalization of the concentration of molecules, ions or colloidal particles in a system as a result of their thermal random motion. Hence, diffusion is a macroscopic exhibition of thermal motion of molecules, and therefore the more rapid it is, the higher is the temperature. Diffusion is irreversible, it occurs until concentrations are completely equalized because the maximum entropy of a system corresponds to the random distribution of the particles. Diffusion can be best understood by the Fick 's first law, given by Ji = - D (dc/dx)

(1.4)

Where Ji= flux of the material, D= diffusion coefficient which depends on the properties of the diffusing particles and the medium, dc/dx = concentration gradient. The significance of negative sign is that the derivative dc/dx has a negative value as the quantity c decreases with an increase in the value of x. The diffusion coefficient is directly proportional to the absolute temperature and inversely proportional to the viscosity of a medium and the particle radius, as given below:

D

RT 1 NA 6 r

KT 6 r

where R is the gas constant, T is temperature, NA is Avogadro's number,

(1.5) is viscosity

and r is the particle radius. The diffusion coefficient is small in colloidal systems because the dimensions of colloidal particles are very large in comparison with those of ordinary molecules. 1.7.3 Osmosis When two solutions of different concentrations are separated by a semi permeable membrane, a flow of the solvent appears from the lower concentration to the higher one that levels out the concentrations. The flow is balanced by counter 12

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pressure gradient developed due to the excess number of impacts of the solvent molecules against the membrane from the side of the more dilute solution. The osmotic pressure P of very diluted colloidal solutions is given by

P

ms RT m pVN A

RT NA

KT

(1.6)

Where ms= mass of the solute, mp =mass of the particle, V= volume of the system, NA= Avogadro's number, T= absolute temperature,

= numerical constant. The

molecular-kinetic equations that are suitable for true solutions can be applied also to colloidal solutions, the only difference being that the mass of a mole of a substance is replaced by the mass of particles in them. The osmotic pressure of a disperse system is determined only by the numerical concentration and does not depend on the nature and dimensions of particles. Hence, the osmotic pressure is very low for colloidal systems since the numerical concentration of a colloidal system is always far less than that of a true solution. 1.7.4 Tyndall effect (optical property) When a beam of light is passed through a true solution, and observed at right angles to the direction of the beam, the path of the light is not clear. At the same time, if the beam of light is passed through a colloidal solution, the path of the light is quite distinct due to scattering of light by the colloidal particles. If an electric potential is applied across two platinum electrodes immersed in a colloidal solution, the colloidal particles move in a particular direction, depending upon the charge of the particles. The phenomenon of scattering of light by the colloidal particles is known as “Tyndall effect”. 1.7.5 Electrophoresis (electrical property) If an electric potential is applied across two platinum electrodes immersed in a colloidal solution, the colloidal particles move in a particular direction, depending upon the charge of the particles. Thus the migration of colloidal particles under the influence of electric field is called electrophoresis. This phenomenon can be demonstrated by placing a layer of arsenic sulphide solution under two limbs of a U-tube. When current is passed through the limbs, it can be observed that the level of the colloidal solution decreases at one end of the limb and rises on the other end.

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1.7.6 Surface Tension and Surface Energy The existence of surface tension can be expected from the difference between the molecules in the bulk and molecules at the surface. The force which acts per unit length of an interface and causes a diminution of the liquid surface is known as surface tension which is always tangential to liquid surface and is measured in Newton / meter. In order to increase the liquid surface, work must be done to overcome forces which cause internal pressure is called surface energy. Surface tension is also regarded as a free energy per unit area i.e. the work required to bring molecules from the interior of the phase on to the surface region to form more surface and the corresponding unit is joule/meter2 which is dimensionally equal to N/m. Surface energy is equal to surface tension only for a single-component liquid. The surface tension decreases with the increase in temperature. This relation is shown by the Gibbs - Helmholtz equation [11] as S=

T (d

/ dT) V

(1.7)

Where S = Total surface energy per unit area of the layer (for a homogeneous surface at V= constant) and

= Surface tension.

Fig 1.6 Temperature dependence of the surface tension

and the total

surface energy S for carbon tetrachloride [12]. From Fig (1.6), it is clear that as the temperature rises, the surface tension of the liquid diminishes almost according to the linear law. This means that the temperature coefficient d /dT has almost constant negative value upto temperatures 14

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which are close to critical temperature. At the critical temperature, the difference between boundary phases disappears and surface tension becomes zero. This applies not only to the liquid-vapour system, but also to the liquid-liquid system when surface tension disappears at the critical temperature of solubility. 1.8 Coagulation of colloids The entire colloidal particles are electrically charged; all are positively charged or negatively charged. Therefore every colloidal particle repel each other and remains stable. In order to coagulate a colloid, these charges have to be nullified. This can be done in three ways: (I) By adding a double salt (electrolyte) (ii) By introducing an electrode of opposite charge (iii) By introducing another colloid of opposite charge After nuetralising the charges, the colloidal particles are brought together and they are large enough to settle down. Thus the process of precipitating a colloidal solution is called coagulation. 1.9 Stability of colloidal systems If the colloidal particles do not aggregate at a significant rate, the system is said to be colloidally stable. An aggregate is, in general, a group of particles (atoms or molecules) held together in any way. When a sol is colloidally unstable, the formation of aggregates is called coagulation or flocculation. Specifically, if the aggregation is compact, it is called as coagulation and a formation of a loose or open network implies flocculation. The reversal of coagulation i.e. dispersion of aggregates to form a colloidally stable suspension or emulsion is called as deflocculation or peptization. The rate at which a sol coagulate depends on the frequency with which the particles encounter one another and the probability that their thermal energy is sufficient to overcome the repulsive potential energy barrier to coagulation when these encounters take place. The rate at which the particles aggregate is given by,

dn dt

k2n 2

(1.8)

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Where 'n' is the number of particles per unit volume of sol at time’t’ and 'k2' is a second order rate constant. The negative sign stands for a decrease in number of free particles. During the course of coagulation, k2 usually decreases and the sol may become only partially coagulated – due to the height of the repulsion energy barrier increasing with increase in particle size. In lyophilic sol, the colloidal particles experience short range repulsive forces and also, as they are surrounded by several solvent molecules, lyophilic sols can be stable even at their isoelectric point [13]. Whereas, lyophobic sols are stable due to the electric double layer at the surface of the colloidal particles [14]. If two particles of an insoluble material do not have a double layer, they can come close due to the attractive van der Waals forces (refer equation 1.1). On the other hand, if the particles possess a double layer, the overall effect is that they repel one another at large distances of separation. This repulsion prevents the close approach of the particles and stabilizes the colloid. Coagulation can be brought about in lyophobic sols by changing the electrolyte concentration. The size of the repulsion barrier between colloidal particles depend upon the magnitude of the surface charge and on the extent of the electrical double layer which in turn depends on the total electrolyte concentration. The electrolyte concentration at which slow coagulation gives way to rapid coagulation is called as the Critical Coagulation Concentration. Addition of small amounts of a lyophilic colloidal sol to a lyophobic sol may make the latter more sensitive to flocculation by electrolyte. This phenomenon is called as Sensitization. The disappearance of the boundary between the aggregated particles followed by change of shape leading to a reduction of the total surface area is called as Coalescence. Settling of suspended particles under the action of gravity is called as sedimentation. Sedimentation depends on the radius of the particle (r), which is given by the formula,

r

(

9 H )1 / 2 2T (d d 0 ) g

(1.9)

Where, : viscosity of the medium, H: height, T: time for settling, d: density of the particle, do: density of the medium and g: acceleration due to gravity.

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1.10 Sol-gel process Sol-Gel process is a colloidal route used to synthesize aerogels or coatings with an intermediate stage of a sol or a gel state. It generally consists of the preparation of a colloidal suspension of a solid into liquid; viz (sol) and then three dimensional structures of solid enclosing the liquid (gel). The gel on further removal of liquid results in the final material which will be in the hydroxylated state [2,15]. The starting material used in the preparation of the sol is usually inorganic metal salts or metal organic compounds such as metal alkoxides. In a typical sol gel process the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension or sol. The sol when cast into mould results in the formation of a gel. With further drying and heat treatment, the gel is converted into dense ceramics or glass articles. If the liquid in a wet gel is removed under supercritical conditions; a highly porous and extremely low density material called aerogel is obtained. Applying sol-gel process, it is possible to fabricate ceramic or glass materials in a wide variety of forms, ultra fine or spherical shaped, powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses or extremely porous aerogel materials. In the sol-gel process, simple molecular precursors are converted into nanometer-sized particles to form a colloidal suspension, or sol. The colloidal nanoparticles are then linked with one another in a 3D, liquid-filled solid network. This transformation to a gel can be initiated in several ways, but the most convenient approach is to change the pH of the reaction solution. Even the method used to remove liquid from a solid will affect the solgel’s properties. 1.10.1 Sol A sol is a stable suspension of colloidal solid particles within a liquid [16]. Particles of a sol are small enough to remain suspended indefinitely by Brownian motion. Sols are classified as lyophobic if there is a relatively weak solvent particle interaction and lyophilic if the interaction is relatively strong. Lyophobic sols exhibit well defined Tyndall effect. In lyophilic sols the particles are largely solvated and this lowers the differences in refractive indices of two phases. The Tyndall effect is due to the scattering of light from the surface of colloidal particles.

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1.10.2 Stability of sols Sol particles are held by Van der Waals forces of attraction or dispersion energy. Van der Waals force is propotional to the polarisabilities of the atoms and inversely related to the sixth power of their separation. This Van der Waals force results from transitory dipole-transitory dipole interactions (London forces). It is this London forces that produce long range attraction between the colloidal particles. The attractive potential for two infinite slabs separated by distance, h is given by

VA

A 12 h 2

(1.10)

For atoms, VA

1

h6

Where A is a material property called Hamaker’s constant. Since this attractive force extends over distances of nanometers, sols are thermodynamically unstable [17,18]. Aggregation can be prevented by erecting necessary barriers of comparable dimensions. i) Electrostatic stabilization Electrostatic stabilization is explained by DLVO theory. According to this theory the net force between particles in suspension is assumed to be the sum of the attractive Van der Waals forces and electrostatic repulsion created by charges adsorbed on particles. The repulsive barrier depends on two types of ions that make up the double layer, charge determining ions that controls the charge on the surface of the particle and counter ions that are in solution in the vicinity of the particle and act to screen charges of potential determining ions [2]. A schematic representation of electrostatic stabilization is provided in Fig. 1.7. For hydrous oxides the charge determining ions are H+ and OH- which establish the charge on the particle by protonating or deprotonating the MOH bonds on the surface of the particle. M-OH + H M-OH + OH-

M - OH2+

(1.11)

M – O- + H2O

(1.12)

The ease with which the protons are added or removed from the oxide depends on the metal atom. The pH at which the particle is neutrally charged is called the Point of Zero Charge (PZC). At pH greater than PZC equation (1.12) predominates and the particle is negatively charged, whereas at pH less than PZC equation (1.11) 18

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gives the particle a positive charge. The magnitude of the surface potential depends on the departure of the pH from the PZC, and that potential attracts oppositely charged ions (counter ions) that may be present in the solution.

Fig. 1.7 Electrostatic stabilization According to the standard theory, from Fig.1.8. the potential drops linearly through the tightly bound layer of water and counter ions, called the Stern layer. Beyond the Helmholtz plane h=H, that is, in the Gouy layer, the counter ions diffuse freely. In this region the repulsive electrostatic potential of the double layer varies with distance from the particle, h, approximately according to

VR

e

K (h H )

,h

H

(1.13)

Where1/K is called the Debye Huckel screening length. When the screening length is large (i.e. K is small), the repulsive potential extends far from the particle. This happens when the counterion concentration is small. When counterions are present, the potential drops more rapidly with distance. Since the repulsive force is propotional to the slope of the potential,

FR

dVR dh

Ke

K (h H )

(1.14)

The repulsive force increases with small additions of electrolyte. (i.e FR increases with K). Large amounts of counter ions collapse the double layer. As the concentration of counter ions increases, the double layer is compressed because the same numbers of charges are required to balance the surface charge and they are now available in a smaller volume surrounding the particle. On further increase in the concentration of counter ions , the double layer repulsions are reduced to the point that net particle potential is attractive and the colloid will coagulate immediately. When an electric field is applied to a colloid, the charged particles move 19

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towards the electrode with the opposite charge. This phenomenon is called electrophoresis. When the particle moves it carries along the adsorbed layer and part of the cloud of counter ions, while the more distant portion of the double layer is drawn towards the opposite electrode. The slip plane or plane of shear separates the region of fluid that moves with the particle from region that flows freely. The rate of movement of particles in the field depends on the potential at the slip plane, known as zeta potential. The pH at which zeta potential is zero is called the isoelectric point (IEP). The stability of the colloid correlates with zeta potential to be around 30-50 mV [2].

Fig. 1.8 Schematic of Stern and Guoy layers. Surface charge on particle is assumed positive ii) Steric stabilization Sols can be stabilized by steric hindrance. For example when short chain polymers are adsorbed onto the surface of particles. There are two components to this stabilization energy. As the sol particles approach one another, the adsorbed polymer loses configurational entropy. This raises the Gibb’s free energy of the system, which is equivalent to the development of a repulsive force between the particles. In addition, as the polymer layers overlap the concentration of the polymer in the overlap region increases. This leads to local osmotic pressure and a repulsive

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force between the particles [19]. A schematic representation of steric stabilization is provided in Fig. 1.9.

Fig. 1.9 Steric Stabilization iii) Stability by solvation Sol stability coming from energy of solvation is most effective in aqueous system. Energy of solvation is the energy required to disrupt the ordered layer of solvent surrounding the sol particles and to desolvate the surface to allow the particles to come into contact with one another. Because of the energy of solvation, lyophilic sols tend to be more stable than lyophobic sols [20]. 1.10.3 Gel A gel is a porous three dimensionally inter-connected solid network that expands in a stable fashion throughout the liquid medium and is only limited by the size of the container [21]. Gel results when the sol loses its fluidity. An important criterion for gel formation is that at least part of the solvent is bound. If the solid network is made up of colloidal particles, the gel is said to be colloidal (particulate). If the solid network is made up of sub colloidal chemical units the gel is called polymeric. In particulate gels, the sol gel transition is caused by physio-chemical effect and in the latter by chemical bonding [20]. The Schematic representation of sol-gel process is shown in Fig. 1.10. 1.10.4 Sol-Gel Precursors The based on the data concerning the development of sol-gel technologies, one can say that a very important moment is the choice of appropriate precursors. Most often these are alkoxides, soluble metal salts, polymers, colloids which depending on their nature, may be combined with suitable solvents, and the aggregation processes stimulating solid phase formation can be controlled. Fig. 1.11 21

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shows schematically the most used variant of the sol-gel process. The more general interpretation is given by Kakihana [22], according to which a chemical process starting from solutions and leading to a solid phase without a precipitate is a sol-gel process even if the system does not represent an infinite solid network. One of the methods leading to colloid dispersions (sols) is based on inorganic salts, water and occurrence of hydrolysis processes at a definite pH. A classical example is the formation of gel of SiO2 whose detailed description is given in the monograph of Iler [23]. Metal salts and alkoxides are the two main groups of precursors. The general formula of metallic salts is MmXm where M is the metal, X is the anionic group. M(OR)n is the general formula of alkoxides. Sol-Gel precursors undergo chemical reactions both with water and other species present in the solution.

Fig. 1.10 Schematic Representation of Sol-Gel Process

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Fig. 1.11 Different routes of the sol-gel processing 1.10.5 Application of Sol-Gel Process Monoliths, fibers, films and nano-sized powders obtained directly from the gel state combined with compositional and microstructural control and low processing temperature finds applications in various fields. Thin films and coatings find applications for optical, electronic, protective and porous thin films or coatings. Monoliths find applications as optical components, transparent super insulation and ultra low expansion glasses. Powders, grains and spheres used as ceramic precursors or abrasive grains. Fibers drawn from viscous sols are used primarily for reinforcement or fabrication of refractory textiles. Gels can be used as matrices for particle-reinforced composites and as hosts for organic, ceramic or metallic phases. Porous gels and membranes found application in filtration, separations, catalysis and chromatography. 1.10.6 Advantages of sol-gel process Advantages of sol-gel process are given below [24]. _ Increased chemical homogeneity in multicomponent system. _ High surface area for the gels or powders produced. _ High purity can be maintained because of the absence of grinding and pressing steps. _ A range of products in the form of fibers, powders, coatings and spheres can be prepared with relative ease starting from simple solutions. 23

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_ Low temperatures for sol-gel process, saving energy and minimizing evaporation losses. _ Gels can be molded in the shape of the final desired object, with dimensions enlarged to allow for shrinkage during drying and sintering. 1.10.7 Disadvantages of sol-gel process _ Hydrolysable organic derivatives of the metals used in sol-gel processing are expensive [24]. _ The moulded body tends to crack during drying and sintering due to considerable shrinkage [24]. _ Long processing times [25].

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References: [1] T. Graham, J. Chem. Soc., 17 (1864) 318. [2] C. J. Brinker, G. W. Scherer, “Sol-gel science: the physics and chemistry of sol-gel processing “, Academic Press, New York (1990) [3] G. I. Brown, “Introduction to Physical Chemistry”, 3 ed. Longman Group (F.E.) Ltd. Hong Kong (1983) [4] A. Sheludko, ‘Colloid Chemistry’, Elsevier, Amsterdam, (1966) [5] P.A. Rehbinder and G.J. Fuchs (eds.): “Uspekhi Kolloidnaya Khimii (Advances in Colloid Chemistry)”, Nauka, Moscow (1973). [6] R. J. Hunter, Foundations of Colloid Science, 2nd edition, Oxford University Press, (2001). [7] D. V. S. Jain, S. P. Jauhar "Physical chemistry: Principles and problems", Tata Mc Graw Hill Publishing Company Ltd., New Delhi, (1988). [8] H. C. Hamker, Rec, Trav. Chim., 55 (1936) 1015. [9] J. Lyklema, Disc. Faraday Soc., 52 (1971) 318. [10] M. V. Smoluchowski, Z. Phys. Chem., 92 (1917) 129. [11] D. A. Fridrikshberg , "A Course in Colloid Chemistry" Mir Publishers, Moscow (1984). [12] J. Escobedo, G.A. Mansoori, AIChE Journal, 42(5) (1996) 1425 [13] G. Frens , J. Th. G. Overbeek, J. Colloid. Interface Science, 38 (1972) 376 [14]E. Matijevic, “Twenty Years of Colloid and Surface Chemistry”, the Kendall Award Addresses, (eds. K.J. Mysels, C.M. Samour and J.H. Hollister) Amer. Chem. Soc., Washington D.C. (1973) p 283 [15] A.C Pierre, Introduction to Sol-Gel Processing, Kluwer Academic publishers, The Netherlands (1998). [16] P.C. Hiemenz, R. Rajagopalan, ‘Principles of Colloidal and Surface Chemistry’, Marcel Dekker, New York (1997). [17] C.K. Narula, ‘Ceramic Precursor Technology and its Applications’, Marcel Dekker, New York (1995). [18] G.Y. Onoda and L.L. Hench eds. Ceramic Processing Before Firing, John Wiley and Sons, New York (1978). [19] C.W. Turner, Am. Ceram. Soc. Bull. 70 (1991)1487. [20] C.F. Baes, R. E. Mesmer, ‘The Hydrolysis of Cations’, Wiley, New York (1976). 25

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[21] Y.O. Roizin, S.A. Gevelyuk, L.P. Prokopovich, D.P. Savin, E. Rysiakiewicz-Pasek, K. Marczuk, J. Porous Mater. 4 (1997) 151. [22] M. Kakihana, J.Sol Gel Sci. Tech. 6 (1996) 7. [23] R. K. Iler, ‘The Chemistry of Silica’, Wiley Interscience, Publ. N. Y. (1979). [24] H. Schmidt, A. Kaiser, M. Rudolph and A Lentz in “Science of Ceramic Chemical Processing”, ed. By L. L. Hench and D. R. Ulrich, Wiley, New York (1986). [25] B.E. Yoldas, J. Mater. Sci. 21 (1986) 1080.

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