11 Quantum Dot Preparation through colloidal methods I

S.S. islam

1. Basics of Colloid

A colloid is a mixture which contains microscopic dispersion of insoluble particles suspended in a solvent. In some cases, the dispersed material itself is termed as colloid, and the mixture containing this material suspended in a solvent is termed as colloidal suspension. A colloid is different from a solution in the sense that while in a solution solute and solvent constitute the same phase, a colloid has a dispersed phase (suspended particles) and a continuous phase (suspension medium). A mixture can be termed as colloid if it does not settle or takes a vey long time to settle.

The diameter of the dispersed phase can range from 1 to 1000 nm. These particles are easily observable under an optical microscope. However, for small sized particles (diameter less than 500 nm), an electron microscope is necessary. Homogeneous mixtures having dispersed phase of such dimensions is usually termed as colloidal aerosols, emulsions, foams, dispersions, or hydrosols. The surface chemistry greatly affect the dispersed phase particles.

Colloids may either be translucent (Tyndall effect), or opaque or slightly colored. The study of colloidal suspensions comes under interface and colloid science and the field was formally introduced by Thomas Graham in 1861.

Figure 1 Milk is an emulsified colloid of liquid butter fat globules dispersed in water based solvent.

IUPAC defines colloid as a synonym of colloidal solution. Colloidal is a state of subdivision so that the molecules or clusters of molecules dispersed in a medium have at least one dimension between 1 nm and 1 μm. It may also be defined as a system wherein discontinuities occur at distance on the order of 1μm to 1nm.

2. Classifications of Colloids

It is usually difficult to measure the size of the dispersed particles. Additionally, the colloids appear much like solutions. For these two reasons, colloids are classified and characterized by their physicochemical as well as transport behaviors. For instance, if a colloid consisting of solid phase dispersed in a liquid is passed through a membrane, the solid particles do not diffuse across the membrane, while in a true solution, the dissolved ions or any molecules also diffuse through the membrane. Due to size exclusion, particles of a colloid do not pass through the pores of ultra fine membranes having pores diameter smaller than the diameter of the dispersed particles. As the size of the pores of the membrane is decreased, the concentration of the colloid particles present in the filtered liquid also decreases. Thus, the measured concentration of a truly dissolved species depends upon the experimental conditions used to separate the dispersed state from the liquid. This has applications in solubility studies of readily hydrolyzed molecules, e.g., Al. Colloidal

solutions are classified as:

On the basis of the interactions between dispersed phase and dispersion medium, colloidal solutions are further divided into hydrophilic and hydrophobic colloids. Hydrophilic colloids are water-loving where the particles of the colloid are attracted to water. These are also termed as reversible sols. Hydrophobic sols, on the other hand, are not water-loving, and there is repulsive interaction between the particles of the colloid and water. These are irreversible sols.

While differentiating solutions as homogeneous or heterogeneous solution, a colloidal suspension may be considered as a homogeneous mixture. This is because in such cases, the distinction between the dissolved and particulate matter can be used to classify the solutions.

3. Interaction Between Particles

The forces responsible for interactions of colloidal particles can be summarized as:

a. Excluded Volume Repulsion: This obscures any overlap between hard particles.

b. Electrostatic Interaction: Usually the colloid particles carry electrical charges. This causes attractive/repulsive Coulombic interactions among particles. These interactions are affected by the charges of both the dispersed phase and dispersion medium, and also on the mobility of these phases.

c. Van der Waals Forces: These occur because of the interactions between two dipoles, which may be either permanent or induced. Although the colloid particles do not have permanent dipoles, any variations in electron density may cause temporary dipoles in the particles. A temporary dipole in one particle can induce dipole in the nearby particles as well. Both these dipoles (temporary and induced) are attracted towards each other, owing to the van der Waals forces. These forces are always present (unless the dispersed and continuous phases have same refractive index). These forces are short range and are attractive.

d. Entropic Forces: Second law of thermodynamics suggests that a system progresses to the state of maximum entropy. This results in some net force between almost all particles.

e. Steric Forces: These forces modulate the inter-particle forces between polymer coated surfaces or in solutions having non-adsorbing polymer. They result in either steric repulsive forces (mostly of entropic origin) or attractive depletion forces between them. These effects are of high importance in superplasticizers wherein they can boost the workability of concrete and also reduce water content.

Colloid Preparation

The most popular methods to prepare colloid are:

a. Dispersing large particles to colloidal dimensions via milling, mixing, etc.

b. Condensing very fine particles to produce larger colloid particles via precipitation, condensation, etc. This approach is very frequently used to prepare colloids of silica and gold.

Stabilization or Peptization

The stability of a colloidal suspension is measured as the ability of the particles to remain suspended in the solution under equilibrium conditions. Figure 2 is the schematic representation of stable and unstable colloids.

Figure 2 Schematic representation of stable and unstable colloids.

Aggregation and sedimentation processes severely affect the stability of colloids. These processes are the result of the colloid’s tendency to minimize the surface energy. The colloids can be stabilized by decreasing the surface tension which can avoid the aggregation of the particles in the colloid.

Aggregation is the result of the interactions between the particles of the colloid. When attractive forces (e.g., van der Waals, etc.) outnumber the repulsive forces (e.g., electrostatic), particles aggregate and form clusters/lumps.

The colloids can be effectively stabilized against clustering via electrostatic and steric stabilizations. These are explained below:

a. Electrostatic Stabilization: This approach makes use of the mutual repulsion between like charges. Usually, the charge affinity is different for different phases, thus, electrical double layer is formed at the interface. These effects are highly enhanced in colloid suspensions, wherein small-sized particles have very large surface areas. In stable colloids, dispersed phase are so light that buoyancy and kinetic energy cannot overcome electrostatic repulsions between the charged layers of dispersed phase.

b. Steric Stabilization: In this approach, the particles are coated with polymers to prevent the particle from coming in the range of attractive forces.

Usually a hybrid of these two methods is used. Both these mechanisms to minimise particle aggregation work by enhancing the repulsions among the particles. However, both these approaches do no address the problem of sedimentation/floating directly.

Sedimentaion and floating (it is less common) arise due to the differences in densities of dispersed phase and the dispersion medium. As this difference in densities increases, sedimentation occurs at a faster rate.

Stabilization via Gel Network

Stabilization by gel formation is a very popular method to prepare stable colloids which are free from both aggregation as well as sedimentation. In this approach, a polymer is added to the colloid. This polymer forms a gel network and is characterized by shear thinking properties. Typical examples include guar gum and xanthan.

Figure 3 Comparison between steric and gel network stabilization mechanisms.

The stiffness of the polymer matrix hinders the particle settlement/precipitation by entrapping them within the matrix. Additionally, the long network of polymeric chains provides both steric and electrostatic stabilizations to keep particles dispersed. The rheological shear shining behavior is advantageous to prepare suspensions and their handling, since low viscosity at high shear rates allow deagglomeration, mixing, and flow of suspensions.

Destabilization

In unstable colloids, particles aggregate and form clusters owing to the inter-particle attractive forces. This property of the colloids can be used to produce photonic glasses. The following methods can be employed to achieve this:

a. Removing the electrostatic barrier which prevents particle aggregation by adding a salt to the colloid or by changing its pH to effectively neutralize the surface charges. This results in the removal of repulsive forces among the particles which keep them suspended. Thus, van der Waals attractions come into play and the process of particle aggregation/coagulation initiates.

b. A charged polymer flocculant can be added to the colloid. The flocculants bridge individual colloid particles via attractive forces.

c. Non-adsorbed polymers termed as depletants can also induce coagulation of the particles by causing entropic effects.

d. Any physical deformation to the particle increase van der Waals attractions, thereby causing aggregation of particles in specific directions.

In low-volume fraction colloids, the clusters of the particles either float to the surface or precipitate at the bottom of the suspension. The gravitational force in this clusters overcomes the Brownian forces (which keep the particles suspended). In higher volume fraction, colloidal gels are formed which have viscoelastic properties. Such gels (e.g., bentonite, toothpaste, etc.) flow as liquids under shear, and regain their shape as the shear is removed. This behaviour is often seen in toothpaste: it squeezes out of the tube like a fluid, and remains on the toothbrush as it is.

Monitoring Stability

The stability or the dispersion state of a colloid can be monitored via multiple light scattering coupled with vertical scanning. It can be used to analyse concentrated colloids without needing dilution. The incident light is backscattered by the particles of the suspension. The backscattered intensity is proportional to the size and volume fraction of the dispersed phase. Thus, the variations in concentration as well as changes in size can be readily analysed.

Accelerating methods for shelf life prediction

The destabilization process can be rather long (on the order of few months to years). Thus, certain accelerating techniques can be used to reduce the development time for designing new products. Among these techniques, thermal processes are most common and involve increasing the temperature to accelerate destabilization. However, the temperature is kept below the critical temperatures of phase inversion or chemical degradation. Temperature influences the viscosity, interfacial tension (in non-ionic surfactants), and the interactions in the system. High temperature methods usually simulate the real life conditions likely to be encountered by the product during its operating life (for example, tube of sunscreen cream kept inside a car in summers). These techniques also accelerate destabilization process by upto 200 times.

Other techniques employed to increase destabilization include mechanical acceleration such as vibration, agitation, centrifugation, etc. They apply different forces on the product to push the particles against each other, thereby causing film drainage. Some emulsions do not coagulate under normal gravity, and require artificial gravitational forces to be applied. Furthermore, different populations of particles can be segregated by centrifugation and vibration.

As an Atomic Model System

Colloidal suspension can also be as model systems for atoms. Microscaled colloid particles are large enough to be observed by optical techniques including confocal microscopy. Forces governing the structure and properties of matter (excluded volume interactions or electrostatic forces), also affect the structure and properties of colloids, such that, the methods employed to model ideal gases are also applicable on hard sphere colloidal suspensions. Additionally, phase transitions in colloidal suspensions can be examined in real time by optical techniques, and are analogous to phase transitions in liquids. Also, optical fluidity can be used to control colloidal suspensions.

Colloidal Crystals

Colloidal crystals, analogous to atomic/molecular crystal, are highly ordered array of particles. They possess long range orders, generally of the order of few mm to cm. Colloidal crystals also occur naturally and the most popular example is opal (a precious stone), wherein closely packed domains of amorphous colloidal spheres of silicon dioxide (SiO2 or silica), cause bright colours of the stone. These spherical particles precipitate in highly siliceous pools to produce highly ordered arrays after sedimentation and compression under hydrostatic and gravitational forces. The periodic arrays of submicron sized spherical particle offer identical arrays of interstitial voids. These voids work as diffraction grating to the light waves in visible region. In this manner, diffraction and constructive interference of visible light produces brilliant colours in these naturally occurring crystals. This is analogous to the Bragg’s diffraction of X-ray used in X-ray diffraction technique in crystalline solids. Over the years, numerous facile synthesis routes have been developed to artificially produce synthetic mono-disperse colloids (both polymers as well as minerals).

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