Crystallization

Definition:

It is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas.

Crystallization is also a chemical solid-liquid separation technique in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.

Process of crystallization:
There are two major events in the process of crystallization

a. Nucleation
b. Crystal growth

Nucleation:
A step where the solute molecules dispersed in the solvent start together into clusters on the nanometer scale. These become stable under the current operating conditions.
However, when the slucters are not stable they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei.
It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal growth.

Crystal growth:
It is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.
Supersaturation:
Nucleation and crystal growth continue to occur simultaneously while the supersaturation exists.

Supersaturation is the driving force of the crystallization hence the rate of nucleation and growth is driven by the existing supersaturation in the solution.

Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete.

Polymorphism:
Many compounds have the ability to crystallize with different crystal structures a phenomenon called polymorphism.

Each polymorph is in fact a different thermodynamic solid state crystal polymorphs of the same compound exhibit different physical properties such as dissolution rate, shape and melting point etc.

So, polymorphism is of major importance in industrial manufacture of crystalline product.

Artificial method for crystallization:
For crystallization to occur from a solution, it must be supersaturated. This can be achieved by solution cooling, addition of a second solvent to reduce the solubility of the solute (techniques known as antisolvent or drawn out), chemical reaction or change in pH being the most common methods used in industrial practice.

Other methods such as solvent evaporation can also be used.

Applications:
Crystal production such as powdered slat for food industry, silicon crystal wafer production and production of sucrose from sugar beet, where the sucrose is crystallized out from an aquous solution.

Purification:
Crystallization separates out a product from a liquid (feedstream) often in extremely pure form by cooling the feedstream or adding precipitants which lower the solubility of the desired product so that it forms crystals.

Well formed crystals are expected to be pure because each molecule or ion must fit perfectly into the crystal as it leaves the solution.

Apparatus for crystallization:
Tank crystallizers:
Saturated solutions are allowed to cool in open tanks. After a period of time the mother liquid is drained and the crystals removed. In this method, nucleation and size of crystal are difficult to control. Labor costs are high.

Scrapped surface crystallizers:
One type of scraped surface crystallizer consists of Swensen-Walker crystallizer consisting an open of an open trough 0.6 meter wide with a semicircular bottom having a cooling jacket outside.

A slow speed spiral agitator rotates and suspends the growing crystals on turning the blades pass close to the walls and break off any deposits or crystals on the cooled wall.

Adsorption

Definition:

“it is a phenomenon in which accumulation of a substance at the boundary on interface between the hetergenous phases takes place.”

Explanation:
It is difficult from absorption, as the absorption is the distribution of a substance throuth the bulk solution while adsorption is a surface phenomenon.

Sorption:
It is sometimes very difficult to define clearly the interface of highly porous solids, so for these system the term sorption is used as we cannot distinguish wether it is adsorption or absorption.

The substance that is attached to the surface of the solid is called adsorbate and the surface on which it gets adsorption is called adsorbent.

Occurance:
Adsorption can occur on following interfaces:

• Solid/Liquid
• Solid/gas
• Liquid/gas
• Liquid/liquid

Since adsorption is a surface phenomenon. The most effective adsorption are those with high surface area e.g. finely divided solids.

Positive adsorption:
Adsorption shows the ratio of a substance at the interface and the bulk phase if the concentration of the substance at the interface is greater. Than the concentration of the substance in bulk phase then it is called as positive adsorption.

Negative adsorption:
If the volume concentration of substance is higher than the concentration of bulk is known as negative adsorption.

Types of adsorption:
There are two types of adsorption:

Physical adsorption

Negative adsorption

1. Physical adsorption:
In physical adsorption the adsorbate is attached with adsorbent by Vander Waals or Electrostatic weak forces and it is characterized by low heat of adsorption.

Physical adsorption of gases is common at low temperature and high pressure. The gas in the adsorbent layer is in equilibrium with the gas molecule. In the bulk gas the equilibrium depends upon the nature of the adsorbent.

2. Chemical adsorption:
This involves the chemical combination of adsorbate at the surface of adsorbent. It is characterized by high heat of adsorption and unlike physical adsorption is irreversible. In many cases the chemical adsorption is slow because the molecule has to acquire an energy of interaction before they can react with the adsorbent, the rate of uptake will increase with increase of temperature.

Factors affecting the adsorption:

Solubility of adsorbate:
The adsorption is inversely proportional to the solubility of the adsorbate in the adsorbent.

Adsorption α 1/Solubility

pH:
it does not effect the adsorption directly pH of the solution affect the degree of ionization.

Usually the drug with a single molecule has more adsorption.

Nature of the adsorption:
Nature of the adsorbent have major effect on the adsorption by increasing the surface area, the adsorption rate could be increased. It can be increased by making it porous or finely divided.

Temperature:
Adsorption is an Exothermic process so increase in temperature will decrease. The adsorption and vice versa.

Pressure:
Adsorbed amount of adsorbate is directly proportional to the pressure applied.

Levigation

Definition:


Levigation is the process of grinding an insoluble substance to a fine powder.

Or

Levigation is the grinding to a powder of a moist or hard substance.

Explanation:

The material is introduced into the mill together with water in which the powdered substance remains suspended and flows from the mill as a turbid liquid or thin pastes. According to the amount of water employed. There is no loss of material as dust nor injury or annoyance to the workmen.

Further any (substances) soluble impurities in the substance are dissolved and the product thereby purified.

Advantages of levigation:

The greatest advantages of levigation is the facility it affords for the subsequent separation of the products into various grades of fitness because of slower. Subsidence of the finer particles from suspension.

The turbid liquid flows into the first of a series of tanks and is allowed to stand for a time. The coarsest and heaviest particles quickly subside leaving the finer material suspended in the water which is drawn from above the sediment into the next tank.

The liquid is passed from tank to tank remaining in each longer than it remained in the proceeding since the finer and higher the particles the more time is necessary for this deposition. In some cases a dozen or more tanks may be used and the process then becomes exceedingly slow.

The term levigation is now often applied to more sedimentation a substance being simply stirred up in water without previous wet grinding.

Calcination

Definition:
The process of heating a substance to a high temperature but below the melting point or fusing point causing loss of moisture reduction or oxidation and dissociation. Into simpler substances, the term was originally applied to the method of driving off carbon dioxide from limestone to obtain lime.

Calcination is also used to extract metals from ores.

Or

To heat a substance to a high temperature but below the melting point or fusing point causing loss of moisture, reduction or oxidation and the decomposition of carbonates and other compounds.

Explanation:
Calcination also referred to as calcining a thermal treatment process applied to ores and other solid materials in order to bring about a thermal decomposition phase transition or removal of a volatile fraction. The calcinations process normally takes place at temperatures below the melting point of the product materials.

Calcination is to be distinguished from roasting. In which more complex gases, solids reactions takes place between the furnace and the solids.

Calcination reactions:
Calcinations reaction usually takes place at or above the thermal decomposition temperature or transition temperature. This temperature is usually defined as the temperature at which the standard energy of reaction for a particular calcination reaction is equal to zero.

Example:
Examples of chemical decomposition reactions common in calcination processes and their respective thermal decomposition temperature include
CaCO3 = CaO + CO2 : 484ºC

Occurance:
Calcination occur under layers of hot volcanic ash.

Physical properties:
The physical properties which involves bulk density, total pore volume, and the pore size distribution of the calcines prepared under different conditions were determine by using a Mercury porosimeter.
It was found that the physical properties of calcines were dramatically affected by the calcinations conditions, at high calcination temperature, because of sintering and shrinking affects, a decrease in properties. An increase in bulk density and average pore radius were observed.

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Implants

This represents the form of radiation therapy or radiotherapy involving placement of radioactive entities inside the body of the patient close to the tumor.

An artificial body part placed in tissues (often to replace missing body parts) with the help of surgery.

Fusion protein

Fusion proteins are produced by the combination of two genes or proteins or peptides. They can exist naturally or can be prepared in the laboratory.

PEGylated dosage forms

PEGylated dosage form refers to the attachment of the polythylene glycol polymer chains to a drug concealing the agent from the immune sytem of the body reducing immunogenicity resulting in prolonged circulatory time.

Examples:
PEGylated interferon alpha is in use for the treatment of hepatitis B and C.
PEGylated liposome having doxorubicin is used in the treatment of cancer.

Ophthalmic Inserts

Ophthalmic inserts are solid or semisolid sterile preparations for placement in the conjunctival sac of the eye. These are sustained release drug delivery systems for the eye.

Osmotic Systems

Osmotic systems make use of osmotic pressure for controlled delivery of active ingredients.
This method can be used both for systemic and targeted drug delivery.

Mechanism:
Osmotic pressure as a result of imbibitions of the fluid by the osmotic agents directly affects the rate of drug delivery from osmotic systems such as osmotic pumps.

Components of Osmotic Pump:
1. Drug
2. Osmotic agent or osmogent such as magnesium sulfate, sodium chloride or sodium bicarbonate
3. Semipermeable membrane
4. Plasticizers such as polyethylene glycols, triethyl citrate or ethylene glycol monoacetate

Factors affecting Osmotic drug delivery:
1. Size of the opening of delivery point
2. Solubility of the components
3. Intrinsic character of the rate controlling membrane
4. Osmotic pressure of the components

These factors are important to develop an optimized and desired osmotic system.

Mucoadhesive system

Mucoadhesion refers to adhesion with with moist lining in the body passages of mammals containing cells which secrete mucus and opens to the external environment.

Mucoadhesive system refers to the adhesion of natural or synthetic polymer and soft tissues of mucous membrane for an extended period. Cross linked polymer device, containing drug, will adhere to biological membrane and will transfer the drug to the body at the given site, decreasing frequency of administration.

Theories of mucoadhesion:
Following theories have been proposed in the mucoadhesion of the drug:
1. The wetting theory
2. The diffusion theory
3. The electronic theory
4. The adsorption theory
5. The cohesive theory
6. The Fracture theory
7. The mechanical theory

Controlled release microchips

It represents the slow and controlled release of medicaments from microfabricated device. These are the types of small programmable devices.

In this microchip, micrometer scale pumps, valves and flow channels are incorporated or microfabricated into the active devices. Microchip of solid state silicon can be used in this technique. These microchips have following properties:

1. They can store a large amount and number of chemicals or medicaments
2. They have the ability of controlling the time of release of chemicals or medicaments
3. They have the ability of controlling the rate of release of chemicals or medicaments

These microchips have a tiny power supply. The control of microchips are done by remote control, microprocessors and/or biosensors.

The chemical release takes place through electrochemical dissolution of membranes of thin anode covering microreservoirs which are filled with chemicals in the form liquid, solid or gel.

Uses:
This microchip technology has found uses in many areas such as chemical detection, drug delivery, medical diagnostics and combinatorial chemistry.

References:
January 20, 1999. A commentary on the controlled release microchip. http://web.mit.edu/newsoffice/1999/microchipcom.html. Accessed March 28, 2011.
Santini, J. T. Jr.; Cima, M. J.et al. 1999. A controlled-release microchip. Nature, 397, Pages 335-338.

Santini, J. T. Jr.; Richards, A. C. et al. 2000. Microchips as Controlled Drug-Delivery Devices. Wiley-VCH, 39, Pages 2396-2407.

Phonophoresis

It represents the use of ultrasound for the delivery of drugs through the skin. Through this process topical application of anti-inflammatory drugs and analgesics can be increased.

Iontophoresis

A therapeutic strategy or technique in which a small amount of electric current is used locally for introduction of of medicine or some other chemicals (heavy metal ions) into the tissues of the body through the skin.

In this process, bipolar electrodes are used to create electric potential and ionized medication is introduced into the body. Moreover, sweat glands temporarily turn off during this procedure.

Anesthetic machine

A machine or device helping in the administration of anesthesia.

Oxygen Mask

A device used for the delivery of oxygen from a storage tank to the lungs.

Dry Powder Inhaler

It is a device for inhalation of the dry powder used to treat respiratory disease.

Pessary

A small medical device made of silicone or plastic and is inserted into the rectum or vagina. It may be used as a contraceptive or for the support of uterus.

Douche

It refers to a device with detachable nozzles used for introducing a stream of water into the body cavity for medical or hygienic purposes.

Enema

It refers to the administration of a liquid through anus in order to evacuate it. It is usually used for the treatment of constipation.