In chemistry, the critical micelle concentration (CMC) is defined as the concentration of surfactants above which micelles are spontaneously formed. Upon introduction of surfactants (or any surface active materials) into the system they will initially partition into the interface, reducing the system free energy by a) lowering the energy of the interface (calculated as area x surface tension) and b) by removing the hydrophobic parts of the surfactant from contacts with water. Subsequently, when the surface coverage by the surfactants increases and the surface free energy (surface tension) has decreased, the surfactants start aggregating into micelles, thus again decreasing the system free energy by decreasing the contact area of hydrophobic parts of the surfactant with water. Upon reaching CMC, any further addition of surfactants will just increase the number of micelles (in the ideal case).

There are several theoretical definitions of CMC. One well-known definition is that CMC is the total concentration of surfactants under the conditions:if C = CMC, (d3F/dCt3) = 0F = a[micelle] + b[monomer]: function of surfactant solutionCt: total concentrationa, b: proportional constants

Therefore, CMC depends on the method of measuring the samples, since a and b depend on the properties of the solution such as conductance and photochemical characteristics. When the degree of aggregation is monodispersion, the CMC is not related to the method of measurement. On the other hand, when the degree of aggregation is multidispersion, CMC is related to both the method of measurement and the dispersion.

CMC is an important characteristic of a surfactant. Before reaching the CMC, the surface tension changes strongly with the concentration of the surfactant. After reaching the CMC, the surface tensions stays more constant.

CMC is the concentration of surfactants in the bulk at which micelles start forming. The word BULK is important because surfactants partition between the bulk and interface and CMC is independent of interface and is therefore a characteristic of the surfactant molecule. In most of the situations like for e.g. in surface tension measurements or conductivity measurements, the amount of surfactant at the interface is negligible compared to that in the bulk and CMC is approximated by the total concentration as is done in most of the textbooks.

There are important situations where interfacial areas are large and the amount of surfactant at the interface cannot be neglected. For example if we take a solution of a surfactant above CMC and start introducing air bubbles at the bottom of the solution, these bubbles, as they rise to the surface, pull out the surfactants from the bulk to the top of the solution creating a foam column thus bringing down the concentration in bulk to below cmc. This is one of the easiest methods to remove surfactants from effluents (foam flotation). Thus in foams with sufficient interfacial area there will not be any micelles. Similar reasoning holds for emulsions.

The other situation arises in detergency. One initially starts off with concentrations greater than CMC in water and on adding fabric with large interfacial area and waiting for equilibrium, the surfactant concentration goes below CMC and no micelles are left. Therefore the solubilization plays a minor role in detergency. Removal of oily soil is by modification of the contact angles and release of oil in the form of emulsion.

The study of the aggregation of lipids (amphiphiles) is known as lipid polymorphism, and forms part of current academic research.

>>>> http://en.wikipedia.org/wiki/Critical_micelle_concentration <<<<<

Surfactant

A surfactant is an amphiphilic molecule that possesses both a hydrophobic group and a hydrophilic group in the molecule itself. The hydrophobic group is derived from the property of affinity for oil, since it is mainly made up of longer alkyl chains. While, the hydrophilic group is ascribed to polar groups which have affinty for water, such as hydroxyl group, ether type oxygen atom and carboxylic group etc. Thus, the surfactant has affinity for both water and oil. In aqueous solution, with an increase in concentration of surfactant, a supontaneous association of molecules makes aggregates termed "micelles" (Fig. 1). The concentration at which surfactants start forming micelle is called the "critical micelle concentration (CMC)". With the aid of micelles, surfactants are used as emulsifiers, dispersants, bubbles, antifoams, and detergents.

Surfactants are classified into four types due to the hydrophilic group; anion surfactant, cationic surfactant, ampholytic surfactant, and nonionic surfactant.

(1) Anionic surfactant When dissociated in water, anionic surfactants become surfactant anions and metalic cations, just like electrolytes. Hydrophobic group of anionic surfactant adosorbs on a hydrophobic surface of substance, generating high detergency and foaming power. Thus. the anionic surfactant is usually used as detergents for clothes and body. (2) Cationic surfactant When dissociated in water, cationic surfactant become surfactant cations and small anions. Since many substances have negative charges on their surfaces, cationic surfactants is adsorbed near there. Thus, cationic surfactants are used as lubricant agents, antistats, fabric care liquid and conditioners. (3) Ampholytic surfactant Ampholytic surfactant is the type of amino acid and changes its charge depending on the pH of aqueous solution. Ampholytic surfactant can be used with other types of surfactant, especially the use with anionic surfactant enhances detergency and foaming power. Ampholytic surfactant is also used as shampoos, fungicides, antistats, fabric softeners and corrosion inhibitors for metal. (4) Nonionic surfactant Nonionic surfactant is the nonelectrolyte type. Generally, hydrophilic group is polyoxyethylene group. Nonionic surfactant is used as detergents for kitchen and clothes, emulsifiers for cosmetics and food additives.

Many studies on the structure and the property of a surfactant have been performed using various electrochemical and optical methods. Infrared (IR) and Raman spectroscopy are well-known methods that can analyze the vibration state of the molecule. Theoretical calculations have been also performed with the development of recent computer techniques, and applied to the analyses of spectra obtained. However, the calculation method has not been established yet: on the calculation process, the parameter used includes user's arbitrarity. Recently, the development of the molecular orbital method was capable of applying a nonempirical calculation. Our purpose is the detailed analysis of vibrational spectra of various surfactant molecules using "density-functional calculation method", comparing the observed IR and Raman spectra.

http://www.ach.nitech.ac.jp/~physchem/taga/1tagag_e.html(written by Takatsugu SHIMOAKI)