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.
(written by Takatsugu SHIMOAKI)
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