Fluorescence in Aqueous Micellar Solutions
One of the earliest principles of physical chemistry which we learn is that ‘like dissolves like.’ Thus polar substances tend to be water soluble (particularly if ionic) and nonpolar substances tend to be oil soluble. Molecules with both polar and nonpolar groups behave in a manner determined by the balance between these groups, the hydrophilic-lyophilic balance (HLB). Such molecules minimize their free energy in solution in two ways. In water, the molecules will at first distribute themselves between the bulk solution and the air-water interface with a predominance at the interface, where the nonpolar group (hydrocarbon tail, say) will be in the air and the polar group in the water. This distribution is given by the Gibbs adsorption isotherm. Second, as the concentration in the water builds up, there will be a critical micelle concentration (which is not particularly sharp) at which the molecules will aggregate, with their nonpolar “chains” together and their polar groups on the outside, so reducing the nonpolar-water interaction while leaving the polar-water interaction intact. Such aggregates are called micelles and are generally spherical, at least at the concentration at which they are initially formed. These micelles are dynamic entities, forming and dissociating in a series of steps on a time-scale of the order of a microsecond. In a hydrocarbon solvent, a similar situation prevails in which inverse micelles form, with the polar groups internal and nonpolar groups external1.
KeywordsCritical Micelle Concentration Sodium Dodecyl Sulphate Constant Ionic Strength Excited Species Nonpolar Group
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