Critical micelle concentration surface forces

FORMATION. Aqueous solutions of highly surface-active substances spontaneously tend to reduce interfacial energy of solute-solvent interactions by forming micelles. The critical micelle concentration (or, c.m.c.) is the threshold surfactant concentration, above which micelle formation (also known as micellization) is highly favorable. For sodium dodecyl sulfate, the c.m.c. is 5.6 mM at 0.01 M NaCl or about 3.1 mM at 0.03 M NaCl. The lower c.m.c. observed at higher salt concentration results from a reduction in repulsive forces among the ionic head groups on the surface of micelles made up of ionic surfactants. As would be expected for any entropy-driven process, micelle formation is less favorable as the temperature is lowered. 

A microscopic part of a monolayer consisting of monododecylpentaethylene glycol surfactants (C12E5) adsorbed at the water surface was simulated with the AMBER force field and the SPC water parameter set (5). The temperature T was set to 298 K, while the number of molecules, A, was composed of 36 surfactant and 1575 water molecules arranged in a cubic box with periodic boundary conditions. The area per surfactant molecule (A) corresponded to a value of A = 0.55 nm, which is somewhat larger than the critical micelle concentration (CMC) of C]2E5 (A = 0.50 0.03 nm ). 

More recent studies of amine adsorption mechanisms at surfaces have been carried out using atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and SFVS techniques (Bakshi et al. 2004 Castro et al. 1986 C hemyshova et al. 2000 Fuerstenau and Renhe 2004 Paruchuri et al. 2004 Schrodle and Richmond 2008 Subramanian and Ducker 2000 Velegol et al. 2000). AFM studies show that spherical micelles of cetyltrimethylammonium bromide/cetyltrimethylammonium chloride (CTAB/CTAC) surfactants form at silica surfaces near the critical micelle concentration (CMC) (Bakshi et al. 2004). Compared with tertiary and quaternary amines, primary amines, such as DDA, would have a different molecular structure at the silica surface. For example, AFM studies suggest that primary amine forms a featureless bilayer at mica surfaces. 

The summation in Equation 4.1 is carried out over aU components. UsnaUy an equimolecular dividing surface with respect to the solvent is introduced for which the adsorption of the solvent is set zero by definition [4,5]. Then the snmmation is carried ont over aU other components. Note that F, is an excess surface concentration with respect to the bulk F is positive for surfactants, which decreases o in accordance with Equation 4.1. On the contrary, F is negative for aqneous solutions of electrolytes, whose ions are repelled from the surface by the electrostatic image forces [5] consequently, the addition of electrolytes increases the surface tension of water [6]. For surfactant concentrations above the critical micellization concentration (CMC) = constant and, consequently, a = constant (see Equation 4.1). 

The above metastable foams are produced from surfactant solutions near or above the critical micelle concentration (c.m.c.). The stability is governed by the balance of surface forces (see below). Film thickness is comparable to the range of intermolecular forces. In the absence of external disturbances, these foams may stay stable indefinitely They are produced using proteins, long-chain fatty acids or solid particles. 

---