Dissociation surfactant molecules

IIA. The Surface Density of Surfactant. The surface density of surfactant is relevant for both the double layer (caused by the surface charge density generated via the dissociation of the surfactant adsorbed at the interface) and the hydration interaction (caused by the ion pair density of the non-dissociated surfactant molecules). The surface density of an anionic surfactant, I, will be related to the saturation surface density, I , via the Frumkin adsorption isotherm... 

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

Surfactants having an appropriate hydrophobic/hydrophilic balance (sodium bis(-2-ethylhexyl)sufosuccinate, or AOT, for example) undergo concentration-dependent self association in apolar solvents to form reversed or inverted micelles (Fig. 33) [256-262]. Reversed micelles are capable of solubilizing a large number of water molecules (AOT reversed micelles in hexane are able to take up 60 water molecules per surfactant molecule, for example). Reversed-micelle-entrapped water pools are unique they differ significantly from bulk water. At relatively small water-to-surfactant ratios (w = 8-10, where w = [H20]/[Surfactant]), all of the water molecules are strongly bound to the surfactant headgroups. Substrate solubilization in the restricted water pools of reversed micelles results in altered dissociation constants [256, 257, 263-265], reactivities [256, 258, 266], and reaction products [267]. 

A surfactant molecule that can dissociate to yield a surfactant ion whose polar group is negatively charged. Example sodium dodecyl sulfate, CH3(CH2)iiS04 Na+. 

The electrostatic contribution to the energy when the only ions present in the solution are those of the counterions of the surfactant molecules, hence in the absence of an added electrolyte, is calculated by integrating the electrostatic pressure from infinity to the distance xi. Denoting the surface charge o, = a, e/A, where a, is the degree of dissociation and A is the area per surfactant molecule adsorbed on the interface, the electrostatic energy per unit area is given by (see Appendix B)... 

It will be assumed that the surfactant molecules in the aqueous phase are completely dissociated and that then-presence in the water phase does not affect the potential given by eq B.2, since their concentration is very low. The condition of equilibrium and the mass balance of the surfactant in the water phase lead to the following expression for the concentration (XXf of surfactant ions near the interface, as a function of the average concentration of the surfactant in the water phase... 

Let us consider that the surface charge arises via the dissociation of surfactant molecules adsorbed on the interfaces and that the surface dipoles are due to the undissociated surfactant molecules adsorbed. In this case, the electric field induced by the surface dipoles is opposite to that generated by the surface charge (dm < 0). Hence, the presence of surface dipoles actually decreases the repulsion. The effect is illustrated in parts c and d of Figure 2, for dm = —0.2 D. Therefore, in this case the total repulsion cannot be obtained (as usually assumed) by adding two independent terms, a double layer force due to the surface charges and a hydration force due to the surface dipoles. 

It will be assumed that the surfece charge is due to the dissociation of surface groups (for instance dissociation of surfactant molecules).8 9 In general, the electrolyte counterions are the most abundant ions in the vicinity of the surfece and therefore they can control the surface charge via reassociation. Denoting by cE the concentration (in the reservoir) of an 1 1 electrolyte, by N the number of sites per unit area, and by x the fraction of dissociated sites, the dissociation equilibrium provides the expression... 

Micelles. It is still a matter of debate whether the dissociation of surfactant molecules from micelles would lead to retardation of adsorption. It appears very unlikely that the time scale involved would be longer than a second. On the other hand, some surfactants have a very small solubility in water (e.g., phospholipids), and they are often present in small solid lumps or as vesicles. In such a case, it may take a very long time before surfactant molecules have reached the interface. 

A popular representation of spherical micelles was devised by Hartley (26). As indicated in Fig. 1, the Hartley model of, e.g., an anionic micelle exhibits a spherical electric double layer composed of bulky, hydrated anionic heads of surfactant molecules and their counterions in the aqueous phase, while the hydrophobic tails, visualized as sticks, form a hydrocarbon-like micellar interior. Because of the high surface charge density of the micelle, there is only little electrolytic dissociation of counterions. The Hartley model explains the low conductivity of micellar solutions and the way surfactants work as detergents by solubilizing (i.e. incorporating) hydroi obic substrates. The model fails to explain certain NMR and fluorescence data that demonstrate some contact of... 

When only one single surface active ion is formed by dissociation of a surfactant molecule we get even a simpler form... 

Vj stoichiometric coefficients of the dissociation reaction of a surfactant molecule V total number of ions (including micelles) arising as a result of dissociation of a surfactant molecule... 

A nanodroplet contains surfactant molecules, of which are dissociated. Due to electroneutrality the numbers of the negatively charged surfactant molecules, N, and the number of positively charged counterions, FT, are equal, i.e., IP = N = N. ... 

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