Ionic surfactants osmotic pressure

The contribution of double-layer forces to the osmotic pressure of HIPEs was also investigated [98], These forces arise from the repulsion between adjacent droplets in o/w HIPEs stabilised by ionic surfactants. It was observed that double-layer repulsive forces significantly affected jt for systems of small droplet radius, high volume fraction and low ionic strength of the aqueous continuous phase. The discrepancies between osmotic pressure values observed by Bibette [97] and those calculated by Princen [26] were tentatively attributed to this effect. 

Solutions of highly surface-active materials exhibit unusual physical properties. In dilute solution the surfactant acts as a normal solute (and in the case of ionic surfactants, normal electrolyte behaviour is observed). At fairly well defined concentrations, however, abrupt changes in several physical properties, such as osmotic pressure, turbidity, electrical conductance and surface tension, take place (see Figure 4.13). The rate at which osmotic pressure increases with concentration becomes abnormally low and the rate of increase of turbidity with concentration is much enhanced, which suggests that considerable association is taking place. The conductance of ionic surfactant solutions, however, remains relatively high, which shows that ionic dissociation is still in force. 

This equation is good for ideal solutions. For an ionic surfactant solution, the solution is nonideal even at very low surfactant concentration and gives a highly nonlinear dependence of osmotic pressure on concentration. This is expected because ionic surfactants have a high affinity for the interfaces of solution-vapor, solution-solid, and solution-membrane as well as for themselves (i.e., micellization). 

Figure 6.21 Solution properties of an ionic surfactant as a function of concentration, c. A, Osmotic pressure (against c) B, solubility of a water-insoluble solubilisate (against c) C, intensity of light scattered by the solution (against c) D, surface tension (against log c) E, molar conductivity (against...

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

For ionic surfactant solution the body force tensor, Pb, is not isotropic - it is the Maxwell electric stress tensor, i.e. Pb = f6bEE - i6jE l2, where E = -V is the electric field (Landau and Lifshitz 1960). The density of the electric force plays the role of a spatial body force, f, in the Navier-Stokes equation of motion (3). In the lubrication approximation the pressure in the continuous phase depends on the vertical coordinate, z, only through its osmotic part generated from the electric potential and the pressure in the middle plane (or the pressure, pn, corresponding to the case of zero potential) ... 

The physical properties of surface active agents differ from those of smaller or non-amphipathic molecules in one major aspect, namely the abrupt changes in their properties above a critical concentration [1], Figure 2.1 illustrates with plots of several physical properties (osmotic pressure, turbidity, solubilisation, magnetic resonance, surface tension, equivalent conductivity and self-diffusion) as a function of concentration for an ionic surfactant [1]. 

The influence of the surfactant structure on the nature of the microemulsion can also be predicted from the thermodynamic theory. The most stable microemulsion would be that in which the phase with the smaller volume fraction forms the droplets (the osmotic pressure increases with increasing />). For a W/O microemulsion prepared using an ionic surfactant such as Aerosol OT, the effective volume (hard-sphere volume) is only slightly larger than the water core volume, since the hydro-... 

The CMC can be measured by observing the change of several properties, e.g. surface tension with concentration (Eigure 5.4). Other properties like osmotic pressure and electrical conductivity also change at CMC. The conductivity increases after CMC and this is a useful CMC estimation method for ionic surfactants. Turbidity and detergency also increase up to the CMC. Solubilization also increases above CMC, as many compounds are readily soluble in the micelles. AU of these observations further verify the existence of micelles after CMC and indicate the importance of micelles in cleaning (detergency). 

The spontaneous curvature of surfactant layers can be controlled in many ways see Table 7.1. For ionic surfactants, one can control the contribution coming from the counter-ions by adjusting the concentration of salt. As the electrolyte concentration increases, the ionic atmosphere approaches the oil-water interface (Debye length decreases), the transverse pressiue moment decreases and the spontaneous curvature faUs. " Increasing the temperature does the opposite, because the osmotic pressxne (jt) of the counter-ions is proportional to temperature due to the osmotic pressure ideal gas law n = cRT. 

Electrolytes in particular can exhibit significant effects on the stability of emulsions prepared with one or more ionic surfactants. There are multiple potential effects, including (1) changes in the role of the surfactant at the various interfaces as a result of changes in their electrical properties, (2) changes in the namre of the interfacial films due to the presence of specific ionic interactions between surfactant and electrolyte, and (3) alterations in the transport properties of the intervening phase due to differences in the osmotic pressure between the two phases. 

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