Surfactant, strongly adsorbed

The topic of spreading rates is of importance in the technology of the use of mono-layers for evaporation control (see Section IV-6) it is also important, in the opposite sense, in the lubrication of fine bearings, as in watches, where it is necessary that the small drop of oil remain in place and not be dissipated by spreading. Zisman and coworkers have found that spreading rates can be enhanced or reduced by the presence of small amounts of impurities in particular, strongly adsorbed surfactants can form a film over which the oil will not spread [48]. 

Figure 2.6 Mass conservation of a strongly adsorbed surfactant results in enrichment on an advancing concave surface due to area reduction, while the opposite is true for a convex profile, tb>ts (source Ref. [67]).

During the process of adsorption of a mixture of adsorbable solutes the uncovered parts of the surface area are free for the attachment of other adsorbable molecules. If the diluted solution of strongly adsorbable surfactant also contains a higher concentration of a less adsorbable one, the latter will cover the electrode surface first and will then be replaced by the diffusion process of the stronger adsorbable substance. If the adsorption of both components is diffusion-controlled, then for < 1 the total capacitance decrease is in first approximation additive ... 

The Gibbs equation serves as a basis for studying the adsorption thermodynamics [11, 24,44]. For the strongly adsorbed surfactants with a small molar fraction in the solution compared to the molar fraction of the solvent (the solvents are mutually insoluble) the Gibbs equation is written as [45] ... 

Alternatively, the adsorption isotherm can be determined from a desorption measurement of a surfactant provided that the surfactant is not too strongly adsorbed. The equations above also apply for this situation now with Co = 0. It should be noted that strongly adsorbed surfactants, such as nonionics on latex surfaces, do not wash out well and hence measuring by desorption is not reeommended. 

This follows the same principles discussed above, namely electrostatic repulsion produced by overlap of the double layers, steric repulsion due to the presence of strongly adsorbed surfactants or polymers, or a combination of both. 

Based on the available data, it can be stated that a fluorosurfactant film at a water-C02 microemulsion interface appears to be less densely packed than the equivalent hydrocarbon surfactant in water-in-oil phases. The value of 110 A represents approximately twice (xl.87) the physical cross section of two fluorocarbon chains (56 A [52]). For n-alkyl sulfosuccinates the equivalent factor is around 1.55X a close-packed chain cross section [44]. The origin of this may be the finite, although small, solubility of water in CO2, which is not the case in water-oil systems. This comparison of molecular areas is valid only for strongly adsorbed surfactants, i.e., a negligible amount of free monomer in the bulk phases. For the CO2 systems, if there were to be a significant loss of surfactant from the interface, then the effective w value would increase because it would become [D20]/([surfactant],oBi - [surfactant]free) [58]. The upshot would be to reduce the gradient of the... 

AH corrosion inhibitors in use as of this writing are oil-soluble surfactants (qv) which consist of a hydrophobic hydrocarbon backbone and a hydrophilic functional group. Oil-soluble surfactant-type additives were first used in 1946 by the Sinclair Oil Co. (38). Most corrosion inhibitors are carboxyhc acids (qv), amines, or amine salts (39), depending on the types of water bottoms encountered in the whole distribution system. The wrong choice of inhibitors can lead to unwanted reactions. Eor instance, use of an acidic corrosion inhibitor when the water bottoms are caustic can result in the formation of insoluble salts that can plug filters in the distribution system or in customers vehicles. Because these additives form a strongly adsorbed impervious film at the metal Hquid interface, low Hquid concentrations are usually adequate. Concentrations typically range up to 5 ppm. In many situations, pipeline companies add their own corrosion inhibitors on top of that added by refiners. 

At the shear plane, fluid motion relative to the particle surface is 2ero. For particles with no adsorbed surfactant or ionic atmosphere, this plane is at the particle surface. Adsorbed surfactant or ions that are strongly attracted to the particle, with their accompanying solvent, prevent Hquid motion close to the particle, thus moving the shear plane away from the particle surface. The effective potential at the shear plane is called the 2eta potential, It is smaller than the potential at the surface, but because it is difficult to determine 01 To usual assumption is that /q is effectively equal to which can be... 

The influence of surfactants on polarization can also have a number of reasons their effect on EDL structure (change in / -potentiai) when an adsorbed oxygen fayer appears on the surface of platinum or the formation of layers fairly strongly adsorbed on the electrode and hindering passage of the reacting species. 

Compared with literature data for adsorption of surfactants from aqueous solutions on oxide surfaces(/4,75), the kinetic data obtained in this work for C18/glass are of similar orders of magnitude to the former systems. The values of 1/k and k.for CIg/Al are greater than those for oxide adsorbent studied, indicating the strong adsorbing ability of metallic aluminum (even oxidized) relative to the mineral oxides. 

---