Adsorbed surfactant layer

The evidence accumulated in the literature suggests that the structure of surfactant adsorbed layers is, in some respects, analogous to that of surfactant micelles. Fluorescence probing techniques - e.g., pyrene and dinaphtylpropane (DNP) fluorescence probes are used to investigate the structure of adsorbed layer of a surfactant - give information on the polarity of the microenvironment in the adsorbed... 

A. Blom, G. G. Warr, and E. J. Wanless. Morphology transitions in nonionic surfactant adsorbed layers near their cloud points. Langmuir, 21(25) 11850-11855, 2005. 

Electrokinetic Study of Layer-by-Layer Polyelectrolyte and Surfactant Adsorbed Layers... 

In this paper we investigate the process of alternate adsorption of cationic polyelectrolyte and anionic surfactant, structure and properties of adsorbed layers depending on different factors (molecular weight of PE, concentration of polyelectrolyte and surfactant, adsorbed layer formation time, the flow rate of the solution) by measuring potential and streaming current using the capillary electrokinetic method. 

The organic additives of the micellar mobile phase affect the surfactant-adsorbed layer. This changes the chromatographic selectivity and efficiency obtained for a set of analytes with the same column. Selectivity and efficiency are studied in other parts of this book. 

The recommendation to use the same column with the same surfactant was done because, in some instances, it may be impossible to fully remove the surfactant-adsorbed layer. It was not possible to eliminate a small part of SDS adsorbed on a 1980 Hypersil ODS phase even when using a pure methanol mobile phase [8]. However, a complete SDS desorption from a 1988 Hypersil ODS phase was possible with a pure methanol mobile phase [24]. Part of the nonionic Brij 35 surfactant could not be removed from a Resolve C18 phase after a 24-hour elution of water-acetonitrile 70-30% v-v [16]. Conversely, there are many examples where the surfactant layer can be completely stripped off the stationary phase [11, 19, 21, 24]. It was suggested that partial irreversible surfactant adsorption was due to a tight insertion of the surfactant alkyl chains in the alkyl moieties of the bonded layer of densely grafted phases [35]. A column could be used with different... 

G.G. Warr, Surfactant adsorbed layer structure at solid/liquid solution interfaces impact and implications of AFM imaging studies, Curr. Opin. CoUoid Inteif. Sci., 2000, 5, 88-94. 

AFM studies can also follow morphology transitions in nonionic, surfactant-adsorbed layers near their cloud points, since temperature plays a significant role in surfactant aggregation states (Figure 12). " ... 

A dynamic surface tension study of gemini surfactant/conventional surfactant mixtures revealed a diffusion-controlled formation of the mixed surfactant adsorbed layer [127]. 

The examples in the preceding section, of the flotation of lead and copper ores by xanthates, was one in which chemical forces predominated in the adsorption of the collector. Flotation processes have been applied to a number of other minerals that are either ionic in type, such as potassium chloride, or are insoluble oxides such as quartz and iron oxide, or ink pigments [needed to be removed in waste paper processing [92]]. In the case of quartz, surfactants such as alkyl amines are used, and the situation is complicated by micelle formation (see next section), which can also occur in the adsorbed layer [93, 94]. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Two kinds of barriers are important for two-phase emulsions the electric double layer and steric repulsion from adsorbed polymers. An ionic surfactant adsorbed at the interface of an oil droplet in water orients the polar group toward the water. The counterions of the surfactant form a diffuse cloud reaching out into the continuous phase, the electric double layer. When the counterions start overlapping at the approach of two droplets, a repulsion force is experienced. The repulsion from the electric double layer is famous because it played a decisive role in the theory for colloidal stabiUty that is called DLVO, after its originators Derjaguin, Landau, Vervey, and Overbeek (14,15). The theory provided substantial progress in the understanding of colloidal stabihty, and its treatment dominated the colloid science Hterature for several decades. 

Phosphoric acid ester was used as a model for the estimation of concentration of a reagent in an adsorbed layer by optical measurements of the intensity of a beam reflecting externally from the liquid-liquid interface. The refractive index of an adsorbed layer between water and organic solution phases was measured through an external reflection method with a polarized incident laser beam to estimate the concentration of a surfactant at the interface. Variation of the interfacial concentration with the bulk concentration estimated on phosphoric acid ester in heptane and water system from the optical method agreed with the results determined from the interfacial tension measurements... 

Aid in the uniform dispersion of additives. Make powdered solids (e.g. particulate fillers with high energy and hydrophilic surface) more compatible with polymers by coating their surfaces with an adsorbed layer of surfactant in the form of a dispersant. Surface coating reduces the surface energy of fillers, reduces polymer/filler interaction and assists dispersion. Filler coatings increase compound cost. Fatty acids, metal soaps, waxes and fatty alcohols are used as dispersants commonly in concentrations from 2 to 5 wt %. 

These include electrostatic interaction between the particles and interaction of particles with the fluid governed by their wettability, morphology and density (17-19) the extent of adsorption of the polymer and its influence on the interaction of particles, the orientation or configuration of the adsorbed polymers (and surfactant when it is present) and resultant interaction of adsorbed layers the hydrodynamic state of the system and its influence on the interaction of floes themselves. 

The fits of experimental data to a Langmuir (or another) adsorption isotherm does not constitute evidence that adsorption is the actual mechanism that accounts for the loss of the sorbate from the solution. Very frequently adsorption to a surface is followed by additional interactions at the surface, e.g., a surfactant undergoes two-dimensional association subsequent of becoming adsorbed or charged ions tend to repel each other within the adsorbed layer. 

Subsequent to the adsorption onto a surface, surfactants, especially long chain fatty acids and alcohols tend to undergo alterations such as two-dimensional associations in the adsorbed layer, presumably at rates kinetically independent of preliminary steps. These intra-layer reactions have been shown to be very slow. 

Figure 2.17 Pair potentials calculated for 50 nm polystyrene particles in 0.1 moldm 3 electrolyte and with a -potential of —30mV. Curve a is the result for a simple polystyrene surface and curve b was calculated from the model of a 1 nm surfactant layer so that the -potential is taken as occurring at the outer edge of the adsorbed layer. A maximum in the potential of 8kBT is insufficient to provide long-term stability and the curves clearly shows how electrosteric stabilisation can achieve this...

Another explanation has been offered for the observed maximum in spreading area with concentration, based on the concept of autophobicity, i.e., the existence of an adsorbed layer of surfactant at the liquid-solid interface that is not wetted by its own kind [44]. However, the short time scale of the spreading ( 2 seconds in the high concentration regime) requires that this autophobic layer is laid down in this short period, which is unlikely considering the kinetic limitations on reorientation of molecules [50]. 

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