Structure adsorbed surfactant layer

Combination of static and dynamic laser light scattering is also useful to determine not only the size distribution but also the particle structure of polymer colloids such as the adsorbed surfactant layer thickness [73] and the formation of nanoparticles [74,75]. A recently developed method of determining the density of polymer particles is outlined below to illustrate the usefulness of laser light scattering as a powerful analytical tool for investigating more sophisticated colloidal problems [76-78]. 

Fig. 3. Schematic representation of the structure of adsorbed surfactants layers deduced from different forms of the adsorption isotherm.

Among the other spectroscopies, either in the direct or reflection mode, fluorescence spectroscopy may be mentioned. A promising variant is Total Internal Reflection Fluorescence spectroscopy (TIRF). The decay rate of an excited fluorescing probe is usually Interpreted in terms of the local fluidity and polarity. The technique has been used to estimate the extent of ordering inside adsorbed surfactant layers, but this is not an absolute method because a fluorescent probe has to be inserted, and such probes themselves affect the local fluidity. More rigorous are fluorescence experiments with molecules that possess such a probe as an intrinsic part of their structure, such as tryptophans in... 

Fig. III-8. A model S-shaped four-region isotherm for the adsorption of ionic surfactant on an oppositely charged surface (a) The structure of the adsorbed surfactant layers corresponding to different regions of the adsorption isotherm (b)...

Phases built up of discrete aggregates include the normal and reversed micellar solutions, micellar-type microemulsions, and certain (micellar-type) normal and reversed cubic phases. However, discrete self-assemblies are also important in other contexts. Adsorbed surfactant layers at solid or liquid surfaces may involve micellar-type structures and the same applies to mixed polymer-surfactant solutions. 

Atomic force microscopy (AFM) [20] has recently been used to image interfacial aggregates directly, in situ and at nanometer resolution [21, 22], The key to this application lies in an unusual contrast mechanism, namely a pre-contact repulsive force ( colloidal stabilization force ) between the adsorbed surfactant layers on the tip and sample. In contrast to previous adsorbate models of flat monolayers and bilayers, AFM images have shown a striking variety of interfacial aggregates - spheres, cylinders, half-cylinders and bilayers - depending on the surface chemistry and surfactant geometry. I review the AFM evidence for these structures and discuss the possible inter-molecular and molecule-surface interactions involved. 

Despite some limitations as seen above, by using AFM soft-contact imaging, it is possible to view the structure of adsorbed surfactant layers on solid surfaces in equilibrium with their solution state. This topic is discussed in more detail in Atomic Force Microscopy (AFM), Techniques. 

AFM is particularly sensitive to the lateral structure in the film, and this has yielded some surprising results, showing that adsorbed surfactant layers contain aggregates that are strikingly similar to elements of complex fluid phases including spherical or globular and cylindrical micelles, branched or mesh structures, as well as (occasionally) conventional bilayers. ... 

Steric stabilization differs from electrostatic stabilization in not being a function of a net force, but of the thickness of an adsorbed layer. When < >, equals 5-10%, stabilizing and destabilizing forces extend beyond the length of the electrostatic, interparticle barrier (Cabane et al., 1989). At this distance, attraction and repulsion are inconsequential, and electrolytes therefore have little effect. Bergenstahl (1988) proposed that the steric stabilization of emulsions by gums in the presence of a surfactant involves adsorption of the gum on the surfactant to form a combined structure constituted by a primary surfactant layer covered by an adsorbed polymer layer. 

The second adsorbed layer was formed due to the pumping through the capillary with preadsorbed polyelectrolyte layer of anionic surfactant solution of different concentrations below erne. To clear out the influence of the first adsorbed PE layer on the formation of the second anionic surfactant layer, we studied the adsorption of SDS on PE layers of different structures (see Deformation of Adsorbed Layers) when the PE molecules adsorbed in flat conformation (CSDAPM at C = 10—4 g/1) and when the extended layer with loops and tails was formed (CSDAPM at C = KT2 g/1 and PDADMAC at C = KT2, KT3 and 10-4g/l). 

A characteristic of the early neutron reflectivity studies of nonionic surfactant adsorption was some variability in the pattern of adsorption. This was investigated in more detail and more systematically by McDermott et al. [55], who compared the adsorption of Ci2E6 onto a range of different substrates, amorphous silica, crystalline quartz, and the oxide layer on a silicon single crystal. The adsorbed surfactant was found to form a bilayer with an overall thickness 49 4 A, with a structure similar to that determined in the previous studies (see Fig. 4). 

McGillivray et al. [71] have also observed stable layered structures adsorbed at the silicon-solution (and air-solution) interface for didodecyl dimethylammo-nium bromide (DDAB) and the corresponding diundecyl (DUDAB) cationic surfactants, in the concentration range 0.2-2 wt.%. Similar to AOT, the surface structures that are found are highly sensitive to temperature, with the repeat distance decreasing with increasing temperature. A notable difference between these systems and AOT [69] is that for the DDAB and DUDAB, the repeat distances are much larger, 600-1500 A. Furthermore, the observed structures... 

Flexible macromolecules, such as proteins, and small molecules, such as surfactants, are amphipathic and may form a layer at the oil-water interface. These molecules may also partly stabilize emulsions by forming a physical barrier to close contact, thereby reducing the attractive van der Waals forces to ineffective levels (Dalgleish, 1989). Repulsion can arise in either of two ways and physico-chemical calculations are available for both mechanisms in oil-in-water systems. Either the approaching protein-coated particles will tend to compress or alternatively interpenetrate the adsorbed protein layer on adjacent particles. The optimum structure of the stabilizing protein will be dealt with in the section on protein as an ingredient. 

The accuracy of thickness measurements with this microinterferometric technique is 0.2 nm. For thinner foam films (< 30 nm) it is necessary to account also for the film structure. The three-layer film model with an aqueous core of thickness /12 and refractive index n2 and two homogenous layers of adsorbed surfactant of thickness h each and refractive index i is... 

Since, however, each model involves some assumptions, the calculation of h2 always renders certain inaccuracy. The most important problem in the three-layer model concerns the position of the plane that divides the hydrophobic and hydrophilic parts of the adsorbed surfactant molecule. In some cases it seems reasonable to have this plane passing through the middle of the hydrophilic head of the molecule, in others the head does not enter into the aqueous core. That is why it is worth comparing film thicknesses determined by the interferometric technique using the three-layer model, to those estimated by other methods. An attempt for such a comparison is presented in [63]. Discussed are phospholipid foam films the thickness of which was determined by two optical techniques the microinterferometric and FT-IR (see Section 2.2.5). The comparison could be proceeded with the results from the X-ray Reflectivity technique that deals not only with the foam film itself but also with the lamellar structures in the solution bulk, the latter being much better studied. Undoubtedly, this would contribute to a more detailed understanding of the foam film structure. 

Redispersion of the flocculate and other evidences for the hydrophilic character of the support coated with the adsorbed surfactant in the neighbourhood of the cmc indicate that bilayer coverage represents complete saturation of the surface. Commonly, the term bilayer is applied to an adsorbed structure in which the surfactant molecules are oriented perpendicular to the surface and fully extended [5,9,20,37,81,89]. The hydrocarbon tails of both layers form a hydrophobic core between the heads. At both sides counterions accumulate between the ionic head-groups. The result looks like a lamellar micelle. For certain physical regimes, the adsorbed amount is only a fraction of what is expected for a tightly packed bilayer [37,48] the structure which best fits the experimental data can... 

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