Surfactants bilayers

Figure 8.7 Schematic diagram of simple monolayer and bilayer surfactant adsorption from aqueous solution.

Fig. 102. Schematics of available sites for organizing colloidal semiconductors in single-bilayer surfactant vesicles [500]...

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

Hemimicelle An aggregate of adsorbed surfactant molecules that may form beyond monolayer coverage, the enhanced adsorption being due to hydrophobic interactions between surfactant tails. Hemimicelles (halfmicelles) have been considered to have the form of surface aggregates or of a second adsorption layer with reversed orientation, somewhat like a bimolecular film. In bilayer surfactant adsorption, the term ad-micelles has also been used (20). 

Whereas all the neutron reflection studies of bilayer surfactant structures have relied on using the overall thickness of the layer to establish that it is a bilayer. Fragneto et al. were able to show this directly by using partially labelled CieTAB molecules [8]. The series of isotopic species 0 Ci6 dC ,dTAB with m = 4, 8 and 12, where 0 indicates... 

L Shen, PE Laibinis, TA Hatton. Bilayer surfactant stabilized magnetic fluids Synthesis and interactions at interfaces. Langmuir 1999 15 447. 

The hexagonal phases consist of rodlike micelles packed in an hexagonal assembly (Fig. 6.1). A lamellar phase consists of bilayer surfactant aggregates sep-... 

In this chapter, the preparation of Ag colloids in aqueous surfactant solutions and their stability were investigated. Depending on the nature of the materials prepared, two types of adsorption onto the surface of particles may be considered. If the surface of particles is hydrophobic, the hydrophobic part of the surfactant will adsorb onto the surface of particles and form a monomolecular film in an aqueous solution. If the surface of particles is hydrophilic, the hydrophilic part will adsorb onto the surface of particles and a bilayer surfactant film will form at the surface of particles in the aqueous solution because the hydrophobic part of the surfactant cannot be oriented toward the aqueous solution. 

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

Rutland M W and Parker J L 1994 Surface forces between silica surfaces in cationic surfactant solutions adsorption and bilayer formation at normal and high pH Langmuir 0 1110-21... 

This range yields more highly tmncated cones. The main mesophase stmcture obtained from these units is a flexible bilayer such as that fonned in vesicles and liposomes. These arrangements are often obtained from doublechain surfactants such as lecithin, double tailed cationic surfactants and AOT. 

This parameter corresponds to cylindrical packing shapes. Surfactants and amphiphiles falling in this range often produce planar bilayers and lamellar mesophases. Such cylindrical building blocks also contribute to many... 

Pashley R M, McGuiggan P M, Ninham B W, Brady J and Evans D F 1986 Direct measurements of surface forces between bilayers of double-chained quaternary ammonium acetate and bromide surfactants J. Phys. Chem. 90 1637-42... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

To complete this overview of chain models, we mention the dimer models, which represent the amphiphiles by just two units attached to each other [153-157]. They have been used to study curved bilayers [153], the kinetics of phase separation between oil and water in the presence of surfactants [155], and some aspects of self-assembled micelles [154,157] (see below). 

Whereas microscopic models for bulk systems incorporate the amphiphihc character and often the orientational properties of the surfactants as basic ingredients, models for bilayers and monolayers are constructed to reproduce internal transitions, such as the gel-fluid transition, and therefore concentrate on rather different aspects of the surfactant structure. 

In the latter the surfactant monolayer (in oil and water mixture) or bilayer (in water only) forms a periodic surface. A periodic surface is one that repeats itself under a unit translation in one, two, or three coordinate directions similarly to the periodic arrangement of atoms in regular crystals. It is still not clear, however, whether the transition between the bicontinuous microemulsion and the ordered bicontinuous cubic phases occurs in nature. When the volume fractions of oil and water are equal, one finds the cubic phases in a narrow window of surfactant concentration around 0.5 weight fraction. However, it is not known whether these phases are bicontinuous. No experimental evidence has been published that there exist bicontinuous cubic phases with the ordered surfactant monolayer, rather than bilayer, forming the periodic surface. 

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

The change in surface wettability (measured by the contact angle) with concentration for the three surfactants is plotted in Fig. 2.54 (Zhang and Manglik 2005). The contact angle reaches a lower plateau around the CMC where bilayers start to form on the surface. Wettability of non-ionic surfactants in aqueous solutions shows that the contact angle data attains a constant value much below CMC. Direct interactions of their polar chain are generally weak in non-ionics, and it is possible for them to build and rebuild adsorption layers below CMC. The reduced contact an-... 

From surfactant molecules it is known that the repeated vertical dipping of a substrate through a floating monolayer of these molecules leads to the formation of an LB multilayer on the substrate. In principle, the same procedure should also allow the preparation of multilayers of latex particles. In Figure 8b, the preparation of a particle bilayer is schematically indicated multiple repetition should result in the formation of an LB multilayer of particles. However, if one tries to realize this concept, one immediately gets into difficulties, because the contact of the particles with the underlying substrate is very poor, and the already deposited particle layer tends to detach from the surface when the substrate is dipped into... 

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