Surfactants monolayer-forming

Figure 4.21 Microstructures of self-assembled surfactant monolayers formed as a function of water content, assuming a fixed surfactant parameter. The graphs illustrate the variations of Gaussian and mean curvature as a function of water content. The lower drawings illustrate schematic cross-section through aggregates where the surfactant parameter is larger than unity (upper axis, internal shaded regions polar) and less than unity (lower axis, internal shaded regions hydrophobic).

As an example, a fluorocarbon surface layer can be obtained by dissolving a small amount (less than 1%) of a polymerizable fluorosurfactant in a lacquer and cross-linking the surfactant monolayer formed at the surface. Figure 17.31 shows two fluorocarbon surfactants, one polymerizable (a) and the other non-reactive (b), used in such an experiment. The surfactants were added to a poly(methyl methacrylate) (PMMA) lacquer. PMMA is more polar than the hydrocarbon part of the surfactant so the surfactant orients at the film-air interface with... 

Surfactants are often used to suppress or depress evaporation from a free liquid surface [13]. Surfactant monolayers formed on the water surface... 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

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 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. 

However, the spreading of a surfactant monolayer from a volatile solvent leaves behind a film that may not be in thermodynamic equilibrium with its bulk crystalline form or the aqueous subphase. It has been proposed that this is a result of the relatively high energy barriers to film collapse or dissolution into the subphase as compared with lowered interfacial free energy when a stable, insoluble surfactant monolayer is formed (Gershfeld, 1976). The rate at which a whole system approaches true equilibrium in such a system is often so slow that the monolayer film can be treated for most purposes as though it were at equilibrium with the subphase. 

The purpose of this paper will be to develop a generalized treatment extending the earlier mixed micelle model (I4) to nonideal mixed surfactant monolayers in micellar systems. In this work, a thermodynamic model for nonionic surfactant mixtures is developed which can also be applied empirically to mixtures containing ionic surfactants. The form of the model is designed to allow for future generalization to multiple components, other interfaces and the treatment of contact angles. The use of the pseudo-phase separation approach and regular solution approximation are dictated by the requirement that the model be sufficiently tractable to be applied in realistic situations of interest. 

The pseudo-phase separation approach has been successfully applied in developing a generalized nonideal multicomponent mixed micelle model (see I4) and it is Interesting to consider whether this same approach can be used to develop a generalized treatment for adsorbed nonideal mixed surfactant monolayers. The preferred form for suoh a model is that it be suitable (at least in principle) for treating multiple components and be extendable to other interfaoes and properties of interest suoh as oontaot angles. Earlier models (5, 18, 27) based on the pseudo-phase separation approach and... 

Transmission electron microscopy (TEM) has been an underutilized yet valuable too in particle size characterization of MC particles in LB films. Monolayer films of trioctylphosphine oxide-capped CdSe (18), spread as a monolayer on an aqueous subphase, were transferred to a TEM grid. A close-packed hexagonal arrangement of 5.3-nm (cr —4%) crystallites was found. TEM images were also obtained for HMP-stabilized CdS incorporated in BeH/octadecylamine films (79) and for CdS formed under an amine-based surfactant monolayer and transferred to a TEM grid (14). In one study, direct viewing of CdS and CdSe particles made from Cd2+-FA films on TEM grids was not possible due to poor phase contrast between the particles and the film (30). Diffraction patterns were observed, however, that were consistent with crystalline (3-CdS or CdSe. Approximately spherical particles of CdSe could... 

As can be seen from Fig. 6.3, it was found that the partially purified, microbubble-surfactant mixture does in fact form stable monomolecular films at an air/(distilled) water interface. During the first compression-expansion cycle a minor degree of hysteresis was observed, but this effect was essentially absent during recompression (Fig. 6.3) and is probably due to the presence of various contaminants in the microbubble-surfactant mixture (see Section 6.3). It was further found that these microbubble-surfactant monolayers remain quite insoluble (cf. Section 6.1.3) when highly compressed, i.e., up to measured surface pressures of 24 dyne/cm. 

Therefore, a surfactant monolayer might be formed during drying already at concentrations below ceff as indicated by the pze (cf. Fig. 8) or the contact angle of the dried photoresist layers. 

At an air-water interface, a monolayer forms with heads lying down and tails up (toward air), whereas at an air-hydrocarbon interface the monolayer lies with tails down. By closing on the tail side, the sheetlike structure can be dispersed in aqueous solutions as spherical, rodlike, or disklike micelles (Fig. 3). Closure on the head side forms the corresponding inverted micelles in oil. Oil added to a micellar solution is incorporated into the interior of the micelle to form a swollen micellar solution. Thus, surfactant acts to solubilize substantial amounts of oil into aqueous solution. Similarly, a swollen inverted micellar solution enables significant solubilization of water in oil. 

In solutions of water and surfactant, the surfactant monolayers can join, tail side against tail side, to form bilayers, which form lamellar liquid crystals whose bilayers are planar and are arrayed periodically in the direction normal to the bilayer surface. The bilayer thickens upon addition of oil, and the distance between bilayers can be changed by adding salts or other solutes. In the oil-free case, the hydrocarbon tails can be fluidlike (La) lamellar liquid crystal or can be solidlike (Lp) lamellar liquid crystal. There also occurs another phase, Pp, called the modulated or rippled phase, in which the bilayer thickness varies chaotically in place of the lamellae. Assuming lamellar liquid crystalline symmetry, Goldstein and Leibler [19] have constructed a Hamiltonian in which (1) the intrabilayer energy is calculated... 

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