Droplets chain extender

Figure 3.7 Stretching of a typical droplet of chain extender during mixing and the structure of the interface. Chemical reactions between the prepolymer and the chain extender occur at the interface. Local direction of stretching (xi) and the direction of concentration variation across the thickness of the droplet (X2) are shown.

Barker and Grimson (1991) modeled the flow of deformable particles after a free-draining floe whose shape, orientation, and internal structure ranged between the extremes of an extended chain and a folded globule. They interpreted the unhindered motions of free-flowing, deformable droplets to result from an unbalanced force imposed by the flow field, resulting in rotations around the particles center of mass this rotation is superimposed on the steady translational motion. 

The PHS chains are highly solvated by the oil molecules and extend at the W/O interface, giving a layer thickness on the order of 7-8 nm, as confirmed using film thickness measurements [3]. A thin film consisting of oil plus surfactant was formed between two aqueous droplets when they were brought into contact, and the thickness of the film was measured using light reflectance a fihn thickness of... 

A plot of 8 versus

distance between the droplets is relatively large. The reduction in 8 with increase in

[Pg.241]

Proteins adsorbed at an oil-water interface may stabilize the oil droplets by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) interactions and/or the steric stabilization mechanism. The proteins may possess or be capable of adopting extended structures, which protrude into a solution for a considerable distance from the interface. This extended hydrated layer may form the basis for steric stabilization of the emulsion. Interactions between the adsorbed protein layers can involve a reduction in conhgurational entropy as molecular chains overlap (Darling and Birkett, 1987). In addition, hydration of adsorbed hydrophilic components can lead to an enthalpic repulsion when two particles are in close proximity. This tends to force the oil droplets apart (Darling and Birkett, 1987). 

The formation of emulsions or microemulsions is conneeted with several dynamic processes the time dependence of surface tensions due to the kinetics of adsorption, the dynamic contact angle, the elasticity of adsorption layers as a mechanic surface property influencing the thiiming of the liquid films between oil droplets, the mass transfer across interfaces and so on. Kahlweit et al. (1990) have recently extended Widom s (1987) concept of wetting or nonwetting of an oil-water interface of the middle phase of weakly-structured mixtures and microemulsions. They pointed out that the phase behaviour of microemulsions does not differ from that of other ternary mixtures, in particular of mixtures of short-chain amphiphiles (cf for example Bourrell Schechter (1988). 

Two different nanofibers of C100H202 were studied. One system (B6) contained 36 independent parent chains, and the other system (f72) contained 72 independent parent chains, both in boxes in which Lx was initially 5.25 nm. The simulation protocol for collapse of the fiber consisted of a cycle of increasing Lx by 0.25 nm, followed by relaxation for 10 Monte Carlo steps at a temperature of 509K. This cycle was continued until collapse was observed. The smaller system with 36 independent parent chains collapsed to droplets when Lx reached 10 nm. The larger system, with 72 independent parent chains, did not collapse even when Lx exceeded the length of the fiilly extended chain, which is 12.5 nm. 

As a good approximation, micelles can, in a wide concentration range above the CMC, be viewed as microscopic liquid hydrocarbon droplets covered with the polar head-groups, which are in interaction with water. It appears that the radius of the micelle core constituted of the alkyl chains is close to the extended length of the alkyl chain, i. e. in the range of 1.5-3.0 nm. Why is this so ... 

We came above to a simple characterization of the micelle core as a hydrocarbon droplet with a radius equalling the length of the extended alkyl chain of the surfactant. We noted also that, since the cross-sectional area per chain decreases radially towards the centre, only one chain can be fully extended while the others are more or less folded. The aggregation number, A, can be expressed as the ratio between the micellar core volume, Vniic and the volume, u, of one chain as follows ... 

With sufficiently high surface loading, long-chain surfactants and high molecular-weight polymers can become adsorbed at the surfaces of emulsion droplets such that a significant amount of adsorbate extends out from the surfaces. In this situation, an entropy decrease can accompany... 

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