Spreading water-fluid interfaces

In another approach to functionalize surfaces we made use of the Langmuir-Blodgett-Kuhn technique a dimyristoyllecithin monolayer at the water/air-interface doped (by co-spreading) with 5 raole% biotinylated lipid (Fig. 4(a)) shows the usual pressure-area (tt-A) isotherm (if compressed) (Fig. 4(b)), with the coexistence of fluid and ordered domains... 

Polymers spread at the interface between two fluid media are of importance in a number of areas, liquid-liquid extraction, stabilisation of emulsions and as model membrane systems being some examples. By far the most extensive body of work reported has been for cases in which one of the fluid media is air, the liquid phase generally being water. Pol)miers spread at the air/water interface can be viewed as pseudo-two-dimensional systems thus there is fundamental interest in ascertaining whether theory can be adapted to explain the observations. One practical advantage of spread polymer films is that film compression leads to an increase in surface concentration, so it is relatively easy to change this quantity experimentally. 

What is of interest for us now is not the storm-soothing mechanism (belonging to the field of fluid mechanics and hydrodynamics), but the ability of a relatively small amount of oil to spread over large areas of water surface, or, to be more precise, to form thin film on the air-water interface. 

From a biopharmaceutical viewpoint, a large sized suppository may be advantageous. In principal, a larger volume spreads over a larger intestinal surface. As a result more active substance is released from the base, more active substance dissolves in the rectal fluid and the absorption is faster. This is especially important for substances that are poorly soluble in both fat and water, such as paracetamol. The release of active substance from the base in this case strongly depends on the extent of the interface between fat and rectal fluid available for active substance release. From a technological point of view a large size suppository must be... 

The surface chemistry of the eye is probably better understood than that of the ear. The cornea is covered with a thin, fluid film the so-called tear film, which is believed to consist of an aqueous phase, approximately 10" cm thick, with an adsorbed lipid and mucin layer at the air-water interface and an adsorbed mucin layer on the corneal side. The latter renders the cornea hydrophilic and enables the tear film to spread. In dry eye syndrome local areas of dewetting occur due to increased contact angle. Dry eye is sometimes precipitated by drug therapy, and there is a search for adequate artificial tear fluids. The surface chemistry of tear film components has been discussed by Holly [331]. Adsorption of cationic surfactants present in eye drops as preservatives can lead to the production of a hydrophobic surface due to electrostatic adsorption of the cations with the hydrocarbon chains oriented towards the tear film. Such a process can itself result in dewetting and thus cationics should be excluded from artificial tear fluids. 

The pressure-induced transition from a fluid to a solid phase in phospholipid mono-layers spread on an air/water interface is associated with aggregation processes leading to smooth solid domain shapes as well as fractal or dendritic morphologies/ depending on the experimental circumstances. In these monolayers, it is the diffusion of a dye impurity only miscible in the fluid phase which is responsible for the tenuous solid domains. The two-dimensional model presented in the present work not only identifies and clarifies the non-equilibrium fractal-forming mechanism underlying the experimental observations but also describes qualitatively the experimentally observed crossover from non-equilibrium fractal growth to equilibrium compact solid domains at late times. 

The surfactant film at the air-mucus interface is probably important for the rheological properties of the extracellular layer by stabilizing the aqueous layer mechanically and by reducing the evaporation of water (54,93). These are well-known functions of surfactant films if spread on top of a water surfaee (94). Fluid balance in the airway lining layer may also be influenced by the recently demonstrated effeet of surfactant in stimulating chloride secretion by airway epithelial cells (95). 

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