Film hydrophobicity

The comparison of the results for foam films with those for emulsion films has proved to be very useful, especially with respect to emulsion films of the O/W type. Reason for such a comparison provides the fact that in both cases the thin liquid film is in contact with two hydrophobic phases. It is anticipated that the effects related to adsorption and orientation of surfactant molecules at the film/hydrophobic phase interface are very similar, and there are examples illustrating it. Hence, some regularities established for foam films can be applied to emulsion films and vice versa. 

Hydrocolloids (proteins and polysaccharides) are common materials for edible films and coatings, since they possess excellent gas barrier, creating a gentle modified atmosphere that helps maintaining quality of foods, such as fresh produces, dry fruits, and so on. However, they exhibit poor moisture barrier due to their hydrophihc nature (Kester and Fennema, 1986 Gennadios and Weller, 1990 Park and Chinnan, 1990 Gennadios et al., 1994). To overcome this problem, hpids are added to increase film hydrophobicity. 

Chitosan has also been used as reinforcement in nanocomposites. Chitosan nanoparticles were used by Kampeerapappun et al. [204] to produce bionanocomposites with cassava starch and MMT nanocomposites. The authors reported that the addition of chitosan, due to its hydrophilicity and ability to attach to the clay surface, played a role in compatibilizing the interface between starch matrix and MMT. As a result, the starch/MMT composite film at low MMT content exhibited an improvement in tensile properties due to a reinforcement effect It was also found that the surface hydrophobicity of the composite film increased with an increase in chitosan content In association with film hydrophobicity, the water vapor transmission rate and moisture absorption were found to decrease with an increase in chitosan content. 

Cheng Q, Brajter-Toth A (1995) Permselectivity and high sensitivity at ultrathin monolayers. Effect of film hydrophobicity. Anal Chem 67 2767-2775... 

A wide variety of other specialty monomers are also used to provide specialized performance properties for coating applications. For example, amine functional monomers can be used to improve adhesion to aged alkyd substrates. Specialized monomers can also be used to improve exterior durability, for example VEOVA (vinyl ester of vesatic acid) monomers can improve the hydrolysis resistance of vinyl acetate polymers, and n-butyl methacrylate can be used to enhance the durabiHty of BA-MMA acrylics. Polymer hydrophobicity can be fine tuned by varying the levels of hydrophobic and hydrophilic monomers in the composition and styrene or ethyl hexyl acrylate are used to increase film hydrophobicity and reduce water permeability in BA-MMA systems. Specialty monomers are also used to provide specific chemical functionality to polymer compositions. For example, hydroxyethyl methacrylate can be used to provide hydroxyl functionality to acrylic resins, allowing these polymers to be used in cross-linkable thermoset coatings which cure via melamine chemistry. While specialty monomers are used at relatively low levels in polymer compositions, they frequently provide the performance features needed for the successful application of emulsion polymers in many coating areas. 

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

Clearly, it is important that there be a large contact angle at the solid particle-solution-air interface. Some minerals, such as graphite and sulfur, are naturally hydrophobic, but even with these it has been advantageous to add materials to the system that will adsorb to give a hydrophobic film on the solid surface. (Effects can be complicated—sulfur notability oscillates with the number of preadsoibed monolayers of hydrocarbons such as n-heptane [76].) The use of surface modifiers or collectors is, of course, essential in the case of naturally hydrophilic minerals such as silica. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

Figure C2.4.5. Horizontal transfer on a hydrophobic substrate. This metliod is useful for very rigid films tliat are in tire solid state in the ji-A-diagram.

The generally low chemical, mechanical and thennal stability of LB films hinders their use in a wide range of applications. Two approaches have been studied to solve this problem. One is to spread a polymerizable monomer on the subphase and to polymerize it either before or following transfer to the substrate. The second is to employ prefonned polymers containing hydrophilic and hydrophobic groups. 

Another approach to the fabrication of LB films from prefonned polymers is to fonn a hydrophobic main chain by reacting monomers tenninated by a vinyl group [102, 103, 104, 105 and 106]. The side groups studied also included perfluorinated hydrocarbon chains, which tilt with respect to the nonnal to the plane of the film, whereas the analogous ordinary hydrocarbon chains do not [105]. 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

AH corrosion inhibitors in use as of this writing are oil-soluble surfactants (qv) which consist of a hydrophobic hydrocarbon backbone and a hydrophilic functional group. Oil-soluble surfactant-type additives were first used in 1946 by the Sinclair Oil Co. (38). Most corrosion inhibitors are carboxyhc acids (qv), amines, or amine salts (39), depending on the types of water bottoms encountered in the whole distribution system. The wrong choice of inhibitors can lead to unwanted reactions. Eor instance, use of an acidic corrosion inhibitor when the water bottoms are caustic can result in the formation of insoluble salts that can plug filters in the distribution system or in customers vehicles. Because these additives form a strongly adsorbed impervious film at the metal Hquid interface, low Hquid concentrations are usually adequate. Concentrations typically range up to 5 ppm. In many situations, pipeline companies add their own corrosion inhibitors on top of that added by refiners. 

The uses of spunbonded fabrics as coverstock in diapers and other personal absorbent devices will most likely remain unchallenged for the near term. Virtually any other nonwoven production method appears to be at a cost disadvantage opposite spunbonded polypropylene. There have been composite products developed from meltblown and spunbonded combinations, where areas of either improved hydrophobicity or hydrophilicity are desired. These products can be produced on-line at relatively low additional cost and offer high value to diaper manufacturers. Any competitive threat is likely to come from advances in film technology such as large improvements in perforated film used in segments of absorbent product appHcations, particularly sanitary napkins. 

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