Amphiphilic materials

The so-called self-assembly technique has its origin in 1946, when a paper was published by Bigelow et a] [116] and tluis is slightly younger tlian tlie LB teclmique. The autliors noted tliat a hydrophilic surface exposed to an amphiphilic compound dissolved in a non-polar solvent induces tlie amphiphilic material to fonn a monolayer on it. 

The combination of hydrophilicity and hydrophobicity into a single pol5meric support can lead to interesting results. Amphiphilic materials, containing both hydrophilic and hydrophobic domains, appreciably swell in a range of solvents of quite different polarity (see for example the characterization of amphiphilic CFPs given in Ref. [82]). This concept has been elegantly applied by Uozumi and Nakao in both reduction [159] and oxidation reactions (see Section 5.2) [70] catalyzed by... 

Surfactants, sometimes called surface active agents or detergents, are amphiphilic materials which contain both apolar, hydrophobic (lipophilic) and polar, hydrophilic (lipophobic) groups (Hartley, 1948, 1977 Fendler and Fendler, 1975 Fendler, 1982 Lindman and Wennerstrom, 1980 Sudholter... 

The technique of producing ordered layers of amphiphilic materials by evaporation in vacuo on to a suitable substrate appears to originate in the work of Agarwal [18] but has only very recently been developed. 

Amphiphilic materials spread at the air/water interface have been the subject of intensive study over a long period of time. The type of apparatus usually used for this purpose has much in common with the apparatus needed to form Langmuir-Blodgett films and, indeed, it is usually possible to adapt the same apparatus for both purposes. In this section the problems which must be overcome if these processes are to be carried out are discussed and the most effective solutions to these problems described. 

The amphiphilic material to be studied is dissolved at a known concentration in a volatile solvent which is not miscible with water and a known quantity is spread at the water surface using a micropipette. In order to study the physical properties of the film thus formed, one needs to be able to confine the film to a definite area and to be able to vary this area at will. It might appear that it would be equally possible to maintain a constant area and vary the amount of material which is spread. For the majority of materials this latter procedure is not satisfactory as equilibrium is not arrived at in a reasonable period of time and this method would not allow one to take the material through successive cycles of compression and expansion. We thus turn to a discussion of the various ways in which a film can be confined and its area varied in a systematic manner. Leaving aside methods which are really only of historical interest, for which reference should be made to the book by... 

Laboratory experiments have shown that radiation processing of simulated presolar ices leads to more complex molecular species [25-27]. Hundreds of new compounds are synthesized, although the starting ices contain only a few simple common interstellar molecules. Many of the compounds formed in these experiments are also present in meteorites and cometary and asteroidal dust (interplanetary dust particles - IDPs), and some are presumably relevant to the origin of life, including amino acids [28,29], quinines [30], and amphiphilic material [31]. 

Impressive materials properties can also be achieved by the solvent-specific aggregation properties of appropriately designed amphiphilic materials. A brief account on the organogel or hydrogel formation by amphiphilic molecules is now presented, citing a few examples from the current literature. Both... 

Typically, these films are deposited using the following procedure. A flat, shallow container, such as a Langmuir-Adam surface balance, is filled with water (or other suitable liquid) and the substrate to be coated is immersed. Then a solution of the amphiphilic material, in a solvent that is insoluble in water, is deposited dropwise onto the water, thereby forming an oriented monomolecular surface film upon evaporation of the solvent. This film can then be compacted... 

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

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