Substrates, hydrocarbon-hydrophobic

On basically hydrocarbon-hydrophobic substrates such as polystyrene, it is well established that even on the negatively charged particles there is adsorption of surfactant anions via the hydrocarbon chains. 

On basically hydrocarbon-hydrophobic substrates such as polystyrene, it is well established that even on the negatively charged particles there is adsorption of surfactant anions via the hydrocarbon chains. This is demonstrated in the work of Kayes (1976), who found a substantial increase in the electrophoretic mobility of polystyrene latices with increase in the concentration of dndecyl sulfate in the system, and in the work of Cebula et al. (1978) on the adsorption of dndecanoate ions on polystyrene latex particles. 

An alternative to the injection method for importing enzymes into a microemulsion is the phase transfer method. In this method, a layer of an aqueous enzyme solution is located under a mixture of surfactant and oil. Upon gentle shaking, the enzyme is transferred into the reverse micelles of the hydrocarbon phase. Finally, the excess of water is removed and the hydrophobic substrates can be added. The main advantage of this method is that it ensures thermodynamically stable micro emulsions with maximum water concentrations. However, the method is very time consuming. The method is often applied in order to purify, concentrate or renaturate enzymes in the reverse micellar extraction process [54-58]. 

Often the enzyme stability can be improved by using a suitable water-immiscible solvent instead of a water-miscible one. Two-phase systems are obtained with the enzyme and other hydrophilic substances present in the aqueous phase while hydrophobic substrates and products mainly partition to the organic phase (Figure 9.1). Water immiscible solvents often used for enzymatic reactions are hydrocarbons, ethers and esters further details on solvents are found in the section 9.5 Selection of solvents , below. In order for the bioconversion to occur, the substrates must be transferred to the enzyme in the aqueous phase after the reaction hydrophobic... 

Solubilization is the formation of a thermodynamically stable, isotropic solution of a substance (the solubilizate), normally insoluble or slightly soluble in water, by the addition of a surfactant (the solublizer). The micelles of the surfactant cause solubilization of the substrate, producing an isotropic solution of the chemical. The solubilizate can be incorporated in the surfactant micelle in different ways, depending on the nature of the substrate and the surfactant micelles. For hydrophobic substrates, the molecules become incorporated in the hydrocarbon core of the micelle. With more polar substrates, the molecules may become incorporated in the hydrophilic PEO chains of the micelle or they may be simply adsorbed at the micelle surface. 

Recently, it has been found that aqueous solutions of two different hydrocarbon chain surfactants can also show superspreading on highly hydrophobic substrates (Rosen, 2002 Zhou, 2003). In these mixtures, the two different hydrocarbon-chain surfactants also interact to produce synergistic enhancement of the total surfactant adsorption at the hydrophobic solid-aqueous solution interface relative to that at the air-aqueous solution interface, and this is accompanied by an enhanced rate of reduction of the contact angle (Zhou, 2003). SF values for these mixtures are also listed in Table 6-3. 

Amphiphilic EPSs are likely to have contributed to the ultimate removal of the oil and to the formation of oil aggregates, which were a dominant feature observed in contaminated surface waters. The potential of Halomonas EPS to influence the biodegradation of hydrocarbons due to amphiphilic properties that allow these macromolecules to interface with hydrophobic substrates was discussed. It effectively increases the... 

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]. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

The feasibility of the fabrication of comb like fluorocarbonpolymer LB films has been shown. These films can be deposited onto different kinds of substrates as y-type layers by the usual LB technique. In this case the deposition procedure is much simpler than the one for polyimide LB films, but the temperature, chemical and mechanical stability, and dielectricproperties of the fluorocarbonpolymers are not inferior to those of polyimides. The fluorocarbons are more hydrophobic than ordinary hydrocarbons, hence shorter hydrophobic chains can be used and thinner monolayers can be prepared (the PFHA-AA LB monolayer thickness investigated was 16.5 x l(L8cm). 

Where nucleic acids are concerned, the enhanced hydrophobicity of abiotic polyfluorinated aromatic bases (e.g., tetrafluorobenzene or tetrafluoroindole deoxyribose derivatives) was exploited as an alternative to natural hydrogen bonding to achieve selective and stable nucleic acid base pairing in duplex DNA [85], The DNA replication was examined using polyfluorinated-nucleotide analogs as substrates. A DNA polymerase active site was able to process the polyfluorinated base pairs more effectively than the analogous hydrocarbon pairs, demonstrating hydrophobic selectivity of polyfluorinated bases for other polyfluorinated bases [86]. 

The efficiency of the catalysis would also depend on the binding of the substrate to the piperazine-2,5-dione by means of weak forces. Imanishi and his group (85BCJ497) have sought to achieve this by means of hydrophobic interactions. They have chosen as substrates p-nitrophenyl esters of long-chain fatty acids the hydrocarbon chain would enter into... 

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

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