Water-hydrophobic interfaces

Water-Hydrophobic Interfaces, WHI A Deceptively Simple System... 

In this section, we first present a summary of previous applications of our QM/MM approach to several problems that include molecules and radicals of atmospheric interest interacting with the air-water interface, and molecules of biological relevance at water-hydrophobic interfaces. Then, we will present some new results for a series of volatile organic compounds (MeCl, MeCN and MeOH) at the air-water interface for these systems, in particular, we will discuss the orientational dynamics and the effect of the water medium on their electronic properties. 

Siano and coworkers [14] report the use of 8-anilino-l-naphthalenesulfonic acid (ANS) as a probe of environment polarity in polyacrylamides containing 1 mole percent n-dodecylacrylamide (estimated from feed ratio) in 2 wt% NaCl. ANS, an amphoteric probe is selectively solubilized at the water-hydrophobe interface. The fluorescence emission maximum is shifted from 520 nm, in aqueous environments to approximately 462 nm in hydrophobic environments [18]. Studies of the polymer described above and a homopolymer of acrylamide were accomplished by varying polymer concentration up to 5000 ppm at constant ANS concentration. In Fig. 2.10 a, a plot of the emission wavelength maximum of ANS vs polymer concentration indicated an increasingly hydro-phobic environment with increasing copolymer concentration. Above approximately 1000 ppm, emission wavelength remained constant, indicating that ANS... 

Mono- and multilayers may be fonned by the LB technique from polymers bearing both hydrophilic and hydrophobic side groups that are already spread as a polymer at the water-air interface. 

Monolayers at the Air—Water Interface. Molecules that form monolayers at the water—air interface are called amphiphiles or surfactants (qv). Such molecules are insoluble in water. One end is hydrophilic, and therefore is preferentially immersed in the water the other end is hydrophobic, and preferentially resides in the air, or in a nonpolar solvent. A classic example of an amphiphile is stearic acid, C H COOH, wherein the long hydrocarbon... 

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

The water structure at the water/surfactant interface depends on the nature of the surfactant head group, whereas the hydrophobic interface plays only a secondary role [91-93],... 

Lipids have the common property of being relatively insoluble in water (hydrophobic) but soluble in nonpolar solvents. Amphipathic lipids also contain one or more polar groups, making them suitable as constituents of membranes at lipidiwater interfaces. 

In 1958 Sarda and Desnuelle [79] discovered the lipase activation at the interfaces. They observed that porcine pancreatic lipase in aqueous solution was activated some 10-fold at hydrophobic interfaces which were created by poorly water-soluble substrates. An artificial interface created in the presence of organic solvent can also increase the activity of the lipase. This interfacial activation was hypothesized to be due to a dehydration of the ester substrate at the interface [80], or enzyme conformational change resulting from the adsorption of the lipase onto a hydrophobic interface [42,81,82]. 

In a number of cases ITIES can be used to separate the products of a photoinduced electron-transfer reaction. An early example is the work by Willner et al. [7] at the water/toluene interface, who studied the photooxidation of [Ru(bpy)3]2+ in the aqueous phase. The excited state was quenched by hexadecyl- 4,4 bipyridinium, which becomes hydrophobic on reduction and crosses to the toluene phase. There are other examples and mechanisms at the present time their main interest resides in their chemistry, and in the separation of products that can be achieved at the interface. 

At the water-air interface hydrophilic groups are oriented toward the water, hydro-phobic groups are oriented toward air. At solid-water interfaces, the orientation depends on the relative affinities for water and for the solid surface. The hydrophilic groups of amphipathic molecules may - if the hydrophobic tendency is relatively small - interact coordinatively with the functional groups of the solid surface (Ulrich et al., 1988) (see Fig. 4.10). 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

The name surfactant refers to molecules that are surface-active , usually in aqueous solutions. Surface-active molecules adsorb strongly at the water-air interface and, because of this, they substantially reduce its surface energy (Gibbs theorem). This is the opposite behaviour from that observed for most inorganic electrolytes, which are desorbed at the air interface and hence raise the surface energy of water (slightly). Surfactant molecules are amphiphilic, that is, they have both hydrophilic and hydrophobic moieties, and it is for this reason that they adsorb so effectively at interfaces (note that amphi means of both kinds in Greek). 

Gustafsson, O., and P. M. Gschwend, Hydrophobic organic compound partitioning from bulk water to the water/air interface , Atmos. Environ., 33,163-167 (1999). 

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