Interface nonpolar-water

These are molecules which contain both hydrophilic and hydrophobic units (usually one or several hydrocarbon chains), such that they love and hate water at the same time. Familiar examples are lipids and alcohols. The effect of amphiphiles on interfaces between water and nonpolar phases can be quite dramatic. For example, tiny additions of good amphiphiles reduce the interfacial tension by several orders of magnitude. Amphiphiles are thus very efficient in promoting the dispersion of organic fluids in water and vice versa. Added in larger amounts, they associate into a variety of structures, filhng the material with internal interfaces which shield the oil molecules—or in the absence of oil the hydrophobic parts of the amphiphiles—from the water [3]. Some of the possible structures are depicted in Fig. 1. A very rich phase... 

Recently, the newly developed time-resolved quasielastic laser scattering (QELS) has been applied to follow the changes in the surface tension of the nonpolarized water nitrobenzene interface upon the injection of cetyltrimethylammonium bromide [34] and sodium dodecyl sulfate [35] around or beyond their critical micelle concentrations. As a matter of fact, the method is based on the determination of the frequency of the thermally excited capillary waves at liquid-liquid interfaces. Since the capillary wave frequency is a function of the surface tension, and the change in the surface tension reflects the ion surface concentration, the QELS method allows us to observe the dynamic changes of the ITIES, such as the formation of monolayers of various surfactants [34]. 

For proteins with comparable surface hydrophobicity, the adsorption uptake correlates strongly with the extent of protein under-wrapping [19]. As an adequate control, only proteins with the same extent of surface hydrophobicity or solvent-exposed nonpolar area were included in the comparative analysis. Hence, the attractive drag exerted by dehydrons on test hydrophobes became accessible. The net gain in Coulomb energy associated with wrapping a dehydron has been experimentally determined to be 4 kJ/mol [19]. The adhesive force exerted by a dehydron on a hydrophobe at 6 A distance is 7.8 pN, a magnitude comparable to the hydrophobic attraction between two nonpolar moieties that frame unfavorable interfaces with water. 

Of more relevance to ITIES work is the interrogation of the interface between pure water and DCE via the same nonlinear spectroscopic techniques The less distinct sum-frequency spectral features were taken as evidence of a rougher, less structured interface compared to interfaces between water and nonpolar organic solvents, consistent with the fluorescent anisotropy work [39]. The transition from a sharp to a blurred interface could be induced by a progressive increase in the mole fraction of DCE in a CC14-DCE mixture. Subsequent MD calculations have been used to gain structural information on... 

Within the narrow interpretation, our results suggest that solubihties at interfaces between water and a nonpolar liquid allow the prediction of potencies for a broad range of anesthetic compounds better than solubilities in environments that model the interior of the membrane. In a broader view, a very good correlation between MAC and interfacial solubilities suggests that the sites of anesthetic action are located near an interface. A natural candidate site is the head group region of the neuronal membrane. Another possibility is an interface between the aqueous solvent and a hydrophobic patch in a water-exposed portion of a membrane receptor. 

Liquid-membrane electrodes include classical ion-exchange, liquid ion-exchange, and electroneutral ionophore-based liquid monbrane electrodes. Of particular interest are systems where the ion-exchanging compounds are dissolved macrocyclic compounds that have a strong selectivity to alkali metals. The stability of the formed complexes in nonpolar solvents far exceeds that found in water and allows for the fabrication of membrane-free micropipettes where the nonpolar/water interface is the membrane. Unfortunately, this leads to higher resistance than that exhibited by crystalline micropipettes and requires the addition of lipophilic salt to the nonpolar solvent to decrease the pipette resistance. 

Forces of Electric Origin between Particles at a Liquid Interface Figure 4.27 shows two particles attached to the interface between water and a nonpolar fluid (oil, air). In general, the particles experience three forces of electric origin —electrodipping force [351] ... 

In further studies of the remarkable water effects on Diels-Alder reactions, we examined the exo-endo selectivity of the processes. We saw that butenone added with a 95.7% preference for endo addition in water, but only an 80% endo preference in cyclopentadiene as solvent. Thus the endo addition is favored not only by secondary orbital overlap , it is even more strikingly favored by the hydrophobic effect. In the transition state for the addition reaction, the endo geometry diminishes the amount of water/hydrocarbon interface more than does the exo geometry. The high energy of a hydrocarbon/water interface is the cause of hydrophobicity, the tendency of nonpolar materials and segments to cluster in water so as to diminish the interface with water. 

Amphiphiles, the representatives of which are soap, surfactant and lipid, have a hydrophilic polar head and lipophilic nonpolar tails. They always remain on the interface between water and oil and form monolayers of surfactants in a water/oil/amphiphile ternary system. This monolayers or interfacial film reduce the surface tension between water and oil domains. In a three-component system the surfactant film exists in various topologically different structures such as micelles, vesicles, bicontinuous microemulsions, hexagonal arrays of cylinders or lamellar structures depending upon the pressure, temperature and the concentration of the components [1,2]. Microemulsions are thermodynamically stable, isotropic and transparent mixtures of ternary amphiphilic systems. When almost equal volume fractions of water and oil are mixed with a dilute concentration of surfactants, they take... 

It is interesting to note that the orientational profiles of the water dipole at the water liquid/vapor interface and at the interface between water and a nonpolar liquid all exhibit similar behavior The water dipole tends to lie parallel to the interface (possibly with a slight tilt toward the bulk) but with a broad distribution. At this orientation, the water molecule is able to maximize its ability to hydrogen bond with other water molecules. The orientation becomes fully isotropic in the bulk at distances that are 1 nm or less from the Gibbs surface. More refined orientational profiles without the capillary wave broadening can be obtained with respect to the intrinsic surface. These intrinsic orientational profiles can better identify the orientational ordering at the interface.Complementary (and sometimes consistent) experimental information has been provided in recent years mainly by nonlinear spectroscopic methods (second harmonic and sum frequency generation), discussed below. ... 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

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... 

Macromolecules exchange internal surface hydrogen bonds for hydrogen bonds to water. Entropic forces dictate that macromolecules expose polar regions to an aqueous interface and bury nonpolar regions. 

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