Hydrophobicity-hydrophilicity, effect interfaces

Interfacial tension studies are particularly important because they can provide useful information on the interfacial concentration of the extractant. The simultaneous hydrophobic-hydrophilic nature of extracting reagents has the resulting effect of maximizing the reagent affinity for the interfacial zone, at which both the hydrophobic and hydrophilic parts of the molecules can minimize their free energy of solution. Moreover, as previously mentioned, a preferential orientation of the extractant groups takes place at the interface. Conse-... 

We have examined two types of organized media that effectively control the charge separation and back reactions of the intermediate photoproducts. These include, (a) charged colloids i.e. SiC>2 and ZrC>2 colloids that introduce electrostatic interactions between the photoproducts and interface (7-10), and (b) water-in-oil microemulsions that provide aqueous-oil two phase systems capable of controlling the reactions by proper design of the hydrophobic-hydrophilic balance of the photoproducts ( 6). 

Fig. 8a, b. Control of photoinduced ET reactions in organized microenvironments a) application of charged interfaces to effect charge separation and retard recombination processes by means of electrostatic interactions b) application of water-oil two phase systems in charge separation and stabilization of photoproducts against back reactions by means of hydrophobic-hydrophilic interactions... 

Other IRRAS applications to peptides and proteins. In addition to the pulmonary surfactant system, a variety of other applications employing IRRAS to study peptide and protein conformation and orientation have appeared. The secondary structure conversion of the amyloid (prion)-protein in the normal form into the abnormal form is the main cause of several human and animal diseases, such as Alzheimer s disease [68]. The secondary structure of the first 40 residues of the amyloid protein was detected by circular dichroism (CD) in aqueous solution and with IRRAS at the interface. A stable /1-sheet-enriched state of the amyloid is formed at the air-water interface, in contrast to the initial bulk solution containing high a-helix/random coil and low /l-sheet parts. The change in the pH going from bulk (alkaline pH) to the interface (neutral or slightly acidic pH) can have effects on the conformation at the interface. Another alternative might be the intrinsic hydrophobicity of the air-water interface, which is a hydrophobic-hydrophilic system with air as the hydrophobic part. 

The area per surfactant molecule at the hydrophobic-hydrophilic interface -the head-group area - is prescribed by the temperature, water content, steric effects and ionic concentration for ionic surfactants. Assume for now that the area per each surfactant "block" making up the assembly is set a priori. This assumption implies that the surface to volume ratio of the mixture (assumed to be homogeneous) is set by the concentration of the surfactant. So the interfacial topology is predetermined by this global constraint, the surface to volume ratio. 

It can be seen from Tables 6 and 7 that, in general, PG systems containing a surfactant produced higher relative values for photocurrent, photopotential, and conversion efficiency than systems studied in which a surfactant was absent. Fendler and Fendler [39] and Atwood and Florence [40] have attributed this to the ability of a surfactant to solubilize certain molecules (i.e., the photosensitizing dye) and the catalytic effect that carefully chosen surfactants induce on particular chemical reactions. Furthermore, Rohatgi-Mukherjee et al. theorized that addition of a surfactant into a PG system increases conversion efficiency via facilitating the separation of photogenerated products by hydrophobic-hydrophilic interaction of the products with the surfactant interface [27]. 

The increased adsorption of certain pollutants on surfactant modified pillared montmorillonites results from the orientation of fte surfactant cations different from that in the non-piUared smectites. As the polyhydroxoaluminium complexes are positively charged, the cations are assumed to be oriented with their positive head groups pointing away from the pillars (Srinivasan and Fogler, 1990). This makes the surface hydrophilic, whereas the space between the alkyl chains retains its hydrophobic character. It is also assumed that an electrostatic shielding against flocculation further increases the effective interface area. 

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. 

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... 

Silane coupling agents may contribute hydrophilic properties to the interface, especially when amino functional silanes, such as epoxies and urethane silanes, are used as primers for reactive polymers. The primer may supply much more amine functionality than can possibly react with the resin at the interphase. Those amines that could not react are hydrophilic and, therefore, responsible for the poor water resistance of bonds. An effective way to use hydrophilic silanes is to blend them with hydrophobic silanes such as phenyltrimethoxysilane. Mixed siloxane primers also have an improved thermal stability, which is typical for aromatic silicones [42]. 

One of the most attractive roles of liquid liquid interfaces that we found in solvent extraction kinetics of metal ions is a catalytic effect. Shaking or stirring of the solvent extraction system generates a wide interfacial area or a large specific interfacial area defined as the interfacial area divided by a bulk phase volume. Metal extractants have a molecular structure which has both hydrophilic and hydrophobic groups. Therefore, they have a property of interfacial adsorptivity much like surfactant molecules. Adsorption of extractant at the liquid liquid interface can dramatically facilitate the interfacial com-plexation which has been exploited from our research. 

When a monolayer of phospholipids is adsorbed at the ITIES, there must be a modification of the electrical structure of the interface [60]. Since we aim at describing the effect of this monolayer on the rate of ion transfer in a simple way, we assume a sharp interface also in the presence of phospholipids. The hydrophobic tails are located in the organic phase (negative x region), and the hydrophilic headgroups are located in the aqueous phase (positive X region). 

According to the colloid scientist Winsor, surfactants are defined as compounds which possess in the same molecule distinct regions of hydrophilic and lipophilic character. For example, in the oleate ion there is an alkyl chain that is basically hydrophobic (lipophilic tail) and a COO" headgroup that is hydrophilic (lipo-phobic). Being amphiphilic in nature, surfactants have the ability to modify the interface between various phases [66]. Their effects on the interface are the result of their ability to orient themselves in accordance with the polarities of the two opposing phases. The polar part can be expected to be oriented towards the more polar (hydrophilic, aqueous) phase, whereas the nonpolar tails should direct towards the nonpolar (lipophilic, oil) phase. 

Microbes were frequently found to synthesise surface-active molecules in order to mobilise hydrophobic organic substrates. These biosurfactants, which are either excreted by the producing organisms or remain bound to their cell surfaces, are composed of a hydrophilic part making them soluble in water and a lipophilic part making them accumulate at interfaces. With respect to their physical effects, one can distinguish two types of biosurfactants firstly, molecules that drastically reduce the surface and interfacial tensions of gas-liquid, liquid-liquid and liquid-solid systems, and, secondly, compounds that stabilise emulsions of nonaqueous phase liquids in water, often also referred to as bioemulsifiers. The former molecules are typically low-molar-mass... 

---