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Interface surface-active substance

The Gibbs equation (see equation 19) predicts that a substance that reduces the surface (interfacial tension) will be the adsorbed at the surface (interface). Surface-active substances (especially long-chain fatty acids, detergents, and surfactants) decrease the surface (interfacial) tension. Amphipathic molecules (which contain hydrophilic and hydrophobic groups) become oriented at the interface. At solid-water interfaces, Ae orientation depends on the relative affinities of the adsorbate for water and the solid surface. The hydrophilic groups (sulfate carboxylate, hydroxyl, etc.) may—if the hydrophobic tendency is relatively small—interact coordinatively with the functional groups of the solid surface (Ochs et al., 1994 Ulrich et al., 1988). [Pg.579]

Adsorption of various organic compounds (e.g., cyclohexanol, adamantanol-1, and camphor) has been studied at a renewed Sn + Pb alloy/electrolyte interface.820-824 The time variation of the surface composition depends on the solution composition, the nature and concentration of the surface-active substance, and on E. The " of cyclohexanol for just-renewed Sn + Pb alloys shifts toward more negative E with time, i.e., as the amount of Pb at the Sn + Pb alloy surface increases. [Pg.144]

The appreciation of the importance of adsorption phenomena at liquid interfaces is probably as old as human history, since it is easily recognized in many facets of everyday life. It is not surprising that liquid interfaces have been a favorite subject of scientific interest since as early as the eighteenth century [3,4], From an experimental point of view, one obvious virtue of the liquid interfaces for studying adsorption phenomena is that we can use surface tension or interfacial tension for thermodynamic analysis of the surface properties. The interfacial tension is related to the adsorbed amount of surface active substances through the Gibbs adsorption equation. [Pg.120]

All molecules that, when dissolved in water, reduce surface tension are called surface-active substances (e.g., soaps, surfactants, detergents). This means that such substances adsorb at the surface and reduce surface tension. The same will happen if a surface-active substance is added to a system of oil-water. The interfacial tension of the oil-water interface will be reduced accordingly. Inorganic salts, on the other hand, increase the surface tension of water. [Pg.43]

Investigations have shown that, if one carefully sucked a small amount of the surface solution of a surfactant, then one can estimate the magnitude of E The concentration of the surface-active substance was found to be 8 pmol/mL. The concentration in the bulk phase was 4 pmol/L. The data show that the surface excess is 8 pmol/mL - 4 pmol/mL = 4 pmol/mL. Further, this indicates that, when there is 8 pmol/L in the bulk of the solution, the SDS molecules completely cover the surface. The consequence of this is that, at a concentration higher than 8 pmol/L, no more adsorption at the interface of SDS takes place. Thus, y remains constant (almost). This means that the surface is completely covered with SDS molecules. The area-per-molecule data (as found to be 50 A2) indicates that the SDS molecules are oriented with the S04- groups pointing toward the water phase, while the alkyl chains are oriented away from the water phase. [Pg.61]

The relationships between the overall mass transfer coefficient and the film mass transfer coefficients in both phases are not as simple as in the case of heat transfer, for the following reason. Unlike the temperature distribution curves in heat transfer between two phases, the concentration curves of the diffusing component in the two phases are discontinuous at the interface. The relationship between the interfacial concentrations in the two phases depends on the solubility of the diffusing component. Incidentally, it is known that there exists no resistance to mass transfer at the interface, except when a surface-active substance accumulates at the interface to give additional mass transfer resistance. [Pg.74]

Adsorption phenomena significantly influence the rate of electrode reactions. The heterogeneous nature of electrode reactions determines that energetics and local activities of reacting species in the vicinity of the electrode may be different from those in the bulk solution, even when mass transport limitations can be regarded as negligible. The structure and properties of the electrode—solution interface then play a key role in the adsorption of electroactive as well as electroinactive surface active substances (SAS) at electrodes. [Pg.58]

On the other hand, electroinactive but surface active substances (SAS) adsorbed at the electrochemical interface affect the rate of electrode reactions and clear examples are organic additives used in metal deposition and inhibitors of metal corrosion. In a few cases, these substances accelerate the rate of electrode reactions [118, 119]. [Pg.64]

An almost overwhelmingly large number of different techniques for measuring dynamic and static interfacial tension at liquid interfaces is available. Since many of the commercially available instruments are fairly expensive to purchase (see Internet Resources), the appropriate selection of a suitable technique for the desired application is essential. Dukhin et al. (1995) provides a comprehensive overview of currently available measurement methods (also see Table D3.6.1). An important aspect to consider is the time range over which the adsorption kinetics of surface-active substances can be measured (Fig. D3.6.5). For applications in which small surfactant molecules are primarily used, the maximum bubble pressure (MBP) method is ideally suited, since it is the only... [Pg.639]

The theoretical foundation of the drop volume technique (DVT) was developed by Lohnstein (1908, 1913). Originally, this method was only intended to determine static interfacial tension values. Over the past 20 years, the technique has received increasing attention because of its extended ability to determine dynamic interfacial tension. DVT is suitable for both liquid/liquid and liquid/gas systems. Adsorption kinetics of surface-active substances at liquid/liquid or liquid/gas interfaces can be determined between 0.1 sec and several hours (see Fig. D3.6.5). [Pg.642]

More commonly, demulsifiers are surface-active substances (surfactants) that have the ability to destabilize emulsions. This involves reducing the interfacial tension at the emulsion interface, often by neutralizing the effect of other surfactants that are stabilizing the emulsion. An example is antagonistic action - the addition of an O/W promoter to break a W/O emulsion (see sensitization in Section 5.4). Mikula... [Pg.216]

Surface-active substances — are electroactive or elec-troinactive substances capable to concentrate at the interfacial region between two phases. Surface-active substances accumulate at the electrode-electrolyte - interface due to -> adsorption on the electrode surface (see -> electrode surface area) or due to other sorts of chemical interactions with the electrode material (see - chemisorption) [i]. Surface-active substances capable to accumulate at the interface between two immiscible electrolyte solutions are frequently termed surfactants. Their surface activity derives from the amphiphilic structure (see amphiphilic compounds) of their molecules possessing hydrophilic and lipophilic moieties [ii]. [Pg.650]

We consider the adsorption and partition of an ionic surfactant i in two inuniscible electrolyte solutions, O and W, in contact, as a function of the phase-boundary potential between O and W, Aq = where and are the inner potentials in W and O. When a surface-active substance partitions between O and W, the partition of the surfactant and the two adsorption processes from both sides of the interface are not independent of each other, as these are all affected by Aq 0 [18]. The dependence of the partition of an ionic substance on Aq is expressed by the Nemst equation. [Pg.156]

The energy state of the interface obviously determines the adsorption of a given surfactant by this inter ce. On the other hand, a given interface may adsorb with different energy surface-active substances of different chemical nature. Rhebinder (1927) was the first to point out that the difference in polarity between boundary phases, which affects the interfedal energies, is the main factor determing adsorption for adsorption of a third component by the interface, the polarity of this component should lie between the polarities of the two boundary phases. [Pg.250]

As is the case for its physical properties, the geochemical characteristics of the reaction medium can also influence the rates and mechanisms of pesticide compound transformation in the hydro-logic system, as well as the health and activity of the organisms capable of transforming these compounds. Such characteristics include redox conditions (discussed earlier), pH, ionic strength, the stracture and concentrations of any surface-active substances, solvents or ligands that may be present, and the chemical properties of any interfaces with which the reactants may come in contact. [Pg.5101]

Vogler 31) developed a mathematical model to derive semiquantitative kinetic parameters interpreted in terms of transport and adsorption of surfactants at the interface. The model was fitted to experimental time-dependent interfacial tension, and empirical models of concentration-de-pendent interfacial tension were compared to theoretical expressions for time-dependent surfactant concentration. Adamczyk (32) theoretically related the mechanical properties of the interface to the adsorption kinetics of surfactants by introducing the compositional surface elasticity, which was defined as the proportionality coefficient between arbitrary surface deformations and the resulting surface concentrations. Although the expressions to describe the adsorption process differed from one another, it was demonstrated that the time-dependent interfacial tensions mirrored the change of surface-active substances at the interface. [Pg.71]

Batina, N., Ruzic, I., and Cosovic, B. (1985) An Electrochemical Study of Strongly Adsorbable Surface-Active Substances. Determinations of Adsorption Parameters for Triton-X-100 at the Mercury/Sodium Chloride Interface, J. Electroanal. Chem. Interfacial Electrochem. 190, 21-32. [Pg.937]

Of the five types of interfaces mentioned above, adsorption at gas-liquid (e.g., air-water) interfaces is of interest in all adsorptive bubble separation methods. In the liquid pool, a molecule is acted upon by molecular attractions, which are distributed more or less symmetrically about the molecule. However, at the air-water interface, a water molecule is only partially surrounded by other like molecules as a consequence, an attraction tends to draw the surface molecules inward, and in doing so makes the water behave as if it were surrounded by an invisible membrane. This behavior of the surface is called surface tension. Surface-active substances possess the ability to lower the surface tension of water even at low concentrations. [Pg.94]

Dispersion in liquid/liquid L/L) systems is associated with the enlargement of the interface area between two immiscible liquids, so that e.g. an extraction process or a chemical reaction (saponification, nitration, etc.) can proceed rapidly or dispersions of particular droplet size are produced (bead and suspension polymerizations, etc.). In this chapter only dispersion by stirrers is considered. If this process is assisted by the addition of surface-active substances, it is termed emulsification, for which completely different laws generally apply, see e.g. [201]. [Pg.244]

It is noteworthy that many proteins in the monolayer state retain their enzymatic activity and are capable of taking part in specific chemical reactions. For this reason the colloid-chemical methods used to investigate the properties of protein films, combined with other techniques, represent valuable tools for the study of the properties of proteins. These methods allow one to examine more closely mechanisms of transport phenomena that take place at cellar interfaces in biological systems. The latter are the interfaces at which the accumulation of surface active substances with biological and physiological activity occur. These substances, when present at such interfaces, reveal their important unique properties (e.q. enzymatic activity). [Pg.111]


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