Surfactant adsorption chemisorption

Practical applications of surfactants usually involve some manner of surfactant adsorption on a solid surface. This adsorption is always associated with a decrease in free-surface energy, the magnitude of which must be determined indirectly. The force with which the adsorbate is held on the adsorbent may be roughly classified as physical, ionic, or chemical. Physical adsorption is a weak attraction caused primarily by van der Waals forces. Ionic adsorption occurs between charged sites on the substrate and oppositely charged surfactant ions, and is usually a strong attractive force. The term chemisorption is applied when the adsorbate is joined to the adsorbent by covalent bonds or forces of comparable strength. 

One of the essential features of the solid-liquid interface is that the adsorbing substance may not only be bound to the surface by relatively weak physical forces, but also may form true chemical bonding with molecules or ions located at the surface of the solid phase. This phenomenon, referred to as the chemisorption, may seem to invalidate the polarity equalization rule at the interface between a polar crystal (e.g. silicate or sulfide) and a polar medium (water) the adsorption due to chemical bond formation may occur in such a way that the hydrocarbon chains are facing the water phase (Fig. III-9, a). At sufficiently high concentrations of chemisorbing surfactant, when the entire solid surface is covered with a monolayer, the formation of a second, oppositely oriented, surfactant layer starts, i.e., regular surfactant adsorption... 

Surfactants may not only stabilize system against coagulation, but may have an opposite effect, i.e. cause destabilization in cases when the surfactant adsorption proceeds against the polarity equalization rule (Chapter III,2), e.g., during chemisorption of surfactants from aqueous medium on a hydrophilic surface. For example, small additives of cationic surfactants cause coagulation of aqueous dispersions of clays and other silicates due to hydrophobization at T< rmax. Further increase in surfactant concentration results in the formation of a second (hydrophilizing) adsorption layer and leads to an increased... 

As noted above, surfactant adsorption may be described in terms of simple interaction parameters. However, in some cases these interaction parameters may involve ill-defined forces, such as hydrophobic bonding, solvation forces and chemisorption. In addition, the adsorption of ionic surfactants involves electrostatic forces, particularly with polar surfaces containing ionogenic groups. Thus, the adsorption of ionic and nonionic surfactants will be treated separately. Surfaces (substrates) can be also hydrophobic or hydrophilic and these may be treated separately. 

Most adsorption processes are exothermic (AH is negative). Adsorption processes involving nonspecific interactions are referred to as physical adsorption, a relatively weak, reversible interaction. Processes with stronger interactions (electron transfer) are termed chemisorption. Chemisorption is often irreversible and has higher heat of adsorption than physical adsorption. Most dispersants function by chemisorption, in contrast to surfactants, which... 

Preferential adsorption of a surfactant from a mixture depends on the structures of the amphiphiles and the substrate. Self assembly by physisorption is reversible, while that by chemisorption is irreversible. Thus, surfactants physisorbed in monolayers can be replaced by surfactants which are able to chemisorb. Such behavior was demonstrated by allowing a donor cyanine surfactant, D (capable of physisorption), and OTS (known to chemisorb) to... 

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]. 

Radio frequency heating, 500 Steam stripping, 500 Vacuum extraction, 500 Aeration, 501 Bioremediation, 501 Soil flushing/washing, 502 Surfactant enhancements, 502 Cosolvents, 502 Electrokinetics, 503 Hydraulic and pneumatic fracturing, 503 Treatment walls, 505 Supercritical Water Oxidation, 507 Solid Solution Theory, 202 Solubility products, 48-53 Metal carbonates, 433-434 Metal hydroxides, 429-433 Metal sulfides, 437 Sorption, 167 See Adsorption Specific adsorption, 167 See Chemisorption Stem Layer, 152-154 Sulfate, 261... 

EDL can be generated not only via redox interactions (path 1), exemplified by schemes (3)-(5), but also via physical adsorption of polar molecules (path 2), specific adsorption of surfactant ions (path 3), and chemisorption of heteroatoms or polar compounds (path 4). Thus, the electrochemical theory also takes into account chemical aspects of surface phenomena, even though they are not as detailed as in the chemical theory. When the adsorption processes occur according to paths 2-A,... 

In mineral-reagent systems, surface precipitation has been proposed as another mechanism for chemisorption. The solubility product for precipitation and the activities of the species at the solid-liquid interface determine the surface precipitation process. Under appropriate electrochemical conditions, the activity of certain species can be higher in the interfacial region than that in the bulk solution and such a redistribution can lead to many reactions. For example, the sharp increase in adsorption of the calcium species on silica around pH 11 has been shown to be due to surface precipitation (Somasundaran and Anan-thapadmanabhan, 1985 Xiao, 1990). Similar correlations have been obtained for cobalt-silica, alumina-dodecylsulfonate, calcite/apatite/dolomite-fatty acid, francolite-oleate and tenorite-salicylaldoxime systems. The chemical state of the surfactant in the solution can also affect adsorption (Somasundaran and Ananthapadmanabhan, 1985). 

Langmuir covers the broad area of surface and colloid chemistry. Topics include micelles, emulsions, surfactants, vesicles, wetting and interfacial films, chemisorption and catalysis, electrochemistry, and physical adsorption on liquid and solid surfaces. Original research articles and letters arc accepted. There are occasional invited review tvpe articles. The journal is intended to provide a comprehensive coverage of the field of chemistry at interfaces. 

Dependence of Adsorption on Temperature. Figures 11 and 12 show the temperature dependence of adsorption for several foam-forming surfactants on sandstone or unconsolidated sand. Physical adsorption is an exothermic process and is expected to decrease with increasing temperature. This trend is observed for the anionic surfactants (Figures 11a and 12) Adsorption decreases up to an order of magnitude when the temperature is raised from 50 to 150 °C. In contrast, adsorption of the amphoteric surfactants is affected very little by temperature and may even show a slight increase with temperature in some cases (Figure lib). An increase in adsorption with temperature has sometimes been taken as an indication of chemisorption (36). 

Most common is the theory of chemisorption of collectors at the surface of solid particles. However, it is, as a rule, at the same time noted that simultaneously concurrent processes take place, which are connected both with adsorption of surfactants at 1/g interfaces and with their interaction with ions in the flotation pulp. Thus, it is shown in [66] that saturated C 2—Cj4 fatty acids are selectively chemisorbed on calcium carbonate surfaces which provides them with an effective hydrophobisation. At the same time, C5—Cg acids are adsorbed mainly at 1/g interfaces which produces a substantial influence on the stability of the flotation foams. Free and Miller [67] have thoroughly investigated the behaviour of sodium oleate in the flotation of a calcium mineral, fluorite. It has been established that calcium dioleates are formed both in the bulk and at 1/s interfaces. In this case, an effective hydrophobisation of the surface of fluorite particles takes place both due to the interaction of oleate with calcium ions on the active sites and by adsorption of calcium dioleates formed in the solution. It has been once more confirmed [68] that classical collecting agents, xanthogenates, e.g. ethyl xanthogenate, form on the... 

Chemical interactions The chemical contribution may result from interactions such as covalent or complex bond formation between the surfactants and the surface sites. Surfactants such as fatty acids, alkyl sulfates, alkyl sulfonates, amines and alkylhydroxa-mates have been proposed to adsorb by means of chemical interactions on a variety of particles. In addition, surfactants containing hydroxyl, phenolic, carboxylic and amine groups can hydrogen-bond with the surface sites. Infrared spectroscopy has been used to understand the chemisorption of surfactants at the surface, by examining the shift in the characteristic peaks of the surfactants upon adsorption. 

For example, as shown in Figure 10.36 (15), no adsorption of myristate (an anionic surfactant) occurs below the isoelectric point (pH 7.0) of chromite (even though the surfaces are oppositely charged), while essentially complete flotation is achieved at about pH 9.0 (when the surfactant and surface are similarly charged). The lack of flotation below the lEP was attributed to the limited solubility of myristic acid under these conditions. A similar behaviour has been observed for the flotation of haematite with myristate. Chemisorption of another anionic collector, octyl hydroxamate, has also been observed on haematite at pH 9.0 (15). 

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