Metal oxides surfaces, surfactant adsorption

When an ionic/nonionic surfactant mixture adsorbs on a metal oxide surface, the admicelle exhibits negative deviation from ideality (74). This means that the adsorption level is higher than it would be if the admicelle were ideal, at a specific surfactant concentration below the CMC. Above the CMC, the adsorption level is dictated by the relative enhancement of micelle formation vs. admicelle formation. In this region, the level of adsorption can be viewed as the result of the competition between micelles and admicelles for surfactant. In analogy, the surface tension above the CMC can be viewed as competition between the monolayer and micelles for surfactant. 

The adsorption of surfactant mixtures on metal oxide surfaces (e.g., minerals) from aqueous solutions is an important process in such applications as enhanced oil recovery and detergency. Since surfactants used in real-world applications are almost always mixtures,... 

Lee, E.M. and Koopal, L.K., Adsorption of cationic and anionic surfactants on metal oxide surfaces Surface charge adjustment and competition effects, J. Colloid Interf. Sci., 177, 478, 1996. 

A good surfactant can also serve as an adjuvant to the lubrication process itself. The adsorption of stearate ion on metal oxide films has been studied [54, 55]. It was shown that the equilibrium for stearate adsorption on a metal surface was high, and the kinetics reasonably fast. Stearate, along with the appropriate cation, has even been shown to form multilayered structures on metal oxides. This added adsorption gives additional protection to the substrate. Adsorbed surfactants alone were able to reduce the friction of one system by -80% [56]. This was rationalized by the extended chains and their ability to easily orient along the direction of shear force. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

When sur-f actants adsorb on metal oxide sur-f aces (e.g., minerals), at low concentrations, the adsorbate molecules are widely dispersed enough that no signi-ficant interactions between adsorbed sur-f actants occurs. Above a certain critical concentration, dense sur-factant aggregates form on the surface (72). These are called admicelles. For ionic surfactants, the admicelles are bilayered structures (72). Above the CMC, the total adsorption of surfactant can increase or decrease slowly. 

Compared with literature data for adsorption of surfactants from aqueous solutions on oxide surfaces(/4,75), the kinetic data obtained in this work for C18/glass are of similar orders of magnitude to the former systems. The values of 1/k and k.for CIg/Al are greater than those for oxide adsorbent studied, indicating the strong adsorbing ability of metallic aluminum (even oxidized) relative to the mineral oxides. 

An important case addressed in numerous studies is the adsorption of anionic and cationic surfactants from aqueous solutions on polar surfaces such as metal oxides [5,6]. The shapes of experimental adsorption isotherms, which represent a relationship between the adsorption, T, and the equilibrium surfactant concentration, c, have been thoroughly investigated and some common features noted. A typical adsorption isotherm plotted on a log-log scale can be subdivided into four regions ( Fig. III-8, a), the interpretation of which and... 

Since the charge of many polar surfaces (especially of metal (hydrous) oxides) is determined by the concentration of H,0+ and OH" ions, the surfactant adsorption is strongly influenced by the solution pH. Changes in the pH may drastically affect the value of T, the concentration ranges corresponding to different regions on the isotherm (Fig. III-8), and the shape of the isotherm itself. 

Depending on the particle-surfactant system, one or more of the above contributions can be responsible for adsorption. The dominating one would depend on the nature and concentration of the surfactant, the surface chemistry of the particle, and solution properties such as pH and ionic strength. Electrostatic and lateral interaction forces are usually the major factors determining the adsorption of surfactants on oxides and other non-metallic minerals. Chemical interactions become more dominant for surfactant adsorption on salt-type minerals, such as carbonates and sulfides. 

Fig. 1 Electrodes for protein voltammetry (a) noble metals (Au, Ag) modified with a SAM. Group X is typically sulfur, while Y is a functionality, such as —CH3, COO, CH2OH, the variety and mixture of which can be designed to optimize the interaction with the protein. Examples are described in Refs. [10-13] (b) a metal oxide electrode. Examples are described in Refs. [9, 15-lSj (c) a carbon electrode, typically pyrolytic graphite with the edge surface projected to the solution. Protein adsorption is often optimized by inclusion of polycations. Examples are described in Refs. [1,14) (d) An electrode coated with a surfactant layer within which the protein is confined. Examples are described in Refs. [19-21].

In hard-surface cleaning, solid particles deposit primarily from air suspensions and embedded in greasy soils, such as silica and metal oxides, will have their wetting characteristics modified by surfactants. Through surfactant adsorption at the water-solid particle interface, the interfacial tension will be reduced, and so will be the adhesion forces binding particles together. Efficient surfactants are anionic surfactants that, when adsorbed, make the surface of the solid particles more negative and induce electrostatic repulsion between adjacent particles [60]. 

Microemulsions have been used as confined reaction media during the past two decades, since, due to the very small size of the droplets, they can act as microreactors capable to control the size of the particles and at the same time to inhibit the aggregation by adsorption of the surfactants on the particle surface when the particle size approaches that of the microreactor droplet. The synthesis of nanoparticles using reactions in microemulsions was first described by Boutonnet and cowoikers They synthesized monodispersed metal particles of Pt, Pd, Rh and Ir by reduction of metal salts with hydrogen or hydrazine in water in oil (w/o) microemulsions. Since then, many different types of materials have been prepared using microemulsions, including metal carbonates, metal oxides, " metal chalcogenides, "" polymers," etc. 

Flux is sometimes thought of as a catalyst that lowers the surface tension between the molten solder and a metal surface [98]. In reality, the chemistry of flux interactions at oxide surfaces can be very complicated and involve acid-base, oxidation-reduction, and coordination-type and adsorption-type reactions discussed in later sections [102-104]. Spalik prefers to think of most fluxes used for electronic soldering as substances that react as Bronsted-Lowry acids with metallic oxides to form their respective salts and water, and that the salts serve as surfactants that promote solder wetting. 

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