Adsorption rock surface effect

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

Experiments were also carried out to determine the effects of salts on the adsorption of actinides on rock surfaces. The effects of the formal cationic charge of the added salts as well as the concentration of the salts were investigated. 

Another interesting result reported by Sarkar and Sharma (55) was that after saturating the core initially with a polar crude oil, the damage ratio was only 1/1.5 and the permeability decline was very slow. Thus, allowing the rock surfaces to come in direct contact with the polar oil did make the rock less susceptible to damage. This effect is apparently related to adsorption of polar components on the rock surfaces and altered wettability of the fines. 

This shows the sacrificial nature of the silicates. Also some work by Somasundaran (30) has shown that the sodium silicates are more effective than the other alkaline chemicals in reducing surfactant adsorption on rock surfaces. 

The data from these tests show that sodium orthosilicate is more effective than sodium hydroxide in recovering residual oil under the conditions studied, both for continuous flooding and when 0.5 PV of alkali was injected. The mechanisms through which sodium orthosilicate produced better recovery than sodium hydroxide in this system have not been completely elucidated. Reduction in interfacial tension is similar for both chemicals, so other factors must play a more important role. Somasundaran (26) has shown that sodium silicates are more effective than other alkaline chemicals in reducing surfactant adsorption on rock surfaces. Wasan (27,28) has indicated that there are differences in coalescence behavior and emulsion stability which favor sodium orthosilicate over sodium hydroxide. Further work is being done in this area in an attempt to define the limits of physically measurable parameters which can be used for screening potential alkaline flooding candidates. 

Since surfactant adsorption can alter rock surface wettability, it is possible that a surfactant could change a water-wet surface to oil-wet and break the foam. Such foam effects on porous media surfaces must be considered in the design of the foam. 

In this section, we consider the problems relevant to equiUbriiim of the two multicomponent phases separated by a curved interface. This is the classical and the most well-studied case of the thermodynamic equilibrium involving surface effects. Such equilibrium is present in macro-porous and mesoporous media, like the porous rocks of petroleum reservoirs, where it accompanies adsorption. In the pores of smaller sizes, the forces produced by the solid surfaces may modify the properties of the bulk (Uquid and gas) phases. However, the present study is also important to the pores of smaller sizes, as it makes it possible to separate the effects connected with the gas-liquid surface tension (and, of course, the contact angle) from additional contributions of the solid walls. The corrections related to the last type of interactions have been considered in, for example. Refs. [13-15]. For brevity, we will apply the term capillary equilibrium to the narrow case being described, but it must be remembered, however, that a wider understanding of the capillary equilibrium is available. 

The data in Table 2.2 are for natural samples. It is notoriously difficult to prepare and maintain a clean solid surface, since any freshly created surface quickly becomes contaminated with adsorbed species even with a rather good laboratory vacuum. Any naturally occurring solid material must be considered to have a surface extensively populated by adsorbed atoms and molecules rather than a pristine surface. Such surfaces are what are examined in most laboratory experiments (e.g., those reported in Table 2.2), and, of coruse, just such surfaces are geochemically relevant for noble gas adsorption. It is interesting to note, however, that in other situations, noble gas adsorption can be rather a stronger effect. Thus, for example, Bernatowicz et al. (1983) examined Xe adsorption on a vacuum-crushed lunar rock and concluded that a small part of the freshly created surface had an adsorption potential as high as 14 kcal/mole but that in a few days at 10 8torr this surface was rendered inaccessible to Xe by other chemical species that were better competitors for the sorbent surfaces. 

Surface area of the porous media has a remarkable effect on surfactant adsorption. Liu (2007) measured surfactant adsorption in three rock samples of the same carbonate porous medium but with different surface areas. He used a TC blend surfactant—1 1 mixture by weight of dodecyl 3 ethoxylated sulfate and iso-tridecyl 4 propoxylated sulfate from Stepan. He found that the adsorptions of the TC blend on the three samples were close to each other if the adsorption was calculated by using surfactant adsorption amount per porous media surface area, as shown in Figure 7.43. However, if the adsorption was... 

Regarding the surfactant type and rock type, nonionic surfactants have much higher adsorption on a sandstone surface than anionic surfactants (Liu, 2007). However, Liu s initial experiments indicated that the adsorption of nonionic surfactant on calcite was much lower than that of anionic surfactant without the presence of NaaCOs and was of the same order of magnitude as that of anionic surfactant with the presence of Na2C03. Thus, nonionic surfactants might be candidates for use in carbonate formations from the adsorption point of view. The role of salinity is much less important, but the temperature effect is much more important for nonionics than for anionics (Salager et al 1979a). More factors that affect adsorption were discussed by Somasundaran and Hanna (1977). 

Other large organic molecules may favourably interact with natural zeolite or clay surfaces. Of interest is the ability of these materials, e.g., clinoptilolite- or montmorillonite-rich rocks, to adsorb on their hydrophilic, negatively charged surfaces complex substances, such as aflatoxins, which arc toxic secondary metabolites of several agricultural products, containing polar functional groups [70,71J. Adsorption, which has been proven either in-vitro or in-vivo, is effective and amounts to some hundred pg per g of adsorber. 

In the mixed-wettability cores, adsorption is affected only slightly by the presence of oil. The compensating effects of a decrease in the accessible solid—water interface and an increase in oil—water interface could be the reason for the relatively small effect. The most notable effect of the asphaltene treatment is the substantially higher adsorption density on the more oil-wet rock. Surfactant adsorption is higher on hydrophobic surfaces than it is on hydrophilic surfaces, a finding that is consistent with literature data. 

O Day PA, Carroll SA, Waychunas GA (1998) Rock-water interactions controlling zinc, cadmium, and lead concentrations in surface waters and sediments, U.S. Tri-State Mining District. I. Molecular identification using X-ray absorption spectroscopy. Env Sci Tech 32 943-955 O Reilly S, Strawn DG, Sparks DL (2001) Residence time effects on arsenate adsorption/desorption mechanisms on goethite. Soil Sci Soc Am J 65 67-77... 

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