Selective adsorption of n-fatty acids at the silica/benzene and silica/n-hexane interface. Part 2.—Heats of adsorption

Abstract

The heats of (selective) adsorption of the n-C6, C,, CIz, CI4 and c 1 6 fatty acids at tlic silica/ benzene interface have been determined from measurements of the heats of immersion of " fully " hydroxylated silicas in benzene + n-fatty acid solutions. Within experimental error, the molar heat of adsorption is independent both of the surface coverage of the fatty acids and of their hydrocarbon chain length. The heats of solution of the fatty acids in benzene at 27°C have also been measured and indicate that the acids exist as monomeric species only at low solution concentrations. At concentrations above about m the species tend to associate due to hydrogen bonding between thecarboxylic head groups as shown by infra-red spectroscopic studies. The present results and those in the preceding paper indicate that the limiting adsorption is observed because only the monomeric fatty acid species is surface active, causing the surface coverage to tend to a limiting value as the concentration of the monomeric species in the equilibrium solution phase approaches an upper limit set by the aggregation process.The thermodynamics of adsorption at the solid/liquid interface with reference to the determination of the differential heat of adsorption of one component from a binary-solution has been considered by Crisp,' Everett and Corkill et aL3 These workers have shown that AH1 = ( a h I / a n S ) P , T , Z , n ; ,(1) where is the differential heat of adsorption at the solid/liquid interface of component 1 from the liquid solution phase, h, is the heat of immersion of the evacuated solid adsorbent into the binary liquid solution, ni and n; are the number of moles of components 1 and 2 respectively in the interfacial phase, and E is the area of the interfacial phase. Thus, to evaluate ml as a function of n: it is necessary to measure the heat of immersion of the adsorbent in a range of binary liquid solutions and also to know the adsorption isotherm of component 1.In practice, when one immerses an evacuated solid in a finite quantity of a binary solution from which one component (1) is selectively adsorbed the wetting process causes a dilution of the solution phase. In this case, where hT is the total heat change measured by the calorimeter during the immersion process, h, is the heat of immersion defined above and hD is the heat change caused by the dilution of the binary liquid phase. In many systems, hD is sufficiently small hT = h1-I-h~(2)

Adsorption of organic molecules on silica surface

“…versus concentration. Calorimetric data at 27°C for the adsorption of n-fatty acid from benzene solution on silica indicates the adsorption to be independent of surface coverage and chain length [277]. It was further postulated that the expected decrease in heat of adsorption due to adsorbateadsorbent interaction is offset by a decreasing enthalpy of mixing of the adsorbed film.…”

“…It was further postulated that the expected decrease in heat of adsorption due to adsorbateadsorbent interaction is offset by a decreasing enthalpy of mixing of the adsorbed film. Heats of adsorption of benzene, pyridine, and furan on to silica were measured calorimetrically [276][277][278][279][280]. Much work has been devoted to the development of infra red spectroscopy as an indirect method of determining heats of adsorption on silica including aromatics from both the liquid [281][282][283] and gas [279,280,[284][285][286].…”

Adsorption of Benzoic and 4-Heptylbenzoic Acid on Different Silica Substrates from Organic and Aqueous Solution

“…Earlier results on the change in max of fatty acids from heptane on silica, where considerable care was taken to exclude water from the system, have shown that the extent of fatty acid adsorption decreases with increasing chain length from hexanoic to palmittic acid (15). It was concluded that this decreased adsorption could not be caused by a parallel orientation of the acid molecules on the surface and that it was probably not related to the structure of the adsorbed phase.…”

“…Armistead et al (15) and Madsen and Lind (9) used carefully dried silica mixed with dried solutions of carboxylic acids in their experiments. Results obtained by Madsen and Lind (9) indicate that there was a small but insignificant (i.e., within the error limits of the experiments) decrease in the adsorption from cyclohexane on silica when the adsorbate was changed from hexanoic (C 6 ) to benzoic (C 7 ) acid.…”

“…In studies of the adsorption of surface-active molecules from organic phase on mineral surfaces, the system has normally been made water-free by extensive drying of the solvent, solute, and solid surfaces (9,10,(13)(14)(15). However, water is an inherent part of all oil-bearing formations, whether it is present as a thick film or as a monolayer on the surface of the reservoir mineral.…”