Adsorption of Ions at Uncharged Insoluble Monolayers

Abstract

A method is proposed for the experimental determination of the adsorption of inorganic electrolytes at a surface covered with insoluble surfactant monolayer. This task is complicated by the fact that the change of the salt concentration alters both chemical potentials of the electrolyte and the surfactant. Our method resolves the question by combining data for the surface pressure versus area of the monolayer at several salt concentrations with data for the equilibrium spreading pressure of crystals of the surfactant (used to fix a standard state). We applied the method to alcohols spread at the surface of concentrated halide solutions. The measured salt adsorption is positive and has nonmonotonic dependence on the area per surfactant molecule. For the liquid expanded film, depending on the concentration, there is one couple of ions adsorbed per each 3-30 surfactant molecules. We analyzed which ion, the positive or the negative, stands closer to the surface, by measuring the effect of NaCl on the Volta potential of the monolayer. The potentiometric data suggest that Na(+) is specifically adsorbed, while Cl(-) remains in the diffuse layer, i.e., the surface is positively charged. The observed reverse Hofmeister series of the adsorptions of NaF, NaCl, and NaBr suggests the same conclusion holds for all these salts. The force that causes the adsorption of Na(+) seems to be the interaction of the ion with the dipole moment of the monolayer.

Barrier kinetics of adsorption–desorption of alcohol monolayers on water under constant surface tension

“…determined a barrier rate constant for myristic acid [18] at pH 2. In our study [17] of dodecanol monolayers spread on pure water or concentrated aqueous electrolytes, we found that the desorption is proceeding under barrier control for the initial 1-2 minutes. The apparent disagreements in the literature regarding the mechanism of adsorption and desorption is further discussed in the review by He et al [22].…”

“…The isobaric regime has been used for determination of the diffusion coefficient D of the surfactant [5] and for evaluation of its solubility [11]; to study the dissociation and the Hofmeister effect on the properties of monolayers of adsorbed acids [7]; to study the processes involved in the collapse of the monolayer [8,12]; phase transitions [14] in monolayers; interaction between adsorbed lipids and proteins [10]; and generallythe mechanism of desorption [4,6,9]. Recently, we used the isobaric regime as an auxiliary experiment to correct the  S vs. A isotherms for the material loss due to dissolution of the monolayer [17] (similarly to Motomura et al [18]).…”

Effective osmotic cohesion due to the solvent molecules in a delocalized adsorbed monolayer

“…5 in Ref. [41]). However, this effect is complicated by the direct screening of the van der Waals attraction between the hydrocarbon tails by the salt.…”

“…S1) to determine the hard-disc area s of the surfactant independently, instead of fitting it. The alcohol group is of cross-section area smaller than that of the hydrocarbon chain, so we used crystallographic data for solid alkanes [38,39] together with area of collapse of insoluble monolayers of alcohols [40,41]: these data yield an average area per molecule in a close-packed structure of  = 18.2±0.4 Å 2 . This value must be corrected by a packing factor: for hexagonal packing,  must be divided by 1.10 (the ratio between the area of a hexagon and the inscribed circle) to obtain the actual area s = 16.5±0.4 Å 2 of the hard disc.…”

“…5 in Ref. [41]), so instead of using s = 34.2 Å 2 , we left s of CnH2n+1EO6 as an adjustable parameter to be determined from the experimental data. The areas  and s are, to a good approximation, independent of n.…”