Measuring Partition Coefficients of Colloids at Air−Water Interfaces

Abstract

Quantification of surface excesses of colloidal particles at gas-liquid interfaces under environmentally relevant conditions has not previously been reported, despite its importance in understanding and predicting partitioning, transport, and transformations of colloids and other organic and metal species complexed onto colloids in many natural environments and engineered systems. In this study, we developed a bubble column method for measuring partition coefficients of colloids at air-water interfaces. The method was validated by comparing the adsorption isotherm constant for a surfactant measured using our method with that determined from conventional surface tension measurements and the Gibbs absorption equation. The first measured partition coefficients of kaolinite and humic colloids at the air-water interfaces are reported. The bubble column method can also be used to identify surface exclusion of colloids at the air-water interface. By extension, this method will permit quantification of surface activities of a wide range of inorganic, organic, and microbial colloids as well as molecular species complexed onto colloids.

Adsorption and Wetting Mechanisms at the Surface of Aqueous Hydrocarbon Solutions as a Possible Source of Atmospheric Pollution

“…According to the theory developed by Lemlich [20] and applied by Wan and Tokunaga [13], the fundamental hypothesis is Equation (5) where K is the proportionality constant; this relation assumes a linear variation of adsorption, Γ, versus the bulk concentration, C. In environmental studies [21], this isotherm equation is often termed the Freundlich isotherm (with exponent parameter equal to 1); but, in fact it is rather the limit form of the Langmuir isotherm at very low concentrations [8]. This approximate form can be used for almost all of the systems with very small adsorption values:…”

“…The parameter J characterises the separation of the solute from the solvent induced by the adsorption upon the ascending bubbles. Moreover, integrating Equation (6) along the column height and expressing the mass conservation during the building of concentration gradient lead finally to: (8) where C 0 is the species homogeneous initial concentration, and H is the height of the liquid column [13]. For each species present in the bubble column, the measurement of C(z) at steady state along the column versus the altitude z gives J from Equation (8).…”

“…With this value of J, additional measurements of A, a, f and D lead to K by solving equation (7), and finally to Γ by using Equation (5). Usually, D can be determined from the expression of the timedependent concentration profile and measurement of such profiles by different methods [13,22]. Actually, we followed Wan and Tokunaga and took always the same value of D (6.1 × 10 -4 m s -1 ) and the same relative uncertainty, 0.1, because the characteristics of our column and system were alike theirs.…”

“…Most of the device characteristics depicted on Figure 2 were taken from the description by Wan and Tokunaga [13]. The column consists of a 2-m-high tube of Pyrex; its inner diameter is 19 mm.…”

“…On the other hand, for very dilute solutions and hexane we applied the bubble column method [13], which is in fact a variant of the depletion method used to study adsorption on solid suspensions; in the case of a three-component mixture, this method allows one to determine the adsorption of each solute. But, since its implementation is quite difficult, it is worth being employed only in very specific cases where no other method is relevant.…”