Hydrophilicity of Nonanoic Acid and Its Conjugate Base at the Air/Water Interface

Abstract

A general adsorption model based on partial dissociation was developed for carboxylic acids. The model was applied to the adsorption of nonanoic acid at the air/water interface. Two cases were selected for experimental verification: acid-only and acid with a constant Na + OH – concentration. The model was applied simultaneously at both conditions, and the hydrophilicity of the ionic states was quantified by the adsorption constants, K A and K AH . It was found that the adsorption constant for the acidic group is significantly higher than that for the carboxylate group, K AH /K A ∼ 272. The model lays important groundwork for modeling and predicting carboxylic/carboxylate adsorption.

Smart Air–Water Interfaces with Arylazopyrazole Surfactants and Their Role in Photoresponsive Aqueous Foam

“…Note that we do not distinguish between two ionic states of AAP-E 4 like Boyer et al as well as Badban and co-workers did for other carboxylic acid surfactants. 37 , 38 For that reason, we report in our study an apparent rate constant, which describes the combined action of both species. The reason why we resort to a simplified model is that it reduces the number of free parameters for fitting the experimental results and enables a comparison of the (overall) kinetics during photoswitching as a function of pH as well as between the IF and the bulk (see below).…”

Insights into the behavior of nonanoic acid and its conjugate base at the air/water interface through a combined experimental and theoretical approach

“…This result is in agreement with previous studies. 20 , 21 It is obvious that HA is much more likely to partition to the surface than the A – and at low concentrations the deprotonated form (A – ) is negligible on the surface.…”

“…For the surface adsorption of A – , a slightly modified version of another ion-binding model reported from Badban et al was used to fit the experimental data. 21 The final equation that links the derivative of surface tension with the concentration of A – eqn (6) below: where γ is surface tension, R is the ideal gas constant, T is temperature, c A – represents the bulk concentration of A – , Γ max is defined as the total available sites on the surface, and K A – represents the adsorption constant for A – . Detailed calculations can be found in the ESI.…”

“…The protonated form of fatty acids is more surface-active while the deprotonated form is relatively surface inactive. 20 , 21 This is of particular interest since studies have shown that the pH of aerosol particles can vary dramatically within a large range (0–8). 22 – 26 Therefore, it is crucial to understand the behavior of protonated and deprotonated medium-chain fatty acids at the surface and in the bulk and how changes in pH alter their behavior in order to understand how this pH range affects the properties of the aerosol surface.…”

“…Two adsorption models were fitted with the experimental data to obtain the partitioning of both HA and A – . 19 , 21 To close the cycle shown in Scheme 1 , we include the surface acid dissociation constant. This constant is usually called the “surface p K a ” 20 , 29 and it is a value of great interest.…”