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.
Ethanol is a common amphiphilic solvent, often used in conjunction with water. However, despite its widespread use, key questions regarding the thickness and composition of molecules at the ethanol/water/air surface remain unclear. Recent thermodynamic analyses, Bagheri and co-authors (2016) and Santos and Reis (2018), indicated that the interfacial thickness is not constant.However, the interfacial thickness from these two analyses follows opposite trends. This study aims to provide a detailed description of the thickness and composition of the interfacial layer by combining neutron reflectivity (NR) experiments with rigorous molecular simulation. The
Since
the early 20th century, the slightly disparate measurements
of a surfactant’s critical micelle concentration, via either
surface tension or electrical conductivity, have been assumed one
and the same. As a consequence, the possibility that micelles can
adsorb at the air/water surface has been disregarded and has led to
some abnormalities in the literature that remain as yet unresolved.
In this paper, we closely examined the two critical concentrations
for a double-chain cationic surfactant. We confirmed that the two
concentrations represent two different physical phenomena. Furthermore,
the results verified the existence of surface micelles, which are
different from the bulk micelles. The formation of the surface micelles
can be explained by the structural changes of the adsorption layer,
which was also corroborated by molecular simulations. The findings
open new challenges to examine the surface adsorption, which offers new
insights into the molecular levels.
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