Difference in Surface Properties between Insoluble Monolayer and Adsorbed Film from Kinetics of Water Evaporation and BAM Image

Abstract

The evaporation rate of water molecules across three kinds of interfaces (air/water interface (1), air/surfactant solution interface (2), and air/water interface covered by insoluble monolayer (3)) was examined using a remodeled thermogravimetric balance. There was no difference in both the evaporation rate and the activation energy for the first two interfaces for three types of surfactant solutions below and above the critical micelle concentration (cmc). This means that the molecular surface area from the Gibbs surface excess has nothing to do with the evaporation rate. In the third case, the insoluble monolayer of 1-heptadecanol decreased the evaporation rate and increased the activation energy, indicating a clear difference between an insoluble monolayer and an adsorbed film of soluble surfactant. This difference was substantiated by BAM images, too. The images of three surfactant solution interfaces were similar to that of just the water surface, while distinct structures of molecular assemblies were observed for the insoluble monolayer. The concentration profile of water molecules in an air/liquid interfacial region was derived by Fix's second law. The profile indicates that a definite layer just beneath the air/liquid interface of the surfactant solution is made mostly of water molecules and that the layer thickness is a few times the root-mean-square displacement %@mt;sys@%%@rl;;@%2%@ital@%Dt%@rsf@%%@rlx@%%@mx@% of the water molecules. The thickness was found to be more than a few nanometers, as estimated from several relaxation times derived from the other kinetics than evaporation of amphiphilic molecules in aqueous systems and a maximum evaporation rate of purified water.

Aerosol hygroscopicity at high (99 to 100%) relative humidities

“…This model of the SDS solution droplet with a complete coating of solute at the droplet-air interface is similar to the "inverted micelle" model of solution droplets with organic solutes that has been previously described (Gill et al, 1983;Ellison et al, 1999), and for which some observational evidence has been obtained (e.g., Husar and Shu, 1975;Tervahattu et al, 2002;Russell et al, 2002). However, this blurs the distinction between soluble (e.g., SDS) and insoluble (e.g., fatty acids) surfactants (Moroi et al, 2004). For a soluble surfactant, the excess surface concentration is not expected to reside exclusively at the interface (e.g., purely in a self-assembled monolayer), but rather as a concentration gradient between the bulk and surface phases.…”

“…This interpretation of the surface excess concentration is supported by measurements of evaporation rates from SDS solutions, which are similar to those of pure water. In contrast, a compressed film of insoluble surfactant can substantially reduce evaporation rates from aqueous solutions (Moroi et al, 2004). However the same effect could result from any mechanism causing a decrease in SDS solution activity that goes as D −1 wet .…”

Evaporation of Droplets of Surfactant Solutions

“…The studies encompass different conditions: constant pressure and temperature, elevated pressure, fast compression, still gas atmosphere and turbulent reacting flows, strongly and weakly pinning substrates [1,2]. The experimental, theoretical and computer simulation studies carried out so far [1][2][3][18][19][20][21][22][23][24][25][26][27] have taken into account different physical processes: heat transfer inside droplets, mass diffusion in bi-and multicomponent fluids, droplet interactions in sprays, turbulence, radiation absorption, thermal conductivity of the solid substrate, Marangoni convection inside the droplets.…”

Molecular insights of adsorption and structure of surfactants and inorganic ions at fluid – liquid interfaces

“…Another model suggests that the structure of the adsorbed layer of soluble surfactants differs from that of conventional surface monolayers of insoluble surfactants. Whereas the latter is located on the outermost layer of the air -water surface, the former is in the form of a bimolecular layer located "at a certain distance below the surface" (Humphry-Baker et al, 2006, Nakahara et al, 2005, Moroi et al, 2004, Rusdi et al, 2005a, Rusdi et al, 2005b. However, this model was later modified by surface potential measurements to consider a soluble monolayer that is adsorbed at the topmost surface layer with an adsorbed bilayer lying some distance beneath this monolayer.…”

“…Recently, alternative adsorption geometries such as bimolecular adsorption at deeper layers of the fluid -liquid interface have been proposed for soluble surfactants using a variety of experimental techniques such as Brewster angle microscopy (Moroi et al, 2004), evaporation rate measurements (Rusdi et al, 2005b), pyrene fluorescence spectroscopy (Humphry-Baker et al,…”