New Evidence of Head-to-Tail Complex Formation of SDS-DOH Mixtures Adsorbed at the Air-Water Interface as Revealed by Vibrational Sum Frequency Generation Spectroscopy and Isotope Labelling

Crucial Role of Surfactants in Improving the Extraction Efficiency of β-Amyrin from Dischidia major Using Ultrasound-Assisted Extraction Coupled with Gas Bubble Flotation

Ultrasound-assisted extraction coupled with gas bubble flotation was developed as a green method for extracting β-amyrin fromDischidia major. The solvent system was water:ethanol (9:1). To improve the adsorption of β-amyrin onto the air/liquid interface during flotation, surfactants were employed; however, the positive effect was only observed with cationic surfactants. High-performance liquid chromatography with spectrophotometric detection (HPLC-PDA) was, for the first time, applied to quantify the β-amyrin content in D. major and its extracts. With the assistance of surfactants, the foam layer collected from flotation showed high selectivity toward β-amyrin. The product content was notably increased after surfactants had been removed from the solution.

Polarization-Dependent Sum-Frequency Generation Spectroscopy for Ångstrom-Scale Depth Profiling of Molecules at Interfaces

The three-dimensional spatial distribution of molecules at soft matter interfaces is crucial for processes ranging from membrane biophysics to atmospheric chemistry. While several techniques can access surface composition, obtaining information on the depth distribution is challenging. We develop a noninvasive, polarization-resolved, surface-specific sum-frequency generation spectroscopy providing quantitative depth information. We demonstrate the technique on formic acid molecules at the air-water interface. With increasing molar fraction from 2.5% to 10%, the formic acid molecules shift, on average, ∼0.9  Å into the bulk. The consistency with the simulation data manifests that the technique allows for probing the Ångstrom-scale depth profile.

Evidence of surfactant sub-monolayer adsorption at the air/water interface provided by laser scattering measurements of ultrafine gas bubbles

As the SDS concentration increases in bubble rich solutions, the surfactant layer alters its size and refractive index. The scattered light enhancement and SFG signal cancellation prove that sub-monolayer adsorption exists at the air/water interface.

Impact of Fluorocarbon Gaseous Environments on the Permeability of Foam Films to Air

A foam film, free to move and stabilized with tetradecyltrimethylammonium bromide or sodium dodecylsulfate surfactants, is deposited inside of a cylindrical tube. It separates the tube into two distinct gaseous compartments. The first compartment is filled with air, while the second one contains a mixture of air and perfluorohexane vapor (C₆F₁₄), which is a barely water-soluble fluorinated compound. This foam film thus acts as a liquid semipermeable membrane for gases equivalent to the solid semipermeable membranes conventionally used in fluid separation processes. To infer the rate of air transfer through the membrane, we measure the displacement of the mobile foam film. From this, we deduce the instantaneous permeability of the membrane. In contrast to the permeability of solid membranes, which inexorably decreases over time because they become clogged, an anticlogging effect is observed with a permeability that systematically increases over time. Because the thickness of the film is constant over time, we attribute this to the possibility of adsorbing or desorbing fluorinated gas molecules on the liquid membrane. Indeed, because the partial pressure of the fluorinated gas is high at the beginning of the experiment, the density of the adsorbed molecules is also high, which leads to a low permeability to air transfer. On the contrary, at the end of the experiment, the partial pressure in fluorinated gas and thus the density of the adsorbed molecules are low. This leads to a higher permeability and a less clogged membrane.

Specific Ion Effects of Dodecyl Sulfate Surfactants with Alkali Ions at the Air-Water Interface

The influence of Li⁺, Na⁺ and Cs⁺ cations on the surface excess and structure of dodecyl sulfate (DS⁻) anions at the air–water interface was investigated with the vibrational sum-frequency generation (SFG) and surface tensiometry. Particularly, we have addressed the change in amplitude and frequency of the symmetric S-O stretching vibrations as a function of electrolyte and DS⁻ concentration in the presence of Li⁺, Na⁺ and Cs⁺ cations. For the Li⁺ and Na⁺ ions, we show that the resonance frequency is shifted noticeably from 1055 cm⁻¹ to 1063 cm⁻¹ as a function of the surfactants’ surfaces excess, which we attribute to the vibrational Stark effect within the static electric field at the air–water interface. For Cs⁺ ions the resonance frequency is independent of the surfactant concentration with the S-O stretching band centered at 1063 cm⁻¹. This frequency is identical to the frequency at the maximum surface excess when Li⁺ and Na⁺ ions are present and points to the ion pair formation between the sulfate headgroup and Cs⁺ counterions, which reduces the local electric field. In addition, SFG experiments of the O-H stretching bands of interfacial H₂O molecules are used in order to calculate the apparent double layer potential and the degree of dissociation between the surfactant head group and the investigated cations. The latter was found to be 12.0%, 10.4% and 7.7% for lithium dodecyl sulfate (LiDS), sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CsDS) surfactants, which is in agreement with Collins ‘rule of matching water affinities’.