Adsorption kinetics of sodium dodecyl sulfate on perturbed air-water interfaces

A flow velocity dependence of dynamic surface tension in Plateau borders of foam

HYPOTHESIS

Liquid drainage through foams is a multiscale process, that primarily occurs through channels known as Plateau borders (PBs). Recent experimental studies of isolated PBs have observed variations in channel surface tension, γ, with liquid flow rate, Q, for systems containing soluble low molecular weight surfactant (LMWS). The current study proposes that the dynamic surface tension (DST) could be responsible for this effect, where the residence time of surfactant molecules in the PB is similar to the time required for their adsorption to the channel interface.

EXPERIMENTS

Profile geometries of isolated 'ideal' PB's were created in a bespoke experimental setup at controlled forced liquid flow rates. Average surfactant residence times, τ_Res, were calculated for solutions of Sodium dodecylsulfate (SDS), Tween 20 (T20) and Tween 80 (T80), and used to calculate corresponding average DST values in discrete regions of measured PB profiles. DST values were combined with microscale drainage theory to assess the potential physical implications on liquid flow.

FINDINGS

Significant variations in the magnitude of γ were calculated based on surfactant characteristics, where only the rapid adsorption of SDS was sufficient to produce DST values approaching equilibrium. These findings seriously question assumptions of near equilibrium surface tension in LMWS foam systems above their critical micelle concentration (CMC). Furthermore, the presence of surface tension gradients identified using this discrete approach, highlights the need to further refine the current theory to a continuous approach incorporating Marangoni effects.

Conversion of Viscous Oil-in-Water Nanoemulsions to Viscoelastic Gels upon Removal of Excess Ionic Emulsifier

Viscous, flowable nanoemulsions stabilized with ionic emulsifier can be transformed into repulsively jammed elastic gels that do not flow under gravity by reducing the droplet size and increasing the interfacial repulsive shell layer thickness. However, a high concentration of emulsifier required to achieve nanodroplets could remain in the continuous phase and lead to oscillatory structural forces, thereby reducing repulsive interaction and forming flowable liquid systems. It was hypothesized that the removal of excess emulsifier from a nanoemulsion could lead to the formation of repulsive gels. Canola oil-in-water nanoemulsions, containing 40 wt % oil, were prepared with a citric acid ester of monoglyceride (Citrem) using a high-pressure homogenizer. The excess emulsifier in the aqueous phase was removed by multiple ultracentrifugation cycles, and the droplet size, rheology, and stability of the nanoemulsions were investigated as a function of excess Citrem concentration. Nanoemulsions with average droplet sizes of 222 and 150 nm were obtained with 3 and 5 wt % Citrem, respectively. The removal of excess Citrem did not change the droplet size significantly. However, the viscosity, yield stress, and storage moduli increased significantly with the reduction of excess Citrem and the decrease in droplet size, converting the flowable weak gel nanoemulsion to a strong viscoelastic gel. The calculated values of oscillatory structural forces decreased with the removal of excess emulsifier, leading to an increase in repulsive interactions and the thickness of the electric double layer. Such an increase in interdroplet separation led to an increase in the effective oil volume fraction beyond the maximum random jamming of oil droplets and the formation of a viscoelastic nanoemulsion gel.

Protein Adsorption at the Air-Water Interface by a Charge Sensing Interferometric Technique

Protein uptake at the interface of a millimeter-sized air bubble in water is investigated by a recently developed differential interferometric technique. The technique allows the study of capillary waves with amplitudes around 10^-9 m, excited at the surface of the bubble by an electric field of intensity on the order of 10 V/cm. When one studies the resonant modes of the bubble (radial and shape modes), it is possible to assess variations of interfacial properties and, in particular, of the net surface charge as a function of bulk protein concentration. Sensing the interfacial charge, the technique enables us to follow the absorption process in conditions of low concentrations, not easily assessable by other methods. We focus on bovine serum albumin (BSA) and lysozyme as representatives of typical globular proteins. To provide comprehensive insight into the novelty of the technique, we also investigated the equilibrium adsorption of sodium dodecyl sulfate (SDS) ionic surfactant for bulk concentrations at hundreds of times lower than the Critical Micelle Concentration (CMC). Results unveil how the absorption of charged molecules affects the amplitudes of the bubble resonant modes even before affecting the frequencies in a transition-like fashion. Different adsorption models are proposed and developed. They are validated against the experimental findings by comparing frequency and amplitude data. By measuring the charging rate of the bubble interface, we have followed the absorption kinetics of BSA and lysozyme recognizing a slow, energy barrier limited phenomena with characteristic times in agreement with data in the literature. The evaluation of the surface excess concentration (Γ) of BSA and SDS at equilibrium is obtained by monitoring charge uptake. At the investigated low bulk concentrations, reliable comparisons with literature data from equilibrium surface tension isotherm models are reported.

Measuring the impact of channel length on liquid flow through an ideal Plateau border and node system

The phenomenon of foam drainage is a complex multi-scale process that unites molecular level interactions with bulk foam characteristics. Foam drainage is primarily governed by the flow of liquid in the channels and junctions that form between bubbles, which are known as Plateau borders (PBs) and nodes respectively. Existing theoretical work predicts the surface rheology of the PB and node air-liquid interface to influence liquid flow rates; however, direct experimental observations of this phenomenon remain scarce. This study recognises the clear need for a reproducible, accurate and standardised approach to directly studying liquid flow at the scale of a theoretically 'ideal' PB-node architecture. Measurements of PB geometric profiles and their apparent surface shear viscosities, μs, were made for an aqueous solution of Sodium Dodecyl Sulphate (SDS) at varying PB lengths, l1, and liquid flow rates in the range 10 μl min-1 ≤ Q ≤ 200 μl min-1. Geometric profiles displayed previously unobserved transitions between PB relaxation and expansion towards the node, with expansion dominating under conditions approaching conventional foam drainage. Average values of μs in the PB relaxation regions showed virtually inviscid behaviour, with magnitudes of 10-8 g s-1 < μs < 10-4 g s-1 for l1 in the range 27.5 mm ⪆ l1 ⪆ 8.0 mm. Decreasing magnitudes of μs and degrees of shear thinning were observed with increasing l1. This was attributed to a compressibility of the interface that was limited by an SDS concentration dependence on l1. Numerical evaluation predicted the appearance of Marangoni forces that scaled strongly with liquid shear rates, and could therefore have been responsible for the apparent shear thinning behaviour.

The Role of Electrostatic Repulsion on Increasing Surface Activity of Anionic Surfactants in the Presence of Hydrophilic Silica Nanoparticles

Hydrophilic silica nanoparticles alone are not surface active. They, however, develop a strong electrostatic interaction with ionic surfactants and consequently affect their surface behavior. We report the interfacial behavior of n-heptane/anionic-surfactant-solutions in the presence of hydrophilic silica nanoparticles. The surfactants are sodium dodecyl sulfate (SDS) and dodecyl benzene sulfonic acid (DBSA), and the diameters of the used particles are 9 and 30 nm. Using experimental tensiometry, we show that nanoparticles retain their non-surface-active nature in the presence of surfactants and the surface activity of surfactant directly increases with the concentration of nanoparticles. This fact was attributed to the electrostatic repulsive interaction between the negatively charged nanoparticles and the anionic surfactant molecules. The role of electrostatic repulsion on increasing surface activity of the surfactant has been discussed. Further investigations have been performed for screening the double layer charge of the nanoparticles in the presence of salt. Moreover, the hydrolysis of SDS molecules in the presence of silica nanoparticles and the interaction of nanoparticles with SDS inherent impurities have been studied. According to our experimental observations, silica nanoparticles alleviate the effects of dodecanol, formed by SDS hydrolysis, on the interfacial properties of SDS solution.

Achieving foaming control smartly: pre-solubilized flavor oil serves as an in situ homogeneous defoamer

In the wide application of aqueous foam, creating abundant foam and processing appropriate foaming control are both essential, depending upon the actual situation; the latter process is not only harder to achieve, but also more complicated to comprehensively understand on the molecular level. In this paper, a type of natural flavor oil, carvone, was solubilized in a micelle solution of sodium dodecyl sulfate (SDS) to study the effect on the foaming properties. The foamability and foam stability of the swollen micelle solutions were experimentally characterized, and the molecular behavior of the surfactant and oil molecules before, during and after the foaming process were investigated. It was found that the solubilized carvone co-adsorbed with SDS at the gas/water interface and caused a prominent effect on the foam film stability in several approaches, thereby making the flavor oil a possible foam controller that would not inhibit foam formation, but could eliminate foam efficiently once foam was undesired. Interestingly, it was found that the release of flavor in the foaming process was promoted. Detailed discussion of the interfacial behavior of carvone and the effect on the foaming properties of surfactants in different stages of foam may provide a theoretical foundation for exploring green and smart approaches in achieving foaming control.