Structure–Behavior–Property Relationship Study of Surfactants as Foam Stabilizers Explored by Experimental and Molecular Simulation Approaches

A Comparative Study on CO2-Switchable Foams Stabilized by C22- or C18-Tailed Tertiary Amines

The CO2 aqueous foams stabilized by bioresource-derived ultra-long chain surfactants have demonstrated considerable promising application potential owing to their remarkable longevity. Nevertheless, existing research is still inadequate to establish the relationships among surfactant architecture, environmental factors, and foam properties. Herein, two cases of ultra-long chain tertiary amines with different tail lengths, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and N-oleicamidopropyl-N,N-dimethylamine (UC18AMPM), were employed to fabricate CO2 foams. The effect of temperature, pressure and salinity on the properties of two foam systems (i.e., foamability and foam stability) was compared using a high-temperature, high-pressure visualization foam meter. The continuous phase viscosity and liquid content for both samples were characterized using rheometry and FoamScan. The results showed that the increased concentrations or pressure enhanced the properties of both foam samples, but the increased scope for UC22AMPM was more pronounced. By contrast, the foam stability for both cases was impaired with increasing salinity or temperature, but the UC18AMPM sample is more sensitive to temperature and salinity, indicating the salt and temperature resistance of UC18AMPM-CO2 foams is weaker than those of the UC22AMPM counterpart. These differences are associated with the longer hydrophobic chain of UC22AMPM, which imparts a higher viscosity and lower surface tension to foams, resisting the adverse effects of temperature and salinity.

Synthesis and application of a green surfactant for the treatment of water containing PFAS/ hazardous metal ions

In this work, the effectiveness of a biodegradable and natural surfactant synthesized through a novel method has been studied through the ion flotation process to treat waters containing Per/Poly Fluoroalkyl Substances (PFAS) and heavy-metal ions. This cysteine-based surfactant, which is environmentally acceptable, showed considerable solubility and foaming ability over a wide range of pH. It also could remove 97-99(%) of 5 mg/L of cadmium, chromium, copper, nickel, zinc, and manganese ions in a single batch physicochemical process. Moreover, for the first time, a foam fractionation method in association with using this cysteine-based surfactant was applied for perfluorooctanoic acid (PFOA) removal from water. This surfactant was used as a co-surfactant and could readily remove 72% of PFOA (40 mg/L) from water. The characterization of the surfactant was undertaken using ¹H NMR, FT-IR, elemental analysis, melting point, and determination of its critical micelle concentration (CMC). This environmentally friendly surfactant has high potential applications in green chemistry especially in the treatment of waters contaminated with PFAS/heavy-metal ions.

Revealing the Origin of the Specificity of Calcium and Sodium Cations Binding to Adsorption Monolayers of Two Anionic Surfactants

The studied anionic surfactants linear alkyl benzene sulfonate (LAS) and sodium lauryl ether sulfate (SLES) are widely used key ingredients in many home and personal care products. These two surfactants are known to react very differently with multivalent counterions, including Ca²⁺. This is explained by a stronger interaction of the calcium cation with the LAS molecules, compared to SLES. The molecular origin of this difference in the interactions remains unclear. In the current study, we conduct classical atomistic molecular dynamics simulations to compare the ion interactions with the adsorption layers of these two surfactants, formed at the vacuum-water interface. Trajectories of 150 ns are generated to characterize the adsorption layer structure and the binding of Na⁺ and Ca²⁺ ions. We found that both surfactants behave similarly in the presence of Na⁺ ions. However, when Ca²⁺ is added, Na⁺ ions are completely displaced from the surface with adsorbed LAS molecules, while this displacement occurs only partially for SLES. The simulations show that the preference of Ca²⁺ to the LAS molecules is due to a strong specific attraction with the sulfonate head-group, besides the electrostatic one. This specific attraction involves significant reduction of the hydration shells of the interacting calcium cation and sulfonate group, which couple directly and form surface clusters of LAS molecules, coordinated around the adsorbed Ca²⁺ ions. In contrast, SLES molecules do not exhibit such specific interaction because the hydration shell around the sulfate anion is more stable, due to the extra oxygen atom in the sulfate group, thus precluding substantial dehydration and direct coupling with any of the cations studied.

Effect of Alkyl Tail Length of Alpha Olefin Sulfonates on Foam Properties

The effect of alkyl tail length on the foam properties of alpha olefin sulfonates (AOS) in aqueous solutions at various concentrations was investigated by measurements of the foamability, foam stability and bubble size and numbers, obtained from conductivity and image analyses techniques based on FoamScan foam analyzer. It was found that foamability and foam stability of Cn AOS (n = 14–16, 16–18, 20–24) first increase and then decrease with increase in the carbon chain length, showing an optimum for a length of n = 16–18. The foamability and foam stability of AOS increase with increasing of surfactant concentration. This is due to the fact that the adsorbed quantity of surfactant molecules increases at the air/water interface with the increase of concentration. In addition, it was found that the bubble size produced by C16–18 AOS is smaller than that of C14–16 AOS.

CO2-switchable foams stabilized by a long-chain viscoelastic surfactant

Smart foams sensitive to external stimulation have gained increasing attention recently. However, reversibly switchable CO₂ foams have been less documented. In this work, a novel kind of CO₂-switchable foams was developed using a long-chain cationic surfactant, N-erucamidopropyl-N,N-dimethylammonium bicarbonate (UC₂₂AMPM⋅H⁺), as both the foaming agent and stabilizer. The foams can be rapidly transformed between stable and unstable states at ambient temperature with CO₂/NH₃·H₂O as the triggers. The foaming properties and switchable performance were examined by a combination of confocal microscopy, cryogenic transmission electron microscopy, and rheological techniques. The results demonstrated that the enhanced foam stability in the presence of CO₂ is attributed to the high bulk phase viscosity and gas/liquid surface viscosity, resulting from the entanglement of wormlike micelles (WLMs) formed from UC₂₂AMPM⋅H⁺. When NH₃·H₂O is added, the network structure of WLMs is disrupted, and the bulk phase viscosity and surface viscosity subsequently drop, consequently leading to an ultimate foam destabilization. Such a CO₂-sensitive viscoelastic surfactant could not only be used to fabricate smart CO₂ foams but can also enable CO₂ to play dual roles as both the dispersed phase, as most gases do, and an "activator" to protonate long-chain tertiary surfactants into cationic analogs to form viscoelastic WLMs to stabilize foams.

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.

Understanding about How Different Foaming Gases Effect the Interfacial Array Behaviors of Surfactants and the Foam Properties

In this paper, the detailed behaviors of all the molecules, especially the interfacial array behaviors of surfactants and diffusion behaviors of gas molecules, in foam systems with different gases (N2, O2, and CO2) being used as foaming agents were investigated by combining molecular dynamics simulation and experimental approaches for the purpose of interpreting how the molecular behaviors effect the properties of the foam and find out the key factors which fundamentally determine the foam stability. Sodium dodecyl sulfate SDS was used as the foam stabilizer. The foam decay and the drainage process were determined by Foamscan. A texture analyzer (TA) was utilized to measure the stiffness and viscoelasticity of the foam films. The experimental results agreed very well with the simulation results by which how the different gas components affect the interfacial behaviors of surfactant molecules and thereby bring influence on foam properties was described.

Enhancement of foamability and foam stability induced by interactions between a hyperbranched exopolysaccharide and a zwitterionic surfactant dodecyl sulfobetaine

Weak hydrogen bonding and electrostatic interactions between a zwitterionic surfactant dodecyl sulfobetaine (DSB) and a hyperbranched exopolysaccharide (EPS) enhanced considerably the stability and foamability of EPS/DSB foam.

Study of the microcharacter of ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant with existence of magnesium ion

In this paper, ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant alkyl polyoxyethylene sulfate (AE3S) with coexisting Mg(2+) was reported. Detailed molecular behaviors of AE3S in foam film with coexisting divalent cationic Ca(2+) or Mg(2+) were investigated by molecular dynamic simulation, comparing with the traditional surfactant sodium dodecyl sulfate (SDS), to find out how the microcharacter and array behavior of molecules in the foam film determined by molecular interaction effect the foam stability. It was found that the ultrastable foam film obtained by the cooperation of magnesium ions and AE3S was driven from two aspects: one is the favorable arrangement of surfactant molecules, and the other is the increase of capacity of foam films for resolutely holding water molecules deduced by a dipolar pair formed by the flexible connecting head groups of AE3S and hydrated Mg(2+) via intermolecular coactions, both related to the presence of magnesium ions. Foam lamella stability measurement and foam decay method were both used to evaluate the stability of foam. Fourier transform infrared (FT-IR) was used to detect the composition variation of foam film in the drainage process; the vibration peak of OH for water molecule shifted from the 3390 cm(-1) (being assigned to the bulk water integrated by hydrogen bonds) to 3685 cm(-1) (being assigned to the vibration of isolated water molecules) for the ultrastable foam film after complete drainage, which agreed very well with the molecular simulation results.