Spreading of aqueous surfactant solutions on oil substrates: Superspreaders vs non-superspreaders

HYPOTHESIS

The question of why aqueous solutions of some surfactants demonstrate a rapid spreading (superspreading) over hydrophobic solid substrates, while solutions of other similar surfactants do not, has no definitive explanation despite numerous previous studies. The suggested hypothesis for this study assumes that once the spreading coefficient of surfactant is positive, there is a concentration range for solutions of any surfactant which demonstrates rapid spreading. As it is impossible to calculate spreading coefficients for solid substrates, we compare the spreading performance of known superspreaders and non-superspreaders on liquid (oil) substrate.

EXPERIMENTS

The kinetics of spreading of aqueous solutions of a series of branched ionic surfactants and non-ionic trisiloxane surfactants on two liquid substrates was studied and compared with the spreading of a surfactant-free liquid, silicone oil. Both dynamic and equilibrium spreading coefficients were calculated using measured surface and interfacial tensions.

FINDINGS

There is no difference in spreading rate on liquid substrate between solutions of surfactants proven as superspreaders (while spreading on solid substrate) or non-superspreaders. A rapid spreading (superspreading) with the characteristic rate of spreading O(102-103) mm2/s occurs if the dynamic spreading coefficients exceeds the positive threshold value. If the dynamic spreading coefficient is negative or slightly positive, complete wetting still occurs, but the spreading is slow with the spreading rate is O(1) mm2/s. Spreading exponents for surfactant solutions in the rapid spreading regime are considerably larger than for the surfactant-free liquid. A number of spreading and dewetting patterns were observed depending on the surfactant type, its concentration and substrate.

Synthesis of Carboxyl Modified Polyether Polysiloxane Surfactant for the Biodegradable Foam Fire Extinguishing Agents

It is necessary to develop novel and efficient alternatives to fluorocarbon surfactant and prepare fluorine-free environmentally-friendly fire extinguishing agent. The carboxyl modified polyether polysiloxane surfactant (CMPS) with high surface activity was synthesized via the esterification reaction using hydroxyl-containing polyether modified polysiloxane (HPMS) and maleic anhydride (MA) as raw materials. The process conditions of the esterification reaction were optimized by orthogonal tests, and the optimum process parameters were determined as follows: reaction temperature of 85 °C, reaction time of 4.5 h, isopropyl alcohol content of 20% and the molar ratio of HPMS/MA of 1/1. The chemical structure, surface activity, aggregation behavior, foam properties, wetting properties and electron distribution were systematically investigated. It was found that the carboxyl group was successfully grafted into silicone molecule, and the conjugated system was formed, which changed the interaction force between the molecules and would affect the surface activity of the aqueous solution. The CMPS exhibited excellent surface activity and could effectively reduce the water's surface tension to 18.46 mN/m. The CMPS formed spherical aggregates in aqueous solution, and the contact angle value of CMPS is 15.56°, illustrating that CMPS had excellent hydrophilicity and wetting performance. The CMPS can enhance the foam property and has good stability. The electron distribution results indicate that the introduced carboxyl groups are more inclined towards the negative charge band, which would be conducive to weak the interaction between molecules and improve the surface activity of the solution. Consequently, new foam fire extinguishing agents were prepared by using CMPS as a key component and they exhibited excellent fire-fighting performance. The prepared CMPS would be the optimal alternative to fluorocarbon surfactant and could be applied in foam extinguishing agents.

The effects of surfactant and oil chemical structures on self-assembly in apolar media

The thermodynamic and chemical structural aspects of surfactant self-assembly in aqueous systems have been much studied. On the other hand, for oil-water interfaces the effects of chemical structures of surfactants and solvents have received less attention. This review focuses on the surfactant chemical effects in low dielectric solvents, in particular formation and properties of surfactant films at oil-water interfaces. For this purpose, reversed micelles (RMs) and water-in-oil (W/O) microemulsions (μEs) serve as model systems, since electrostatic effects are minimized, allowing a focus on chain architecture of the surfactants and oil solvents themselves. It is noted that chemical structure can have profound effects on stability and self-assembly, suggesting a possibility of identifying unified chemical principles for designing and formulating systems across various thermodynamic conditions.

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

Microemulsions, as thermodynamically stable mixtures of oil, water, and surfactant, are known and have been studied for more than 70 years. However, even today there are still quite a number of unclear aspects, and more recent research work has modified and extended our picture. This review gives a short overview of how the understanding of microemulsions has developed, the current view on their properties and structural features, and in particular, how they are related to applications. We also discuss more recent developments regarding nonclassical microemulsions such as surfactant-free (ultraflexible) microemulsions or ones containing uncommon solvents or amphiphiles (like antagonistic salts). These new findings challenge to some extent our previous understanding of microemulsions, which therefore has to be extended to look at the different types of microemulsions in a unified way. In particular, the flexibility of the amphiphilic film is the key property to classify different microemulsion types and their properties in this review. Such a classification of microemulsions requires a thorough determination of their structural properties, and therefore, the experimental methods to determine microemulsion structure and dynamics are reviewed briefly, with a particular emphasis on recent developments in the field of direct imaging by means of electron microscopy. Based on this classification of microemulsions, we then discuss their applications, where the application demands have to be met by the properties of the microemulsion, which in turn are controlled by the flexibility of their amphiphilic interface. Another frequently important aspect for applications is the control of the rheological properties. Normally, microemulsions are low viscous and therefore enhancing viscosity has to be achieved by either having high concentrations (often not wished for) or additives, which do not significantly interfere with the microemulsion. Accordingly, this review gives a comprehensive account of the properties of microemulsions, including most recent developments and bringing them together from a united viewpoint, with an emphasis on how this affects the way of formulating microemulsions for a given application with desired properties.

Aggregation Behavior and Intermolecular Interaction of Cationic Trisiloxane Surfactants: Effects of Unsaturation

Imidazolium/pyridinium-based trisiloxane surfactants containing a phenyl or vinyl group in the hydrophobic siloxane chain, bis(vinyldimethylsiloxy)methylsilylpropyl-pyridinium chloride (Vi-Si₃pyrCl), bis(vinyldimethylsiloxy)methylsilylpropyl-imidazolium chloride (Vi-Si₃minCl), and bis(phenyldimethylsiloxy)methylsilylpropylimidazolium chloride (Ph-Si₃minCl), were synthesized and confirmed by nuclear magnetic resonance (NMR) (¹H, ¹³C, and ²⁹Si NMR), mass spectrometry, and Fourier transform infrared spectrometry. The effect of the phenyl/vinyl group on their micellization behavior was studied by surface tension, electric conductivity, dynamic light scattering, 2D nuclear Overhauser effect spectroscopy (NOESY) NMR, and transmission electron microscopy. Owing to the hydrophobicity of the siloxane groups and cationic head groups, the critical micelle concentration (cmc) values follow the order Ph-Si₃minCl < Vi-Si₃pyrCl < Vi-Si₃minCl < Si₃pyrCl. Ph-Si₃minCl has a larger γ_cmc value, resulting from the introduction of the phenyldimethylsiloxy unit (π-π stacking interaction). The β values of Vi-Si₃minCl and Ph-Si₃minCl increase with the increase in temperature, which is attributed to the intermolecular interaction which hinders the association of Cl^- with the imidazolium ring and confirmed by 2D NOESY NMR. In aqueous solutions, the investigated cationic trisiloxane surfactants can self-assemble into spherical aggregates.

Design of Surfactant Tails for Effective Surface Tension Reduction and Micellization in Water and/or Supercritical CO2

The interfacial properties and water-in-CO₂ (W/CO₂) microemulsion (μE) formation with double- and novel triple-tail surfactants bearing trimethylsilyl (TMS) groups in the tails are investigated. Comparisons of these properties are made with those for analogous hydrocarbon (HC) and fluorocarbon (FC) tail surfactants. Surface tension measurements allowed for critical micelle concentrations (CMC) and surface tensions at the CMC (γ_CMC) to be determined, resulting in the following trend in surface activity FC > TMS > HC. Addition of a third surfactant tail gave rise to increased surface activity, and very low γ_CMC values were recorded for the double/triple-tail TMS and HC surfactants. Comparing effective tail group densities (ρ_layer) of the respective surfactants allowed for an understanding of how γ_CMC is affected by both the number of surfactant tails and the chemistry of the tails. These results highlight the important role of tail group chemical structure on ρ_layer for double-tail surfactants. For triple-tail surfactants, however, the degree to which ρ_layer is affected by tail group architecture is harder to discern due to formation of highly dense layers. Stable W/CO₂ μEs were formed by both the double- and the triple-tail TMS surfactants. High-pressure small-angle neutron scattering (HP-SANS) has been used to characterize the nanostructures of W/CO₂ μEs formed by the double- and triple-tail surfactants, and at constant pressure and temperature, the aqueous cores of the microemulsions were found to swell with increasing water-to-surfactant ratio (W₀). A maximum W₀ value of 25 was recorded for the triple-tail TMS surfactant, which is very rare for nonfluorinated surfactants. These data therefore highlight important parameters required to design fluorine-free environmentally responsible surfactants for stabilizing W/CO₂ μEs.

Aggregation Behavior of Polyether Based Siloxane Surfactants in Aqueous Solutions: Effect of Alkyl Groups and Steric Hindrance

A series of polyether based siloxane surfactants with different branched chain and alkyl groups were synthesized by thiol-ene reaction and Piers-Rubinsztajn reaction. The effect of the siloxane structures (alkyl groups and branched chains) on the adsorption and aggregation behavior in aqueous solution was investigated by surface tension, fluorescence, dynamic light scattering (DLS), freeze-fracture transmission electron microscopy (TEM), and TEM. The molecular structures of siloxane can obviously influence their surface activities and thermodynamics. Replacing the methyl of trimethylsiloxyl groups with longer alkyl groups (ethyl, propyl, and butyl) and branching trimethylsiloxyl resulted in an obvious decrease of the values of critical micelle concentration (CMC) and surface tension at CMC (γ_CMC). Dense surface films packed with CH₃ groups result in the lower surface tensions being disordered by longer alkyl groups or branched chains of siloxane hydrophobic groups. And the minimum surface area per surfactant molecule ( A_min) values of Si₃-PG, Et-Si₃-PG, Pro-Si₃-PG, and But-Si₃-PG successively decrease about 3.5 Å with each increasing -CH₂- group. All polyether based siloxane surfactants can form nonuniform size spheroidal aggregates in aqueous solution. Concerning the driving force, the micellization process was spontaneous but less spontaneous compared with adsorption.