Role of Counterions in the Adsorption and Micellization Behavior of 1:1 Ionic Surfactants at Fluid Interfaces─Demonstrated by the Standard Amphiphile System of Alkali Perfluoro-n-octanoates

Molecular insights: How counterions determine surfactant aggregation

The morphology of surfactant aggregates inherently determines their properties and functions. Hence, the modulation of morphology holds significant importance in their applications. Opting for an appropriate counterion emerges as an efficient and convenient method to shape the aggregate morphology of ionic surfactants. Both inorganic and organic counterions exert influence on surfactant aggregation by neutralizing the repulsive headgroup charges and synergistically adjusting the hydrophobic association among the alkyl tails. In this process, the charge property, polarizability, hydrophobicity, configuration, and functional groups of counterions are of pivotal influencing factors. A delicate adjustment of counterion structures can yield dramatic differences in the aggregation behavior of ionic surfactants. This review provides a comprehensive overview of the fundamental principles governing the impact of inorganic and organic counterions on the formation of diverse surfactant aggregates, encompassing spherical micelles, wormlike micelles, vesicles, coacervates, and other aggregates. It aims at providing insightful understanding on how to select counterions for surfactants to achieve desired aggregate structures and properties.

A Coarse-Grained Model Describing the Critical Micelle Concentration of Perfluoroalkyl Surfactants in Ionic Aqueous Phase

In this study, dissipative particle dynamics (DPD) simulations were employed to determine the critical micelle concentration (CMC) of perfluoroalkyl and polyfluoroalkyl substances (PFAS) in ionic aqueous solutions. This approach provides precise CMC data for PFAS surfactants in the presence of various ionic species, thereby addressing a gap in the current literature. Additionally, this study contributes to the development of open-source molecular force fields for charged perfluorinated compounds, which are currently limited. These models incorporate hydration free energy values obtained from density functional theory (DFT) and account for ionic interactions through a well-established linear relationship. Hydrophobic interactions between the surfactant tail and water were fine-tuned to match the CMC of chosen surfactants. Then, the DPD models successfully predicted CMC values for a diverse range of surfactants, including those based on hydrocarbons and PFAS, demonstrating the ability to represent realistic salinities encountered in natural waters. Experimental validation of the methodology was conducted using sodium n-nonyl sulfate (SNS) and sodium n-dodecyl sulfate (SDS) via interfacial tension measurements, confirming the accurate representation of the CMC changes with salinity. This study enhances our understanding of the behavior of PFAS surfactants in ionic aqueous solutions and provides a valuable tool for predicting CMC values in complex environmental systems.

Structure, orientation, and dynamics of per- and polyfluoroalkyl substance (PFAS) surfactants at the air-water interface: Molecular-level insights

HYPOTHESIS

Understanding the intricate molecular-level details of toxic per- and polyfluoroalkyl substances (PFAS) partitioning to the air-water interface holds paramount importance in evaluating their fate and transport, as well as for finding safer alternatives for various applications, including aqueous film forming foams. The behavior of these substances at interfaces strongly depends on molecular architecture, chemistry, and concentration, which define molecular packing, self-assembly, interfacial diffusion, and the surface tension.

SIMULATIONS

Modeling of three PFAS surfactants, namely, longer-tail (perfluorooctanoate (PFOA)) and shorter-tail (perfluorobutanoate (PFBA) and 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy) propanoate (GenX)) has been conducted using atomistic molecular dynamics simulations. A systematic comparison between these representative PFAS of different sizes and structure reveals factors influencing their association behavior, mechanism of surface tension reduction, and interfacial mobility as a function of surface coverage.

FINDINGS

Shorter-chain PFAS surfactants (GenX or PFBA) require lower surface coverage compared to longer chain (PFOA) PFAS to achieve the same decrease in surface tension. However, a higher concentration of GenX and PFBA is necessary in the bulk aqueous solution to achieve the same surface coverage as PFOA, due to their higher solubility in water. The PFAS molecular orientation and mobility at the interface are found to be vastly influenced by the length and architecture of the hydrophobic fluorocarbon tail. A significant ordering of the water dipole moment near the anionic headgroup is apparent at high surface concentration. A direct correlation is established between the PFAS interfacial properties and PFAS-PFAS, PFAS-counterion, and PFAS-water interactions.

Role of Counterion in the Adsorption of Ionic Amphiphiles at Fluid Interfaces

This communication represents the chemical alternative to the previous two papers dealing with the influence of positively charged alkali cations on the adsorption properties of the series of the standard surfactant system of alkali-perfluorocarbon octanoates. Now, this contribution describes the adsorption properties of the negatively charged cationic surfactant series of trimethyldodecyl-ammonium halides. In our latest contributions, we have put forward a new model of adsorption of ionic surfactants. It says that the surface excess of the adsorbed anionic surfactant is exclusively determined by the cross-sectional area of the positive counterion. This, however, has been demonstrated by applying relevant, positively charged (alkali) counterions only, i.e., by anionic surfactants. In this article, we extend the new model to negatively charged counterions (halides) applying the cationic standard surfactant series of the trimethyldodecylammonium-halides. A big difference between the hydration behavior of the positively charged alkali and the negatively charged counterions has become striking. Thus, for example, whereas the ratio between the naked ion radius of the cesium and of the lithium cation is almost 2-fold, it is practically equal for the chloride and the iodide anion. Surprisingly, however, the relevant adsorption data are practically identical. This means that the bigger, negatively charged halide counterions interact considerably more strongly with their residual ionic surfactant group than the positively charged alkali cations with theirs. Due to this, the size of the hydrated negative halide ions is considerably greater than that of the relevant positive alkali ions. These specialties can well be explained by the Stern model of charge distribution across a naked ion's surface. It shows that for the electrostatic interaction between counterion and ionic surfactant headgroup, the peculiarities of the polar solvent of water will play a crucial role, too. By these investigations our new model of adsorption of ionic amphiphiles is further extended and gives finally evidence that it is of general validity.

Influence of counterions on the thermal and solution properties of strong polyelectrolytes

Strong polyelectrolytes (i.e., macromolecules whose charge density is independent of the medium's pH) are invaluable assets in the soft matter toolbox, as they can readily disperse in aqueous media, complex to oppositely charged species - polymers and small molecules alike - and can be implemented in a plethora of applications, ranging from surface modification to chelating agents and lubricants. However, the direct synthesis of strong polyelectrolytes in a controlled fashion remains a challenging endeavour, and their in-depth characterisation is often limited. Additionally, producing a set of charged macromolecules with the same chain length but varying counterions would open doors towards a fine control of the polymer's chemistry and physical properties. Unfortunately, this either necessitates the direct polymerisation of several monomers with potentially varying reactivities, or a time-consuming ion exchange from a single batch. Herein we explore the facile and efficient production of strong polyanions through the deprotection of a poly(3-isobutoxysulphopropyl methacrylate) using a range of inorganic and organic iodide-containing salts. Owing to the contrasting nature of their counterions, the resulting polyanions exhibit a wide range of glass transition temperatures, which follow a non-monotonic trend with increasing counterion size. While all polymers readily dissolve in water, some can also be dissolved in non-aqueous media as well. This strategy, applied to block copolymers, permits the production of a library of amphiphilic macromolecules with consistent hydrophilic and hydrophobic blocks, yet varying nature of their polyanionic segments. All amphiphiles, regardless of their counterions, readily disperse in aqueous media and form well-defined micelles featuring a hydrophobic core and a charged hydrophilic shell, as evidenced by dynamic light scattering, ζ-potential and transmission electron microscopy. Additionally, a handful of block copolymers are capable of yielding polymer micelles in organic solvents, opening an avenue to the build-up of nanostructured soft matter in non-aqueous media.

Relationship between Molecular Structure and Surface Activity of Ionic Surfactants

A new model is developed to quantify the surface tension of ionic surfactants from surface affinity and ionization equilibrium. The model successfully predicts two important molecular structure-surface activity factors: the length of single-branch homologues and the nature of counterions. The modeling results also clarify the underlying mechanisms of the two processes. Changing the counterion only affects the ionization, not the affinity. On the other hand, increasing carbon length dramatically increases the affinity while having a small effect on ionization. The modeling framework consistently resolves structure-activity observations, some of which have been reported since the 19th century. The model can be extended for surfactants with more than one ionic state and surfactant/electrolyte mixtures.

Prediction of critical micelle concentration for per- and polyfluoroalkyl substances

In this study, we focus on the development of Quantitative Structure-Property Relationship (QSPR) models to predict the critical micelle concentration (CMC) for per- and polyfluoroalkyl substances (PFASs). Experimental CMC values for both fluorinated and non-fluorinated compounds were meticulously compiled from existing literature sources. Our approach involved constructing two distinct types of models based on Support Vector Machine (SVM) algorithms applied to the dataset. Type (I) models were trained exclusively on CMC values for fluorinated compounds, while Type (II) models were developed utilizing the entire dataset, incorporating both fluorinated and non-fluorinated compounds. Comparative analyses were conducted against reference data, as well as between the two model types. Encouragingly, both types of models exhibited robust predictive capabilities and demonstrated high reliability. Subsequently, the model having the broadest applicability domain was selected to complement the existing experimental data, thereby enhancing our understanding of PFAS behaviour.

The Impact of Adding a Cationic Metal Salt and Curcumin to Monoammonium Glycyrrhizic Acid on Its Solubilizing Capacity and Gelation

Monoammonium glycyrrhizic acid (MAG), a glycyrrhizic acid monoammonium salt, is a naturally derived low-molecular-weight gelling agent with surface-active properties. It has the capacity to individually facilitate the preparation of gel-solubilized drugs. As MAG is an anionic surfactant with carboxyl groups, the addition of counterions may affect micelle formation and gelation. In this study, the solubilization and gelling properties of MAG were investigated following the addition of metal salts (NaCl and KCl). The addition of metal salts resulted in a decrease in the critical micelle concentration and an increase in gel hardness. Supersaturation of curcumin (CUR) was maintained by the addition of metal salts because of increased micelle number and viscosity. When the gel hardness was compared between formulations with and without CUR, a significant reduction in hardness was observed with the solubilization of CUR. The addition of KCl prevented the decrease in the hardness of gels containing CUR compared to the addition of NaCl. Put together, the addition of metal salts had a noteworthy impact on micelle and gel formation of MAG. In particular, the addition of KCl was more effective in the preparation of gel-solubilized CUR.