Molecular Interactions Leading to the Coadsorption of Surfactant Dodecyltrimethylammonium Bromide and Poly(styrenesulfonate) at the Oil/Water Interface

A Tale of Two Tails: Tail Ordering of Stoichiometric 1:1 DTAB:SDS Pairs Adsorbed at the Oil-Water Interface

Cationic:anionic surfactant mixtures adsorbed at an oil-water interface stabilize foams in the presence of oil, making them essential to the oil, gas, and firefighting industries. The oil tolerance of foams stabilized by surfactant mixtures, relative to pure (unmixed) cationic and anionic surfactants, results from the mixtures' enhanced flexibility in tailoring the physicochemical properties of the interface. To judiciously employ these mixtures, it is necessary to characterize the structure-function property relationship of their surfactant monolayers that lend to oil-tolerant/intolerant foams. In this work, we employ interfacial tensiometry and vibrational sum frequency spectroscopy to determine the composition (surfactant population and cationic:anionic ratio) and the structure (surfactant alkyl tail conformation) of monolayers prepared at the oil-water interface by 1:1 DTAB:SDS (dodecyltrimethylammonium bromide:sodium dodecyl sulfate) mixtures. We show that the interfacial surfactant density of 1:1 DTAB:SDS mixtures greatly exceeds that of pure DTAB and SDS at similar concentrations up to and beyond their respective critical micelle concentration. The enhanced interfacial adsorption of these mixtures is due to the adsorption of stoichiometric 1:1 DTAB:SDS surfactant pairs that form through the attractive electrostatic interactions between surfactant headgroups. We find that these paired surfactants preferentially adsorb at the interface, causing the interfacial DTAB:SDS ratio to be nearly 1:1. Additionally, we find that the SDS tail is more conformationally ordered than the DTAB tail, even though they are expected to be conformationally identical along the entire tail, since they are likely conjoined through van der Waals interactions. This leads to the conclusion that the surfactant pairs are in a staggered arrangement at the interface. These findings help to uncover molecular factors that contribute to the enhanced oil tolerance, and in some cases oil intolerance, of foams stabilized by cationic:anionic surfactant mixtures.

Photoresponsive arylazopyrazole surfactant/PDADMAC mixtures: reversible control of bulk and interfacial properties

In many applications of polyelectrolyte/surfactant (P/S) mixtures, it is difficult to fine-tune them after mixing the components without changing the sample composition, e.g. pH or the ionic strength. Here we report on a new approach where we use photoswitchable surfactants to enable drastic changes in both the bulk and interfacial properties. Poly(diallyldimethylammonium chloride) (PDADMAC) mixtures with three alkyl-arylazopyrazole butyl sulfonates (CnAAP) with -H, -butyl and -octyl tails are applied and E/Z photoisomerization of the surfactants is used to cause substantially different hydrophobic interactions between the surfactants and PDADMAC. These remotely controlled changes affect significantly the P/S binding and allows for tuning both the bulk and interfacial properties of PDADMAC/CnAAP mixtures through light irradiation. For that, we have fixed the surfactant concentrations at values where they exhibit pronounced surface tension changes upon E/Z photoisomerization with 365 nm UV light (Z) and 520 nm green (E) light and have varied the PDADMAC concentration. The electrophoretic mobility can be largely tuned by photoisomerisation of CnAAP surfactants and P/S aggregates, which can even exhibit a charge reversal from negative to positive values or vice versa. In addition, low colloidal stability at equimolar concentrations of PDADMAC with CnAAP surfactants in the E configuration lead to the formation of large aggregates in the bulk which can be broken up by irradiation with UV light when the surfactant's alkyl chain is short enough (C0AAP). Vibrational sum-frequency generation (SFG) spectroscopy reveals changes at the interface similar to the bulk, where the charging state at air-water interfaces can be modified with light irradiation. Using SFG spectroscopy, we interrogated the O-H stretching modes of interfacial H2O and provide qualitative information on surface charging that is complemented by neutron reflectometry, from which we resolved the surface excesses of PDADMAC and CnAAP at the air-water interface, independently.

Molecular Adsorption and Physicochemical Properties at Liquid/Liquid Nanoemulsion Soft Interfaces: Effect of Charge and Hydrophobicity

Contrary to the popular adage, "Oil and water do not mix", evidence of mixtures comprising the two "immiscible" liquids is universal. In the presence of an emulsifier, oil and water mix to form a colloidal suspension known as emulsion. Their utility in many areas such as food chemistry, biomedical health sectors, catalysis, and the petroleum industry is well recognized. While their application in our society is pervasive, tantalizing fundamental questions regarding the chemistry that takes place at the oil/water soft interface still linger. For instance, do organic compounds show proclivity for this molecularly thin boundary and, if so, what forces, hydrophobic or pure electrostatic among others, drive the molecular interactions? The focus of this Article is on molecular adsorption at the interface of oil-in-water (O/W) nanoemulsion (NE) droplets. The effect of the interfacial surfactant charge (positive, negative, zwitterionic, and neutral) on the affinity of aromatic organic compounds on the O/W NEs has been studied. Using a second harmonic generation (SHG), a nonlinear light scattering technique, we have explored the adsorption equilibrium of charged and neutral organic dyes. By variation of the surfactant functional group and thereby the interfacial charge properties, the source of the adsorption interaction, if any, has been deduced. The population of surfactants containing a charged functional group at the O/W interface is found to be sparse, yet adsorption at some of these interfaces has been observed. A purely electrostatic Coulomb interaction plays a key role, but the presence of a charged interface does not necessitate molecular adsorption. Hydrophobic interactions are not a major driving force of adsorption for the SHG dyes studied. However, a possible pi-interaction is likely in explaining the accumulation of neutral aromatic compounds at the O/W NE interface. These intricate adsorption features are discussed in the context of NE interfacial charge properties and their stability upon molecular adsorption.

Spectroscopic Studies of Zwitterionic DDAPS at Planar and Droplet Oil/Water Interfaces

Developing the knowledge on surfactant interfacial phenomena is highly valuable for the advancement of technological, commercial, and industrial products, as these applications often rely on interfacial and colloidal chemistry. Zwitterionic surfactants are a less toxic alternative to standard charged surfactants. With both positively charged quaternary ammonium and negatively charged sulfonate constituents, zwitterionic DDAPS can have diverse interfacial interactions with various coadditives. In this work, we investigate DDAPS adsorption to a planar oil/water interface and its stabilization of oil-in-water nanoemulsions. By studying both interfacial geometries with surface-specific, nonlinear spectroscopy, we gain deeper insights and a molecular perspective into DDAPS's behavior in the presence of various salts and cosurfactants. From an application standpoint, zwitterionic surfactants are often mixed with other chemicals or used in an environment with pre-existing chemicals (e.g., ocean water during oil remediation). Thus, it is important to understand how such coadditives alter DDAPS's behavior and its performance as an emulsifier. Our results show that DDAPS is nearly uninfluenced by coadditives at a planar oil/water interface, but the identical coadditives are crucial for DDAPS to form and stabilize nanoemulsions. Additionally, the surfactant packing properties vary between interfaces as well as coadditives, indicating that certain interactions with the DDAPS headgroup are stronger and play a greater role in tuning DDAPS's interfacial behavior.

Interfacial Polyelectrolyte-Surfactant Complexes Regulate Escape of Microdroplets Elastically Trapped in Thermotropic Liquid Crystals

Polyelectrolytes adsorbed at soft interfaces are used in contexts such as materials synthesis, stabilization of emulsions, and control of rheology. Here, we explore how polyelectrolyte adsorption to aqueous interfaces of thermotropic liquid crystals (LCs) influences surfactant-stabilized aqueous microdroplets that are elastically trapped within the LCs. We find that adsorption of poly(diallyldimethylammonium chloride) (PDDA) to the interface of a nematic phase of 4-cyano-4'-pentylbiphenyl (5CB) triggers the ejection of microdroplets decorated with sodium dodecylsulfate (SDS), consistent with an attractive electrical double layer interaction between the microdroplets and LC interface. The concentration of PDDA that triggers release of the microdroplets (millimolar), however, is three orders of magnitude higher than that which saturates the LC interfacial charge (micromolar). Observation of a transient reorientation of the LC during escape of microdroplets leads us to conclude that complexes of PDDA and SDS form at the LC interface and thereby regulate interfacial charge and microdroplet escape. Poly(sodium 4-styrenesulfonate) (PSS) also triggers escape of dodecyltrimethylammonium bromide (DTAB)-decorated aqueous microdroplets from 5CB with dynamics consistent with the formation of interfacial polyelectrolyte-surfactant complexes. In contrast to PDDA-SDS, however, we do not observe a transient reorientation of the LC when using PSS-DTAB, reflecting weak association of DTAB and PSS and slow kinetics of formation of PSS-DTAB complexes. Our results reveal the central role of polyelectrolyte-surfactant dynamics in regulating the escape of the microdroplets and, more broadly, that LCs offer the basis of a novel probe of the structure and properties of polyelectrolyte-surfactant complexes at interfaces. We demonstrate the utility of these new insights by triggering the ejection of microdroplets from LCs using peptide-polymer amphiphiles that switch their net charge upon being processed by enzymes. Overall, our results provide fresh insight into the formation of polyelectrolyte-surfactant complexes at aqueous-LC interfaces and new principles for the design of responsive soft matter.

Playing Favorites: Preferential Adsorption of Nonionic over Anionic Surfactants at the Liquid/Liquid Interface

While many studies have investigated synergic interactions between surfactants in mixed systems, understanding possible competitive behaviors between interfacial components of binary surfactant systems is necessary for the optimized efficacy of applications dependent on surface properties. Such is the focus of these studies in which the surface behavior of a binary surfactant mixture containing nonionic (Span-80) and anionic (AOT) components adsorbing to the oil/water interface was investigated with vibrational sum-frequency (VSF) spectroscopy and surface tensiometry experimental methods. Time-dependent spectroscopic studies reveal that while both nonionic and anionic surfactants initially adsorb to the interface, anionic surfactants desorb over time as the nonionic surfactant continues to adsorb. Concentration studies that vary the ratio of Span-80 to AOT in bulk solution show that the nonionic surfactant preferentially adsorbs to the oil/water interface over the anionic surfactant. These studies have important implications for applications in which mixed surfactant systems are used to alter interfacial properties, such as pharmaceuticals, industrial films, and environmental remediation.

Surfactant Adsorption to Different Fluid Interfaces

Surfactant adsorption to fluid interfaces is ubiquitous in biological systems, industrial applications, and scientific fields. Herein, we unravel the impact of the hydrophobic phase (air and oil) and the role of oil polarity on the adsorption of surfactants to fluid interfaces. We investigated the adsorption of anionic (sodium dodecyl sulfate), cationic (dodecyltrimethylammonium bromide), and non-ionic (polyoxyethylene-(23)-monododecyl ether) surfactants at different interfaces, including air and oils, with a wide range of polarities. The surfactant-induced interfacial tension decrease, called the interfacial pressure, correlates linearly with the initial interfacial tension of the clean oil-water interface and describes the experimental results of over 30 studies from the literature. The higher interfacial competition of surfactant and polar oil molecules caused the number of adsorbed molecules at the interface to drop. Further, we found that the critical micelle concentration of surfactants in water correlates to the solubility of the oil molecules in water. Hence, the nature of the oil affects the adsorption behavior and equilibrium state of the surfactant at fluid interfaces. These results broaden our understanding and enable better predictability of the interactions of surfactants with hydrophobic phases, which is essential for emulsion, foam, and capsule formation, pharmaceutical commodities, cosmetics, and many food products.

Peeling back the layers: Investigating the effects of polyelectrolyte layering on surface structure and stability of oil-in-water nanoemulsions

HYPOTHESIS

Layer-by-layer deposition of polyelectrolytes is a useful technique for modifying surface functionalities. For drug delivery systems, alternating layers of biopolymers coat nanoemulsions, which house and protect the cargo until the time and destination of delivery. Here, we investigate molecular factors contributing to the stability and interfacial properties of nanoemulsions prepared by a co-adsorption of polymers poly(styrene sulfonate) and polyethylenimine (PEI), and surfactant dodecyltrimethylammonium bromide. We hypothesize the interplay between electrosteric and hydrophobic effects upon multi-polymer co-adsorption contributes to both macroscopic and molecular-level interfacial properties of nanoemulsions.

EXPERIMENTS

To probe interfacial layering properties, we use vibrational sum frequency scattering spectroscopy with ζ-potential measurements to determine the adsorptive behavior and molecular conformational arrangement of the polymer layers. Complementing these interfacial studies are dynamic light scattering experiments measuring the nanoemulsion size distribution and polydispersity index over a 30-day period.

FINDINGS

Our light scattering, ζ-potential, and spectroscopic results of the nanoemulsion surface show that the duration of droplet stability and the degree of molecular orientation of adsorbed polymers can be tuned by surfactant concentration, PEI concentration, and pH. These results illustrate how molecular surface properties of multi-polymer coated nanoemulsions contribute to synergistic effects and droplet stability, enabling advancements in applications surrounding biopharmaceuticals, cosmetics, and food sciences.

An Advanced Material with Synergistic Viscoelasticity Enhancement of Hydrophobically Associated Water-Soluble Polymer and Surfactant

In order to prepare materials with controllable properties, changeable microstructure, and high viscoelasticity solution with low polymer and surfactant concentration, a composite is constituted by adding surfactant (sodium dodecyl sulfate, SDS) to hydrophobically associated water-soluble polymer (abbreviated as PAAC) solution. The viscoelasticity, aggregate microstructure, and interaction mechanism of the composite are investigated by rheometery, Cryo-transmission electron microscopy (Cryo-TEM), and fluorescence spectrum. The results show that when the mass ratio of polymer to surfactant is 15:1, the viscosity of the composite reaches the maximum. The viscosity of the composite system increases hundredfold. The viscosity plateau under dynamic shear is generated. The composite has the properties of high viscoelasticity, strong shear thinning behavior, and good salt tolerance, and temperature resistance. The maximum viscosity of the composite is shown at the salinity of 20000 mg L^-1 . In addition, there is no phase separation in the composite with the increase of polymer and surfactant concentration, which indicates the good stability of the system. It is proposed a method to obtain a high viscoelasticity solution by adding surfactants without wormlike micelles to a hydrophobically associated water-soluble polymer solution.

Probing the Molecular Structure of Coadsorbed Polyethylenimine and Charged Surfactants at the Nanoemulsion Droplet Surface

Nanoemulsions, nanoscale oil droplets dispersed in an aqueous medium, can be stabilized by polymer-surfactant (PS) mixtures, making them ubiquitous in commercial, industrial, and pharmaceutical applications. It is well-known that the presence of PS layers coadsorbed at the droplet surface plays a significant role in droplet stability and functionality; however, little is understood about the molecular nature of this coadsorption. Such insights are especially important for application in drug delivery where physiological conditions can vary the environmental pH and significantly impact stabilization. Hence, the focus of this study examines the surface properties of ∼300 nm nanoemulsions stabilized by the coadsorption of polyethylenimine (PEI) and charged alkyl surfactants sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB). PEI is a common charge-tunable polymer used in nanocarrier templates. This study employs vibrational sum frequency scattering spectroscopy, coupled with ζ-potential and surface pressure measurements performed as a function of varying concentrations and pH. The surface specific spectroscopic results reported herein reveal that PEI adsorption and molecular ordering is influenced by both electrostatic and hydrophobic interactions. While the degree of PEI adsorption is stronger in the presence of anionic SDS than cationic DTAB, for both surfactants, PEI is molecularly disordered in acidic conditions and adopts a persistent net ordering as the solution pH becomes more basic. Both surfactants also display degrees of interfacial conformational ordering that is altered by the presence of the coadsorbed polymer. These results demonstrate the molecular-level diversity in PEI behavior at the droplet interface and provide insight into how such behavior can be controlled to yield nanocarrier technology with specific functions and enhanced efficacy.

Coming to Order: Adsorption and Structure of Nonionic Polymer at the Oil/Water Interface as Influenced by Cationic and Anionic Surfactants

Polymer-surfactant mixtures are versatile chemical systems because of their ability to form a variety of complexes both in bulk solution and at surfaces. The adsorption and structure of polymer-surfactant complexes at the oil/water interface define their use surface chemistry applications. Previous studies have investigated the interactions between charged polyelectrolytes and surfactants; however, a similar level of insight into the interfacial behavior of nonionic polymers in mixed systems is lacking. The study herein uses vibrational sum frequency (VSF) spectroscopy to elucidate the molecular details of nonionic polyacrylamide (PAM) adsorption to the oil/water interface in the presence of surfactant. The polymer's adsorption and conformational structure at the interface is investigated as it interacts with cationic and anionic surfactants. Where the polymer will not adsorb to the interface on its own in solution, the presence of either cationic or anionic surfactant causes favorable adsorption of the polymer to the oil/water interface. VSF spectra indicate that the cationic surfactant interacts with PAM at the interface through charge-dipole interactions to induce conformational ordering of the polymer backbone. However, conformational ordering of polymer is not induced at the interface when anionic surfactant is present.

Helping Strands: Polyelectrolyte Assists in Surfactant Assembly below Critical Micelle Concentration

Strongly adsorbing polymer/surfactant (P/S) combinations have been proposed for long-term applications such as emulsion stabilization. However, P/S systems are known to exhibit nonequilibrium behavior despite steady-state surface characteristics. This work examines the coadsorption of dodecyltrimethylammonium bromide and sodium poly(styrene sulfonate) (PSS) using oil/water tensiometry, UV absorption, and vibrational sum frequency spectroscopy. To determine which features do not represent true equilibrium, the molecular details of PSS adsorption are compared for fresh and aged samples. At surfactant concentrations concurrent with bulk precipitation, significant differences between fresh and aged samples indicate that the strong initial coadsorption within this system is a nonequilibrium feature. We conclude that the long equilibration timescales arise from the slow assembly of non-adsorbing polyelectrolyte/micelle complexes below the critical micelle concentration. This study resolves a recent debate regarding system equilibria of surface-active P/S combinations at a water surface.