Come Together: Molecular Details into the Synergistic Effects of Polymer–Surfactant Adsorption at the Oil/Water Interface

Using the pH sensitivity of switchable surfactants to understand the role of the alkyl tail conformation and hydrogen bonding at a molecular level in elucidating emulsion stability

HYPOTHESIS

The monoalkyl diamine surfactant, N-dodecylpropane-1,3-diamine (DPDA), is expected to exhibit a pH-dependent charge switchability. In response to pH changes, the interfacial self-assembly of DPDA becomes an intermediary constituent that can potentially modify the interfacial interactions and structural assembly of both the oil and water phases. Hence, we hypothesize that as we change the pH, DPDA will respond to it by changing its charge and alkyl tail conformation as well as the conformation of adjacent phases at the molecular level, consequently affecting emulsion formation and stability. A neutral pH, resulting in a mono-cationic dialkyl amine, affects the conformation, driving an ordered self-assembly and stable emulsion.

EXPERIMENTS

The pH-sensitivity and interfacial activity of DPDA were evaluated through pH titration and interfacial tension measurements. Subsequently, a molecular-level study of DPDA, as a pH-sensitive switchable surfactant, was performed at the dodecane-water interface using SFG spectroscopy. The interpretation of the vibrational spectra was further reinforced by determining the gauche defects in the interfacial alkyl chain organization and the extent of hydrogen (H) bonding between the interfacial water molecules.

FINDINGS

By adjusting the pH of water, the charge of the adsorbed DPDA molecules, their self-assembly, the organization of interfacial molecules, and ultimately the stability of the emulsion were tuned. At pH 7.0, the SFG spectra of DPDA showed that the interfacial alkyl chains were relatively well-ordered, while water molecules also had stronger H-bonding interactions. As a result, the oil-water emulsion showed improved stability. When water was at a high pH, the water molecules had fewer H-bonding interactions and relatively disordered alkyl chains at the interface, providing desirable conditions for demulsification. These observations were compatible with the observation in bulk emulsion preparation, confirming that alkyl chain packing and water H-bonding interactions at the interface contribute to overall emulsion stability.

In situ study of structural changes: Exploring the mechanism of protein corona transition from soft to hard

HYPOTHESIS

The process of protein corona changes has been widely believed to follow the Vroman effect, while protein structural change during the process is rarely reported, due to the lack of analytical methods. In-situ interpretation for protein structural change is critical to processes such as the recognition and transport of nanomaterials.

EXPERIMENTS

Molecular dynamics (MD) simulation was used to predict the deflection and twist of the protein tertiary structure. The structural changes of the surface protein corona during the interaction of nanoparticles (NPs) with lipid bilayer were probed in situ and real-time by sum frequency generation (SFG) spectroscopy.

FINDINGS

The ring tertiary structure of the protein corona is altered from vertical to horizontal on particle surface, a process of the soft-to-hard structural transition, which is contributed by the hydrogen bonding force between the protein and water molecules. The negatively charged protein corona can induce the redistribution of interfacial charge, leading to a more stable hydrogen bond network of the interfacial water. Our findings suggest that the structural change from flexible to rigid is a crucial process in the soft-to-hard transition of the protein corona, which will be a beneficial supplement to the Vroman effect of protein adsorption.

Synergistic Interaction of Hyperbranched Polyglycerols and Cetyltrimethylammonium Bromide for Oil/Water Interfacial Tension Reduction: A Molecular Dynamics Study

Applying surfactants to reduce the interfacial tension (IFT) on water/oil interfaces is a proven technique. The search for new surfactants and delivery strategies is an ongoing research area with applications in many fields such as drug delivery through nanoemulsions and enhanced oil recovery. Experimentally, the combination of hyperbranched polyglycerol (HPG) with cetyltrimethylammonium bromide (CTAB) substantially reduced the observed IFT of oil/water interface, 0.9 mN/m, while HPG alone was 5.80 mN/m and CTAB alone IFT was 8.08 mN/m. Previous simulations in an aqueous solution showed that HPG is a surfactant carrier. Complementarily, in this work, we performed classical molecular dynamics simulations on combinations of CTAB and HPG with one aliphatic chain to investigate further the interaction of this pair in oil interfaces and propose the mechanism of IFT decrease. Basically, from our results, one can observe that the IFT reduction comes from a combination of effects that have not been observed for other dual systems: (i) Due to the CTAB-HPG strong interaction, a weakening of their specific and isolated interactions with the water and oil phases occurs. (ii) Aggregates enlarge the interfacial area, turning it into a less ordered interface. (iii) The spread of individual molecules charge profiles leads to the much lower interfacial tension observed with the CTAB+HPG systems.

Comparative Thermodynamic and Structural Analysis of Polyfluorinated Dodecylphosphonic Acid Adsorption to Distilled and River Water Interfaces

As concerns rise about the health risks posed by per- and polyfluoroalkyl substances (PFAS) in the environment, there is a need to understand how these pollutants accumulate at environmental interfaces. Untangling the details of molecular adsorption, particularly when there are potential interactions with other molecules in environmental systems, can obscure the ability to focus on a particular contaminant with molecular specificity. Often adsorption studies of environmental interfaces require a reductionist approach, where laboratory experiments may not be fully tractable to environmental systems. In this work, we study polyfluorinated dodecylphosphonic acid (F₂₁-DDPA) at the aqueous surfaces of distilled water (the most reduced "environmental" surface) and river water to explore the use of vibrational sum-frequency (VSF) spectroscopy as an experimental probe of fluorinated contaminants at natural environmental surfaces. We demonstrate how VSF spectroscopy offers advantages over nonspecific surface tension measurements when measuring PFAS adsorption isotherms at river water surfaces. VSF spectra of the C-F stretching region selectively probe the presence of F₂₁-DDPA and can be used to extract meaningful structural insights and calculate surface concentrations, even at the complex river water surface. This study highlights the potential for VSF spectroscopy to be developed as a probe of fluorinated contaminants at natural environmental interfaces.

Comparison between Cashew-Based and Petrochemical Hydroxyoximes: Insights from Molecular Simulations

Solvent extraction has been ubiquitously used to recover valuable metals from wastes such as spent batteries and electrical boards. With increasing demands for energy transition, there is a critical need to improve the recycling rate of critical metals, including copper. Therefore, the sustainability of reagents is critical for the overall sustainability of the process. Yet, the recycling process relies on functional organic compounds based on the hydroxyoxime group. To date, hydroxyoxime extractants have been produced from petrol-based chemical feedstocks. Recently, natural-based cardanol has been used to produce an alternative hydroxyoxime. The natural-based oxime has been employed to recover valuable metals (Ga, Ni, Co) via a liquid/liquid extraction process. The natural compound has a distinctive structure with 15 carbons in the alkyl tail. In contrast, petrol-based hydroxyoximes have only 12 or fewer carbons. However, the molecular advantages of this natural-based compound over the current petrol-based ones remain unclear. In this study, molecular dynamics simulation was employed to investigate the effect of extractant hydrocarbon chains on the extraction of copper ions. Two hydroxyoxime extractants with 12 and 15 carbons in the alkyl chain were found to have similar interactions with Cu²⁺ ions. Yet, a slight molecular binding increase was observed when the carbon chain was increased. In addition, lengthening the carbon chain made the extracting stage easier and the stripping stage harder. The binding would result in a lower pH in the extraction step and a lower pH in the stripping step. The insights from this molecular study would help design the extraction circuit using natural-based hydroxyoxime extractants. A successful application of cashew-based cardanol will improve the environmental benefits of the recycling process. With cashew-producing regions in developing countries, the application also improves these regions' social and economic sustainability.

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces

Conventional wisdom suggests that cations play a minimal role in the assembly of cationic amphiphiles. Here, we show that at liquid/liquid (L/L) interfaces, specific cation effects can modulate the assemblies of hydrophobic tails in an oil phase despite being attached to cationic headgroups in the aqueous phase. We used oligo-dimethylsiloxane (ODMS) methyl imidazolium amphiphiles to identify these specific interactions at hexadecane/aqueous interfaces. Small cations, such as Li⁺, bind to the O atoms in the ODMS tail and pin it to the interface, thereby imposing a kinked conformation─as evidenced by vibrational sum frequency generation spectroscopy and molecular dynamics simulations. While larger Cs⁺ ions more readily partition to the interface, they do not form analogous complexes. Our data not only point to ways for controlling amphiphile structure at L/L interfaces but also suggest a means for the separation of Li⁺, or related applications, in soft-matter electronics.

Responsive Material and Interfacial Properties through Remote Control of Polyelectrolyte-Surfactant Mixtures

Polyelectrolyte/surfactant (P/S) mixtures find many applications but are static in nature and cannot be reversibly reconfigured through the application of external stimuli. Using a new type of photoswitchable surfactants, we use light to trigger property changes in mixtures of an anionic polyelectrolyte with a cationic photoswitch such as electrophoretic mobilities, particle size, as well as their interfacial structure and their ability to stabilize aqueous foam. For that, we show that prevailing hydrophobic intermolecular interactions can be remotely controlled between poly(sodium styrene sulfonate) (PSS) and arylazopyrazole tetraethylammonium bromide (AAP-TB). Shifting the chemical potential for P/S binding with E/Z photoisomerization of the surfactants can reversibly disintegrate even large aggregates (>4 μm) and is accompanied by a substantial change in the net charging state of PSS/AAP-TB complexes, e.g., from negative to positive excess charges upon light irradiation. In addition to the drastic changes in the bulk solution, also at air-water interfaces, the interfacial stoichiometry and structure change drastically on the molecular level with E/Z photoisomerization, which can also drive the stability of aqueous foam on a macroscopic level.

Nanoscale Effects on the Surfactant Adsorption and Interface Charging in Hexadecane/Water Emulsions

Nanoscale properties at interfaces play a key role in the colloidal stability of emulsions and other soft matter materials where physical properties need to be controlled from the nano to macroscopically visible length scales. Our molecular level understanding of oil-water interfaces arises mostly from results at extended interfaces and the common view that emulsions are stabilized by a large number of surfactant molecules at the droplet's interface which, however, has been recently challenged. In this work, we show that the particle size and the curvature of oil droplets at the nanoscale is of great importance for the interface adsorption of dodecyl sulfate surfactants and possible counterion condensation at the charged hexadecane-water interface. Using second-harmonic scattering, we have studied the surface charge of oil droplets in nanoemulsions where we systematically varied the particle size R between 80 and 270 nm and demonstrate that the surface charge density σ changes drastically with size: For sizes >200 nm, σ is similar to what can be expected at flat extended interfaces, while σ is dramatically reduced by almost an order of magnitude when the particle size of the oil droplet is 80 nm. Using a theoretical approach that considers counterion condensation, we quantify the nanoscale effects on the change in surface charge with particle size and find excellent agreement with our experimental result. Modeling of the experimental results also implies that the charge per particle remains constant and depends on a critical balance of surfactant adsorption and ion condensation.

Understanding Self-Assembly and the Stabilization of Liquid/Liquid Interfaces: The Importance of Ligand Tail Branching and Oil-Phase Solvation

HYPOTHESIS

Organophosphorus-based ligands represent a versatile set of solvent extraction reagents whose chemical makeup plays an important role in extraction mechanism. We hypothesize that the branching of the extractant hydrophobic tail and its oil-phase solvation affect the liquid/liquid interfacial structure. Understanding the structure mediated adsorption and interfacial ordering becomes key in designing ligands with enhanced selectivity and efficiency for targeted extractions.

EXPERIMENT

We employed vibrational sum frequency generation spectroscopy and interfacial tension measurements to extract thermodynamic adsorption energies, map interfacial ordering, and rationalize disparate behaviors of model di-(2-ethylhexyl) phosphoric acid and dioctyl phosphoric acid ligands at the hexadecane water interface.

FINDINGS

With increased surface loading, ligands with branched hydrophobic tails formed stable interfaces at much lower concentrations than those observed for ligands with linear alkyl tails. The lack of an oil phase and associated solvation results in markedly different interfacial properties, and thus measurements made at air/liquid surfaces cannot be assumed to correlate with the processes occurring at buried liquid/liquid interfaces. We attribute these differences in the surface mediated self-assembly to key variations in hydrophobic interactions and tail solvation taking place in the oil phase demonstrating that interactions in both the polar and nonpolar phases are essential to understand self-assembly and function.

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.

Nanoparticle-Induced Disorder at Complex Liquid-Liquid Interfaces: Effects of Curvature and Compositional Synergy on Functional Surfaces

The self-assembly of surfactant monolayers at interfaces plays a sweeping role in tasks ranging from household cleaning to the regulation of the respiratory system. The synergy between different nanoscale species at an interface can yield assemblies with exceptional properties, which enhance or modulate their function. However, understanding the mechanisms underlying coassembly, as well as the effects of intermolecular interactions at an interface, remains an emerging and challenging field of study. Herein, we study the interactions of gold nanoparticles striped with hydrophobic and hydrophilic ligands with phospholipids at a liquid-liquid interface and the resulting surface-bound complexes. We show that these nanoparticles, which are themselves minimally surface active, have a direct concentration-dependent effect on the rapid reduction of tension for assembling phospholipids at the interface, implying molecular coassembly. Through the use of sum frequency generation vibrational spectroscopy, we reveal that nanoparticles impart structural disorder to the lipid molecular layers, which is related to the increased volumes that amphiphiles can sample at the curved surface of a particle. The results strongly suggest that hydrophobic and electrostatic attractions imparted by nanoparticle functionalization drive lipid-nanoparticle complex assembly at the interface, which synergistically aids lipid adsorption even when lipids and nanoparticles approach the interface from opposite phases. The use of tensiometric and spectroscopic analyses reveals a physical picture of the system at the nanoscale, allowing for a quantitative analysis of the intermolecular behavior that can be extended to other systems.

Assessing the Impact of Solvent Selection on Vibrational Sum-Frequency Scattering Spectroscopy Experiments

The development of vibrational sum-frequency scattering (S-VSF) spectroscopy has opened the door to directly probing nanoparticle surfaces with an interfacial and chemical specificity that was previously reserved for planar interfacial systems. Despite its potential, challenges remain in the application of S-VSF spectroscopy beyond simplified chemical systems. One such challenge includes infrared absorption by an absorptive continuous phase, which will alter the spectral lineshapes within S-VSF spectra. In this study, we investigate how solvent vibrational modes manifest in S-VSF spectra of surfactant stabilized nanoemulsions and demonstrate how corrections for infrared absorption can recover the spectral features of interfacial solvent molecules. We also investigate infrared absorption for systems with the absorptive phase dispersed in a nonabsorptive continuous phase to show that infrared absorption, while reduced, will still impact the S-VSF spectra. These studies are then used to provide practical recommendations for anyone wishing to use S-VSF to study nanoparticle surfaces where absorptive solvents are present.

Role of Water in CaCO3 Biomineralization

Biomineralization occurs in aqueous environments. Despite the ubiquity and relevance of CaCO₃ biomineralization, the role of water in the biomineralization process has remained elusive. Here, we demonstrate that water reorganization accompanies CaCO₃ biomineralization for sea urchin spine generation in a model system. Using surface-specific vibrational spectroscopy, we probe the water at the interface of the spine-associated protein during CaCO₃ mineralization. Our results show that, while the protein structure remains unchanged, the structure of interfacial water is perturbed differently in the presence of both Ca²⁺ and CO₃^2– compared to the addition of only Ca²⁺. This difference is attributed to the condensation of prenucleation mineral species. Our findings are consistent with a nonclassical mineralization pathway for sea urchin spine generation and highlight the importance of protein hydration in biomineralization.

Hydrogen-Bond-Driven Chemical Separations: Elucidating the Interfacial Steps of Self-Assembly in Solvent Extraction

Chemical separations, particularly liquid extractions, are pervasive in academic and industrial laboratories, yet a mechanistic understanding of the events governing their function are obscured by interfacial phenomena that are notoriously difficult to measure. In this work, we investigate the fundamental steps of ligand self-assembly as driven by changes in the interfacial H-bonding network using vibrational sum frequency generation. Our results show how the bulk pH modulates the interfacial structure of extractants at the buried oil/aqueous interface via the formation of unique H-bonding networks that order and bridge ligands to produce self-assembled aggregates. These extended H-bonded structures are key to the subsequent extraction of Co²⁺ from the aqueous phase in promoting micelle formation and subsequent ejection of the said micelle into the oil phase. The combination of static and time-resolved measurements reveals the events underlying complexities of liquid extractions at high [Co²⁺]:[ligand] ratios by showing an evolution of interfacially assembled structures that are readily tuned on a chemical basis by altering the compositions of the aqueous phase. The results of this work point to new principles to design-applied separations through the manipulation of surface charge, electrostatic screening, and the associated H-bonding networks that arise at the interface to facilitate organization and subsequent extraction.

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.

Insight into the Mechanisms Driving the Self-Assembly of Functional Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers

Polymer-stabilized liquid/liquid interfaces are an important and growing class of bioinspired materials that combine the structural and functional capabilities of advanced synthetic materials with naturally evolved biophysical systems. These platforms have the potential to serve as selective membranes for chemical separations and molecular sequencers and to even mimic neuromorphic computing elements. Despite the diversity in function, basic insight into the assembly of well-defined amphiphilic polymers to form functional structures remains elusive, which hinders the continued development of these technologies. In this work, we provide new mechanistic insight into the assembly of an amphiphilic polymer-stabilized oil/aqueous interface, in which the headgroups consist of positively charged methylimidazolium ionic liquids, and the tails are short, monodisperse oligodimethylsiloxanes covalently attached to the headgroups. We demonstrate using vibrational sum frequency generation spectroscopy and pendant drop tensiometery that the composition of the bulk aqueous phase, particularly the ionic strength, dictates the kinetics and structures of the amphiphiles in the organic phase as they decorate the interface. These results show that H-bonding and electrostatic interactions taking place in the aqueous phase bias the grafted oligomer conformations that are adopted in the neighboring oil phase. The kinetics of self-assembly were ionic strength dependent and found to be surprisingly slow, being composed of distinct regimes where molecules adsorb and reorient on relatively fast time scales, but where conformational sampling and frustrated packing takes place over longer time scales. These results set the stage for understanding related chemical phenomena of bioinspired materials in diverse technological and fundamental scientific fields and provide a solid physical foundation on which to design new functional interfaces.

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

The strong synergistic adsorption of mixed polymer/surfactant (P/S) systems at the oil/water interface shows promise for applications such as oil remediation and emulsion stabilization, especially with respect to the formation of tunable mesoscopic multilayers. There is some evidence that a combination of dodecyltrimethylammonium bromide (DTAB) and sodium poly(styrenesulfonate) (PSS) exhibits the adsorption of a secondary P/S layer, though the structure of this layer has long eluded researchers. The focus of this study is to determine whether the DTAB-assisted adsorption of PSS at the oil/water interface occurs as a single layer or with subsequent multilayers. The study presented uses vibrational sum-frequency spectroscopy and interfacial tensiometry to determine the degree of PSS adsorption and orientation of its charged groups relative to the interface at three representative concentrations of DTAB. At low and intermediate DTAB concentrations, a single mixed DTAB/PSS monolayer adsorbs at the oil/water interface. No PSS adsorbs above the system critical micelle concentration. The interfacial charge is found to be similar to that of P/S complexes solvated in the aqueous solution. The surface adsorbate and P/S complexes in the bulk both exhibit a charge inversion at around the same DTAB concentration. This study demonstrates the importance of techniques which can differentiate between coadsorbing species and calls into question current models of P/S adsorption at an oil/water interface.

Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge

The stabilization of nanoemulsions, nanosized oil droplets dispersed in water, is commonly achieved through the addition of surfactants and polymers. However, nanoemulsions in the absence of emulsifiers have been observed to acquire a significant negative charge at their surface, which ultimately contributes to their stability. While the source of this negative charge is disputed to this day, its presence is taken as an inherent property of the aqueous-hydrophobic interface. This report provides a look at the molecular structure and bonding characteristics of bare aqueous-hydrophobic nanoemulsion interfaces. We report the creation of bare nanoemulsions with near zero surface charge, which are marginally stable for several days. The process of creating these low-charge nanoemulsions (LCNEs) required rigorous cleaning procedures and proper solvent storage conditions. Using vibrational sum-frequency scattering spectroscopy, we measure the structure and bonding of the interfacial aqueous and hydrophobic phases. The surfaces of these LCNE samples possess a measurable free OH vibration, not found in previous studies and indicative of a clean interface. Tuning the nanoemulsion charge through addition of anionic surfactants, modeling potential surface-active contaminants, we observe the free OH to disappear and a reorientation of the interfacial hydrophobic molecules at micromolar surfactant concentrations. Notably, the free OH vibration provides evidence for stronger dispersion interactions between water molecules and the hydrophobic phase at the LCNE surface compared with similar planar water-alkane interfaces. We propose the stronger bonding interactions, in addition to an ordered interfacial aqueous layer, contribute to the delayed droplet coalescence and subsequent phase separation.

CnTAB/polystyrene sulfonate mixtures at air-water interfaces: effects of alkyl chain length on surface activity and charging state

Binding and phase behavior of oppositely charged polyelectrolytes and surfactants with different chain lengths were studied in aqueous bulk solutions and at air-water interfaces. In particular, we have investigated the polyanion poly(sodium 4-styrenesulfonate) (NaPSS) and the cationic surfactants dodecyltrimethylammonium bromide (C₁₂TAB), tetradecyltrimethylammonium bromide (C₁₄TAB) and cetyltrimethylammonium bromide (C₁₆TAB). In order to reveal the surfactant/polyelectrolyte binding, aggregation and phase separation of the mixtures, we have varied the NaPSS concentration systematically and have kept the surfactant concentration fixed at 1/6 of the respective critical micelle concentration. Information on the behavior in the bulk solution was gained by electrophoretic mobility and turbidity measurements, while the surface properties were studied using surface tension measurements and vibrational sum-frequency generation (SFG). This has enabled us to relate bulk to interfacial properties with respect to the charging state and the surfactants' binding efficiency. We found that the latter two are strongly dependent on the alkyl chain length of the surfactant and that binding is much more efficient as the alkyl chain length of the surfactant increases. This also results in a different phase behavior as shown by turbidity measurements of the bulk solutions. Charge neutral aggregates that are forming in the bulk adsorb onto the air-water interface - an effect that is likely caused by the increased hydrophobicity of C_nTAB/PSS complexes. This conclusion is corroborated by SFG spectroscopy, where we observe a decrease in the intensity of O-H stretching bands, which is indicative of a decrease in surface charging and the formation of interfaces with negligible net charge. Particularly at mixing ratios that are in the equilibrium two-phase region, we observe weak O-H intensities and thus surface charging.