Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy

Experimental and simulation-based characterization of surfactant adsorption layers at fluid interfaces

Adsorption of surfactants to fluid interfaces occurs in numerous technological and daily-life contexts. The coverage at the interface and other properties of the formed adsorption layers determine the performance of a surfactant with regard to the desired application. Given the importance of these applications, there is a great demand for the comprehensive characterization and understanding of surfactant adsorption layers. In this review, we provide an overview of suitable experimental and simulation-based techniques and review the literature in which they were used for the investigation of surfactant adsorption layers. We come to the conclusion that, while these techniques have been successfully applied to investigate Langmuir monolayers of water-insoluble surfactants, their application to the study of Gibbs adsorption layers of water-soluble surfactants has not been fully exploited. Finally, we emphasize the great potential of these methods in providing a deeper understanding of the behavior of soluble surfactants at interfaces, which is crucial for optimizing their performance in various applications.

Photo-Responsive Control of Adsorption and Structure Formation at the Air-Water Interface with Arylazopyrazoles

Smart interfaces that are responsive to external triggers such as light are of great interest for the development of responsive or adaptive materials and interfaces. Using alkyl-arylazopyrazole butyl sulfonate surfactants (alkyl-AAP) that can undergo E/Z photoisomerization when irradiated with green (E) and UV (Z) lights, we demonstrate through a combination of experiments and computer simulations that there can be surprisingly large changes in surface tension and in the molecular structure and order at air-water interfaces. Surface tensiometry, vibrational sum-frequency generation (SFG) spectroscopy, and neutron reflectometry (NR) are applied to the study of custom-synthesized AAP surfactants with octyl- and H-terminal groups at air-water interfaces as a function of their bulk concentration and E/Z configuration. Upon photoswitching, a drastic influence of the alkyl chain on both the surface activity and the responsiveness of interfacial surfactants is revealed from changes in the surface tension, γ, where the largest changes in γ are observed for octyl-AAP (Δγ ∼ 23 mN/m) in contrast to H-AAP with Δγ < 10 mN/m. Results from vibrational SFG spectroscopy and NR show that the interfacial composition and the molecular order of the surfactants drastically change with E/Z photoisomerization and surface coverage. Indeed, from analysis of the S-O (head group) and C-H vibrational bands (hydrophobic tail), a qualitative analysis of orientational and structural changes of interfacial AAP surfactants is provided. The experiments are complemented by resolution of thermodynamic parameters such as equilibrium constants from ultra-coarse-grained simulations, which also capture details like island formation and interaction parameters of interfacial molecules. Here, the interparticle interaction ("stickiness") and the interaction with the surface are adjusted, closely reflecting experimental conditions.

Is there a difference between surfactant-stabilised and Pickering emulsions?

What measurable physical properties allow one to distinguish surfactant-stabilised from Pickering emulsions? Whereas surfactants influence oil/water interfaces by lowering the oil/water interfacial tension, particles are assumed to have little effect on the interfacial tension. Here we perform interfacial tension (IFT) measurements on three different systems: (1) soybean oil and water with ethyl cellulose nanoparticles (ECNPs), (2) silicone oil and water with the globular protein bovine serum albumin (BSA), and (3) sodium dodecyl sulfate (SDS) solutions and air. The first two systems contain particles, while the third system contains surfactant molecules. We observe a significant decrease in interfacial tension with increasing particle/molecule concentration in all three systems. We analyse the surface tension data using the Gibbs adsorption isotherm and the Langmuir equation of state for the surface, resulting in surprisingly high adsorption densities for the particle-based systems. These seem to behave very much like the surfactant system: the decrease in tension is due to the presence of many particles at the interface, each with an adsorption energy of a few k_BT. Dynamic interfacial tension measurements show that the systems are in equilibrium, and that the characteristic time scale for adsorption is much longer for particle-based systems than for surfactants, in line with their size difference. In addition, the particle-based emulsion is shown to be less stable against coalescence than the surfactant-stabilised emulsion. This leaves us with the conclusion that we are not able to make a clear distinction between the surfactant-stabilised and Pickering emulsions.

All-experimental analysis of doubly resonant sum-frequency generation spectra for Franck–Condon and Herzberg–Teller vibronic modes

The transform technique applied to the analysis of doubly resonant sum-frequency generation (DR-SFG) spectra is extended to include Herzberg–Teller (HT) vibronic modes. The experimentally measured overlap spectral function generates all the energy resonant amplitudes of the DR-SFG excitation function for both Franck–Condon (FC) and HT modes. When FC modes dominate the DR-SFG spectra, a methodology is provided to perform efficient curve fitting and orientation analysis in order to extract FC activities of the various vibration modes from experimental spectra with the help of a molecular model. Determination of the FC or HT natures of the vibration modes from DR-SFG data is also shown to be possible through their visible line shapes with an appropriate choice of polarizations. As an example, experimental DR-SFG data suggest that a known HT-active mode in the vibronic structure of Rhodamine 6G monomers exhibits a FC behavior in molecular aggregates.

Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO₄) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K⁺ to the fatty sulfate interface showed 10 times greater affinity than for Li⁺ and approximately 3 times greater than for Na⁺. All three cations formed solvent shared ion pairs when the mole fraction of ESO₄ was 0.33 or lower. Above this threshold charge density, Li⁺ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K⁺ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na⁺ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.

Fatty Acids Can Induce the Formation of Proteinaceous Particles in Monoclonal Antibody Formulations

The presence of subvisible or visible particles in mAb formulations can pose significant challenges to pharmaceutical development as it can lead to reduced shelf life, batch rejection, and recalls. Among all type of particles, proteinaceous particles are the most concerning due to their potential role in immunogenicity. Nevertheless, the underlying mechanism for protein particle formation remains poorly understood. Past research highlighted the importance of interfaces and mechanical agitation in causing protein particle formation. Current research suggests that fatty acids, as impurities present in excipients or as a result of polysorbate degradation, can also induce protein assembly and promote particle formation. In this work, we assessed oleic and lauric acid for their impact on particle formation as each represents the main hydrolysis product of PS80 or PS20, respectively. It was found that co-existence of either fatty acids with 10 mg/mL mAb A can cause protein particles, with a similar morphology to those observed previously in mAb formulations. FTIR spectra showed that the particles are proteinaceous, heterogeneous in its composition, but contain corresponding fatty acids. Interestingly, it was found that oleic acid is significantly more effective in causing protein particles than lauric acid in these experiments. This suggests that PS20 containing formulations might have a lower likelihood to have protein particles compared to PS80 containing mAb formulations if hydrolysis of polysorbate were to occur. Lastly, the presence of 0.01% polysorbate in the mAb A formulation was able to fully mitigate the effect of fatty acids and reduce the protein particles significantly, suggesting a potential mechanism where interfacial action is involved. The present study can help to understand the root cause for protein particles in a mAb formulation where fatty acids are introduced because of polysorbate hydrolysis. With further work, it will help to shed light into product control strategy as well as design approaches for robust mAb products.

Effects of Salt-Induced Charge Screening on AOT Adsorption to the Planar and Nanoemulsion Oil-Water Interfaces

Nanoemulsions, nanosized droplets of oil, are easily stabilized by interfacial electric fields from the adsorption of ionic surfactants. While mean-field theories can be used to describe the impact of these interfacial fields on droplet stability, the influence of these fields on the adsorption properties of ionic surfactants is not well-understood. In this work, we study the adsorption of the surfactant sodium dioctyl sulfosuccinate (AOT) at the nanoemulsion and planar oil-water interfaces and investigate how salt-induced charge-screening affects AOT adsorption. In the absence of salt, vibrational sum-frequency scattering spectroscopy measurements reveal the ΔG_ads and the maximum surface density is the same for AOT at the hexadecane nanoemulsion surface as at the planar hexadecane-H₂O interface. Upon the addition of NaCl, an increase in AOT surface density is detected at both interfaces, indicating that repulsive electrostatic interactions between AOT monomers are the dominant force limiting surfactant adsorption at both interfaces. The bulky alkyl chains of AOT molecules cause our observations in this study to differ from those found in previous studies investigating the adsorption of linear-chain ionic surfactants to the nanoemulsion surface. These results provide necessary information for understanding factors limiting the adsorption of ionic surfactants to nanodroplet surfaces and highlight the need for further studies into the adsorption properties of more complex macromolecules at nanoemulsion surfaces.

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films

A new method is proposed to analyze Doubly Resonant infrared-visible Sum-Frequency Generation (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG line shapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H- and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.

Evidence of surfactant sub-monolayer adsorption at the air/water interface provided by laser scattering measurements of ultrafine gas bubbles

As the SDS concentration increases in bubble rich solutions, the surfactant layer alters its size and refractive index. The scattered light enhancement and SFG signal cancellation prove that sub-monolayer adsorption exists at the air/water interface.

Exploring the nature of the interactions between the molecules of the sodium dodecyl sulfate and water in crystal phases and in the water/vacuum interface

The nature of the interaction between the molecules of the sodium dodecyl sulfate surfactant forming two crystal phases, one anhydrous, NaC₁₂H₂₅O₄S and the other, NaC₁₂H₂₅O₄S.H₂O, hydrated with one water molecule for unit cell, has been studied in detail using the quantum theory of atoms in molecules and a localized electron detector function. It was found that for the anhydrous crystal, the head groups of the surfactant molecules are linked into a head-to-head pattern, by a bond path network of Na–O ionic bonds, where each Na⁺ atom is attached to four SO4− groups. For the hydrated crystal, in addition to these four bonds for Na+, two additional ones appear with the oxygen atoms of the water molecules, forming a bond paths network of ionic Na–O bonds, that link the Na⁺ atoms with the SO4− groups and the H₂O molecules. Each H₂O molecule is bonded to two SO4− groups via hydrogen bonds, while the SO4− groups are linked to a maximum of four Na+ atoms. The phenomenon of aggregation of the sodium dodecyl sulfate molecules at the liquid water/vacuum interface was studied using NVT molecular dynamics simulations. We have found that for surfactant aggregates, the Na+ ions are linked to a maximum of three SO₄^- groups and three water molecules that form Na–O bonds. Unlike hydrated crystal, each of the O atoms that make these Na–O bonds is linked to only one Na+ ion. Despite these differences, like the crystal phases, the surfactant molecules tend to form a head-to-head network pattern of ionic Na–O bonds that link their heads. The present results indicate that the clustering of anionic surfactant at the water/vacuum interface is a consequence of the electrostatic alignment of the cationic and anionic groups as occurs in the crystalline phases of sodium dodecyl sulfate.

Electric-Field-Induced Interface Behavior of Dodecyl Sulfate with Large Organic Counterions: A Molecular Dynamics Study

Dodecyl sulfate with tetramethylammonium counterions has been employed to systematically investigate the influence of different static electric fields on molecular structural properties, surface tension, by adopting molecular dynamics (MD) simulations with IR and sum frequency generation (SFG) spectrum calculations. The results indicated that dodecyl sulfate (DS^-) and large organic TMA⁺ counterions can form a mixed adsorption layer in which one head group of DS^- is surrounded by two tetramethylammonium (TMA⁺) and one water molecule. Additionally, it was observed that the surface tension significantly decreases with the increasing static electric field strength since the surfactant stands straighter at the interface as the electric field increases. The result can be instructively adopted in the manufacturing field to control surface tension. Moreover, it was found that the SFG stretch intensities of methylene decrease and the stretch intensities of the methyl group increase with increasing static electric fields. The result indicated that the static electric fields can make DS^- more orderly and upright at the interface.

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.

Synthesis of the cationic fluorescent probes for the detection of anionic surfactants by electrostatic self-assembly

Anionic surfactants were widespread used in car cleaning agents, household detergents, agricultural and industrial processes, and considered as a major source of environmental pollutant. Therefore, it is necessary to develop a fast, simple, highly selective and sensitive probe for the detection of anionic surfactants. Here, we synthesized two aggregation induced emission (AIE)-active molecules 4,4',4″,4‴-(ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis (1-(4-bromobenzyl)pyridin-1-ium) bromide (TPE-Br) and 4,4',4″,4‴-(ethene-1,1,2,2-trayltetrakis(benzene-4,1-diyl))tetrakis(1-methylpyridin-1-ium)iodide (TPE-I), which were then applied as fluorescence probes for detecting sodium dodecyl sulfate (SDS) with high selectivity and sensitivity. In the presence of SDS, a multi-fold fluorescence emission intensity enhancement was observed in both two probes (TPE-Br and TPE-I) due to the electrostatic self-assembly of AIE molecular. The limits of detection are 71.5 and 120 nM for TPE-Br and TPE-I, respectively. This study may provide a new strategy for environmental monitoring by AIE-based fluorescent probe.

The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency Generation Spectroscopy

The plasmonic amplification of nonlinear vibrational sum frequency spectroscopy (SFG) at the surfaces of gold nanoparticles is systematically investigated by tuning the incident visible wavelength. The SFG spectra of dodecanethiol-coated gold nanoparticles chemically deposited on silicon are recorded for 20 visible wavelengths. The vibrational intensities of thiol methyl stretches extracted from the experimental measurements vary with the visible color of the SFG process and show amplification by coupling to plasmon excitation. Because the enhancement is maximal in the orange-red region rather than in the green, as expected from the dipolar model for surface plasmon resonances, it is attributed mostly to hotspots created in particle multimers, in spite of their low surface densities. A simple model accounting for the longitudinal surface plasmons of multimers allows the recovery of the experimental spectral dispersion.

Specific Ion Effects of Dodecyl Sulfate Surfactants with Alkali Ions at the Air-Water Interface

The influence of Li⁺, Na⁺ and Cs⁺ cations on the surface excess and structure of dodecyl sulfate (DS⁻) anions at the air–water interface was investigated with the vibrational sum-frequency generation (SFG) and surface tensiometry. Particularly, we have addressed the change in amplitude and frequency of the symmetric S-O stretching vibrations as a function of electrolyte and DS⁻ concentration in the presence of Li⁺, Na⁺ and Cs⁺ cations. For the Li⁺ and Na⁺ ions, we show that the resonance frequency is shifted noticeably from 1055 cm⁻¹ to 1063 cm⁻¹ as a function of the surfactants’ surfaces excess, which we attribute to the vibrational Stark effect within the static electric field at the air–water interface. For Cs⁺ ions the resonance frequency is independent of the surfactant concentration with the S-O stretching band centered at 1063 cm⁻¹. This frequency is identical to the frequency at the maximum surface excess when Li⁺ and Na⁺ ions are present and points to the ion pair formation between the sulfate headgroup and Cs⁺ counterions, which reduces the local electric field. In addition, SFG experiments of the O-H stretching bands of interfacial H₂O molecules are used in order to calculate the apparent double layer potential and the degree of dissociation between the surfactant head group and the investigated cations. The latter was found to be 12.0%, 10.4% and 7.7% for lithium dodecyl sulfate (LiDS), sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CsDS) surfactants, which is in agreement with Collins ‘rule of matching water affinities’.

Cooperative Effects of Zwitterionic-Ionic Surfactant Mixtures on the Interfacial Water Structure Revealed by Sum Frequency Generation Vibrational Spectroscopy

Cooperative effects of a series of equimolar binary zwitterionic-ionic surfactant mixtures on the interfacial water structure at the air-water interfaces have been studied by sum frequency generation vibrational spectroscopy (SFG-VS). For zwitterionic surfactant palmityl sulfobetaine (SNC₁₆), anionic surfactant sodium hexadecyl sulfate (SHS), and cationic surfactant cetyltrimethylammonium bromide (CTAB) with the same length of alkyl chain, significantly enhanced ordering of interfacial water molecules was observed for the zwitterionic-anionic surfactant mixtures SNC₁₆-SHS, indicating that SNC₁₆ interacts more strongly with SHS than with CTAB because of the strong headgroup-headgroup electrostatic attraction for SNC₁₆-SHS. Meanwhile, the SFG amplitude ratio of methyl and methylene symmetric stretching modes was used to verify the stronger interaction between SNC₁₆ and SHS. The conformational order indicator increased from 0.64 for SNC₁₆ to 7.17 for SNC₁₆-SHS but only 0.94 for SNC₁₆-CTAB. In addition, another anionic surfactant sodium dodecyl sulfate (SDS) was introduced to study the influence of chain-chain interaction. Decreased SFG amplitude of interfacial water molecules for SNC₁₆-SDS was observed. Therefore, both the headgroup-headgroup electrostatic interaction and chain-chain van der Waals attractive interaction of the surfactants play an important role in enhancing the ordering of interfacial water molecules. The results provided experimental and theoretical bases for practical applications of the surfactants.

Electroosmosis near surfactant laden liquid-air interfaces

Generation of an electroosmostic (EO) flow near a liquid-gas interface covered with ionic surfactants is experimentally investigated. A combination of microscopic flow measurements with a molecular characterization of the interface by second harmonic generation (SHG) shows that under an electrical forcing, although a liquid flow is generated below the free surface, the surfactants remain immobile. The zeta potential was then determined and compared to the surfactant surface coverage. This combination of experimental techniques opens the route to simultaneously probe the liquid flow near a soapy interface and the corresponding surfactant repartition affecting the hydrodynamic boundary condition.

Ion Pairing and Adsorption of Azo Dye/C16TAB Surfactants at the Air-Water Interface

Mixed layers of 6-hydroxy-5-[(4-sulfophenyl)azo]-2-naphthalenesulfonate (Sunset Yellow, SSY) and cetyltrimethylammonium bromide (C16TAB) at the air-water interface were studied using vibrational sum-frequency generation (SFG) and dynamic surface tension measurements. In the bulk, addition of C16TAB to SSY aqueous solution causes substantial changes in UV/vis absorption spectra, which originate from strong electrostatic interactions between the anionic SSY azo dye with the cationic C16TAB surfactant. These interactions are a driving force for the formation of SSY/C16TAB ion pairs. The latter are found to be highly surface active while free SSY molecules show no surface activity. Dynamic SFG as well as surface tension measurements at low SSY concentrations reveal that free C16TAB surfactants adsorb at the air-water interface on time scales <1 s where they initially form the dominating surface species, but on longer time scales free C16TAB is exchanged by SSY/C16TAB ion pairs. This causes a dramatic reduction of the surface tension to 35 mN/m but also in foam stability. These changes are accompanied by a substantial loss in SFG intensity from O-H stretching bands around 3200 and 3450 cm-1, which we relate to a decrease in surface charging due to adsorption of ion pairs with no or negligible net charges. For SSY/C16TAB molar ratios >0.5, the O-H bands in SFG spectra are reduced to very low intensities and are indicative to electrically neutral SSY/C16TAB ion pairs. This conclusion is corroborated by an analysis of macroscopic foams, which become highly instable in the presence of neutral SSY/C16TAB ion pairs. From an analysis of SFG spectra of air-water interfaces, we show that the electrostatic repulsion forces inside the ubiquitous foam films are reduced and thus remove the major stabilization mechanism within macroscopic foam.

Electrokinetic transport in liquid foams

Investigating electrokinetic transport in a liquid foam is at the confluence of two well developed research areas. On one hand, the study of electrokinetic flows (i.e. surface-driven flows generated close to a charged interface) is fairly well understood in regards the solid/liquid interface. On the other hand, the flow of liquid in a 3D deformable network, i.e a foam, under a volume force such as gravity has been thoroughly studied over the past decade. The overlapping zone of these two frameworks is of great interest for both communities as it gives rise to challenging new questions such as: what is the importance of the nature of the charged interface, created by mobile and soluble surfactants in the case of foam, on electrokinetic transport? How does a foam behave when submitted to a surface-driven flow? Can we compensate a volume-driven flow, i.e. gravity, by a surface-driven flow, i.e. electroosmosis? In this review, we will explore these questions on three different scales: a surfactant laden interface, a foam film and a macroscopic foam.

The ionic strength dependent zeta potential at the surface of hexadecane droplets in water and the corresponding interfacial adsorption of surfactants

An anomalous maximum in the ionic strength dependent electrophoretic mobility curves has been observed in previous reports from particles dispersed in colloids. This maximum has been considered anomalous because it is contradictory with the Gouy-Chapman model. The existence of such a maximum has been attributed to specific ionic adsorption, a hairy layer at the surface, or the effect of the anomalous change of surface conductivity in different studies. It was also pointed out that the O'Brien-White approach based on the Gouy-Chapman model could be used to understand this maximum in electrophoretic mobility curves and lead to understandable zeta potential curves. This implied that the observed maximum was actually not "anomalous". In this work we report our simulation of ionic strength dependent zeta potential curves based on the O'Brien-White approach and experimental studies of the ionic strength dependent electrophoretic mobility of the hexadecane droplets in the hexadecane-water emulsions at different pH or in the presence of sodium dodecyl sulphate at varied concentrations. In some cases, the simulation shows that the calculation with the O'Brien-White approach does change the trend in the concerned ionic strength dependent curves. However, the simulation in some other cases also leads to similar trends in the ionic strength dependent electrophoretic mobility curves and zeta potential curves. In the experiments, both the existence and non-existence of such a maximum were observed and demonstrated to be system dependent. The corresponding molecular structure of the oil-water interface was then discussed with the analyses of the zeta potential curves and second harmonic generation signals recorded at the hexadecane-water interface.

Modulation of Buckling Dynamics in Nanoparticle Laden Droplets Using External Heating

Dynamics of contact free (levitated) drying of nanofluid droplets is ubiquitous in many application domains ranging from spray drying to pharmaceutics. Controlling the final morphology (macro to micro scales) of the dried out sample poses some serious challenges. Evaporation of solvent and agglomeration of particles leads to porous shell formation in acoustically levitated nanosilica droplets. The capillary pressure due to evaporation across the menisci at the nanoscale pores causes buckling of the shell which leads to ring and bowl shaped final structures. Acoustics plays a crucial role in flattening of droplets which is a prerequisite for initiation of buckling in the shell. Introduction of mixed nanocolloids (sodium dodecyl sulfate + nanosilica) reduces evaporation rate, disrupts formation of porous shell, and enhances mechanical strength of the shell, all of which restricts the process of buckling. Although buckling is completely arrested in such surfactant added droplets, controlled external heating using laser enhances evaporation through the pores in the shell due to thermally induced structural changes and rearrangement of SDS aggregates which reinitializes buckling in such droplets. Furthermore, inclusion of anilinium hydrochloride into the nanoparticle laden droplets produces ions which adsorb and modify the morphology of sodium dodecyl sulfate crystals and reinitializes buckling in the shell (irrespective of external heating conditions). The kinetics of buckling is determined by the combined effect of morphology of the colloidal particles, particle/aggregate diffusion rate within the droplet, and the rate of evaporation of water. The buckling dynamics leads to cavity formation which grows subsequently to yield final structures with drastically different morphological features. The cavity growth is controlled by evaporation through the nanoscale pores and exhibits a universal trend irrespective of heating rate and nanoparticle type.

Specific effects of monovalent counterions on the structural and interfacial properties of dodecyl sulfate monolayers

Shows that NH₄⁺ ions dehydrate the DS⁻ headgroup by displacing hydrogen bonded waters from the interface.

Suppressing interfacial water signals to assist the peak assignment of the N⁺-H stretching mode in sum frequency generation vibrational spectroscopy

Amines are one of the common functional groups of interest due to their abundant presence in natural proteins, surfactants and other chemicals. However, their accurate spectral assignment of vibrational modes, critical to interpreting SFG signals for characterizing various bio-interfaces such as protein-membrane interaction and surfactant adsorption, still remains elusive. Herein we present a systematic study to identify and justify the correct peak assignment of the N(+)-H stretching mode at the air-water interface. We used three special surfactants: hexadecylamine (a primary amine without counterions), dodecylamine hydrochloride (a primary amine with counterions) and hexadecyltrimethylammonium bromide as a control (the N(+)-H stretching mode is absent in this quarternary amine). We suppressed the SFG interfacial water signals using saturated NaCl solutions. Our designed experiments resolved the current controversy and concluded that the 3080 cm(-1) peak is from the N(+)-H vibrations, while the 3330 cm(-1) peak is not due to ammonium species but rather originates from the interfacial water vibrational modes or the backbone amide modes.

Bovine serum albumin-sodium alkyl sulfates bioconjugates as drug delivery systems

Precipitation of bovine serum albumin (BSA) by anionic surfactants with alkyl chains of increasing lengths (octyl, decyl, dodecyl sulfates) was studied at room temperature, at pH 3.0, in isotonic sodium chloride solution. The particle size of albumin, the zeta potential, the surface charge and fluorescent properties of BSA-surfactant composites were investigated concerning addition of increasing amount of surfactant. The thermal stability of the systems was monitored by calorimetric analysis (DSC). The formation of the well-ordered structure in the self-assembly process in liquid phase was studied by XRD measurement. The structure of the precipitated BSA-surfactant nanocomposites was characterized by small-angle X-ray scattering (SAXS). Finally, ibuprofen (IBU) molecules were enclosed in BSA-surfactant bioconjugate systems and the release properties of the drug were investigated. It has been found out that, as a consequence to the increasing number of carbon atoms in the alkyl chains of the surfactant, the structure and the fluorescent properties of the aggregates formed can be controlled due to the increase in the hydrophobicity of BSA-surfactant composites. The bioconjugates are well applicable as carrier to realize controlled release of drug molecules.

Stochastic methods for aerosol chemistry: a compact molecular description of functionalization and fragmentation in the heterogeneous oxidation of squalane aerosol by OH radicals

A compact, experimentally validated model of organic aerosol oxidation enables the ageing process to be connected to specific chemical reactions.

Temperature-dependent phase transition and desorption free energy of sodium dodecyl sulfate at the water/vapor interface: approaches from molecular dynamics simulations

Adsorption of surfactants at the water/vapor interface depends upon their chemical potential at the interface, which is generally temperature-dependent. Molecular dynamics simulations have been performed to reveal temperature influences on the microstructure of sodium dodecyl sulfate (SDS) molecule adsorption layer. At room temperature, SDS molecules aggregate at the interface, being in a liquid-expanded phase, whereas they tend to spread out and probably transit to a gaseous phase as the temperature increases to above 318 K. This phase transition has been confirmed by the temperature-dependent changes in two-dimensional array, tilt angles, and immersion depths to the aqueous phase of SDS molecules. The aggregation of SDS molecules accompanies with larger immersion depths, more coordination of Na(+) ions, and less coordination of water. Desorption free energy profiles show that higher desorption free energy appears for SDS molecules at the aggregate state at low temperatures, but no energy barrier is observed. The shapes of desorption free energy profiles depend upon the distribution of SDS at the interface, which, in turn, is related to the phase state of SDS. Our study sheds light on the development of adsorption thermodynamics and kinetics theories.

Sodium dodecyl sulfate adsorption onto positively charged surfaces: monolayer formation with opposing headgroup orientations

The adsorption and structure of sodium dodecyl sulfate (SDS) layers onto positively charged films have been monitored in situ with vibrational sum-frequency-generation (SFG) spectroscopy and surface plasmon resonance (SPR) sensing. Substrates with different charge densities and polarities used in these studies include CaF2 at different pH values as well as allylamine and heptylamine films deposited onto CaF2 and Au substrates by radio frequency glow discharge deposition. The SDS films were adsorbed from aqueous solutions ranging in concentration from 0.067 to 20 mM. In general the SFG spectra exhibited well resolved CH and OH peaks. However, at SDS concentrations between 1 and 8 mM the SFG CH and OH intensities decreased close to background levels. Combined data sets from molecular conformation, orientation, and order sensitive SFG with mass sensitive SPR suggest that the observed changes in SFG intensities above 0.2 mM are related to structural arrangements in the SDS layer. A model is proposed where the SFG intensity minimum between 1 and 8 mM is associated with a monolayer containing two headgroup orientations, one pointing toward the substrate and one pointing toward the solution phase. The SFG peaks observed at concentrations below 0.2 mM are dominated by the presence of adsorbed contaminants such as fatty alcohols (e.g., dodecanol), which are more surface active than SDS. As SDS solution concentration is increased above 1 mM SDS molecules are incorporated in the surface layer, with dodecanol continuing to be present in the surface layer for solution concentrations up to at least the critical micelle concentration.

Conductivity Measurements as a Method for Studying Ionic Technical Grade Surfactants

The critical micellar concentrations (cmc) of cationic alkyltrimethylammonium bromides (C10TAB – C16TAB) and of anionic alkyl sulfates (SDS, technical grade Sulfopon 12G and Sulfopon 1214G) were determined using specific conductivity and surface tension measurements. While the cationic surfactants were purified, the anionic surfactants were used as received. In the former case, surface tension and conductivity measurements lead to the same cmc values. In the latter case, measuring the specific conductivities (bulk property) proved to be superior compared to measuring the surface tensions (surface property) because the presence of highly surface active impurities creates a pronounced minimum in the surface tension isotherm, which makes it impossible to determine an accurate value for the cmc. On the other hand, these impurities do not influence the conductivities, i. e. that the conductivities can be used to determine cmc values of impure and technical grade ionic surfactants, respectively. Moreover, an evaluation of the specific conductivities allows for the determination of the degree of micelle ionization (α) of the ionic surfactant solutions and thus provides additional useful information.

Environmental chemistry at vapor/water interfaces: insights from vibrational sum frequency generation spectroscopy

The chemistry that occurs at surfaces has been an intense area of study for many years owing to its complexity and importance in describing a wide range of physical phenomena. The vapor/water interface is particularly interesting from an environmental chemistry perspective as this surface plays host to a wide range of chemistries that influence atmospheric and geochemical interactions. The application of vibrational sum frequency generation (VSFG), an inherently surface-specific, even-order nonlinear optical spectroscopy, enables the direct interrogation of various vapor/aqueous interfaces to elucidate the behavior and reaction of chemical species within the surface regime. In this review we discuss the application of VSFG to the study of a variety of atmospherically important systems at the vapor/aqueous interface. Chemical systems presented include inorganic ionic solutions prevalent in aqueous marine aerosols, small molecular solutes, and long-chain fatty acids relevant to fat-coated aerosols. The ability of VSFG to probe both the organization and reactions that may occur for these systems is highlighted. A future perspective toward the application of VSFG to the study of environmental interfaces is also provided.

Effect of Side Chains on Competing Pathways for β-Scission Reactions of Peptide-Backbone Alkoxyl Radicals

High-level quantum chemistry calculations have been carried out to investigate beta-scission reactions of alkoxyl radicals located at the alpha-carbon of a peptide backbone. This type of alkoxyl radical may undergo three possible beta-scission reactions, namely C-C beta-scission of the backbone, C-N beta-scission of the backbone, and C-R beta-scission of the side chain. We find that the rates for the C-C beta-scission reactions are all very fast, with rate constants of the order 10(12) s(-1) that are essentially independent of the side chain. The C-N beta-scission reactions are all slow, with rate constants that range from 10(-0.7) to 10(-4.5) s(-1). The rates of the C-R beta-scission reactions depend on the side chain and range from moderately fast (10(7) s(-1)) to very fast (10(12) s(-1)). The rates of the C-R beta-scission reactions correlate well with the relative stabilities of the resultant side-chain product radicals (*R), as reflected in calculated radical stabilization energies (RSEs). The order of stabilities for the side-chain fragment radicals for the natural amino acids is found to be Ala < Glu < Gln approximately Leu approximately Met approximately Lys approximately Arg < Asp approximately Ile approximately Asn approximately Val < Ser approximately Thr approximately Cys < Phe approximately Tyr approximately His approximately Trp. We predict that for side-chain C-R beta-scission reactions to effectively compete with the backbone C-C beta-scission reactions, the side-chain fragment radicals would generally need an RSE greater than approximately 30 kJ mol(-1). Thus, the residues that may lead to competitive side-chain beta-scission reactions are Ser, Thr, Cys, Phe, Tyr, His, and Trp.

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.