Mixtures of n-dodecyl-beta-D-maltoside and hexaoxyethylene dodecyl ether--surface properties, bulk properties, foam films, and foams

Interface Adsorption versus Bulk Micellization of Surfactants: Insights from Molecular Simulations

Surfactants play essential roles in many commonplace applications and industrial processes. Although significant progress has been made over the past decades with regard to model-based predictions of the behavior of surfactants, important challenges have remained. Notably, the characteristic time scales of surfactant exchange among micelles, interfaces, and the bulk solution typically exceed the time scales currently accessible with atomistic molecular dynamics (MD) simulations. Here, we circumvent this problem by introducing a framework that combines the general thermodynamic principles of self-assembly and interfacial adsorption with atomistic MD simulations. This approach provides a full thermodynamic description based on equal chemical potentials and connects the surfactant bulk concentration, the experimental control parameter, with the surfactant surface density, the suitable control parameter in MD simulations. Self-consistency is demonstrated for the nonionic surfactant C12EO6 (hexaethylene glycol monododecyl ether) at an alkane/water interface, for which the adsorption and pressure isotherms are computed. The agreement between the simulation results and experiments is semiquantitative. A detailed analysis reveals that the used atomistic model captures well the interactions between surfactants at the interface but less so their adsorption affinities to the interface and incorporation into micelles. Based on a comparison with other recent studies that pursued similar modeling challenges, we conclude that the current atomistic models systematically overestimate the surfactant affinities to aggregates, which calls for improved models in the future.

Synthesis of d-Gluconic Acetal Surfactants and Their Foaming Behaviors

Sugars are natural and environmentally benign substances, which can offer various hydroxyl groups. The understanding of details of the hydroxyl interactions in the hydrophilic groups of sugar-based surfactants, as well as the related properties, is still indistinct. Here, novel d-gluconic acetal surfactants with bicyclic and monocyclic structures in the head group were designed and synthesized. The obtained surfactant with a bicyclic architecture exhibited excellent foamability and a multistimulus-responsive behavior toward foam stabilization. In addition, the control of foamability from defoaming and foaming could be achieved by changing pH values or bubbling gas of CO₂/N₂. To explore the structural effects such as hydroxyl groups and rigidity of the head group on the properties of sugar-based surfactants, another kind of amphiphilic molecule with various OH- groups and a monocycle in the head group was designed for comparison. These two series of amphiphilic molecules both exhibited good surface activity. However, only the d-gluconic acetal surfactant with a bicyclic structure and a smaller number of OH- groups exhibited excellent foamability. Further studies showed that the foam behaviors were attributed to the conformation and arrangement of the surfactant molecule at the surface layer with the assistance of hydrogen bonds formed by hydroxyl groups and H₂O molecules. In addition, the surfactant could provide an environmentally friendly foamer in many potential applications.

From Individual Liquid Films to Macroscopic Foam Dynamics: A Comparison between Polymers and a Nonionic Surfactant

Foams can resist destabilizaton in ways that appear similar on a macroscopic scale, but the microscopic origins of the stability and the loss thereof can be quite diverse. Here, we compare both the macroscopic drainage and ultimate collapse of aqueous foams stabilized by either a partially hydrolyzed poly(vinyl alcohol) (PVA) or a nonionic low-molecular-weight surfactant (BrijO10) with the dynamics of individual thin films at the microscale. From this comparison, we gain significant insight regarding the effect of both surface stresses and intermolecular forces on macroscopic foam stability. Distinct regimes in the lifetime of the foams were observed. Drainage at early stages is controlled by the different stress-boundary conditions at the surfaces of the bubbles between the polymer and the surfactant. The stress-carrying capacity of PVA-stabilized interfaces is a result of the mutual contribution of Marangoni stresses and surface shear viscosity. In contrast, surface shear inviscidity and much weaker Marangoni stresses were observed for the nonionic surfactant surfaces, resulting in faster drainage times, both at the level of the single film and the macroscopic foam. At longer times, the PVA foams present a regime of homogeneous coalescence where isolated coalescence events are observed. This regime, which is observed only for PVA foams, occurs when the capillary pressure reaches the maximum disjoining pressure. A final regime is then observed for both systems where a fast coalescence front propagates from the top to the bottom of the foams. The critical liquid fractions and capillary pressures at which this regime is obtained are similar for both PVA and BrijO10 foams, which most likely indicates that collapse is related to a universal mechanism that seems unrelated to the stabilizer interfacial dynamics.

Emergence of Electric Fields at the Water-C12E6 Surfactant Interface

We study the properties of the interface of water and the surfactant hexaethylene glycol monododecyl ether (C12E6) with a combination of heterodyne-detected vibrational sum frequency generation (HD-VSFG), Kelvin-probe measurements, and molecular dynamics (MD) simulations. We observe that the addition of the hydrogen-bonding surfactant C12E6, close to the critical micelle concentration (CMC), induces a drastic enhancement in the hydrogen bond strength of the water molecules close to the interface, as well as a flip in their net orientation. The mutual orientation of the water and C12E6 molecules leads to the emergence of a broad (∼3 nm) interface with a large electric field of ∼1 V/nm, as evidenced by the Kelvin-probe measurements and MD simulations. Our findings may open the door for the design of novel electric-field-tuned catalytic and light-harvesting systems anchored at the water-surfactant-air interface.

Multiscale Microstructure, Composition, and Stability of Surfactant/Polymer Foams

Inclusion of polymer additives is a known strategy to improve foam stability, but questions persist about the amount of polymer incorporated in the foam and the resulting structural changes that impact material performance. Here, we study these questions in sodium dodecyl sulfate (SDS)/hydroxypropyl methylcellulose (HPMC) foams using a combination of flow injection QTOF mass spectrometry and small-angle neutron scattering (SANS) measurements leveraging contrast matching. Mass spectrometry results demonstrate polymer incorporation and retention in the foam during drainage by measuring the HPMC-to-SDS ratio. The results confirm a ratio matching the parent solution and stability over the time of our measurements. The SANS measurements leverage precise contrast matching to reveal detailed descriptions of the micellar structure (size, shape, and aggregation number) along with the foam film thickness. The presence of HPMC leads to thicker films, correlating with increased foam stability over the first 15-20 min after foam production. Taken together, mass spectrometry and SANS present a structural and compositional picture of SDS/HPMC foams and an approach amenable to systematic study for foams, gathering mechanistic insights and providing formulation guidance for rational foam design.

How Promoting and Breaking Intersurfactant H-Bonds Impact Foam Stability

On the basis of previous results revealing that intersurfactant H-bonds improve foam stability, we now focus on how foams stabilized by two different N-acyl amino acid surfactants are affected by different salts (NaF, NaCl, NaSCN), which can promote or break intersurfactant H-bonds. The chosen surfactants, namely, sodium N-lauroyl sarcosinate (C₁₂SarcNa) and sodium N-lauroyl glycinate (C₁₂GlyNa), differ only by one methyl group at the nitrogen of the amide bond that blocks intersurfactant H-bonds in the case of C₁₂SarcNa. The salts were chosen because they are kosmotropic (NaF), chaotropic (NaSCN), and in between (NaCl) and thus influence the formation of an H-bond network in different ways. Surface tension measurements showed that the addition of salts decreased the cmcs of both surfactants and increased the packing density, as expected. Moreover, in presence of the salts, the head groups of the H-bond forming surfactant C₁₂GlyNa were more tightly packed at the surface than the C₁₂SarcNa head groups. The effect of the salts on foam stability was studied by analysis of the foam height, the foam liquid fraction, and by image analysis of the foam structure. As expected, the salts had no significant effect on foams stabilized by C₁₂SarcNa, which is unable to form intersurfactant H-bonds. In contrast, the stability of C₁₂GlyNa-containing foams followed the trend NaF > NaCl > NaSCN, which is in agreement with NaF promoting and NaSCN breaking intersurfactant H-bonds. Surface rheology measurements allowed us to correlate foam stability with surface elasticity. This study provides new insights into the importance of H-bond promoters and breakers, which should be used in the future design of tailor-made surfactants.

Quantifying Double-Layer Potentials at Liquid-Gas Interfaces from Vibrational Sum-Frequency Generation

Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential ϕ₀ at liquid-gas interfaces. For this, mixed solutions of nonionic tetraethyleneglycol-monodecylether (C₁₀E₄) and cationic hexadecyltrimethylammonium bromide (C₁₆TAB) surfactants were investigated using SFG spectroscopy and a thin-film pressure balance (TFPB). Derjaguin-Landau-Verwey-Overbeek analysis of disjoining pressure isotherms obtained with the TFPB technique provides complementary information on ϕ₀, which we apply to validate the results from SFG spectroscopy. By using a single ϕ₀ value, we can disentangle χ⁽²⁾ and χ⁽³⁾ contributions to the O-H stretching modes of interfacial water molecules in the SFG spectra. Having established the latter, we show that unknown double-layer potentials at the liquid-gas interface from solutions with different C₁₆TAB/C₁₀E₄ mixing ratios can be obtained from an analysis of SFG spectra and are in excellent agreement with the complementary results from the TFPB technique.

On the Influence of Intersurfactant H-Bonds on Foam Stability: A Study with Technical Grade Surfactants

From the literature on the foam stability of various surfactants with C12 alkyl chains but different head groups a clear picture emerges: Foams are more stable when hydrogen bonds can form between the head groups, i. e. when the polar head group has a hydrogen bond donor and a proton acceptor. These observations suggest that hydrogen bonds between neighbouring molecules at the surface enhance foam stability. To support this hypothesis, we carried out a systematic foaming study of two types of technical grade surfactants, one of them being capable of forming H-bonds and the other one not. As was the case for the pure surfactants we found again that more stable foams are formed when the head group is capable of forming intersurfactant H-bonds: These results will certainly affect the future design of surfactants.

Probing foam with neutrons

Foams are multiscale materials that have an enormous number of uses. As the relevant structural length-scales span from a few nanometres up to millimetres a number of characterisation methods need to be combined to obtain the full material structure. In this review we explain how foams can be explored using Small Angle Neutron Scattering (SANS). We remind the reader of the basics of SANS and contrast variation before we describe the different types of experiments that have been carried out on foams emphasising the specific role of neutrons in learning about the systems. To date SANS has been used to measure different foam structural parameters, such as the film thickness and the bubble size. Several studies have also been carried out to elucidate the organisation of the stabilising objects in the bulk solution. Finally we show how SANS measurements can be used to measure foam composition. Some of the accessible information is unique to SANS experiments, but as the method is still not very widely used on foams the review is also aimed to act as an introduction on how to carry out such measurements on foams.

On how hydrogen bonds affect foam stability

Do intermolecular H-bonds between surfactant head groups play a role for foam stability? From the literature on the foam stability of various surfactants with C12 alkyl chains but different head groups a clear picture emerges: stable foams are only generated when hydrogen bonds can form between the head groups, i.e. when the polar head group has a hydrogen bond donor and a proton acceptor. Stable foams can therefore be generated with surfactants having a sugar unit, a glycine, an amine oxide (at pH~5), or a carboxylic acid (at pH~pK_a) as polar head group. On the other hand, aqueous foams stabilized with surfactants having oligo(ethylene oxide), phosphine oxide, quaternary ammonium, sulfate, sarcosine, amine oxide (at pH≠5), or carboxylic acid (at pH≠pK_a) are not very stable. These observations suggest that hydrogen bonds between neighbouring molecules at the surface enhance foam stability. Formation of hydrogen bonds between surfactant head groups gives rise to a short-range attractive interaction that may restrict the surfactant's mobility while providing a more elastic surfactant layer which can counteract deformations. To support our hypothesis we carried out a systematic foaming study of two types of surfactants, one of them being capable of forming H-bonds and the other one not. Generating foams of all surfactants mentioned above with the same foaming conditions we found that stable foams are obtained when the head group is capable of forming intersurfactant H-bonds. The outcome of this study constitutes a new step towards the implementation of H-bonds in the future design of surfactants.

On the influence of surfactant on the coarsening of aqueous foams

We review the coarsening process of foams made with various surfactants and gases, focusing on physico-chemical aspects. Several parameters strongly affect coarsening: foam liquid fraction and foam film permeability, this permeability depending on the surfactant used. Both parameters may evolve with time: the liquid fraction, due to gravity drainage, and the film permeability, due to the decrease of capillary pressure during bubble growth, and to the subsequent increase in film thickness. Bubble coalescence may enhance the bubble's growth rate, in which case the bubble polydispersity increases. The differences found between the experiments reported in the literature and between experiments and theories are discussed.

Effects of protonation on foaming properties of dodecyldimethylamine oxide solutions: a pH-study

The critical micelle concentration (cmc), the surface excess (Γ), as well as the micelle aggregation number (m) of the surfactant dodecyldimethylamine oxide (C12DMAO) have been reported to strongly depend on the pH-value of the aqueous surfactant solution. At high ionic strength, the cmc displays a minimum, while both Γ and m have a maximum at a pH-value close to the pKa of the surfactant. These experimental observations have been explained as being due to specific hydrogen bonds between the head groups, which are formed once the surfactant is partly or fully protonated. This investigation addresses the question of whether the pH also affects the foaming properties of C12DMAO solutions. To answer this question we measured the foamability and the foam stability of C12DMAO solutions at a fixed C12DMAO concentration of 5 cmc for five different pH-values, namely pH = 2, 3, 5, 8, and 10. We found that the foamability is hardly affected by the pH-value, while the foam stability strongly depends on the pH. As is the case for the above mentioned properties, the foam stability also displays an extremum in the studied pH-range, namely a maximum at pH = 5. We discuss our results in terms of the hydrogen bond hypothesis and show that this hypothesis indeed is in line with the observed trend for the foam stability. Moreover, we discuss that hydrogen bond formation may rationalize how the molecular structure of a surfactant affects foam stability.

Comparison between generations of foams and single vertical films--single and mixed surfactant systems

The purpose of this article is to compare experiments carried out with single vertical foam films and with foams. We focus on the generation of films and foams and measure (i) the quantity of water entrained and (ii) the stability of the systems. The surfactants we used are C12E6, β-C12G2 and their 1 : 1 mixture because those systems are very well characterised in the literature and are known to stabilise foams with very different properties. We show that the quantity of water uptake in foams and single vertical films scales in the same way with the velocity of generation. However, the different surfactant solutions have different foamabilities, whereas the films they stabilise have exactly the same thickness. Moreover, the foamability of a C12E6 solution is much lower than that of a β-C12G2 solution or of a solution of the 1 : 1 mixture. This is due to the rapid rupture of the C12E6 foam films during foam generation. Surprisingly, the isolated films have exactly the same lifetime for all the surfactant solutions. We conclude that, though drawing a correlation between films and foams is tempting, the results obtained do not allow correlating of film and foam stability during the generation process. The only difference we observed between the single films stabilised by the different solutions is the stability of their respective black films. We thus suggest that the stability of black films during foam generation plays an important role which should be explored further in future work.

A study of generation and rupture of soap films

What are the lifetime and maximum length of a soap film pulled at a velocity V out of a bath of soapy solution? This is the question we explore in this article by performing systematic film rupture experiments. We show that the lifetime and maximal length of the films are fairly reproducible and controlled only by hydrodynamics. For surfactants with high surface elastic modulus, we argue that the rupture is triggered by the expansion of a thinning zone at the top of the film. The length ltz of this zone expands with time at a velocity equal to V/2, which can be obtained by a balance between gravity and viscous forces. The film lifetime is then found to decrease with the pulling velocity V, which implies that the surface tension gradient along the film increases with V. This surface tension gradient is found to be surprisingly small. Finally, the lifetime of films stabilised by solutions with small surface elastic modulus turns out to be much shorter than the ones for films with rigid interfaces.

Spontaneous transformation of lamellar structures from simple to more complex states

Spontaneous transformation of lamellar structures, such as multilamellar vesicles from micelles or unilamellar vesicles, is an important challenge in the field of amphiphile molecules, which may serve as models to understand biologically relevant bilayer membranes. Herein, we report a progressive self-assembly progress of N-tetradecyllactobionamide (C14G2) and tetraethylene glycol monododecyl ether (C12EO4) mixtures in aqueous solution. Increasing temperature or surfactant compositions causes spontaneous transformation from simple to high-level aggregates, i.e., from unilamellar vesicles, to coexisting multilamellar vesicles, terraced planar bilayers, and finally terraced planar bilayers. Deuterium nuclear magnetic resonance ((2)H NMR), freeze-fracture transmission electron microscopy (FF-TEM), and small-angle X-ray scattering (SAXS) measurements clearly demonstrate the spontaneously progressive self-assembly process. The interlamellar spacing (d) of the bilayers decreases from unilamellar vesicles to the terraced planar bilayers with an increase of the temperature or surfactant compositions. Lamellar samples consisting of terraced planar bilayers at higher temperature still show viscoelastic properties, being Bingham fluids, and both the viscoelasticity and yield stress increase with the composition and decrease with the temperature. The spontaneous transformation of the progressive self-assembly progress of C14G2 and C12EO4 aqueous mixtures is due to a balance of three driving forces, hydrophobic interactions, hydrogen bonding, and steric effects.

Dilational surface rheology studies of n-dodecyl-β-D-maltoside, hexaoxyethylene dodecyl ether, and their 1:1 mixture

It is time to review latest activities on the dilational surface rheology of the two nonionic surfactants n-dodecyl-β-D-maltoside (β-C12G2) and hexaoxyethylene dodecyl ether (C12E6) and their 1:1 mixture as a lot of different data generated with different techniques have been published in the last years. As the data are scattered throughout different papers and were generated with different techniques, we carried out an extensive study with one technique, which we will use as reference for the discussion of different data sets. We found that the results are in most of the cases in line with already published data as regards the general trends. However, a quantitative comparison reveals differences, which may result in different interpretations of the data. In the review at hand, we summarize, compare and discuss our latest and previously published data.

Colloid and Interfacial Chemistry at Stuttgart University

The research work carried out in our group can be referred to as “Colloid and Interfacial Chemistry”. We subdivide this rather broad research area into four main topics which are covered by the projects presented in this overview. The surfaces we study are surfactant-loaden water-air surfaces, the films are mainly free-standing thin foam films of less than 100 nm thickness, and the foams are 3D aqueous foams whose stability and drainage we investigate. As regards the topic “Complex Fluids” we study lyotropic liquid crystalline phases and microemulsions. In the past, we were able to establish two new tuning parameters for the formation and destruction of lyotropic liquid crystals, while current research focuses on the lyotropic mesomorphism of new surfactants and of surfactant mixtures. Apart from lyotropic liquid crystals microemulsions are a central theme in the group. Due to their unique properties and fascinating structure variety microemulsions offer a great potential as templates for the synthesis of new functional materials, which is a further research topic in our group. These studies involve the gelation of and the polymerisation in microemulsions preserving their nanostructure to create high surface area polymers. Currently, we also use microemulsions as tailor-made nano-compartmented reaction media. The studied reactions are either enzyme-catalysed conversions of substrates or the reduction of metal salts to synthesize mono- or bimetallic nanoparticles. In this context we focus on bicontinuous and water-in-oil droplet microemulsions. Last but not least we also synthesize new surfactant structures such as inositol-based surfactants and explore the properties.

Self-assembly, surface activity and structure of n-octyl-β-D-thioglucopyranoside in ethylene glycol-water mixtures

The effect of the addition of ethylene glycol (EG) on the interfacial adsorption and micellar properties of the alkylglucoside surfactant n-octyl-β-d-thioglucopyranoside (OTG) has been investigated. Critical micelle concentrations (cmc) upon EG addition were obtained by both surface tension measurements and the pyrene 1:3 ratio method. A systematic increase in the cmc induced by the presence of the co-solvent was observed. This behavior was attributed to a reduction in the cohesive energy of the mixed solvent with respect to pure water, which favors an increase in the solubility of the surfactant with EG content. Static light scattering measurements revealed a decrease in the mean aggregation number of the OTG micelles with EG addition. Moreover, dynamic light scattering data showed that the effect of the surfactant concentration on micellar size is also controlled by the content of the co-solvent in the system. Finally, the effect of EG addition on the microstructure of OTG micelles was investigated using the hydrophobic probe Coumarin 153 (C153). Time-resolved fluorescence anisotropy decay curves of the probe solubilized in micelles were analyzed using the two-step model. The results indicate a slight reduction of the average reorientation time of the probe molecule with increasing EG in the mixed solvent system, thereby suggesting a lesser compactness induced by the presence of the co-solvent.

Rotational diffusion of coumarin 153 in nanoscopic micellar environments of n-dodecyl-β-D-maltoside and n-dodecyl-hexaethylene-glycol mixtures

The microstructure of mixed micelles containing n-dodecyl-β-D-maltoside and n-dodecyl-hexaethylene-glycol, two nonionic surfactants belonging to the alkyl polyglucoside and polyoxyethyelene alkyl ether families, respectively, has been investigated. With the aim of understanding how the micellar composition affects the microenvironmental properties of micelles, we have examined the photophysics and dynamics of the neutral probe coumarin 153 in the binary mixtures of the surfactants across the entire composition range. We present data on the steady-state absorption and emission spectra of the probe, as well as fluorescence lifetimes and both steady-state and time-resolved fluorescence anisotropies. These data indicate that the participation of the ethoxylated surfactant in the mixed micelle induces an increasing hydration in the palisade layer of the micelle, which forces the probe to migrate toward the inner micellar region, where it senses a slightly less polar environment. The time-resolved fluorescence anisotropy data were analyzed on the basis of the two-step and wobbling-in-cone model. The average reorientation time of the probe molecule was found to decrease with the presence of the ethoxylated surfactant, in good agreement with steady-state fluorescence anisotropy data, suggesting a reduction of the microviscosity in the solubilization site of the probe. The behavior of all diffusion reorientation parameters was analyzed on the basis of two factors: the micellar hydration and the headgroup flexibility of both surfactants. It was concluded that the increasing participation of the ethoxylated surfactant induces a greater hydration in the micellar palisade layer, producing the formation of a less compact microenvironment where the probe experiences a faster rotational reorientation.

On how surfactant depletion during foam generation influences foam properties

Although it is known that foaming a surfactant solution results in a depletion of the surfactant in the bulk phase, this effect is often overlooked and has never been quantified. Therefore, the influence of surfactant depletion on foam properties using solutions of the two nonionic surfactants, n-dodecyl-β-D-maltoside (β-C(12)G(2)) and hexaethyleneglycol monododecyl ether (C(12)E(6)), were investigated. These investigations were conducted in two steps. First, different foam volumes were generated with the same surfactant solution at a concentration of c = 2 cmc. It was found that the higher the foam volume, the larger the surfactant depletion. Second, two different bulk concentrations (c = 2 and 1.33 cmc) were used for the generation of 50 and 110 mL of foam, respectively. For a foam volume of 50 mL, no differences were observed, whereas generating 110 mL led to different results. The surfactant loss in the bulk solution was measured via surface tension measurements and then compared to the results of purely geometric considerations that take into account the amount of interface created in the foam. Both results were in very good agreement, which means that surfactant depletion can be calculated in the way suggested here. Under conditions where depletion plays a role, our approach can also be used to estimate the bubble size of a foam of known volume by measuring the surfactant concentration in the bulk solution after foaming.