Surfactant Packing Research Articles

SURFACE PROPERTIES AND MICELLIZATION OF SODIUM ALKYLSULPHATES IN THE PRESENCE OF LOW-MOLECULAR ALCOHOLS

The process of micellization of sodium alkyl sulfates (SAS : sodium decyl- (SDS) and dodecylsulfate (SDDS)) of formation in aqueous solution without or with alcohols including propanol-1, propanol-2 and butanol-1 and their adsorption behavior at air-liquid interface were investigated with both the tensiometry and the conductometry at 298 K. The effect of chain length of alcohol on micellar and interfacial properties were discussed. Some parameters including surface excess concentration (Γmax), minimum area per surfactant molecule (Smin) at air-liquid interface, degree of ionization of micelle, and standard free energy of adsorption and micellization, etc. are estimated. It was experimentally established that the introduction of 0.5-1.0 mol/dm3 of alcohols into surfactant solutions contributes to the process of micellar formation of SAS and increases the stability of the micellar phase, as evidenced by the observed synergistic effect on the critical micelle concentration (CMC). The addition of different alcohols changes the values of Γmax, Smin, CMC of SDS and SDDS and induces an increase in degree of ionization (β) of micelle relative to that in pure water. When alcohols are introduced into SDS and SDDS solutions, the degree of ionization of micelles at a constant length of the alkyl radical of surfactant moderately increases with increasing alcohol content in the solution and ranges from 0.50-0.73 and 0.38-0.75, respectively. Also, the effect of alcohol was discussed. The obtained thermodynamic parameters were used to confirm these behaviors of interfacial adsorption or micellization. An increase in chain length of alcohol promotes the micellization process. The CMC values decrease with increasing length of the alkyl radical of alcohols. Propanol-1, compared to propanol-2, somewhat reduces the value of SDS and SDDS CMC to a greater extent. Standard free energy of micellization ∆G0mic, also confirms the micellization behavior. Standard free energy of adsorption (∆G0адс) indicates that an increase in the chain length of alcohol in aqueous solution is favorable to the adsorption of SDS and SDDS at air-liquid interface. A comparison of the values of the standard free energy of adsorption and micelle formation in sodium alkyl sulfates – alcohols – water systems showed that adsorption is the most thermodynamically advantageous process in the studies systems, and the packing of surfactant “molecules” in micelles is less dense, compared to packing in a mixed adsorption layer.

Synergisms between Surfactants, Polymers, and Alcohols to Improve the Foamability of Mixed Systems

In order to produce detergents with improved performance and good market acceptability, it is crucial to develop formulations with improved foamability and cleaning performance. The use of a delicate balance of surfactants and additives is an appealing strategy to obtain good results and enables a reduction in the amount of chemicals used in formulations. Mixtures of hydrophobically modified linear polymers and surfactants, as well as balanced mixtures with co-surfactants and/or hydrotropes, are the most effective parameters to control foamability and foam stability. In the present study, the effect of the addition of hydrophobically modified linear polymers, nonionic co-surfactants and hydrotropes, and their mixtures to anionic and zwitterionic surfactant aqueous solutions was evaluated. It was found that the presence of the hydrophobically modified polymer (HM-P) prevented the bubbles from bursting, resulting in better stability of the foam formed using zwitterionic surfactant solutions. Also, the surfactant packing was inferred to be relevant to obtaining foamability. Mixtures of surfactants, in the presence of a co-surfactant or hydrotrope led, tendentially, to an increase in the critical packing parameter (CPP), resulting in higher foam volumes and lower surface tension for most of the studied systems. Additionally, it was observed that the good cleaning efficiency of the developed surfactant formulations obtained a higher level of fat solubilization compared to a widely used brand of commercial dishwashing detergent.

Lessons Learned in Interfacial Tension Prediction Using a Mixture of Sulfonate- and Ethoxylate-based Surfactants in a Waxy Oil-brine System

The chemical-enhanced oil recovery (CEOR) method is applied to change reservoir rock or fluid characteristics by injecting alkaline, surfactant, and polymer or a combination of two or three of the compounds. Surfactant flooding improves oil recovery by reducing the interfacial tension between oil and water. Selecting reservoir surfactants, especially microemulsions, requires careful screening. This study predicted waxy oil system interfacial tension using surfactant mixtures at below- and above-optimum salinity. To predict the interfacial tension, microemulsion types, HLB, ideal salinity, and HLD were used. The study predicted oil-surfactant-water interfacial tension using SAE, FEO, and their mixtures. We improved the Huh equation by adding a fitting parameter, β, to accommodate the transition from type III to type II microemulsions as salinity increases. With increasing salinity, anionic surfactant’s hydrophilic-hydrophobic interactions change, affecting the values and surfactant layer thickness. This study improved hydrophilic-lipophilic deviation (HLDN) by establishing a fixed interval for nonionic surfactants. Van der Waals attraction, values and interface surfactant layer thickness are connected, reflecting the fact that lower values reduce interfacial tension better. This study also found that surfactant packing at the oil-water interface increases the order of the oil-solution ratio and the microemulsion values with polarity.

Hidden Beneath the Layers: Extending the Core/Shell Model of Reverse Micelles.

Reverse micelles (RMs) provide a unique and highly tunable model system to study water in confined environments. The complex properties of water within RMs arise from the disruption of extended hydrogen bond (H-bond) networks that mediate local and long-range dynamics in bulk aqueous systems. Modulating the water pool size influences its H-bond dynamics, with smaller RMs increasingly restricting the H-bond network rearrangements leading to slower dynamics; however, within small confined systems, the dynamics of the surfactants also influence the water dynamics. Using ultrafast two-dimensional infrared spectroscopy, we investigate the effects of RM size on the surfactant headgroup rotamer populations and picosecond interfacial H-bond dynamics of aerosol-OT surfactants. We find that the increased water penetration accelerates H-bond dynamics, with larger RMs showing faster dynamics. These results imply that the changes in the RM structure alter the physical structure of the RM interface and thus alter the solvation dynamics. The findings in this study can be used for developing models for structure-specific solvation dynamics that account for the surfactant packing and hydration at the interface.

Effect of Glycosylation on Self-Assembly of Lipid A Lipopolysaccharides in Aqueous Solutions.

Lipopolysaccharides (LPSs) based on lipid A produced by bacteria are of interest due to their bioactivity in stimulating immune responses, as are simpler synthetic components or analogues. Here, the self-assembly in water of two monodisperse lipid A derivatives based on simplified bacterial LPS structures is examined and compared to that of a native Escherichia coli LPS using small-angle X-ray scattering and cryogenic transmission electron microscopy. The critical aggregation concentration is obtained from fluorescence probe experiments, and conformation is probed using circular dichroism spectroscopy. The E. coli LPS is found to form wormlike micelles, whereas the synthetic analogues bearing six lipid chains and with four or two saccharide head groups (Kdo2-lipid A and monophosphoryl lipid A) self-assemble into nanosheets or vesicles, respectively. These observations are rationalized by considering the surfactant packing parameter.

Dilution and packing of anionic liquid surfactant in presence of divalent and trivalent counter-ions

Alkyl ether carboxylates are part of a new class of ionic liquids based on the Concept of Melting Point Lowering due to Ethoxylation (COMPLET) and possess unique aggregation behaviour due to the simultaneous anionic and non-ionic nature of the surfactant anion. Previous publications on H[C8E8c] (Polyoxyethylene(8) octyl ether carboxylic acid) and its monovalent salts have shown a variety of phases in aqueous systems with a transition from small spherical micelles at high water contents towards a phase of interdigitated head groups at low water contents. In the present paper, we examine the shorter homologue H[C8E5c] (Polyoxyethylene(5) octyl ether carboxylic acid) and its di- and trivalent transition and rare earth metal salts using X-ray scattering methods in the small- and wide-angle range (SWAXS). These measurements confirm the presence of spherical micelles in diluted aqueous systems which aggregate and whose head groups interdigitate at higher concentrations of the isotropic, non-birefringent ionic liquid. Swelling experiments yield two-dimensional swelling of the microstructure, and combined with the scattering pattern, the data infer prolate spheroidal direct (o/w) micelles with interdigitated head groups, which align to form short linear chains. These chains are offset to form a localised hexagonal close-like packing of the micelles. Most interesting, within this packing, the metal cations are confined into channels between the micelles in linear chains with ion-ion distances of 0.4–1.1 nm. In particular, in the water-free ionic liquid, this confinement may be responsible for some of the unique properties of these systems. It is suggested that the confinement of the cations leads to strong ion-ion aggregation that could be at the origin of the very peculiar magnetic properties of these simple one-component liquids, which contrasts with classical ferrofluids, in which magnetic nanoparticles are dispersed in another liquid.

Insight into the role of anions in the formation of mesoporous silicates

Cationic surfactants can form micellar structures in solution and the condensation of silicate around these micellar structures results in the inorganic/organic mesostructured composites. Anions other than silicates in the solution were believed to be located closely to the headgroup of the cationic surfactant micelle and thus affected the effective headgroup area, a component of the surfactant packing parameter g (g = V/a0l, where V is the chain volume, a0 is the effective headgroup area, and l is the chain length). In the presence of H2SO4, HCl, HBr, and HNO3, the surfactant cetyltriethylammonium bromide (CTEABr) directed the well-ordered mesophases of three-dimensional (3D) hexagonal P63/mmc, cubic Pm3‾n, 2D hexagonal p6mm, and cubic Ia3‾d, respectively. The diversity of the mesophases from CTEABr was attributed to the anion tuning of the effective headgroup area of CTEA+ in surfactant packing parameter g by closely located anions of SO42−, Cl−, Br−, and NO3− with various radii and hydratability. In this work, we discovered that part of Cl−, Br−, and NO3− can penetrate into the micellar system of CTEA+, which may increase the chain volume V of CTEA+ to some degree. The distance of anions to the center of the micelle, determined by the liquid 1H NMR analyses, was in the order of SO42− > Cl− > Br− > NO3−, which is consistent with the Hofmeister series. The deeper penetration of anions into the micelle may lead to the larger chain volume V and surfactant packing parameter g. Such insight into the location of anions in micellar system provided us a new picture on the formation mechanism of mesoporous silica and other mesostructured materials in solution.

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

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

Effect of salt on the adsorption of ionic surfactants at the air-water interface

• NaCl has significant effect on the equilibrium and dynamic surface tension of ionic surfactant solutions. • The SRT model predicts the adsorption dynamics more accurately than the DKC model. • Salt bridging leads to the compact packing of ionic surfactant at the air-water interface. The combined effect of ionic surfactants and inorganic salts on the interfacial properties is of great importance to various industrial processes. In this work, the effect of ionic surfactants (SDS and CTAB), in the presence of inorganic salt (NaCl), on the surface tension is systematically investigated using experiments, theoretical models, and all-atom molecular dynamics simulations. The equilibrium and dynamic surface tension of surfactant-salt-water systems in a salinity range of 0.01–0.1 M are measured using the pendant drop method. The theoretical models viz., statistical rate theory and diffusion kinetic controlled, suggest that adsorption drives the migration of surfactant molecules to the interface. Finally, the molecular dynamics simulations illustrate increased surfactant packing at the air-liquid interface in the presence of salt due to counterions bridging between the surfactant molecules, which results in a lower value of surface tension for both the surfactants.

Langmuir-Blodgett Deposition of Cellulose Nanocrystal Surfactants into Ordered Monolayers.

The cellulose nanocrystals (CNCs) are shown to interact with amine-functionalized polyhedral oligomeric silsesquioxane (POSS-NH2) strongly at the water/oil interface, forming the CNC-POSS assemblies, that is, CNC surfactants that decrease the interfacial tension of the water/chloroform greatly. When bringing the CNC aqueous solution and POSS chloroform solution into a Langmuir trough, they form a monolayer of the CNC surfactants. Upon applying a continuous compression, a distinct transition appears in the surface pressure-area curves, and during this transition, the packing of the CNC surfactants in the produced monolayers transits from network-like patterns to ordered alignment.

The advantages of nanoparticle surfactants over Janus nanoparticles on structuring liquids.

The nanoparticle (NP) surfactants generated in situ by binding NPs and polymers can assemble into an elastic NP monolayer at the interface of two immiscible liquids, structuring the liquids. Janus NPs can be more strongly bound to the interface than the NP surfactants, but they are unable to structure liquids into complex shapes due to the difficulty of assembling the jamming arrays. By molecular dynamics simulations, we give an insight into the better performance of NP surfactants than Janus NPs on dynamically structuring liquids. The high energy binding of Janus NPs to the interface will drive the Janus NPs to assemble into micelles in binary liquids. The micelles are stabilized in one liquid by encapsulating a little of the other liquid, hindering interfacial adsorption when the interface is marginally extended upon liquid deformation. In contrast, the in situ formed NP surfactants can rapidly fill the enlarged interfacial area to arrest the consecutive shape changes of the liquids. Moreover, NP surfactants can be designed with an appropriate coverage ratio (≤50%) of NP surface bearing host-guest sites to avoid dissolution and impart a desirable mechanical elasticity to their assembly.

Concentrated phases of an ionic surfactant (Aerosol OT) in polar solvents

AbstractWe have studied concentrated phases of Aerosol OT (AOT) in polar (water, ethylene glycol, formamide, N,N‐dimethylformamide) and apolar (isooctane) solvents. We investigated two surfactant volume fractions (φ = 0.2 and φ = 0.6) with small angle x‐ray scattering (SAXS), rheology and electrical conductivity experiments. AOT self‐assembles differently depending on solvent type and concentration. SAXS experiments show that the AOT/water system displays a lamellar phase. In the other cases, only formamide displays a lamellar phase for φ = 0.6. The other solvents (and formamide at φ = 0.2) promote the self‐assembly of AOT in other microstructures. The SAXS spectra display correlation peaks consistent with a disordered array of cylindrical aggregates. The microstructure of the AOT/isooctane system at φ = 0.2 is that of an arrangement of spherical aggregates. The results are explained in terms of surfactant packing models and solvent properties. For instance, the ability of ethylene glycol to form hydrogen bonds with AOT promotes the formation of cylindrical aggregates by increasing the area per surfactant polar head. As N,N‐dimethylformamide is slightly miscible in hydrocarbons it increases the volume of the surfactant tail promoting reverse structures.

Tensiometry and FTIR study of the synergy in mixed SDS:DDAO surfactant solutions at varying pH

The interactions between a model anionic and amphoteric surfactant pair in aqueous solution are examined as a function of composition, at floating and fixed pH, employing a combination of tensiometry, regular solution theory analysis, and FTIR spectroscopy. An extensive series of pure and mixed ratios of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO), ranging from 0.0016 to 100 mM, yielding 77 data points below and above the critical micelle concentrations (CMC), is investigated. Compared to either pure surfactant solutions, the CMC of mixed SDS:DDAO solutions is found to decrease by up to 20-fold, and the surface tension (γ) at CMC down to ≃ 23 mN/m. At all concentrations, the most prominent effects are observed at equimolar SDS:DDAO ratios. Further, the pH of mixed micellar solutions is found to increase with respect to the pure surfactant solutions (from ≃ 7 up to ≃ 9.5), which is attributed to the enhanced protonation of DDAO in the presence of SDS, and supported by FTIR frequency shifts of isolated O‒H stretching vibrations. Vibrational responses from CH2 stretching of the methylene tails, and the S‒O stretching modes for the sulfate headgroups indicate strong lateral interaction and enhanced packing between SDS and DDAO. From regular solution theory analysis of tensiometry data, the molecular interaction parameters are found to have a larger magnitude (i.e., more negative) at the interface as compared to within micelles. At fixed solution pH, a decrease from pH 9.5 to 7.5 results in minimal changes in both interfacial and micellar parameters, indicating the intrinsic origin of these pairwise interactions. Overall, our findings demonstrate a pronounced synergistic interaction between SDS and DDAO, arising from diminished electrostatic and steric repulsions in, respectively, SDS and DDAO, accompanied by enhanced lateral surfactant packing.

Ions Slow Water Dynamics at Nonionic Surfactant Interfaces.

Nonionic surfactant interfaces are often considered to be unaffected by aqueous ions. However, ions often localize to interfaces and, as such, can interact with surfactants either via direct contact or by affecting interfacial hydrogen bond structures in water. Characterizing these effects is essential to understanding how ions affect interfacial properties at the oil-water interface in the presence of nonionic surfactants. Here, we use two-dimensional infrared (2D IR) spectroscopy in combination with atomistic molecular dynamics (MD) simulations to study the effects of high-concentration Na+ and Ca2+ ions on interfacial hydrogen bond dynamics in heterogeneous sorbitan stearate reverse micelles. Experiments show only minor changes in interfacial hydrogen bond populations when salts are added but those interfacial water network dynamics are slowed by nearly 300%. Molecular dynamics simulations show the slowdown results from an increased disorder in surfactant headgroup orientation and packing density, which stabilizes hydrogen bonding interactions between surfactants and interfacial water.

Thermodynamics and Kinetics of Pretilachlor Adsorption on Organobentonites for Controlled Release.

Thermodynamics and kinetics of pretilachlor adsorption on organobentonites modified with hexadecyltrimethyl ammonium chloride were investigated to reveal the structural effects of organobentonites on the interaction with pretilachlor and the diffusion of the herbicide and were related to the controlled release from organobentonites. The adsorption of pretilachlor was entropically driven by hydrophobic interaction. The entropy change dropped with increasing surfactant loading from 0.4 to 1.50 times the cation exchange capacity (CEC) of the bentonite used, corresponding to a decrease in the degree of freedom of pretilachlor molecules due to the enhanced order of surfactant in the interlayer. The kinetics of pretilachlor adsorption was well fitted to the pseudo-second-order model and related to the structural features of organobentonites. The enhanced packing density of the surfactant in the interlayer generally resulted in a reduction of the rate constant of the pretilachlor adsorption onto organobentonites. However, the stepwise increase in the basal spacing due to the surfactant arrangement transition, from lateral-monolayer to lateral-bilayer at a loading level of more than 0.8 × CEC, benefited the diffusion of pretilachlor and diminished the influence of the increase in surfactant packing density. The release of pretilachlor from organobentonites was predominated by Fickian diffusion, which could be understood from the adsorption thermodynamics and kinetics. The time taken for the release of 50% of active ingredient was 16–23 times that for the control formulation and exhibited a linear increase with the relative value of the equilibrium constant to the rate constant of pretilachlor adsorption.

Effect of Polar Solvents on Surfactant Membranes

We have studied how a surfactant (Aerosol OT or AOT) self-assembles in different polar (Water, Ethylene glycol, Formamide, N, N-Dimethylformamide) and non-polar (Isooctane) solvents. AOT is a widely used surfactant in biological and industrial applications. The lamellar phase of the AOT/water system is often used as a model of lipidic membrane. We investigated two surfactant volume fractions (0.2 and 0.6) with visual inspection, SAXS, rheology and electrical conductivity experiments. The results indicate that AOT self-assembles differently depending on solvent type and concentration. SAXS experiments show that the AOT/water systems display lamellar phases. In the other cases, only formamide displays a lamellar phase for φ = 0.6. The other solvents (and formamide at φ= 0.2) promote the self-assembly of AOT in other microstructures. In these cases, the SAXS spectra display correlation peaks consistent with a disordered array of cylindrical aggregates. The visual inspection, rheology and electrical conductivity results are consistent with the deduced self-assembled structures. We explain most of our results in terms of surfactant packing models and solvent properties.

Micellar Fd3m cubosomes from monoolein – long chain unsaturated fatty acid mixtures: Stability on temperature and pH response

HypothesisControl of the nanostructure of self-assembled systems may be achieved through manipulation of surfactant molecular packing and interfacial curvature. In order to phase engineer the inverse micellar cubosomes in some monoolein-fatty acid systems, lipids with wedge shaped molecular geometry were incorporated to promote the formation of this phase, that is of interest as potential sustained released nanocarriers. ExperimentsLiquid crystalline nanoparticle dispersions of monoolein with some cis unsaturated fatty acids were prepared and their partial temperature-composition phase diagrams and structure were established using high throughput Small Angle X-ray Scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). The pH responsiveness of these systems was evaluated in the presence of phosphate buffered saline (PBS). FindingsThe partial temperature-composition phase diagrams of five nanoparticle formulations containing monoolein and unsaturated fatty acids were established and identified the presence of micellar cubosomes in each of these systems. The results indicate that temperature, fatty acid concentration and structure, as well as pH all directly impact the formation and stability of this phase. Low energy inverse micellar cubic to emulsion phase transformations were identified in the monoolein with oleic acid and vaccenic acid systems at physiological temperatures that may be advantageous for staged therapeutic release strategies in nanomedicine.

Catanionic and chain-packing effects on surfactant self-assembly in the ionic liquid ethylammonium nitrate

HypothesisMicelle formation, particularly the formation of compact, globular micelles in the high ionic strength environment of an ionic liquid, seems to be at odds with the principle of opposing forces and the concept of the surfactant packing parameter. Here we examine how interactions between polar head-groups, with solvent ions, and chain packing affect self-assembly structure in ionic liquids. ExperimentsThe self-assembly of pure cationic and anionic surfactants, their catanionic mixtures, and the effect of the cosurfactant tetradecanol in the ionic liquid ethylammonium nitrate (EAN) is investigated by small-angle neutron scattering (SANS). FindingsThe critical micelle concentrations of cationic surfactants are much higher and micelles much smaller in EAN than in water, whereas anionic micelles are less dramatically affected. Catanionic surfactant mixtures form small micelles in EAN at all compositions, unlike their counterparts in water which form vesicle dispersions or precipitates near 1:1 mol ratios. This contrasts with the behavior of cationic surfactants when mixed with double-chained analogs or alkanol cosurfactants, which induce micelle growth and shape transitions to rod-like and bilayer aggregates through chain packing effects similar to aqueous systems. This enables the aggregate morphology in the ionic liquid to be controlled through the composition of the surfactant mixtures.

Molecular packing of surface active ionic liquids in a deep eutectic solvent: a small angle X-ray scattering (SAXS) study.

During the past decade, deep eutectic solvents (DESs) have shown promising application in the self-assembly of surfactants. Various aggregates such as micelles, vesicles, lyotropic liquid crystals, microemulsions and gels have been reported. In this research, the phase behaviours of imidazolium surface active ionic liquids (SAILs) CnmimBr (n = 12, 14, 16) were investigated in ChG. With the help of small angle X-ray scattering (SAXS), the types and structure parameters of aggregates were determined. The molecular packing of SAILs was influenced by the solvophobic chain length, surfactant concentration, temperature and solvent, accounting for their different aggregation behaviours. This study would give a good description of the molecular packing of surfactants in DESs.

Controlling Foam Stability with the Ratio of Myristic Acid to Choline Hydroxide.

The interfacial and foam properties of a model system based on the mixture between myristic acid and choline hydroxide have been investigated as a function of the molar ratio ( R) between these two components and temperature. The aim of this study was to obtain insight on the links between the self-assemblies in bulk and in the foam liquid channels, the surfactant packing at the interface, and the resulting foam properties and stability. A multiscale approach was used combining small angle neutron scattering, specular neutron reflectivity, surface tension measurements, and photography. We highlighted three regimes of foam stability in this system by modifying R: high foam stability for R < 1, intermediate at R ∼ 1, and low for R > 1. The different regimes come from the pH variations in bulk linked to R. The pH plays a crucial role at the molecular scale by setting the ionization state of the myristic acid molecules adsorbed at the gas-liquid interface, which in turn controls both the properties of the monolayer and the stability of the films separating the bubbles. The main requirement to obtain stable foams is to set the pH close to the p Ka in order to have a mixture of protonated and ionized molecules giving rise to intermolecular hydrogen bonds. As a result, a dense monolayer is formed at the interface with a low surface tension. R also modifies the structure of self-assembly in bulk and therefore within the foam, but such a morphological change has only a minor effect on the foam stability. This study confirms that foam stability in surfactant systems having a carboxylic acid as polar headgroup is mainly linked to the ionization state of the molecules at the interface.