Influence of critical micelle concentration of choline-based long chain fatty acid soaps on their antibacterial activity against Methicillin resistant Staphylococcus aureus.

HYPOTHESIS: Antimicrobial resistance (AMR) is a pressing global health concern. ESKAPEE pathogens, such as Methicillin-resistant Staphylococcus aureus (MRSA) are notable of concern in healthcare settings due to their resistance to critical antibiotics. To combat AMR, the development of alternatives such as bacterial membrane-active agents is crucial. Fatty acids (FAs) have emerged as a sustainable, antibiotic-free solution with inherent antibacterial activity. However, long chain saturated fatty acids (LCFAs) sodium soaps exhibit poorly antibacterial properties in comparison to short chain FAs, believed to be linked to limited solubility in aqueous media. EXPERIMENTS: We employed choline as a chaotropic organic counter-ion to enhance the solubility of LCFAs and investigated their antibacterial effects against MRSA. The optimal medium conditions for micelle formation for LCFAs was first investigated. Then, we determined the critical micelle concentration (CMC), micellar morphology, and aggregation number through surface tension measurements and small angle neutron scattering experiments. Antimicrobial activity was assessed using minimum bactericidal concentration (MBC) assays and time-kill experiments. FINDINGS: We have identified conditions where LCFAs are effective against MRSA for the first time, providing valuable insights for developing new antibacterial agents to fight AMR. LCFAs need to be used above their Krafft temperatures and CMC to exhibit antibacterial efficacy.

Stimulus-Responsive Gas Marbles as an Amphibious Carrier for Gaseous Materials.

Gas marbles are a new family of particle-stabilized soft dispersed system with a soap bubble-like air-in-water-in-air structure. Herein, stimulus-responsive character is successfully introduced to a gas marble system for the first time using polymer particles carrying a poly(tertiary amine methacrylate) (pKa ≈7) steric stabilizer on their surfaces as a particulate stabilizer. The gas marbles exhibited long-term stability when transferred onto the planar surface of liquid water, provided that the solution pH of the subphase is basic and neutral. In contrast, the use of acidic solutions led to immediate disintegration of the gas marbles, resulting in release of the inner gas. The critical minimum solution pH required for long-term gas marble stability correlates closely with the known pKa value for the poly(tertiary amine methacrylate) stabilizer. It also demonstrates amphibious motions of the gas marbles.

12-hydroxystearic acid-mediated in-situ surfactant generation: A novel approach for organohydrogel emulsions.

HYPOTHESIS: Organohydrogel emulsions display unique rheological properties and contain hydrophilic and lipophilic domains highly desirable for the loading of active compounds. They find utility in various applications from food to pharmaceuticals and cosmetic products. The current systems have limited applications due to complex expensive formulation and/or processing difficulties in scale-up. To solve these issues, a simple emulsification process coupled with unique compounds are required. EXPERIMENTS: Here, we report an organohydrogel emulsion based only on a low concentration of 12-hydroxystearic acid acting as a gelling agent for both oil and water phases but also as a surfactant. The emulsification process is based on in-situ surfactant transfer. We characterize the emulsification process occurring at the nanoscale by using tensiometry experiments. The emulsion structure was determined by coupling Small Angle X-ray and neutron scattering, and confocal Raman microscopy. FINDINGS: We demonstrate that the stability and unique rheological properties of these emulsions come from the presence of self-assembled crystalline structures of 12-hydroxystearic acid in both liquid phases. The emulsion properties can be tuned by varying the emulsion composition over a wide range. These gelled emulsions are prepared using a low energy method offering easy scale-up at an industrial level.

Comparison of the Antibacterial Activity of Selected Deep Eutectic Solvents (DESs) and Deep Eutectic Solvents Comprising Organic Acids (OA-DESs) Toward Gram-Positive and Gram-Negative Species.

Deep eutectic solvents (DESs) have been intensively investigated in recent years for their antibacterial properties, with DESs that comprise organic acids (OA-DESs) showing promising antibacterial action. However a majority of the reports focused only on a limited number strains and techniques, which is not enough to determine the antibacterial potential of a substance. To bridge this gap, the antibacterial activity of classical DESs and OA-DESs is assessed on twelve Gram-negative and Gram-positive bacteria strains, with some of them exhibiting specific resistance toward antibiotics. The investigated formulations of OA-DESs comprise glycolic, malic, malonic, and oxalic acids as representatives of this group. Using a range of microbiological assays as well as physicochemical characterization methods, a major difference of the effectiveness between the two groups is demonstrated, with OA-DESs exhibiting, as expected, greater antibacterial effectiveness than classical DESs. Most interestingly, slight differences in the minimum inhibitory and bactericidal concentration values as well as time-kill kinetics profiles are observed between Gram-positive and Gram-negative strains. Transmission electron microscopy analysis reveals the effect of the treatment of the bacteria with the representatives of both groups of DESs, which allows us to better understand the possible mechanism-of-action of these novel materials.

Oleofoams: The impact of formulating air-in-oil systems from a lipid oxidation perspective.

Air-in-oil foams, or oleofoams, have a great potential for food applications as they can at least partially replace animal or hydrogenated fats, without compromising on textural properties. Yet, there are some challenges to tackle before they can largely be implemented for real-life applications. One of those is the lack of data regarding their oxidative stability. This is an important point to consider, as although using oils rich in polyunsaturated fatty acids (PUFAs) is highly desirable from a nutritional perspective, these fatty acids are particularly prone to oxidation, which leads to major degradations of food quality. This work thus aimed to investigate the oxidative stability of oleofoams prepared with omega-3 PUFA-rich vegetable oils (rapeseed or flaxseed oil) and various types of high melting point lipid-based oleogelators (stearic acid, glyceryl monostearate and stearyl alcohol) when incubated at room temperature. The physical structure and stability of the oleofoams was monitored by various techniques (visual observations, microscopy, DSC, NMR, SAXS and WAXS). Lipid oxidation was assessed by combined measurements of primary (conjugated diene hydroperoxides) and secondary (thiobarbituric acid reactive substances - TBARS) products. We found that the oxidative stability of oleofoams was higher compared to that of the corresponding bulk oil. This protective effect was also found when the oil was simply mixed with the oleogelator without incorporation of air bubbles (i.e., forming an oleogel), and was somewhat modulated depending on the type of oleogelator. These results suggest that oleogelators and the structural changes that they induce limit the cascaded propagation of lipid oxidation in oil-continuous matrices, which is promising in the perspective of future applications.

One step generation of single-core double emulsions from polymer-osmose-induced aqueous phase separation in polar oil droplets.

Water-in-oil-in-water emulsions (W/O/W) are aqueous droplet(s) embedded within oil droplets dispersed in a continuous water phase. They are attracting interest due to their possible applications from cosmetic to food science since both hydrosoluble and liposoluble cargos can be encapsulated within. They are generally prepared using a one-step or a two-step method, phase inversion and also via spontaneous emulsification. Here, we describe a general and simple one-step method based on hydrophilic polymers dispersed in polar oils to generate osmose-induced diffusion of water into oil droplets, forming polymer-rich aqueous droplets inside the oil droplets. Polyethylene glycol, but also other hydrophilic polymers (branched polyethylene imine or polyvinyl pyrrolidone) were successfully dispersed in 1-octanol or other polar oils (oleic acid or tributyrin) to produce an O/W emulsion that spontaneously transformed into a W1/O/W2 emulsion, with the inner aqueous droplet (W1) only containing the hydrophilic polymer initially dispersed in oil. By combining single drop experiments, with macroscopic viscosity measurements, we demonstrated that the double emulsion resulted of water diffusion, which amplitude could be adjusted by the polymer concentration. The production of high internal phase emulsions was also achieved, together with a pH-induced transition from multiple to single core double emulsion. We expect this new method for producing double emulsions to find applications in domains of microencapsulation and materials chemistry.

Liquid foams as sensors for the detection of biomarkers.

Bioassays are widely used in healthcare to detect and quantify biomarkers, such as molecules or enzymes, which are crucial in monitoring diseases and health conditions. In developed countries, healthcare professionals use specialized reagents and equipment's to perform these bioassays. However, in less-industrialized countries, the creation of low cost, fast, and technically simple bioassays is required. Herein, we propose a simple approach for detecting biochemical markers using host-guest complexes containing a surfactant. When the biochemical marker is present, the host-guest complex is disrupted, releasing the surfactant and producing foam. The read-out mechanism relies on the change of foam volume as function of biomarker concentration. This change is quantifiable by the naked eye and can be measured with a simple ruler. We claim that the use of foams as sensing tool is an attractive, inexpensive, fast, and easy to handle on-site detection method.

Saturated long chain fatty acids as possible natural alternative antibacterial agents: Opportunities and challenges.

The spread of new strains of antibiotic-resistant pathogenic microorganisms has led to the urgent need to discover and develop new antimicrobial systems. The antibacterial effects of fatty acids have been well-known and recognized since the first experiments of Robert Koch in 1881, and they are now used in diverse fields. Fatty acids can prevent the growth and directly kill bacteria by insertion into their membrane. For that, a sufficient amount of fatty acid molecules has to be solubilized in water to transfer from the aqueous phase to the cell membrane. Due to conflicting results in the literature and lack of standardization methods, it is very difficult to draw clear conclusions on the antibacterial effect of fatty acids. Most of the current studies link fatty acids' effectiveness against bacteria to their chemical structure, notably the alkyl chain length and the presence of double bonds in their chain. Furthermore, the solubility of fatty acids and their critical aggregation concentration is not only related to their structure, but also influenced by medium conditions (pH, temperature, ionic strength, etc.). There is a possibility that the antibacterial activity of saturated long chain fatty acids (LCFA) may be underestimated due to the lack of water solubility and the use of unsuitable methods to assess their antibacterial activity. Thus, enhancing the solubility of these long chain saturated fatty acids is the main goal before examining their antibacterial properties. To increase their water solubility and thereby improve their antibacterial efficacy, novel alternatives may be considered, including the use of organic positively charged counter-ions instead of the conventional sodium and potassium soaps, the formation of catanionic systems, the mixture with co-surfactants, and solubilization in emulsion systems. This review summarizes the latest findings on fatty acids as antibacterial agents, with a focus on long chain saturated fatty acids. Additionally, it highlights the different ways to improve their water solubility, which may be a crucial factor in increasing their antibacterial efficacy. We finish with a discussion on the challenges, strategies and opportunities for the formulation of LCFAs as antibacterial agents.

Ultrastable and Responsive Foams Based on 10-Hydroxystearic Acid Soap for Spore Decontamination.

Currently, there is renewed interest in using fatty acid soaps as surfactants. Hydroxylated fatty acids are specific fatty acids with a hydroxyl group in the alkyl chain, giving rise to chirality and specific surfactant properties. The most famous hydroxylated fatty acid is 12-hydroxystearic acid (12-HSA), which is widely used in industry and comes from castor oil. A very similar and new hydroxylated fatty acid, 10-hydroxystearic acid (10-HSA), can be easily obtained from oleic acid by using microorganisms. Here, we studied for the first time the self-assembly and foaming properties of R-10-HSA soap in an aqueous solution. A multiscale approach was used by combining microscopy techniques, small-angle neutron scattering, wide-angle X-ray scattering, rheology experiments, and surface tension measurements as a function of temperature. The behavior of R-10-HSA was systematically compared with that of 12-HSA soap. Although multilamellar micron-sized tubes were observed for both R-10-HSA and 12-HSA, the structure of the self-assemblies at the nanoscale was different, which is probably due to the fact that the 12-HSA solutions were racemic mixtures, while the 10-HSA solutions were obtained from a pure R enantiomer. We also demonstrated that stable foams based on R-10-HSA soap can be used for cleaning applications, by studying spore removal on model surfaces in static conditions via foam imbibition.

Decontamination of Spores on Model Stainless-Steel Surface by Using Foams Based on Alkyl Polyglucosides.

In the food industry, the surfaces of processing equipment are considered to be major factors in the risk of food contamination. The cleaning process of solid surfaces is essential, but it requires a significant amount of water and chemicals. Herein, we report the use of foam flows based on alkyl polyglucosides (APGs) to remove spores of Bacillus subtilis on stainless-steel surfaces as the model-contaminated surface. Sodium dodecyl sulfate (SDS) was also studied as an anionic surfactant. Foams were characterized during flows by measuring the foam stability and the bubble size. The efficiency of spores' removal was assessed by enumerations. We showed that foams based on APGs could remove efficiently the spores from the surfaces, but slightly less than foams based on SDS due to an effect of SDS itself on spores removal. The destabilization of the foams at the end of the process and the recovery of surfactant solutions were also evaluated by using filtration. Following a life cycle assessment (LCA) approach, we evaluated the impact of the foam flow on the global environmental footprint of the process. We showed significant environmental impact benefits with a reduction in water and energy consumption for foam cleaning. APGs are a good choice as surfactants as they decrease further the environmental impacts.

Photo- and Thermoresponsive Liquid Marbles Based on Fatty Acid as Phase Change Material Coated by Polypyrrole: From Design to Applications.

Responsive liquid marbles (LMs), which can change their shape, stability, and motion by the application of stimuli, attract a growing interest due to their wide range of applications. Our approach to design photo- and thermoresponsive LMs is based on the use of micrometer-sized fatty acid (FA) particles as phase change material covered with polypyrrole (PPy) overlayers with photothermal property. The core-shell particles were synthesized by aqueous chemical oxidative seeded dispersion polymerization. First, we investigated the effect of the alkyl chain length of FA on the resulting FA/PPy core-shell particles by characterizing their size and its distribution, shape, morphology, chemical composition, and photothermal behavior. Then LMs were fabricated by rolling water droplets on the dried FA/PPy particle powder bed and their light and temperature dual stimuli-responsive nature was studied as a function of the FA alkyl chain length. For all FAs studied, LMs disrupted in a domino manner by light irradiation as the first trigger: the temperature of the FA/PPy particles on the LM surface increased by light irradiation, followed by phase change of FA core of the particles from solid to liquid, resulting in disruption of the LM and release of the encapsulated water. The disruption time was closely correlated to the melting point of FA linked to the alkyl chain length and light irradiation power, and it could be controlled and tuned easily between quasi instantaneous and approximately 10 s. Finally, we showed potential applications of the LMs as a carrier for controlled delivery and release of substances and a sensor.

Photo/Thermo Dual Stimulus-Responsive Liquid Marbles Stabilized with Polypyrrole-Coated Stearic Acid Particles.

In this study, we report on the fabrication of photo/thermo dual stimulus-responsive liquid marbles (LMs) that can be disrupted by light irradiation and/or heating. To stabilize the LMs, we synthesized micrometer-sized stearic acid (SA) particles coated with overlayers of polypyrrole (PPy) by aqueous chemical oxidative seeded dispersion polymerization. The SA/PPy core-shell particles could adsorb at the air-water interface to stabilize LMs by rolling water droplets on the particle powder bed. The presence of SA, known as a phase-change material, which undergoes a transition from solid to liquid by heating, and PPy, which can transduce light to heat, gives rise to the photo and thermo dual stimulus-responsive characters of the LMs. The disruption of the LMs could be induced in a cascade manner: light irradiation on the LM induced a temperature increase, followed by melting of the SA component on the LM surface, leading to its disruption and release of the inner water. The disruption time is linked to the PPy loading and light irradiation power, and it can be tuned from quasi-instantaneous to a few tens of seconds. The melting of SA due to a light-induced phase change from the solid to liquid state is a new mechanism to trigger the disruption of LMs. We finally demonstrated two applications of the LMs as a light-responsive microreactor and a sensor.

Foamitizer: High ethanol content foams using fatty acid crystalline particles.

Aqueous foams are encountered in many commercial products used in our everyday lives and are widely studied. However, the formation and stabilization of foams using high alcohol content (>75%) solvents such as ethanol is still a scientific challenge. Herein, we report for the first-time foams based on high ethanol content showing long-term stability by using natural fatty acid crystals. The platelet-shape crystals are adsorbed at the air-water surface protecting the bubbles against coalescence. The melting of crystals triggers the foam destabilization leading to thermostimulable high ethanol content foams. These foams can be used as a new formulation strategy for alcohol-based hand sanitizers to better clean hands, protect the skin by the presence of fatty acids, and limit the transmission of virus and other pathogens.

Aqueous and Oil Foams Stabilized by Surfactant Crystals: New Concepts and Perspectives.

Surfactant crystals can stabilize liquid foams. The crystals are adsorbed at bubble surfaces, slowing down coarsening and coalescence. Excess crystals in the liquid channels between bubbles arrest drainage, leading to ultrastable foams. The melting of crystals upon raising the temperature allows thermoresponsive foams to be designed. In the case of oil foams, the stabilization by crystals received substantial renewed interest in the last 5 years due to their potential applications, particularly in the food industry. For aqueous foams, several reports exist on foams stabilized by crystals. However, these two kinds of liquid foams possess similarities in terms of stabilization mechanisms and the design of surfactant crystal systems. This field will certainly grow in the coming years, and it will contribute to the engineering of new soft materials not only for food but also for cosmetics, pharmaceuticals, and biomedical applications.

Surface science of cosmetic substrates, cleansing actives and formulations.

The development of shampoo and cleansing formulations in cosmetics is at a crossroads due to consumer demands for better performing, more natural products and also the strong commitment of cosmetic companies to improve the sustainability of cosmetic products. In order to go beyond traditional formulations, it is of great importance to clearly establish the science behind cleansing technologies and appreciate the specificity of cleansing biological surfaces such as hair and skin. In this review, we present recent advances in our knowledge of the physicochemical properties of the hair surface from both an experimental and a theoretical point of view. We discuss the opportunities and challenges that newer, sustainable formulations bring compared to petroleum-based ingredients. The inevitable evolution towards more bio-based, eco-friendly ingredients and sustainable formulations requires a complete rethink of many well-known physicochemical principles. The pivotal role of digital sciences and modelling in the understanding and conception of new ingredients and formulations is discussed. We describe recent numerical approaches that take into account the specificities of the hair surface in terms of structuration, different methods that study the adsorption of formulation ingredients and finally the success of new data-driven approaches. We conclude with practical examples on current formulation efforts incorporating bio-surfactants, controlling foaming and searching for new rheological properties.

The Effect of Vegetable Oil Composition on the Structural Properties of Oleogels Based on Behenyl Alcohol/Behenic Acid Oleogelator System.

Recently, we described that the weight ratio (R) between behenyl alcohol (BO) and behenic acid (BA) in sunflower oil effects the textural and structural properties of the oleogel system. One R (7:3) was found as optimal since it led to an enhancement of the oleogel properties for both the hardness and the stability in terms of oil-binding capacity. However, what remains unknown is the effect of other vegetable oils. Therefore, in this study, we aim to test a range of different vegetable oils that are widely used in the cosmetic industry. All the oleogels were prepared by heating together at 85°C the oil and the fatty components under magnetic stirring. After heating, the samples were allowed to cool down quiescently to room temperature without any stirring. The oil properties tested included viscosity, density, and surface tension. The oleogel properties (hardness, oil loss, and gel stability) and their structure as a function of R were characterized at different length scales by coupling optical microscopy, differential scanning calorimetry (DSC), Small-Angle X-ray Scattering (SAXS), and Wide-Angle X-ray Scattering (WAXS) experiments. The same crystal structure evolution determined by SAXS and WAXS as a function of R was observed whatever the oil. In the DSC profiles and optical microscopy pictures, no oil effect was detected. However, our results highlighted two different optimal ratios, giving rise to the best oleogels in terms of stability (oil loss) and hardness as a function of the oil. For sunflower, apricot, and rapeseed oils, R = 7:3 was the optimal ratio, whereas R = 8:2 was the optimal ratio for olive and camelina oil. These observations were correlated with the fatty acid chain length composition of the oil. The results obtained have practical applications for the cosmetic industry since it establishes formulation rules for oleogel systems. Oleogels are based on BO and BA components, which are raw materials widely used for hair and skin applications.Different oils have different fatty acid chain lengths composition and as a result, the ratio between BO and BA needs to be adjusted in order to obtain the best oleogel in terms of texture and stability, which can then be used also to produce oil foams.

Effect of the ratio between fatty alcohol and fatty acid on foaming properties of whipped oleogels.

Oil foams that are based on oleogels are stabilized by the presence of crystalline particles at the air bubble surface and in bulk. The size of crystalline particles is an important parameter in oil foam stabilization. The creation process (cooling and shear rate) can tune its properties. The aim of this study was to determine the effect of altering the weight ratio (R) between long chain fatty acids and fatty alcohols on the oil foam. Two optimal weight ratios R = 7:3 and R = 8:2, for which mixed crystals were present, produced the best foams in terms of overrun, foam firmness and foam stability. R not only affected the crystal size, but also the number of crystalline particles present in the oleogel. Mixed crystals help to produce and stabilize the foams. We highlighted that there is a link between the oleogel stability and hardness with their resulting oleofoam properties.

Adsorption of Fatty Acid Molecules on Amine-Functionalized Silica Nanoparticles: Surface Organization and Foam Stability.

The crucial roles of the ionization state and counterion presence on the phase behavior of fatty acid in aqueous solutions are well-established. However, the effects of counterions on the adsorption and morphological state of fatty acid on nanoparticle surfaces are largely unknown. This knowledge gap exists due to the high complexity of the interactions between nanoparticles, counterions, and fatty acid molecules in aqueous solution. In this study, we use adsorption isotherms, small angle neutron scattering, and all-atom molecular dynamic simulations to investigate the effect of addition of ethanolamine as a counterion on the adsorption and self-assembly of decanoic acid onto aminopropyl-modified silica nanoparticles. We show that the morphology of the fatty acid assemblies on silica nanoparticles changes from discrete surface patches to a continuous bilayer by increasing concentration of the counterion. This morphological behavior of fatty acid on the oppositely charged nanoparticle surface alters the interfacial activity of the fatty acid-nanoparticle complex and thus governs the stability of the foam formed by the mixture. Our study provides new insights into the structure-property relationship of fatty acid-nanoparticle complexes and outlines a framework to program the stability of foams formed by mixtures of nanoparticles and amphiphiles.

Effect of the ratio between behenyl alcohol and behenic acid on the oleogel properties.

HYPOTHESIS: In oleogel food systems (based on the mixture between stearic acid and stearyl alcohol) the strong effect of the weight ratio (R) between these two components on the textural and structural properties is well described. The effect of R for other fatty acids and fatty alcohols is less explored. Moreover, they do not show an enhancement of the oleogel properties for specific R. The effect of R on the oleogel properties, for a mixture of fatty acid and fatty alcohol with longer alkyl chains (behenyl alcohol and behenic acid) in sunflower and soybean oils, which are raw materials widely used in cosmetic and pharmaceutical industries, was investigated. EXPERIMENTS: We characterized the oleogel properties as a function of R in terms of structuring potential: hardness, oil loss and gel stability. This information was correlated with microstructural data obtained at different length scales by coupling optical microscopy, DSC, SFC, SAXS and WAXS experiments. FINDINGS: Our results highlight that R tunes the oleogel properties in a comparable manner to previous results obtained for stearic acid and stearyl alcohol-based oleogels. Two specific R (8:2 and 7:3) close to the 3:1 molecular ratio gave oleogels with both the highest hardness and stability. The morphology and size of the mixed crystals obtained for these R cannot solely explain why they are stronger gels with low oil loss in comparison to the other R. The almost complete crystallization for these two R is one of the key parameters controlling the oleogel properties. As described in the literature, we also suggest that the differences in oleogel properties come from the spatial distribution of the crystalline mass. In this study, we confirm that the effect of the 3:1 molecular ratio in mixed surfactant systems described more than 50 years ago for foams, emulsions and Langmuir monolayers occurs also on the crystallization of mixed fatty alcohol and fatty acid in oils leading to better oleogels properties.

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.