Titration of Fatty Acids Solubilized in Cationic and Anionic Micelles. Calorimetry and Thermodynamic Modeling

Water-mediated catalyst-free synthesis of lysine-based ampholytic amphiphiles for multipurpose applications: Characterization and pH-responsive emulsifying properties

A novel series of lysine-based ampholytic amphiphiles, with alkylsuccinic anhydrides of varying chain lengths as hydrophobic acylating agents, were synthesized in medium to high yield (50.23-90.15%) based on a facile, catalyst-free method in water medium; and structurally confirmed by mass spectrometry (MS), Fourier transform infra-red (FTIR) spectroscopy, and ¹H/¹³C nuclear magnetic resonances (NMR) analysis. The resulting compounds were subjected to pH-dependent amphiphilic property, ferrous ion chelating, DPPH antioxidant capacity, and cytotoxicity analyses. Results showed that CMC values decrease, γ value increase, and emulsion stability increase with the increase of medium pH, suggesting that the surface activity of synthetic compounds at air/water and oil/water interface under neutral and alkaline conditions was remarkably higher than that under acidic condition. Lauryl O-acylated malic lysine (compound 4b) presented excellent foaming ability close to commercial detergent sodium dodecyl sulphate; dodecyl succinic lysine (compound 4a) afforded highly stable o/w nanoemulsion. Moreover, compound 4b displayed comparable ferrous ion chelating property to lysine and 2,2-diphenyl-1-picrylhydrazyl (DPPH) antioxidative capacity similar to a commercial food ingredient, diacetyl tartaric acid esters of mono- and di-glycerides (DATEM), indicating its multi-faceted functionalities. A cytotoxicity study of compounds 3b &4b showed that they were non-toxic. Thus, these novel ampholytic amphiphiles may find multi-purpose applications in food, detergent, pharmaceutical, and cosmetic industry.

Temperature-responsive self-assembled nanostructures from lysine-based surfactants with high chain length asymmetry: from tubules and helical ribbons to micelles and vesicles

Stimuli-sensitive self-assembled nanostructures are of great relevance for the templating of nanomaterials and the design of efficient systems for the controlled delivery of molecules. Amino acid-based surfactants often display such fascinating self-assembly due to a combination of molecular features such as critical packing parameter, chirality and H-bonding interactions. Herein, we focus on a family of newly synthesized double-chained alkylcarboxylates derived from l-lysine, and designated by 8Lysn, mLys8, with n, m = 12, 14 and 16, and 12Lys16 and 16Lys12, where the numbers represent the number of C atoms in each hydrocarbon chain. The effects of the chain length asymmetry and structural isomerism of the surfactants on their interfacial properties, thermal behavior and self-assembly in water were investigated by a comprehensive toolbox, including surface tension, DSC, imaging (light microscopy, SEM, TEM and AFM) and SAXS. All the surfactants below their Krafft temperature self-organize into tubular structures of various morphologies (flat structures, twisted and coiled ribbons and hollow tubes), forming hydrogels at low surfactant concentration. Upon the solubilization phase transition, micelles or vesicles are formed depending on the surfactant structure, and the tubule-micelle or tubule-vesicle transition is thermoreversible. A molecular-level rationalization of the observed self-assembly and phase transition features is put forth.

A renaissance of soaps? - How to make clear and stable solutions at neutral pH and room temperature

Soaps are the oldest and perhaps most natural surfactants. However, they lost much of their importance since "technical surfactants", usually based on sulfates or sulfonates, have been developed over the last fifty years. Indeed, soaps are pH- and salt-sensitive and they are irritant, especially to the eyes. In food emulsions, although authorized, they have a bad taste, and long-chain saturated soaps have a high Krafft temperature. We believe that most or perhaps all of these problems can be solved with modern formulation approaches. We start this paper with a short overview of our present knowledge of soaps and soap formulations. Then we focus on the problem of the lacking soap solubility at neutral pH values. For example, it is well known that with the food emulsifier sodium oleate (NaOl), clear and stable aqueous solutions can only be obtained at pH values higher than 10. A decrease in the pH value leads to turbid and unstable solutions. This effect is not compatible with the formulation of aqueous stable and drinkable formulations with neutral or even acidic pH values. However, the pH value/phase behavior of aqueous soap solutions can be altered by the addition of other surfactants. Such a surfactant can be Rebaudioside A (RebA), a steviol glycoside from the plant Stevia rebaudiana which is used as a natural food sweetener. In a recent paper, we showed the influence of RebA on the apKa value of sodium oleate in a beverage microemulsion and on its clearing temperature. In the present paper, we report on the effect of the edible bio-surfactant RebA, on the macroscopic and microscopic phase behavior of simple aqueous sodium oleate solutions at varying pH values. The macroscopic phase behavior is investigated by visual observation and turbidity measurements. The microscopic phase behavior is analyzed by acid-base titration curves, phase-contrast and electron microscopy. It turned out that even at neutral pH, aqueous NaOl/RebA solutions can be completely clear and stable for more than 50days at room temperature. This is for the first time that a long chain soap could be really solubilized in water at neutral pH at room temperature. At last, these findings were applied to prepare stable, highly translucent and drinkable aqueous solutions of omega-3-fatty acids at a pH value of 7.5.

Solubilization of octane in cationic surfactant-anionic polymer complexes: effect of polymer concentration and temperature

Polymers may alter the ability of oppositely charged surfactant micelles to solubilize hydrophobic molecules depending on surfactant-polymer interactions. This study was conducted to investigate the effects of polymer concentration and temperature on the solubilization thermodynamics of an octane oil-in-water emulsion in mixtures of an anionic polymer (carboxymethyl cellulose) and cationic cetyltrimethylammonium bromide (CTAB) surfactant micelles using isothermal titration calorimetry (ITC). Results showed that the CTAB binding capacity of carboxymethyl cellulose increased with increasing temperature from 301 to 323 K, and correspondingly the thermodynamic behavior of octane solubilization in CTAB micelles, either in the absence or presence of polymer, was found to depend on temperature. The addition of carboxymethyl cellulose caused the solubilization in CTAB micelles to be less endothermic, and increased the solubilization capacity. Based on the phase separation model, the solubilization was suggested to be mainly driven by enthalpy gains. Results suggest that increasing concentrations of the anionic polymer gave rise to a larger Gibbs energy decrease and a larger unfavorable entropy increase for octane solubilization in cationic surfactant micelles.

pH-responsive micelles based on caprylic acid

Free fatty acids play a vital role as fuel for cells and in lipid metabolism. During lipid digestion in the gastrointestinal tract, triglycerides are hydrolyzed, resulting in free fatty acid and monoglyceride amphiphilic products. These components, together with bile salts, are responsible for the transport of lipids and poorly water-soluble nutrients and xenobiotics from the intestine into the circulatory system of the body. In this study, we show that the self-assembly of digestion products from medium-chain triglycerides (tricaprylin) in combination with bile salt and phospholipid is highly pH-responsive. Individual building blocks of caprylic acid within the mixed colloidal structures are mapped using a combination of small-angle X-ray and neutron scattering combined with both solvent contrast variation and selective deuteration. Modeling of the scattering data shows transitions in the size and shape of the micelles in combination with a transfer of the caprylic acid from the core of the micelles to the shell or into the bulk water upon increasing pH. The results help to understand the process of lipid digestion with a focus on colloidal structure formation and transformation for the delivery of triglyceride lipids and other hydrophobic functional molecules.

Mixed monolayer of DPPC and lysine-based cationic surfactants: an investigation into the antimicrobial activity

In this paper, we report studies which aim to elucidate the mechanisms involved in the antimicrobial activity of three cationic lysine-based surfactants: LLM, LALM, and C6 (LL)2. To this end, a simple membrane model (i.e., 1,2-dipalmitoyl-sn-phosphatidylcholine, DPPC) was used to explore the monolayer properties at the air/liquid interface. Compression π-A isotherms of mixtures of DPPC/lysine surfactants at different pH showed an expansion of the DPPC monolayer, suggesting cationic lysine surfactant/DPPC interactions, which strongly depend on surfactant structure and hydrophobic interactions. Antimicrobial activity of the three surfactants has also been assessed with transmission electron microscopy, observing the effects on Staphylococcus aureus and Escherichia coli . The three surfactants caused various kinds of damage to the bacteria tested, such as structural alterations, leakage of internal material, and cell destruction.

Self assembly of pH-sensitive cationic lysine based surfactants

Three cationic surfactants of the type N(ε)-acyl lysine methyl ester hydrochloride have been studied with respect to solution behavior and adsorption on the air/water interface, as well as the thermolyotropic behavior. The self-assembly of these surfactants, which have the cationic charge on amine protonated groups, was assessed by different physicochemical methods. Depending on the pH value, these surfactants can dissociate in aqueous solutions, losing the cationic charge. Therefore, knowledge of the pK(a) of these compounds is essential to explain their behavior in aqueous solutions. The bulk techniques, conductivity, and nuclear magnetic resonance diffusion (NMR) obtained similar critical micellar concentration (CMC) values, which were well above those obtained from surface tension. Surface tension measurements were strongly dependent on the technique used, namely, Wilhelmy plate and pendant drop. The phase behavior at medium to high concentrations has been studied by optical polarizing microscopy and small angle x-ray scattering (SAXS). The X-ray studies showed that the lysine-based surfactants at low hydration have rich thermotropic liquid crystalline behavior. The results are discussed in terms of the structure of the compounds and the cationic charge of the molecule. We will show how apparently small changes in molecule structure have a large influence on phase behavior.

Phenomenological approaches in the thermodynamics of mixed micelles with electric charges

The stability of mixed micelles in general has been extensively studied by the molecular thermodynamic approaches as well as by the phenomenological or thermodynamic approaches. In this article, phenomenological approaches in the thermodynamics of charged mixed micelles, mostly on ionic/nonionic mixed micelles, are reviewed. The electrostatic interaction constitutes a main contribution to the excess free energy per monomer (in kT unit) g(ex) in the case of ionic/nonionic mixed micelles and the corresponding contribution is generally negative, known as the electric synergism. The origin of the electric synergism is shown to reside in positive curvatures of g(ex,el) (the electric part of g(ex)) when plotted against the mole fraction of the ionic species x. Two types of the micellar Gibbs-Duhem (MGD) relations with (type 1) or without (type 2) the contribution from counterions are discussed to clarify various confusions found in the literature. Effects of varying ionic strengths with the micelle composition in the case of charged mixed micelles without any supporting electrolyte are discussed and a relevant way to correct for the effects is proposed. For ionic/nonionic mixed micelles, the regular solution model (RSM) and some two-parameter models to overcome the limitations inherent to the RSM are discussed. For mixed micelles subject to type 2 MGD relation, hydrogen ion titrations could provide g(el) experimentally just as in the case of linear polyelectrolytes and for those micelles subject to RSM, the titration curve becomes a straight line. Useful information is presented originating from the thermodynamic analysis on the difference of the intrinsic proton dissociation constants between the micelle and the monomer. An analytical expression of the dependence of the degree of counterion binding on x is discussed in comparison with the molecular thermodynamic results. The Corrin-Harkins relation is compared with the degree of counterion binding for ionic/nonionic mixed micelles. Mixed micelles in the concentration range higher than the critical micelle concentration (cmc) are discussed for three cases, the general method of Funasaki-Hada, the ideal mixing case, and the RSM.

Protolytic equilibrium in lyophilic nanosized dispersions: Differentiating influence of the pseudophase and salt effects

The so-called apparent ionization constants of various acids (mainly indicator dyes) in versatile organized solutions are analyzed. Aqueous micellar solutions of colloidal surfactants and related lyophilic colloidal systems display a strong differentiating influence on the acidic strength of indicators located in the dispersed pseudophase, i.e., non-uniform changes of pKa on going from water to the given system. This concept allows the influence of such media on acid-base properties of dissolved reagents to be rationalized. It is demonstrated that the differentiating phenomenon is the main reason for limitation of the common electrostatic model of acid-base interactions, and is the principal hindrance to exact evaluations of the interfacial electrical potentials of ionic micelles by means of acid-base indicators. Salt effects, i.e., the influence of supporting electrolytes on the apparent ionization constants of acid-base indicators in the Stern region of ionic micelles, are considered. These salt effects can be conventionally divided into two kinds, namely, general (normal) and special (specific) effects. While the first type adds up to screening of the surface charge, the second one consists in micellar transitions caused by hydrophobic counterions.