Amino Acid-derived Surfactant Research Articles

Effect of pH on the Structure and Dynamics of Wormlike Micelles in an Amino Acid-Derived Surfactant Composition.

We studied the impact of pH as a thickening mechanism on the structure and dynamics of wormlike micelles in a mixture of sodium lauroyl sarcosinate (SLSar) and cocamidopropyl hydroxysultaine (CAHS). The viscoelastic properties were obtained using mechanical rheometry and diffusing wave spectroscopy, which provided access to a wide range of frequencies. By using a mesoscopic simulation method [Zou; Larson. J. Rheol. 2014, 58 (3), 681-721], characteristic micelle lengths and times were extracted including contour length, persistence length, entanglement length, reptation time, breakage time, breakage rate, and breakage rate constant. The interplay of pH-dependent reptation times (10-1000 ms) and breakage times (4-38 ms) leads to a minimum in the ratio of reptation time to breakage time of about 0.02 at pH 4.8. This minimum was closely associated with the sharp increase and decrease of the observed viscosity maximum at pH 4.8 in this system. These values may be contrasted with much longer breakage times (20-300 ms) that have been measured in more easily thickened sulfate-based systems. The low breakage times of the SLSar/CAHS system were attributed to the high and pH-sensitive breakage rate constants (0.01-0.17 ms-1 μm-1).

Aggregation Behavior, Antibacterial Activity and Biocompatibility of Catanionic Assemblies Based on Amino Acid-Derived Surfactants.

The surface activity, aggregates morphology, size and charge characteristics of binary catanionic mixtures containing a cationic amino acid-derived surfactant N(π), N(τ)-bis(methyl)-L-Histidine tetradecyl amide (DMHNHC14) and an anionic surfactant (the lysine-based surfactant Nα-lauroyl-Nεacetyl lysine (C12C3L) or sodium myristate) were investigated for the first time. The cationic surfactant has an acid proton which shows a strong pKa shift irrespective of aggregation. The resulting catanionic mixtures exhibited high surface activity and low critical aggregation concentration as compared with the pure constituents. Catanionic vesicles based on DMHNHC14/sodium myristate showed a monodisperse population of medium-size aggregates and good storage stability. According to Small-Angle X-Ray Scattering (SAXS), the characteristics of the bilayers did not depend strongly on the system composition for the positively charged vesicles. Negatively charged vesicles (cationic surfactant:myristate ratio below 1:2) had similar bilayer composition but tended to aggregate. The DMHNHC14-rich vesicles exhibited good antibacterial activity against Gram-positive bacteria and their bactericidal effectivity declined with the decrease of the cationic surfactant content in the mixtures. The hemolytic activity and cytotoxicity of these catanionic formulations against non-tumoral (3T3, HaCaT) and tumoral (HeLa, A431) cell lines also improved by increasing the ratio of cationic surfactant in the mixture. These results indicate that the biological activity of these systems is mainly governed by the cationic charge density, which can be modulated by changing the cationic/anionic surfactant ratio in the mixtures. Remarkably, the incorporation of cholesterol in those catanionic vesicles reduces their cytotoxicity and increases the safety of future biomedical applications of these systems.

Biocompatible thermosensitive nanostructures and hydrogels of an amino acid-derived surfactant and hydroxyethyl cellulose polymers

Aqueous surfactant/polymer mixtures form colloidal structures of great fundamental interest and practical relevance, such as nanostructured hydrogels for biomedical and pharmaceutical uses. In this work, we investigated the phase behavior, structure and cytotoxicity of mixtures of a double-tailed lysine-based surfactant, 16Lys12, and two hydroxyethyl cellulose (HEC) derivatives, JR400 and LM200. The surfactant, S−, is anionic and self-assembles into tubular structures at room temperature, undergoing a tubule-to-vesicle transition at ≈44 °C. JR400 is a cationic homopolymer, P+, longer and more densely charged than LM200, a closely related hydrophobically modified polymer, HMP+. Electrostatic and hydrophobic interactions play a crucial role in the observed phase behavior and resulting colloidal structures. Both the S−/P+ and S−/HMP+ mixtures show three main phase regions: at surfactant charge excess, bluish dispersions containing mixed polymer/tubular aggregates and, upon heating, polymer/vesicle clusters; a white precipitate near charge equimolarity, coexisting with either a solution or a gel; and highly viscous hydrogels, at polymer charge excess. In the bluish dispersions, the S−/P+ and S−/HMP+ systems show relevant differences in thermal behavior and type of aggregates present. Cryogenic scanning electron microscopy shows that the hydrogels consist of honeycomb-like structure of surfactant and polymer moieties. Cytotoxicity assays in the bluish dispersion region indicate good levels of cytocompatibility for both types of surfactant/polymer systems. Overall, these dispersions and hydrogels can be further explored for the encapsulation and temperature-triggered release of biomolecules.

Thickening mechanisms for an amino acid-derived surfactant composition

Wormlike micelles in mixed surfactant mixture containing an amino acid-derived anionic surfactant, sodium lauroyl sarcosinate (SLSar), and a zwitterionic surfactant, cocoamidopropyl hydroxysultaine (CAHS) were investigated. The total surfactant concentration was maintained at 15% wt/v, while the ratio of SLSar and CAHS was varied and the pH was lowered from 7.5 to 4.5. Steady-shear rheology, potentiometric titration, cross-polarized microscopy, zeta potential and cryo-TEM were used to obtain zero-shear viscosity and analyze the structure-rheology relationship. As pH dropped, viscosity increased to a maximum followed by a viscosity decrease in four out of five tested systems. The synergistic effect was strongest in the SLSar 6 %:CAHS 9 % composition or an equivalent molar ratio of 1:1 between the two surfactants, for which the viscosity maximum was 9.6 Pa.s at pH 4.8. The viscosity peak of each system occurred in the pH range 4.8–5.0, close to the system’s experimental pKa. The intermolecular forces between the SLSar acid form, SLSar anionic form and CAHS reduced the charge repulsion and contributed to the thickening behavior of these systems.

Facile synthesis of uniform LaSrCoO4 using amino acid-derived surfactant and its utilization as an excellent cathode material for intermediate temperature solid oxide fuel cell

In this paper, a uniform and defective LaSrCoO4 was prepared by using amino acid derived surfactant (N-(2-hydroxylalkyl)-l-Phenylalanine). Owing to rich active groups of N-(2-hydroxylalkyl)-l-Phenylalanine, the resultant LaSrCoO4 showed enhanced specific surface area and good electro-catalytic property. The porous K2NiF4-type LaSrCoO4 was verified as a good cathode material for intermediate temperature solid oxide fuel cells (SOFCs). The LaSrCoO4 electrode demonstrated good electrochemical performance, including good electrical conductivity (138.7 S cm−1, in air at 800 °C), low polarization resistance Rp (0.108 Ω cm2, symmetric cells in air at 800 °C) and high power densities (688 mW cm−2 at 800 °C). The as-prepared LaSrCoO4 shows a promising application as the cathode materials in the intermediate temperature SOFCs.

Generation of a Chiral Giant Micelle.

Over the past few years, chiral supramolecular assemblies have been successfully used for recognition, sensing and enantioselective transformations. Several approaches are available to control chirality of discrete assemblies (e.g., cages and capsules), but few are efficient in assuring chirality for micellar aggregates. Optically active amino acid-derived surfactants are commonly used to generate chiral spherical micelles. To circumvent this limitation, we benefited from the uniaxial growth of spherical micelles into long cylindrical micelles usually called wormlike or giant micelles, upon the addition of cosolutes. This paper describes the unprecedented formation of chiral giant micelles in aqueous solutions of cetyltrimethylammonium bromide (CTAB) upon increasing addition of enantiopure sodium salt of 1,1'-bi-2-naphthol (Na-binaphtholate) as a cosolute. Depending on the concentrations of CTAB and Na-binaphtholate, chiral gel-like systems are obtained. The transition from spherical to giant micellar structures was probed using rheology, cryo-transmission electron microscopy, polarimetry, and electronic circular dichroism (CD). CD can be effectively used to monitor the incorporation of Na-binaphtholate into the micelle palisade as well as to determine its transition to giant micellar structures. Our approach expands the scope for chirality induction in micellar aggregates bringing the possibility to generate "smart" chiral systems and an alternative asymmetric chiral environment to perform enantioselective transformations.

Enhanced interfacial properties of novel amino acid-derived surfactants: Effects of headgroup chemistry and of alkyl chain length and unsaturation

Amino acid-derived surfactants have increasingly become a viable biofriendly alternative to petrochemically based amphiphiles as speciality surfactants. Herein, the Krafft temperatures and critical micelle concentrations (cmc) of three series of novel amino acid-derived surfactants have been determined by differential scanning microcalorimetry and surface tension measurements, respectively. The compounds comprise cationic molecules based on serine and tyrosine headgroups and anionic ones based on 4-hydroxyproline headgroups, with varying chain lengths. A linear dependence of the logarithm of cmc on chain length is found for all series, and in comparison to conventional ionic surfactants of equal chain length, the new amphiphiles present lower cmc and lower surface tension at the cmc. These observations highlight their enhanced interfacial performance. For the 18-carbon serine-derived surfactant the effects of counterion change and of the presence of a cis-double bond in the alkyl chain have also been investigated. The overall results are discussed in terms of headgroup and alkyl chain effects on micellization, in the light of available data for conventional surfactants and other types of amino acid-based amphiphiles reported in the literature.

Spontaneous formation of temperature-responsive assemblies by molecular recognition of a β-cyclodextrin containing block copolymer and poly(N-isopropylacrylamide).

We report the construction of novel temperature-responsive assemblies based on a double hydrophilic block copolymer (consisting of a PEG block and a β-cyclodextrin-containing block, PEG-b-PCD) and poly(N-isopropylacrylamide) (PNIPAm). Thus formed nano-assemblies exhibit a spherical morphology and have a temperature-responsive loose core. The driving force for the formation of these assemblies was found to be the inclusion complexation interaction between the hydrophobic cavity of β-cyclodextrin and the isopropyl group of PNIPAm. The particle size of these assemblies changed reversibly in response to the external temperature change. The particle size also changed with the PNIPAm/PEG-b-PCD weight ratio. A model hydrophobic drug (indomethacin) was loaded into these assemblies with a high efficiency. An in vitro release study showed that the payload could be released in a sustained manner after an initial burst release. The release rate could be switched between high and low in an ON/OFF fashion by temperature. These results demonstrate that the nano-assemblies have high potential for applications in controlled drug delivery and biomedicine when temperature responsiveness is desired.

Self-Assembly in a Catanionic Mixture with an Aminoacid-Derived Surfactant: From Mixed Micelles to Spontaneous Vesicles

The aqueous self-assembly of a novel lysine-derived surfactant with a gemini-like architecture, designated here as 12-Lys-12, has been experimentally investigated for the amphiphile alone in water and in a mixture with dodecyltrimethylammonium bromide (DTAB). The neat surfactant forms interesting micrometer-sized rigid tubules in the dilute region, resulting in very viscous solutions. For the catanionic mixture with DTAB, various single and multiphase regions were identified (up to a total surfactant concentration of 1.5 wt %) by means of combined polarizing light microscopy, cryo-TEM, and NMR. In the DTAB-rich side, for a mixing molar ratio in the range 2 < DTAB/12-Lys-12 < 4, a region of stable, unilamellar vesicles can be found. Furthermore, it was found that upon addition of 12-Lys-12 to pure DTAB solutions, the mixed micelles grow and beyond a given mixing ratio, vesicles assemble and coexist with small micelles. The transition is not continuous, since there is a narrow mixing range where phase separation occurs. Self-diffusion measurements and cryo-TEM imaging show that the average vesicle radius is on the order of 30-40 nm.

Assessment of the potential irritation and photoirritation of novel amino acid-based surfactants by in vitro methods as alternative to the animal tests.

The ultraviolet-A radiation damage effects on skin and eyes will be increased by phototoxic compounds which could be present in pharmaceutical or cosmetic formulations. Great efforts have been made in the last years to find surfactants to replace those with phototoxic potential in commercial use. Series of different in vitro models for phototoxicity, included to validated neutral red uptake (NRU) 3T3 phototoxicity assay are useful screening tools. The phototoxic effects of a novel family of glycerol amino acid-based surfactant compounds were examined via these assays. Human red blood cells and two immortalised cell lines, murine fibroblast cell line 3T3, and one human keratinocyte cell line, HaCaT, were the in vitro models employed to predict potential photoirritation. The phototoxic end-points assessed were hemolysis (human red blood cell test) and resazurin transformation to resorufin and NRU in cell culture methods. The results suggest that no phototoxic effects by any new amino acid derived-surfactants, could be identified.

Assessment of the potential irritation and photoirritation of novel amino acid-based surfactants by in vitro methods as alternative to the animal tests

The ultraviolet-A radiation damage effects on skin and eyes will be increased by phototoxic compounds which could be present in pharmaceutical or cosmetic formulations. Great efforts have been made in the last years to find surfactants to replace those with phototoxic potential in commercial use. Series of different in vitro models for phototoxicity, included to validated neutral red uptake (NRU) 3T3 phototoxicity assay are useful screening tools. The phototoxic effects of a novel family of glycerol amino acid-based surfactant compounds were examined via these assays. Human red blood cells and two immortalised cell lines, murine fibroblast cell line 3T3, and one human keratinocyte cell line, HaCaT, were the in vitro models employed to predict potential photoirritation. The phototoxic end-points assessed were hemolysis (human red blood cell test) and resazurin transformation to resorufin and NRU in cell culture methods. The results suggest that no phototoxic effects by any new amino acid derived-surfactants, could be identified.