N-acyl Amino Acid Surfactants Research Articles

Interfacial dilational rheological behaviors of N-acyl amino acid surfactants: effects of amino acid residues

Amino acid surfactants have attracted increasing interest in recent years. As biological raw materials, amino acids have diverse molecular structures, which grant the derived surfactants various properties. Herein, the effects of N-methyl and polar, nonpolar, and aromatic α-substituents of amino acids on the interfacial behaviors of amino acid surfactants were systematically studied using interfacial dilational rheological experiments. The amide groups in the glycinate surfactant formed hydrogen bonds, which enhanced the intermolecular interactions. In comparison, an extra N-methyl group in sarcosinate surfactants prevented intermolecular hydrogen bonding and thus decreased the interfacial dilational modulus. The nonpolar α-substituents of alaninate and valinate surfactants enhanced the tendency of surfactant molecules to adsorb to the interface, but caused steric repulsion for hydrogen bond formation, which decreased the dilational modulus but promoted the relaxation processes. The polar α-substituents of serinate and threoninate surfactants enhanced the intermolecular hydrogen bonding and slowed down relaxation processes. The aromatic α-substituent in phenylalanate surfactant improved its interfacial activity. Moreover, the phenyl enhanced the intermolecular interactions through NH-π and π-π interactions. As a result, the interfacial dilational modulus was significantly enhanced. The systematic study of the relationship between amino acid structure and the interfacial behaviors of surfactants provides the research basis for the development and application of N-acyl amino acid surfactants.

Implementation of Amino Acid as a Natural Feedstock in Production of N-Acylamides as a Biocompatible Surfactants: A Review on Synthesis, Behavioral, Application and Scale-up Process

The use of surfactants is extremely widespread for human life. The development of the use of surfactants has not reached detergents anymore, but for drug delivery, biolubricant, emulsifier, cosmetics, enhanced oil recovery (EOR), dispersants and even virus vectors. Unfortunately, the past history of surfactants had given a bad impression since many surfactants are difficult to decompose in nature, are toxic and are not suitable as biological materials. This article will examine the research development and production, synthesis pathway, classification, behaviour and application of N-Acyl amino acid (NAAAc) surfactants. Amino acid-based N-Acylamides (AAc) or NAAAc surfactants are next-generation biological surfactants. NAAAc can be synthesized by chemical and enzymatic pathway. NAAAc can also be combined with ionic liquids (ILs) to become green surfactants NAAAc ILs which is low in toxicity unlike conventional ILs. The conclusion of this article studied was NAAc production process that had the highest efficiency so far was the production through a catalytic chemical reaction, namely the fatty acid amidation or amino acid acylation process. The application of AAc-based N-Acylamides is so promising that it can be considered for scale-up processes in the future.

Contrastive Study of the Foaming Properties of N-Acyl Amino Acid Surfactants with Bovine Serum Albumin and Gelatin.

A detailed study on the foamability, foam stability, foam liquid-carrying capacity, and foam morphology of two N-acyl amino acid surfactants with bovine serum albumin (BSA) and gelatin were performed by foam scanning. The results showed that the foamability of the mixed system increased gradually and then tended to be stable with increasing surfactant concentration. The foamability of the high-concentration BSA system was stronger than that of the low-concentration BSA system. The foamability and foam stability of sodium N-lauroyl phenylpropanoic acid (N-C12P)/BSA were better than those of sodium N-lauroyl propylamino acid (N-C12A)/BSA, and the foamability and foam stability of N-C12A/gelatin was better than those of N-C12P/gelatin. The liquid-carrying capacity of the foam initially increased and then decreased with increasing time, and the maximum liquid-carrying capacity increased with increasing surfactant concentration. When the concentration of the surfactant was 8 mM, the drainage rate of N-C12A/protein was higher than that of N-C12P/protein. The morphology of the bubble gradually changed from spherical to polyhedron and the number of bubbles gradually decreased with time increasing. Differences in surfactant structure and protein type had an important effect on the number and area of foam.

Effects of fatty acyl chains on the interfacial rheological behaviors of amino acid surfactants

Amino acid surfactants derived from vegetable oils have attracted significant interest in the scientific community due to their excellent biocompatibility and low environmental impact. Because of the wide variety of vegetable oils, derived N-acyl amino acid surfactants may feature various fatty acyl groups and therefore offer diverse properties. According to the composition of commonly used vegetable oils, three main characteristics of fatty acyl structures, namely fatty acyl chain length, the number of CC bonds, and hydroxyl substituent, were summarized in this paper and a series of N-fatty acyl glycinate surfactants were synthesized. The specific effects of these three structural factors on the surface tension and interfacial rheological properties were systematically studied. In doing so, we found that an increase of fatty acyl chain length enhanced the interfacial activity and intermolecular interactions of N-fatty acyl glycinate surfactants. Thus, glycinate surfactants with longer fatty acyls could generate more compact adsorption films with a higher dilational modulus. The cis CC bonds in oleoyl and linoleoyl chains were found to bend the long hydrophobic tails, which might affect the compact arrangement of surfactant molecules and increased the viscoelasticity of interfacial films. In addition, the hydroxyl group on ricinoleoyl was found to inhibit the close packing of the hydrophobic tails and reduced intermolecular interactions. As a result, more viscoelastic films were formed by sodium N-ricinoleoylglycinate. The obtained results gained insights into the relationships between fatty acyl structural characteristics and the interfacial arrangements of N-acyl amino acid surfactants, which present a theoretical foundation for surfactant design and further application.

Analysis of Derivatized N-Acyl Amino Acid Surfactants Using HPLC and HPLC/MS

A method for the analysis of weak anionic surfactants based on N-acyl amino acids was developed. The surfactants were derivatized using 2,4′-dibromoacetophenone yielding 4′-bromophenacyl esters suitable for spectrophotometric detection. Surfactants containing glycine, threonine and glutamic acid were analyzed after derivatization using reversed-phase liquid chromatography with UV/Vis and MS detection. The gradient profile was optimized using isocratic retention data of N-acyl-linked fatty acid homologues. The relative content of the homologues of N-acyl-linked fatty acids was expressed using the determined method. The intraday repeatability and stability of the prepared derivatives was tested. The relative content of fatty acids in the surfactants was correlated with the most common sources of fatty acids, showing high Pearson’s correlation coefficients with the typical fatty acids profile of a coconut oil.

Interfacial rheological behavior of N-acyl amino acid surfactants derived from vegetable oils

Surfactants based on natural building blocks instead of traditional petroleum chemicals have attracted increasing attention in recent years. N-acyl amino acid surfactants derived from vegetable oils and amino acids are popular because of their excellent interfacial properties, biodegradability and low toxicity. Due to the variety of vegetable oils, it is significant to understand the effect of the composition and fatty acids structure of the vegetable oils on the interfacial properties of the synthesized amino acid surfactants. In the study presented herein, two N-acyl amino acid surfactants (Cas-Gly-Na and Cot-Gly-Na) were synthesized using glycine and different vegetable oils, i.e. castor oil and cottonseed oil. The interfacial dilational rheological behavior of the resulting surfactants were investigated. The long hydrophobic tails of Cot-Gly-Na enhanced the intermolecular van der Waals interactions, while the hydroxyl group present on the ricinoleate acyl moiety of Cas-Gly-Na inhibited the interactions between the hydrophobic tails. Thus, Cot-Gly-Na exhibited a higher interfacial activity and could generate stronger adsorption films. Moreover, the hydroxyl group on the hydrophobic tails of Cas-Gly-Na may have caused a molecular reorientation during adsorption at the interface. The obtained results demonstrated that the fatty acyls of amino acid surfactants introduced from different vegetable oils exhibited a significant influence on the interfacial properties.

How Promoting and Breaking Intersurfactant H-Bonds Impact Foam Stability.

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

Study on foaming properties of N-acyl amino acid surfactants: Sodium N-acyl glycinate and sodium N-acyl phenylalaninate

A detailed study on the foaming properties of sodium N-acyl glycinate(N-CnG, n = 12, 14,16,18)and sodium N-acyl phenylalaninate(N-CnP, n = 12, 14,16,18)with different carbon chain lengths were performed. The effects of hydrophilic group, hydrophobic group, surfactant concentration and temperature were evaluated on the foamability, foam morphology and foam stability by foam scanning method. The results showed that the foamability and foam stability of N-C12G were better than that of N-C12P. In addition to N-C14P, with the increase of alkyl carbon chain, the foamability of sodium N-acyl amino acid decreased, but the foaming stability gradually increased. With the increase of concentration, the foamability of the two kinds of surfactants increased first and then decreased. The increase of temperature was beneficial to the foamability of N-CnP, but the foam stability decreased. At higher temperatures, the longer alkyl chain was, the stronger the foamability and foam stability of N-CnP were. With the increasing of alkyl carbon, the maximum liquid carrying capacity and liquid drainage rate of N-CnG decreased. At the same concentration and alkyl carbon number, N-CnP had higher maximum liquid carrying capacity than N-CnG. The number of bubbles initially increased and then decreased, and the morphology of bubbles gradually changed from sphere to polyhedron.

Interfacial activities and aggregation behaviors of N-acyl amino acid surfactants derived from vegetable oils

The development of amino acid surfactants which are considered to be biodegradable and less toxic than traditional surfactants has been a subject of growing interests among chemists for the past 20 years. Within this category, N-acyl amino acid surfactants are popular due to their excellent interfacial properties and antimicrobial activities. In the present work, six new N-acyl amino acid surfactants were synthesized using vegetable oils (castor oil and cottonseed oil) and amino acids (glycine, alanine, and serine). Surface active properties of these surfactants were investigated. With the amide bonds acting as hydrogen bond donors and acceptors, globular and tubular vesicles were observed in the aqueous solutions of some prepared surfactants. The results indicated that hydroxyl groups on the hydrophobic tails for castor oil derivatives were associated with spherical vesicles formation, whereas serine residues bearing hydroxyl groups may be associated with the tubular vesicles.

A comparison of the self-assembly behaviour of sodium N-lauroyl sarcosinate and sodium N-lauroyl glycinate surfactants in aqueous and aqueo-organic media

Self-assembly of surfactants is influenced by various intermolecular interactions and molecular structure, which dictate packing of molecules in the aggregate and its microstructure. Hydrogen-bonding between amide groups plays a key role in the self-assembly process of N-acyl amino acid surfactants (NAAS). The self-assembly properties of two NAAS, sodium N-lauroyl sarcosinate (SLS) and sodium N-lauroyl glycinate (SLG) that differ only in the head-group structure were compared in aqueous and aqueo-organic media by using a number of methods, including surface tension fluorescence, dynamic light scattering, calorimetry, and microscopy. It was observed that aggregate formation is more favoured in SLG. Studies revealed that while SLS formed small spherical micelles, SLG produced unilamellar vesicles in pH 7 buffer above critical micelle concentration at 25 °C. The stability of SLG vesicles with respect to pH and temperature was also investigated. Furthermore, both SLG and SLS were found to gelify aquo-organic mixtures of varying composition upon heat-cool treatment. Their gelation behaviour was compared by measuring minimum gelation concentration, molecular packing, and morphology and mechanical stability of the thermoreversible gels. The difference in self-assembly behaviour in water as well as in aqueo-organic mixtures was attributed to the steric repulsion and hydrogen-bonding interaction at the head-group of the molecules.

New Lyotropic Mixtures with Non-Chiral N-Acylamino Acid Surfactants Presenting the Biaxial Nematic Phase Investigated by Laser Conoscopy, Polarized Optical Microscopy and X-ray Diffraction.

Amino acid-based surfactants were used as the main surfactants to prepare new lyotropic mixtures presenting three nematic phases. One of them is biaxial (NB), and the two others are uniaxial, discotic (ND) and calamitic (NC). These surfactants were the non-chiral molecules, potassium N-dodecanoyl-dl-alaninate (dl-KDDA), potassium N-dodecanoyl-dl-serinate (dl-KDDS), disodium N-dodecanoyl-dl-aspartate (dl-NaDDAs) and potassium N-dodecanoyl-glycinate (KDDGly). Measurements of the optical birefringences and X-ray diffraction analysis were used to characterize the nematic phases and phase transitions. Mixtures with dl-KDDS exhibited the largest biaxial phase domain (~9 °C) with respect to the other mixtures in this study. The results obtained with the KDDGly mixture showed that the existence of hydrogen bonding between the head groups of the surfactant molecules seems to hinder the orientation of the micelles under the action of an external magnetic field.

Effect of hydrogen-bonding interactions on the self-assembly formation of sodium N-(11-acrylamidoundecanoyl)- l-serinate, l-asparaginate, and l-glutaminate in aqueous solution

Aggregation behavior of three N-acyl amino acid surfactants, sodium N-(11-acrylamidoundecanoyl)- l-serinate (SAUS), sodium N-(11-acrylamidoundecanoyl)- l-asparaginate (SAUAS), and sodium N-(11-acrylamidoundecanoyl)- l-glutaminate (SAUGL), was studied in aqueous solution by use of surface tension, fluorescence, dynamic light scattering, and transmission electron microscopic techniques. The amphiphiles have been shown to initially form flexible bilayer structures, which upon increase of surfactant concentration transform into closed spherical vesicles. The transmission electron micrographs of the aqueous solutions of the surfactants confirmed the existence of spherical vesicles. Dynamic light scattering measurements were performed to obtain hydrodynamic radii of the vesicles. Circular dichroism spectra of the amphiphiles indicated formation of chiral helical aggregates in the case of SAUS. The self-assembly formation of the amphiphiles has been discussed in light of the intermolecular hydrogen bonding interaction of the amide groups.

Self-Organization Properties and Microstructures of Sodium N-(11-Acrylamidoundecanoyl)-l-valinate and -l-threoninate in Water

Abstract A new class of N-acylamino acid surfactants, sodium N-(11-acrylamidoundecanoyl)-l-valinate (SAUV) and -l-threoninate (SAUT), were synthesized and characterized in aqueous solutions. The self-assembly properties were studied by surface tension, fluorescence probe, light scattering, and microscopic techniques. The interfacial properties of the amphiphiles have been investigated at the air/water interface. The critical aggregation concentration (cac) and free energy change of aggregation for both amphiphiles were determined. The results of fluorescence probe and surface tension studies have indicated that initially flat bilayer structures are formed, which upon a further increase of surfactant concentration transform into closed vesicles. The pH and temperature dependence of the bilayer aggregates have been studied. The role of intermolecular hydrogen bonding between amide groups upon aggregation towards microstructure formation in solution has been elucidated. The micropolarity of the hydrophobic domains of the bilayer self-assemblies was estimated by fluorescence probe methods. Dynamic light scattering measurements were performed to determine the mean size of the aggregates. The circular dichroism spectra of SAUV and SAUT suggested the formation of chiral aggregates in dilute solution. The transmission electron micrographs revealed the presence of closed vesicles and twisted ribbons in the aqueous solutions of the amphiphiles. Optical microscopic images also showed the existence of twisted ribbons and rope-like helical structures in the case of SAUV.

Spontaneously Formed Vesicles of SodiumN-(11-Acrylamidoundecanoyl)-glycinate andl-Alaninate in Water

Two N-acyl amino acid surfactants, sodium N-(11-acrylamidoundecanoyl)-glycinate (SAUG) and L-alaninate (SAUA), were synthesized and characterized in aqueous solution. A number of techniques, such as surface tension, fluorescence probe, light scattering, and transmission electron microscopy were employed for characterization of the amphiphiles in water. The surface and interfacial properties were measured. The amphiphiles have two critical aggregation concentrations. The results of surface tension and fluorescence probe studies suggested formation of bilayer self-assemblies in dilute aqueous solutions of the amphiphiles. The magnitudes of free energy change of aggregation have indicated that bilayer formation is more favorable in the case of SAUG. Steady-state fluorescence measurements of pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH) were used to study the microenvironment of the molecular self-assemblies. Temperature-dependent fluorescence anisotropy change of DPH probe revealed phase transition temperature of the bilayer self-assemblies. The effects of pH on the structure of the self-assemblies of SAUG and SAUA have been studied. The role of intermolecular hydrogen bonding between amide groups upon aggregation toward microstructure formation in solution has been discussed. Circular dichroism spectra suggested the presence of chiral aggregates in an aqueous solution of SAUA. The transmission electron micrographs revealed the presence of closed spherical vesicles in aqueous solutions of the amphiphiles. Dynamic light scattering measurements were performed to obtain average size of the aggregates.

Microviscosity of bilayer membranes of some N-acylamino acid surfactants determined by fluorescence probe method

The microviscosities of the self-assemblies formed by sodium N-(11-acrylamido-undecanoyl)-aminoacidate, and N-(4- n-dodecyloxybenzoyl)-aminoacidate surfactants were estimated by fluorescence anisotropy measurements using 1,6-diphenyl-1,3,5-hexatriene as probe molecule. The self-assemblies have high microviscosities in consistence with their bilayer membrane structures. In order to test the validity of the method, the results have been compared with those of micelle forming surfactants, sodium dodecylsulfate, dodecyltrimethylammonium bromide, cetyltrymethylammonium bromide, and Triton X-100 for which the microviscosity data are available in the literature.

Krafft temperature and enthalpy of solution of N-acyl amino acid surfactants and their racemic modifications: effect of the amino acid residue

The Krafft temperatures and the enthalpies of solution of six kinds of N-hexadecanoyl amino acid surfactant (Gly, Ala, Val, Leu, Ile, and Phe) were obtained from both solubility measurements and differential scanning calorimetry. It was shown that the Krafft temperature of N-hexadecanoyl amino acid surfactant increased with decreasing size of the amino acid residue except for the case of phenylalanine. On the other hand, the enthalpy of solution was endothermic and increased with decreasing size of the amino acid residue except for the cases of glycine and phenylalanine. It was found from these results that the D-L interaction was superior to the L-L interaction in solid state of N-hexadecanoyl amino acid surfactant salt for both the alanine and phenylalanine systems. It was suggested by ab initio calculations that the difference of the magnitude of the peptide-peptide hydrogen bonding was the dominant factor for the chiral effect.

Effect of amino acid surfactants on phase transition of poly( N-isopropylacrylamide) gel

The volume phase transition behavior of a poly( N-isopropylacrylamide) gel (NIPA gel) in solutions of N-acyl amino acid surfactants were studied as a function of surfactant concentration. The addition of a surfactant beyond the critical micelle concentration (cmc) produced elevation in the transition temperature of the NIPA gel and its swelling. The changes in the volume phase transition temperature and in the swelling of the NIPA gel became more significant with the decreasing size of the amino acid side chain. This result could almost be explained only by the binding amount of surfactant onto the NIPA gel regardless of molecular structure of the amino acid. The binding amount increased in the order of sodium N-lauroyl-glycinate>-alaninate>-valinate>-leucinate⩾-phenylalaninate. For an N-acyl amino acid surfactant to bind onto the NIPA gel, to increase the transition temperature, and to facilitate swelling of the gel, the steric hindrance of the amino acid side chain was more effective than its hydrophobicity.

Synthesis and Surface Properties of Amino Acid Surfactants from Industrial Waste Proteins

N-Acyl amino acid surfactants (AAS) were chemically derived from industrial waste protein hydrolysates [cottonseed (CSD), silk residue (SR), and silk chrysalis (SC)] according to two methods: (1) reacting hydrolysates with alkyl acyl chloride, followed by purification and neutralization with alcoholic sodium hydroxide; (2) reacting hydrolysate with fatty alcohols in organic solvents followed by purification. The yield of purified mixed AAS (sodium salt) was ∼60−75%; amino acid ester from glutamic acid was considerably higher (85−92%) than the sodium salt derivatives. Results indicate that as acyl chain length (i.e., C12−C18) increased, surface tension of AAS increased, critical micelle concentration (cmc) decreased, and Krafft point increased. The emulsifying power of AAS in O/W emulsion was better with n-decanol as an oil phase than liquid paraffin. The C12 derivatives of all the mixed AAS showed high foaming power. Mixed AAS from CSD exhibited the best lime soap dispersing requirement (5.7−6.5 g/100 g). The diameter of micelle increased for glutamic acid AAS (GA-AAS) and CSD-AAS as the acyl chain length increased (i.e, C12−C18). The hydrodynamic diameter of AAS followed the order SC > CSD > GA. Generally, AAS with C12 produced good surface properties. Keywords: Amino acid surfactants; synthesis; surface properties; industrial waste proteins

Critical micelle concentration in mixtures of N-acyl amino acid surfactants

The critical micelle concentrations (CMC) were determined in the mixed solutions of N-lauroyl amino acid sodium salts. The results suggested that steric hindrance of its amino acid residue was not always significant on formation of the mixed micelle. In addition, a model was proposed to predict the CMC in the mixed system without an adjustable parameter. It was found that the model was applicable to the systems that deviated significantly from ideal mixing.