Salt-Induced Vesicle to Micelle Transition in Aqueous Solution of Sodium N-(4-n-Octyloxybenzoyl)-l-valinate

Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA⁺) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA⁺ ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA⁺ addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA⁺ entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide-amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.

Monitoring Local pH of Membranous Aggregates via Ratiometric Color Changing Response

A series of oxidized di(indolyl)arylmethanes (DIAM) with polyaromatic signaling moieties have been designed for monitoring local pH at interfacial region of surfactant aggregates, such as micelles and vesicles etc. The oxidized DIAMs show changes in solution color from red to yellow when incorporated in cationic surfactants (at pH 7.4) and yellow to reddish pink when exposed to negatively-charged surfactants (at pH 5.0). The changes in surface charge can influence the interfacial pH (distinct from bulk pH of the medium) of the surfactant aggregates. The mechanistic studies indicate the red-shifted absorption maxima observed in the presence of anionic amphiphiles (acidic local pH) was originated from the protonated species. On the contrary, maxima in the blue region, triggered by positively charged amphiphiles (basic local pH), can be attributed to the zwitterionic species. Such prototropic equilibrium affects charge transfer states of the molecules along with their self-assembly properties. Thus, it is evident that probes can predict as well as quantify the local pH change using the pseudophase ion exchange formalism. Also, the probes can detect the presence of anionic amphiphiles even when bound to phospholipid membranes.

Vesicle formation of single-chain amphiphilic 4-dodecylbenzene sulfonic acid in water and micelle-to-vesicle transition induced by wet-dry cycles

Simple single-chain amphiphiles (SCAs) can form vesicular structures in their single-component aqueous solutions, which has attracted great attention, but the understanding of their aggregation behavior is still limited. In this work, the aggregation behavior of 4-dodecylbenzene sulfonic acid (DBSA), a typical simple SCA, in water was investigated. The structure and properties of the aggregates formed were determined. In particular, the effect of wet-dry cycles on the structures of aggregates was examined. The mechanisms of aggregate formation and structural transition were discussed. It was found that the increase of DBSA concentration can drive the occurrence of a micelle-to-vesicle transition, showing a critical micelle concentration and critical vesicle concentration of ∼0.53 and 2.14 mM, respectively. The vesicles formed coexist with micelles in solution, with a unilamellar structure and ∼80 nm size, and exhibit size-selective permeability. In addition, the vesicles show remarkable stability upon long-term storage, exposure to high temperature, and freeze-thaw cycles. The H-bonding interaction between DBSA species and the interdigitated structure of alkyl chains in bilayers play a key role in the formation and stability of DBSA vesicles. Interestingly, it was found that the wet-dry cycle can induce a micelle-to-vesicle transition and an obvious increase in the size of the original vesicles, accompanied by the formation of some multilamellar vesicles. This work provides a better understanding of the aggregation behavior of simple SCAs in their single-component aqueous solutions.

Factors affecting the morphology of some organic and inorganic nanostructures for drug delivery: characterization, modifications, and toxicological perspectives

Introduction: In this review, we aim to highlight the impact of various processes and formulation variables influencing the characteristics of certain surfactant-based nanoconstructs for drug delivery. Areas covered: The review includes the discussion on processing parameters for the preparation of nanoconstructs, especially those made up of surfactants. Articles published in last 15 years (437) were reviewed, 381 articles were selected for data review and most appropriate articles (215) were included in article. Effect of variables such as surfactant concentration and type, membrane additives, temperature, and pH-dependent transitions on morphology has been highlighted along with effect of shape on nanoparticle uptake by cells. Various characterization techniques explored for these nanostructures with respect to size, morphology, lamellarity, distribution, etc., and a separate section on polymeric vesicles and the influence of block copolymers, type of block copolymer, control of block length, interaction of multiple block copolymers on the structure of polymersomes and chimeric nanostructures have been discussed. Finally, applications, modification, degradation, and toxicological aspects of these drug delivery systems have been highlighted. Expert opinion: Parameters influencing the morphology of micelles and vesicles can directly or indirectly affect the efficacy of small molecule cellular internalization as well as uptake in the case of biologicals.[Figure: see text].

Temperature-induced reversible micelle–vesicle transition in aqueous solution of a pseudogemini surfactant without any additive

Temperature-induced reversible micelle–vesicle transition is achieved in aqueous solution of a single pseudogemini surfactant without any additive.

Controlling the release of hydrophobic compounds by a supramolecular amphiphilic assembly

Here, we report a novel approach of using a supramolecular system based on calix[4]resorcinarene and surfactant to facilitate the release of hydrophobic compounds.

Synthesis and self-assembly behaviour of poly(Nα-Boc-l-tryptophan)-block-poly(ethylene glycol)-block-poly(Nα-Boc-l-tryptophan)

We report for the first time the use of Nα-Boc-l-tryptophan for the synthesis of amphiphilic BAB triblock copolymers for potential drug delivery applications.

Temperature-induced vesicle to micelle transition in cationic/cationic mixed surfactant systems

Temperature-induced vesicle to micelle transition (VMT), which has rarely been reported in cationic/cationic mixed surfactant systems, was systemically studied in a didodecyldimethylammonium bromide (DDAB)/dodecyltrimethylammonium chloride (DTAC) aqueous solution. We investigated the effect of temperature on DDAB/DTAC aqueous solutions by means of turbidity, conductivity, cryo-TEM, a UV-vis spectrophotometer, and a steady-state fluorescence spectrometer. It was found that increasing temperature could induce the transformation from the vesicle to the micelle in this cationic/cationic mixed surfactant system. The degree of transformation can be easily controlled by the operation temperature. Additionally, by adjusting the proportion of the mixed cationic/cationic systems and employing cationic surfactants with different chain-lengths, we were able to conclude that the hydrophobic tail length of the surfactant affects the aggregation behavior of cationic/cationic mixed surfactant systems as a function of temperature. It is universal to induce the transformation from the vesicle to the micelle by temperature in cationic/cationic mixed surfactant systems. A possible mechanism for the temperature-induced VMT was proposed based on the experimental results.

Dual stimuli-responsive self-assembly transition in zwitterionic/anionic surfactant systems

Temperature and pH responsiveness is important for biological applications in protein reconstitution, gene delivery and controlled drug release. The temperature and pH dual responsive self-assembly transition, vesicle-to-micelle transitions (VMTs) and micelle-to-vesicle transitions (MVTs), in dodecyl sulfonatebetaine (DSB)/sodium bis(2-ethylhexyl) sulfosuccinate (AOT) aqueous solution are investigated. Various experimental techniques including cryogenic transmission electronic microscopy, UV-vis spectroscopy, fluorescence spectroscopy, conductivity, and zeta potential were employed to verify the transformation process. Encapsulation of calcein was further applied in this study. The results showed that the self-assembly transition in DSB/AOT aqueous solution is reversible and can be controlled by temperature and pH. It is anticipated that utilizing simple stimuli methods to realize unique self-assembly behaviour in dilute aqueous solution may offer new possibilities in cancer diagnosis and therapy.

Aggregation behavior of poly(ethylene glycol) chain-containing anionic amphiphiles: Thermodynamic, spectroscopic and microscopic studies

Two novel amphiphilic molecules were synthesized by the reaction between poly(ethylene glycol) methyl ether (mPEG) of different chain lengths and sodium-2-mercapto ethane sulfonate (mesna). Different techniques, such as surface tensiometry, conductometry, fluorescence spectroscopy, dynamic light scattering, UV-vis spectroscopy, transmission electron microscopy, and isothermal titration calorimetry were employed to investigate the self-assembly properties of the PEG-based single-tailed amphiphiles in aqueous buffer. Despite having so called polar tail the amphiphiles exhibit aggregate formation in aqueous buffer as well as in water. The shorter chain amphiphile was shown to form bilayer vesicles in contrast to small micelles by its longer chain counterpart. The helicity of the PEG chain was taken into consideration to interpret the difference in self-assembled microstructure formation. The thermodynamics of the self-assemblies were also thoroughly examined. The thermodynamic parameters clearly suggested that the hydrophobic interaction among the PEG chains is the main driving force for aggregate formation. The self-assembled microstructures were observed to be fairly stable with respect to increase of surfactant concentration, aging time and temperature.

The coexisting phase behavior of thermo-responsive copolymer solutions

Using a molecular theory for dilute PEO-b-PNIPAm solutions, we first take the formation of hydrogen bonds between copolymer monomers and water molecules into account, which enables us to study the impact of temperature on PEO-b-PNIPAm self-assembly effectively by quantitatively describing the different changes in water affinities of two blocks. With the increase of temperature, hydrogen bonds between PNIPAm and water break down dramatically, resulting in the hydrophobic character of PNIPAm while PEO remains hydrophilic. Amphiphilic copolymers in the aqueous surrounding can aggregate into various structures: micelles and vesicles. According to the equilibrium criterion of the excess grand potential under the conditions of the grand canonical ensemble, we find that both structures are stable and can coexist. Theoretically calculated potentials of mean force of aggregates further verify the coexistence of micelles and vesicles, although the low critical solution temperatures of different aggregates are different under these conditions. A phase diagram as functions of temperature and the weight fraction of PEO (fPEO) is obtained, which shows different regions of micelles, vesicles and their coexistence. It implies the appearance of two types of micelle-vesicle transition: spontaneous and temperature-induced. Since PEO-b-PNIPAm as a thermoresponsive material has a broad range of applications, a systematic investigation of the phase behavior is very useful not only for the scientific interest but also for the practical applications.

Chain length dependence of non-surface activity and micellization behavior of cationic amphiphilic diblock copolymers

The cationic and anionic amphiphilic diblock copolymers with a critical chain length and block ratio do not adsorb at the air/water interface but form micelles in solution, which is a phenomenon called "non-surface activity". This is primarily due to the high charge density of the block copolymer, which creates a strong image charge effect at the air/water interface preventing adsorption. Very stable micelle formation in bulk solution could also play an important role in the non-surface activity. To further confirm these unique properties, we studied the adsorption and micellization behavior of cationic amphiphilic diblock copolymers of poly(n-butyl acrylate)-b-poly(3-(methacryloyloxy)ethyl)trimethylammonium chloride) (PBA-b-PDMC) with different molecular weights of hydrophobic blocks but with the same ionic block length. These block copolymers were successfully prepared via consecutive reversible addition-fragmentation chain transfer (RAFT) polymerization. The block copolymer with the shortest hydrophobic block length was surface-active; the solution showed surface tension reduction and foam formation. However, above the critical block ratio, the surface tension of the solution did not decrease with increasing polymer concentration, and there was no foam formation, indicating lack of surface activity. After addition of 0.1 M NaCl, stable foam formation and slight reduction of surface tension were observed, which is reminiscent of the electrostatic nature of the non-surface activity. Fluorescence and dynamic and static light scattering measurements showed that the copolymer with the shortest hydrophobic block did not form micelles, while the block copolymers formed spherical micelles having radii of 25-30 nm. These observations indicate that micelle formation is also important for non-surface activity. Upon addition of NaCl, cmc did not decrease but rather increased as observed for non-surface-active block copolymers previously studied. The micelles formed were very stable, and their size decreased by only ∼5 nm after addition of 0.1 M NaCl.

Realization of the reversible vesicle-micelle transition of vitamin-derived bolaamphiphiles by heat change monitoring

The real-time energetics involved in the structural change of a zwitterionic vitamin-derived bolaamphiphiles (DDO) vesicles, which were induced by conventional surfactants, such as hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX100), was characterized by isothermal titration calorimetry (ITC). Interactions of both CTAB and SDS with DDO were accompanied with considerable heat release whereas the interaction energetics between TX-100 and the vesicles were small. However, the transition of DDO vesicles to micelles did occur upon the addition of all of the three surfactants. Fine inflection points were observed in heat flow enthalpograms, which indicated systematically the change of vesicle structure. By monitoring the interaction of CTAB with DDO, we found that heat release kept constant over a certain concentration range at higher temperatures. The repairing effect of heating was revealed and a reversible transition from micelles to vesicles of DDO was thus realized. Further encapsulation of fluorescein in DDO vesicles proved that the reversible vesicle-micelle transition was controllable. This research demonstrates that ITC combined with complementary analytical methods such as dynamic light scattering (DLS) and transmission electron microscopy (TEM) helps to get the real-time information of the structural changes of vesicles. It also shows that these synthetic novel bolaamphiphiles offer great promise for designing controllable release system.

pH-responsive surface activity and solubilization with novel pyrrolidone-based Gemini surfactants

A new series of pH-responsive Gemini surfactants with 2-pyrrolidone head groups, N,N'-dialkyl-N,N'-di(ethyl-2-pyrrolidone)ethylenediamine (Di-C(n)P, where n = 6, 8 10, 12), were synthesized and characterized by (1)H NMR, (13)C NMR, ESI-MS, and elemental analysis. The surface activity and micellization behavior at acidic, neutral, and basic conditions were characterized by equilibrium surface tension and fluorescence techniques. It was found that the surface activity of Di-C(n)P depends on the pH of aqueous solutions due to the protonation state of surfactant molecules when pH was varied. The new compounds have lower cmc and γ(cmc) in comparison with that of m-2-m type conventional cationic Gemini surfactants and gluconamide-type nonionic Gemini surfactants. Fluorescence data confirm that micelles are formed when the concentration is above the cmc. Since micellization is of fundamental importance in surfactant applications such as solubilization, microemulsion, and related technologies, the significant difference in cmc at different pH of this new Gemini surfactant is employed to solubilize cyclohexane. The preliminary result indeed shows that the solubilization capacity of Di-C(n)P can be tuned by pH.

Non-surface activity and micellization behavior of cationic amphiphilic block copolymer synthesized by reversible addition-fragmentation chain transfer process

Cationic amphiphilic diblock copolymers of poly(n-butylacrylate)-b-poly(3-(methacryloylamino)propyl)trimethylammonium chloride) (PBA-b-PMAPTAC) with various hydrophobic and hydrophilic chain lengths were synthesized by a reversible addition-fragmentation chain transfer (RAFT) process. Their molecular characteristics such as surface activity/nonactivity were investigated by surface tension measurements and foam formation observation. Their micelle formation behavior and micelle structure were investigated by fluorescence probe technique, static and dynamic light scattering (SLS and DLS), etc., as a function of hydrophilic and hydrophobic chain lengths. The block copolymers were found to be non-surface active because the surface tension of the aqueous solutions did not change with increasing polymer concentration. Critical micelle concentration (cmc) of the polymers could be determined by fluorescence and SLS measurements, which means that these polymers form micelles in bulk solution, although they were non-surface active. Above the cmc, the large blue shift of the emission maximum of N-phenyl-1-naphthylamine (NPN) probe and the low micropolarity value of the pyrene probe in polymer solution indicate the core of the micelle is nonpolar in nature. Also, the high value of the relative intensity of the NPN probe and the fluorescence anisotropy of the 1,6-diphenyl-1,3,5-hexatriene (DPH) probe indicated that the core of the micelle is highly viscous in nature. DLS was used to measure the average hydrodynamic radii and size distribution of the copolymer micelles. The copolymer with the longest PBA block had the poorest water solubility and consequently formed micelles with larger size while having a lower cmc. The "non-surface activity" was confirmed for cationic amphiphilic diblock copolymers in addition to anionic ones studied previously, indicating the universality of non-surface activity nature.

Interaction between zwitterionic and anionic surfactants: spontaneous formation of zwitanionic vesicles

The physicochemical properties, such as critical micelle concentration (cmc), surface tension at cmc (γ(cmc)), and surface activity parameters of the mixtures of a new amino acid-based zwitterionic surfactant, N-(n-dodecyl-2-aminoethanoyl)-glycine (C(12)Gly) and an anionic surfactant, sodium dodecyl sulfate (SDS) at different molar fractions, X(1) (= [C(12)Gly]/([C(12)Gly] + [SDS])) of C(12)Gly were studied. A synergistic interaction was observed between the surfactants in mixtures of different X(1). The self-organization of the mixtures at different molar fractions, concentrations, and pH was investigated. Fluorescence depolarization studies in combination with dynamic light scattering, and transmission electron microscopic and confocal fluorescence microscopic images suggested the formation of bilayer vesicles in dilute solutions of SDS rich mixtures with X(1) ≤ 0.17 in the pH range 7.0 to 9.0. However, the electronic micrographs showed structures with fingerprint-like texture in moderately dilute to concentrated C(12)Gly/SDS mixture at X(1) = 0.50. The vesicles were observed to transform into small micelles upon lowering the solution pH and upon increase of total surfactant concentration in mixtures with X(1) ≤ 0.17. However, decrease of SDS content transformed vesicles into wormlike micelles. The structural transitions were correlated with bulk viscosity of the binary mixtures.

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.

Effect of hydrogen bonding on the physicochemical properties and bilayer self-assembly formation of N-(2-hydroxydodecyl)-L-alanine in aqueous solution

The aggregation behavior of N-(2-hydroxydodecyl)-L-alanine (C12HAla) and N-(n-dodecyl)-L-alanine (C12Ala) was studied in aqueous buffer (pH 12) over a concentration range above their critical aggregation concentration (cac). The C12HAla amphiphile has two cacs in contrast to only one cac value for C12Ala. The micropolarity and microviscosity of the aggregates were studied by use of pyrene and 1,6-diphenyl-1,3,5-hexatriene, respectively, as fluorescent probes. Dynamic light scattering was used to measure the average hydrodynamic diameter and size distribution of the aggregates. Large size, high microviscosity, and low micropolarity values of the aggregates suggested the formation of bilayer structures in dilute solutions of C12HAla. In contrast, C12Ala was observed to form micelles. Transmission electron micrographs of dilute and moderately concentrated solutions of C12HAla revealed the existence of spherical vesicles and branching tubular structures, respectively. Comparison of the aggregation behavior of these amphiphiles to that of C12Ala and the FT-IR spectrum suggested that intermolecular hydrogen-bonding interactions between adjacent hydrocarbon chains through the -OH and -NH- groups of C12HAla are responsible for bilayer formation. The mechanism of nanotube formation was discussed. The temperature dependence of aggregate formation of the amphiphile also was investigated.