Femtosecond study of ultrafast fluorescence resonance energy transfer in a catanionic vesicle

Ultrafast fluorescence resonance energy transfer (FRET) in a catanionic [sodium dodecyl sulfate (SDS)-dodecyltrimethyl ammonium bromide (DTAB)] vesicle is studied by femtosecond up-conversion. The vesicles (diameter ～400 nm for SDS-rich and ～250 nm for DTAB-rich vesicles) are much larger than the SDS and DTAB micelles (diameter ～4 nm). In both micelle and vesicles, FRET occurs in multiple time scales and the time scales of FRET correspond to a donor-acceptor distance varying between 12 and 36 A?.     

Electrokinetic chromatographic characterization of novel catanionic surfactants vesicle as pseudostationary phase

A novel catanionic surfactants vesicle system composed of octyltriethylammonium bromide/ sodium dodecyl benzene sulfonate (C₈NE₃Br/SDBS) has been developed as pseudostationary phase (PSP) in EKC. The C₈NE₃Br/SDBS system possesses a large vesicle phase region and none of agglomeration phenomena appeared while mixing cationic and anionic surfactants at any molar ratio. Electrophoretic and chromatographic parameters including elution window, hydrophobic selectivity, polar group selectivity, and shape selectivity were characterized using the vesicle at molar ratio of C₈NE₃Br to SDBS of 3:7 as PSP. Compared with SDS micelles, the vesicle PSP possessed a wider elution window and a better selectivity. The retention behavior and selectivity differences between the novel vesicle and SDS micelles were evaluated through linear solvation energy relationship (LSER) analysis. Though the cohesiveness and the hydrogen bond acidity have greatest influences on the solutes retention and selectivity in both the vesicle and SDS micelle, the vesicle PSP demonstrated a higher hydrophobicity and a lower hydrogen bonding donating capability owing to compact bilayer structure of vesicle. Additionally, the vesicle system had a stronger hydrogen bond accepting capability than SDS micelle. Consequently, according to LSER analysis, the bigger coefficients for v, b, and a revealed the vesicle PSP had a better separation selectivity than conventional SDS micelle.     

Fluorescence Dynamics of Coumarin C522 as a Function of Micelle Confinement along with Cyclodextrin Supramolecular Complex Formation

Our aim is to doubly confine a molecule of coumarin C522 in a host–guest supramolecular complex with β‐cyclodextrin in a reverse sodium dioctyl sulfosuccinate (AOT) micelle using nonpolar n‐heptane and polar water solvents. Varying the volumes of coumarin C522 and β‐cyclodextrin dissolved in water allows us to control the water‐pool diameters of the reverse micelle in n‐heptane with values of w=3, 5, 10, 20, and 40, where w is the ratio of water concentration to AOT concentration in n‐heptane. To study the fluorescence dynamics of coumarin C522, the spectral steady‐state and time‐resolved dependences are compared for the two systems coumarin C522(water)/AOT(n‐heptane), denoted C522/micelle, and coumarin C522/β‐cyclodextrin(water)/AOT(n‐heptane), referred to as C522/CD/micelle. The formation of the supramolecular host–guest complex CD–C522 is indicated by a blue shift, but in the micelle, the shift is red. However, the values of the fluorescence maxima at 520 and 515 nm are still way below the value of 535 nm representing bulk water. The interpretation of the red shift is based on two complementary processes. The first one is the confinement of CD and C522 by the micelle water pool and the second is the perturbation of the micelle by CD and C522, resulting in an increase of the water polarity. The fluorescence spectra of the C522/micelle and C522/CD/micelle systems have maxima and shoulders. The shoulder intensities at 440 nm, representing the C522 at n‐heptane/AOT interface, decrease as the w values decrease. This intensity shift suggests that the small micelle provides a stronger confinement, and the presence of CD shifts the equilibrium from n‐heptane towards the water pool even more. The fluorescence emission maxima of the C522/micelle and C522/CD/micelle systems for all w values clearly differentiate two trends for w=3–5, and w=10–40, suggesting different interaction in the small and large micelles. Moreover, these fluorescence maxima result in 7 and 13 nm differences for w=3 and w=5, respectively, and provide the spectral evidence to differentiate the C522 confinement in the C522/micelle and C522/CD/micelle systems as an effect of the CD molecule, which might be interpreted as a double confinement of C522 in CD within the micelle. The ultrafast decay in the case of w=3 ranges from 9.5 to 16 ps, with an average of 12.6 ps, in the case of the C522/micelle system. For C522/CD/micelle, the ultrafast decay at w=3 ranges from 9 to 14.5 ps, with an average of 11.8 ps. Increasing w values (from 10 to 40) result in a decrease of the ultrafast decay values in both cases to an average value of about 6.5 ps. The ultrafast decays of 12.6 and 11.8 ps for C522/micelle and C522/CD/micelle, respectively, are in the agreement with the observed red shift, supporting a double confinement in the C522/CD/micelle(w=3) system. The dynamics in the small and large micelles clearly show two different trends. Two slopes in the data are observed for w values of 3–5 and 10–40 in the steady‐state and time‐resolved data. The average ultrafast lifetimes are determined to be 12.6 and 6.5 ps for the small (w=3) and the large (w=40) micelles, respectively. To interpret the experimental solvation dynamics, a simplified model is proposed, and although the model involves a number of parameters, it satisfactory fits the dynamics and provides the gradient of permittivity in the ideal micelle for free water located in the centre (60–80) and for bound water (25–60). An attempt to map the fluorescence dynamics of the doubly confined C522/CD/micelle system is presented for the first time.     

Micelle-vesicle transitions in catanionic mixtures of SDS/DTAB induced by salt,temperature, and selective solvents: a dissipative particle dynamics simulation study

Dissipative particle dynamics (DPD) simulations are performed to study the factors that lead to the transition between micelle and vesicle in catanionic mixtures composed of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB), with the aim of understanding and controlling the structures of this system. The phase behavior, kinetics of vesicle formation, and micelle–vesicle transitions induced by salt, temperature, and selective solvents are investigated systematically. In this research, phase diagram of SDS/DTAB mixture is constructed by simulations at different concentrations and composition fractions. It is consistent with experimental results. The kinetic process of catanionic vesicle formation is illustrated. It is clarified that the transition between micelle and vesicle can be controlled by properly adjusting the external conditions. More interestingly, the evolution condition and transition mechanism between micelle and vesicle induced by various conditions are revealed. The membrane thickness differences between vesicles formed at different external conditions are compared. Here, the predicted phenomenon is compared with experimental results whenever possible, and we try to make a connection between the simulation model and the reality of the experiments. These studies help to shed light on the microscopic details of micelle–vesicle transition in catanionic mixtures.     

Ultrafast photoinduced electron transfer from dimethylaniline to coumarin dyes in sodium dodecyl sulfate and triton X-100 micelles

The primary steps of photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to five coumarin dyes are studied in an anionic micelle [sodium dodecyl sulfate (SDS)] and a neutral micelle [triton X-100 (TX-100)] using femtosecond upconversion. The rate of PET in micelle is found to be highly nonexponential. In both the micelles, PET displays components much faster (approximately 10 ps) than the slow components (180-2900 ps) of solvation dynamics. The ultrafast components of electron transfer exhibit a bell-shaped dependence on the free energy change. This is similar to Marcus inversion. The rates of PET in TX-100 and SDS micelle are, in general, faster than those in cetyltrimethylammonium bromide (CTAB) micelle. In the SDS and TX-100 micelle, the Marcus inversion occurs at -DeltaG0 approximately 0.7 eV which is lower than that (approximately 1.2 eV) in CTAB micelle. Possible causes of variation of PET in different micelles are discussed.     

Ultrafast FRET to Study Spontaneous Micelle‐to‐Vesicle Transitions in an Aqueous Mixed Surface‐Active Ionic‐Liquid System

The spontaneous micelle‐to‐vesicle transition in an aqueous mixture of two surface‐active ionic liquids (SAILs), namely, 1‐butyl‐3‐methylimidazolium n‐octylsulfate ([C₄mim][C₈SO₄]) and 1‐dodecyl‐3‐methylimidazoium chloride ([C₁₂mim]Cl) is described. In addition to detailed structural characterization obtained by using dynamic light scattering, transmission electron microscopy (TEM), and cryogenic TEM techniques, ultrafast fluorescence resonance energy transfer (FRET) from coumarin 153 (C153) as a donor (D) to rhodamine 6G (R6G) as an acceptor (A) is also used to study micelle–vesicle transitions in the present system. Structural transitions of SAIL micelles ([C₄mim][C₈SO₄] or [C₁₂mim]Cl micelles) to mixed SAIL vesicles resulted in significantly increased D –A distances, and therefore, increased timescale of FRET. In [C₄mim][C₈SO₄] micelles, FRET between C153 and R6G occurs on an ultrafast timescale of 3.3 ps, which corresponds to a D –A distance of about 15 Å. As [C₄mim][C₈SO₄] micelles are transformed into mixed micelles upon the addition of a 0.25 molar fraction of [C₁₂mim]Cl, the timescale of FRET increases to 300 ps, which suggests an increase in the D –A distance to 31 Å. At a 0.5 molar fraction of [C₁₂mim]Cl, unilamellar vesicles are formed in which FRET occurs on multiple timescales of about 250 and 2100 ps, which correspond to D –A distances of 33 and 47 Å. Although in micelles and mixed micelles the obtained D –A distances are well correlated with their radius, in vesicles the obtained D –A distance is within the range of the bilayer thickness.     

Excitation wavelength dependence of solvation dynamics in a supramolecular assembly: PEO-PPO-PEO triblock copolymer and SDS

The triblock copolymer (PEO)20-(PPO)70-(PEO)20 (P123) forms a supramolecular aggregate with sodium dodecyl sulfate (SDS). The solvation dynamics and anisotropy decay of coumarin 480 (C480) in different regions of a P123-SDS aggregate are studied through variation of the excitation wavelength (lambdaex) using femtosecond upconversion. In a P123 micelle, because of the drastic differences in polarity between the hydrophilic corona region (PEO block) and the hydrophobic PPO core, C480 exhibits a pronounced red edge excitation shift (REES) of emission maximum by 24 nm. In the P123-SDS aggregate, SDS penetrates the core of the P123 micelle. This increases the polarity of the core and reduces the difference in the polarity between the core and the corona region. In a P123-SDS aggregate, the REES is much smaller (5 nm) which suggests a reduced difference between the core and the corona. Solvation dynamics in a P123 micelle displays a bulklike ultrafast component (<0.3 and 1 ps) in the PEO corona region, a 200 ps component arising from dynamics of polymer segments, and a very long component (5000 or 3000 ps) due to the highly restricted PPO core. In a P123-SDS aggregate, at lambdaex = 375 and 405 nm, the solvation dynamics is found to be faster than that in P123 micelle. In this case, the component (3000 ps) arising from the core region is faster than that (5000 ps) in P123 micelle. In both P123 micelle and P123-SDS aggregate, the relative contribution of the core region decreases and that of the corona region increases with an increase in lambdaex. At lambdaex = 435 nm, which probes the hydrophilic corona, the solvation dynamics for both P123 micelle and P123-SDS aggregate are almost similar.     

Vesicle formation induced by layered double hydroxides in the catanionic surfactant solution composed of sodium dodecyl sulfate and dodecyltrimethylammonium bromide

In this paper, it is reported that positively charged Mg₃Al layered double hydroxide (LDH) nanoparticles can induce the spontaneous formation of vesicles in micelle solution of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) with a mass ratio of 8:2. The formation of vesicles was demonstrated by negative-staining transmission electron microscopy observations. The size of the vesicles increased with the increase in the concentration of Mg₃Al-LDH nanoparticles. A composite of LDH nanoparticles encapsulated in vesicles was formed. A possible mechanism of LDH-induced vesicle formation was suggested. The positively charged LDH surface attracts negatively charged micelles or free amphiphilic molecules, which facilitates their aggregation into bilayer patches. These bilayer patches connect to each other and finally close to form vesicles. It was also found that an adsorbed compound layer of SDS and DTAB micelles or molecules on the LDHs surface played a key role in vesicle formation.     

Micellar Behavior of Polystyrene-Poly(Ethylene Oxide) Diblock Copolymers in Aqueous Media: Effect of Copolymer Composition,Temperature, Salt,and Surfactants

Formation and structure of micelles from two amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers (PS mol.wt. 1000; PEO mol.wt. 3000 and 5000) were examined by surface tension, viscosity, steady state fluorescence, dynamic light scattering (DLS), small angle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). The critical micelle concentration (CMC) of the copolymers in aqueous solution was ca. 0.05%; micelle hydrodynamic diameter was 30–35 nm with a narrow size distribution. SANS studies show that the copolymers form ellipsoidal micelles with semi major axis ～23 nm and semi minor axis ～8 nm. No significant change in the structure was found with temperature and presence of salt. The copolymer micelles interaction with the ionic surfactants sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DTAB) was also examined by DLS and SANS.     

The pH induced vesicle to micelle morphology transition of a THP‐protected polymer

A diblock copolymer consisting of tetrahydropyranyl acrylate (THPA) as a pH‐deprotectable block, and a permanently hydrophobic block, methyl acrylate, was synthesized by RAFT polymerization using a quaternary amine functionalized, hydrophilic, RAFT chain transfer agent. The polymer self‐assembled in water to form vesicles with D_h = 130 nm, as determined by DLS and cryogenic transmission electron microscopy. Acid catalyzed deprotection of the THPA units to yield acrylic acid resulted in a vesicle to micelle morphology transition, as evidenced by the decrease in hydrodynamic diameter to D_h = 19 nm and the observation of micelles by dry state transmission electron microscopy. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3026–3031     

Solvation dynamics in ionic liquid swollen p123 triblock copolymer micelle: a femtosecond excitation wavelength dependence study

Femtosecond solvation dynamics of coumarin 480 (C480) in a mixed micelle is reported. The mixed micelle consists of a triblock copolymer (PEO)20-(PPO) 70-(PEO)20 (Pluronic P123) and an ionic liquid (IL), 1-pentyl-3-methylimidazolium tetrafluoroborate ([pmim][BF4]). At a low concentration (0.3 M), the sparingly water soluble IL ([pmim][BF4]) penetrates the hydrophobic PPO core of the P123 micelles. Thus emission maximum of C480 in the core (accessed at lambdaex=375 nm) in 0.3 M IL is red-shifted by 8 nm from that in its absence and the red edge excitation shift (REES) is large (19+/-1 nm). At a high concentration (0.9 M), the ionic liquid [pmim][BF4] invades both the core and corona region and the mixed micelle exhibits very small REES (3+/-1 nm). Anisotropy decay and solvation dynamics in different regions of the mixed micelle are studied by variation of excitation wavelength (lambda ex). In P123 micelle, the average rotational time () is 2800 ps in the core (at lambdaex=375 nm) and 1350 ps in the corona region (at lambdaex=435 nm). In 0.3 M [pmim][BF4], tau rot at the core of the mixed micelle decreases to 1950 ps while that in the corona remains unaffected. In 0.9 M IL, both the core and corona (lambda ex=375 and 435 nm) exhibit similar and short approximately 600 ps. In 0.3 M IL, solvation dynamics in the core region (lambdaex=375 nm) of P123 micelle is about 2 times faster than in its absence. In 0.3 M IL, solvation dynamics in the corona region (lambdaex=435 nm) is approximately 100 times faster than that in the core. In 0.9 M IL, the solvation dynamics in the core and in the corona is, respectively, approximately 9 times and 4 times faster than that in 0.3 M IL.     

Small-angle neutron scattering study of the effect of n -alkanols on interacting micelles

Small-angle neutron scattering cross-section distributions of sodium dodecyl sulphate (SDS) and dodecyl trimethyl ammonium bromide (DTAB), each 0·3 M in D₂O were obtained in the absence and presence of 0·1 M 1-pentanol, 1-hexanol, and 1-octanol at 25°C. The Hayter-Penfold type analysis was adopted. An ellipsoidal model with semiminor axis (a=16·5 ?) and semimajor axes (b=40·7 ? and 29·8 ?) for pure SDS and DTAB micelles has produced best fits. On increasing alkanol chain lengths an increase inb values was found. Micellar parameters like effective radius (R), (a, b), fraction of counterions per micelle, and intermicellar distances were obtained. Surfactant aggregation number, additive aggregation number intermicellar interaction potentials and values of Debye screening length were obtained for SDS and DTAB in the presence of alkanols. Implications of partitioning effect, surfactant ionicity and intermicellar potentials on the microstructures are rationalised.     

Micelle-to-vesicle transition induced by organic additives in catanionic surfactant systems

A micelle-to-vesicle transition (MVT) induced by the addition of a series of apolar hydrocarbons (n-butylbenzene, n-hexane, n-octane, and n-dodecane) to the catanionic surfactant system n-dodecyltriethylammonium bromide/sodium n-dodecylsulfate (DTEAB/SDS) has been investigated for the first time by means of rheology and turbidity measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Interestingly, a MVT can take place within certain micellar regions, which are dependent on the structure and chain length of the hydrocarbon. However, these hydrocarbons are unable to induce a MVT in another catanionic surfactant system, namely, n-dodecyltriethylammonium bromide/sodium n-dodecylsulfonate (DTEAB/SDSO(3)), in which the molecular interactions are weaker than in the DTEAB/SDS system. On the other hand, polar additives, such as n-octanol and n-octylamine, exhibit much higher efficiency and activity in inducing MVT than hydrocarbons in both DETAB/SDS and DTEAB/SDSO(3). Moreover, DLS, TEM, and time-resolved fluorescence quenching (TRFQ) results demonstrate that the ratio of vesicles to micelles in the system can be actively controlled by addition of polar additives. Possible mechanisms for the above phenomena are presented, and the potential application of controllable micelle/vesicle systems in the synthesis of tailored bimodal mesoporous materials is discussed.     

Ultrafast fluorescence resonance energy transfer in a bile salt aggregate: Excitation wavelength dependence

Fluorescence resonance energy transfer (FRET) from Coumarin 153 (C153) to Rhodamine 6G (R6G) in a secondary aggregate of a bile salt (sodium deoxycholate, NaDC) is studied by femtosecond up-conversion. The emission spectrum of C153 in NaDC is analysed in terms of two spectra-one with emission maximum at 480 nm which corresponds to a non-polar and hydrophobic site and another with maximum at ∼530 nm which arises from a polar hydrophilic site. The time constants of FRET were obtained from the rise time of the emission of the acceptor (R6G). In the NaDC aggregate, FRET occurs in multiple time scales — 4 ps and 3700 ps. The 4 ps component is assigned to FRET from a donor (D) to an acceptor (A) held at a close distance (R _DA ∼ 17 ?) inside the bile salt aggregate. The 3700 ps component corresponds to a donor-acceptor distance ∼48 ?. The long (3700 ps) component may involve diffusion of the donor. With increase in the excitation wavelength (λ _ex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (∼4 ps) increases from 3 to 40% with a concomitant decrease in the contribution of the ultraslow component (∼3700 ps) from 97 to 60%. The λ _ex dependence is attributed to the presence of donors at different locations. At a long λ _ex (435 nm) donors in the highly polar peripheral region are excited. A short λ _ex (375 nm) ‘selects’ donor at a hydrophobic location.     

Reversible Switching of a Micelle‐to‐Vesicle Transition by Compressed CO2

The study of the micelle‐to‐vesicle transition (MVT) is of great importance from both theoretical and practical points of view. Herein, we studied the effect of compressed CO₂ on the aggregation behavior of dodecyltrimethylammonium bromide (DTAB)/sodium dodecyl sulfate (SDS) mixed surfactants in aqueous solution by means of direct observation, turbidity and conductivity measurements, steady‐state fluorescence, time‐resolved fluorescence quenching (TRFQ), fluorescence quantum yield, and template methods. Interestingly, all these approaches showed that compressed CO₂ could induce the MVT in the surfactant system, and the vesicles returned to the micelles simply by depressurization; that is, CO₂ can be used to switch the MVT reversibly by controlling pressure. Some other gases, such as methane, ethylene, and ethane, could also induce the MVT of the surfactant solution. A possible mechanism is proposed on the basis of the packing‐parameter theory and thermodynamic principles. It is shown that the mechanism of the MVT induced by a nonpolar gas is different from the MVT induced by polar and electrolyte additives.     

Distribution of the methyl radical adduct of N‐benzylidene‐1‐diethoxyphosphoryl‐1‐methylethylamine N‐oxide‐type nitrones in water–sodium dodecyl sulfate micelles: an electron paramagnetic resonance spin trapping study

The behavior of the methyl radical adduct of six β‐phosphorylated nitrones in the N‐benzylidene‐1‐diethoxyphosphoryl‐1‐methylethylamine N‐oxide series in the presence of sodium dodecyl sulfate (SDS) micelles was followed by electron paramagnetic resonance spectroscopy. Except when the highly hydrophilic trap 4‐PyOPN (2) was used, all the adducts were found to partition significantly between micelles and the bulk aqueous phase. The average correlation time τ of the exchange of spin adducts between SDS micelles and water was found to be in the range 5 × 10^?8—4 × 10^?7 s, which is in the region of the life time of an SDS monomer in the micelle structure. In each case, the adduct affinity for the micelles has been quantified by evaluating its micelle–water distribution coefficient K_d. Copyright © 2002 John Wiley & Sons, Ltd.     

A femtosecond study of excitation wavelength dependence of solvation dynamics in a PEO-PPO-PEO triblock copolymer micelle

Excitation wavelength (lambdaex) dependence of solvation dynamics of coumarin 480 (C480) in the micellar core of a water soluble triblock copolymer, PEO20-PPO70-PEO20 (Pluronic P123), is studied by femtosecond and picosecond time resolved emission spectroscopies. In the P123 micelle, the width of the emission spectrum of C480 is found to be much larger than that in bulk water. This suggests that the P123 micelle is more heterogeneous than bulk water. The steady state emission maximum of C480 in P123 micelle shows a significant red edge excitation shift by 25 nm from 453 nm at lambdaex=345 nm to 478 nm at lambdaex=435 nm. The solvation dynamics in the interior of the triblock copolymer micelle is found to depend strongly on the excitation wavelength. The excitation wavelength dependence is ascribed to a wide distribution of locations of C480 molecules in the P123 micelle with two extreme environments-a bulklike peripheral region with very fast solvent response and a very slow core region. With increase in lambdaex, contribution of the bulklike region having an ultrafast component (< or =2 ps) increases from 7% at lambdaex=375 nm to 78% at lambda(ex)=425 nm while the contribution of the ultraslow component (4500 ps) decreases from 79% to 17%.     

Electrochemical Behavior of Cationic‐Anionic Surfactant Solutions by Cyclic Voltammetry

The electrochemical behavior of cationic tetradecyltrimethylammonium bromide (TTABr), anionic sodium dodecylsulfate (SDS), cationic‐anionic (catanionic) mixed surfactant and self‐assembled solutions at Pt wire electrode has been studied by cyclic voltammetry (CV). On the basis of the cyclic voltammograms and determining the self‐assembled structures by using freeze‐fracture transmission electron microscopy (FF‐TEM), the mechanisms of the electrochemical reactions near the electrode for the two surfactant self‐assembled solutions, i.e., micelles and vesicles, are presented. When mixing TTABr and SDS, at the right mixing ratio of TTABr:SDS, vesicles are established spontaneously. The redox behavior of two vesicle‐phase solutions at a constant total concentration of 25 mmol·L^?1 with the ratios of TTABr:SDS 9.35:0.65 of positive charges of bilayer membranes and 1.25:8.75 of negative charges of bilayer membranes are investigated by cyclic voltammetry. These cyclic voltammograms of vesicles with different charges are compared with those of 100 mmol · L^?1 TTABr and 100 mmol · L^?1 SDS micelle solutions. This CV study on surfactant self‐assembled solutions should open up a new method of study in surfactant science.     

Synthesis and self‐assembling of poly(N‐isopropylacrylamide‐block‐poly(L‐lactide)‐block‐poly(N‐isopropylacrylamide) triblock copolymers prepared by combination of ring‐opening polymerization and atom transfer radical polymerization

Novel thermo‐responsive poly(N‐isopropylacrylamide)‐block‐poly(l ‐lactide)‐block‐poly(N‐isopropylacylamide) (PNIPAAm‐b‐PLLA‐b‐PNIPAAm) triblock copolymers were successfully prepared by atom transfer radical polymerization of NIPAAm with Br‐PLLA‐Br macroinitiator, using a CuCl/tris(2‐dimethylaminoethyl) amine (Me₆TREN) complex as catalyst at 25 °C in a N,N‐dimethylformamide/water mixture. The molecular weight of the copolymers ranges from 18,000 to 38,000 g mol^?1, and the dispersity from 1.10 to 1.28. Micelles are formed by self‐assembly of copolymers in aqueous medium at room temperature, as evidenced by ¹H NMR, dynamic light scattering (DLS) and transmission electron microscopy (TEM). The critical micelle concentration determined by fluorescence spectroscopy ranges from 0.0077 to 0.016 mg mL^?1. ¹H NMR analysis in selective solvents confirmed the core‐shell structure of micelles. The copolymers exhibit a lower critical solution temperature (LCST) between 32.1 and 32.8 °C. The micelles are spherical in shape with a mean diameter between 31.4 and 83.3 nm, as determined by TEM and DLS. When the temperature is raised above the LCST, micelle size increases at high copolymer concentrations due to aggregation. In contrast, at low copolymer concentrations, decrease of micelle size is observed due to collapse of PNIPAAm chains. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3274–3283     

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.