Cetyltrimethylammonium Chloride Micelles Research Articles

Tri-templating Synthesis of Multilevel Mesoporous Silica Microspheres with a Complex Interior Structure for Efficient CO2 Capture and Catalysis.

Multilevel porous architectures with microscopic shape control and tailor-made complex structures offer great potential for various innovative applications, but their elaborate design and synthesis have remained a scientific and technological challenge. Herein, we report a simple and effective tri-templating method, in which microscale Pickering droplets, nanoscale polystyrene colloids (PS), and molecular cetyltrimethylammonium chloride micelles are synchronously employed, for the fabrication of such micro-nanohierarchical mesoporous silica microspheres. In this protocol, Pickering droplet-directed interfacial sol-gel growth and its spatially confined surfactant assembly-directed sol-gel coating on PS suspensions are coupled together, enabling the successful formation of structured mesoporous silica that consists of numerous nanocompartments enclosed by a permeable shell. By varying the quantity of PS colloidal templates, rational regulation of the complex interior structure is achieved. Also, ascribed to the multilevel arrangement, this peculiar architecture not only shows desirable fast mass transport of external molecules but also possesses easy handling ability. After loading with tetraethylenepentamine or enzyme species, the yielded microspherical CO2 sorbents or immobilized biocatalysts, respectively, exhibit enhanced CO2 capture capacity and enzymatic catalysis efficiency. Notably, taking advantage of their microscopic characteristics, the immobilized biocatalysts could be ideally packed in a fixed-bed reactor for long-term continuous-flow enzymatic reactions. This tri-templating strategy provides a new synthetic route to access other multilevel microscopic materials with fascinating complex structures and paves a way to promote their practical applications.

Surfactant desorption and scission free energies for cylindrical and spherical micelles from umbrella-sampling molecular dynamics simulations

HypothesisThe free energies associated with adsorption/desorption of individual surfactants from micelles and the fusion/scission of long micelles can be used to estimate the rate constants for micellar kinetics as functions of surfactant and salt concentration. ExperimentsWe compute the escape free energies △Gesc of surfactant from micelles and the scission free energies △Gsciss of long micelles from coarse-grained molecular dynamics simulations coupled with umbrella sampling, for micelles of both sodium dodecylsulfate (SDS) in sodium chloride (NaCl) and cetyltrimethylammonium chloride (CTAC) in sodium salicylate (NaSal). FindingsFor spherical micelles, △Gesc values have maxima at certain aggregation numbers, and at salt-to-surfactant molar concentration ratios R near unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in △Gesc, both at R ~ 0.7, while △Gsciss of CTAC micelles also peaks at around R ~ 0.7 and that of SDS micelles increases monotonically with R. We explain the non-monotonic dependence of escape and scission free energies on R by a combination of electrostatic screening and the decrease of micelle radius with increasing R. Transitions from predominantly spherical to cylindrical micelles, and between adsorption/desorption and fusion/scission kinetics with changing salt concentration can be inferred from the free energies for CTAC/NaSal.

Fluorescence quenching of chloroquine by Cu2+ in micelles

Abstract Fluorescence quenching, very often, has been employed as a suitable technique for characterization of the immediate environment of a fluorophore. This paper reports the absorption and fluorescence characteristics of an antimalarial drug, chloroquine, in presence of the salt copper chloride (CuCl2) in aqueous and micellar systems. Chloroquine fluorescence was quenched considerably by the copper ion (Cu2+); quenching ability of Cu2+ in micellar solution of the cationic surfactant cetyltrimethylammonium chloride (CTAC) is higher than that in water while for the nonionic surfactant polyoxyethylene (20) cetyl ether (Brij 58) it has much lower quenching efficiency. Downward deviations in Stern-Volmer (SV) plots were observed for all the systems. Possibly, the chloroquine molecules are inserted somewhat deeper inside the micelles of Brij 58 as compared to the CTAC micelles and in Brij 58 micelles the fluorophore might be less exposed to the aqueous phase to become available for the metal ion quenching.

Assembly of Lyotropic Liquid Crystals with Solid Crystal's Structural Order Translated from the Lipid Rafts in Cell Membranes.

Self-assembly offers a powerful way to control the complexity and hierarchy of nanoscale materials, and promises to create a diverse range of emergent properties. Successful syntheses that allow a delicate structural design of building units play an important role. However, as can be learned from many cellular processes and functions, coself-assembly using logically chosen additives should be equally effective in designing self-assembly. Herein I show that, translated from the dynamic nanoscale assemblies in cell membranes known as lipid rafts, coself-assembly of 1-decanol into cetyltrimethylammonium chloride micelles for the assembly of lyotropic liquid crystals generates new structural complexity and hierarchy, and a surprising property that is emerging from it. Designing the intermolecular forces in the way that cholesterol interacts with sphingolipids promotes the synergistic balance between the flexibility and rigidity, and the unique molecular recognition for silicic acid, followed by the micelle coalescence. This very much resembles the assembly process of the lipid rafts in cell membranes and triggers orders of magnitude of sharp increases in X-ray diffraction intensity. The analysis of the diffraction patterns shows that the structural order of these liquid crystals matches that of solid crystals, often of single crystals. Furthermore, the assembly of the liquid crystals promotes a substantial increase in the condensation rate of silicic acids by guiding them to form a silicate trimer along the surface of micelles. This very much resembles the role of the lipid rafts that sharply increases the reaction rate of biomolecules by guiding them to form discrete species along the surface of membranes. This finding demonstrates that it is possible to translate the key features of cellular processes and functions into artificial self-assembling systems of our choice using the building units that are readily available, thus creating novel soft materials.

Formation of Nanogrooves with Sub-5 nm Periodicity Using Local Silicification at the Interspace between a Si Substrate and Lyotropic Liquid Crystals

Bottom-up fabrication of nanopatterns with single nanometer-scale periodicity is quite challenging. In this study, we have focused on the use of the outermost convex surfaces of lyotropic liquid crystals (LLCs) as a template. Periodically arrayed single nanometer-scale nanogrooves consisting of silica are successfully formed on a Si substrate covered with LLCs composed of cylindrical micelles of cetyltrimethylammonium chloride. Soluble silicate species are generated from the Si substrate by a treatment with an NH3-water vapor mixture, infilling the interspaces between the Si substrate and the LLCs. The cross section of the nanogrooves has a symmetrical sawtooth-like profile with a periodicity of 4.7 nm, and the depth of each nanogroove is around 2 nm. Uniaxial alignment of the nanogrooves can be achieved using micrometer-scale grooves fabricated by a focused ion beam technique. Although formed nanogrooves contain defects, such as bends and discontinuities, this successful concept provides a novel fabrication method of arrayed concave patterns with sub-5 nm periodicity on the surfaces of Si substrates.

Self-Assembly of Nanoparticle-Surfactant Complexes with Rodlike Micelles: A Molecular Dynamics Study.

The self-assembly of nanoparticles (NPs) with cationic micelles of cetyltrimethylammonium chloride (CTAC) is known to produce stable nanogels with rich rheological and optical properties. Coarse-grained molecular dynamics (MD) simulations are performed to explore the molecular mechanisms underlying this self-assembly process. In an aqueous solution of CTAC surfactants, a negatively charged NP with a zeta potential of less than -45 mV is observed to form a stable vesicular structure in which the particle surface is almost entirely covered with a double layer of surfactants. In comparison, surfactants form a monolayer, or a corona, around an uncharged hydrophobic NP with the tailgroups physically adsorbed onto the particle. In the presence of sodium salicylate salt, such NP-surfactant complexes (NPSCs) interact with rodlike CTAC micelles, resulting in the formation of stable junctions through the opening up of the micelle end-cap followed by surfactant exchange, which is diffusion-limited. The diffusive regime spans several hundred nanoseconds, thereby necessitating MD simulations over microsecond time scales. The energetics of NPSC-micelle complexation is analyzed from the variation in the total pair-potential energy of the structures.

Uniaxial Extension of Surfactant Micelles: Counterion Mediated Chain Stiffening and a Mechanism of Rupture by Flow-Induced Energy Redistribution

We study the configurational dynamics in uniaxial elongational flow of rodlike and U-shaped cationic surfactant micelles of cetyltrimethylammonium chloride (CTAC) in the presence of sodium salicylate (NaSal) counterions in water using molecular dynamics simulations. Above the critical strain rate, approximately equal to the inverse of the micelle relaxation time, hydrodynamic forces overcome the conformational entropy of the micelle and a configurational transition from a folded to a stretched state occurs. As the accumulated strain exceeds a critical value of O(100), the micelle ruptures through a midplane thinning mechanism facilitated by the advection of the counterions toward the micelle end-caps. The change in the total pair-potential energy as a function of micelle elongation is well described by a Hookean spring model that allowed to estimate the stretching modulus of the micelle. Micelle stiffness depends greatly on the degree of screening of electrostatic repulsion among the CTA+ head groups by t...

A Simple Method for Nanostructure Engineering of Mesoporous Zinc Silicate Particles

We report a novel method to engineer a nanostructure of zinc silicate particles. In this method, a mixture of tetraethoxysilane, zinc acetate, and cetyltrimethylammonium chloride (CTAC) was reacted in water at 80 °C for ∼3 h, followed by calcination. This method produced mesoporous zinc silicate particles with a core–shell structure in which the core contained a mixed oxide of ZnO and SiO2, whereas the shell was pure SiO2. We found a faster formation of mixed oxide than pure SiO2, which is believed to be responsible for the core–shell structure. On the basis of this understanding, we engineered the nanostructure of the synthesized particles: (1) zinc oxide in the core was dissolved by citrate buffer to produce hollow mesoporous silica particles, and (2) a layer-by-layer deposition technique was used to grow mesoporous shells on the existing particles, producing multishell mesoporous particles with various morphologies. Using a nitrogen sorption method, the average pore diameter of mesoporous zinc silicate particles was found to be 3.4 nm, which is similar to the diameter of spherical CTAC micelles. We also tested the adsorption capacity of hollow mesoporous silica particles using water-soluble anionic (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt) and cationic (rhodamine B) dyes, and we found a high adsorption capacity for the cationic dye but negligible adsorption for the anionic dye. Finally, we compared release profiles of rhodamine B from hollow mesoporous silica particles with different morphologies.

Effects of CTAC micelles on the molecular structures and separation performance of thin-film composite (TFC) membranes in forward osmosis processes

In this article, cetyltrimethylammonium chloride (CTAC) was used to produce novel thin-film composite (TFC) forward osmosis (FO) membranes through interfacial polymerization with 1,3,5-benzenetricarbonyl trichloride (TMC) and m-phenylenediamine (MPD). The effects of the CTAC concentrations on the structure of the polyamide skin layer were investigated by ATR–FTIR, XPS, WAXD, NMR and SEM analyses. FO performance of TFC membranes was detected using 2M each of four different salt solutions (NaCl, Na2SO4, MgCl2 and MgSO4) as draw solutions. The results indicate that the skin layer is significantly affected by the CTAC micelles. CTAC micelles in aqueous solution have ionic interactions with MPD molecules and affect interfacial polymerization process of MPD and TMC. With an increase of the CTAC concentration, the linear molecular structure of the skin layer gradually increases and the more microcrystalline structure appears. The pure water flux of the TFC FO membranes has been negatively affected in FO process by the special polyamide structures as a result of CTAC; however, the rates of rejection and reverse selectivity of salts are significantly improved with the increase of the CTAC concentrations (maximum 99.9% of MgCl2), and they are all greater than 90% (MgSO4>MgCl2>Na2SO4>NaCl).

Effect of NaCl on ESPT‐Mediated FRET in a CTAC Micelle: A Femtosecond and FCS Study

Femtosecond upconversion, single-molecule fluorescence resonance energy transfer (sm-FRET) and fluorescence correlation spectroscopy (FCS) are applied to study the competition between excited-state proton transfer (ESPT) and FRET [to rhodamine 6G (R6G)] of 8-hydroxypyranine-1,3,6-trisulfonate (HPTS) in cetyltrimethylammonium chloride (CTAC) micelles. Pyranine exhibits dual emission at λ(em)=430 nm for ROH and 520 nm for RO(-). The absorption spectrum of R6G (acceptor) has very good overlap with the RO(-) emission and poor overlap with ROH emission. It is observed that FRET occurs readily from the RO(-)* state of HPTS (donor) to R6G (acceptor). Multiple timescales of FRET were detected from the rise time of acceptor emission. The different timescales correspond to different donor-acceptor distances. The ultrafast components (8.5 and 13 ps) are assigned to FRET at a close contact of donor and acceptor (≈20 Å). The longer components (500 and 800 ps) arise from long-distance FRET from the donor to the acceptor (≈40 Å) located in different regions of the CTAC micelle. The larger donor-acceptor distances agree with those obtained from an sm-FRET study. On addition of 4 M NaCl to CTAC, the rate of proton transfer (k(PT)) slowed by about eight and two times, respectively, for the fast and slow sites of the CTAC micelle. As a result, the intensity of the ROH emission increases and that of RO(-) decreases. The decrease in the intensity of the RO(-) emission causes a decrease in the efficiency of FRET.

Conductivity and Surface Tension Study for Cucurbit[7]uril Inclusion Complex of Cetyltrimethylammonium Chloride in Aqueous Solution

Conductivity and surface tension measurements have been carried out at temperatures between 298.15 K for cetyltrimethylammonium Chloride (CTAC) + H2O and CTAC + Cucurbit[7]uril (CB[7]) + H2O systems. The apparent critical micelle concentrations, the dissociation degree of the micelle, the hydrophobic contribution of the transfer free energy for the hydrocarbon chain of CTAC, the stoichoimetry and inclusion constants for the inclusion complex of CTAC with CB[7] have been determined. The influence of CB[7] and its complex on the micellization processes of CTAC is analyzed. It is shown that CB[7] partly screened the hydrophobic hydrocarbon chain of CTAC molecules from contact with the surrounding medium, and retarded the formation of CTAC micelles in a certain extent. CB[7] and the inclusion complex do not have any surface activity. The CB[7] and its complexes do not participate the formation of micelles of CTAC, and the complex has no effect on the micelle properties once the micelles are formed. The result suggests that β-CB[7] forms strong complex with CTAC, and the stoichoimetry is found to be 1:1, and the inclusion constants of CB[7]-CTAC complex are almost the same in different CB[7] solutions.

Formation of micelles of cetyltrimethylammonium chloride and cetyltrimethylammonium bromide in N-methyl acetamide—alcohol and N,N-dimethyl acetamide—alcohol mixtures

The critical micelle concentrations of cetyltrimethylammonium chloride and cetyltrimethylammonium bromide in solutions of in N-methylacetami de and in N,N-dimethyl acetamide with added methanol, ethanol, n-propanol, n-butanol and n-pentanol were determined using electrical conductivity and surface tension measurements at various temperatures. Both methods show that micelles are formed in N-methyl acetamide and N, N-dimethyl acetamide solutions in a presence of n-alcohols. Critical micelle concentrations were also determined as functions of concentration of added alcohol. The data suggest that alcohol adding leads to an enhancement of penetration of alcohol into the micelle external shell that depends on the alcohol chain length. Thermodynamic parameters for micellar systems in a presence of n-alcohols were also calculated.

Minimal motif peptide structure of metzincin clan zinc peptidases in micelles

It is well known that the functions of metalloproteins generally originate from their metal-binding motifs. However, the intrinsic nature of individual motifs remains unknown, particularly the details about metal-binding effects on the folding of motifs; the converse is also unknown, although there is no doubt that the motif is the core of the reactivity for each metalloprotein. In this study, we focused our attention on the zinc-binding motif of the metzincin clan family, HEXXHXXGXXH; this family contains the general zinc-binding sequence His-Glu-Xaa-Xaa-His (HEXXH) and the extended GXXH region. We adopted the motif sequence of stromelysin-1 and investigated the folding properties of the Trp-labeled peptides WAHEIAHSLGLFHA (STR-W1), AWHEIAHSLGLFHA (STR-W2), AHEIAHSLGWFHA (STR-W11), and AHEIAHSLGLFHWA (STR-W14) in the presence and absence of zinc ions in hydrophobic micellar environments by circular dichroism (CD) measurements. We accessed successful incorporation of these zinc peptides into micelles using quenching of Trp fluorescence. Results of CD studies indicated that two of the Trp-incorporated peptides, STR-W1 and STR-W14, exhibited helical folding in the hydrophobic region of cetyltrimethylammonium chloride micelle. The NMR structural analysis of the apo STR-W14 revealed that the conformation in the C-terminus GXXH region significantly differred between the apo state in the micelle and the reported Zn-bound state of stromelysin-1 in crystal structures. The structural analyses of the qualitative Zn-binding properties of this motif peptide provide an interesting Zn-binding mechanism: the minimum consensus motif in the metzincin clan, a basic zinc-binding motif with an extended GXXH region, has the potential to serve as a preorganized Zn binding scaffold in a hydrophobic environment.

Effect of β-Cyclodextrin on the Micellization of Cetyltrimethylammonium in Aqueous Solution at High Temperature

Molar volume and conductivity measurements have been carried out at 338.2 K for cetyltrimethylammonium chloride (CTAC) + H2O and CTAC + β-cyclodextrin (β-CD) + H2O systems. The apparent critical micelle concentrations, the dissociation degree of the micelle, the transfer free energy for the hydrocarbon chain of CTAC, the standard partial molar volumes of CTAC in aqueous β-CD solution and the stoichoimetry for the inclusion complex of CTAC with β-CD have been determined. The influence of β-CD and its complex on the micellization processes of CTAC are analyzed under this temperature. It is shown that β-CD partly screened the hydrophobic hydrocarbon chain of CTAC molecules from contact with the surrounding medium, and retarded the formation of CTAC micelles in a certain extent. The thermodynamic activity of CTAC is decreased. The β-CD and its complexes do not participate the formation of micelles of CTAC, and the complex have no effect on the micelle properties once the micelles are formed. Based on a simple model, the number of CH2 groups entered the cavity of β-CD was calculated. The result suggests that β-CD forms strong complex with CTAC, and the stoichoimetry is found to be 2:1. This supports our conductivity results.

Molecular Dynamics Simulations of Threadlike Cetyltrimethylammonium Chloride Micelles: Effects of Sodium Chloride and Sodium Salicylate Salts

We use atomistic molecular dynamics simulations to probe the effects of added sodium chloride (NaCl) and sodium salicylate (NaSal) salts on the spherical-to-threadlike micelle shape transition in aqueous solutions of cetyltrimethylammonium chloride (CTAC) surfactants. Long threadlike micelles are found to be unstable and break into spherical micelles at low concentrations of NaCl, but remain stable for 20 ns above a threshold value of [NaCl] approximately 3.0 M, which is about 2.5 times larger than the experimental salt concentration at which the transition between spherical and rodlike micelles occurs. The chloride counterions associate weakly on the surface of the CTAC micelles with the degree of counterion dissociation decreasing slightly with increasing [NaCl] on spherical micelles, but dropping significantly on the threadlike micelles at high [NaCl]. This effect indicates that the electrolyte ions drive the micellar shape transition by screening the electrostatic repulsions between the micellar headgroups. The aromatic salicylate counterions, on the other hand, penetrate inside the micelle with their hydrophilic groups staying in the surfactant headgroup region and the hydrophobic groups partially embedded into the hydrophobic core of the micelle. The strong association of the salicylate ions with the surfactant headgroups leads to dense packing of the surfactant molecules, which effectively reduces the surface area per surfactant, and increases intramicellar ordering of the surfactant headgroups, favoring the formation of long threadlike micelles. Simulation predictions of the geometric and electrostatic properties of the spherical and threadlike micelles are in good agreement with experiments.

Adsorption Behavior of the Cetyltrimethylammonium Chloride Reverse Micelles on Porous Silica Gels

The adsorption behavior of reverse micelles of cetyltrimethylammonium chloride (CTAC) onto porous silica gels of various pore sizes has been investigated. A decrease in the amount of CTAC in the reversed micellar bulk solution in chloroform/water added to the silica gel in a column was observed, indicating the CTAC adsorption onto the silica surface. The adsorption profile obtained showed a maximum of the adsorbed surfactant amount, where the point of surface saturation may be reached, beyond which the adsorption amount remained unchanged. The adsorption of CTAC was also dependent on the specific surface area of the silica gel used. These results suggest that a CTAC monolayer may be formed and completed on the silica surface. The adsorbed CTAC amount slightly decreased with increasing the water to surfactant molar ratio, R, in the reversed micellar bulk solution, although at R < 2 the CTAC adsorption remained constant with silica gels having larger pore diameters. Even in the presence of traces of water moisture in the bulk solution used for the CTAC adsorption, the adsorption of water on silica gel was also observed and the molar ratios, R(Gamma), of the water to surfactant adsorbed on different silica gels were almost unity, implying that the silica gel may tend to hold some of water molecules originally adsorbed on its surface. The adsorption of CTAC molecules appears to occur through the adsorbed water molecules, probably polarized at the silica surface, and a CTAC-water-silica nanointerface seems to be formed. The adsorbed water amount or R(Gamma) also increased with an increase in R and on silica gels with larger pore diameters and a plateau was observed in the R range of 3 - 4, beyond which an increase in R(Gamma) occurred again. The water molecules present in the vicinity of the adsorbed CTAC and the silica surface may possess different properties at different R and R(Gamma) values.

Photophysics of the phenoxazine dyes resazurin and resorufin in direct and reverse micelles

The photophysics of the phenoxazin-3-one dyes resazurin and resorufin was studied in a micellar solution of cetyltrimethylammonium chloride and in reverse micelles of 1,4-bis(2-ethylhexyl)sulfosuccinate and benzylhexadecyldimethylammonium chloride. Absorption and fluorescence emission spectra, as well as fluorescence lifetimes and T–T transient absorption spectra were determined as a function of surfactant concentration. In the presence of direct micelles of cetyltrimethylammonium chloride, both dyes displayed red shifts in the absorption and fluorescence spectra together with a simultaneous fluorescence lifetime increase. The electrostatic attraction between the anionic dyes and the positive micellar interface favors the location of the dyes closer to the head groups of the surfactant molecules. In reverse micellar systems the spectral properties depended upon the charge of the surfactant and water content. In the case of 1,4-bis(2-ethylhexyl)sulfosuccinate, at low water content both dyes were incorporated into the interface; as the water content increased their spectral properties tended to those in pure water. In contrast, in the case of cationic surfactant, the dyes were located in the interfacial pseudophase as a result of electrostatic interactions.

Structural Evolution of Complexes of Poly(styrenesulfonate) and Cetyltrimethylammonium Chloride

Structural evolution is tracked as a charged surfactant, cetyltrimethylammonium chloride (CTAC), is titrated into a solution of oppositely charged polyelectrolyte, sodium poly(styrenesulfonate), to form polyelectrolyte−surfactant complexes. At surfactant-to-polymer molar charge ratio [r] less than 0.5, small-angle neutron scattering of the soluble complexes reveals bound spherical CTAC micelles of radii 21−24 A, slightly less than for pure CTAC. At such small [r], the number of spherical micelles per chain grows with [r] but never becomes large (<2). At [r] ∼ 0.5, if salt concentration is low (<100 mM), cylindrical micelles of radii 20−22 A and aspect ratio at least 5:1 replace the spherical micelles. Irrespective of salt concentration, at [r] ∼ 0.7 precipitation occurs, with X-ray scattering revealing CTAC arranged into hexagonally close-packed cylinders. At even higher [r], the hexagonal phase transforms into the Pm3n cubic phase. Raising ionic strength diminishes NaPSS−CTAC attraction, delaying onset o...

The Initial Transformation Mechanism of Gold Seeds on Indium Tin Oxide Surfaces

In this study, the initial transformation of gold seed on indium tin oxide (ITO) surfaces was investigated. Gold nanoparticles (AuNPs) grew on ITO surfaces using a seed-mediated method. The reduction rate of gold(I) salt and the shapes of AuNPs were affected by the concentration of cetyltrimethylammonium bromide (CTAB). Gold nanorods (AuNRs) appeared when the CTAB concentration was above 75 mM, and the shapes agreed with the dispersing state of CTAB micelles in solution. CTAB was replaced by cetyltrimethylammonium chloride (CTAC) in the growth procedure. Only small gold nanospheres (AuNSs) appeared when the CTAC concentration was increased. This result agreed with the dispersing state of CTAC micelles in solution and implied that gold salts were not free in either CTAB or CTAC solution. A red shift of 14 nm in the UV–vis spectra of AuBr4− was found in the presence of CTAB, and furthermore, AuBr2− became stable. This indicated that the gold salt strongly bound to cetyltrimethylammonium ion (CTA+) and was c...

Ultrafast Proton Transfer of Pyranine in a Supramolecular Assembly: PEO−PPO−PEO Triblock Copolymer and CTAC

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) is studied in a polymer-surfactant aggregate using femtosecond emission spectroscopy. The polymer-surfactant aggregate is a supramolecular assembly consisting of a triblock copolymer (PEO)(20)-(PPO)(70)-(PEO)(20) (P123) and a cationic surfactant, cetyltrimethylammonium chloride (CTAC). ESPT of the protonated species (HA) in HPTS leads to the formation of A(-). The dynamics of ESPT may be followed from the decay of the HA emission (at approximately 440 nm) and rise of the A(-) emission (at approximately 550 nm). Both steady-state and time-resolved studies suggest that ESPT of HPTS in P123-CTAC aggregate is much slower than that in bulk water, in P123 micelle, or in CTAC micelle. The ratio of the steady-state emission intensities (HA/A(-)) in P123-CTAC aggregate is 2.2. This ratio is approximately 50, 12, and 2 times higher than that respectively in water, in P123 micelle, and in CTAC micelle. Retardation of ESPT causes an increase in the rise time of the A(-) emission of HPTS. In P123-CTAC aggregate, A(-) displays three rise times: 30, 250, and 2400 ps. These rise times are longer than those in CTAC micelle (23, 250, and 1800 ps), in bulk water (0.3, 3, and 90 ps), and in P123 micelle (15 and 750 ps). The rate constants for initial proton transfer, recombination, and dissociation of the ion pair are estimated using a simple kinetic scheme. The slow fluorescence anisotropy decay of HPTS in P123-CTAC aggregate is analyzed in terms of the wobbling-in-cone model.