Reversible pH-triggered encapsulation and release of pyrene by adsorbed block copolymer micelles

The well-established ability of copolymer micelles to encapsulate and release hydrophobic molecules has been investigated following their adsorption onto silica particles. Here, a pH-responsive copolymer, poly(2-(dimethylamino)ethyl methacrylate)- b-poly(2-(diethylamino)ethyl methacrylate) (PDMA(106)- b-PDEA(25)), has been used to study the formation and dissociation of adsorbed micelles through pH variation. This copolymer behaves as free unimers in aqueous solutions below pH 8 and forms micelles 29 nm in hydrodynamic diameter above this pH. Encapsulation and release of a model hydrophobic compound (pyrene) by in situ adjustment of the solution pH has been compared for both free and adsorbed micelles using fluorescence spectrophotometry, epifluorescence microscopy, and zeta potential measurements. At basic pH values, pyrene is solubilized within the cores of micelles adsorbed on silica particles: addition of acid leads to micelle dissociation and release of the pyrene into the bulk aqueous solution. Micelle adsorption does not appear to hinder the extent of pyrene uptake/release. Moreover, this pH-responsive behavior is both reversible and reproducible over multiple pH cycles.     

Comparison of the adsorption of cationic diblock copolymer micelles from aqueous solution onto mica and silica

The similarities and differences in the adsorption behavior of diblock poly(2-(dimethylamino)ethyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate) (XqPDMA-PDEA, where X refers to a mean degree of quaternization of the PDMA of either 0, 10, 50, or 100 mol%) copolymers at the mica/ and silica/aqueous solution interfaces have been investigated. These diblock copolymers form core-shell micelles with the PDEA chains located in the cores and the more hydrophilic PDMA chains forming the cationic micelle coronas at pH 9. These micelles adsorb strongly onto both mica and silica due to electrostatic interactions. In situ atomic force microscopy (AFM) has demonstrated that the mean spacing and the dimension of the adsorbed micelles depend on both the substrate and the mean degree of quaternization of the PDMA blocks. In particular, the morphology of the adsorbed nonquaternized 0qPDMA-PDEA copolymer micelles is clearly influenced by the substrate type: these micelles form a disordered layer on silica, while much more close-packed, highly ordered layers are obtained on mica. The key reasons for this difference are suggested to be the ease of lateral rearrangement for the copolymer micelles attached to the solid substrates and the relative rates of relaxation of the coronal PDMA chains.     

Adsorption characteristics of zwitterionic diblock copolymers at the silica/aqueous solution interface

The adsorption of a zwitterionic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-block-poly(methacrylic acid) (PDEA59-PMAA50), at the silica/aqueous solution interface has been characterised as a function of pH. In acidic solution, this copolymer forms core-shell micelles with the neutral PMAA chains being located in the hydrophobic cores and the protonated PDEA chains forming the cationic micelle coronas. In alkaline solution, the copolymer forms the analogous inverted micelles with anionic PMAA coronas and hydrophobic PDEA cores. The morphology of the adsorbed layer was observed in situ using soft-contact atomic force microscopy (AFM): this technique suggests the formation of a thin adsorbed layer at pH 4 due to the adsorption of individual copolymer chains (unimers) rather than micelle aggregates. This is supported by the remarkably low dissipation values and the relatively low degrees of hydration for the adsorbed layers, as estimated using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). In alkaline solution, analysis of the adsorption data suggests a conformation for the adsorbed copolymers where one block projects normal to the solid/liquid interface; this layer consists of a hydrophobic PDEA anchor block adsorbed on the silica surface and an anionic PMAA buoy block extending into the solution phase. Tapping mode AFM studies were also carried out on the silica surfaces after removal from the copolymer solutions and subsequent drying. Interestingly, in these cases micelle-like surface aggregates were observed from both acidic and alkaline solutions. The lateral dimension of the aggregates seen is consistent with the corresponding hydrodynamic diameter of the copolymer micelles in bulk solution. The combination of the in situ and ex situ AFM data provides evidence that, for this copolymer, micelle aggregates are only seen in the ex situ dry state as a result of the substrate withdrawal and drying process. It remains unclear whether these aggregates are caused by micelle deposition at the surface during the substrate withdrawal from the solution or as a result of unimer rearrangements at the drying front as the liquid recedes from the surface.     

Effects of copolymer concentration and chain length on the pH-responsive behavior of diblock copolymer micellar films

The pH-responsive behavior of adsorbed diblock copolymer films of PDMA-PDEA (poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate)) on silica has been characterized using a quartz crystal microbalance with dissipation monitoring (QCM-D), an optical reflectometer (OR) and an atomic force microscope (AFM). The copolymer was adsorbed at pH 9 from various copolymer concentrations; QCM-D measurements indicate that the level of desorption when rinsed at pH 9 depends on the initial copolymer concentration. The adsorbed films produced at pH 9 generally have low charge densities; adjusting the solution pH to 4 results in a significant protonation of the constituent copolymers and a related interfacial structural change for the copolymer film. OR studies show no significant change during pH cycling, while QCM-D measurements indicate that the adsorbed mass and dissipation alter dramatically in response to the solution pH. The difference between the QCM-D adsorbed masses and dissipation values at pH 4 and 9 were found to be dependent on the initial copolymer concentration. This is due to differences in the initial conformations within the adsorbed copolymer layers at pH 9. The effect of the PDMA chain length on the pH-responsive behavior has also been studied; both the QCM-D adsorbed mass and dissipation of PDMA54-PDEA24 (shorter PDMA block) at pH 4 and 9 were observed to be greater than those of PDMA9X-PDEA2Y (longer PDMA block). This suggests that the normal extension of the adsorbed PDMA54-PDEA24 copolymer films is more significant than that of the PDMA9X-PDEA2Y films on silica.     

Incorporation of block copolymer micelles into multilayer films for use as nanodelivery systems

This work demonstrates the potential application of stimulus responsive block copolymer micelles as triggerable delivery systems for use within multilayer films. Cationic, pH-responsive micelles of poly[2-(dimethylamino)ethyl methacrylate-block-poly(2-(diethylamino)ethyl methacrylate)] (PDMA-PDEA) were deposited on anionic polystyrene latex particles. The charge reversal of the surface and the amount of adsorbed polymer were monitored by zeta potential measurements and colloidal titrations, respectively. Prior to adsorption, the PDMA-PDEA micelles were loaded with a hydrophobic dye, and UV-vis spectroscopy was used to determine the amount of dye encapsulated within a monolayer of micelles. It was found that subtle chemical modification of the PDMA-PDEA diblock copolymer via permanent quaternization of the PDEA block results in micelles with tunable loading capacities. Multilayers of cationic micelles of partially quaternized PDMA-PDEA and anionic polyelectrolyte (poly(sodium 4-styrene sulfonate)) were deposited on the surface of polystyrene latex particles by sequential adsorption. UV-vis analysis of the dye present within the multilayer after the addition of each layer demonstrates that the micelles are sufficiently robust to retain encapsulated dye after multiple adsorption/washing cycles and can thus create a film that can be increasingly loaded with dye as more micelle layers are adsorbed. Multiple washing cycles were performed on micellar monolayers of PDMA-PDEA to demonstrate how such systems can be used to bring about triggerable release of actives. When performing several consecutive washing steps at pH 9.3, the micelle structure of the PDMA-PDEA micelles in the monolayer is retained, resulting in only a small reduction in the amount of encapsulated dye. In contrast, washing at pH 4, the structure of the micelle layers is severely disrupted, resulting in a fast release of the encapsulated dye into the bulk. Finally, if a sufficient number of micelle/homopolyelectrolyte layers are adsorbed, it is possible to selectively dissolve the latex template, resulting in hollow capsules.     

pH-responsive behavior of selectively quaternized diblock copolymers adsorbed at the silica/aqueous solution interface

The desorption and subsequent pH-responsive behavior of selectively quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) films at the silica/aqueous solution interface has been characterized. The copolymer films were prepared at pH 9, where micelle-like surface aggregates are spontaneously formed on silica. The subsequent rinse with a copolymer-free electrolyte solution adjusted to pH 9 causes partial desorption of the weakly or non-quaternized copolymers, but negligible desorption for the highly quaternized copolymers. Further rinsing with a pH 4 electrolyte solution results in additional desorption and extension (swelling) of the remaining adsorbed copolymer film normal to the interface. This pH-responsive behavior is reversible for two pH cycles (9-4-9-4) as monitored by both quartz crystal microbalance with dissipation monitoring (QCM-D) and also zeta potential measurements. The magnitude of the pH-responsive behavior depends on the mean degree of quaternization of the PDMA block. Moreover, a combination of contact angle data, zeta potential measurements and in situ atomic force microscopy (AFM) studies indicates that the pH-responsive behavior is influenced not only by the number of cationic binding sites on the adsorbed copolymer chains but also by the adsorbed layer structure.     

Characterization of layer-by-layer self-assembled multilayer films of diblock copolymer micelles

The in situ layer-by-layer (LbL) self-assembly of low Tg diblock copolymer micelles onto a flat silica substrate is reported. The copolymers used here were a cationic poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (50qPDMA-PDEA; 50q refers to a mean degree of quaternization of 50 mol % for the PDMA block) and zwitterionic poly(methacrylic acid)-block-poly(2-(diethylamino)ethyl methacrylate) (PMAA-PDEA), which has anionic character at pH 9. Alternate deposition of micelles formed by these two copolymers onto a silica substrate at pH 9 was examined. The in situ LbL buildup of the copolymer micelle films was monitored using zeta potential measurements, optical reflectometry, and a quartz crystal microbalance with dissipation monitoring (QCM-D). For a six layer deposition, complete charge reversal was observed after the addition of each layer. The OR data indicated clearly an increase in adsorbed mass with each additional micelle layer and suggest that some interdiffusion of copolymer chains between layers and/or an increase in the film roughness, and hence in the effective surface area of the micellar multilayers, must take place as the film is built up. QCM-D data indicated that the self-assembled micellar multilayers on a flat silica substrate undergo structural changes over a prolonged period. This is attributed to longer-term interdiffusion of the copolymer chains between the outer two layers after the initial adsorption of each layer is complete. The QCM-D data further suggest that the outer adsorbed layers adopt a progressively more extended conformation, particularly for the higher numbered layers. The morphology of each successive layer was characterized using in situ soft-contact atomic force microscopy, and micelle-like surface aggregates are clearly observed within each layer of the complex film, suggesting the persistence of aggregate structures throughout the multilayer structure.     

Synthesis and characterisation of new shell cross-linked micelles with amine-functional coronas

Shell cross-linked (SCL) micelles with amine-functional coronas have been constructed in aqueous solution by exploiting the micellar self-assembly of new thermo-responsive ABC triblock copolymers. These copolymers were prepared via atom transfer radical polymerisation (ATRP) in convenient one-pot syntheses and comprised a thermo-responsive core-forming poly(propylene oxide) [PPO] block, a cross-linkable central poly(glycerol monomethacrylate) [GMA] block and an amine-functional outer block based on either poly(2-(dimethylamino)ethyl methacrylate) [DMA] or poly([2-(methacryloyloxy)ethyl]trimethyl ammonium chloride) [QDMA]. DMF GPC analysis indicated an M_n of 17,700 and an M_w/M_n of 1.46 for the PPO-PGMA-PDMA triblock copolymer. The DMA residues of the PPO-PGMA-PDMA triblock copolymer were reacted with methyl iodide to prepare copolymers with differing degrees of quaternisation. Each triblock copolymer dissolved molecularly in aqueous solution at 5 °C and formed micelles with amine-functional coronas above a critical micelle temperature (CMT) of around 12 °C, which corresponded closely to the cloud point of the PPO macro-initiator. Cross-linking of the GMA residues in the inner shell using divinyl sulfone produced SCL micelles that remained intact at 5 °C, i.e. below the cloud point of the core-forming PPO block. Aqueous electrophoresis studies confirmed that these SCL micelles had considerable cationic surface charge, as expected. The cationic SCL micelles were adsorbed onto a near-monodisperse anionic silica sol, which was used as a model colloidal substrate. Thermogravimetric analyses indicated SCL micelle mass loadings of 6.1-15.5 wt.%, depending on the initial micelle concentration. Aqueous electrophoresis studies confirmed that surface charge reversal occurred on adsorption of the SCL micelles and scanning electron microscopy studies revealed the presence of SCL micelles on the silica particles.     

Novel fluoroalkyl end-capped amphiphilic diblock copolymers with pH/temperature response and self-assembly behavior

A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by (1)H NMR and (19)F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA(30)-b-PDEA(10) diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA(10)-b-PDMA(30) diblock copolymers can form flowerlike micelles in basic aqueous solution.     

Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface

The pH-responsive behavior of cationic diblock poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) copolymer micelles adsorbed at the silica/aqueous solution interface has been characterized. The micellar morphology of this copolymer, initially adsorbed at pH 9, can be dramatically altered by lowering the solution pH. The original micelle-like morphology of the adsorbed copolymer chains at pH 9 completely disappears as the pH is decreased to 4, and a brush-like layer structure is produced. This change results from protonation of the copolymer chains: the subsequent electrostatic repulsions within the film drive the copolymer chains to expand into the aqueous phase. Returning the solution pH from 4 to 9 causes this brush-like layer to collapse, with atomic force microscopy images suggesting degradation of the film. Hence, the pH-responsive behavior of the copolymer film exhibits irreversible morphological changes. Measurements of the adsorbed/desorbed amounts of the copolymer film were conducted using both a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). After an initial rinse at both pH values, the OR adsorbed mass becomes almost constant during subsequent pH cycling, whereas the corresponding QCM-D adsorbed mass changes significantly but reversibly in response to the solution pH. Since the QCM-D measures a bound mass that moves in tandem with the surface, the discrepancy with the OR data is due to changes in the amount of bound water in the copolymer film as a result of the pH-induced changes in surface morphology. The larger effective mass observed at pH 4 suggests that the brush-like layer contains much more entrapped water than the micellar films at pH 9. The pH dependence of the contact angle of the adsorbed film is consistent with the changes observed using the other techniques, regardless of whether the solution pH is altered in situ or the aqueous solution is completely replaced. In fact, comparison of these two approaches provides direct evidence of the exposure of adsorbed micelle core blocks to the solution during pH cycling and the concomitant impact upon all the other measurements.     

Synthesis and characterization of shell cross-linked micelles with hydroxy-functional coronas: a pragmatic alternative to dendrimers?

Shell cross-linked (SCL) micelles with hydroxy-functional coronas have been constructed in aqueous solution by exploiting the micellar self-assembly behavior of a new thermoresponsive ABC triblock copolymer. This copolymer was prepared via atom transfer radical polymerization in a convenient one-pot synthesis and comprised a thermoresponsive core-forming poly(propylene oxide) (PPO) block, a cross-linkable central poly(2-(dimethylamino)ethyl methacrylate) (DMA) block, and a hydroxy-functional outer block based on poly(glycerol monomethacrylate) (GMA). DMF GPC analysis confirmed a unimodal molecular weight distribution for the PPO-PDMA-PGMA triblock copolymer precursor, with an M(n) of 12 100 and a polydispersity of approximately 1.26. This copolymer dissolved molecularly in aqueous solution at 5 degrees C but formed micelles with hydroxy-functional coronas above a critical micelle temperature of around 12 degrees C, which corresponded closely to the cloud point of the PPO macroinitiator. Cross-linking of the DMA residues using 1,2-bis(2-iodoethoxy)ethane produced SCL micelles that remained intact at 5 degrees C, i.e., below the cloud point of the core-forming PPO block. Dynamic light scattering studies confirmed that the SCL micelle diameter could be varied depending on the temperature employed for cross-linking: smaller, more compact SCL micelles were formed at higher temperatures, as expected. Since cross-linking involved quaternization of the DMA residues, the SCL micelles acquired cationic surface charge as judged by aqueous electrophoresis studies. These cationic SCL micelles were adsorbed onto near-monodisperse anionic silica sols, which were used as a model colloidal substrate. Thermogravimetric analyses indicated a SCL micelle mass loading of 2.5-4.4%, depending on the silica sol diameter and the initial micelle concentration. Aqueous electrophoresis measurements confirmed that surface charge reversal occurred after adsorption of the SCL micelles, and scanning electron microscopy studies revealed a uniform coating of SCL micelles on the silica particles.     

温敏性含氟两亲接枝共聚物的制备及胶束化行为

以聚乙二醇单甲醚甲基丙烯酸酯(MPEGMA)为大分子单体, 甲基丙烯酸六氟丁酯(HFMA)为含氟单体, N-异丙基丙烯酰胺(NIPAAm)为功能性单体, 采用大分子单体接枝共聚法, 制备了一种温敏性含氟两亲接枝共聚物P(NIPAAm-co-HFMA)-g-PEG. 利用FTIR, ¹H NMR, ¹⁹F NMR和GPC对共聚物的结构进行表征; 采用紫外-可见分光光度计测定了共聚物的低临界溶解温度(LCST)约为38.9 ℃, 高于人体正常的生理温度; 利用荧光探针技术测定了共聚物的临界胶束浓度(cmc), 结果表明, 当共聚物溶液温度高于LCST时, 其cmc明显变小; 利用激光光散射粒度仪(LLS)测定了共聚物胶束的水合粒径及其分布, 当温度达到LCST时, 胶束粒径明显变小, 温度过高时, 粒径又有所增大; 利用透射电子显微镜(TEM)研究了共聚物胶束的形貌, 结果表明, P(NIPAAm-co-HFMA)-g-PEG在水溶液中可自组装成球状胶束粒子, 随着温度的升高, 共聚物胶束由松散的核壳结构转变成更加紧凑的球状结构, 且粒径明显变小.     

Non-surface activity and micellization of ionic amphiphilic diblock copolymers in water. Hydrophobic chain length dependence and salt effect on surface activity and the critical micelle concentration

We reported previously (Macromolecules 2003, 36, 5321; Langmuir, 2004, 20, 7412) that amphiphilic diblock copolymers having polyelectrolytes as a hydrophilic segment show almost no surface activity but form micelles in water. In this study, to further investigate this curious and novel phenomenon in surface and interface science, we synthesized another water-soluble ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa by living anionic polymerization. Several diblock copolymers with different hydrophobic chain lengths were synthesized and the adsorption behavior at the air/water interface was investigated using surface tension measurement and X-ray reflectivity. A dye-solubilization experiment was carried out to detect the micelle formation. We found that the polymers used in this study also formed micelles above a certain polymer concentration (cmc) without adsorption at the air-water interface under a no-salt condition. Hence, we further confirmed that this phenomenon is universal for amphiphilic ionic block copolymer although it is hard to believe from current surface and interface science. For polymers with long hydrophobic chains (more than three times in length to hydrophilic chain), and at a high salt concentration, a slight adsorption of polymer was observed at the air-water interface. Long hydrophobic chain polymers showed behavior "normal" for low molecular weight ionic surfactants with increasing salt concentration. Hence, the origin of this curious phenomenon might be the macroionic nature of the hydrophilic part. Dynamic light scattering analysis revealed that the hydrodynamic radius of the block copolymer micelle was not largely affected by the addition of salt. The hydrophobic chain length-cmc relationship was found to be unusual; some kind of transition point was found. Furthermore, very interestingly, the cmc of the block copolymer did not decrease with the increase in salt concentration, which is in clear contrast to the fact that cmc of usual ionic small surfactants decreases with increasing salt concentration (Corrin-Harkins law). These behaviors are thought to be the special, but universal, characteristics of ionic amphiphilic diblock copolymers, and the key factor is thought to be a balance between the repulsive force from the water surface by the image charge effect and the hydrophobic adsorption.     

Enhanced adsorption of alkyl glucosides on the silica/water interface by addition of amine oxides

The effect of adding a small amount of dodecyl dimethylamine oxide (DDAO) on adsorption on silica from an aqueous solution of dodecyl maltoside (C12G2) has been investigated. The C12G2 itself does not adsorb significantly on silica at any concentration. DDAO on the other hand readily adsorbs in a bilayer-like structure at concentrations approaching the critical micelle concentration (cmc), but the adsorbed amount at the concentrations it has been applied in these mixtures is small. In contrast, by combination of the two surfactants, significant adsorption is observed at concentrations where the adsorption of the pure DDAO, as well as pure C12G2, is very low. We thus see a strong enhancement of the adsorption from the mixed system. The adsorption is suggested to be a two-step process, where individual DDAO molecules first adsorb to the silica surface through electrostatic interactions, and then C12G2 adsorbs at the hydrophobic sites the DDAO tails constitute through hydrophobic bonding. A minimum concentration of DDAO is required to induce adsorption from a solution with constant C12G2 concentration. This concentration is lower for C12G2 solutions below and equal to cmcC12G2 than above cmcC12G2. In addition, the total adsorbed amount shows a maximum around cmcC12G2 of the mixture for a solution with low DDAO contents. Both these effects are explained by incorporation of DDAO in the mixed micelles above cmc(mix), which leads to a desorption of DDAO from the surface.     

Purely salt-responsive micelle formation and inversion based on a novel schizophrenic sulfobetaine block copolymer: structure and kinetics of micellization

A novel sulfobetaine block copolymer, poly(N-(morpholino)ethyl methacrylate)-b-poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl)pyridinium betaine) (PMEMA-b-PSVBP), was synthesized via reversible addition-fragmentation chain transfer polymerization. In aqueous solution, PMEMA homopolymer becomes insoluble in the presence of Na2SO4 (>0.6 M), whereas PSVBP homopolymer molecularly dissolves in the presence of NaBr (>0.2 M). Thus, PMEMA-b-PSVBP diblock copolymer exhibits purely salt-responsive "schizophrenic" micellization behavior in aqueous solution, forming two types of micelles with invertible structures, that is, PMEMA-core and PSVBP-core micelles, depending on the concentrations and types of added salts (Scheme 1). The equilibrium structures of these two types of micelles were characterized via a combination of 1H NMR and laser light scattering (LLS). We further investigated the kinetics of salt-induced formation/dissociation of PMEMA-core and PSVBP-core micelles and the structural inversion between them employing the stopped-flow light scattering technique. In the presence of 0.5 M NaBr, the addition of Na2SO4 (>0.6 M) induces the formation of PMEMA-core micelles stabilized with well-solvated PSVBP coronas. Dilution-induced dissociation of PMEMA-core micelles into unimers occurs within the dead time of the stopped-flow apparatus (approximately 2-3 ms) when the final Na2SO4 concentration drops below 0.3 M, while salt-induced breakup of PSVBP-core micelles is considerably slower. The structural inversion from PMEMA-core to PSVBP-core micelles proceeds first with the dissociation of PMEMA-core micelles into unimers, followed by the formation of PSVBP-core micelles. On the other hand, structural inversion from PSVBP-core to PMEMA-core micelles exhibits different kinetic sequences. Immediately after the salt jump, PMEMA corona chains are rendered insoluble, and unstable PSVBP-core micelles undergo intermicellar fusion; this is accompanied and/or followed by the solvation of PSVBP cores and structural inversion into colloidally stable PMEMA-core micelles.     

Adsorption of block polyampholyte micelles in monolayers at the silicon water interface

 The adsorption of the diblock polyampholyte poly (methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) from aqueous solution on silicon substrates was investigated as a function of polymer concentration and pH. Dynamic light scattering and electrokinetic measurements were used to characterize the polyampholyte in solution. The amount of polymer adsorbed was determined by ellipsometry and lateral structures of the polymer layer were investigated by scanning force microscopy. The amount of polymer adsorbed was found to be strongly influenced by the pH of the polymer solution, while the size of the polyampholyte micelles adsorbed on the surface was hardly affected by pH during adsorption. From investigations by scanning force microscopy well-seperated micelles were seen in the dried monolayers adsorbed directly from solution. The structures at the surface are correlated to structures in solution, and the adsorbed amount depends on the relative charge of the micelles and the surface. Received: 13 September 1999 Accepted in revised form: 8 December 1999     

Role of the tracer in characterizing the aggregation behavior of aqueous block copolymer solutions using fluorescence correlation spectroscopy

The hydrodynamic radii of micelles formed by amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution determined using fluorescence correlation spectroscopy (FCS) depend on the nature of the fluorescent tracer used. We have compared the values of the hydrodynamic radii of the unimers and the micelles as well as the critical micelle concentrations (CMC), using as tracers (1) the identical diblock copolymers being fluorescence-labeled at the hydrophilic or the hydrophobic block terminus [Bonné et al. Colloid Polym Sci (2004) 282:833–843], and (2) a low molar mass fluorescence dye, rhodamine 6G. Whereas similar values for the CMC were found for both probes, the hydrodynamic radius of micelles is significantly underestimated using a free dye as a tracer in FCS, especially near the CMC. We attribute this discrepancy to the fast exchange of the dye between micelles and solution.     

New thermodynamic/electrostatic models of adsorption and tension equilibria of aqueous ionic surfactant mixtures: application to sodium dodecyl sulfate/sodium dodecyl sulfonate systems

The nonideal adsorbed solution (NAS) theory has been formally extended to adsorption at the air/water interface from aqueous mixtures of ionic surfactants, explicitly accounting for the surface potential of the adsorbed monolayer with the Gouy-Chapman theory. This new ionic NAS (iNAS) theory is thermodynamically consistent and, when coupled to a micellization model, is valid for concentrations below and above the mixed cmc. Counterion binding is incorporated into the model using two fractional binding parameters, beta(sigma) for the adsorbed monolayer and beta(m) for the micelles. The regular solution theory is used to model the nonideal interactions within the adsorbed monolayer and within the mixed micelles. New tension data for an equimolar mixture of sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) at two salinities fit this model well when mixing is ideal. The total surface densities, the surface compositions, and the surface potentials for the mixed monolayers are calculated. When there is no added salt, at total surfactant concentrations below the mixed cmc, the adsorbed monolayer is enriched in SDSn, but at total concentrations at and above the mixed cmc, the adsorbed monolayer is nearly an equimolar mixture. In the presence of 100 mM NaCl, the adsorbed monolayer is nearly an equimolar mixture, independent of the total surfactant concentration.     

Structure of a hydrophilic-hydrophobic block copolymer and its interactions with salt and an anionic surfactant

The aggregation of a hydrophilic-hydrophobic diblock copolymer consisting of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(methyl methacrylate) (PMMA) in aqueous solution has been investigated by small-angle neutron scattering. This polybase is extensively protonated at low pH and forms micelles with a dense core of PMMA and a diffuse coronal layer of cationic PDMAEMA. Addition of salt induced micellar growth, brought about by charge screening and more efficient packing of the chains. As a result, the aggregation number increased from 8 up to 31. A similar effect was observed at low concentrations of an anionic surfactant, sodium dodecyl sulfate (SDS) since the net cationic charge in the hydrophilic coronal layer was reduced due to surfactant binding. However, at higher surfactant concentrations, a drastic structural reorganization occurred, as the PMMA became solubilized into the SDS micellar cores and the PDMAEMA chains interacted with the surfactant micelles, resulting in a "pearl-necklace" structure. The presence of the cationic polyelectrolyte significantly increased the population of SDS micelles by effectively lowering the critical micelle concentration of this anionic surfactant.