Françoise M. Winnik

Fluorescence methods in the study of the interactions of surfactants with polymers

In this review we present the application of fluorescence techniques to study polymer-surfactant systems, discussing the various methods and materials in use. In the first section we consider the use of low molecular weight fluorescence probes to measure the critical aggregation concentration and aggregation number of polymer-surfactant complexes and to characterize the microenvironment within the complexes. The next section deals exclusively with the association of fluorescently-labeled polymers and surfactants. The final part of the review is dedicated to a description of selected recent investigations, with particular emphasis on the study of the complexes formed between surfactants and hydrophobically-modified polymers.

Interaction of water-soluble collagen with poly(acrylic acid)

Interactions between poly(acrylic acid) labeled with pyrene (PAA-Py) and succinylated calfskin collagen (type I) (SCSC) were studied by fluorescence spectroscopy. PAA-Py exhibits a strong emission from pyrene monomer (intensity, I(M)) when it exists in an extended conformation. It exhibits another broad emission from pyrene excimer (intensity, I(E)) when it adopts a collapsed globule conformation. At pH 3, a value that is lower than the isoelectric point of SCSC, the ratio I(E)/I(M) value decreased cooperatively with increasing concentration of SCSC at constant PAA-Py concentration, under salt-free condition. On the other hand, this effect was not observed in the presence of 0.1 M NaCl. At pH 7, a value higher than the isoelectric point of SCSC, the ratio I(E)/I(M) was not affected by the presence of SCSC in the absence and presence of salt. From electrophoretic light scattering experiments, it was found that at pH 3 PAA-Py was negatively charged, while SCSC had a positive charge. Thus it is strongly suggested that the two polymers interact by electrostatic attraction at low pH where they are oppositely charged, and that PAA-Py adopts an extended conformation in the complex formed with SCSC. Similar interactions are believed to occur between dentinal collagen and the polycarboxylate component of glass-ionomer cements.

Fluorescent dyes as model 'hydrophobic modifiers' of polyelectrolytes : a study of poly(acrylic acid)s labelled with pyrenyl and naphthyl groups

Abstract A commercial poly(acrylic acid) (PAA, Mv = 150 000) has been labelled with low levels (0.4 to 3.1 mol%) of either pyrene, naphthalene or both chromophores introduced randomly along the chain. The photophysical properties of the polymers have been investigated by steady-state and time-resolved fluorescence spectroscopy. The pH-behaviour of the polymers has been examined by non-radiative energy transfer (NRET) measurements and by the ratio ( I E I M ) of excimer to monomer intensities. The pH-induced expansion of the pyrene-labelled PAA is accompanied by a sharp decrease in the ratio I E I M , signalling the disruption of pyrene excimer forming sites. In the case of the doubly-labelled PAA, the extent of NRET, which monitors the increase in the naphthalene to isolated pyrene separation distance, occurs over a wide pH range (3.0 to 4.6). In contrast, the decrease in I E I M , which reports on the fraction of pyrene groups located in excimer forming sites, takes place sharply at pH 4.5 which is also the polymer pKa derived from potentiometric titrations.

Mechanism of the interaction of hydrophobically-modified poly-(N-isopropylacrylamides) with liposomes.

Interactions of hydrophobically-modified poly-(N-isopropylacrylamides) (HM PNIPAM) with phospholipid liposomes were studied as a function of the lipid type, the lipid bilayer fluidity, and the polymer conformation. Fluorescence experiments monitoring non-radiative energy transfer (NRET), between naphthalene attached to the HM PNIPAM and 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated into the lipid bilayer, confirmed the direct penetration of hydrophobic anchor groups linked to the polymer into the liposome hydrophobic core. Contraction of the polymer backbone above the lower critical solution temperature (LCST) resulted in a partial withdrawal of the anchor groups from the lipid bilayer. Analysis of polymer/lipid mixtures by centrifugation and quasi-elastic light scattering (QELS) revealed the polymer-induced fission of liposomes in the liquid-crystalline state, resulting in the formation of vesicles 150-230 nm in diameter. The process is reversible and upon transition of the bilayer into the gel state these vesicles are converted into larger aggregates. According to the results of gel-filtration experiments the HM PNIPAM is in dynamic exchange between the liquid-crystalline lipid bilayer and the water solution, while the binding to the bilayer in the gel state is more static in nature. The binding constant for mixture of HM PNIPAM with DMPC liposomes, evaluated from the centrifugation experiments, was found to be 120 M(-1).

Effect of the surfactant head group length on the interactions between polyethylene glycol monononylphenyl ethers and poly(acrylic acid)

Abstract The behavior of homogeneous and polydisperse ethoxylated nonylphenols in the presence of poly(acrylic acid) (PAA) was investigated by fluorescence, surface tension, viscometric and pH measurements. Critical micelle concentration (CMC), critical aggregation concentration (CAC or T 1 ) and various other critical points of interaction such as the appearance of free surfactant molecules at the air/water interface ( T 2 ′) and of micelles into solution ( T 2 ) were determined. For polydisperse surfactants, the surface excess, areas per molecule and the composition of polymer–surfactant complexes were also established. T 1 was lower than CMC and was not influenced significantly by the length and dispersity of the poly(ethylene oxide) (PEO) chain. For the polydisperse surfactants, T 2 ′ and T 2 decrease with increasing PEO chain length. The PAA practically does not affect the area per molecule, irrespective of the ethoxylation degree. The results suggest that complexes originate from hydrogen bonding and are further stabilized by hydrophobic interactions of the surfactant tails.

Surface viscoelasticity in mixed polycation anionic surfactant systems studied by a simple test

Abstract The talc particle test, performed on mixed solutions of polyelectrolyte and oppositely charged surfactant, was used to examine the changes in rheology of the air/water interface of these solutions over a large range of compositions. The systems scrutinized consisted of sodium dodecyl sulfate (SDS, concentration ranging from 0.005 to 0.5 g L −1 ) and either the cationic cellulosic polymer, Polymer JR400, or the corresponding hydrophobically modified polymer, Quatrisoft ™ LM200, of concentration ranging from 0.001 to 1 g L −1 . Clear indications of viscoelasticity at the air/water interface were obtained over a wide composition domain in both instances. Viscoelasticity provides evidence of a synergistic adsorption of the two components at the air/water interface. When the SDS was increased sufficiently with respect to the polyelectrolyte concentration, the viscoelasticity vanished, and the interface became fluid; the fluidity recovery is an indication that the interface is denuded of polyelectrolytes by growing competition from micellar interfaces in solution.

Destabilization of cationic lipid vesicles by an anionic hydrophobically modified poly(N-isopropylacrylamide) copolymer: a solid-state 31P NMR and 2H NMR study.

The effect of binding PNIPAM-Py-Gly, a copolymer of N-isopropylacrylamide, N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide and N-glycydyl-acrylamide, on membrane stability in cationic multilamellar vesicles (MLVs) was examined using solid-state phosphorus (31P) and deuterium (2H) nuclear magnetic resonance (NMR) spectroscopy. For MLVs of composition n-octadecyldiethylene oxide (ODEO)+cholesterol (CHOL)+1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)+dimethyldioctadecylammonium bromide (DODAB) (molar ratios 75:10.5:10.5:4), PNIPAM-Py-Gly induced a complete conversion from a bilayer-type 31P NMR spectrum to one characteristic of lipids undergoing isotropic motional averaging, indicating the existence of regions of high local membrane curvature. This response was sustained even at elevated temperatures. For MLVs of composition POPC+1,2-dioleoyloxy-3-(trimethylammonio)-propane (DOTAP), only at high levels of DOTAP and ionic strength did PNIPAM-Py-Gly induce even a partial conversion to an isotropic-type 31P NMR spectrum. At lower pH this effect was diminished. Raising the temperature eliminated the isotropic 31P NMR spectral component, and this effect was not reversible upon returning to room temperature. 2H NMR spectroscopy of headgroup-deuterated DOTAP and POPC confirmed the 31P NMR results, but did not provide specific surface electrostatic information. We conclude that the binding of PNIPAM-Py-Gly to phospholipid-based vesicles is dominated by electrostatic attraction between cationic lipids and the polymers glycine residues. At high binding levels, the polymer assumes a collapsed conformation at the surface, resulting in regions of high local curvature of the lipid assembly. For ODEO-based liposomes, these effects are magnified by the additional contribution of hydrogen bonding to the strength of polymer binding.

Analysis of ferrofluids by capillary electrophoresis

Abstract A series of commercial and experimental water-based ferrofluids was analyzed by capillary electrophoresis (CE) coupled with diode array UV–visible detection. The ferrofluids examined include a neutral dextran–iron oxide colloid, a cationic iron oxide colloid and four negatively charged ferrofluids. The effects of pH, ionic strength and applied voltage on the electropherograms were monitored. The electrophoretic mobility of the iron oxide particles was calculated from the CE data and compared to the corresponding values obtained from microelectrophoresis. Capillary electrophoresis provides a new, efficient and sensitive method to characterize ferrofluids in terms of surface charge, electrophoretic mobility and the level of purity.

Stabilization of aqueous foams by polymer/surfactant systems: effect of surfactant chain length

Abstract The foaming of mixed polymer/surfactant solutions was investigated as a function of surfactant chain length for the system consisting of the cationic cellulose ether Polymer JR400 and a series of sodium alkyl sulfates (C n H 2 n +1 SO 4 Na, n =8, 10 and 12) and sodium decane sulfonate. Foam enhancement by the polymer was observed with all of these surfactants, but the largest effect and best foaming results were found with the shorter chain length surfactants/Polymer JR400 systems, especially the sodium octyl sulfate/Polymer JR400 system. The foaming results correlated with surface viscoelasticity measurements and also with film drainage observations on selected systems.

Effect of polymer architecture on the interactions of hydrophobically-modified poly-(N-isopropylamides) and liposomes

Abstract The interactions of hydrophobically-modified poly-( N -isopropylacrylamides) with phospholipid liposomes have been studied as a function of the bilayer phase composition and the polymer architecture, using fluorescence spectroscopy, dynamic light scattering, size exclusion chromatography, and centrifugation assays. Measurements were carried out with polymers carrying hydrophobic groups, either at random along the main chain, or specifically at one chain end. Anchoring of the hydrophobic substituent within the lipid bilayer was ascertained in both systems. Binding constants of the polymer to dimyristoyl phosphatidylcholine (DMPC)/cholesterol, 7/3 molar ratio) liposomes and dipalmitoyl phosphatidylcholine (DPPC)/cholesterol, 7/3 molar ratio) liposomes were evaluated from the centrifugation assays.