Small-angle neutron scattering study of micellization of ionic copolymers in aqueous solutions. The effects of side-chain length and molecular weight

Structure of polymerizable surfactant micelles: insights from neutron scattering

Although polymerization of reactive surfactants (surfmers) in micelles and other self-assembled phases has been studied for at least 30 years, the last decade or so has seen substantial advances in understanding both the structure and dynamics of these systems. In this review we highlight the new insights yielded primarily by small-angle neutron scattering (SANS) using high-flux sources, the perspective this provides for realizing topochemical polymerization in micellar systems, and the prospects and new developments for further exploiting SANS in this field. We present some new neutron contrast variation results exemplifying these elements.

Well-defined critical association concentration and rapid adsorption at the air/water interface of a short amphiphilic polymer, amphipol A8-35: a study by Förster resonance energy transfer and dynamic surface tension measurements

Amphipols (APols) are short amphiphilic polymers designed to handle membrane proteins (MPs) in aqueous solutions as an alternative to small surfactants (detergents). APols adsorb onto the transmembrane, hydrophobic surface of MPs, forming small, water-soluble complexes, in which the protein is biochemically stabilized. At variance with MP/detergent complexes, MP/APol ones remain stable even at extreme dilutions. Pure APol solutions self-associate into well-defined micelle-like globules comprising a few APol molecules, a rather unusual behavior for amphiphilic polymers, which typically form ill-defined assemblies. The best characterized APol to date, A8-35, is a random copolymer of acrylic acid, isopropylacrylamide, and octylacrylamide. In the present work, the concentration threshold for self-association of A8-35 in salty buffer (NaCl 100 mM, Tris/HCl 20 mM, pH 8.0) has been studied by Förster resonance energy transfer (FRET) measurements and tensiometry. In a 1:1 mol/mol mixture of APols grafted with either rhodamine or 7-nitro-1,2,3-benzoxadiazole, the FRET signal as a function of A8-35 concentration is essentially zero below a threshold concentration of 0.002 g·L(-1) and increases linearly with concentration above this threshold. This indicates that assembly takes place in a narrow concentration interval around 0.002 g·L(-1). Surface tension measurements decreases regularly with concentration until a threshold of ca. 0.004 g·L(-1), beyond which it reaches a plateau at ca. 30 mN·m(-1). Within experimental uncertainties, the two techniques thus yield a comparable estimate of the critical self-assembly concentration. The kinetics of variation of the surface tension was analyzed by dynamic surface tension measurements in the time window 10 ms-100 s. The rate of surface tension decrease was similar in solutions of A8-35 and of the anionic surfactant sodium dodecylsulfate when both compounds were at a similar molar concentration of n-alkyl moieties. Overall, the solution properties of APol "micelles" (in salty buffer) appear surprisingly similar to those of the micelles formed by small, nonpolymeric surfactants, a feature that was not anticipated owing to the polymeric and polydisperse nature of A8-35. The key to the remarkable stability to dilution of A8-35 globules, likely to include also that of MP/APol complexes, lies accordingly in the low value of the critical self-association concentration as compared to that of small amphiphilic analogues.

Alkylated poly(L-lysine citramide) as models to investigate the ability of amphiphilic macromolecular drug carriers to physically entrap lipophilic compounds in aqueous media

Poly(L-lysine citramide) was synthesized to serve as a polymeric bioresorbable drug carrier. It was previously shown that low molecular weight poly(L-lysine citramide) hydrophobized with heptyl and lauryl side chains (PLCA-C7(p) with p=43 and 60%; and PLCA-C12(p), with p=68, 75 and 100%) formed aggregates in aqueous media. The size of these aggregates was found to depend on the balance between repulsive electrostatic charges and attractive hydrophobic interactions, on the degree of ionization, and on the ionic strength. In this paper, the formation of these aggregates was further investigated by fluorescence probing, using two polarity sensitive molecules, pyrene and Nile Red, which were physically entrapped within the lipophilic core of the aggregates. In contrast to other micellar structures formed by surfactants and amphiphilic block copolymers, aggregates were observed even at very low polymer concentrations. The capacity of the hydrophobic domains to accommodate lipophilic molecules via physical entrapment was demonstrated with progesterone.