title: Interplay of electrostatic and hydrophobic effects with binding of cationic gemini surfactants and a conjugated polyanion: experimental and molecular modeling studies.
abstract: Understanding factors responsible for the fluorescence behavior of conjugated polyelectrolytes and modulation of their behavior are important for their application as functional materials. The interaction between the anionic poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl}copolymer (PBS-PFP) and cationic gemini surfactants alpha,omega-(CmH2m+1N+(CH3)2)2(CH2)s(Br-)2 (m-s-m; m=12, s=2, 3, 5, 6, 10, and 12) has been studied experimentally in aqueous solution. These surfactants are chosen to see whether molecular recognition and self-assembly occurs between the oppositely charged conjugated polyelectrolyte and gemini surfactant when the spacer length on the surfactant is similar to the intercharge separation on the polymer. Without surfactants, PBS-PFP exists as aggregates. These are broken up upon addition of gemini surfactants. However, as anticipated, the behavior strongly depends upon spacer length (s). Fluorescence measurements show three surfactant concentration regimes: At low concentrations (<2x10(-6) M) quenching occurs and is most marked with the small spacer 12-2-12; at intermediate concentrations (approximately 2x10(-6)-10(-3) M), fluorescence intensity is constant, with a 12-carbon spacer 12-12-12 showing the strongest fluorescence; above the critical micelle concentration (CMC; approximately 10(-3) M) increases in emission intensity are seen in all cases and are largest with the intermediate spacers 12-5-12 and 12-6-12, where the spacer length most closely matches the distance between monomer units on the polymer. With longer spacer length surfactants, surface tension measurements for concentrations below the CMC reveal the presence of polymer-surfactant aggregates at the air-water interface, possibly reflecting increased hydrophobicity. Above the CMC, small-angle neutron scattering experiments for the 12-6-12 system show the presence of spherical aggregates, both for the pure surfactant and for polyelectrolyte/gemini mixtures. Molecular dynamics simulations help rationalize these observations and show that there is a very fine balance between electrostatic and hydrophobic interactions. With the shortest spacer 12-2-12, Coulombic interactions are dominant, while for the longest spacer 12-12-12 the driving force involves hydrophobic interactions. Qualitatively, with the intermediate 12-5-12 and 12-6-12 systems, the optimum balance is observed between Coulombic and hydrophobic interactions, explaining their strong fluorescence enhancement.
authors.affiliation: Departamento de Química and Faculdade de Farmácia, Universidade de Coimbra, 3004-535 Coimbra, Portugal. burrows@ci.uc.pt
authors.forename: Hugh D
authors.initials: HD
authors.lastname: Burrows
authors.forename: María J
authors.initials: MJ
authors.lastname: Tapia
authors.forename: Claudia L
authors.initials: CL
authors.lastname: Silva
authors.forename: Alberto A C C
authors.initials: AA
authors.lastname: Pais
authors.forename: Sofia M
authors.initials: SM
authors.lastname: Fonseca
authors.forename: João
authors.initials: J
authors.lastname: Pina
authors.forename: J Seixas
authors.initials: JS
authors.lastname: de Melo
authors.forename: Yujie
authors.initials: Y
authors.lastname: Wang
authors.forename: Eduardo F
authors.initials: EF
authors.lastname: Marques
authors.forename: Matti
authors.initials: M
authors.lastname: Knaapila
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Monkman
authors.forename: Vasil M
authors.initials: VM
authors.lastname: Garamus
authors.forename: Swapna
authors.initials: S
authors.lastname: Pradhan
authors.forename: Ullrich
authors.initials: U
authors.lastname: Scherf
mesh_terms: D000476:Alkanesulfonates / Q000737:chemistry; D002412:Cations / Q000737:chemistry*; D003198:Computer Simulation; D005449:Fluorenes / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D015394:Molecular Structure; D009502:Neutrons; D010777:Photochemistry; D000071228:Polyelectrolytes; D011108:Polymers / Q000737:chemistry*; D055672:Static Electricity; D013499:Surface Properties; D013501:Surface-Active Agents / Q000737:chemistry*
chemical_list: D000476:Alkanesulfonates; D002412:Cations; D005449:Fluorenes; D000071228:Polyelectrolytes; D011108:Polymers; D013501:Surface-Active Agents; C521413:poly(1,4-phenylene-(9,9-bis(4-phenoxybutylsulfonate))fluorene-2,7-diyl)