Surfactant/Nonionic Copolymer Interaction: A SLS, DLS, ITC, and NMR Investigation Pablo Taboada,* Emilio Castro, and Vı ´ctor Mosquera Laboratorio de Fı ´sica de Coloides y Polı ´meros, Grupo de Sistemas Complejos, Departamento de Fı ´sica de la Materia Condensada, Facultad de Fı ´sica, UniVersidad de Santiago de Compostela, Spain ReceiVed: June 14, 2005; In Final Form: August 26, 2005 The interactions between an oxyphenylethylene-oxyethylene nonionic diblock copolymer with the anionic surfactant sodium dodecyl sulfate (SDS) have been studied in dilute aqueous solutions by static and dynamic light scattering (SLS and DLS, respectively), isothermal titration calorimetry (ITC), and 13 C and self-diffusion nuclear magnetic resonance techniques. The studied copolymer, S 20 E 67 , where S denotes the hydrophobic styrene oxide unit and E the hydrophilic oxyethylene unit, forms micelles of 15.6 nm at 25 °C, whose core is formed by the styrene oxide chains surrounded by a water swollen polyoxyethylene corona. The S 20 E 67 / SDS system has been investigated at a copolymer concentration of 2.5 g dm -3 , for which the copolymer is fully micellized, and with varying surfactant concentration up to approximately 0.15 M. When SDS is added to the solution, two different types of complexes are observed at various surfactant concentrations. From SLS and DLS it can be seen that, at low SDS concentrations, a copolymer-rich surfactant mixed micelle or complex is formed after association of SDS molecules to block copolymer micelles. These interactions give rise to a strong decrease in both light scattering intensity and hydrodynamic radius of the mixed micelles, which has been ascribed to an effective reduction of the complex size, and also an effect arising from the increasing electrostatic repulsion of charged surfactant-copolymer micelles. At higher surfactant concentrations, the copolymer-rich surfactant micelles progressively are destroyed to give surfactant-rich-copolymer micelles, which would be formed by a surfactant micelle bound to one or very few copolymer unimers. ITC data seem to confirm the results of light scattering, showing the dehydration and rehydration processes accompanying the formation and subsequent destruction of the copolymer-rich surfactant mixed micelles. The extent of interaction between the copolymer and the surfactant is seen to involve as much as carbon 3 (C3) of the SDS molecule. Self-diffusion coefficients corroborated light scattering data. 1. Introduction A great deal of fundamental investigations have been gener- ated to study the interaction between different polymers and surfactants as a consequence of the varied applications found in technical processes for this class of mixtures: 1-4 paints, coatings, laundry detergents, cosmetic products, and pharma- ceutical formulations. One group of interesting polymers is the nonionic, water soluble, low molecular weight block copolymers consisting of a hydrophilic poly(oxyethylene) block (PEO, (-OCH 2 CH 2 )) and a second hydrophobic block. The combina- tion of hydrophilic and hydrophobic blocks confers to these block copolymers interesting and useful surface active and micellization properties in dilute aqueous solution and gelation in concentrated solutions. Most studies on this type of block copolymer have been focused on those whose hydrophobic block is formed by units of oxypropylene (-OCH 2 CH(CH 3 ), denoted as PPO or P) or 1,2-oxybutylene (-OCH 2 CH(C 2 H 5 ) denoted as BO or B), which are commercially available from BASF and Chemical Dow, respectively. However, less attention has been paid to other oxyalkylene block copolymers. For example, we have recently studied the micellization and gel properties of block copolymers containing styrene oxide ((-OCH 2 CH- (C 6 H 5 ), denoted as SO or S), 5,6 recently released onto the market by Goldsmischdt AG, and phenyl glicidyl ether ((-OCH 2 CHO- (C 6 H 5 ), denoted as GO or G), 7 not commercially available. In many copolymer applications surfactants are present, which is likely to influence the self-assembly properties of the copolymers to improve the properties of this kind of material. 8 The interaction mechanisms in these systems are dependent on surfactant type, polymer molecular weight, chemical structures of polymer and surfactant, hydrophobic content of the polymer, electrolyte, temperature, and solvent quality. The surfactant- copolymer interactions have been mainly focused on mixtures formed by PEO-PPO-PEO copolymers and classical anionic, cationic, and nonionic surfactants, 9-13 as a result of their commercial availability. For this class of systems, formation of mixed surfactant-copolymer micelles was found accompa- nied with a progressive reduction of the number of copolymer molecules and subsequent size decrease of these mixed ag- gregates. 9,10 On the other hand, some studies have been made to characterize the interactions between surfactants and other types of block copolymers, as the diblocks formed by polyb- utadiene-poly(ethylene oxide) (PB-PEO), 14,15 and polystyrene- poly(ethylene oxide) (PS-PEO) blocks. 16-18 In recent articles, we have analyzed the interactions between cationic and anionic surfactants with two diblock copolymers whose hydrophobic block is formed by styrene oxide units, 19-21 for which up to four different regions in the behavior of the system could be defined as the surfactant concentration increased. In the present article, we extend these previous studies and analyze the effect of copolymer hydrophobic block length and surfactant-copolymer mixed micelle structure on the interac- tions between diblock styrene oxide-poly(ethylene oxide) * To whom correspondence should be addressed. Phone: 0034981563100, ext. 14042. Fax: 0034981520676. E-mail: fmpablo@usc.es. 23760 J. Phys. Chem. B 2005, 109, 23760-23770 10.1021/jp0532061 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/16/2005