Phase behavior of electrostatic complexes in systems containing polymers and surfactants in the presence of inorganic salts

Author(s)

Marcos Vinícius Aquino Queirós

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Química
• Defense date: 2022-03-04

Examining board members

Watson Loh; Luis Henrique Mendes da Silva; Fernando Luis Barroso da Silva; Frank Herbert Quina; Edvaldo Sabadini

• Advisor: Watson Loh

Abstract

This work presents the study of the phase behavior of electrostatic complexes formed by the association of polymers and oppositely charged surfactants and polymers upon addition of inorganic salts. Poly(acrylate) with molar masses of 2 and 450 kDa were used with the cationic surfactant dodecyltrimethylammonium in the presence of NaBr or NaCl. For concentrations of complexes around 20 wt%, two important phase separation events were observed: associative phase separation, where polymer and surfactant remain in the same phase, and segregative phase separation, where both species are concentrated in different phases. Surfactant aggregates are initially packed in a cubic micellar structure that undergoes a transition to a micellar phase, which can change its size and shape as the salt concentration increases. For complex concentrations around 50 wt%, the systems present a single-phase with surfactant aggregates in a hexagonal or cubic micellar arrangement. The increase in the salt concentration causes all aggregates to form a hexagonal phase. Upon further addition, a new phase separation occurs, generating a surfactant-rich phase with a hexagonal structure and an aqueous phase that becomes polymer-rich at higher salt concentrations. Poly(acrylate) complexes with molar masses of 2 and 100 kDa and the polycation poly(diallyldimethylammonium) were also prepared and studied in the presence of NaCl and Na2SO4. For these complexes, associative and segregative phase separations are also observed in systems containing NaCl, but segregative separation is not observed in systems with Na2SO4. Phase analyses in NaCl-segregated systems not only found that polymers undergo strong segregation, but also that the small counterions partition between the phases. Each small ion concentrates on the phase that is the richest in its oppositely charged poly-ion. The interactions between oppositely charged polymers were also studied through Monte Carlo simulations. The screening Coulomb potential was applied in order to simulate the presence of a salt, whose concentrations were parameterized through the screening length (?D). We have found that at higher values of ?D, polymers tend to associate. This association tends to weaken as ?D decreases. On the other hand, at lower values of ?D, polymers tend to segregate. Finally, the effect of different charged groups from the polymers on the complexation selectivity was studied experimentally. We have verified that the sulfonate complexes preferentially over the carboxylate group, while that quaternary ammonium binds more favorably than primary ammonium groups. These systems also form multiphasic complexes (AU)

• FAPESP's process: 18/09099-3 - Phase behavior of electrostatic complexes in systems containing polymers and surfactants in the presence of inorganic salts
• Grantee: Marcos Vinícius Aquino Queirós
• Support Opportunities: Scholarships in Brazil - Doctorate