Effect of charge and telechelic polymer on the structural ordering and stability of neutral lamellar phases: insights from small-angle neutron scattering on TX100/TX35 systems

Abstract

Small-angle neutron scattering (SANS) experiments were conducted in this study to investigate the structure and stability of a neutral lamellar phase. This phase consists of Triton X₁₀₀ (TX₁₀₀) as the primary surfactant, Triton X₃₅ (TX₃₅) as the co-surfactant, and water. To prepare samples within the lamellar phase region, the mass ratio was fixed at Ω = TX₃₅/TX₁₀₀ = 1.26. All scattered intensities, I(q), were fitted using the Nallet model, which provides insight into the spatial arrangement of membranes in the system. We first focused on the effect of the volume fraction ϕ on the structure and stability of the lamellar phase. Subsequently, a cationic surfactant, cetylpyridinium chloride (CpCl), was added at a fixed concentration of Γ = 1% to samples with different volume fractions in order to assess the influence of introducing charges on the lamellar structure. Finally, a telechelic polymer, polyethylene oxide modified with hydrophobic alkyl (dodecyl) groups at both chain ends (denoted PEO-2m), was incorporated at a fixed concentration of Ψ = 10% into samples of various volume fractions. The results reveal that neither the variation of the volume fraction nor the addition of charges or PEO-2m alters the phase type: all samples remain in the lamellar phase. However, the internal structure is strongly affected. At low volume fractions (ϕ = 5% and ϕ = 10%), the membranes are disordered and lack a well-defined lamellar arrangement, whereas at higher volume fractions (ϕ = 20% and ϕ = 30%), they adopt an ordered state typical of a well-organised lamellar structure. The introduction of charges or PEO-2m significantly enhances the structural order: a well-defined lamellar phase is obtained even at low volume fractions (ϕ = 5% and ϕ = 10%) in the presence of charges, and at ϕ = 10% when PEO-2m is added.