In the present work we provide experimental evidence on the influence of ionic strength and concentration of different salts, belonging to the Hofmeister series for CTA+ head-group, with mono (NaF), di ((COONa)2) and tri (Na3PO4) valent characteristics [1], on the rheo-optical properties of 5 % wt. aqueous wormlike micellar solutions (WLMs) of cetyltrimethylammonium tosylate (CTAT) at, measured simultaneously under simple shear flow with a Small-Angle Light Scattering (SALS) technique. We reveal a relationship between the fast-breaking regime and development of SALS “butterfly-like” patterns for CTAT solutions and these salts. Firstly, we analyse the effects of three salts in a non-destructive test, Small-Amplitude Shear Oscillatory (SAOS), where the di and tri valent salts cause a relatively sharper decrease in the relaxation time, complex viscosity magnitude and contour length. Under simple shear flow, η follows a similar trend to |η∗|, where the increase in salts concentration and shift to the salting-out side promotes a smaller first-Newtonian-plateau level. The comparison between η and |η∗| reveals that the Cox-Merz rule is satisfied at lower molar ratios [2,3], as there is a displacement towards salting-out salts. BMP model estimates of the structural relaxation time, λs, and the Maxwell relaxation time at high shear rates, λ∞ [4], show a faster structural recovery with di and trivalent salts, in contrast to mono-valent salts. In addition, the SALS-pattern analysis reveals that an earlier transition to the fast breaking-regime generates an appearance of the scattering patterns at lower shear rates, suggesting that the salting-out salts promote an enhancement in concentration fluctuations. Finally, the structure factor plots display that salting-out salts cause the formation of a second shoulder in relatively smaller salt concentrations and shear rates than salting-in salts.
References
[1] E. Macias, F. Bautista, J. Perez-Lopez, P. Schulz, M. Gradzielski, O. Manero, J. Puig, and J. Escalante, Effect of ionic strength on rheological behaviour of polymer-like cetyltrimethylammonium tosylate micellar solutions, Soft Matter 7, 2094–2102 (2011).
[2] C. Fierro, L. Medina-Torres, F. Bautista, E. E. Herrera-Valencia, F. Calderas, and O. Manero, The structure factor in flowing wormlike micellar solutions, J. Non-Newton. Fluid Mech. 289, 104469 (2021).
[3]
M. Romero-Ureña, L. Medina-Torres, O. Manero, J. E. López-Aguilar, Rheo-optics
of giant micelles: SALS patterns of cetyltrimethylammonium tosylate solutions
in presence of sodium bromide, J. Non-Newton. Fluid Mech. (2024) 105286.
[4] O. Manero, F. Bautista, J. F. A. Soltero, and J. E. Puig, Dynamics of wormlike micelles: the Cox–Merz rule, J. Non-Newton. Fluid Mech. 106, 1–15 (2002).
JEL-A acknowledges the support from Secretaría de Ciencia, Humanidades, Tecnología y Innovación (SECIHTI, Mexico - grant number CF-2023-I-318) and from Universidad Nacional Autónoma de México UNAM (grant numbers PAPIIT IA102022, IN106424 and PAIP 5000-9172 Facultad de Química). JEL-A and OM acknowledge the support from UNAM under the project with Grant No. PAPIIT IN100623. MR-U acknowledges the support from Secretaría de Ciencia, Humanidades, Tecnología y Innovación (SECIHTI, Mexico) for the scholarship (CVU number 1084375) to fund his post-graduate studies.