Comparative study of organic co-solvents on micellar and surface behavior of ionic surfactants: effects of DMSO, acetonitrile, and 1-propanol

Abstract This study investigates the effects of 10 % v/v dimethyl sulfoxide (DMSO), acetonitrile (ACN), and 1-propanol (1-PrOH) on the micellar and surface behavior of three ionic surfactants: sodiumdodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and cetylpyridinium chloride (CPC). The addition of co-solvents increased the critical micelle concentration (CMC), with 1-PrOH causing the largest increase due to its strong disruption of the hydrogen bonding network of water. Surface tension measurements revealed a decreased surface excess concentration ( Γ max ) indicating reduced surface adsorption. The minimum area per molecule ( A min ) increased with organic solvents, particularly in 1-PrOH-water mixtures, reflecting less efficient packing of the surfactant molecules at the surface. The packing parameter ( P ) decreased in mixed solvents, especially with 1-PrOH, suggesting a shift from spherical to elongated micellar structures. Aggregation numbers ( N agg ) were lower in mixed solvent systems, indicating smaller and less stable micelles. The free energy of adsorption ( <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mo>∆</m:mo> <m:msubsup> <m:mi>G</m:mi> <m:mrow> <m:mi>a</m:mi> <m:mi>d</m:mi> <m:mi>s</m:mi> </m:mrow> <m:mi>o</m:mi> </m:msubsup> </m:mrow> </m:math> ${\increment}{G}_{ads}^{o}$ ) became less negative, indicating reduced spontaneity for micelle formation and adsorption, with the strongest impact seen in 1-PrOH. The overall trend of solvent effects was 1-PrOH &gt; ACN &gt; DMSO, highlighting the role of solvent polarity and amphiphilicity in altering surfactant behavior. These results provide valuable insights for the optimization of surfactant-based formulations.