Tuesday February 13, 2018
"Ion-specific Effects on Micelles Reflect Interfacial Headgroup-counterion Pairing and Dehydration: a Molecular Dynamics/Chemical Trapping Study"
After ~130 years and counting, consensus is still absent on the nature of the ion-specific interactions responsible for the Hofmeister series commonly observed in the properties of association colloids and proteins in aqueous solution. We developed a combined molecular dynamics (MD)/chemical trapping (CT) approach to estimate counterion distributions and extent of ion-pairing in the interfacial regions of cationic micelles (CT reaction (Figure 1A) and location (Figure 1B)). We estimated interfacial molarities of counterions and water and the fractions of headgroup-counterion pairs in micelles of dicationic, 10-2-10 2X, gemini or mono cationic, CTAX, surfactants, with, respectively, three or two Hofmeister counterions, X–
= Cl–, Br–, and AcO–. The measured physical properties of the micelles, e.g., cmc and ?, follow the traditional Hofmeister anion order: Br– > Cl– > AcO––although AcO– has intriguing amphiphilic properties. Surprisingly, the computed radial distribution functions from the micelle center of mass of the surfactant headgroups and the reactive arenediazonium ion headgroup (-N2 +) are virtually identical (Figure 1C). Thus, the shell thickness, ca. 15 Å, and volumes demarked by the gemini and probe headgroups are identical. The counterion and water molarities and fractions of headgroup-counterion pairs within that region were computed from the MD simulations and compared with those obtained by CT. The counterion and water molarities and fractions of interfacial ion-pairs obtained by both methods are qualitatively consistent, follow the Hofmeister order (water molarity orders are the converse of the counterions), and confirm the essential validity of the force fields chosen for the MD simulations and the primary assumptions of the CT method. Our results show that the interfacial region is broader than the micellar core (Figure 1C), is a concentrated salt solution, ca. 1-2 M, and is composed of significant fractions of headgroup-counterion pairs, free ions, water, and a mixed hydrocarbon-water layer at the core boundary. The interfacial region’s composition should be sensitive to changes in headgroup structure, counterion type and concentration caused by changes in bulk concentrations and changes in interfacial concentrations should influence the balance-of-forces that determine association colloid formation, size and shape.