BEGIN:VCALENDAR VERSION:2.0 PRODID:-//jEvents 2.0 for Joomla//EN CALSCALE:GREGORIAN METHOD:PUBLISH BEGIN:VEVENT UID:7b94b2df16eff68c8518f54b5718cbe9 CATEGORIES:Colloquium CREATED:20180117T165422 SUMMARY:Professor Laurence Romsted DESCRIPTION:Professor Laurence RomstedRutgers University Department of Chemistry and Ch emical BiologyTuesday February 13, 2018\n11:00AM, WL260\n "Ion-specific Eff ects on Micelles Reflect Interfacial Headgroup-counterion Pairing and Dehyd ration: a Molecular Dynamics/Chemical Trapping Study"\nAfter ~130 years and counting, consensus is still absent on the nature of the ion-specific inte ractions responsible for the Hofmeister series commonly observed in the pro perties of association colloids and proteins in aqueous solution. We develo ped a combined molecular dynamics (MD)/chemical trapping (CT) approach to e stimate counterion distributions and extent of ion-pairing in the interfaci al regions of cationic micelles (CT reaction (Figure 1A) and location (Figu re 1B)). We estimated interfacial molarities of counterions and water and t he fractions of headgroup-counterion pairs in micelles of dicationic, 10-2- 10 2X, gemini or mono cationic, CTAX, surfactants, with, respectively, thre e or two Hofmeister counterions, X–\n= Cl–, Br–, and AcO–. The measured phy sical properties of the micelles, e.g., cmc and ?, follow the traditional H ofmeister anion order: Br– > Cl– > AcO––although AcO– has intriguing amphiphilic properties. Surprisingly, the computed radial distribution func tions from the micelle center of mass of the surfactant headgroups and the reactive arenediazonium ion headgroup (-N2 +) are virtually identical (Figu re 1C). Thus, the shell thickness, ca. 15 Å, and volumes demarked by the ge mini and probe headgroups are identical. The counterion and water molaritie s and fractions of headgroup-counterion pairs within that region were compu ted from the MD simulations and compared with those obtained by CT. The cou nterion and water molarities and fractions of interfacial ion-pairs obtaine d by both methods are qualitatively consistent, follow the Hofmeister order (water molarity orders are the converse of the counterions), and confirm t he essential validity of the force fields chosen for the MD simulations and the primary assumptions of the CT method. Our results show that the interf acial region is broader than the micellar core (Figure 1C), is a concentrat ed salt solution, ca. 1-2 M, and is composed of significant fractions of he adgroup-counterion pairs, free ions, water, and a mixed hydrocarbon-water l ayer at the core boundary. The interfacial region’s composition should be s ensitive to changes in headgroup structure, counterion type and concentrati on caused by changes in bulk concentrations and changes in interfacial conc entrations should influence the balance-of-forces that determine associatio n colloid formation, size and shape.\n X-ALT-DESC;FMTTYPE=text/html:
Tuesday February 13, 2018
< strong>11:00AM, WL260
"Ion-sp ecific Effects on Micelles Reflect Interfacial Headgroup-counterion Pairing and Dehydration: a Molecular Dynamics/Chemical Trapping Study"
< p>After ~130 years and counting, consensus is still absent on the nature of the ion-specific interactions responsible for the Hofmeister series c ommonly observed in the properties of association colloids and protein s in aqueous solution. We developed a combined molecular dynamics (MD)/chem ical trapping (CT) approach to estimate counterion distributions and e xtent 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-co unterion pairs in micelles of dicationic, 10-2-10 2X, gemini or mono c ationic, CTAX, surfactants, with, respectively, three or two Hofmeister cou nterions, X–