Rules for Ion Pairing in Aqueous Interfacial Structures

With the support of the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry, Professor Paul S. Cremer of Pennsylvania State University will explore the rules for ion pairing at chemically treated interfaces. The surfaces of many organic-based substances are charged in water. Examples include everything from colloids in water-based paints to proteins residing in cell membranes. Negatively charged surfaces attract positively charged metal ions from the adjacent solution. This study will help determine how metal ions from salts interact with negatively charged surfaces. The answers to this question have relevance in the biological and environmental sciences on the one hand, and materials science and engineering problems on the other. This research will also train a new generation of students using a combination of experimental physical chemistry techniques that involve laser light. The students will also get experience from working with theoretical collaborators who will help model ion-pairing data. The results are expected to provide information not only on how tightly ions bind to charged surfaces but also give important details on how the chemical composition of a surface influences ion binding. The results will be applicable to a wide range of practical problems ranging from the most effective way to soften drinking water to the guidance of pharmaceutical drug manufacturing. Work will also be conducted in association with high school chemistry teachers to develop new week-long laboratories for high school students that take advantage of ion pairing to partition food coloring into salt rich liquid phases. The goal will be to create experiments that can be conducted without the need for complex equipment or expensive chemicals.Experiments will be performed using infrared-visible sum frequency vibrational spectroscopy (SFVS) to probe the interactions of Hofmeister ions with Langmuir monolayers of long chain surfactants at the air/water interface. The vibrational spectra should contain information on interfacial water structure, alkyl chain ordering and headgroup hydration. In these experiments, both positively and negatively charged headgroups will be employed. This will include studies of carboxylate, sulfate, sulfonate, phosphate, and phosphate anionic headgroups as well as amine and trimethylamine cationic headgroups. A key feature of these experiments will be to conduct measurements as a function of interfacial charge density. This will be accomplished by using long chain fatty alcohols as filler surfactants to tune the mole fraction of charged headgroups. The order of binding for counterions will be noted and equilibrium binding constants will be measured for each counterion that is employed. Special attention will also be paid to conditions that lead to contact ion pair formation as opposed to solvent-shared ion pairing. Where appropriate, complementary data in bulk solution on ion pairing will be obtained by using multicurve resolution Raman spectroscopy. These types of systematic measurements should enable the elucidation of a set of rules that govern counterion pairing at macromolecular interfaces as a function of chemical identity and interfacial structure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.