Polyelectrolytes (PEs) are widely used in applications such as water purification, wastewater treatment, and mineral recovery. Although much has been learned in past decades about the behavior of PEs in bulk aqueous solutions, their molecular behavior at a surface, and particularly an oil-water interface where many of their applications are most relevant, is largely unknown. From these surface spectroscopic and thermodynamics studies we report the unique molecular characteristics that several common polyelectrolytes, poly(acrylic acid) and poly(methylacrylic acid), exhibit when they adsorb at a fluid interface between water and a simple insoluble organic oil. These PEs are found to adsorb to the interface from aqueous solution in a multistepped process with a very thin initial layer of oriented polymer followed by multiple layers of randomly oriented polymer. This additional layering is thwarted when the PE conformation is constrained. The adsorption/desorption process is highly pH dependent and distinctly different than what might be expected from bulk aqueous phase behavior. macromolecular assembly | surface spectroscopy | water surfaces | hydrophobic surfaces | surfactants

Understanding the interfacial adsorption of polymers has become increasingly important because a wide range of scientific disciplines utilize these macromolecular structures to facilitate processes such as nanoparticle assembly, environmental remediation, electrical multilayer assembly, and surfactant adsorption. Structure and adsorption characteristics for poly(acrylic acid) at the oil-water interface have been studied using vibrational sum frequency spectroscopy and interfacial tension to increase the comprehension of polyelectrolyte structure at interfaces. The adsorption of poly(acrylic acid) to the oil-water interface from the aqueous phase is found to be highly pH dependent and occurs in a multistep process, with the initial polymer adsorption displaying a high degree of conformational ordering.

From enhanced oil recovery to fertile soils and facial cosmetics, carboxylate containing organics are pervasive in our everyday lives. The carboxyl moiety whose charge and metal binding properties can be readily altered with pH often dictates the behavior of the organic to which it is attached. The focus of this study is on understanding how the carboxylate group controls the overall molecular behavior of two alkyl surfactants, Na-dodecanoate and Na-octanoate, at an oilÀwater interface. Detailed spectroscopic studies using vibrational sum frequency spectroscopy and interfacial tension measurements provide important new insights into the solvation, orientation, and hydrogen bonding of the adsorbed carboxylate headgroup and interfacial water in the oilÀwater boundary region and the influence of the attached alkyl tails on the surfactant characteristics. The results show that the molecular characteristics, from head to tail, are distinctly different than what has been observed for these surfactants at another hydrophobic/water interface, namely the air/water interface. This difference is attributed in large part to the solvating environment around the chains, which permits a more disordered monolayer and allows the carboxylates to adopt a wider variety of orientations at the interface and a larger number of hydrogen bonding scenarios.

With Benjamin Franklin's oil on water experiments as a historical example, people have long been fascinated with the physical characteristics of the interface between water and an organic liquid and the unique chemistry that can occur at that interface. In this paper, we present our current understanding of the structure, orientation, and bonding characteristics of this fluid and dynamic interfacial region based on the efforts in our laboratory over the past decade and the important research of others in this field. In our studies, in which we have used a combination of surface specific nonlinear vibrational spectroscopy in conjunction with molecular dynamics simulations, we find that a general feature of organic-water interfaces is that of weak bonding interactions between adjacent water molecules and between the water and organic molecules. These weak water-organic interactions, present at all of the interfaces that we have studied, result in significant interfacial structuring and molecular orientation on both sides of the interface. How the structuring of both the interfacial water and organic molecules is dependent on the nature of the organic media is discussed as well as how this interfacial structuring can facilitate molecular and ion transport across the interface. The discussion of the neat organic-water interface is followed by a summary of how this picture is changed with the addition of ions, surfactants, and biomolecules, and how the presence of the organic media plays a role in the adsorption and conformation of adsorbates relative to the vapor-water interface. Examples of recent applications for the oil-water interface to synthesis and future perspectives are discussed as well. † 2008 marked the Centennial of the American Chemical Society's Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem.