Geraldine L. Richmond

Second harmonic generation studies of interfacial structure and dynamics

Abstract This review surveys the current state of the field of optical second harmonic generation as it pertains to the study of interfacial phenomena. The experimental and theoretical studies discussed examine fundamental issues regarding the source of the nonlinear polarizability at the interface. Also included are both equilibrium and time-resolved measurements of the structural and thermodynamic properties therein. Experimental investigations of a variety of solid materials in air, vacuum, solution, and in contact with other solids are considered. A discussion of recent studies of the liquid/liquid interface concludes this review.

Integration or Segregation: How Do Molecules Behave at Oil/Water Interfaces?

It has been over 250 years since Benjamin Franklin, fascinated with the wave-stilling effect of oil on water, performed his famous oil-drop experiments; nevertheless, the behavior of water molecules adjacent to hydrophobic surfaces continues to fascinate today. In the 18th century, the calming of the seas seemed the most pertinent application of such knowledge; today, we understand that oil-on-water phenomena underlie a range of important chemical, physical, and biological processes, including micelle and membrane formation, protein folding, chemical separation, oil extraction, nanoparticle formation, and interfacial polymerization. Beyond classical experiments of the oil-water interface, recent interest has focused on deriving a molecular-level picture of this interface or, more generally, of water molecules positioned next to any hydrophobic surface. This Account summarizes more than a decades work from our laboratories aimed at understanding the nature of the hydrogen bonding occurring between water and a series of organic liquids in contact. Although the common perception is that water molecules and oil molecules positioned at the interface between the immiscible liquids want nothing to do with one another, we have found that weak interactions between these hydrophilic and hydrophobic molecules lead to interesting interfacial behavior, including highly oriented water molecules and layering of the organic medium that extends several molecular layers deep into the bulk organic liquid. For some organic liquids, penetration of oriented water into the organic layer is also apparent, facilitated by molecular interactions established at the molecularly thin region of first contact between the two liquids. The studies involve a combined experimental and computational approach. The primary experimental tool that we have used is vibrational sum frequency spectroscopy (VSFS), a powerful surface-specific vibrational spectroscopic method for measuring the molecular structures of aqueous surfaces. We have compared the results of these spectroscopic studies with our calculated VSF spectra derived from population densities and orientational distributions determined through molecular dynamics (MD) simulations. This combination of experiment and theory provides a powerful opportunity to advance our understanding of molecular processes at aqueous interfaces while also allowing us to test the validity of various molecular models commonly used to describe molecular structure and interactions at such interfaces.

Water at Hydrophobic Surfaces: When Weaker Is Better

The structure of water molecules at the interface of four hydrophobic phases:  carbon tetrachloride, chloroform, dichloromethane, and air have been studied using molecular dynamics simulations. We discover that hydrophobic phases with weaker dipoles are more successful in orienting water molecules in the vicinity of the aqueous-hydrophobic interface. We create a visual layer-by-layer representation of how water molecules are structured next to these phases. Our findings contribute to an increased understanding of aqueous interfacial phenomena involving salts, ions, surfactants, biomolecules, and nanoparticle assembly.

Non-linear vibrational sum frequency spectroscopy of atmospherically relevant molecules at aqueous solution surfaces

Surface vibrational sum frequency spectroscopy has been shown to be a powerful surface probe of molecules adsorbed at solid and liquid surfaces. Studies described herein apply this method to studying heterogeneous airraqueous solution interfaces to understand surface adsorption and structure of several solute molecules adsorbed at aqueous surfaces. The . .

Ordered polyelectrolyte assembly at the oil–water interface

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.

Assembly and molecular order of two-dimensional peptoid nanosheets through the oil-water interface.

Significance Peptoid nanosheets are an emerging class of 2D nanomaterials that have the potential for use in a variety of applications ranging from molecular sensors to artificial enzymes. Because peptoids are highly designable polymers, nanosheets provide a general platform on which to display an enormous diversity of functionalities. Nanosheets are known to form through a unique monolayer compression mechanism, catalyzed by the air–water interface. Here we demonstrate that nanosheets can be formed via adsorption of peptoids at an oil–water interface. Using vibrational sum frequency spectroscopy, we show that electrostatic interactions are essential in the formation of an ordered peptoid monolayer at the interface, a critical intermediate in the nanosheet assembly pathway. These findings open the door for enhancing the complexity and functionality of 2D nanomaterials. Peptoid nanosheets are a recently discovered class of 2D nanomaterial that form from the self-assembly of a sequence-specific peptoid polymer at an air–water interface. Nanosheet formation occurs first through the assembly of a peptoid monolayer and subsequent compression into a bilayer structure. These bilayer materials span hundreds of micrometers in lateral dimensions and have the potential to be used in a variety of applications, such as in molecular sensors, artificial membranes, and as catalysts. This paper reports that the oil–water interface provides another opportunity for growth of these unique and highly ordered peptoid sheets. The monolayers formed at this interface are found through surface spectroscopic measurements to be highly ordered and electrostatic interactions between the charged moieties, namely carboxylate and ammonium residues, of the peptoid are essential in the ability of these peptoids to form ordered nanosheets at the oil–water interface. Expanding the mechanism of peptoid nanosheet formation to the oil–water interface and understanding the crucial role of electrostatic interactions between peptoid residues in nanosheet formation is essential for increasing the complexity and functionality of these nanomaterials.

Examination of the surface second harmonic response from noble metal surfaces at infrared wavelengths

Contributing factors to the second harmonic response from Ag(111) and Au(111) surfaces have been examined in the long wavelength limit where nonresonant conditions prevail. The rotational anisotropy in the nonlinear response is found to persist for both metals in the absence of surface resonances and to show convergent behavior in both the relative phase and intensity as the incident energy is lowered from resonant to nonresonant conditions. The phase difference between the in‐plane and out‐of‐plane responses in the long wavelength limit can be described by linear Fresnel theory. Whereas the tensor elements corresponding to the out‐of‐plane response are found to dominate, the in‐plane contributions are found to be non‐negligible.

Unique Assembly of Charged Polymers at the Oil-Water Interface

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.

Spiers Memorial Lecture. Recent experimental advances in studies of liquid/liquid interfaces.

Liquid/liquid interfaces play a key role in many important processes. Studying the molecular structure and interactions that occur at these interfaces can aid in our understanding of more complicated processes such as molecular transport across cell membranes. A variety of techniques have been applied to this pursuit. Here we present selected examples of exciting recent studies using different techniques to examine liquid/liquid interfaces.

Tunable picosecond infrared laser system based on parametric amplification in KTP with a Ti:sapphire amplifier

A picosecond laser system that will generate high-power tunable IR pulses with bandwidths suitable for spectroscopic applications is discussed. The system is based on white-light continuum generation in ethylene glycol and optical parametric amplification in potassium titanyl phosphate. The nonlinear-optical processes are driven by a regeneratively amplified Ti:sapphire laser that produces 1.7-ps pulses at a repetition rate of 1 kHz. Energies as high as 40 and 12 microJ have been achieved over the signal (1.02-1.16-microm) and idler (2.6-3.7-microm) tuning ranges, respectively. The IR beam temporal and spatial characteristics are also presented.