Langmuir Monolayer Project

X-ray Scattering and Optical Studies of Langmuir Monolayers

Former Project Members:

The Langmuir monolayer work at Harvard is currently supported by the National Science Foundation through Grant No. NSF-DMR-98-72817. Measurements at the NSLS are supported by DOE grant DE-AC02-76CH00016. Investigators from other institutions that collaborate with us receive other support.

Project summary:

Langmuir monolayers (LMs), i.e. one-molecule thick films at the gas/water interface, provide one example of real physical systems which should allow an experimental study of two-dimensional (2D) physics. The principal goal of this project is to study the statistical and structural properties of 2D phases formed by LMs by utilizing various surface-sensitive x-ray scattering techniques.

Through the grazing incidence diffraction (GID) studies aided by high-intensity synchrotron-generated x-rays, the microscopic structures of ordered LM phases of simple surfactant molecules (e.g., fatty acids, fatty alcohols, and phospholipids) were characterized in the late 80s through mid 90s by different research groups, with our group being one of them. These studies demonstrated that the long-chain amphiphilic molecules form close packed 2D structures on water in which the chains are oriented either normal to the surface or tilted at relatively small angles to the surface normal. These structures are similar to the various tilted and untilted phases of smectic liquid crystals.

Our more recent efforts are directed towards studies of LMs formed by larger molecules, which are not necessarily amphiphilic like the aformentioned simple surfactant molecules and are shaped either like a sphere or a rod. The larger size of the molecules implies that the constructive interference in the GID patterns that can be used to characterize intermolecular packing occurs at smaller angles with larger amplitudes. The combination of this and the large scattering power due to a greater number of electrons per molecule has the important consequence that broad GID peaks may be observed from disordered LM phases. This would make it possible to characterize the structural changes in not only ordered but also noncrystalline part of 2D phase diagram.

One class of LM systems of current interest are sphere-like molecules, consisting of derivatized gold nanoparticles which are "hairy" due to the attachment of hydrocarbon chains around them. For these LMs, we collaborate with Prof. R.B. Lennox and his group at McGill University. The large number of electrons contained in the gold core provides a high x-ray contranst that is advantageous for x-ray scattering studies. The metal nanoparticle LMs are interesting also from a practical standpoint; for example, the observation of a 2D metal/insulator transition has been reported recently for LMs of silver nanoparticle coated with short thiol chains.

Another class of LMs currently being studied consist of a-helical poly(glutamates) which are synthetic polypeptides with a rigid rod-like molecular shape. For this project, we collaborate with Prof. D.A. Tirrell and his group at Caltech. Unlike the conventional long-chain amphiphilic molecules, these rod-like molecules are oriented parallel to the water surface. That is, their molecular axes lie within the LM plane, just as in a 2D plane perpendicular to the helical axis of a cholesteric liquid crystal.

Our recent work on these different LM systems include:

Examples of studied systems:


(1) C60-propylamine adduct monolayers (sphere-like)
In collaboration with Prof. David Vaknin of Ames Lab and Iowa State University.

(2) PBLG (poly(g-benzyl L-glutamate)) monolayers (rod like)
In collaboration with Prof. David A. Tirrell of Caltech.

(3) C16-O-PBLG (poly(g-4-(hexadecyloxy)-benzyl L-glutamate)) monolayers ("hairy rod")
In collaboration with Prof. David A. Tirrell of Caltech.

(4) Derivatized gold nanoparticle monolayers (sphere-like)
In collaboration with Prof. R. Bruce Lennox of McGill University.

Experimental techniques used:

Related publications from our group: