Langmuir Monolayer Project

X-ray Scattering and Optical Studies of Langmuir Monolayers

Former Project Members:

• Masafumi Fukuto
• Oleg Gang
• Daniel Schwartz
• Mark Schlossman
• Marco Bommarito
• William Foster
• Mingchih Shih
• Ralf Heilmann

Project summary:

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:

• Observation of liquid-like 2D structure factor from amorphous LMs.
• Characterization of first-order mono/bilayer transitions.
• Characterization of microscopic inhomogeneities within Langmuir films via excess off-specular diffuse scattering.

Examples of studied systems:

• J. Chem. Phys. 107, 5531 (1997). PDF

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

• Phys. Rev. Lett. 81, 3455 (1998). PDF
• J. Chem. Phys. 111, 9761 (1999). PDF

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

• Phys. Rev. E 66, 010601(R) (2002). PDF
• J. Chem. Phys. 119, 6253 (2003). PDF

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

• J. Chem. Phys. 120, 3446 (2004). PDF

Experimental techniques used:

• Surface pressure vs area/molecule isotherm measurements, both relaxation and continuous methods.
• Brewster angle microscopy.
• X-ray specular reflectivity.
• Grazing incidence x-ray diffraction (GID).
• Off-specular x-ray diffuse scattering.

Related publications from our group:

• M. Fukuto, R.K. Heilmann, P.S. Pershan, A. Badia, and R.B. Lennox, "Monolayer/Bilayer Transition in Langmuir Films of Derivatized Gold Nanoparticles at the Gas/Water Interface: An X-ray Scattering Study", J. Chem. Phys. 120, 3446 (2004).

• M. Fukuto, R.K. Heilmann, P.S. Pershan, S.M. Yu, C.M. Soto, and D.A. Tirrell, "Internal Segregation and Side Chain Ordering in Hairy-Rod Polypeptide Monolayers at the Gas/Water Interface: An X-ray Scattering Study", J. Chem. Phys. 119, 6253 (2003).

• M. Fukuto, R.K. Heilmann, P.S. Pershan, S.M. Yu, C.M. Soto, and D.A. Tirrell, "Confinement Induced Order of Tethered Alkyl Chains at the Water/Vapor Interface", Phys. Rev. E 66, 010601(R) (2002).

• M. Fukuto, R.K. Heilmann, P.S. Pershan, S.M. Yu, J.A. Griffiths, and D.A. Tirrell, "Structure of poly(g-benzyl-L-glutamate) monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study", J. Chem. Phys. 111, 9761 (1999). Abstract

• M. Fukuto, R.K. Heilmann, P.S. Pershan, J.A. Griffiths, S.M. Yu, and D.A. Tirrell, "X-Ray Measurements of Noncapillary Spatial Fluctuations from a Liquid Surface", Phys. Rev. Lett. 81, 3455 (1998). Abstract

• M. Fukuto, K. Penanen, R.K. Heilmann, P.S. Pershan, and D. Vaknin, "C₆₀-propylamine adduct monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study", J. Chem. Phys. 107, 5531 (1997). Abstract

• G.M. Bommarito, W.J. Foster, P.S. Pershan and M.L. Schlossman, "A determination of the phase diagram of relaxed Langmuir monolayers of behenic acid", J. Chem. Phys. 105, 5265 (1996).

• W.J. Foster, M.C. Shih and P.S. Pershan, "The structure of a langmuir monolayer of methyl eicosanoate as determined by x-ray diffraction and Brewster-angle microscopy", J. Chem. Phys. 105, 3307 (1996).

• W.J. Foster, M.C. Shih and P.S. Pershan, "The structure and phases of a relaxed Langmuir monolayer of methyl eicosanoate as determined by grazing incidence x-ray diffraction and Brewster angle microscopy", Materials Research Society, Symposium AA: Applications of Synchrotron Radiation Techniques to Materials Science II 375, 187 (1995).

• D.K. Schwartz, M.L. Schlossman and P.S. Pershan, "Re-entrant appearance of phases in a relaxed monolayer of tetraconsanoic acid as determined by x-ray scattering", J. Chem. Phys. 96, 2356 (1992).

• M.L. Schlossman, D.K. Schwartz, E.H. Kawamoto, G.J. Kellogg, P.S. Pershan, M.W. Kim and T.C. Chung, "X-ray Reflectivity of a Polymer Monolayer at the Water/Vapor Interface", J. Phys. Chem. 95, 6628 (1991).

• M.L. Schlossman, D.K. Schwartz, P.S. Pershan, E. H. Kawamoto, G.J. Kellogg and S. Lee, "Relaxation and Appearance of a Reentrant Phase in a Molecular Monolayer", Phys. Rev. Lett. 66, 1599 (1991).

• M.L. Schlossman, D.K. Schwartz, E.H. Kawamoto, G.J. Kellogg, P.S. Pershan, B.M. Ocko, M.W. Kim and T.C. Chung, "X-ray studies of the liquid/vapor interface: water and polymer and fatty acid monolayers on water", Mat. Res. Soc. Symp. Proc. 177, 351 (1990).

• D.K. Schwartz, M.L. Schlossman, E.H. Kawamoto, G.J. Kellogg, P.S. Pershan and B.M. Ocko, "Thermal diffuse scattering studies of the water-vapor interface", Phys. Rev. A 41, 5687 (1990).

• D.K. Schwartz, A. Braslau, B.M. Ocko, P.S. Pershan, J. Als-Nielsen and J.S. Huang, "X-ray reflectivity studies of a microemulsion surface", Phys. Rev. A 38, 5817 (1988).