AVS 65th International Symposium & Exhibition
    Nanometer-scale Science and Technology Division Tuesday Sessions
       Session NS+AM+MI+MN+SS+TR-TuA

Paper NS+AM+MI+MN+SS+TR-TuA11
Effects of Dimensionality on the Reactivity of Carboxylic-Acid-Terminated Monolayers

Tuesday, October 23, 2018, 5:40 pm, Room 102B

Session: SPM – Probing and Manipulating Nanoscale Structures
Presenter: Dominic Goronzy, University of California, Los Angeles
Authors: D.P. Goronzy, University of California, Los Angeles
E. Avery, University of California, Los Angeles
N.M. Gallup, University of California, Los Angeles
J. Staněk, Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic
J. Macháček, Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic
T. Baše, Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic
K.N. Houk, Chemistry and Biochemistry, University of California, Los Angeles
P.S. Weiss, University of California at Los Angeles
Correspondent: Click to Email
Self-assembled monolayers (SAMs) are an advantageous construct to modify surfaces and thereby to tune material properties. Three major determinants affect the spontaneous assembly of monolayers: the substrate-monolayer interface, the interactions between the adsorbate molecules, and the monolayer-environment interface. By controlling the exposed functional groups in the SAM, this interface between the surface and the environment can be probed and manipulated. To examine how exposed carboxyl functional groups modulate the properties of SAMs, we have tethered thiol-functionalized carborane cage molecules with and without a carboxyl group to Au{111} surfaces. Using scanning tunneling microscopy (STM), visualization of the para-functionalized carboxylic acid carboranethiol was only possible in the form of a mixed monolayer of functionalized and unfunctionalized molecules. These experiments demonstrated that the functionalized molecule adopted the same nearest-neighbor spacings on the surface as the unfunctionalized para-carboranethiol, approximately 7.2 Å. By comparison, in our study of carboranethiols with a carboxylic acid functional group attached in the meta position, we have been able to image pure functionalized monolayers via STM with these molecules showing nearest-neighbor spacings of 8.4 Å, approximately 1 Å larger than the unfunctionalized carboranethiols. Studies of two different isomers of meta-carboranethiolate carboxylic acids yielded similar results. In order to examine the effects of changing dimensionality from a 3D solvent system to a 2D thin film, we applied contact angle titration to probe the reactivity of the carboxylic acid head group. These experiments demonstrated a substantial shift from a pKa of 3-3.2 in solution (3D) to an apparent surface pKa of approximately 6.5-7.5. Density functional theory calculations were performed to test the effects of desolvation by 2D confinement and showed shifts in pKa consistent with the experimental data. Together, these results support the concept that the confinement in a 2D environment induces significant changes in reactivity in the molecules, as evidenced by the pKa shift. Thus, precise positioning of functional groups in SAMs is an important tool to investigate the behavior of surface-confined molecules. This system may also serve as a model to explore dimensionality effects in biological systems, including proteins confined within membranes.