J. Phys. Chem. B 2002, 106, 593-598
593
Molecular Tracer Dynamics in Crystalline Organic Films at the Solid-Liquid Interface David F. Padowitz,* David M. Sada, Eric L. Kemer,† Michael L. Dougan, and W. Andrew Xue Department of Chemistry, Amherst College, Amherst, Massachusetts 01002 ReceiVed: May 10, 2001; In Final Form: September 11, 2001
A molecular tracer method for scanning tunneling microscopy allowed the observation of individual molecular events at a crystal surface in solution. Long-chain alkanes or ethers were coadsorbed with similar length thioethers on a graphite surface. Due to the distinctive contrast of the sulfur atoms, it was possible to track individual thioether tracer molecules. The exchange of tracer molecules between the adsorbed monolayer and the overlying solution was measured at equilibrium. Rates depended on chain length, temperature, and neighboring molecules. Tracers at boundaries between crystal domains revealed specific molecular processes driving boundary fluctuations and domain growth. In addition to adsorption-desorption driven rearrangements, collective motion within the monolayer has been seen. Longer chain ethers exhibit two phases having different molecular configurations at grain boundaries and different microstructural habits. Transition between phases was promoted by the scanning probe.
Introduction Organic thin films are ubiquitous in nature and technology. They have also been fruitful subjects for scanning tunneling microscopy (STM). Molecules containing long alkane chains often form crystalline monolayers on graphite that image well even in solution. Crystal structures and self-assembly have been studied in a number of substituted alkanes, long-chain ethers, fatty acids, and liquid crystals.1-4 The appearance of different functional groups in tunneling images is well studied in these films and has been used for detailed analysis of molecular orientation.5-7 The dynamics of crystalline organic monolayers have also been observed by STM. Nucleation and growth of crystal domains as well as rearrangements at crystal defects and boundaries have been examined.8-14 Equilibration of the adsorbed layer with solution is evident in replacement of a layer by a more stable adsorbate.15-17 Stevens and Beebe saw exchange of thiols and alcohols in mixed monolayers, demonstrating that even well-ordered regions are not static.18 Unfortunately, changes in conformation at the ends of the molecules interfered with identification of the thiols and alcohols. We have found well-matched pairs of molecules that cocrystallize in all proportions and remain clearly distinguishable.19 We used long-chain alkane or ether hosts with an admixture of the corresponding thioether as tracer, one set is illustrated in Figure 1. Sulfur is similar in size and conformation to a methylene group in an alkane chain, but images as a distinctly bright spot. Located in the middle of the chain, the sulfur is not obscured by flexibility at the ends. With a small concentration of thioether in a monolayer of ether or alkane, it is easy to identify individual tracer molecules. By revealing processes otherwise undetectable, tracers make possible unique kinetic and dynamic observations. For instance, STM images of molecular crystals give a misleading impression of stability. If a molecule in a crystal is replaced quickly, the * To whom correspondence should be addressed. Present address: Symyx Technologies, 3100 Central Expressway, Santa Clara, CA 95051. Telephone: (408)764-2000.Fax: (408)748-0175.E-mail:
[email protected]. † St. Andrew’s School, 350 Noxontown Road, Middletown, DE 197091605.
Figure 1. Hosts and tracer. From top, the 33 carbon alkane tritriacontane, hexadecyl ether, and the thioether hexadecyl sulfide. Alkane or ether served as host, with a small admixture of thioether as tracer. Twenty-five and 45 atom chains were also studied.
image remains well defined even to atomic details. Exchange of molecules between monolayer and solution or rearrangements within a crystal may leave no sign. Molecular tracers expose the change. Kinetics cannot be conventionally measured at equilibrium, since concentrations remain constant. By observing individual tracer molecules, however, we determined adsorption and desorption rates at equilibrium. Even when a change is apparent, as in crystal grain growth or domain boundary motion, the paths by which molecules rearrange within a crystal can be difficult to identify. Tracers have allowed us to better track molecules at crystal boundaries and recognize specific molecular events that are pivotal in domain motion and grain growth. Experimental Section Thioether tracers were matched with alkane or ether hosts having chain lengths of 25, 33, or 45 atoms. The tracer-host pairs were dodecyl sulfide in pentacosane, hexadecyl sulfide in hexadecyl ether or tritriacontane, and docosyl sulfide in docosyl ether. A typical sample was 2 mM thioether and 2 mM alkane in phenyloctane solvent. A few microliters of solution was deposited on freshly cleaved graphite and STM images taken through the liquid layer. The surface ratio of tracer to host depended on solution ratio, but varied considerably among experiments. For an equimolar solution of hexadecyl sulfide and tritriacontane, the ratio of thioether to alkane on the surface ranged from 1:5 to 1:15. Pentacosane CH3(CH2)23CH3 (99%) and dodecyl sulfide [CH3(CH2)11]2S (93%) from Aldrich and hexadecyl sulfide [CH3(CH2)15]2S, hexadecyl ether [CH3(CH2)15]2O (95%), and tritria-
10.1021/jp011790h CCC: $22.00 © 2002 American Chemical Society Published on Web 12/01/2001
594 J. Phys. Chem. B, Vol. 106, No. 3, 2002
Padowitz et al. TABLE 1: Surface Residence Timesa host
length
1/k at 10 °C (s)
1/k at 25 °C (s)
25 atom
