Crystallization of medium-length 1-alcohols in mesoporous silicon: An x-ray diffraction study

The linear 1-alcohols $n{\text{-C}}_{16}{\text{H}}_{33}\text{OH}$, $n{\text{-C}}_{17}{\text{H}}_{35}\text{OH}$, $n{\text{-C}}_{19}{\text{H}}_{39}\text{OH}$ have been imbibed and solidified in lined up, tubular mesopores of silicon with 10 and 15 nm mean diameters, respectively. X-ray diffraction measurements reveal a set of six discrete orientation states (“domains”) characterized by a perpendicular alignment of the molecules with respect to the long axis of the pores and by a fourfold symmetry about this direction, which coincides with the crystalline symmetry of the Si host. A Bragg peak series characteristic of the formation of bilayers indicates a lamellar structure of the spatially confined alcohol crystals in 15 nm pores. By contrast, no layering reflections could be detected for 10 nm pores. The growth mechanism responsible for the peculiar orientation states is attributed to a nanoscale version of the Bridgman technique of single-crystal growth, where the dominant growth direction is aligned parallel to the long pore axes. Our observations are analogous to the growth phenomenology encountered for medium length $n$-alkanes confined in mesoporous silicon [A. Henschel, T. Hofmann, P. Huber, and K. Knorr, Phys. Rev. E 75, 021607 (2007)] and may further elucidate why porous silicon matrices act as an effective nucleation-inducing material for protein solution crystallization.

Figure 1

(Color online) X-ray diffraction pole figure of the (20) Bragg reflection of C19OH confined in mesoporous silicon at 298 K. As illustrated in the upper panel, the orientation of the sample sheet with respect to the scattering vector $\mathbf{q}$ is specified by the polar angle $\Phi$ and the azimuth $\chi$. The two orientational domains of the molecules about the $\chi$ axis are schematically sketched in the figure: The orientation of the long axes of the molecules about the $\chi$ axis differ by $90°$ resulting in a view on the lateral herringbone-type ordered in-plane structure of the alcohols for the first domain (left pore) and a side view on the lamellar order of the chains for the second domain (right pore). Note that the long axes of the molecules in both $\chi$ domains are perpendicular to the long pore axis, which coincides with the [100] direction of the Si host.

Figure 2

(Color online) A series of $\Phi$-rocking scans on C17OH crystallized in mesoporous silicon at 245 K. The scattering vector $q$ lies in the $\chi =90°$ plane. $\left|\mathbf{q}\right|$ is held constant along an individual scan at a value specified in the figure. The three peaks observed are the fundamental triple mentioned in the text.

Figure 3

(Color online) A series of radial scans on C17OH solidified in mesoporous silicon showing the $\left(11\right)/\left(1–1\right)$ and the (20) reflections at selected temperatures $T$ indicated in the figure, both on cooling and heating through the melting or freezing and the C-$R$ transition. The scattering vector is parallel to the pore axis $\left(\Phi =0\right)$.

Figure 4

(Color online) Radial scan on C16OH solidified in mesoporous silicon. The scattering vector is perpendicular to the pore axis $\left(\Phi =90°\right)$. The reflection comb characteristic of the equidistant Bragg peaks due to bilayer formation is included as a guide for the eye. Solid lines of the comb mark clearly observed bilayer Bragg peaks, whereas dotted lines indicate allowed bilayer Bragg peak positions, which, within our $q$-resolution, coincide with the in-plane reflections $\left(11\right)/\left(1–1\right)$, (20), (31), and ($3–1$), respectively.