**Crystallization on Self Assembled Monolayers**

Michal Ejgenberg and Yitzhak Mastai *Bar Ilan University Israel* 

### **1. Introduction**

38 Advances in Crystallization Processes

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Over the past two decades, self assembled monolayers (SAMs) (Love *et al.*, 2005; Smith *et al.*, 2004; Ulman, 1996) have been extensively studied due to their many applications in various fields such as electrochemistry (Eckerman *et al.*, 2010), biosensors (Nyquist *et al.*, 2000), protein separation (Chun and Stroeve, 2002) and enantiomer separation (Mastai, 2009). SAMs are organic assemblies formed by the adsorption of molecules from solution or from the gas phase onto the surfaces of solids. These molecules spontaneously organize into highly ordered, crystalline (or semicrystalline) two dimensional films. The molecules composing the SAMs adsorb to the surface through a "headgroup", a functional group with a high affinity for the solid surfaces. Two of the most widely studied systems of SAMs are alkanethiols adsorbed on metals, including gold, silver and platinum and alkylsilane chains formed on silicon dioxide surfaces, including glass and mica. In these cases, the "headgroups" are thiols, which have a high affinity for metal surfaces and silanes, which have a high affinity for silicon-dioxide surfaces. SAMs have become so popular since they offer a unique combination of physical properties that allow fundamental studies of interfacial chemistry, solvent molecule interactions and self-organization. Their wellordered arrays and ease of functionalization make them ideal model systems in many fields.

One of the important advantages of SAMs is that they can be prepared in the laboratory by dipping the desired substrate in the required solution for a specified time followed by thorough washing with the same solvent and drying, often using a jet of dry nitrogen. Gasphase evaporation of the adsorbate can also form good monolayers, although structural control is difficult. Several factors affect the formation and packing density of the self assembled monolayers including the solvent, temperature, nature of adsorbate, adsorbate concentration and the nature and roughness of the substrate. SAM substrates range from planar surfaces to highly curved surfaces (which will not be discussed here). The most common planar substrates for alkanethiol SAMs are thin films of metals supported on flat surfaces, such as glass, silica wafers and mica. These substrates can be easily prepared using methods like physical vapor deposition (PVD) and chemical vapor deposition (CVD). In some cases, an additional layer of titanium or chromium between the solid and the metal is needed in order to improve the adhesion between them. The composition of the thin films (grain size, etc.) is affected by the properties of the metal used (for example, melting point), the solid surface roughness and the experimental conditions. This, in turn, affects the organization and density of the SAM. The most common alkylsilane substrates are silicon dioxide surfaces such as glass, which are usually pre-treated with sulfuric acid.

Crystallization on Self Assembled Monolayers 41

SAMs interact with the crystallizing molecules and thus, affect their organization, ultimately affecting the crystallization outcome. SAMs have been reported to affect crystal orientation, morphology, polymorphism and crystal size. The interaction between SAMs and molecules in solution has usually been rationalized on the basis of hydrogen bonding and/or strong

During crystallization, the SAM constituent exposed at the SAM surface interacts with molecules present in the crystallizing solution. These interactions are specific and in turn, can cause the stabilization of specific crystal faces. When this happens, the crystals grow in a specific orientation, causing changes in crystal morphology. Many researchers have reported specific crystal orientation on SAMs, brought about by interactions of the

For example, Lee *et al*. employed SAMs of rigid thiols on gold surfaces in order to investigate the effects of interfacial molecular recognition on nucleation and growth of L-Alanine and DL-Valine crystals (Lee *et al.*, 2002). L-alanine crystallizes from water in the orthorhombic space group P21212 (a=6.025Å, b=12.324Å and c=5.783Å), with bipyramidal

In order to examine the effect of the SAMs on L-Alanine crystallization, L-Alanine was crystallized from solutions and also onto a variety of rigid SAMs of thiols: 4'-methyl-4 mercaptobiphenyl, 4'-hydroxy-4-mercaptobiphenyl and 4-(4-mercaptophenyl) pyridine. Powder X-ray diffraction patterns and interfacial angle measurements of the L-alanine crystals are shown in Figure 2. In methyl-terminated SAMs, L-alanine selectively nucleated on the (020) plane on the SAM surface while on the OH-terminated SAMs, Lalanine nucleated on an unobserved (200) side face. In both cases, the area of each crystal face was substantially larger than the other faces on the crystal. Crystallization of Lalanine on 4-(4-mercaptophenyl) pyridine resulted in the (011) face as the plane corresponding to nucleation. The preferential interaction was explained on the basis of hydrogen bonding between the pyridine surface and the amino and methyl groups protruding out of the (011) plane. Figure 2 reveals that L-alanine crystals nucleating on SAM surfaces crystallize in an orthorhombic space group with similar unit cell dimensions. However, the functionalized SAMs have an effect on the nucleating plane

morphology, dominated by the {020}, {120}, {110} and {011} growth forms (Figure 1).

Fig. 1. L alanine crystal grown from aqueous solution. (Lee *et al.*, 2002)

and ultimately, on L-alanine crystal growth.

ionic interactions.

**2.1 Effects on crystal orientation** 

crystallizing molecules with SAMs.

The self assembled adsorbate has a great influence on the SAM outcome and can be tailored according to the desired SAM properties. The molecule used can possess a number of functional groups in addition to the molecule's headgroup. These functional groups can be distributed within the monolayer interior and located at the terminus of the molecule. Manipulation of the monolayer interior affects its degree of order and how easily electrons are conducted through it. For example, the molecular chain length and the steric crowding of the organic groups affect the density of the organic layer and the tilt angle of the molecule away from the surface normal. In general, longer chains and less robust organic groups yield denser, more organized SAMs, allowing high degrees of van der Vaals interactions (and in some cases, hydrogen bonds) with the neighboring molecules. The molecular constituent exposed at the SAM surface is critical to the SAM's interfacial properties. It affects the surface's general hydrophobic/hydrophilic character, adhesive properties and reactivity. In addition, it determines the surface interaction with other molecular species that come in contact when placed on the surface of the SAM.

SAMs are of prime technological interest, as the presence of molecules chemically bound to the surface renders the properties of the modified interface (i.e., wetting, conductivity, adhesion, and chemistry) to be entirely different than those of the bare substrate. The incorporation of functional moieties such as chromophores, electroactive groups, or molecules that can bond within the SAM (i.e., covalent cross-linking between adjacent molecules or non-covalent hydrogen bonding) enable capabilities in sensing, electron transfer, molecular recognition, and other areas (Smith *et al.*, 2004).

This review will focus on the use of SAMs in crystallization processes. We will begin with a short introduction on crystallization on SAMs. Then, we will review the latest advances in crystallization on patterned SAM's and effects of SAMs on crystal morphology and crystal polymorphism. This chapter will also include a description of chiral SAMs and their role in enantioselective crystallization.
