Morphology of and dislocation movement in n-C₄₀H₈₂ paraffin crystals grown from solution

Abstract

Ex situ atomic force microscopy, supplemented with in situ optical microscopy, was applied to the {0 0 1} surface of n-C₄₀H₈₂ paraffin crystals grown from n-hexane solution. Platelets of monomolecular thickness, small ultra-thin crystals originating from one or two growth spiral(s), as well as aggregates of thicker crystals were found. The monomolecular plates are the actual growth form of perfect n-paraffin crystals, the growth of which is determined by one- and two-dimensional nucleation. Owing to post-growth stresses acting on the thin crystals slip movement by screw dislocations occurred, which is visible as crossing of as-grown and post-growth steps on the crystal surface. The shear stresses for dislocation movement result from adhesion forces acting on the crystals after evaporation of the liquid.

Introduction

Four different modifications are known for n-paraffin crystals: triclinic, monoclinic, orthorhombic and hexagonal [1]. The hexagonal or rotator phase modification, in which the molecules can freely rotate around their long axes, only occurs just below the melting point. For higher even alkanes with chain length m>28, apart from the rotator phase, two basic crystal structures are known: monoclinic and orthorhombic. One of the even n-paraffins of this length scale that has been studied in detail is n-C₃₆H₇₄. For pure n-C₃₆H₇₄, the space group in which it crystallizes is monoclinic P2₁/a (Z=2) [2] or the related polytypic Pcab (Z=4). The latter is orthorhombic, but is denoted here as polytypic, to prevent confusion with the orthorhombic Pca2₁ structure mentioned below [3]. Because it is difficult to separate the higher n-paraffins from each other, n-C₃₆H₇₄ is often contaminated with impurities, being mostly other n-paraffins with a slightly different number of carbon atoms. With too many impurities, the crystals do not manage to crystallize in P2₁/a anymore and instead the space group becomes orthorhombic Pca2₁ (Z=4) [4]. It has been shown [5], [6] that large even n-paraffins with almost the same length (e.g. n-C₃₆H₇₄ and n-C₃₈H₇₈) solidify in homologously isomorphous crystal structures. Hence, these space groups are expected to apply to n-C₄₀H₈₂ crystals as well. Fig. 1 gives a schematic view of the three structures and presents the unit cell parameters of each. For Pca2₁, the molecules are stacked perpendicularly to the molecular layers and d₀₀₂=c/2=0.127 m+0.185 nm=5.26 nm [7], with m=40 the number of C atoms. For P2₁/a, the n-C₄₀H₈₂ molecules are stacked tilted with an angle of about 65° to the molecular layers; as a result d₀₀₁=0.113 m+0.175 nm=4.68 nm [7] is lower. For the polytypic Pcab, the tilting angle of the molecules is the same, but the direction is reversed for alternating layers. The monomolecular layer thickness d₀₀₂ is 4.68 nm as well.

The surface morphology of the {0 0 1} faces of n-paraffin has been studied intensively. Already, before the invention of atomic force microscopy (AFM), monomolecular steps could be observed by optical microscopy [1], [8] and electron microscopy [1], [9], [10], [11] with relative ease, because of the large step height. Attention was paid to space-group-dependent symmetry of spirals [1], interlacing and polytypic step patterns [1], [8]. The first observations of dislocation movement in n-paraffin crystals were already done in 1953 [11]. Using transmission electron microscopy, patterns resulting from screw dislocation movement by glide in n-C₃₉H₈₀ crystals were observed.

In the present study, the surfaces of n-C₄₀H₈₂ crystals grown from solution are examined using AFM. This technique, being able to measure steps down to 1 Å, experiences no difficulties at all in imaging the molecular height steps of about 5 nm on {0 0 1} n-C₄₀H₈₂, and therefore is well suited for imaging this crystal surface in a quantitative way without any further preparation. These observations are supplemented with in situ optical microscopy. The morphology corresponds with the orthorhombic space group Pca2₁. AFM revealed very flat surfaces: the crystals of monomolecular thickness did not reveal any growth steps; the thicker crystals are covered with growth spirals originating from single $\text{b}\text{=}\text{1}\text{2}\text{[0}\text{0}\text{1]}$ screw dislocations. A most striking observation was the occurrence of monomolecular steps crossing each other, which is shown to result from post-growth dislocation movement.