Multicomponent Hollow Tubules Formed Using Phytosterol and γ-Oryzanol-Based Compounds: An Understanding of Their Molecular Embrace Michael A. Rogers,* ,† Arjen Bot, ‡ Ricky Sze Ho Lam, † Tor Pedersen, § and Tim May § Department of Food and Bioproduct Sciences, UniVersity of Saskatchewan, Saskatoon, SK, Canada, S7N5A8, UnileVer Research and DeVelopment Vlaardingen, OliVier Van Noortlaan 120, NL-3133 AT Vlaardingen, The Netherlands, and Canadian Light Source, Saskatoon, SK, S7N0X4, Canada ReceiVed: May 5, 2010; ReVised Manuscript ReceiVed: July 1, 2010 The formation kinetics of self-assembling tubules composed of phytosterol:γ-oryzanol mixtures were investigated at the Canadian Light Source on the mid-IR beamline using synchrotron radiation and Fourier transform infrared spectroscopy (FT-IR). The Avrami model was fitted to the changing hydrogen bonding density occurring at 3450 cm -1 . The nucleation process was found to be highly dependent on the molecular structure of the phytosterol. The nucleation event for cholesterol:γ-oryzanol was determined to be sporadic whereas 5R-cholestan-3-ol:γ-oryzanol and -sitosterol:γ-oryzanol underwent instantaneous nucleation. One- dimensional growth occurred for each phytosterol:γ-oryzanol mixture and involved the evolution of highly specific intermolecular hydrogen bonds. More detailed studies on the cholesterol:γ-oryzanol system indicated that the nucleation activation energy, determined from multiple rate constants, obtained using the Avrami model, was at a minimum when the two compounds were at a 1:1 weight ratio. This resulted in drastic differences to the microscopic structures and affected the macroscopic properties such as turbidity. The formation of the phytosterol:γ-oryzanol complex was due to intermolecular hydrogen bonding, which was in agreement with the infrared spectroscopic evidence. Introduction Molecularly self-assembled one-dimensional nanostructures have numerous potential applications including medical ap- plications, chemical synthesis, external pressure responsive systems, and electronics. Self-assembly into one-dimensional fibers exhibit distinct features whereby the assembling mol- ecules, within the cross-section of the fiber are highly ordered, similar to crystalline materials. 1 However, along the fiber axis, disorder may be incorporated, resulting in crystallographic mismatches causing the fiber to branch. 1 Self-assembly has been an area of active research attempting to acquire a fundamental knowledge which focuses on supramolecular chemistry and on gaining an understanding of the tailorability of physical parameters including size, shape, and external and internal structures. 2 However, the process of self-assembly still has a plethora of unanswered questions such as “why” and “how” do these materials assemble into fibrillar networks. 3 Molecular self-assembly may occur either spontaneously or under the influence of external forces such as shear, 4 pH, 5 temperature, 6 light, 7 or electric fields. 8 By varying the magnitude of these external forces, it is possible to control the size, dimensionality of growth, and the spatial distribution of mass. In many instances, molecular self-assembly occurs via a supersaturated state whereby the “gelator”-solvent melt is cooled below the melting point of the gelator, triggering the gelator to microscopically phase-separate and self-assemble via stochastic nucleation events driven by enthalpic forces. 9 Depending on the degree of supercool- ing, nucleation may occur either sporadically or instantaneously and is then followed by crystal growth causing gelator to accrete on the surface of the stable nuclei forming fibers via highly specific noncovalent interactions. The process of one-dimensional growth requires a meticulous balance between the contrasting parameters of solubility and those which control epitaxial growth, leading to an elongated axis. 10 This unique one-dimensional supramolecular configuration presents useful properties such as alignment, con- ductivity, biological interactions, and templating of novel soft materials. 2 In specific cases, the supramolecular assemblies may form organogels which aggregate to form three-dimensional networks entrapping the solvent phase. Considerable attention has recently focused on the ability of γ-oryzanol and different phytosterols to form hollow tubules of 7 nm diameter and just under 1 nm wall thickness. 11-15 Mixtures of phytosterol and γ-oryzanol establish a continuous three-dimensional network of hollow tubules capable of immobilizing apolar solvents such as polyunsaturated triglyceride oils over macroscopic length scales. 11,12,14 The tubule formation is a synergistic phenomenon between the phytosterol and the γ-oryzanol, because individually neither are capable of entrapping the oil phase. 11 Previous research also demonstrated that -sitosterol-rich organogels tend to be more turbid, which was attributed to enhanced aggregation of the tubules, leading to larger junction zones in the organogel since the diameter of the tubule does not change as a function of phytosterol:γ- oryzanol ratio. 12 At a 8:8 wt %/wt% ratio, these compounds produce a transparent gel. 12 While acknowledging the many regulatory restrictions on applications in foods, cosmetics, and medicine, we envisage numerous potential applications of these tubules, such as drug carriers for transdermal applications, encap- sulation of nutraceuticals, and perhaps most intriguing a trans/ saturated fat replacer. The use of the γ-oryzanol:phytosterol mixtures for hardstock replacement is interesting because it circumvents the need for trans or saturated fats, normally required to structure edible oils as a colloidal dispersion which provides * Corresponding author: E-mail: michael.rogers@usask.ca. Tel: + 306 966 5028. Fax: + 306 966-8898. † University of Saskatchewan. ‡ Unilever Research and Development Vlaardingen. § Canadian Light Source. J. Phys. Chem. A 2010, 114, 8278–8285 8278 10.1021/jp104101k  2010 American Chemical Society Published on Web 07/28/2010