CURVES IN CHEMISTRY

LANGUAGE OF SHAPE On viewing the curved silica shape Jehn Marie Lehn commented “you have synthesized a facsimile of a Brancouzi sculpture that I have seen in the New York museum of modern art”. This brought to mind Jehn Marie Lehn’s axiom in his classic text Supramolecular Chemistry, “if it exists it can be synthesized”. Having synthesized such uniquely curved shapes we felt compelled to investigate their growth and form, and coined the term morphosynthesis.

LANGUAGE OF SHAPE Both materials are comprised “only” of silica. The one on the top is dodecasil 3C and the one on the right is hexagonal mesoporous silica. The familiar Platonic octahedral shape has crystalline microporosity (IUPAC convention, length scale of pores < 2 nm) while the unfamiliar curved object has crystalline mesoporosity (2-50 nm) but is amorphous at the microscale. Single organic molecules facilitate the production of micropores in dodecasil 3C, whereas a “supramolecular assembly of non-covalently bound organic amphiphiles” enables the creation of mesopores in hexagonal mesoporous silica.

LANGUAGE OF SHAPE Morphology and microstructure of the former is determined by crystallographic physics, space groups, planar symmetry elements, and thermodynamics and kinetics of crystal nucleation and growth. Traditional concepts of this type however do not explain the periodic mesoscale porosity, growth and highly curved form of the latter.

REPRESENTATIVE SEM IMAGES OF HEXAGONAL MESOPOROUS SILICA FIBER, DISCOID AND SPHERE MORPHOLOGIES Better than 90% yield of a particular shape

REPRESENTATIVE TEM IMAGES OF WHOLE MOUNTED HEXAGONAL MESOPOROUS FIBERS AND DISCOIDS Fiber with channels running along the axis Discoid with channels whirling around main rotation axis

SURFACE TOPOLOGY OF MESOPOROUS SILICA MORPHOLOGIES Summary of AFM, SEM, TEM imaging studies

MORPHOGENESIS OF MESOPOROUS SILICA

MORPHOKINETICS SHAPE IN TIME Dynamic light scattering, DLS provides R h from D Early micellar species rapidly grow by ca. 1 nm Incorporation of silicate into headgroup region Beyond ca. 30 mins, very fast growth process

MORPHOKINETICS SHAPE IN TIME Time dependence of the crystalline mass yield of hexagonal mesoporous silica fibers and gyroids/discoids Data are fitted to the three parameter Avrami Equation

NUCLEATION AND GROWTH OF HEXAGONAL MESOPOROUS SILICA

LOOKING FOR EARLY-STAGE GROWTH NUCLEI A copper grid (TEM) and a piece of graphite (AFM) dip into the reaction mixture at early stages

INCUBATING THE SEED Tapping mode AFM image with phase detection Earliest stage growth nuclei on freshly cleaved graphite observed in a hexagonal mesoporous silica fiber, gyroid and sphere synthesis ~ nm soft objects

INCUBATING THE SEED TEM image Earliest stage growth nuclei on a copper grid observed in a hexagonal mesoporous silica fiber, gyroid, sphere synthesis ~50-70 nm mesostructured nanoparticles

MORPHOGENESIS OF HEXAGONAL MESOPOROUS SILICA GYROID SHAPE

SPLAY, TWIST, BEND  G d = 1/2K 1 (div n) 2 + 1/2K 2 (n curl n) 2 + 1/2K 3 [n curl n] 2 Theory of elasticity applied to nematic liquid crystals for three basic splay, twist, bend distortions K 1, K 2, K 3 Master equation for theoretical treatment of defects and textures in liquid crystals Minimum free energy yields stable distortions of the director field n K 1 K 2 K3K3

TOPOLOGICAL DEFECTS Maltese cross optical birefringence pattern observed for a surfactant-free and annealed hexagonal mesoporous silica discoid between crossed polars in an optical microscope

COLLOIDAL CRYSTALLIZATION Tuning the charge and EDL on silicate micelle and liquid crystal seeds EDL repulsion balanced against VDW attraction DLVO theory of colloidal interactions Acidity Dielectric constant Ionic strength Temperature CH 3 (CH 2 ) n N(CH 3 ) 3 + Cl - SILH x X+ xCl -

COLLOIDAL CRYSTALLIZATION Ionic atmosphere developed around a charged colloidal particle Electrical potential  in solution drops off exponentially with distance z from the colloidal particle  =  O exp(-  z) Electrical double layer thickness 1/  1/  =  kT/e 2  c i 2 z i 2 Reduced by increasing ionic strength or decreasing dielectric constant of solvent z

CURVING THE SEED Effect of surface positive charge on the structure of hexagonal lyotropic silicate liquid crystal growth nucleus (top) highly protonated (bottom) lightly protonated

GROWTH OF HEXAGONAL MESOPOROUS SILICA FIBER AND GYROID MORPHOLOGIES End-on addition of a highly charged silicate rod micelle to a highly charged silicate/silica fiber shaped seed Side-on addition of a lightly charged silicate rod micelle to a highly charged silicate/silica gyroid shaped seed.

COLLOIDAL CRYSTALLIZATION GROWTH MECHANISM Results of systematic variations of pH, ionic strength and dielectric constant provide compelling evidence for the role of colloidal interactions in controlling the nucleation, growth and form of mesoporous silica pH = 0.1 SEED FIBERGYROIDSPHERE ethanol, Et 4 N + pH = 0.6 pH = 1.0