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The long term goal of the ACS-PRF proposal is focused on synthesizing nanorods of desired aspect ratio and subsequently developing methods for a suspension based assembly of these particles into membrane like structure consisting of one rod length thick monolayers of aligned rods. The exact assembly pathway of nanorods is highly dependent on numerous microscopic parameters such as the rod length and diameter as well as the range and the strength of the attractive depletion potential. Because of this complex behavior our first goal was to develop theoretical models of colloidal membranes. These models will serve as a guide for the synthesis of nanorods of predetermined geometries and aspect ratios that will be assembled into desired 2D membranes.  

To establish general and widely applicable design principles for assembling 2D nanorod membranes we have developed detailed computer simulations. These efforts were undertaken in collaboration with the group of Michael Hagan at Brandeis University. Our collaborative effort has elucidated the molecular forces leading to assembly of two dimensional membrane-like structures composed of a one rod-length thick monolayer of aligned rods from an immiscible suspension of hard rods and depleting polymers. The results from out computer simulations predict that monolayer membranes are thermodynamically stable above a critical rod aspect ratio and below a critical depletion interaction length scale. Outside of these conditions alternative structures such as stacked smectic columns or nematic droplets are thermodynamically stable. Our work demonstrates that collective molecular protrusion fluctuations alone are sufficient to stabilize membranes composed of homogenous rods with simple excluded volume interactions. These results are currently being prepared for a publication.

In summary, our computational efforts so far have demonstrated for the first time that entropic forces are sufficient to stabilize monolayer colloidal membranes at equilibrium. Our simulations predict that the width of the isolated colloidal membrane phase depends strongly on aspect ratio and depletant size. While most previous simulations of hard rods considered small aspect ratios, our prediction of a critical aspect ratio below which the colloidal membrane phase disappears suggests that large aspect ratios dramatically alter the phase behavior. The predicted critical aspect ratio is only qualitative, but can be tested by monitoring the phase behavior of depletant and rods with varying lengths. This will be the goal of our grant in the subsequent year.