# Effect of Small Molecular Additives on Growth Rates of Molecular Crystals from the Melt near Glass-Transition Temperature

## TL;DR

This paper shows how small-molecule additives can change crystal growth rates based on differences in glass-transition temperatures and hydrogen bonding.

## Contribution

The novel finding is that small-molecule additives affect crystal growth rates depending on their glass-transition temperature relative to the host crystal.

## Key findings

- Growth rate increases when additive's Tg is lower than host crystal's Tg.
- Growth rate decreases when additive's Tg is higher than host crystal's Tg.
- Effect is observed only in host crystals capable of forming intermolecular hydrogen bonds.

## Abstract

Frequently, small
concentrations of additives can substantially
modify crystal growth rates from solutions, while a substantial concentration
of additives is required to likewise affect crystallization from the
melt. Recently, it was discovered that even 1 wt % of polymeric additive
can significantly either increase or decrease growth rates of molecular
crystals depending on the differences between glass-transition temperatures T
g of host crystals and polymers [C. Huang et
al., J. Phys. Chem. B
2017, 121, 1963–1971].
A similar effect is illustrated here for small-molecule additives.
It is shown that the growth rate of a molecular crystal increases
if the T
g of an additive is lower than
that of a host crystal and growth rate decreases if the T
g of an additive is higher than that of a host crystal.
This effect is observed only for host crystals that can form intermolecular
hydrogen bonds in the melt. We speculate that in this case, the additive
can efficiently disrupt the hydrogen bond network and affect the liquid
dynamics in the melt.