Influence of headgroup and chain length on the surface freezing behavior of long chain amphiphiles

Long-chain polar amphiphiles such as alcohols and amines form a finite contact angle on high-energy solid surfaces on account of formation of an oriented monolayer, as opposed to expected complete wetting. Previous experiments with octadecanol (and hexadecanol) suggest that the monolayer formed on alumina displays surface freezing behavior on account of strong headgroup-substrate interactions. However, the data for different headgroups and chain lengths is elusive in literature. In the present study, we investigate the combined effect of headgroup-substrate interactions and chain length on the observed surface freezing behavior using interface-sensitive sum frequency generation spectroscopy (SFG). SFG provides information about ordering of amphiphilic molecules at the sapphire-melt interface along with interaction strength of different headgroups with sapphire. Our study provides insights into the fundamental mechanism of monolayer formation.