Langmuir monolayers of diacyl glycerol amino acid-based surfactants. Effect of the substitution pattern of the glycerol backbone

The Langmuir monolayers of diacyl glycerol amino acid-based surfactants with different substitution patterns of the glycerol backbone have been studied by means of surface pressure versus molecular area isotherms and Brewster angle microscopy (BAM). The isotherms show that all the studied compounds form stable monolayers on a water subphase. Concerning the compounds with alkyl chains in adjacent positions, the effect of the nature of the headgroup and the length of alkyl chains on their monolayer behavior is similar to what is observed in naturally occurring phospholipids. BAM images of the condensed phase show initially lobulated domains that relax to rounded-shaped structures with a singularity at the center (star defect) or at the boundary of the condensed droplets. The compound with alkyl chains bound to the ends of the glycerol backbone shows a markedly different behavior. Although condensed domains are favored in this case, some hindrance in the interaction between alkyl chains of different molecules leads to the formation of compact and rough growing droplets, reminiscent of those predicted by the Eden model of cluster growth. This hindrance is attributed to the lower hydrophobic interaction between intra-alkyl chains due to the carbon atom separating them.