Effect of head groups on the phase transitions in Gibbs adsorption layers at the air-water interface

The adsorption kinetics and the surface phase behavior of four different amphiphiles, which are 2-hydroxyethyl laurate (2-HEL), dodecanoyl N-ethanolamide (NHEA-12), dodecanoyl N-methylethanolamide (NMEA-12) and tetradecanoyl N-methylethanolamide (NMEA-14), have been investigated at the air-water interface by film balance, surface tensiometer and Brewster angle microscopy (BAM). The former two amphiphiles show a first-order phase transition from a lower density liquid like phase to a higher density condensed phase in Gibbs adsorption layers. On the other hand, the latter two amphiphiles are unable to show such characteristics under any experimental conditions. The presence of a methyl group in the head group of NMEA-12 sterically hinders the molecules and resists the formation of any condensed phases. This steric hindrance is so high that even an increase in the chain length by two CH(2) groups in NMEA-14 does not allow the formation of condensed domains. Although, both 2-HEL and NHEA-12 are able to form the condensed phase, the domain morphology formed in these monolayers is different from each other. The domains of 2-HEL at lower temperatures are circular having a stripe texture, while those at higher temperatures show fingering patterns having uniform brightness. On the other hand, the domains of NHEA-12 are dendritic in shape. The presence of hydrogen bonding sites close to the interface should be responsible for the formation of such domains in NHEA-12.