ISSN:
0392-6737
Keywords:
Surface energy (surface tension, interface tension, angle of contact, etc.)
;
Phase transitions and critical phenomena
;
Constant-composition solid-solid phase transformations: polymorphic
;
massive, and order-disorder
;
Conference proceedings
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Summary 1,2-sn-diglyceride monolayers exhibit unique and complex phase transitions as a function of surface pressure. The dynamical response of the layer on expanding the film has been investigated by computer simulations, (π-A) isotherms and grazing-incidence X-ray diffraction. Good agreement is found between the simulations and experiments. The Langmuir film undergoes two phase transitions occurring at 38.3 and 39.8 Å2/molecule. The transition at low surface density is associated with a tilt of ≈14° in the direction close to nearest neighbour. The first transition is unique for the diglyceride molecules and has not been observed for other amphiphilic molecules. It is driven by a competition of hydrophobic/hydrophilic and intra/intermolecular forces and can be pictured as a «seesaw» mechanism. Due to the close packing of the chains at high surface pressure, thesn-1 ester group aligns with the alkyl chain, and only thesn-2 ester group is favoured to lay at the aqueous interface. Hydrophilic forces attract thesn-1 ester group, but its motion towards the aqueous subphase is hindered by intra- and intermolecular chain interactions. On expansion, the intermolecular interaction decreases, and at the first transition, the intramolecular interaction between the two chains is strong enough to cause a «swelling» of the molecules. The diffraction pattern determined in the different mesophases reveals that the layer maintains its hexagonal structure up to the second phase. Concomitant with the tilt, the structure relaxed from a hexagonal to a distorted hexagonal lattice.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF02462032
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