Morphology and phase behaviour of monoglyceride monolayers on aqueous sugar substrates

Morphology and phase behaviour of palmitoyl glycerol (PmGl) and hexadecyl glycerol (HdGl) monolayers on glucose, fructose and sucrose solution subphases is studied by means of surface pressure–area (π–A) isotherms, Brewster Angle Microscopy (BAM) and Grazing Incidence X-ray Diffraction (GIXD). The presence of the sugars in the subphase shifts the π–A isotherms to higher areas per molecule compared with those on water subphase and increases the pressure πt and the latent heat ΔHt of the main phase transition from the fluid to the condensed state. The monolayer expansion is greatest if the sucrose is used as a solute and decreases in the order sucrose>fructose>glucose. The addition of sugars in the subphase affects the morphology of the condensed phase domains of both PmGl and HdGl. The segment texture of the PmGl domains is distorted, the segment number is reduced and only small areas with different reflectivity appear on the periphery of the domain. Domains without inner structure are seen as well. In case of HdGl the segment texture is preserved, but domains are more compact and a certain number of circular domains exists. The differences in the morphology of the single domains are seen mainly in the coexistence region between the fluid and condensed monolayer state and vanish when the compact condensed phase is formed. The effect of the sugar type on domain morphology diminishes in the same order, as revealed by the isotherm measurements. At the same time, GIXD shows that the packing mode of the alkyl chains of PmGl and HdGl is not affected by the subphase composition. The expanding effect of sugars on the PmGl and HdGl monolayers is related to the strong tendency towards hydrogen bond formation between the sugar molecules and the head groups of the PmGl or HdGl. Monoglyceride–sugar interactions could reduce the electrostatic repulsion between the amphiphilic molecules, thus increasing the latent heat of fluid to condensed transition and the line tension at the contact line between the fluid and condensed phases in PmGl and HdGl monolayers. As a result, more compact domains with altered segment texture appear. However, the hydrogen bond formation in the head group region is not sufficient to change the crystal structure and the alkyl chains packing in the PmGl and HdGl condensed phase.