Morphology and phase behaviour of monoglyceride monolayers on aqueous sugar substrates

Morphology and phase behaviour of palmitoyl glycerol (PmGl) and hexadecyl g lycerol (HdGl) monolayers on glucose, fructose and sucrose solution subphas es is studied by means of surface pressure-area (pi-A) isotherms, Brewster Angle Microscopy (BAM) and Grazing Incidence X-ray Diffraction (GIXD). The presence of the sugars in the subphase shifts the pi-A isotherms to higher areas per molecule compared with those on water subphase and increases the pressure pi(t) and the latent heat Delta H-t of the main phase transition f rom the fluid to the condensed state. The monolayer expansion is greatest i f the sucrose is used as a solute and decreases in the order sucrose > fruc tose > glucose. The addition of sugars in the subphase affects the morpholo gy of the condensed phase domains of both PmGl and HdGl. The segment textur e of the PmGl domains is distorted, the segment number is reduced and only small areas with different reflectivity appear on the periphery of the doma in. Domains without inner structure are seen as well. In case of HdGl the s egment texture is preserved, but domains are more compact and a certain num ber of circular domains exists. The differences in the morphology of the si ngle domains are seen mainly in the coexistence region between the fluid an d 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, GI XD 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 hydroge n bond formation between the sugar molecules and the head groups of the PmG l or HdGl. Monoglyceride-sugar interactions could reduce the electrostatic repulsion between the amphiphilic molecules, thus increasing the latent hea t 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 r esult, more compact domains with altered segment texture appear. However, t he hydrogen bond formation in the head group region is not sufficient to ch ange the crystal structure and the alkyl chains packing in the PmGl and HdG l condensed phase. (C) 2000 Elsevier Science B.V. All rights reserved.