Swelling of a lecithin lamellar phase induced by small carbohydrate solutes

In this paper, we consider the effect of adding small carbohydrate solutes (small sugars) to DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) L(alpha) dispersions and the consequences on the force balance at zero osmotic pressure (maximal swelling). We show the importance of long incubations required to obtain samples at thermodynamic equilibrium where molecular diffusion has been completed. The monotonic increase of maximal swelling versus sugar content occurs as a combined effect of the screening of the van der Waals contribution and fluctuations in the lamellar stacks. According to this new approach, it is shown that changes in dielectric properties result in a much less pronounced effect than entropic forces (undulations) generated by the softening of the membranes at high sugar content. However, this sugar-induced swelling cannot be explained quantitatively by adding an entropic contribution to molecular interactions. Quantitative disagreement between the proposed mechanism and our observations is due either to nonadditivity of molecular interactions with entropic forces or to the relation used to account for the entropic contribution.