By vibrating tube densitometry the volumetric behaviour of dimyristoyl-phosphatidylcholine (DMPC) in the fluid phase was investigated when perturbed at 40 C with different types of solutes at different concentrations. The specific volume of DMPC upon addition of the fatty acids stearic acid (SA), lauric acid (LA) and 1,12-dodecanedioic acid (DDA) as well as the detergent sodium dodecyl sulfate (SDS) was determined. Both SA and LA showed a volume increment of the DMPC-FA system indicating a looser packing of the bilayer. In contrast SDS caused a monotonous decreasing effect on the volume. Furthermore, the effect on the specific volume of DMPC perturbed with the sugars trehalose, sucrose, glucose and the polymer poly(ethylen glycol) (PEG) was examined, where the three former showed the same tendency, i.e. an initial slight increase followed by a decrease, after which the volume increased at high concentrations. PEG initially showed the same trend, although more extreme, and no final increase was observed. Finally, perturbation of POPC with the solutes SA, DDA and trehalose was examined, and the volume effects were parallel displaced above DMPC. In conclusion the regular fatty acids caused a looser andmore disordered structure of theDMPCwhereas both SDS and DDA eventually lead to a denser bilayer. The sugars all though different in size initially induced a loser packing of DMPC followed by a more dense packing at sugar concentrations >0.05 w/w probably caused by osmotic pressure effect and shifting dominance patterns throughout the concentration range
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