Study On The Surface Wetting Properties Of Hydroxyl-terminated Self-assembled Membranes

Water on the microscopic scale,no matter in quasi-one-dimensional,two-dimensional or three-dimensional confined space,or on surface of materials and biological bodies,has very different properties from that on the macroscopic scale.In particular,we have insufficient understanding of the behavior of biological interface water,such as the surface water of biofilm.Its microstructure,dynamic behavior,and whether or not to participate in biological processes are worthy of discussion.For example,in the direct simulation of phospholipid bilayer membranes,although the structural characteristics of the phospholipid membrane can be well described,it was found that the membrane surface water behavior showed a significant difference by using different force fields.In this paper,in order to describe the behavior of water on the biofilm surface more clearly,we use a biomimetic self-assembling membrane terminated with hydroxyl groups as a simplified physical model to study the surface water behavior,aim to increase the understanding of water behavior on the interface of biofilm.Based on the structure of phospholipid membrane,we constructed a hydroxyl-terminated self-assembled monolayer membrane.The hydrophobic alkyl chain was used to simplify the two hydrophobic tails of the phospholipid membrane.Instead of the hydrophilic head groups of phospholipid membrane molecule,the common and simplest hydroxyl group in biomolecules was used.Molecular dynamics simulation method was used to study the surface water behavior by changing the packing density of hydroxyl-terminated self-assembled molecules.First,we consider single-hydroxyl self-assembled monolayers.It was found that at a moderate packing density,the hydroxyl surface is not completely hydrophilic.Water droplets with a contact angle of about 40° appear.Second,bishydroxyl self-assembled membrane was constructed with an extra hydroxyl group added to the monohydroxyl membrane molecule,in order to simulate the effect of hydrophilic groups at different positions in the phospholipid membrane.It is generally believed that surface containing multiple hydrophilic groups should be completely hydrophilic.However,our simulation results show that with the change of packing density,the water behavior of the surface of bishydroxyl self-assembled monolayers can be roughly divided into three regions:in the lower packing density region,water molecules are embedded in the membrane surface.The hydrophobicity of the self-assembled monolayers is enhanced,and droplets with a contact angle of about 35 degree appear on it.This is in line with the previous studies on the carboxyl self-assembled membranes.It was found that the embedded water molecules help the membrane to form a stable structure and make the composite surface exhibit hydrophobicity.However,unlike carboxyl groups,in the region with moderate packing densities,the bishydroxyl membrane molecules can form hydrogen bonds through the interaction between the hydroxyl groups of adjacent membrane molecules by actively selecting the appropriate configuration even if the water molecule is not embedded.The hydrogen bond network greatly reduced the hydrogen bonding sites where the hydroxyl group can bind to the upper water molecules,exhibiting more pronounced hydrophobic characteristics,and the contact angle of the water droplets on the membrane surface can be up to about 82 degree.In areas with higher packing densities,the conditions for the formation of hydrogen-bonding networks between the hydroxyl groups are broken.The water layer spreads completely over the surface of the film,exhibiting the hydrophilic nature of the hydroxyl group itself.In sparse areas with quite lower packing density,the membrane molecules cannot form a complete surface,local cyclic or chain hydrogen bonds are formed between the hydroxyl groups,and the hydrophobic tails are exposed,which corresponds to a composite surface in which hydrophilic and hydrophobic groups are mixed.In this dissertation,biomimetic hydroxyl-terminated self-assembled monolayers terminated with hydroxyl(s)were used as simplified physical models to study the behavior of their interface water.We found that although hydroxyl groups are supper hydrophilic,they can exhibit very hydrophobic properties under different packing densities.The contact angle of water droplets on the surface can reach 82 degree.These results are not the same as those of the previous carboxyl-terminated self-assembled monolayers.Although similar to the case of carboxyl-terminated self-assembled monolayers,it is also observed that the intercalation of water molecules shows an increase in hydrophobicity at the interface,that is,embedded water and droplets coexist,but the more pronounced hydrophobicity was found to derive from the complete ordered hydrogen bonding network formed by the hydroxyl groups.The latter does not require the water molecules to be embedded in the molecular interface,but completely formed by the biological hydrophilic groups themselves exhibit more significant hydrophobicity,which is more easily confirmed in the experiment.This idea can also be applied to the introduction of common groups in other biomolecule molecules such as amino groups for simulation to further expand the behavior of biological interface water.The results of this study will facilitate the understanding of the understanding of the true biological interface water behavior and the molecular-scale affinity properties in biophysics.