Molecular lubricants

We report the results from a molecular dynamics simulation of the serine protease y-chymotrypsin (y-CT) in hexane. The active site of chymotrypsin contains the "catalytic triad" which consists of Ser-His-Asp. y-CT suspended in nearly anhydrous solvents has been found to be catalytically active. In order for proteins to retain their activity in anhydrous solvents some water molecules are required to be present. These "essential waters" have been suggested to function as a molecular lubricant for the protein. Hexane, having a dielectric constant of 1.89, is a suitable non-aqueous solvent for enzymatic reactions. The low dielectric constant of hexane allows it to not compete with the protein for the essential water and allows enzymes to retain their catalytic activity. y-CT in hexane is thus an ideal system to further explore the effect of non-aqueous solvation on protein structure, function and dynamics. 

The plasticizing and swelling effect of CO2 dissolved in the polymeric matrix can accelerate the diffusion of additives into the matrix. The molecular lubrication [20] provided by dissolved CO2 molecules within the polymer matrix enables the diffusion of solutes to proceed in a less hindered manner than that in the absence of CO2. The ease of diffusion of the solute through the matrix is due to the increase in the free volume as a result of the polymer being in a... 

Small polar organic molecules (e.g., A, A -dimethyl formamide and formamide) are known to enhance reaction rates by acting as molecular lubricants [95-97]. 

Palacio M, Bhushan B (2010) A review of ionic liquids for green molecular lubrication in nanotechnology. Tribol Lett 40(2) 247-268. doi 10.1007/sll249-010-9671-8... 

This chapter provides a concise review of the current status of the self-assembled monolayers as molecular lubricants for microelectromechanical systems operating in harsh environments, including elevated temperatures and in fluids. In particular, we focus on the similarities and differences in the structure-property relationships of two SAMs commonly employed to prevent in-use stiction in MEMS, namely octadecyltrichlorosilane (OTS, CH3(CH2)i7SiCl3) and perfluorodecyl-trichlorosilane (FDTS, CF3(CF2)7(CH2)2SiCl3). We discuss the effect of harsh environments on these monolayers and how their degradation impacts their properties and the range of conditions for which these monolayers can be employed effectively for MEMS. 

Self-assembled monolayers (SAMs) are commonly used as molecular lubricants in microelectromechanical systems (MEMS) [1, 2]. They have been shown to reduce dramatically the adhesion (stiction) of free-standing or moving microstructures. The most common SAMs deposited on silicon are made from chlorosilane end-groups and alkyl or perfluorinated chains of variable length, with the chemical form RCI3. The antistiction properties of a variety of monolayers have been reviewed extensively (see, for example, [3-5]). Relatively few investigations have dealt with the performance of molecular lubricants in harsh environments (such as elevated temperature, liquid and corrosive media, and high electric bias). It is of paramount practical importance to determine to what extent these films can be employed for antistiction in these conditions. 

Conventional oil-based lubricants are not suitable for application in MEMS, as the viscous forces can be quite large when compared with the forces involved in operating these components because the size of the oil molecules is of the same order as those of MEMS components [5], Therefore, many researchers have proposed ultrathin organic molecular layers as the lubricants for Si-based MEMS systems [3, 5, 8]. These ultrathin molecular lubricant layers are generally formed by two methods (a) the Langmuir-Blodgett (LB) method and (b) the self-assembly method [9]. The LB monolayers have two main difficulties for their use as the lubricant layers for MEMS components (a) LB films are less wear resistant, as the interaction forces between LB films and substrate (van der Waals) are weaker [10], and (b) application of the LB method is restricted to flat surfeces only, and it is not practical to coat three-dimensional surfaces of a structure [11-14],... 

Several theories have been developed to account for the observed characteristics of the plasticization process Daniels has recently published a review of plasticization mechanisms and theories [8]. Although most mechanistic studies of plasticization have focused on PVC, much of this information can be adapted to other polymer systems. The lubricating theory of plasticization holds that plasticizers act as molecular lubricants to facilitate polymer chain movement when a force is applied to the plastic. It starts with the assumption that the unplasticized polymer chains do not move freely because of surface irregularities and van der Waals attractive forces. As the system is heated and mixed, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions. Portions of the plasticizer molecule are strongly attracted to the polymer while other parts of the plasticizer molecule can shield the polymer chain and act as a lubricant. This reduction in intermolecular or van der Waals forces among the polymer chains increases the flexibility, softness, and elongation of the polymer. 

Xu et al. [983] used a special friction tester that allows contact sizes in between FFM and SFA. They could observe that transition between high (hundreds of MPa) and low (some 10 MPa) shear strength can occur for contact radii of only 20-30nm, depending on whether there is intimate contact of the sliding surfaces or whether there is still a monolayer of lubricating molecules present. This is in line with the quantized friction behavior found in SFA experiments depending on the number of molecular lubrication layers [644]. 

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