Effect of compression on self-assembled monolayers: a molecular dynamics study

Molecular dynamics simulation has been used to study the effect of pressure on self-assembled monolayers (SAM) of n-alkanethiols [CH3(CH2)(n-1), n = 12-15] on Au(1 1 1) substrates. Mechanical properties of the monolayer compressed by another bare Au substrate (SAM-Au contact) or by another SAM (SAM-SAM contact) have been calculated. The effects of pressure and molecular length on structural and physical properties such as tilt angle, radius of gyration and gauche defects have been studied. We find odd-even effects in response to pressure for SAM-Au contacts where even alkanethiols show larger resistance against compression. For SAM-Au contacts, odd alkanethiols compared with even ones show considerably larger gauche defects at the functional end of the chains. However, for SAM-SAM systems the odd-even effect is weak and molecular structural deformations are lower than the SAM-Au contacts. For all SAMs examined here we found a fully elastic response in compression and decompression stages under pressures between 0.5 and 4 GPa. The calculated Young's moduli (E) are reported for C12, C13, C14 and C15 for both SAM-Au and SAM-SAM contacts. The results show that the modulus is mostly independent of the chain length and of SAM being a SAM-Au or SAM-SAM contact.