Rheological Properties of Self-Assembled Monolayers Formed from Oxygen-to-Carbon-Substituted Oligo(ethylene oxide) Molecules

Self-assembled monolayers (SAMs) are widely used in biosensors to prevent the nonspecific adsorption of both target and nontarget molecules. The performance of SAMs strongly depends on their molecular structure. Therefore, understanding the relationship between molecular structure and rheological properties is essential. In this study, we investigated the rheological properties of SAMs formed from a new compound, CH₃O(CH₂CH₂O)₆(CH₂)₅(OCH₂CH₂)₃SH (C5-(OEO)₁₁), where one oxygen atom of CH₃(OCH₂CH₂)₁₁SH ((OEO)₁₁) was replaced with one carbon atom. To systematically evaluate the effects of this structural modification, we compared the temperature-dependent properties of SAMs prepared from three different compounds: (OEO)₁₁, CH₃(OCH₂CH₂)₁₂SH ((OEO)₁₂), and C5-(OEO)₁₁. Our results revealed that a single oxygen-to-carbon substitution significantly altered the relative water content and shear modulus of the SAMs. The C5-(OEO)₁₁ SAM exhibited a higher relative water content than those formed from (OEO)₁₁ and (OEO)₁₂. Its shear modulus was higher than that of the (OEO)₁₁ SAM and comparable to that of the (OEO)₁₂ SAM. Additionally, the thickness of the C5-(OEO)₁₁ SAM was slightly greater than that of the (OEO)₁₁ SAM. These findings indicated that even a single oxygen-to-carbon substitution can markedly influence the rheological properties of SAMs. This insight provides a new molecular design strategy for the development of functional SAMs.