We reported self-assembled monolayers (SAMs) as a model system for understanding interfacial phenomena and investigating physicochemical properties of SAM-coated surfaces, such as water wettability. The studies reported herein have a great potential in the applications including microarrays, sensors, stimuli-responsive surfaces, and nanotechnology.
Reactions that occur at interfaces often show different behaviors from their solution analogues. Herein, we demonstrated how proximity effect, one of the unique phenomena at interfaces, could control interfacial reactions. Terminal carboxylic acids in SAMs of 16-mercaptohexadecanoic acid on gold surfaces were treated with reagents (cyanuric fluoride and pyridine) that are used for forming acid fluorides from carboxylic acid in the solution reaction analogue. After the treatment, each of two different products, acid fluoride and interchain anhydride, was obtained on surfaces under different reaction conditions with keeping the reagents the same. Various factors, such as concentrations of reactants and reaction time, affected the product distribution (or the reaction pathway) on surfaces. The key factors for controlling the reaction pathway are the proximity effect of adjacent carboxylic acid chains in the SAMs and equilibrium shift. Products on surfaces and their reaction behaviors were characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and cyclic voltametry (CV).
We also report the formation of SAMs presenting imidazolium ions at the tail ends on Au and $Si/SiO_2$, and the effects of counter anions and alkyl chain length on surface hydrophilicity/phobicity, i.e., wettability, to demonstrate the possible applications of ILs in surface sciences. The surface wettability of SAMs presenting imidazolium ions at the tail ends could be extrapolated to the water miscibility of the related ionic liquids. As a proof-of-principle, via direct ion exchange on a monolayer surface, changes in water contact angles of the SAM surfaces presenting imidazolium ions have been observed. This experiment was also conducted with an aim of controlling water wettability of Au and $Si/SiO_2$ surfaces by anion exchange. Such stimuli-responsive surfaces have a great potential in various technological applications as well as fundamental understanding of interfacial phenomena. The anion exchange was confirmed by X-ray photoelectron spectroscopy and the water wettability was measured as a water contact angle by contact angle goniometry.
본 연구에서는 계면에서 특징적으로 나타나는 현상을 이해하고 또한 분자의 물리화학적 성질을 파악하기 위한 모델로 자기조립 단분자막을 이용하였다.
계면에서 일어나는 반응은 용액상에서와 다른 양상을 나타내는데 본 연구에서는 이러한 현상을 일으키는 원인중의 하나인 인접효과에 대해서 조사하였다. 우선 금 표면에 카르복시산 말단을 가지는 자기조립 단분자막을 형성한 후 이를 아실플로라이드로 바꾸는 시약인 시아누릭 플로라이드, 피리딘과 반응하였다. 이때 반응 조건에 따라 체인간 카르복시산무수물과 아실플로라이드, 이 두 생성물을 선택적으로 얻을 수 있었다. 이러한 용액상에서와 다른 반응의 양상은 카르복시산 작용기의 인접효과에 기인하며 이와 평형의 조절을 통해 표면 생성물을 조절할 수 있다.
또한 본 연구에서는 금이나 실리콘 위에 이미다졸염의 단분자막을 형성하고 표면의 접촉각측정을 통해 이미다졸염의 소수성의 정도를 정량화할 수 있음을 확인하였다. 이미다졸 염은 대표적인 이온용액으로써 구조에 따른 소수성의 정도를 규명하는 것은 매우 의미있는 일이다. 이미다졸염의 소수성의 정도는 음이온과 알킬사슬의 길이에 영향을 받으며 본 연구에서는 이를 음이온 교환, 알킬사슬의 길이조절에 따른 표면 접촉각의 변화를 통해 조사하였다. 이러한 접촉각의 변화는 표면특성의 조절이라는 면에서도 중요한 의미를 가진다.
