Homogeneously Mixed Monolayers: Emergence of Compositionally Conflicted Interfaces

Abstract

The ability to manipulate interfaces at the nanoscale via a variety of thin-film technologies offers a plethora of avenues for advancing surface applications. These include surfaces with remarkable antibiofouling properties as well as those with tunable physical and electronic properties. Molecular self-assembly is one notably attractive method used to decorate and modify surfaces. Of particular interest to surface scientists has been the thiolate-gold system, which serves as a reliable method for generating model thin-film monolayers that transform the interfacial properties of gold surfaces. Despite widespread interest, efforts to tune the interfacial properties using mixed adsorbate systems have frequently led to phase-separated domains of molecules on the surface with random sizes and shapes depending on the structure and chemical composition of the adsorbates. This feature article highlights newly emerging methods for generating mixed thin-film interfaces, not only to enhance the aforementioned properties of organic thin films, but also to give rise to interfacial compositions never before observed in nature. An example would be the development of monolayers formed from bidentate adsorbates and other unique headgroup architectures that provide the surface bonding stability necessary to allow the assembly of interfaces that expose mixtures of chains that are fundamentally different in character (i.e., either phase-incompatible or structurally dissimilar), producing compositionally "conflicted" interfaces. By also exploring the prior efforts to produce such homogeneously blended interfaces, this feature article seeks to convey the relationships between the methods of film formation and the overall properties of the resulting interfaces.

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…The calculated values suggest that the F8H10-Cm films examined here possess similar roughness/heterogeneity as their monothiol analogues. The contact angle measurements of the F8H10-Cm series suggest a mixed interface composed of both phase incompatible groups, proving to be a unique new class of adsorbate in the pursuit of homogeneously mixed dual-component monolayers on metal substrates [ 51 ].…”

“…Much of the previously mentioned research has been obtained from ω-terminated alkanethiols (monothiol-based adsorbates), which provide a single point of contact between the sulfur headgroup and the metal surface. Alternatively, other types of adsorbates having multiple headgroup-metal interaction points have also been developed and studied [ 49 , 50 , 51 ]. Bidentate adsorbates have been introduced into surface chemistry, due to their ability to enhance the stability of thin films formed on solid substrates [ 52 ].…”

“…A recent example includes the development of film compositions where two phase-incompatible groups were incorporated into the same molecule, yielding homogeneously mixed SAMs [ 62 , 63 ]. Important to note are previous efforts toward the development of mixed-chained adsorbates originating from a single point, including the use of dialkyl sulfides by Whitesides [ 70 ], as well as other methods highlighted in the literature [ 51 ]. However, SAMs generated from dialkyl sulfides, asymmetrical disulfides, and other methods described in the literature used to generate mixed SAMs suffer from poor stability, disordered films, and/or preferential adsorption, which can be circumvented when the SADT structure is incorporated into the architecture of the adsorbate [ 25 , 62 , 63 ].…”

Effect of surface structure on peptide adsorption on soft surfaces

“…This makes it experimentally difficult to relate protein adsorption to the nanoscale topology. Recently the use of bidentate ligands with both hydrophobic and hydrophilic end groups has allowed for the construction of surfaces with incompatible ligands with controlled nanoscale topology [11][12][13] , with feature sizes on the molecular length scales. As this is comparable in size to proteins it may be expected that the interaction between these surfaces and biomolecules may be significantly different to uniform surfaces.…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…Alternatively, other types of adsorbates having multiple headgroup-metal interaction points have also been developed and studied [37][38][39]. Bidentate adsorbates have been introduced into surface chemistry due to their ability to enhance the stability of thin films formed on solid substrates [40].…”