Self-assembly of alkoxysilanized humic substances into multidomain adlayers at the water–solid interface: linking surface morphology to the molecular structure of the adsorbate

Abstract

Self assembly of natural organic materials (e.g., humic substances) on mineral surfaces leads to the formation of adlayers with physically different domains. However, direct evidence to explain the molecular interactions responsible for the formation of these domains is still missing. Here, we developed alkoxysilanized humic probes capable of self-assembling onto hydroxylated surfaces. Using modified humic substances from vastly differed sources (peat, water, coal) and DNA array glass slides as a flat surrogate for mineral surfaces, we create humic adlayers under aqueous conditions and show them to be homogeneous on the macroscale, but consisted of nanosize domains in coal and water humics and submicron-size domains in peat humics. The surface charge and hydrophilic–lipophilic balance of humic associates were proposed as major molecular drivers of the self-assembly with the former causing the formation of thin adlayers composed of separated nanosize domains (e.g., coal humics) and the latter enhancing the formation of thicker adlayers composed of interconnected domains linked in chains up to the submicron-size (e.g., peat humic materials). To the best of our knowledge, this is the first time alkoxysilanized humic probes were used to examine how humic adsorbates and their molecular structure influences the surface morphology of assembled adlayers. In this fashion, the humic adlayer with molecularly-defined functional elements can be assembled on any hydroxylated surface in situ, at the water–solid interface.