The effect of alkyl chain length on the wetting properties of alkyltrichlorosilane coated cellulose-based paper is reported for four different reagents: methyltrichlorosilane (MTCS; -CH3), butyltrichlorosilane (BTCS; -C4H9), dodecyltrichlorosilane (DTCS; -C12H25) and octadecyltrichlorosilane (OTCS; -C18H37). SEM analysis reveals that by systematically varying alkyl chain length, films with different surface morphologies can be generated on flat silicon wafer control samples and on cellulose-based paper samples. The variation in surface morphology leads to different wetting behavior, as determined by measuring static water and oil contact angles. Due to the nano- and micron- scale roughness on MTCS coated substrates, paper samples coated with MTCS display superhydrophobicity with a water contact angle of 152.2°, which is the highest water contact angle among these four alkyltrichlorosilanes. However, additional nano-scale roughness from MTCS coating reduces the oil resistance of coated paper samples, while paper samples coated with long-chain alkyltrichlorosilanes have lower surface energy and also lack nano-scale roughness. As a result, paper samples coated with OTCS display the highest resistance against oils (ethylene glycol contact angle 125.5°; diiodomethane contact angle 101.3°). The intrinsic porosity of paper is largely retained after coating, as indicated by the fact that low surface tension fluids like methanol can easily penetrate coated paper samples.
Tang, Zhenguan, Hanyang Li, Dennis W. Hess, Victor Breedveld