Effect of chain length on the wetting properties of alkyltrichlorosilane coated cellulose-based paper

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.