Reactivity of oxidising radicals with functionalised surfaces Wolstenholme-Hogg, Amy;a Iqbal, Naeem;a and Chechik, Victora aDepartment of Chemistry, University of York, Heslington, York YO10 5DD U.K. 1 Background 3 Computational modelling 6 Conclusions 1. Ulman A. Formation and Structure of Self-Assembled Monolayers. Chemical Reviews. 1996;96(4):1533-54. 2. De Keer L, Van Steenberge P, Reyniers M-F, Gryn'ova G, Aitken HM, Coote ML. New mechanism for autoxidation of polyolefins: kinetic Monte Carlo modelling of the role of short-chain branches, molecular oxygen and unsaturated moieties. Polymer Chemistry. 2022;13(22):3304-14. 3. Smith LM, Aitken HM, Coote ML. The fate of the peroxyl radical in autoxidation: how does polymer degradation really occur? Accounts of chemical research. 2018;51(9):2006-13. • The studies using mixed monolayers with radical initiators have shown degradation of the neighbouring chains, suggesting autoxidation does occur in monolayer systems. • These findings counteract the kinetic model, which showed no alkane degradation should occur due to the rate of peroxyl radical termination being much higher than H-abstraction by a peroxyl radical. • However, the results support the statistical model which suggest the majority of peroxyl radicals would be too far away from each other to react. Therefore, they can only react with neighbouring organic chains or atmospheric molecules. Non-porous silica nanoparticles have been used as model substrates. Approx. 2.5 molecules/nm2 TGA 0 20 40 60 80 100 120 140 f1 (ppm) 0 50000 100000 150000 200000 250000 i6042c.1.fid AVIIIHD400WB Carbon-13 100MHZ 2.5mm i6042c Amy Wolstenholme-Hogg AWH-C8 20 Solid state 13C NMR Si O O O Silica NPs 10-20nm Radical generation within monolayers Oxidative cleavage of the molecules from the silica surface enables the analysis and quantification of the molecular structures after reactivity by 1H NMR and GC-MS. GC-MS 1H NMR Using 1H NMR and GC-MS, the degradation of neighbouring chains can be monitored to give an understanding of the extent to which radical reactions propagate throughout the monolayer systems after exposure to UV light. The critical step is whether the newly formed peroxyl radicals continue to abstract a H from a neighbouring chain and the propagation of radical reactions occurs or whether they undergo termination to form stable products. Radical propagation Peroxyl termination • Reaction of the sensor surface with an •OH can result in the oxidation of several molecules in the monolayer through autoxidation. • This could increase the sensor sensitivity but also reduce its lifetime. • The aim is to assess the degree of autoxidation – the spreading of radical reactions laterally in the monolayer. 1 2 a a b c c c c b a b c 1) Initiation 2) Reaction with O2 3) Peroxyl radical formation 4) Peroxyl radical termination 4) Radical propagation OR Silica surface 2 Project Aims Functionalisation of silica surfaces •OH 1.8 x10-9 L mol-1 s [2] Statistical model Kinetic model 4 Methodology Dissociation of molecules from silica surfaces Si O Si O Si MeO OMe O O NP surface UV radical initiator (Surface bound) Si O Si O Si MeO OMe N N N N N N N N Thermal radical initiator (Surface bound) Si O Si O Si MeO OMe N N N N N N N N N N N N Thermal radical initiator (Not surface bound) NP surface NP surface UV radical initiation Silica nanoparticles have been functionalised with mixed monolayers of an organic molecule with a radical initiator. Silica surface • Assume 10% of the molecules in a monolayer form active radicals at a given time. • Random generation of 10% points in a grid. Minimum distances plotted as distribution. • 5 molecules/nm2. • Distance between molecules 0.6 nm. • Length of monolayer chains ~0.6 nm • 46% ROO• are close enough to react. • 54% ROO• are more likely to propagate than terminate in monolayer systems. O O O O O O O O F F F F 5 F F F F F 5 F NP surface NP surface NP surface ether alkane perfluoro • Functionalised nanoparticles