A biocide-free mineral oil nanoemulsion exhibiting strong bactericidal activity against Mycobacterium immunogenum and Pseudomonas aeruginosa

Mycobacterium immunogenum and Pseudomonas aeruginosa currently present one of the highest health concerns in metalworking processes. The former is a Gram positive bacterium strongly associated with hypersensitivity pneumonitis outbreaks and the latter is a Gram negative famous for its opportunistic pathogenesis and frequent isolation from in-service metalworking fluids (MWFs). Thus far, biocide addition is the only microbial control measure in MWFs, but it often fails on preventing mycobacteria proliferation. This study aimed to formulate an inherently bacteria-resistant MWF with industrial grade chemicals. Using a phase inversion temperature method, we created a low-cost mineral oil nanoemulsion (MONE) which has extraordinarily high bactericidal activity against M. immunogenum and P. aeruginosa and has the smallest oil droplet size ever reported in the literature. A short-term biocidal test showed that a 5% dilution of MONE (5% MONE) exhibited high emulsion stability and strong bactericidal effects. The first-order biocidal rate constants of 5% MONE are higher than 72% of the 32 different test combinations of commercially available MWFs amended with two industrial biocides, KATHON™ LXE and Preventol® CMK-NA. A further longer term test on an aged 5% MONE showed it achieved 100% elimination of all tested bacteria either in pure cultures or a mixed culture up to 28 days. The biocidal kinetics of MONE seemed to follow a new equation, ln(N/N₀)=−k₀’(exp(n’C))t, much better than the currently most widely accepted Chick–Watson law equation. Among the reported antimicrobial nanomaterials and industrial biocides, this nanoemulsion formulation may offer a low-cost solution in sustainable utilization of mineral oil in MWF applications and a new research perspective on biodeterioration prevention of MWFs.