Paths to lowering critical point in a two-dimensional order–disorder transition by Au nanoparticle ‘decoration’

Abstract

Effect of mixing dodecanethiol-capped Au nanoparticles (AuNPs) on the critical point of the liquid ordered ($L_{\mathrm{o}}$) to liquid disordered (${\textit{L}}_{\mathrm {d}}$) phase transition of myristic acid (MyA) Langmuir monolayer has been studied through quantitative evaluation of the two-dimensional patterns of AuNP clusters created through de-mixing and observed through Brewster angle microscopy. The critical temperature (${\textit{T}}_{\mathrm {c}}$), marked by the emergence of a Bethe lattice-like (BLL) pattern of ‘fingers’ and ‘arms’, was brought down from $38^{\circ }\hbox {C}$ in pristine MyA monolayers to 28 and $10^{\circ }\hbox {C}$ for 20 and 40% w/w AuNP concentrations. Analysis of the BLL at the length scales of these ‘fingers’ and ‘arms’ showed that the lowering of ${\textit{T}}_{\mathrm {c}}$ follows two different paths for the two concentrations, through a repulsive force for the lower and an attractive force for the higher concentration at the ‘fingers’ length scale, while at the scale of ‘arms’ the force between NPs is always repulsive. Based on the observations that the repulsive force operates at larger interparticle separation and the attractive one acts at smaller separations, we tentatively assign the first to be a dipolar repulsion and the second to be lipophilic force of quantum mechanical origin. We have also indicated qualitatively how this realignment of forces between nanoparticles can affect the lipophilic force between the hydrocarbon chains of the NP capping and those chains in the monolayer.