Liquid–liquid phase separation in organic particles containing one and two organic species: importance of the average O : C

Mijung Song,Ryan J Andrews,Allan K Bertram,Yuan You,Suhan Ham

doi:10.5194/acp-18-12075-2018

Abstract

Abstract. Recently, experimental studies have shown that liquid–liquid phase separation (LLPS) can occur in organic particles free of inorganic salts. Most of these studies used organic particles consisting of secondary organic materials generated in environmental chambers. To gain additional insight into LLPS in organic particles free of inorganic salts, we studied LLPS in organic particles consisting of one and two commercially available organic species. For particles containing one organic species, three out of the six particle types investigated underwent LLPS. In these cases, LLPS was observed when the O : C was ≤ 0.44 (but not always) and the relative humidity (RH) was between ∼ 97 % and ∼ 100 %. The mechanism of phase separation was likely nucleation and growth. For particles containing two organic species, 13 out of the 15 particle types investigated underwent LLPS. In these cases, LLPS was observed when the O : C was ≤ 0.58 (but not always) and mostly when the RH was between ∼ 90 % RH and ∼ 100 % RH. The mechanism of phase separation was likely spinodal decomposition. In almost all cases when LLPS was observed (for both one-component and two-component particles), the highest RH at which two liquids was observed was 100±2.0 %, which has important implications for the cloud condensation nuclei (CCN) properties of these particles. These combined results provide additional evidence that LLPS needs to be considered when predicting the CCN properties of organic particles in the atmosphere.

Highlights

Depending on location, organic materials comprise 20 %– 80 % of the mass of submicrometer particles in the atmosphere (Zhang et al, 2007; Jimenez et al, 2009)
Three out of the six particle types studied underwent LLPS as relative humidity (RH) was cycled from ∼ 100 % to lower values and back to ∼ 100 %
As RH decreased from ∼ 100 %, the amount of the inner phase decreased and eventually the two liquid phases were merged into one liquid phase at 99 % RH–97 % RH

Summary

Introduction

Organic materials comprise 20 %– 80 % of the mass of submicrometer particles in the atmosphere (Zhang et al, 2007; Jimenez et al, 2009). Organic particles can affect the Earth’s energy budget directly by scattering and/or absorbing solar radiation and indirectly by serving as nuclei for cloud formation (Kanakidou et al, 2005; Hallquist et al, 2009; IPCC, 2013; Knopf et al, 2018). They can affect air quality and human health (Jang et al, 2006; Baltensperger et al, 2008) and provide a medium for multiphase reactions (George and Abbatt, 2010; Shiraiwa et al, 2011; Abbatt et al, 2012; Houle et al, 2015; Reed et al, 2017). One possible phase transition that particles can undergo as the relative humidity (RH) varies in the atmosphere is liquid– liquid phase separation (LLPS)

Methods

Results

Conclusion