LAUSR

The mixed stearic acid (SA)/sterol systems were used as sea spray aerosol mimics to get more insights into the alterations in surface properties of aerosols induced by sterols. By means of surface pressure (π)–area (A) isotherms and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS), the effect of cholesterol (chol), stigmasterol (stig) and ergosterol (erg) on the lateral packing and chain conformation of SA monolayer was explored. The fact that the excess areas of mixing of the mixed monolayers exhibit significant deviations from ideally mixed film proves that, the sterols are miscible with SA throughout all the monolayer compositions and surface pressures examined. The lift-off areas in π–A isotherms were found to increase with increasing mole fraction of sterols, indicating that expulsive interactions exist between SA and sterols, which are more pronounced when the mole fraction of sterols is 0.7. In addition, the peak intensities of νa(CH2) and νs(CH2) in IRRAS spectra decrease with increasing sterols levels, which is consistent with our findings in the π–A isotherms, that the addition of sterols leads to a looser chain packing in SA monolayer. The proportion of gauche defects in SA monolayer induced by the sterols follows the order cholesterol < stigmasterol < ergosterol at a certain sterol level, as reflected by the decreasing peak intensities of νa(CH2) and νs(CH2). Consequently, the sterols generally give rise to considerable expanding effects on SA monolayer, which are particularly pronounced for stigmasterol and ergosterol, suggesting that the additional alkyl side chains and double bonds of the sterols play a role on disordering SA monolayer. The present study is likely to shed light on many boundary processes take place at the interface of SSAs, in particular, transport processes of water and trace gases across the interface.