Journal of Colloid and Interface Science 280 (2004) 234–243 www.elsevier.com/locate/jcis Selectivity and stability of organic ﬁlms at the air–aqueous interface J.B. Gilman, T.L. Eliason, A. Fast, V. Vaida ∗ Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO 80309, USA Received 9 March 2004; accepted 23 July 2004 Available online 26 August 2004 Abstract It has recently been determined that organic compounds represent a signiﬁcant percentage of the composition of certain atmospheric aerosols. Amphiphilic organics, such as fatty acids and alcohols, partition to the interface of aqueous aerosols. In this way, the air–aqueous interface of an aerosol has the ability to act as both a concentrator and a selector of organic surfactants. Isotherms of nonanoic acid, stearic acid, 1-octadecanol, and a binary of mixture of nonanoic and stearic acids were used to infer the packing ability and molecular orientation of the surfactants at the interface. The selectivity of the air–aqueous interface was studied by monitoring the composition of binary organic ﬁlms as a function of ﬁlm exposure time. The ﬁlms were formed, aged, and collected with the use of a Langmuir trough. The composition of the aged ﬁlm was determined via GC-MS. Surfactants with differing carbon number and chemical functionalities were studied. These included stearic acid, lauric acid, 1-octadecanol, and octadecane. The stability and packing ability of stearic and lauric acid ﬁlms were examined as a function of subphase pH. The relevance of these ﬁndings as they relate to the composition and structure of organic aerosols as well as recent surface-sensitive aerosol ﬁeld measurements is discussed.  2004 Elsevier Inc. All rights reserved. Keywords: Surfactants; Amphiphiles; Fatty acids; Organic surface ﬁlm; Mixed surfactant system; Air–aqueous interface; Interfacial selectivity; Film stability; Organic aerosol 1. Introduction Organic ﬁlms on aqueous surfaces have many interesting and unique optical, morphological, and chemical properties. Some of these properties were ﬁrst recognized by studying the somewhat ordinary occurrence of oil spreading on water [1,2]. This phenomenon has interested many noteworthy sci- entists throughout history, including Pockels [2,3] and Lang- muir, Schaefer [4]. Recent water conservation efforts have revived the interest in these classical experiments. Standing water sources, such as lakes, can be covered with organic ﬁlms as a means to reduce the loss of water by reducing the evaporation rate [5]. It has been determined that organic ﬁlms do indeed reduce evaporation rates [6–10], but due to other naturally occurring factors it was deemed an impracti- * Corresponding author. Fax: +1-303-492-5894. E-mail addresses: vaida@spot.colorado.edu, vaida@colorado.edu (V. Vaida). cable conservation tool [11]. Recently, the ability of organic ﬁlms to act as a hydrophobic barrier has been conjectured to be important to other environmental systems such as aque- ous aerosols [12,13]. Atmospheric aerosols have wide-ranging and lasting ef- fects on our health and climate [14,15]. In order to accurately assess these effects, it is imperative that their size, composi- tion, and morphology be known. New instrumentation has allowed for real-time determination of the molecular speci- ation of individual atmospheric aerosols using mass spec- trometry [16–24]. It was found that organics represent a signiﬁcant percentage of the composition of these aerosols, up to 50% by mass [25]. Amphiphilic organics are a signif- icant fraction of the organic content found on collected at- mospheric aerosols [26–30]. Amphiphiles are unique mole- cules that contain two regions of directly opposing polarities, hydrophobic tails and hydrophilic head groups, resulting in an innate ability to partition to an interface and subsequently form a self-assembled ﬁlm. It is in this way that the air– 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.07.019