DOI: 10.1021/la1017642 15265 Langmuir 2010, 26(19), 15265–15271 Published on Web 09/02/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Surface Chemistry and Spectroscopy of UG8 Asphaltene Langmuir Film, Part 2 Jhony Orbulescu, † Oliver C. Mullins, ‡ and Roger M. Leblanc* ,† † Department of Chemistry, University of Miami, 1301 Memorial Drive, Cox Science Center, Coral Gables, Florida 33146, and ‡ Schlumberger-Doll Research, Cambridge, Massachusetts 02139 Received May 3, 2010. Revised Manuscript Received August 18, 2010 While there has been much focus on asphaltenes in toluene, there has been much less focus on asphaltenes in other solvents. It is important to quantify characteristics of asphaltenes in solvents besides toluene in order to better assess their molecular architecture as well as their fundamental aggregation characteristics. The present work focuses on the investigation of UG8 asphaltene Langmuir films at the air-water interface using chloroform as spreading solvent. The results are compared to the results recently obtained using toluene as spreading solvent. Surface pressure-area isotherms and UV-vis spectroscopy indicate that asphaltenes form smaller nanoaggregates in chloroform than in toluene in similar concentration ranges. Still these nanoaggreates share common features with those in toluene. From the surface pressure-area and compression-decompression isotherms, Brewster angle microscopy, and p-polarized infrared reflection-absorption spectroscopy, it was concluded that small size aggregates are spread on the water surface and the compression of the film leads to formation of large aggregates. The films (Langmuir-Schaefer and Langmuir-Blodgett) studied by atomic force microscopy reveal the existence of nanoaggregates spread on the water surface that coexist with large aggregates formed during compression. In addition to these findings, the spreading solvent, chloroform, allows the determination of asphaltene absorption bands using in situ UV-vis spectroscopy at the air-water interface after 15 min waiting time period. The absorbance data carried out after waiting a time period of 1 h shows similar features with the ones carried out after only 15 min; therefore, there is no need to wait 1 h as in the case when toluene is used as spreading solvent. A comparison of the data obtained from chloroform and toluene shows that smaller aggregate sizes are obtained from chloroform as suggested from the surface pressure-area isotherm, in situ UV-vis spectroscopy, and atomic force microscopy. Nevertheless, the similarity of these nanoaggregates in different solvents suggests this formation is a fundamental property of asphaltenes. Moreover, the lack of the isolated absorption band for one-ring aromatics and only a small peak for two-ring aromatics in the UV spectrum of asphaltenes indicate that these groups are not present in asphaltenes in significant quantities. Introduction Asphaltenes constitute a class of polyaromatic hydrocarbons known to be soluble in toluene and insoluble in n-heptane. 1,2 In recent years, there has been a renewed interest in asphaltenes. Current research involves the study of the asphaltenes in solution and films at the oil-water 3-7 and air-water interface. 8-12 The controversy 13 in the literature regarding the actual molecular weight of asphaltene is now settled, and it is agreed that the molecular weight is between 500 and 1000 Da (full-width, half- maximum) with a higher population at 750 Da based on molec- ular diffusion confirming the later finding, including fluorescence correlation and depolarization spectroscopy and mass spectro- metry (MS). 14-21 Measurements using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry 22-25 are also confirming this result. While there has been much focus on asphaltenes in toluene, there has been much less focus on asphal- tenes in other solvents. It is key to establish solvent-invariant properties of asphaltenes in particular in order to understand *To whom correspondence should be addressed. E-mail: rml@miami.edu. 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