The zeta potential and surface properties of asphaltenes obtained with different crude oil/n-heptane proportions q Hilda Parra-Barraza a , Daniel Herna ´ndez-Montiel a , Jaime Lizardi b , Javier Herna ´ndez b , Ronaldo Herrera Urbina c , Miguel A. Valdez d, * a Departamento de Ciencias Quı ´mico Biolo ´gicas, Universidad de Sonora, Rosales y Transversal, C. P. 83000, Hermosillo, Sonora, Mexico b Centro de Investigacio ´n en Alimentacio ´n y Desarrollo, A. C., Apartado Postal 1735, C. P. 83000, Hermosillo, Sonora, Mexico c Departamento de Ingenierı ´a Quı ´mica y Metalurgia, Universidad de Sonora, Rosales y Transversal, C. P. 83000, Hermosillo, Sonora, Mexico d Departamento de Fı ´sica, Departamento de Investigacio ´n en Polı ´meros y Materiales, Universidad de Sonora, Rosales y Transversal, C. P. 83000, Hermosillo, Sonora, Mexico Received 18 December 2002; revised 18 December 2002; accepted 28 December 2002; available online 21 January 2003 Abstract We have investigated some surface properties of asphaltenes precipitated from crude oil with different volumes of n-heptane. According to the crude oil/n-heptane proportions used, asphaltenes are identified as 1:5, 1:15 and 1:40. Zeta potential results indicate that the amount of n-heptane determines the electrokintetic behaviour of asphaltenes in aqueous suspensions. Asphaltene 1:5 exhibits an isoelectric point (IEP) at pH 4.5 whereas asphaltenes 1:15 and 1:40 show an IEP at about pH 3. Surface charge on asphaltenes arises from the dissociation of acid functionalities and the protonation of basic functional groups. The presence of resins remaining on the asphaltene molecules may be responsible for the different IEP of asphaltene 1:5. Both sodium dodecyl sulfate (an anionic surfactant) and cetylpyridinium chloride (a cationic surfactant) were found to adsorb specifically onto asphaltenes. They reverse the sign of the zeta potential under certain conditions. These surfactants may be potential candidates to aid in controlling the stability of crude oil dispersions. Critical micelle concentration, interfacial tension measurements, and Langmuir isotherms at the air –water interface confirm the different nature of asphaltene 1:5, which also showed more solubility and a larger molecular size. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Asphaltenes; Zeta potential; Surfactants; Monolayers; Critical micelle concentration 1. Introduction Asphaltenes precipitation is one of the most undesirable situations in crude oil production and processing, causing severe technical problems and significant economic losses. In order to effectively control asphaltene deposition, many research groups have been working towards gaining a better understanding of the phenomena involved in asphaltene precipitation and adhesion onto both metallic and non- metallic substrates. Usually, crude oil is considered to be a colloidal dispersion of asphaltene and resins, which constitute the discrete and polar components, dispersed in a continuous phase made of non-polar compounds. Resins in crude oil act as peptizing agents keeping asphaltenes in suspension and maintaining the stability of crude oil [1]. When resins are separated from crude oil, the stability of this colloidal system is lost and asphaltene deposition takes place [2]. The stability of crude oil dispersions is closely related to the electrical characteristics of the interface between the discrete and continuous phases. Asphaltenes are defined according to their solubility in non-polar solvents and not by their chemical structure. The common definition is that they are the crude oil components insoluble in n-heptane and soluble in toluene or benzene [3]. However, other definitions are based on the use of a different precipitating solvent, such as n-hexane and n- pentane [4]. An important aspect in the preparation of asphaltenes is the proportion of crude oil/n-heptane used to precipitate them. Some investigators [5,6] follow a modification of ASTM’s procedure 2007D to precipitate asphaltenes using a ratio 1:10, whereas other researchers use 0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00002-4 Fuel 82 (2003) 869–874 www.fuelfirst.com q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com * Corresponding author. Tel.: þ52-6622-592-108; fax: þ 52-6622-592- 109. E-mail address: valdez@fisica.uson.mx (M.A. Valdez).