Technical note The role of viscosity in the fluorescence behavior of the diesel/ biodiesel blends Anderson R.L. Caires a, * , Marisa D. Scherer a , José E. De Souza a , Samuel L. Oliveira b , Jean-Claude M’Peko c a Grupo de Óptica Aplicada, Faculdade de Ciências Exatas e Tecnologia, Universidade Federal da Grande Dourados, CP 364, 79804-970 Dourados, MS, Brazil b Grupo de Óptica e Fotônica, Universidade Federal de Mato Grosso do Sul, CP 549, 79070-900 Campo Grande, MS, Brazil c Grupo Crescimento de Cristais e Materiais Cerâmicos, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369,13560-970 São Carlos, SP, Brazil article info Article history: Received 4 January 2013 Accepted 25 September 2013 Available online 18 October 2013 Keywords: Diesel Biodiesel Blend Fluorescence Viscosity abstract Recently, the potentiality of fluorescence spectroscopy to be used in the quantification of biodiesel content in diesel/biodiesel blends (DBB) was demonstrated. However, the source of the fluorescence dependence of the DBB with biodiesel concentration remains unanswered. In the present paper, a close analysis of the optical properties of the DBB was performed over a wide composition range. The findings suggest that the alterations in the fluorescence intensity can be accounted for only after taking into account changes in viscosity as well as absorbance, in a model where the fluorophores were considered as molecular rotors. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Biodiesel is a biodegradable fuel produced from renewable sources that can partly or completely replace petroleum-derived diesel fuel because its properties are quite similar to the ones of the diesel [1,2]. Consequently, the mandatory use of diesel and biodiesel blends (DBB) is growing around the world due to their environmental, economic, and social advantages [3,4]. In this worldwide scenario, to ensure compliance with legislation, it is necessary to either develop or improve methods able to determine the biodiesel content in the DBB. Of course, it is desirable that this determination be achieved via experimental methods that are easy to handle, provide rapid and accurate results, and have a low cost per sample analysis. In a recent study, our research group proposed an alternative method based on the fluorescence spectroscopy to quantify the biodiesel content in DBB [5]. We showed that a linear increase in the fluorescence intensity of the blends occurs when the biodiesel content in the DBB increases from 0 to 10% (v/v). In consequence, this linear relationship can be used as a calibration curve to determine the biodiesel percentage [5]. This method has the advantage of working without a sample pre-preparation and can potentially be applied directly at the gas station by using a por- table device. However, from a theoretical viewpoint, the origin of the fluorescence increase in these blends when raising the bio- diesel concentration is not yet understood and deserves to be accounted for. Here, to gain further insights into the fluorescence behavior of the DBB, a close analysis of the optical properties of these blends was performed by measuring and analyzing their UVeVis absorp- tion, molecular fluorescence, and viscosity data over a wide composition range. 2. Materials and methods Biodiesel was obtained from the transesterification process of refined soybean oil, using a 6:1 M ratio of methanol/oil. The NaOH catalyst (0.4 wt.% with respect to oil weight) was dissolved in methanol and then added to the preheated soybean oil at 60  C. The solution was stirred for 60 min and then placed in a separating funnel for 24 h. Two phases were observed, one containing mostly biodiesel and the other containing glycerol. After that, the biodiesel was rotary-evaporated under reduced pressure to eliminate excess methanol. Subsequently, the biodiesel was washed four times using tap water (30% v/v) at room temperature and intervals of 30 min. Finally, DBB samples were prepared by using a certified diesel provided by Petrobras (Petróleo Brasileiro S.A.). Henceforth, DBB * Corresponding author. Tel./fax: þ55 67 34102072. E-mail addresses: andersoncaires@ufgd.edu.br, andercaires@yahoo.com (A.R.L. Caires). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.09.041 Renewable Energy 63 (2014) 388e391