International Journal of Pharmaceutics 327 (2006) 73–77 Evaluation of the physical stability of two oleogels Isabel F. Almeida, M. Fernanda Bahia ∗ Department of Pharmaceutical Technology, Faculty of Pharmacy, Oporto University, Rua An´ ıbal Cunha 164, 4050-047 Porto, Portugal Received 9 June 2005; received in revised form 14 July 2006; accepted 20 July 2006 Available online 25 July 2006 Abstract Oleogels are semisolid systems obtained with an organogelator and a hydrophobic liquid that have been investigated over the past few years and that could play an important role as dermatological bases. Recently, we have developed an oleogel of sorbitan monostearate (19wt.%) and sweet almond oil (SM–SAO) and another one of cholesterol (3.5 wt.%) and liquid paraffin (Ch–LP). The aim of this work is to access their physical stability using three different methodologies. The gels were stored at different temperatures (20 and 40 ◦ C) over a 3-month period. Appearance and textural properties were assessed on each month. An accelerated test was also performed where the temperature changed between 4 and 40 ◦ C every 24 h, during 7 days. Rheological tests were also carried out as they could provide useful elements to predict stability. The gels were quite stable at 20 ◦ C, being the SM–SAO gel the most stable. The textural properties of both gels were influenced by temperature. The decrease of the textural parameters, observed after storage at 40 ◦ C and in the cycling test, was more significant for the SM–SAO gel. A good correlation was found between rheological analysis and conventional stability tests. The heating/cooling cycle test provided useful information in a short period of time. © 2006 Elsevier B.V. All rights reserved. Keywords: Stability test; Rheology; Oleogels 1. Introduction Oleogels are gel systems obtained with a gelling agent and a hydrophobic liquid. Interest in this field has increased due to the strikingly rise of the discovery of substances that are able to gel organic solvents. Initially, organogelators were frequently discovered serendipitously while now new strategies of chemi- cal syntheses are being explored with increasing success in the design of new gelators (Van Esch and Feringa, 2000). Terech and Weiss (1997) reviewed the different classes of organogela- tors. Some pharmaceutical excipients were also identified as organogelators, namely sorbitan esters (Murdan et al., 1999) and gelators with the cholesterol moiety (Terech et al., 1995). The applications of oleogels were investigated in several areas such as organic chemistry, environmental chemistry and also in pharmaceutical and cosmetic fields. Some of these poten- tial applications are discussed in a review by Hinze et al. (1996). ∗ Corresponding author. Tel.: +351 222078949; fax: +351 222003977. E-mail address: fgbahia@ff.up.pt (M.F. Bahia). The majority of the applications reported in the pharma- ceutical area were related to transdermal systems (Willimann, 1992), topical bases (Jurgens and Becker, 1974) and prepara- tions intended for percutaneous absorption (Henmi et al., 1994; Hori et al., 1998). In previous studies, four different organogelators (ethylcel- lulose, cholesterol, sorbitan monostearate and lanolin alcohols) were tested with several liquid phases, including vegetable oils (castor oil and sweet almond oil), mineral oil and synthetic esters (isopropyl miristate and isopropyl palmitate) (Almeida and Bahia, 2005). Based on the criteria of short-term stability, homogeneity, gelling ability at concentrations below 30%, and consumer preferences, the oleogels of cholesterol (3.5 wt.%) and liquid paraffin (Ch–LP) and sorbitan monostearate (19 wt.%) and sweet almond oil (SM–SAO) were selected as candidates for topical formulations. Cholesterol was able to gel liquid paraffin at concentrations as low as 1.5 wt.%, while the minimum appar- ent gelling concentration of sorbitan monostearate was around 17 wt.% for sweet almond oil. These hydrophobic gel systems do not require extensive man- ufacturing expertise to be produced, present batch to batch con- sistency and can be formulated in a wide variety of viscosities, 0378-5173/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ijpharm.2006.07.036