Solar Energy Materials & Solar Cells 221 (2021) 110898 0927-0248/© 2020 Elsevier B.V. All rights reserved. Formulation and processing of novel non-aqueous polyethylene glycol-in-silicone oil (o/o) phase change emulsions Clara Delgado-S´ anchez, Antonio Abad Cuadri, Francisco Javier Navarro * , Pedro Partal Departamento de Ingeniería Química, Centro de Investigaci´ on en Tecnología de Productos y Procesos Químicos (Pro 2 TecS), Universidad de Huelva, Campus de “El Carmen”, 21071, Huelva, Spain A R T I C L E INFO Keywords: Viscosity Non-aqueous emulsions Silicone surfactant Phase change material (PCM) Stability ABSTRACT Oil-in-oil phase change emulsions, composed of phase change materials (PCMs) dispersed in a continuous oily medium, have never been reported in the literature. The current study involves the formulation, stabilisation and characterisation of novel anhydrous emulsions of polyethylene glycol (PEG4000) as PCM in silicone oil. Emul- sions with different dispersed phase concentrations were evaluated by analysing the viscous fow properties, optical microscopy and modulated differential scanning calorimetry. The results evidence that the emulsions can be pre-stabilised at 80 ◦ C, under high shear conditions, by selecting an adequate silicone surfactant but undergo a severe destabilisation after a cooling and heating cycle. Nevertheless, a post-processing protocol (low shear agitation at 80 ◦ C for 24 h) leads to thermal cycle resistant and storage stable non-aqueous dispersions. This process gives rise to a reduction of the shear-thinning character of the emulsion, shifts the melting and crys- tallization temperature of the PEG4000 to lower values and reduce its crystallinity. This is attributed to partial compatibility of the crystalline fraction of PEG4000 with other compounds of the emulsion. 1. Introduction Non-aqueous emulsions, also known as oil-in-oil, anhydrous or waterless emulsions, have been rarely studied and with relatively few publications available, despite having many real and potential applica- tions [1]. Since the frst papers in the sixties [2,3], most of the studies on this topic have been focused on the research of cosmetic and pharma- ceutical applications, mainly in the formulation of anhydrous disper- sions used as controlled release vehicles of active principles, and as bases for topical applications or as emollients in formulations. Examples of potential therapeutic applications are occlusive preparations, oily depot emulsions for intramuscular or subcutaneous injections and preparation of oral controlled release dosage [4–7]. In the case of the preparation of porous materials, it has been also reported the development of some non-aqueous oil-in-oil High Internal Phase Emulsions (HIPE) emulsions and, more recently similar systems have been formulated with an ionic liquid as dispersed phase [8]. In another remarkable feld of application, they are employed to replace the emulsions where there are water-sensitive reactions or where higher temperatures than the boiling point of water need to be reached [9–11] and in developing dispersions of water labile drugs i.e. where the water itself is undesirable. Recently, enormous research interest has been drawn to the devel- opment of dispersions of Phase Change Materials (PCM) in a continuous low viscosity medium (mainly water) for thermal energy storage and transport, known as phase change emulsions or slurries. However, whereas most of the scientifc effort has been driven to the development of encapsulated phase change materials [12,13], little work has been done in incorporating PCMs in water with the help of emulsifers [14–18]. Besides, to the best of our knowledge, there are no reports so far studying non-aqueous dispersions with such application. This opens the door for a new feld of scientifc research and innovative applications for solar and geothermal thermal energy storage and industrial waste heat recovery to increase the energy storage capacity of conventional heat transfer fuids. These non-aqueous phase change emulsions would not only have the advantages that slurries bring to heat transfer appli- cations such as the possibility of using the same medium for transport and storage or the high heat transfer rate due to the increase of the specifc surface area of the PCM [19], but also all those of the use of oil phases, making a difference. Anhydrous emulsions would allow the transport of matter and heat in systems where the pressure and tem- perature conditions are more extreme [20], as well as, provide a better long term chemical stability. * Corresponding author. E-mail addresses: frando@diq.uhu.es, clara.delgado@diq.uhu.es (F.J. Navarro). Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat https://doi.org/10.1016/j.solmat.2020.110898 Received 29 May 2020; Received in revised form 20 November 2020; Accepted 21 November 2020