## 1 Introduction  
Solid lipid nanoparticles (SLNs) were developed in the 1990s and were proposed as an alternative to traditional carriers (e.g., polymeric nanoparticles, liposomes). Since then SLNs have been receiving the attention of researchers from both academia and industry for a set of applications [1; 2]. Solid lipid nanoparticles (SLNs) are colloidal carriers composed of a lipid matrix that is solid at body and room temperature (i.e., of melting point above 40 \({}^{\circ}\)C), and stabilized by appropriate surfactants [2; 3]. In the last years, efforts have been made to develop SLN formulations with distinctive properties, e.g., capacity to improve the solubility of poorly water-soluble drugs and consequently their bioavailability, to enhance their targeting potential and cellular uptake, as well as their biological compatibility. SLNs are regarded as carriers with reduced toxicity [2; 3; 4], and have become popular in several areas, aiming to physicochemically protect the loaded drugs and to minimize their side-effects. In this regard, the components of nanoparticles aiming to deliver pharmaceutically active substances by routes, such as intravenous, oral, nasal pulmonary, must be of high grade and biocompatible to physiological systems. Other features to consider are the physicochemical properties of nanoparticles that influence stability and shelf-life of formulations, which can be improved by selecting the appropriate lipids and surfactants in the right combinations [2; 5; 6; 7]. Surfactants are used to disperse the solid-lipid matrix in water thereby reducing the surface tension and energy to control the shape and prevent the risk of nanoparticles' aggregation under shelf life [6; 8; 9]. Surfactants are defined as molecules with amphiphilic/amphiphatic properties, having separate hydrophobic or solvophobic groups and hydrophylic or solvophilic properties. This feature arises from the chemical structure that consists of both a typically long alkyl chain (non-polar or hydrophobic groups, being named as \"tails\") and a polar or hydrophilic group (named as \"heads\") [10; 11]. Surfactants are characterized by the presence or absence of charged groups in the head portion, being classified as ionic and non-ionic, respectively [9; 10]. Depending on the total charge in the head group, ionic surfactants are further classified as (i) cationic, if positively charged, (ii) anionic, if negatively charged, and (iii) amphoteric, if presenting both a positive and a negative charge [11; 12; 13]. The behavior of amphoteric surfactants is pH-dependent, being able of depicting a net positive charge (cationic) at low pH, a net negative charge (anionic) at high pH, and null net charge (zwitterionic) at intermediate pH values [13]. Surfactants have been used for a wide range of industrial applications in different sectors, e.g., foods, cosmetics, pharmaceuticals, with emphasis in the production of micro and nanoparticles [14]. The positive surface charge provided by the use of cationic surfactants may improve the cell-particle interaction as SLNs are attracted by the negatively charged membrane phospholipid. If negatively charged, such interaction is prevented or restrained. However, the interaction of cationic SLNs with the cells may be followed by some degree of toxicity. Indeed, it has been documented that non-ionic surfactants are less toxic, than ionic surfactants [3; 15], while cationic surfactants have been reported to produce higher toxicity, as a result of their higher interaction with cell membranes [3; 14; 16]. Among cationic surfactants, CTAB (cetyltrimethylammonium bromide) and DDAB (dimethyldioctadecylammonium bromide) have been used in various industrial processes (e.g., as biocides and antiseptic agents [17]), and in the production of drug-loaded nanoparticles for various applications [4; 18; 19; 20; 21; 22; 23]. While CTAB has a single chain and a single positive charged head-group, DDAB on the other hand is a double chain cationic lipid showing higher lipophilicity [24].