Driven by the desire to develop novel catalyst formulations which are applicable for localised, more sustainable routes, the area of heterogeneously catalysed ammonia synthesis has attracted much attention in the academic literature in recent times. One of the key incentives for this has been the idea that ammonia synthesis for the production of synthetic fertiliser can be conducted on, for example, a farm close to its point of application with the required hydrogen feedstream being derived from sustainable sources such as electrolysis of water accomplished using electricity produced using wind turbines or solar energy sources. Further drivers are the possible application of ammonia as a non-fossil based fuel and also as a means to indirectly store intermittent over-supply of sustainably derived electricity. In the literature, the energy intensive nature of the Haber-Bosch process, frequently quoted to be 1–2% of global energy demand, and its carbon dioxide footprint, stated to comprise 2.5% of fossil fuel based emissions, are statistics that are often quoted in justification for the search for new routes to ammonia production (1, 2). However, due recognition has to be given to the highly efficient nature of the Haber-Bosch process as currently operated. In relation to this, large scale synthesis of ammonia is highly optimised and it can be credited with the sustenance of ca. 40% of the global population. These considerations, coupled to the recently reported UK CO₂ supply chain shortage, related to a reduction in commercial fertiliser production (3), underline the importance of the highly integrated nature of the process.

In developing smaller localised sustainable ammonia synthesis units operating under milder reaction conditions, a step change in catalyst technology would be necessary requiring the development of catalysts being able to operate in the highly desirable lower temperature–lower pressure reaction regimes which have proved elusive for so long. Such catalysts should also be able to withstand a series of rapid start-up and shut-down procedures corresponding to the intermittent nature of sustainably derived electricity supply. A further consideration, not generally acknowledged nor widely explored, is the desirability of such new catalysts to withstand poisoning. In the following, we detail some of the approaches which have been described in the literature in recent years to bypassing the inherent limitations of the more conventional catalysts. While electrocatalysis and photocatalysis are increasingly explored in relation to nitrogen activation, we have concentrated on heterogeneous catalysis.

The paradoxical nature of scaling relationships in relation to conventional metal-based ammonia synthesis catalysts is widely acknowledged. This relates to the occurrence of a Sabatier volcano type relationship whereby catalysts of optimum activity possess intermediate nitrogen binding energies. In this way, the relative performance of metals known to be effective ammonia synthesis catalysts (such as iron, ruthenium and osmium) can be rationalised and the design of new active catalyst formulations can be undertaken by alloying metals of low nitrogen adsorption strength with those of high nitrogen adsorption strength (4). This rationale has been applied to explain the comparatively high performance of the Co₃Mo₃N ammonia synthesis catalyst (5, 6) on the basis of the combination of cobalt (low nitrogen adsorption enthalpy) with molybdenum (high nitrogen adsorption enthalpy) as expressed in the (111) surface plane (4). This explanation implies structure-sensitivity, which has not generally been explored with this material, and also considers the role of the lattice nitrogen to be induction of ordering such that the active surface crystallographic plane is exhibited (4).

An alternative viewpoint of the origin of the activity of this system is the potential occurrence of a Mars-van Krevelen mechanism. The Mars-van Krevelen mechanism, which is perhaps most frequently encountered in oxidation catalysis using metal oxide catalysts, involves the direct participation of reactive lattice species generating transient vacancies which are replenished to complete the catalytic cycle (7). For ammonia synthesised from metal nitrides, this would involve the synthesis of ammonia directly by hydrogenation of lattice nitrogen leading to vacancy sites where nitrogen is activated. To this end, direct hydrogenation of Co₃Mo₃N to produce ammonia occurs and can lead to the previously unprecedented Co₆Mo₆N phase (8). Co₃Mo₃N and Co₆Mo₆N are two line phases (with no intermediate stoichiometries being observed) with the transformation involving the loss of 50% of the lattice nitrogen upon reduction, with the residual lattice nitrogen relocating from the 16c to 8a crystallographic site (8). At ammonia synthesis temperature, using a hydrogen/nitrogen reaction feed, the rapid regeneration of Co₆Mo₆N to Co₃Mo₃N occurs. Such regeneration can also be accomplished by treatment with nitrogen alone (9) thereby opening the possibility of a chemical looping system for ammonia production (albeit with a very low gravimetric nitrogen content). Heterolytic isotopic exchange studies wherein ¹⁵N₂ is exchanged with the lattice nitrogen of Co₃Mo₃N (in which the nitrogen is predominantly ¹⁴N reflecting normal isotopic abundance) demonstrate the lattice nitrogen to be highly exchangeable with 40% exchange occurring at 600ºC within 40 min (10). Computational modelling lends further support to the possibility of the participation of reactive lattice nitrogen for this system in ammonia synthesis with high concentrations of surface nitrogen vacancies being predicted at the temperatures applied (11). Recently, the possibility of an associative mechanistic pathway occurring has been demonstrated through modelling (12) and this proposal is consistent with the observation that Co₃Mo₃N is not effective for homomolecular ¹⁴N₂/¹⁵N₂ exchange (which directly relates to the material’s ability to dissociate nitrogen) at ammonia synthesis temperature despite being an active catalyst (10). Generally, for heterogeneously catalysed ammonia synthesis, dissociative pathways (wherein nitrogen is dissociated on the catalytic surface often in the rate determining step) are invoked and the occurrence of an associative pathway (in which hydrogenation of surface bound nitrogen occurs prior to its dissociation) is an exciting development since it potentially provides a premise for catalyst design and a link to enzymatic nitrogen fixation which operates, albeit comparatively very slowly, under ambient conditions.

The role of nitrogen vacancies as being of interest for the development of novel nitride-based catalysts has been exemplified in the recently reported nickel/lanthanum nitride catalyst (13). In this catalyst, which is reported to exhibit high activity and extended stability, it is proposed that the scaling limitation is circumvented via the occurrence of a dual site mechanism in which nitrogen activation occurs at the nitrogen vacancies of the lanthanum nitride with hydrogen dissociation being accomplished by the nickel component. This concept has been expanded to cerium nitride and yttrium nitride also, with cerium nitride being an effective catalyst in its own right in which both nitrogen activation and hydrogen activation can be accomplished on nitrogen vacancy sites (14). The application of dual sites to circumvent limiting scaling relations has also been applied to the development of catalysts based on the combination of metals or metal nitrides with lithium hydride in which activated N is believed to transfer to the lithium hydride component which is progressively hydrogenated to LiNH₂ and then ammonia, regenerating the lithium hydride component (15). In terms of this strategy, combination of various alkali and alkaline earth hydrides with manganese nitride has been undertaken with LiH-Mn₄N being particularly effective (16). Elsewhere, in a chemical looping based study, lithium-ion doping has been shown to significantly enhance lattice nitrogen lability in Mn₆N_5+x where computational modelling indicates that its presence reduces the nitrogen vacancy formation energy (17). Overall, it is arguable that the role of alkali metal doping and hydrogen activation have been less extensively studied for novel catalysts. For Co₃Mo₃N, low levels of Cs⁺ ion doping have significant promotional effects but there are challenges in the preparation of the material related to phase stability issues (5).

High activities for ammonia synthesis have been reported for solid state hydrides and oxyhydrides in recent years. To this end, the performance of TiH₂ (18), BaTiO_2.5H_0.5 (18), BaCeO_3–x H_y N_z (19), VH_0.39 (20) and NbH_0.6 (20) is documented within the literature. In a number of cases, hydrogen-based Mars-van Krevelen mechanisms have been invoked as the source of activity. The surfaces of some of these materials may possibly nitride in operation, as demonstrated by small shifts in lattice parameters and elemental analyses. In the case of BaTiO_2.5H_0.5, the formation of BaTiO_2.5N_0.2H_0.3 is documented although BaTiO_2.5N_0.3 when tested exhibited no activity, thereby emphasising the initial requirement for the hydride containing phase to be present. Lattice mobility is an important consideration in the Mars-van Krevelen mechanism wherein diffusion of species within the bulk of materials to their surfaces occurs. This consideration was applied to the development of the vanadium and niobium hydride based systems on the basis that their ‘more open’ body-centred crystal structures facilitate diffusion to a greater extent than the face-centred cubic based crystal structures based upon TiH₂ and the perovskite structure based BaTiO_2.5H_0.3 and BaCe_3–x H_y N_z systems. This consideration, coupled with advantageous metal-nitrogen bond strengths, is invoked to explain the enhanced performance of these materials (20). This is an interesting design consideration from which novel catalyst compositions of apparent high activity have been developed from components which, in isolation, might not be expected to display good performance. Lanthanum oxyhydrides have been applied as supports for ruthenium where it is reported that catalytic performance relates to high surface hydride ion mobility leading to low work function electrons at hydride ion vacancies near the ruthenium interface enhancing nitrogen activation and reducing the effect of hydrogen poisoning on the ruthenium component (21). Surface hydride is proposed to be formed by the reaction of electrons at vacancies reacting with hydrogen adsorbed on the ruthenium component forming hydride ions which subsequently react with adsorbed nitrogen species releasing electrons back to the vacancy sites. It was also stated that the choice of oxyhydrides is important in reduction of the deleterious effect of nitridation. Ternary ruthenium hydrides have also very recently been reported as effective catalysts with Ba₂RuH₆/MgO outperforming the most active catalysts published to date (22). It is proposed that an associative pathway occurs for this system, which is stabilised by the [RuH₆] centres, lattice hydrogen and alkaline earth metal cation. Experiments performed for Li₄RuH₆/MgO demonstrated the requirement for feeding both nitrogen and hydrogen for the generation of ammonia.

In addition to interest in the role of nitrogen and hydrogen vacancies, activation of dinitrogen at dicarbide vacancies has been proposed to occur in relation to nickel-loaded rare earth dicarbides (23). In the case of Ni/CeC₂, the nickel component was reported to be important in promoting the formation of C₂ vacancies in cerium carbide by virtue of its enhanced hydrogen activation functionality. Furthermore, differences in nitrogen adsorption energy and configuration were noted in comparing Ni/CeN and Ni/CeC₂ although they were found to possess comparable ammonia synthesis activity under the testing conditions applied (23). In relation to the catalytic performance of carbides, earlier studies had shown β-Mo₂C to be an effective catalyst and more active than γ-Mo₂N, unlike the α-MoC_1–x phase which was not stable under reaction conditions (24). In a comparison between the performance of Co₃Mo₃N and Co₃Mo₃C, which had been carefully prepared so as to avoid any complicating effects of differing morphology, the carbide was shown to be less active requiring a higher reaction temperature and with activity developing after an initial lag period (25). Upon extended testing, gradual nitridation of the Co₃Mo₃C phase was observed to occur. Ammonia synthesis was related to the presence of nitrogen occupying the 16c crystallographic site although it was not possible to distinguish whether the presence of lattice nitrogen arose from the production of ammonia or was the cause of it (25). Co₆Mo₆C, which comprises carbon in the 8a lattice site, was found to be inactive under the conditions tested and the phase remained stable (25). This is contrary to the observations for Co₆Mo₆N referred to previously.

In concluding this brief summary of some of the recent advances in relation to the development of ammonia synthesis catalysts, it is fair to say that there are very exciting and tantalising opportunities for the design of novel catalysts. The ability to synthesise more complex nitrides such as the quaternary phases NiCoMo₃N (26) and (NiM)₂Mo₃N (M = copper or iron) (27) provides the possibility to tune catalytic performance. Taking ternary metal nitrides as an example, in the comparison of the behaviour of Co₃Mo₃N, Fe₃Mo₃N, Ni₂Mo₃N and Co₂Mo₃N it can be observed that the role of composition and structure is complex (28) and computational modelling has a role to play in rationalising the origin of performance leading to the design of new catalysts. It is also noteworthy that the possibility of associative pathways is being invoked more widely. Such pathways are closer to enzymatic systems and, as such, may be related to the possibility of lower temperature catalytic routes. While surveying the literature uncovers many interesting and exciting advances in the area of alternative materials which can be extended to the further discovery of novel catalysts, the development of technology suitable for small scale sustainable ammonia synthesis has yet to be accomplished and remains an exciting and potentially highly rewarding challenge. In terms of application, consideration would need to be given to longevity of performance and also to poison tolerance as well as catalyst handling, storage and preparation. In relation to preparation, it is interesting to draw attention to the considerations of heat transfer in relation to the synthesis of active materials as has been discussed, for example, for binary molybdenum nitrides where nitridation of MoO₃ via ammonolysis and treatment with nitrogen/hydrogen mixtures has been compared (29, 30).

“Sustainable Materials for Transitional and Alternative Energy” is a 294-page, five-chapter book which forms one part of the Modern Materials and Sensors for the Oil and Gas Industry Series. The book is authored and edited by Mufrettin Murat Sari (Texas A&M University, USA), Cenk Temizel (Saudi Aramco, Kingdom of Saudi Arabia), Celal Hakan Canbaz (Ege University, Turkey), Luigi A. Saputelli (ADNOC Frontender Corp, USA) and Ole Torsæter (Norwegian University of Science and Technology, Norway), in collaboration with a further 15 contributors.

Mufrettin Murat Sari is a Chemistry Professor at the Department of Chemistry, Texas A&M University, and Life and Health Science Department, University of North Texas at Dallas, USA. Beginning with his PhD degree from Hacettepe University, Turkey, in 2005, he has 20 years of experience in the fields of materials chemistry, applied biochemistry and nanotechnology. Throughout his illustrious career, he has published about 40 scientific articles and proceedings with hundreds of citations. Cenk Temizel is a Senior Reservoir Engineer with Saudi Aramco. His career spans around 15 years in industry working on reservoir simulation, smart fields, heavy oil, optimisation, geomechanics, integrated asset modelling, unconventionals and enhanced oil recovery (EOR) across the Middle East, USA and UK, prior to which he was a teaching and research assistant at the University of Southern California, USA, and Stanford University, USA. Like Temizel, Celal Hakan Canbaz is a Senior Reservoir Engineer with 16 years industrial experience. He is highly regarded in the fields of special core analysis, reservoir wettability characterisation, well testing analysis, perforation and testing design, multiphase flow meters, carbon dioxide/oil/water interactions, wellbore flow dynamics and pressure-volume-temperature data interpretation. He has made contributions to industrial projects, conference and journal papers, two books and a US patent. Luigi A. Saputelli is a Reservoir Engineering Expert Advisor with 30 years of experience as a reservoir, drilling and production engineer. Alongside his industrial contributions, he is a researcher, lecturer and active volunteer in the Society of Petroleum Engineers, and has published in excess of 100 industry papers on digital oilfield, reservoir management and real-time production optimisation. Ole Torsæter is a Professor of reservoir engineering at the Norwegian University of Science and Technology and Adjunct Professor at the University of Oslo, Norway, as well as a research associate at PoreLab, a Norwegian Centre of Excellence. He has supervised a staggering 220 Masters and 25 PhD candidates, alongside publication of 200 research papers himself, with the most recent focusing on nanofluids for EOR.

“Sustainable Materials for Transitional and Alternative Energy” addresses today’s energy needs in a fast-paced world in which nanotechnology is of vital importance for maintaining environmental sustainability and protecting human health. This book keeps pace with advancements in the energy industry by addressing the latest research involving advanced nanomaterials which engineers can apply to nanoparticle applications beyond the petroleum industry. Additional topics in Volume 2 include carbon capture-focused, green-based nanomaterials, the importance of coal gasification in terms of fossil fuels and advanced materials for fuel cells.

This book has been written with the intention of targeting a wide range of readers from academics to researchers, and undergraduate to graduate students, from various backgrounds, including petroleum engineers and researchers, nanotechnology researchers in the oil and gas industry, chemical engineers and material scientists.

This book will be reviewed in order of chapter due to its methodological breakdown starting with the oil and gas industry, before moving onto ‘greener’ technologies involving nanomaterials.

The objective of this chapter is to define the field of smart and state-of-the-art materials together with their current status and potential benefits. In the future, more focus will be placed on additives, nanoparticles, shape memory materials and piezoelectric materials: those that can generate electricity on the application of mechanical stress.

Chapter coverage is equally divided among smart materials and state-of-the-art materials. The former refers to those materials that can change their composition or structure, their electrical and mechanical properties or even their functions in response to environmental stimuli. The state-of-the-art materials included in this chapter are additives (bacterial control, corrosion inhibitor, fluid loss, lubricants, fluid viscosifiers, synthetic-based muds, clay stabilisers, antifreeze, odorisation and defoamers) and nanoparticles (for improving sweep efficiencies, stabilised foam, polymer flooding and reduced water production). Their properties and applications are discussed in detail throughout this book, and specifically, this chapter.

Geothermal energy is one of the largest renewable energy sources in existence. Chapter 2 addresses investigation methods used in the exploration, discovery and monitoring phases of geothermal systems, the structural characterisation of which can be implemented through the use of geophysical tools. The latest technology is compared with conventional counterparts in terms of efficiency and cost-effectiveness. This includes the use of nanotechnological materials and advanced tools (for example, fibre optic sensors, distributed temperature sensing systems, thermal infrared remote sensing tools and advanced technology carriers such as drones, aircrafts and satellites) for measuring physical properties of geothermal fluids and rocks. Furthermore, advanced materials and nanomaterials starting to be used in geothermal downstream parts, such as heat transfer and energy conversion in thermoelectrical power plants, are introduced and discussed in appropriate detail.

CO₂ capture and sequestration (CCS) technologies have received increasing interest in recent years due to the pressing global demand to reduce CO₂ emissions and slow down global warming. This chapter explores the development of reusable, low-cost and green-based adsorbents which are important for efficient and environmentally-friendly removal of CO₂ from the atmosphere. The absorbents discussed in detail include halloysite nanotubes, nanofibrillated cellulose, enzyme immobilised on bioinspired nanosorbents, green metal-organic frameworks and bio-derived porous carbons. The advantages of each are given, alongside research study results and their many uses in CCS, which are well summarised in Sub-chapter 3.7, ‘Conclusion and Outlook’. Looking forward to the future, there is a need to investigate CO₂ selectivity of nanomaterials over other gases (for example nitrogen and methane) as research currently focuses on adsorption characteristics.

As global energy needs increase, so does the importance of addressing energy security, environmental problems and the cost of energy. Since renewable energy sources are yet to reach the required energy demand, and issues of radioactive waste from nuclear energy remain, the gasification process (conversion of hydrocarbon fuels into gases) stands out as an alternative technology that enables the production of clean gas products that can be used in many areas. This book’s penultimate chapter provides a comprehensive background for nanocatalysts and sensors in the coal gasification process. Emphasis is placed on the use of catalysts, specifically nanocatalysts with their superior physical properties (namely, large surface area to volume ratio), in the gasification process which have the potential to increase carbon conversion rates and economically reduce processing times.

Inconsistent with the first three chapters of this book and the subsequent final one, Chapter 4 does not have a ‘Conclusions’ section. This emphasises the point raised at the end of this review which highlights the book’s lack of editorial consistency and disruptive flow of content.

The fifth and final chapter of this book introduces the reader to fuel cell technology and its importance as one of the main solutions for the next generation of environmentally friendly energy. It provides a general perspective on fuel cell types, namely polymer electrolyte membrane, microbial, alkaline, phosphoric acid, solid oxide and protonic ceramic, their mechanisms and applications, alongside the nanostructures used to produce catalysts in this field.

For background context, fuel cells are electrochemical devices that directly convert chemical energy from oxygen and hydrogen into electrical energy in a single step. Several improvements and developments regarding the increase in hydrogen production, improvements in cell design, removal of membrane, utilisation of microbial catalysts or platinum-free catalysts have been successfully demonstrated in the field of electrocatalysts. These are addressed in this chapter and summarised in Figure 1 and Table I.

Fig. 1.

While proton exchange membrane fuel cell development is the most highly anticipated area, traditional catalysts, such as platinum nanoparticles on carbon (Pt/C) are still being used in current fuel cells as well as nonprecious metal catalysts. Nanoscale materials used in fuel cell technology include nanoparticles, nanoframes, nanorods and core-shell structures – the latter of which has shown enhanced results in the cathodes and anodes in fuel cells electrodes – which have contributed immensely to fuel cell commercialisation expansion. Current and potential fuel cell applications are discussed in detail in this chapter and summarised in Figure 2.

Fig. 2.

In summary, this book provides a detailed introduction and assessment of the use of sustainable materials in the energy industry. Scientific papers and research are heavily referenced throughout; these are highly detailed and often require the reader’s full attention and specialist background knowledge in order to be fully understood. This book is best suited to a subject matter expert interested in making technological advances in the energy industry using smart and state-of-the-art materials, namely nanomaterials.

In general, this is a poorly written book which made for an unnecessarily difficult read. It is riddled with grammatical errors, such as the omission of conjunction words which greatly impacts upon the flow of the material. Material flow is also disrupted by poor figure placement throughout; there are multiple examples where figures are placed in the middle of sentences rather than at the end of paragraphs making it difficult for the reader to link information.

Inconsistencies exist between chapters due to having multiple contributory authors. The most significant of these concerns the level of background or introductory information provided. For example, in Sub-chapter 4.2, fossil fuels, CO₂ formation and global warming are explained in detail, but assumptions are often made in other chapters of a high level of understanding on much more complex and less familiar topics. This is inconsistent with the fact that the book claims to be targeted to undergraduate students from various backgrounds.

Furthermore, the impact of multiple authors on the fluidity of this book is evident through the inclusion of several introductions to nanotechnology and nanomaterials across numerous chapters. This is tedious for the reader when reading the book as a whole; however, if chapters are viewed in isolation, a case can be made for the need for several introductions. Nevertheless, repeated content could be removed, thus saving on space, by cross-referencing information between chapters; this would also have the added benefit of improving the fluidity of the book’s content.

Resultantly, this book could have been better edited to ensure consistencies between chapters and eliminate repetitive information.

Refined, white beeswax pearls and retinyl palmitate (vitamin A) of 1.7 MIU g^–1 (MIU = milli-international units) were purchased from Bulk Apothecary (Aurora, OH, USA) and Fisher Scientific USA (Pittsburg, PA, USA), respectively. Ethanol was obtained from Decon Laboratories, Inc (King of Prussia, PA, USA). Compression fabric (warp knit: 77% nylon and 23% spandex) was obtained from the Marena Group (Lawrenceville, GA, USA). Pampers^® Aqua Pure^TM nonwoven wipes were also used as a carrier to transfer microparticles from the substrate to skin.

We microencapsulated retinyl palmitate by melt dispersion technique and investigated the effect of four process variables on the produced microcapsules, such as different theoretical loading capacity (10%, 15%, 25%), types of wax (beeswax, carnauba wax, paraffin wax), emulsifier concentrations (0%, 1%, 2%) and stirring speeds (180 rpm, 230 rpm, 280 rpm) in our previous study (12). The statistical analysis showed that theoretical loading capacity and surfactant (%) were the most significant factors and we were able to determine that the highest theoretical loading (25%) and highest surfactant (2%) selected in that study can provide us high actual loading with the small size of the particles. There was no significant difference found among the effects of type of wax on loading capacity, encapsulation efficiency, antioxidant activity or mean size of particles. Hence we decided to conduct further study selecting beeswax as the shell material because of its natural skincare benefits as well as operational convenience due to low melting point (65°C). We selected 280 rpm stirring speed to facilitate dispersion of the oil-in-water emulsion and formation of small size particles.

Thermal analysis of the beeswax, retinyl palmitate and retinyl palmitate-loaded beeswax microcapsules was carried out by using Mettler-Toledo GmbH DSC821e (Greifensee, Switzerland) instrument, where a standard empty aluminium pan was used as the reference. The weight of the samples was within 2–9 mg, and the samples were scanned from 25°C to 100°C under nitrogen atmosphere with a heating rate of 10°C min^–1.

After preparing the microcapsules with 25% theoretical loading, we looked into the shelf life of microcapsules by measuring the actual loading percentage, i.e. the content of retinyl palmitate in a fixed amount of capsules over a period of time, both in powder and dispersion forms. We evaluated the shelf life of the beeswax microcapsules (approximately 71% encapsulation efficiency) in powder form, where they were filtered and dried before storing in an enclosed petri dish under room temperature; and also in dispersion form (approximately 75% encapsulation efficiency), where the particles were kept dispersed within the emulsion during preparation, stored inside dark vials in refrigerator and a portion was filtered on each day of measurement (Day 1, Day 4, Day 8, Day 15 and Day 31).

An extraction from 0.1 g of microcapsules was performed, by heating the capsule in 20 ml of ethanol solution to release the vitamin content and then filtering the wax residue. The concentration of supernatant aliquots was measured at 327 nm by a Shimadzu Corporation UV-2401PC spectrophotometer (Kyoto, Japan). The amount of retinyl palmitate was determined from a standard curve of known concentrations.

We conducted an in vitro kinetic release study similar to prior literature (27, 43) with some modification based on particle content, solvent type and machine parameters. The retinyl palmitate release profile from 3 g of suspended particles (approximately 77% encapsulation efficiency) was examined in 600 ml of pure ethanol. The study was performed in a New Brunswick Scientific C24 (Eppendorf, Germany) incubator shaker with a speed of 100 rpm and temperature set at 37±2°C. Supernatant aliquots of 2 ml were withdrawn and replaced by the fresh medium at appropriate time intervals (1 min, 5 min, 15 min, 30 min, 1 h, 2 h, 4 h). The supernatants containing dissolved retinyl palmitate were diluted and analysed by ultraviolet-visible (UV–vis) spectroscopy at 327 nm. The results were compared with a standard to calculate the vitamin A concentration and to evaluate the release ratio.

We used a robotic transfer replicator (Figure 1) to simulate the transfer of microparticles from a nonwoven wipe to the skin and evaluate the transfer percentage, by means of a similar method as described by Yu et al. (44). 1 g of microparticles was spread as evenly as possible by a spatula over a commercial nonwoven wipe containing 99% water that acted as a donor surface with a diameter of 133 mm. The receptor material was a compression fabric, i.e. a warp knit with a composition of 77% nylon/23% spandex (fabric weight 276 g cm^–2). This fabric was chosen because the study by Yu et al. (44) regarding transfer of particulates from carpet surface to human skin-like receptors revealed that this fabric replicated the human skin, particularly finger pads best as a receptor material. The receptor fabric was attached to an aluminium nose piece with the help of O-ring made of rubber. After the activation of the replicator, the nose piece descended the receptor fabric onto the donor surface and rubbed the receptor against the donor by executing a certain number of motions (imitating an hourglass pattern) under a constant pressure maintained by the programmed hydraulic system. After performing this rub cycle, the nose piece was raised and delivered onto a glass jar containing 20 ml of ethanol. The fabric was released into ethanol and shaken vigorously, followed by sonication for 2 h so that all the particle content is released into ethanol. Then aliquots were removed for assay in an UV–vis spectrophotometer to measure the content of retinyl palmitate. Finally, the amount of transfer of retinyl palmitate was calculated in percentage.

Fig. 1.

All the measurements for shelf life study were performed in triplicates, whereas the measurements of kinetic release study and simulated transfer study were performed in duplicates. The results have been reported as the mean values and their corresponding standard deviations.

Figure 2 shows differential scanning calorimetry (DSC) scans of beeswax, retinyl palmitate and beeswax microcapsules with 25% theoretical loading capacity. In the thermogram of retinyl palmitate, a sharp endothermic peak is observed at 34.33°C, which corresponds to its melting point. However, it is observed that the microcapsules show no endotherms corresponding to the melting point of retinyl palmitate. This implies that retinyl palmitate dissolved within the matrix of beeswax when the temperature reached its melting point (45). This observation was consistent with the result found by Milanovic et al. (46), where encapsulated ethyl vanillin dissolved in the carnauba wax matrix. Untreated beeswax and retinyl palmitate-loaded beeswax microcapsules show their melting peaks at 65.67°C and 63°C, respectively. The slight decrease in the melting point of the particle should be due to the mixing of retinyl palmitate and wax because of plasticisation. A second peak is observed for microcapsules at higher temperature (slightly higher than melting temperature), which could be because of fraction of large crystallites formed after encapsulation process that showed higher melting.

Fig. 2.

Figure 3 shows the shelf life study of the beeswax microcapsules in: (a) powder form stored under room temperature; (b) dispersion form stored in a refrigerator. When the particles were evaluated in powder form under room temperature, the microcapsules lost their active content within 8 days (Figure 3(a)). This phenomenon can be attributed to the diffusion of retinyl palmitate through the wax shell. The high compatibility between lipophilic, low molecular weight active ingredients with wax is the major cause of diffusion (47). Diffusion can be accelerated in small-sized particles due to the availability of larger contact areas as well as due to pores existing in the shell matrix (48).

Fig. 3.

Djordjević et al. (31) described the internal structure of particles produced by melt dispersion with the wax shell to be nonhomogeneous with matrix or hollow-shell morphology. Therefore in the prepared microcapsules, retinyl palmitate is distributed within the wax shell matrix. With the course of time, the core content comes up to the surface and diffuse through the shell. From Figure 4(a), the gradual change in the colour of beeswax microcapsules supports the phenomenon of diffusion as a plausible explanation. The particles stored as powder form appear to be bright yellow after the retinyl palmitate diffuses to the surface and they turn white (beeswax) when almost all of the core content leaches out.

Fig. 4.

On the other hand, when the retinyl palmitate-beeswax particles were stored in the dispersed aqueous emulsion in a refrigerator, they retained the core material and showed no significant decrease in retinyl palmitate content until the 15th day (Figure 3(b)). The variability in size distribution of different batches of filtered particles may account for the slight increase observed in actual loading capacity (Figure 3(b)). After 30 days, a decrease in loading was observed, which can be explained by ester hydrolysis of the beeswax while stored in aqueous emulsion resulting in the release of the content (49). retinyl palmitate-beeswax particles stored in the dispersed aqueous emulsion in the refrigerator do not show a significant visual difference in colour when filtered (Figure 4(b)).

The release profile (Figure 5) of retinyl palmitate-beeswax microcapsule showed an initial burst followed by a slower release of the vitamin entrapped inside the beeswax matrix. Due to the initial burst effect, 7% of the retinyl palmitate released at the first minute, leading to around 55% release in the first half an hour. After the initial rapid release, the release profile showed a sustained release over time. Within 4 h, approximately 98% of retinyl palmitate was released. A similar pattern of release was found by Kheradmandnia et al. (49) from ketoprofen-loaded solid lipid nanoparticles incorporated in the matrix of beeswax-carnauba wax mixture. Zigoneanu et al. (50) described the phenomenon of such initial burst as the result of the cumulative effect of diffusion of the core through the matrix, penetration of dissolution medium into the particle, and degradation of the shell matrix. As retinyl palmitate is soluble in ethanol, this explanation is agreeable to our result. Permeation of ethanol through the pores of the shell matrix and simultaneous diffusion of retinyl palmitate through the matrix facilitated the fast dissolution of the vitamin into ethanol. Duclairoir et al. has reported similar release profile for α-tocopherol from wheat gliadin nanoparticles, where mathematical models were demonstrated for the bistep release, i.e. the burst effect and the slower diffusion process (51). While the initial burst could not be described by their model, the time-dependent slow release showed a good fit (R² = 0.90) for the model in Equation (i):

(i)

Fig. 5.

Here, M₀ is the amount of active content incorporated, M_t is the amount of release core at time t, D is the diffusion coefficient and R is the radius of the particle. Thus the sustained release was related to the diffusivity of the active core inside the matrix system, the surface area of the particle and the loaded content.

From this result, we can understand that alcohol-based cosmetic formulations will not be stable over time as the core content would be released in the carrier substrate during the storage period, making retinyl palmitate susceptible to oxidation and degradation. On the contrary, as we already observed in the shelf life study, an aqueous medium prevents the active content from releasing from the capsule because of having no affinity to the lipophilic content. As a result, a water-based formulation would be suitable to contain the particles for cosmetic applications.

From the transfer study, we found that 21.7±0.02% of retinyl palmitate was transferred to the receptor material from the donor surface of wet nonwoven wipe after the preprogrammed rubbing cycle. The percentage falls within the range reported by Yu et al. in their study of transfer of particulates from carpet surfaces to human skin. Although this amount may vary depending on encapsulation efficiency, method of particle incorporation, and the amount of particle incorporated, this study demonstrates the potential of using such microparticles into facial wipes to impart skincare properties. Knaggs, in his skin-ageing handbook, mentioned that 0.05–0.1% tretinoin (retinoic acid) was effective to reduce signs of ageing in Asians (52). Oliveira et al. demonstrated in their study that topical application 1% retinyl palmitate has promising results for the treatment of skin ageing (53). According to Gangurde et al., the recommended concentration for topical semisolid formulation of vitamin A palmitate is 0.05%–0.3% (27). Thus, considering the approved dosage of retinoids, absorption and conversion rate of retinyl palmitate to retinoic acid within the skin, a proper formulation has to be developed in further study.