Chapter 1: Introduction
2.3 Results and Discussion
2.3.2 Comparison between the surface areas of hydroxyapatite nanoparticles and
Surface areas were calculated using the Brunauer–Emmet–Teller (BET). The BET instrument determined the specific surface area (m²/g) of HA nanoparticles/ nano-hydroxyapatite powder samples. The sample is initially degassed to remove any gas or vapours which may have adsorbed onto the surface of the samples from the ambient air, enhancing adsorption of nitrogen gas (adsorbate). If samples are not degassed, the surface area results can be low and non-reproducible because the surface area has already been adsorbed by other gas molecules/vapours. The sample is then dried with nitrogen purging or in a vacuum applying elevated temperatures. The volume of gas adsorbed to the surface of the particles is measured at the boiling point of nitrogen (-196°C). The amount of adsorbed gas is correlated to the total surface area of the particles including pores in the surface. The surface area is calculated using BET method over the range of P/P0 = 0.02 - 0.2 where a linear relationship was maintained. Traditionally, nitrogen is used as adsorbate gas because it is chemically inert and the experimental temperature to perform a complete
BET is an extended version of the Langmuir model. Langmuir assumed that energy of absorption for the first monolayer is generally considerably larger than that of the second and higher layers, thus forming multilayer is only possible at much higher pressures than the pressure required for formation of the first monolayer (Roquerol, Roquerol and Singh, 1999).
According to the BET model, the molecules in the first layer were assumed to act as sites for the second-layer molecules, and so on to infinite layers. It is also assumed that the adsorption behaviour of all layers above the first monolayer is the same. Moreover, assuming that the multilayer has an infinite thickness at p/p0 = 1, Brunauer, Emmet and Teller were able to derive their famous BET equation, which is usually expressed in the following linear form (Brunauer, Emmet and Teller, 1938):
𝑝
𝑛(𝑝0−𝑝)=𝑛1
𝑚𝐶+𝑛𝐶−1
𝑚𝐶×𝑝𝑝0 Equation 2.1
Where n is the total adsorbed number of molecules, nm is the monolayer capacity and C is an empirical constant that is assumed to be exponentially related to the net heat of adsorption (energy of adsorption by the first monolayer minus the energy of adsorption by the subsequent layers) as the following simplified equation:
𝐶 ≈ 𝐸𝑋𝑃 (𝐸1−𝐸𝐿
𝑅𝑇 ) Equation 2.2
Using the BET method over the range P/P0 = 0.03–0.18, where a linear relationship was maintained, surface areas were calculated based on the following equation 2.3:
𝑎𝐵𝐸𝑇 = 𝑛𝑚𝐿𝜎 Equation 2.3
Where BET surface area is related to n through the effective molecular cross-sectional area, σ, which is equal to 0.162 nm2 for N2 at 77 K and L, is Avogadro’s number (Table 2.5).
Table 2.5 Definition of Symbols used in the calculated BET surface areas
Symbol Definition
P partial vapour pressure of adsorbate gas in equilibrium with the surface at 77.4 K (b.p. of liquid nitrogen), in pascals,
Po saturated pressure of adsorbate gas, in pascals,
Va volume of gas adsorbed at standard temperature and pressure (STP) [273.15 K and atmospheric pressure (1.013 × 105 Pa)], in millilitres,
Vm volume of gas adsorbed at STP to produce an apparent monolayer on the sample surface, in millilitres,
C Dimensionless constant that is related to the enthalpy of adsorption of the adsorbate gas on the powder sample.
Appendix I, II, III and IV are adsorption/desorption isotherms for un-optimised HA, 20nm, 40nm and 60nm nano-hydroxyapatite powders. Adsorption occurs when a gas is brought into contact with a solid, part of it is taken up and remains on the outside attached to the surface.
In physical adsorption (physisorption), there is a weak Van der Waals attraction between the adsorbate and the solid surface. An adsorption isotherm is obtained by measuring the amount of gas adsorbed across a wide range of relative pressures at a constant temperature (typically liquid N2, 77K). Conversely, desorption isotherms are achieved by measuring gas removed as pressure is reduced.
Appendix I, II, III and IV illustrate the characteristics of type II adsorption isotherms, which describe adsorption on macro-porous adsorbents, (in this case, hydroxyapatite), with strong adsorbate-adsorbent interaction or affinity. The hydroxyapatite nanoparticles used in this study were in the form of loose powders and thus had interparticulate pores between the particles. Such isotherms also indicate indefinite multi-layer adsorption after completion of the first monolayer and are found in adsorbents with a wide distribution of pore sizes. Near to the first point of inflexion, a monolayer is completed, successfully followed by adsorption
Un-optimised hydroxyapatite nanocores (AQUA1) which has a large nanoparticle size distribution with a size range between 925-1100nm had a surface area of 22.742 m2g-1.
Nano-hydroxyapatite powders with particle sizes 20nm, 40nm and 60nm had surface areas of 54.42, 58.18 and 65.37 m2g-1 respectively illustrating a directly proportional relationship between smaller nanoparticle size and larger surface area. These calculated BET surface areas are summarised in table 2.6 showing a trend of decreased surface area with an increase in HA core size
Table 2.6 Table showing the calculated BET surface areas of the hydroxyapatite nanocore samples
Sample Calculated BET surface areas (m2g-1)
20nm nanopowder 65.377 synthesize tri-calcium phosphate. After subsequent steps, the resultant powder was heated to at least 600°C to be calcined to obtain a high purity nano-crystalline powder. The different temperatures yielded different crystalline phases of CaPs. Nano-CaPs calcined at 600°C showed the highest average BET surface area not only because of their smaller particle size (48-69nm), but also because of their higher particle aspect ratio. The BET specific average surface area for the powders calcined at 600°C and 800°C were 73 and 57 m2g1, respectively, the difference being statistically significant. The results of this study relate to the results in this chapter agreeing with theory that the smaller the size of nanoparticles (20nm, 40nm and 60nm), the larger the surface-to-volume ratio which is available for drug/protein adsorption in the fabrication of aquasomes.
In the study by Dasgupta, Bandyopadhyay and Bose (2009), BSA was used as a model protein. The adsorptive property of BSA was investigated by the change in BET surface area of CaP nanoparticles. Results from the study showed that the adsorbed amount of BSA increased with increasing surface area of the nano-CaPs immersed in the BSA solutions.
results clearly demonstrate that surface area affects drug/protein loading and thus a smaller hydroxyapatite nanoparticle size translates to a larger surface area, which increases drug/protein loading capacity in aquasome formulation.