Oxide Particle Size, Loading, and Specific Surface Area Effects on Adsorption Isotherms
The specific surface area of mineral particles is usually determined by the BET gas adsorption isotherm method, and is a key property needed to normalize surface concentrations, mineral dissolution and growth rates, etc. per surface area, in studies of mineral surface reactions. However, aggregation and settling of particles could affect the available surface area for surface reactions, thus influencing the measured adsorption isotherms and surface reaction rates, with significant implications for the interpretation of the data. In this study, we have examined the effects of particle loading (mass of oxide per liter of solution) and size (micron- versus nanometer-sized) on adsorption isotherms of dipalmitoylphosphatidylcholine (DPPC), a common cell-membrane phospholipid (PL), at pH 7.2 and 55°C. The oxides included two nanoparticulate amorphous silicas, one nanoparticulate anatase (b-TiO2), and micron-sized quartz (a-SiO2), rutile (a-TiO2) and corundum (a-Al2O3). Phospholipids are amphiphilic molecules that comprise biological cell membranes, and self-assemble to vesicles in solution, serving as model proto-cells for studies relating to the origin of cellularity in the early evolution of life. The interactions and stability of PL vesicles and bilayers adsorbed at mineral surfaces has relevance for understanding the potential role of minerals in promoting or inhibiting the evolution of cellularity. Amphipihile-mineral interactions are also relevant for bacterial cell surfaces in contact with soil minerals. We have previously established the effects of oxide mineral surface chemistry and PL head-group chemistry on adsorption and self-assembly (Oleson and Sahai, 2008a). In the present work, adsorption isotherms apparently showed less DPPC adsorption at higher particle loadings for micron-sized particles, indicating that a greater fraction of the suspended solid settles out, so that the specific surface area available for adsorption is less than that used for normalizing the adsorption isotherms. This effect is most apparent at higher solution concentrations of DPPC. The effect of particle aggregation on supression of isotherms is even more pronounced when comparing micron- versus nanometer-sized particles, with much lower apparent adsorption on the nanometer sized particles. Particle size analyses by HRTEM and Dynamic Light Scattering suggest that the amorphous silica and anatase nanoparticles aggregate into secondary particles of size from ~ 100-1000s of nanometers. There is considerably less aggregation of the micrometer-sized quartz and rutile. Furthermore, the extent of nanoparticle aggregation depends on the solution pH and ionic strength, which affect surface charge and van der Waals forces, as expected rom DLVO theory. Aggregation of the nanoparticles reduces the available surface area for adsorption than the specific surface area value used for the isotherms. Our results have implications beyond the adsorption of PLs. For any surface studies involving particle suspensions, our data indicate the critical need to determine (a) the particular mass loading for micron-sized particles that minimizes settling, (b) secondary particle size of aggregated nanoparticles, and (c) to develop relationships between particle size, shape, and effective specific surface area.