(225b) The Influence of Surface Chemistry on the Electrostatic Properties of Particles

Electrostatic phenomena are frequently observed whenever powder comes in contact with different surfaces, particularly when the motion of particles is involved. The resulting charge accumulation and its potential prolonged retention become a serious concern when insulating materials, such as pharmaceutical powders, are involved in the process. Due to a large number of factors influencing the process of charge generation, quantification and prediction of electrostatic properties is still one of the greatest challenges in this field.

Aims

The aim of this work is to assess the influence of surface chemistry on the ability of a material to acquire and subsequently dissipate electrostatic charge. The main hypothesis is that surface functional groups can strongly dictate the overall charging behaviour of the material and therefore could provide some indication of materialsâ?? electrostatic properties.

Methods

The surface of glass beads, a model material in this study, was modified using a silanisation reaction to produce chemically functionalised samples. Contact angle measurements were used to confirm the extent of surface modification of the samples. Electrostatic properties were studied using a purpose build instrument, which incorporates a capacitive probe¹ within a Dynamic Vapour Sorption instrument. The modified glass beads were tribocharged by contact with a stainless steel surface and their maximum potential as well as decay curves were measured at 25 °C at three different humidity levels, i.e. 20% RH, 50% RH and 75% RH to evaluate the impact of environmental conditions on charge dissipation.

The influence of surface hydrophobicity on the electrostatic behaviour was further investigated for mannitol crystals. Mannitol was recrystallized from water at four supersaturation levels (Ï? = 3%, 13%, 23% and 33%) to produce needle-shaped crystals with varied aspect ratio and hence different overall hydrophobic character. Strong correlation between crystal shape and crystal surface chemistry for mannitol has been determined previously.² The electrostatic properties of the crystals were determined using the capacitive probe method and compared with industrially available instrument called Charge Decay Time Analyser.

Results

Successful surface modification of glass beads produced four samples with distinct surface chemistries, which exhibited differing degrees of surface hydrophobicity. Electrostatic measurements showed that the magnitude of charging followed the trend in hydrophobicity with the most hydrophobic particles acquiring the largest charge. Up to six times larger charge was accumulated by fluoro functionalised glass beads, acquiring charge of -11.2 ± 2.8 V/mg, compared to hydroxyl functionalised sample with overall charge of -1.9 ± 0.3 V/mg. In addition, the unique properties of heteroatoms, such as electronegativity, seem to have further impact on the overall charge adopted as observed by significant charging of the fluoro functionalised sample and charge inversion in case of amino functionalised surface. For all surfaces, increasing the external humidity resulted in faster charge dissipation; however, the actual effect of humidity on charge decay rate seems to be surface dependent. Under insulated charge conditions, increase in humidity from 20% RH to 75% RH resulted in lowering charge half-life time from 21.4 min to 4.9 min for hydrophilic sample and from 45.1 min to 13.3 min for amino functionalised surface.

Crystallisation of mannitol from solutions under varied conditions produced a range of crystals â?? from thin, long needle crystals at low supersaturation to short wide plate-like crystals at high supersaturation. These showed to have different surface energy properties and hence different electrostatic characteristics.

The data generated was repeatable and reproducible indicating that by controlling particle size, shape and chemistry the variation in electrostatic charge can be limited, but also shows that the capacitive probe setup used for electrostatic measurements is reliable and capable of detecting electrostatic charge of powders.

Conclusion

The results obtained show that surface chemistry and hence surface hydrophobicity play an important role in powder electrostatics. It was shown that materials having different chemical groups on the surface can exhibit different electrostatic behaviour. This includes not only the overall charge acquired under the same conditions, but also charge polarity and charge decay characteristics. Such observations could be beneficial in predicting a materialâ??s propensity towards charge accumulation during processing based on its chemical structure and molecular arrangement within a crystal lattice.

Acknowledgments

The authors acknowledge Dr Daryl Williams and Dr Jin Wang Kwek for their comments and useful discussions.

References

(1) Kwek, J. W.; Jeyabalasingam, M.; Ng, W. K.; Heng, J. Y. Y.; Tan, R. B. H., Comparative Study of the Triboelectric Charging Behavior of Powders Using a Nonintrusive Approach. Ind. Eng. Chem. Res. 2012, 51 (50), 16488-16494.

(2) Ho, R.; Wilson, D. A.; Heng, J. Y. Y., Crystal Habits and the Variation in Surface Energy Heterogeneity. Crystal Growth & Design 2009, 9 (11), 4907-4911.