An atomic force microscopy study of the effect of nanoscale contact geometry and surface chemistry on the adhesion of pharmaceutical particles.
OBJECTIVE
To understand differences in particle adhesion observed with increasing humidity between samples of salbutamol sulfate prepared by two different methods.


METHODS
Atomic force microscopy (AFM) force measurements were performed as a function of humidity (<10% to 65% RH) using two systems. The first system used clean AFM tips against compressed disks of micronized and solution enhanced dispersion by supercritical fluid (SEDS) salbutamol. The second system involved particles of both salbutamol samples mounted onto the apexes of AFM cantilevers, and force measurements being performed against a highly orientated pyrolytic graphite (HOPG) substrate. Following these measurements, the contact asperities of the tips were characterized.


RESULTS
The first system showed a maximum in the observed adhesion at 22% relative humidity (RH) for the SEDS salbutamol compared to 44% RH for the micronized salbutamol. The second system showed a mix of peaks and continual increases in adhesion with humidity. The predicted Johnson-Kendall-Roberts forces were calculated and divided by the actual forces in order to produce a ratio.


CONCLUSIONS
By relating the nature of the asperities to the force measurements, we propose a model in which adhesion scenarios range from single asperity nanometer-scale contact in which peaks in the adhesion were observed, to multiasperity contact where a continuous increase in adhesion was seen with humidity.
Hooton
Jennifer C
JC
Laboratory of Biophysics and Surface Analysis, School of Pharmacy, University of Nottingham, NG7 2RD, United Kingdom.
German
Caroline S
CS
Allen
Stephanie
S
Davies
Martyn C
MC
Roberts
Clive J
CJ
Tendler
Saul J B
SJ
Williams
Philip M
PM
D000268:Adhesiveness; D000420:Albuterol / Q000737:chemistry; D000465:Algorithms; D002626:Chemistry, Pharmaceutical / Q000379:methods; D025924:Chromatography, Supercritical Fluid / Q000379:methods; D017276:Friction; D006108:Graphite / Q000737:chemistry; D006813:Humidity; D018625:Microscopy, Atomic Force / Q000295:instrumentation* / Q000379:methods*; D036103:Nanotechnology / Q000379:methods; D010316:Particle Size; D011208:Powders / Q000737:chemistry; D012995:Solubility; D013678:Technology, Pharmaceutical / Q000379:methods
D011208:Powders; D006108:Graphite; D000420:Albuterol