title: Determination of the surface free energy of crystalline and amorphous lactose by atomic force microscopy adhesion measurement.
abstract: OBJECTIVE
  This study was conducted to accurately measure the dispersive surface free energy of lactose solids in ordered and disordered states.
  
  
  METHODS
  Atomic force microscopy (AFM) was used to determine the contact adhesion force between an AFM tip and lactose under low humidity (ca. 1% RH). The geometry of the tip contacting apex was characterized by scanning a porous aluminum film with ultrasharp spikes (radius 2-3 nm). A sphere vs. flat surface model was employed to relate the adhesion force determined to the surface energy based upon the Johnson-Kendal-Roberts theory. Spray-dried amorphous lactose in a compressed-disk form and single crystals of alpha-lactose monohydrate were prepared as model samples.
  
  
  RESULTS
  The condition of the smooth sample surface and sphere-shaped tip used was shown to be appropriate to the application of the JKR model. The surface energy of crystalline [(0,-1,-1) face] and amorphous lactose was determined to be 23.3 +/- 2.3 and 57.4 +/- 7.9 mJ m(-2), respectively.
  
  
  CONCLUSIONS
  We have demonstrated the capability of AFM to measure the dispersive surface free energy of pharmaceutical materials directly through a blank probe at the nanometer scale. These data, although consistent with results from more traditional methods, illustrate some unique attributes of this approach, namely, surface energies are directly derived from solid-solid interactions, measurements may be made on specific crystalline faces, and the potential exists to identify the submicron heterogeneity of organic solids in terms of their molecular energy states (such as ordered and disordered lactose).
authors.affiliation: Molecular Profiles Ltd., 8 Orchard Place, Nottingham Business Park, Nottingham, NG8 6PX, UK.
authors.forename: Jianxin
authors.initials: J
authors.lastname: Zhang
authors.forename: Stephen
authors.initials: S
authors.lastname: Ebbens
authors.forename: Xinyong
authors.initials: X
authors.lastname: Chen
authors.forename: Zheng
authors.initials: Z
authors.lastname: Jin
authors.forename: Shen
authors.initials: S
authors.lastname: Luk
authors.forename: Claire
authors.initials: C
authors.lastname: Madden
authors.forename: Nikin
authors.initials: N
authors.lastname: Patel
authors.forename: Clive J
authors.initials: CJ
authors.lastname: Roberts
mesh_terms: D000465:Algorithms; D003460:Crystallization; D007785:Lactose / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D011208:Powders; D013499:Surface Properties; D000267:Tissue Adhesions
chemical_list: D011208:Powders; D007785:Lactose