ABSTRACT
RONALD R. BURNETTE / Universi ty of Wisconsin, Madison, Wisconsin
I . In t roduct ion 96
I I . Random Walk In te rp re ta t ion of Diffusion 96
III . Fick 's Firs t and Second Law 97
A. Free Energy In te rpre ta t ion of Diffusion 97 B . Diffusion as an Effective T ranspo r t Mechanism
over Small Dis tances 101 C. The Size of a Molecule and I t s Ability to
Diffuse 104 D. Solute Binding and I t s Effect on Diffusion 108 E. The Variable Diffusion Coefficient and the
Concept of a Mean Diffusion Coefficient 109 F . Fick 's Second Law in Three Dimensions and
I ts Coordinate Transformation 111
IV. Passive Diffusion Through a Membrane-The Part i t ion Coefficient 113
V. Passive Diffusion Through a Membrane-The Stagnant Diffusion Layer 119
VI. Application of FickTs Second Law to the Determination of the Non-Steady-Sta te Output Flux T h r o u g h t h e Skin 127
VII. Application of Fick fs Firs t Law to the Determination of D r u g Release from a Polymeric Matrix or Ointment 130
VIII. Diffusion with Simultaneous Reaction 135
IX. Additional Concerns in Diffusional Mass T ranspo r t 13 5 References 136
I. INTRODUCTION
Mass t r a n s p o r t can be due to e i ther random motion of molecules (diffusion) or bulk fluid motion (convect ion) . The emphasis in th is chapt e r will be on diffusional t r a n s p o r t , since in the systems to be descr ibed bulk flow effects can generally be neglec ted . The approach to be taken is to place diffusional p rocesses on a firm thermodynamic foundation, with a discussion of all assumptions employed. Numerical examples will be provided to fu r the r i l lus t ra te t he points being made. Some excellent t ex t s descr ib ing t he necessa ry thermodynamic background [1 ,2] and those dealing with t he general subject of mass t r a n s p o r t and diffusional p rocesses [3-61 a re available. The re a re also some excellent summaries and reviews [7-10] dealing specifically with mass t r a n s p o r t and how it pe r t a ins to pharmaceutical sys tems and biological membranes . It is hoped that th i s in t roduc to ry chapte r on mass t r a n s p o r t will provide an appreciat ion for the wide applicability of mass t r a n s p o r t analysis and will develop an intui t ive feel for diffusional p r o c e s s e s . In addi t ion, the references cited should provide a good s t a r t ing point for fu r the r s t u d y .
