% WIlke-Chang equation to calculate the diffusivity for small
% concentrations of A in B.  An empirical expression for diffusion
% in liquids
%
% Argument List:
%   T      [=] temperature in units of Tdeg
%   VA     [=] molar volume of the solute in cm^3/g-mole
%   index  [=] associated number of solute liquid species as follows
%              1 - 'water'
%              2- 'methanol'
%              3- 'ethanol'
%              4- 'propanol'
%              5- 'other'  (other unassociated solvents)
%  
%   Note if you choose to use databank, you must enter in the
%   solvent first and the solute second
%  
%   Ex: 17.4-1
%      >> clear
%      >> start301 (specify new session, E&M balances, ceng301 data base, 
%        degC as temp, solvent compound:  benzene)
%      >> ldiff(15,5)
%   If you want to try to use the CENG301 data bank, type 1,
%   otherwise type 2: 2
%   What is the molar volume of the solute: 140
% 
%        ans =
%        1.3884e-05   [in cm^2/sec]

  global mw
  global lrho
  
A= input('If you want to try to use the CENG301 data bank, type 1, otherwise type 2: ');

if A==1
  VA=1/(lrho(2,2)/1000/mw(2));
else
  VA=input('What is the molar volume of the solute: ');
end;
  
  mu=liqmucalc(T,1);
  T=at(T);
  molwt=mw(1);
  if index==1
    psiB=2.26;
  elseif index==2
    psiB=1.9;
  elseif index==3
    psiB=1.5;
  elseif index==4
    psiB=1.2;
  elseif index==5
    psiB=1.0;
  end;
    
  DAB=(7.4E-8)*sqrt(psiB*molwt)*T/mu/(VA^.6);