chemical physics of discharges - Argonne National Laboratory

INTRODUCTION
I' Attachment of polar molecules to ions in the gas phase occurs in a
mber of systems: gases under high energy irradiation, gas discharges,
IlameS, etc. The polar molecules may be present as an accidental imkrity
or as a deliberate addmixture. The attachment has a number of
$Portant effects such as change of mobility, change of rektivity with
2gard to ion-molecule reactions, change of the positive (negative) ion
tlectron) recombination coefficient and change in the nature of the
-0ducts resulting from the positive negative ion recombination. The
+dy of ion-polar molecule interactions in the gas phase can also pro-
:de information on ionic solvation since the formation of ion clusters
2nstitutes the first and most important step in the solvation of anion.
; A systematic mass spectrometric study of ion-polar molecule inter-
:tiOnS was started in this laboratory some five years ago.8-12
The mass spectrometric gas phase studies are based on measurement
! the relative concentrations of the clustered ionic species: A+.nS,
+.(n+l)S, etc. The measurement of the relative concentrations is obfined
by bleeding a probe of the gas into an ion mass analysis system,
.,e. a vacuum chamber attached to a mass spectrometer. In the vacuum
hamber .> the gas is pumped out while the ions are captured by electric
ields, accelerated and focused and then mass analysed by some convenional
means (magnetic separation quadrupole filter, etc.). After mass
qalysis, the ion beam intensities are detected as electrical currents.
Several types of solvation studies can be undertaken if the relaive
concentrations of the ionic species are known.
1. Solvation Enthalpies and Entropies of Individual Solvent Mole-
ule Additions Steps: Consider the ion A+ produced in the gas phase by
3me form of ionizing radiation or thermal means. If the atmosphere
urrounding the ion contains the vapor of a polar molecule (solvent S),
number of clustering reactions will occur.
i
A+ + S + A+.S (0,1)
I A+.S + S + A+.2S (1,2)
,~
~ where
+
A+.(n-l)S + S + A+.nS (n-l,n)
At equilibrium the following relations will hold
AFo = AFo + AFo 1,2 ... + AFon-l,n
O,n 0,1
(1)
AFon-l , n
= -RT In
'A+. nS
'A+. (n-1) s-'s
= -RT In Kn-l,n (11)
P, is the partial pressure of X.
Thus knowledge of the equilibrium concentrations of the clustered
pecies A+.nS obtained from experiments at different pressures of s w ill
llow the determination Of Xn-1 n and AFn-l,n. Such measurements done
k different temperatures will lead to the evaluation of AHn-l
and
Sn-lIn. The availability of such detailed information will, #or In-
,tance, reveal the shell structure since a discontinuous change of the
Hn-1 n and ASon-l,n values will occur whenever a shell is completed.
,inally, the total heat of solvation of the ion can also be obtained
rom the expression 111, with equations of the same form holding for the
/ m