Diffusion Across Interfaces

284 Pages·23.934 MB·English

by  Brooks, Lionel H

TIlESIS
presented to the
VICTORIA UNIVERSITY OF MANCHESTER
for the degree of
DOCTOR OF PHILOSOPHY
toy
Lionel H* Brooks, M.So* ^eoh.
October, 1951#
t*
ProQuest Number: 11009452
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SUMMARY,
of
DIFFUSION ACROSS INTERFACES.
THESIS
for the degree of
Doctor of. Philosophy.-
Manchester', University.-
October o 1951.
Lionel H. Brooks. M.Be,Tech.
SUMMARY,
The measurement of diffusion in solution has heen
discussed b rie fly in order to assess which method was most
suitable for the present work, which enquires into the diffusion
of fa tty acids across the w ater/o il in terface. The method
considered to be most suitable has been discussed in d e ta il
and its use in the present work described.
Using the Lamm scale method, the diffusion coeff­
ic ie n t, concentration immediately on each side of the interface
and mass of solute which had crossed the interface a t a given
time, have been measured on the system s:-
Water and hexane' as solvents with acetic and
butyric acids, separately, as solutes, a t 25°C.
Water and toluene as solvents with ac etic ,
propionic, butyric and v aleric acids, separately, as solutes,
a t 25°G.
Water and toluene as solvents with propionic
and butyric acids, separately, as solutes, at 40°C.
The acid, a t the beginning of the experiment, was in
the water layer, with no acid in the o il layer.
I t was found that conditions of re frac tiv e index,
density,so lu b ility and p a rtitio n co efficien t, severely lim it
the range of concentration over which the experiments may be
performed. The lim iting conditions make i t impossible to use
many organic compounds as solutes.
The work on diffusion across in terfaces performed
by other workers has been discussed in b rief.
The concentration of solute in each solvent immediately
at the interface has been compared with the concentration taken
from the p a rtitio n curve which was obtained by an independent
method.
A special type of c e ll was developed from the Lamm c e ll
in order to perform the experiments. A technique was also
devised for rendering the meniscus between aqueous solution and
toluene perfectly f la t. As far as experim ental accuracy would
allow the concentrations immediately on each side of the interface
are the p a rtitio n concentrations.
The mass of solute which had crossed the interface a t a
p articu lar time was compared with the mass of solute which had
flowed across the position of the o riginal boundary in a c e ll
containing aqueous solution of acid and w ater. The methods of
calculation of the required re s u lts , from experim ental data, have
been discussed in d e ta il.
An attem pt has been made to represent the conditions in
the c e ll, m athem atically, using three v ariab les, x, distance
from the in terfa ce, U, concentration and t, the time since the
interface was formed. I t was found th at a simple mathematical
solution could be found which represented conditions in the c e ll
only so long as the w ater/o il p a rtitio n ra tio was greater than
about 5* for solutes with a p a rtitio n ra tio less than th is,
boundary conditions for the mathematical solution have to be more
exacting, which makes the solution so complex that i t is only
discussed in b rie f.
The existence of a free energy b arrier to diffusion across
the interface has been discussed and compared with th at obtained
in a diffusion process with solute diffusing in one solvent only.
In order to obtain values for the magnitude of one of the b a rrie rs,
i t was necessary to perform experiments to measure the in te rfa c ia l
tension of aqueous acetic and butyric acid solutions and toluene,
a t in fin ite d ilu tio n .
The mathematical expression representing diffusion acros
an in te rfa c e , both solutions being s tirre d , has been reviewed and
some experiments of th is kind have been performed so that the rate
of diffusion of solute across the in terface, could be compared
with the Lamm scale experiments, which yield the rate of diffusion
when each layer is u n stirred . The concentration in the toluene
layer in the stirre d experiments was measured with an in te rfe r-
ome t e r .
Experiments have also been carried out to measure the
concentration of n-butyl acetate, benzoic acid and sa lic y lic acid,
respectively, a t the interface when each of these solutes was in
contact with water.
The effect of Increase in tem perature, from 25°C to 40°C,
has been investigated and at 40°G, the diffusion of fa tty 'a c id in
toluene was seen to have an anomalous effect which has been
inve stig ated .
Some new concepts in the diffusion process, put forward
by other workers, have been discussed in b rie f and th eir usefulness
when applied to the present work, discussed.
I am glad to take th is opportunity to expx’ess my boat
thanks to Dr* A.E’.H.Ward, under whoso d irectio n this
work was done, fo r hie valuable help and advice.
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Chapter. , . •' * • Page
Summary . . I
Introduction ... .IV
I Various'Physical .Properties., of the X
. Materials- used in the fcperimenta*
II I* .Density of the Solutions* * 4;
2 Refractive.- index Xtfeagureraenta.* ■ ' 5 ••
, *-
3. Us©;, of tho* Intei^o^omater.* • * '.X I•.
4 # Mea suremont of the; Bart i tIoh Ra11 ov; IS
5. B etem lnatlon o f:InterfuolaX tensions-.- SI
XXX . Work Performed by Various; Observers, f
sim ilar to th at • Performed In th©- P resent':
WO&, ;■ ' _ 39
IV ,.(l) -Discussion on tho-.Diffusion. Coeffielent 55
(S) . Brief Review of 'I$9thGdg,:.of' 'Observing
■ Diffusion. Quantitatively* I- .. 4G. .
(3) Choice of Method* ••'■.*■'- 44
(4) iixapry of th© larmn Beale. Method*. 55
(5) , lim itation of .the Method* 68
■.(6)'.Inyoht of Apparatus*. , I . /7IX-
V- Experimental' Procedure* " • • / -,87.
. Methods of Calculation*' : . 91
VI Ixpeflmehtal Be suits* . 1 0 0
VII Mathematical Representation of
■Diffusion across’ 'an Interface* 135
VIII .-■■'■ Comparison of - s tirre d ancD-Hon-stirred
• . D iffusion i&torosB. th© Interface* 145
... IX..'" Ihe Uatur© of the. Process of Diffusion* 176
X Xher mo dynamic a of-D iffusion across
' the -Interface* * ' 191-
XX D iffusion from an Interface# 215
'XII S ffeqt of. Incr©.as©^.ln\f©mp©ratur©* ’33.3 •
XIII ’Some Bow Concepts In,D iffusion. , - . ■ r 335'.
General Conclusions* ' 341.’
Suggestions fo r further-work 245
References " :'" • E44,
. ■ ■ fee' ma&swembnt of diffusion to solution has boon
discussed briefly tovovdo# to assesstwhich method was moot■ •■“
suitable.-for the present'workj.' which enquires Into the diffusion
of fatty. aeMs aoroaa ^tti©:-wator/oil .Interface* feo method . '
considered to to most suitable bag vb©©n-- discussed. to detail
and'Its -us© -la-the' present worktoloserlted* - ~
: ■ ' Using the .-.lamia' scale., method, • ikm 'diffusion apotf-
i a lent, concentration•-imodlatoly’oii each aide. of Wxo' Interface
and mm® of eblute which had' crossed1 the- 'totorf&c© - at & given1’
tooii'toasm?©cVon-;tli0(.syetom0S-. •
;’. ' • % . '■ Water arnt.ho^ane 'as. solvents with aeotle- and ‘
butyric acids, rogpobtiTOly,:;as:- solutes, • to- '- ■ ■
- V' .- ;. ‘ . ‘ 4fet6r:,and-:t0lubne as; solve mtawife add tie* -. ■
pmpiomioi butyric and‘‘Valeria acids, re^eotively, as-solutes,/
at 85 ®p* - ■ * -'■ ": ■ •
•' , Water an& toluene as salvento. 4vith’'pwplonio
and butyric acids, reepedtlvelf, .a© .solutes* at 40 S* ■ ' .
•fee aold,- at.;'tto’'tegtoutog of was inA-
tkm layer, with-mo acid:to th© oil layer* ...
/it was found th a t• conditions of TOfs?aotlw;Indes,.;-
density, "solubility .-bud ‘partition, coefficient,’ severely limit'
Wm wcmgo of conooiiteatiom .over wMdvtho experiments may
.'to
' . “ ■ • " .to v
performed* fe© limiting ‘ conditions mate It impossible to use
ffiany- orgaaio amBpoumte as solutes*-
■ - -fee work on diffusion across toterf&ceo -performed
by otoor,wortors''has;toem :dis0ttesod to brief* \, ' •