Aspects of the physical
properties and the
visco-elastic features
of the sea water — oil
system*
S. S a e i d K
B
o g u m ił
h a l if a ,
L in d e , A
S t a n is ł a w P
ntoni
O C E A N O L O G IA , No. 32
pp. 1 9 -2 8 , 1992.
PL ISSN 0078-3234
Oil pollution
Visco-elastic properties
Sea water surface
o g o r z e l s k i,
Ś l iw iń s k i
Institute o f Experimental Physics,
University o f Gdansk,
Gdansk
Manuscript received O ctober 3, 1991, in final form May 15, 1992.
A b stra ct
The influence o f the physical properties o f commonly-used oils entering the marine
environment from various sources on the visco-elastic features o f sea-water samples
was examined under laboratory conditions. The physical properties o f the sea water
samples collected from the Baltic sea were determined as well. The visco-elastic
features o f sea water samples in the presence of oil films, the surface pressure - area
(7r — A ) isotherm during, compression and dilation and the surface pressure - time
— t) dependence were examined by means o f the Langmuir trough system. The
elasticity modulus e, the reversibility values R and the observed relaxation time r
were computed from the above dependences.
1. Introduction
T h e physical properties o f five oil substances (G asoline 94, Gasoline 86 ,
Diesel oil, Selectol plus engine oil and extra 15 oil) were determ ined in the la­
b ora tory (Pogorzelski, 1991). T he oil-sea w ater interfacial tension 7 0/w with
insoluble oil substances floating on sea w ater was measured as well. Inso­
luble oil substances spread over bulk sea water as a m onolayer, the excess
oil (particularly heavy oils) rem aining as lenses. T h e equilibrium thickness
<S2 o f these floating lenses (A d a m , 1941; A dam son, 1960; Pogorzelski et al.,
1986) m ay be expressed as
* The investigations were carried out as part o f the research programme C PB P 02.03,
co-ordinated by the Institute o f Oceanology o f the Polish Academ y o f Sciences.
s 2 = - 2S° ' w pw,
(1 )
9 P oP
where
So/w
g
pw
p0
~
-
spreading coefficient,
acceleration due to gravity,
density o f solvent (sea w ater),
density o f solute (oil substance).
T h e spreading coefficient S0/w o f oil films is given by (A d am son , 1960;
H oult, 1969; Pogorzelski et al., 1986)
^ 'o/w
^o f W)
(2 )
where
£u , £0 - are the free surface energies o f water and oil respectively,
£o/w ~ is the interfacial free energy at the oil-w ater interface.
Substances form ing m onolayers will usually spread until the available surface
is covered, the excess m aterial remaining at the surface as lenses or solid
particles. T h e pressure o f the spread m onolayer film (llarknis and B oyed,
1941) is given by
7T = lw ~ lo/wi
(3 )
where
7™
- the surface tension o f water,
lo/w ~ the surface tension in the presence o f m onolayer oil fdms.
In a Langm uir film balance a know n am ount o f m aterial can be spread
on a w ater surface area per adsorbed m olecule; follow ing this, the surface
pressure is m easured. Such a curve for constant tem perature is called an
isotherm . T h e com pressional and dilational conditions have a significant
influence (at the pressure-area ( tt A ) isotherm ) on the conform ation o f the
head-group (group o f oil m olecules) on a water surface (Griesser et al., 1987).
T h e transition occurring at a lower pressure corresponds to the change into
tw o dimensions from the liquid to the vapour ( L /V ) state, and it is wellknow n that this transition is a first-order one. A t higher pressures and
densities a second transition is usually described as representing the change
from a ‘ liq u id -e x p a n d e d ’ to a ‘ liq u id -con d en sed ’ state (L e /L c ). This
transition is regarded as a ‘ higher order ’ one (A d a m , 1941; Harknis, 1952).
T h e susceptibility o f the m onolayer to surface deform ation is expressed
in its visco-elastic features, i.e. the surface elasticity m odulus and surface
viscosity m odulus. T h e elasticity m odulus o f a spreading m onolayer film
is tim e-dependent only if relaxation within the m onolayer has taken place.
E lasticity only occurs as a result o f adsorption (at the interface with the
film ), and arises from the non-uniform ity o f the surface system ; it is given
by the surface stress and strain relationships at the com pression-dilation
surface area. It m ay b e com pu ted b y graphical derivation o f the adsorption
isotherm curve and expressed by (B eckm ann, 1958; Lucassen and Lucassen,
1969; G ilbert, 1971; A lpers and G arret, 1983)
E 0 = -A (d T r/ d A ).
(4)
In a com plex system , the m ost im portan t p rop erty o f the surface phase
is the surface free energy. D uring recom pression, the surface phase passes
through the same equilibrium states through which it went during expan­
sion, releasing precisely the same am ount o f heat at each stage. T h e work
o f com p ression /m ol o f adsorbed surface (Hiihnerfuss and A lpers, 1983) is
given by
W = 7T(d A )N A,
(4a)
where
dA - the change in area/m olecu le,
7T
- the surface pressure,
Na
- the A v o g a d ro num ber.
This w ork done can b e determ ined for the com pression - dilation surface
area relationship and described as the reversibility R :
R = (£ d ii/£ co m )1 0 0 % .
(4 b )
The relaxation tim e o f the physico-chem ical processes in m onolayers o b ­
served during com pression/d ilation deform ations is defined as the time
after which the surface pressure has dropped to 1 / e -(3 7 % ) o f the diffe­
rence betw een the initial and equilibrium surface pressures, (where e is the
Euler con stan t) recorded after a sufficiently long period (Hiihnerfuss and
A lpers, 1983; Hiihnerfuss and W alter, 1984). T h e tim e dependence o f the
surface pressure decrease allows the relaxation tim e to be determ ined from
the expression (Hiihnerfuss and A lpers, 1983; Hiihnerfuss and W alter, 1984)
7r(t) = 7T( t 0) ■ e a t,
(5)
where
a = 1 /r ,
t - the relaxation tim e.
2. Experimental conditions
T h e physical properties o f the five crude oils investigated and the collec­
ted sea water samples were determ ined in the laboratory at a room tem pe­
rature o f 293.0 K and an atm ospheric pressure o f 101.325 P a (Pogorzelski,
in press a ). T h e o il/s e a water surface and interfacial tensions were m easu­
red by the W ilhelm y detachm ent plate m eth od (B row n , 1969; Barger et
al., 1974; C h attora j and Birij, 1984; Brian et al., 1986). T h e viscosity was
m easured using a U N IPA N 504 A ultrasonic viscom eter. T h e weight o f the
oil substances was m easured in a density b ottle o f know n volum e in order
to determ ine the densities o f the selected oils (N elkon, 1978).
T h e Langm uir trough system (Jam es and Prichard, 1974) was used to
m easure the pressure vs. area isotherm and the surface pressure-tim e rela­
tionship o f the films (P ogorzelski, in press a, b ). T h e surface pressure 7r is
m easured as a function o f the trough area occu pied by the oil films spread
over the sea w ater samples. In practice, any change in surface pressure follo­
w ing area com pression was read o ff the deflection angle on the torsion-w ire
balance scale. Glass barriers were placed at b o th ends o f the trough when
the sea w ater samples and the resting point o f the torsion wire balance scale
were being calibrated. T h e oil derivatives were dissolved in hexane to m ake
volatile solutions and were allowed to evaporate for at least 10 minutes.
This tim e interval appeared to be sufficient for the oils to form a m onolayer
with a stable "film pressure. T h e solution was pipetted carefully on to the
sea water sample surface using an 8 m m (j> capillary tube. T h e m onolayer
oil films were com pressed by the glass barriers: they were m oved towards
the centre o f the glass trough; the m onolayer surface area was calculated for
each stage and the surface pressure measured simultaneously. T h e trough
was cleaned several times with a m ethanol - distilled water solvent. T he
pressures o f the various oil film areas in b oth com pression and dilation were
m easured directly by means o f barriers m oved towards and away from the
centre o f the trough. T h e reversibility R and the elasticity m odulus E 0
were m ost often m easured during (n — A ) isotherm variation for each oil
substance (Lucassen and Hansen, 1967). A similar procedure was used to
measure the decrease in surface pressure as a function o f tim e after steep,
rapid com pression o f the film using the barriers. T h e relaxation tim e was
com pu ted on the basis o f the tim e dependence o f the com pressed m onolayer
surface pressure decrease on the (w — t ) variation (Lucassen and Lucassen,
1969).
3. Results and discussion
T h e physical properties o f the crude oil substances and sea water samples
were determ ined. T h e oils are com m ercially available and are com m on
pollutants o f coastal zones. T h e la boratory results are presented in Table 1
(see also Pogorzelski, 1991).
T h e surface tension o f the sea water samples is very low at 34.54 m N · m - 1 .
This indicates that the concentration o f the surface-active m olecules exi­
sting naturally in the sea is to o high to reduce the surface tension. T he
surface pressure o f Selectol plus oil films with fairly long carbon chains o f
negative spreading coefficient gives rise to a 0.39 cm thick layer. T h e posi­
tive spreading coefficients o f the other oil films suggest that m onolayer films
T a b le 1. Physical properties and spreading coefficient o f the substances used in
the experiment at 293 K and 101.325 Pa
Oil
substances
(p)
Density
[ kg · m - 3 ]
Visco­
sity
(7 )
Surface
tension
[:m P as ]
[mN - m " 1]
(? )
Gasoline
94
Gasoline
86
760.02
0.67
23.65
761.31
0.59
20.59
Diesel
847.30
3.54
28.154
877.38
10.504
30.82
854.4
997.87
997.87
11.938
1.004
1.004
31.076
34.54
61.082
( T o / id )
{ S o/w)
Inter­
facial
tension
[mN · m -1 l]
Spread­
ing
coefficient
[mN · m - 1 ]
4.25
(3.178)**
2.66
(3.934)**
8.234
(* )
.
Equilibrium
thickness
[cm]
6.64
monolayer
11.29
monolayer
24.70
monolayer
-8.24
0.39
oil
Engine oil
(used oil)
Extra 15
Seawater*
Seawater**
11.96
(2.921)**
2.1248
_
_
27.89
_
_
monolayer
_
_
Note:
The values in brackets are computed from the seawater** samples collected from
Gdynia beach where the surface tension was 61.082 [m N -m - 1 ]. The seawater*
samples were collected from Navy Port in Gdynia.
o f these oils can form on the sea w ater surface. T h e surface pressure - area
isotherm s during com pression and dilation m easured in the laboratory are
summarized in Figure 1 (Pogorzelski, in press a).
T h e ( 7r — A ) isotherm relationships presented exhibited similar hystere­
sis behaviour. T h e elasticity m odulus was determ ined from (n — A ) curves.
T he surface pressure as a function o f the elasticity m odulus o f the crude oils
is presented in Figure 2.
T h e low surface pressures o f 3 -1 0 · 10-3 m N · m -1 are oceanographically
relevant to the surface pressures at the sea surface.
T h e pressure o f m onolayer oil films gives rise to intens h ydroph obic and
hydrophilic interactions with the adjacent sea water layer. T hese interac­
tions are related to losses o f surface energy as heat during expansion and
com pression and were evaluated from reversibility values. These show that
energy losses are m uch smaller for light oils than for heavy oils. T h e filmform ing oil substances can be classified on the basis o f reversibility in the
order: E xtra 15 oil (6 2 .2 3 % ), Diesel oil (7 1 .3 7 % ), Selectol plus oil (8 1 .8 1 % ),
Gasoline 86 (8 5 .0 9 % ) and Gasoline 94 (9 5 .5 7 % ). T h e relaxation tim e ( r ) ,
can be determ ined from the surface pressure vs. tim e dependence (F ig. 3
(Pogorzelski, in press b )).
Fig. 1. Surface pressure - area isotherms for oil substances spread ontef the surface
o f sea water samples (reproduced by permission o f Deep-Sea Research)
Fig. 2. Dilational elasticity modulus of the spread oil substance films versus surface
pressure. The solid line corresponds to the best-fit polynomial approximation
Fig. 3. Surface pressure - time dependence o f the spread monomolecular oil sub­
stance films (reproduced by permission o f Continental Shelf Research)
T h e relaxation tim e, which was in the 2 -1 0 min range, m ay have been
due to the presence o f unknow n chem ical com pounds in the oil substances.
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