Recent work on spread monolayers, adsorption and desorption

JOURNAL OF COLLOID SCIENCE 11, 398-418 (1956)

RECENT WORK ON SPREAD MONOLAYERS, ADSORPTION AND DESORPTION L. Ter Minassian-Saraga Laboratoire de Chimie Physique, Facult~ des Sciences, Paris Received April 15, 1956 INTRODUCTION

The work carried out at our laboratory during the last ten years 1 on the properties of spread or adsorbed monolayers can be divided into three categories: I. The behavior of protein films spread on aqueous substrates at various pH and salt concentrations. II. The study of the surface potential of very dilute ("gaseous") spread or adsorbed films and of heterogeneous films, for substances showing a two-dimensional phase change. III. The study of isotherms, of the laws and mechanism of desorption for a slightly soluble spread monolayer; the study of the validity of Gibbs equation. This work is a continuation of the research on spread monolayers carried out by J. Guastalla with the two surface balances or "manometers" devised by this author.

Surface Manometer and Micromanometer These apparatuses (Figs. 1 and 2) are of the Langmuir "differential" type and they are direct-reading. A float Separates two parts of the surface of the substrate. The film is spread on one of them (left compartment on the figures). The pressure exerted by the film on the float gives rise to a restoring force which is measured by the displacement of a spot on a scale. In the case of the surface manometer (3) (Fig. 1), the float is made of paraffin-coated mica and is attached to the frame by two vaseline-coated silk threads. The float displaced by the film causes the deviation of a "pendulum-like element" from its equilibrium position; the deviation of this element (magnified by an optical device) is proportional to the hori1 The s t u d y of spread monolayers is an old t r a d i t i o n at t h e L a b o r a t o i r e de Chimie P h y s i q u e de la Facult~ des Sciences, Paris ; A. Marcelin h a d u n d e r t a k e n this research as early as 1913. 398

SPREAD MONOLAYERS~ ADSORPTION~ AND DESORPTION

399

FIG. 1. Surface m a n o m e t e r .

cA_

/ FIG. 2. Surface m i c r o m a n o m e t e r .

zontal restoring force. The weight of the pendulum-like element determines the sensitivity of the apparatus, which can attain 0.02 dyne/cm. In the case of the surface micromanometer (2) (Fig. 2) the float is substituted by a vaseline-coated silk thread floating at the surface of the substrate and kept under a known tension. The thread takes a curved shape owing to the film pressure, and the displacement of the mid-point of the thread is proportional to the pressure. The tension of the silk thread determines the sensitivity of the apparatus, which can attain 0.001 dyne/cm. J. Guastalla (2-7) has verified the following results (already predicted by N. K. Adam (1)) from a study of very dilute films of several fatty substances and proteins; at extreme film dilutions the equation of state for monolayers becomes analogous to the three-dimensional Boyle-Mariotte law: p¢ = kT,

[1]

400

L. TER MINASSIAN-SARAGA

where p -- surface pressure in dynes/cm.; a = surface area in A2/molecule, k = Boltzmann constant; T = absolute temperature. This result embodies the fundamental principle of the method suggested by J. Guastalla for the determination of the molecular weights of spread proteins (4-6). I. STUDY OF THE BEHAVIOR OF PROTEIN FILMS SPREAD ON THE SURFACE OF AQUEOUS SUBSTRATES AT VARIOUS PH~S OR SALT CONCENTRATIONS (8, 9)

Certain molecular properties, such as size and weight, of proteins spread on substrates at various pH's or salt concentrations, can be studied by means of surface manometers. N. Benhamou has investigated several proteins, for example, oxyhemoglobin. She has shown that the pH ranges of stability for the spread protein and for the protein in solution are about the same (note that in the first case the pH of the substrate has to be considered). J. Guastalla's method (4) was used for molecular weight determinations. The method subsequently suggested by Bull (10, 12) is analogous. P

rnillidynes,/cm 20'

10

C |

,

0.05 0.01

%

rng / m 2 extrapolatJon

0.005 O.

1~" " " N" 7

0.05

0,1

~ " .-... ~."

~b

r

c mg/m 2

Fie. 3. Compression results for very dilute films of oxyhemoglobin. Substrate pt:I 3.1.

SPREAD MONOLAYERS, ADSORPTION, AND DESORPTION

40]

The compression curves of the very dilute spread film were studied. The corresponding isotherms can usually be represented by the equation: p(S -

b) -

RT M'

where S --- area/g, of protein, b -- cosufface, M = molecular weight, and R = gas constant. This can be rearranged to give: (1 -

p,

b e ) =-

where C --- concentration in g./unit area. If C / p is plotted as a function of C (Fig. 3), a straight line is obtained which, when extrapolated to C = 0, permits the calculation of the molecular weight and, when extrapolated to C / p = O, leads to the "cosurface" concentration (l/b). Figures 4 and 5 show, for example, the behavior of oxyhemoglobin molecules spread on the surface of substrates having a pH < 8. The molecules of this protein dissociate at about pH 6. Between pH 4 and 5 stable units having a molecular weight of about 35,000 are found. When substrates of lower pH are used, a gradual splitting occurs which leads to smaller units (probably globin of molecular weight equal to 11,000 at pH = 2). The very compressed protein films show a "collapse point" ( 1 i , 13); N. Benhamou has pointed out that the variation of the molecular area and of the average film thickness at the collapse point can lead to an interpretation of the mechanism of the splitting which she observed while making molecular weight determinations in very dilute films. MX IO'L) 1 oo 0

80

•

.

.

.

.

.

.

2

i/

60

40

20

IT

I• 2

pH 3

4

5

6

7

8

FIG. 4. Molecular weight of oxyhemoglobinas a function of substrate pH.

402

L. TER MINASSIAN-SARAGA b ,

'/m9

10 O 0

5

O

O

', iok I

I

1 2 3 4 5 6 FIG. 5. Variation of "cosurface" as a function of substrate pH (oxyhemoglobin).

TABLE I Results for Oxyhemoglobin Pc collapse,

Substrate

pH

KH2P04 ~- K~HPO~) 0.04 molar 6.9 K Biphthalate 0.025 molar and K0H 4.5 HC1 0.01 molar 2

essure~ (~es/cm.) Cc (mg./,n.~) 70,000 22 3.4 35,000 22 1.8 11,000 20-21 2 Mol. wt.

G

(~.)

(,t.)s

26 14 15

3,400 3,200 900

The molecular area zc is equal to M/Co N, where C° is the film concentration at the collapse point and N is Avogadro's constant. If the density of the protein in the film is equal to the density d of the crystal, the film mean thickness e is given by the relation: e = 10 Cc/d in A. The e and ¢~ values calculated by N. Benhamou (9) are shown in Table I. At the first splitting the molecule breaks up horizontally so t h a t each resultant molecule has half the height of the original molecule (¢c of the resulting molecule is substantially equal to ¢~ for oxyhemoglobin at the isoelectric point); when the final splitting occurs the molecules could be broken u p vertically, the thickness of the film remaining unchanged (14-15 A.). II. SURFACE lPOTENTIAL MEASUREMENTS (20--23) The surface potentials are determined by the well-known method of the ionizing electrode (16). The device used (23) is very accurate and permits, in particular, the determination of very low surface potential values of monolayers, constituted by f a t t y acids (myristic acid and lauric acid), in the state called "gaseous." Simultaneously, the surface pressures of the same monolayers have been studied using surface manometers. Figure 6 shows the plot of the surface potential AV against the mono-

SPREAD MONOLAYERS, ADSORPTION~ AND DESORPTION

mV

substr'ates H CI ~ ' "

400

3OO2oo oo - 50

Ax03

2.101'*

/ff

N/IO0

N/l.ooo N/SO.OOO

N/,OO.OOO

4.10I¢

FIG. 6. Surface potential (millivolts) of myristie acid with substrates of ttC1 solution. layer concentration a for the whole range of concentrations of stable myristic acid films spread on substrates at different pH values. The surface potential of fatty acid monolayers in the "gaseous state" adsorbed or spread on the surface either of distilled water (19-21) or of very dilute hydrochloric acid (concentration lower than 10-~ N) is negative; when the acidity increases, the concentration range of films showing a negative potential diminishes and when the pH of the substrate becomes lower than the value 4, the values of the negative potentials are of the same order of magnitude as the experimental error. The analytical form of the curves of surface potential has been predicted by J. Guastalla (17). This relation can be written as follows: V = 1 2 ~ r m ( a - K ~ 3/2) where m is the apparent vertical dipole moment of the carboxyl group. The value of m may be calculated from the slope of the tangent at the origin to the curves of Fig. 7. When the substrate contains very little acid (less than 2 X 10-SM HCI), the value m = 1.7-1.8 Debye is obtained at any pH of the substrate. Several authors (14, 15, 18) have observed that measurements of AV are very difficult in the "transition" region when the mean concentration of the film is intermediate between the "evaporation" point and the "condensation" point. ft. Michel (23) has shown that the results may depend on the measuring technique adopted. Thus a small electrode placed near the surface registers values of AV which diminish as a function of time and tend to the value obtained for the monolayer in the so-called "saturated vapor" state, whatever the mean concentration of the film may be (Fig. 8, curve I).

404

L. T E R M I N A S S I A N - S A R A G A

substrate concentratio n ~ 0.01 L. 0.001

AV (. mV )" ." ~r

T

-~

O ~.~ • ~ t ~

~

~ ~-------'~

" U

o.ooo,

10,.101, d_CmolA~ _ COCO04

~

0.00003

-10 -'~.

~_ ~

~ ' ~ . ~ . ~ -20

0.00002

0.0000125

~...<~:'----e- o.ooool

water3 D

mV

FIG. 7. Surface potential of myristic acid. Substrates: HC1 solutions.

m V small electrode close to the surface o electrode as large as the monolayer a r e a

300

200

/

/

/

~o/ o//° ozQ/'° II

I00

io /

Y

7" ,

0

] I

1xl0TM

i

2xl 014

&"m°leC,/cm2 ,

3x1014

FIG. 8. Effect of electrode type on surface potential of heterogeneous films of myristic acid. Substrate: 10-~N HC]. However, if the measurements are carried out quickly with an electrode placed away from the surface and having a surface area as large as t h a t of the film, the AV values obtained for heterogeneous monolayers v a r y as a continuous function of the mean monolayer concentration (Fig. 8, curve I I ) .

SPREAD MONOLAYERS, ADSORPTION, AND DESORPTION

zJ:05

After having shown that this phenomenon was independent of the presence of the Polonium source and was caused only by the electrode, J. Michel has assumed that the electrode itself (the grating) has driven away the molecular aggregates (portions of the condensed phase), and that the film in the state called "saturated vapor" is the only one able to maintain itself in the presence of the electrode. Finally J. Michel has shown that the surface potential curves of adsorbed films have the same characteristics as the potential of spread films. J. Michel (21), like several other authors (24), has measured the variation of surface potential of a solution as a function of time, in order to study the kinetics of adsorption during the first few seconds after the formation of a fresh surface. I I I , STUDY OF THE SPREAD MONOLAYER, OF ADSORPTION AND DESORPTION

All experiments were carried out on the same substance (lauric acid), the liquid substrate or solvent consisting of a 10-2 M hydrochloric acid solution.

1. Spreading and Adsorption of Lauric Acid The study of the isotherm of the spread monolayer, initiated by N. K. Adam (25), was taken up again using the surface manometers of J. Guastalla (see Introduction). Since the film is slightly soluble, its surface pressure p tends to diminish as a function of time. The experiment should be performed "point by point," i.e., for each film concentration the substance is spread, the film thus obtained is compressed immediately, the pressure is plotted as a function of time, and the resulting curve is extrapolated to zero time. The

30 20

~P dynes/cm 11 IO 9

10

s 4

3

3 2

2

0~-5

10 15 30

60

T20 W50 180

tsec

FIG. 9. V a r i a t i o n of surface pressure as a f u n c t i o n of time. Lauric acid. S u b s t r a t e : 10-~N HC1; 20°C. The curves 2-11 correspond to original molecular areas equal to: 49.7, 40.2, 37.1, 35.5, 34, 32, 31.1, 28.6, 27.1, 22.6 A. ~.

406

L. TER MINASSIAN-SARAGA

desorption studies (see below) have led to a method of extrapolation according to which log p is plotted as a function of the square root of time •(~v/t), (Fig. 9). The P0 values thus obtained are then plotted as a function of the area (0) of the molecules making up the original spread monolayer, and the isotherm is thus established (Figs. 10 and 11). The equation of

1.0 E0.8 ol

o

.o.6 O.

0.4 0.2

2000

4000 6000

FIG. 10. Laurie acid in the gaseous state. Substrate: 10-2N HC1. [] Adam (14-16°C.). • present study (19°C~).

30'

P dynes,/crn

20

\.

0

i

,

20

30

,

40

|

$0 6-A2/rnol~

FIG. 11. Laurie acid at 20°C. Substrate: 10-2N 110]. I: extrapolation of p-t plot; I I extrapolation of log p - %/'{ plot; I1][ 11arldns (31); O Adam (25).

SPREAD IVIONOLAYERS~ ADSORPTION~ AND DESORPTION

P dyn es,~c m

30

20

I

Jl

~//"

1o

0

407

_.--" 0

.

,

IOx 10~z+

20×1014

, 30"101'*

c~ (.molec/cm~,) ~ 40x101"

FIG. 12. A d s o r p t i o n curves for laurie acid. S o l v e n t : 10-2N HC1; 21°C. 4- 1 (46); I : experimental; I I : calculated.

state for a film of laurie acid in the state called "expanded" has the analytical form suggested by J. Guastalla (7), as follows. p = ~Ta + Ba 3/2 + Aa 5/2,

[2]

where A and B are constants. The experimental study of the adsorption, i.e., the measuring of the surface tension lowering p -- Ay at equilibrium for different laurie acid solutions (28, 40), leads to the adsorption curve (p is plotted against the concentration of the solution c~) (Fig. 12).2 Let us consider now the verification of the Gibbs equation which can be written: dp = ~kTd In c~,

[3]

where a -- 1/z is equal to the surface concentration of the monolayer in molecules/cm?. The verification of the Gibbs equation was attempted by two different methods: 1, The value of a is obtained by graphical differentiation (25, 40) of the adsorption curve. The variation of z as a function of p is shown on Fig. 13, curve I, which is called the calculated isotherm of the adsorbed monolayers. 2 A similar s t u d y was carried out for several cationic detergents, which belonged to a homologous series a n d were strong electrolytes. I n p a r t i c u l a r , experiments were carried o u t in the presence of m i n e r a l salt a t different c o n c e n t r a t i o n s (49, 50).

408

L. TER MINASSIAN-SARAGA p dyn es//c m

30

20

10

--

0

..,

20

...

,

30

,

40

,

motet

~,

SO

FIG. 13. Isotherms of laurie acid adsorbed film (I) and spread film (II). Let us suppose with Langmuir (34) that the spread and the adsorbed monolayers are identical. In order to verify the Gibbs equation, the calculated isotherm of the adsorbed monolayer is compared with the isotherm of the spread monolayer. The agreement between the two curves was found to be satisfactory, in comparison with the results of other authors (36). 2. M t e r elimination of p between Eqs. [2] and [3] the relation [3] is integrated to give (40, 46): log c~ = 1.105 X 10-2153/2-6.15 X 10-751/2 + log ~ + log A,

[4]

where A is the integration constant. Finally after elimination of ~ between Eqs. [2] and [4] p is obtained as a function of c~/A; this last function is the calculated adsorption curve, provided the integration constant is evaluated; this can be done with the help of one point taken from the experimental curve; it has thus become possible to compare the calculated adsorption curve with the experimental one. The agreement is satisfactory, as can be seen from Fig. 12. The two series of measurements (adsorption curve and isotherm) allow the calculation of the ratio Kd = co/~ corresponding to a given surface pressure and concentration ca ; this ratio is called the desorption coefficient; its variation as a function of p (46) is shown in Fig. 14.

2. Desorption of Spread Monolayers of Lauric Acid (30, 42, 43, 47) This phenomenon was studied in two different ways, either by keeping the pressure of the film constant, in which case the area of the monolayer

SPREAD MONOLAYERS, ADSORPTION, AND DESORPTION

409

decreases, or by keeping the area constant, in which case the pressure of the film decreases as a function of time. These two kinds of experiments were carried out at constant temperature with the help of a temperature-controlling device which was originated in this laboratory (45). (A) Desorption of a Monolayer Kept under Constant Pressure. The desorption experiments under constant pressure were carried out with the help of a surface barostat (30) (Fig. 15), a modified surface manometer with a pendulum-like element on which a platinum wire is fixed. The wire forms part of an electrical relay circuit which sets a motor going whenever the pendulum-like element is displaced because of a slight decrease of the surface pressure (0.05 dyne/cm.). The motor moves the piston which compresses the film until the initial pressure is obtained. During a desorption experiment, the displacement of the piston permits the measurement of the area S of the film as a function of time. The results of a typical experiment are represented by plotting the logarithm of S as a function of t (Fig. 16). The plot thus obtained, called the desorption curve, has two parts: a curved part at the beginning and a linear portion for higher values of t. Kd

cm J

10

p clynes,,/cm 0 10 20 30 FIG. 14. I)esorption ratio (K~ = c,,/~) for ]auric acid dissolved in 10-~N HC] at 20°C.

FIG. 15. Surface barostat.

410

L. T E R MINASSIAN-SARAG2~

2.4

t log ,5 p ( dynes~ rn

2.3 2.2 1

2.1 2.0

.

,

0

,

1200

,

t sec]=

24O0

FIG. 16. Desorption curves for lauric acid at 19.5 =t=O.5°C.Substrate: 10-2N HCI.

5x10-4

20xi0-s

4x10-4 T

T

3x! 0-v

I

I0xi0-s

• ~, k v

,,

kp

i

2,,10_~ o_

v

l i

xlO-4

T

0

10 20 P dynes/cm FIG. 17. Influence of pressure on desorption of lauric acid monolayers maintained at constant pressure with a substrate 10-~N HC1; 19.5 =t= 0.5°C. The results obtained for a substrate at given p H and temperature are reproducible, provided t h a t the depth of the liquid substrate is always the same. This condition is in agreement with the mechanism of desorption. T h e steepness of the desorption curve increases with pressure, but the lapse of time corresponding to the first portion of the desorption curve seems to be independent of the pressure (15-20 minutes) (Fig. 16). Physical meaning of the desorption curves, a Let N be the n u m b e r of molecules of the monolayer. I t s area is then equal to S: S = ~N. 3 T h e r e f e r e n c e (47) c o n t a i n s a m o r e d e t a i l e d d i s c u s s i o n of d e s o r p t i o n .

[5]

SPREAD MONOLAYERS, ADSORPTION~ AND DESORPTION

411

The slope K ( t ) o f the tangent to the desorption curve at any point is called the rate of desorption. It can be defined by the following relationship:

K(t) -

dS S dt -

(r dN S dt - z~(t),

[6]

where (~(t) = :-(1/S)(dN/dt); this last term is by definition the number of molecules lost per unit time and per unit surface, i.e., the rate of diffusion at the surface. The integral of K(t) between zero and t gives:

fot K(t) dt = In ~S ° 1=[ ~ o t ~(t) dt,

[7]

which represents the desorption curve. The quantity ln(So/S) stands for the number of molecules that have crossed a unit surface in time t. Desorption experiments at constant pressure and constant molecular area permit the calculation of the diffusion rate by means of relation [6]. The particular form of the desorption curve indicates that the rate of diffusion varies as a function of time at the beginning of an experiment; therefore the desorption rate is also variable. After a certain lapse of time, the rate of desorption becomes stationary, independent of time, and is indicated by the linear form of the desorption curve. Mechanism of desorption. At the beginning of a desorption experiment, just after spreading, the film is in contact with the pure substrate. The molecules of the film may acquire the activation energy corresponding to a possible potential barrier and thus may become able to penetrate the substrate and mix with the adjacent molecules of this phase. This process (a) is called dissolution. By a consecutive process (b), the dissolved molecules may diffuse away from the surface into the liquid substrate. The rate of desorption is controlled by the slower rate of the two processes (a) and (b). We have adopted the following working hypothesis: Because of the rapidity with which dissolution occurs, the desorption is diffusion-controlled; the dissolved molecules may then accumulate close to the surface, and from the beginning of an experiment an equilibrium could be established between the film and an infinitely thin region of solution immediately below the surface. This equilibrium is supposed to be identical to the adsorption equilibrium between a solution of concentration ca and an adsorption film of the same p and ~ as those of the spread desorbing film (the identity of the two kinds of films has been assumed previously (34)). The diffusion occurring after dissolution would proceed into the substrate away from the region of concentration ca. Kinetics of desorption. Stationary state of desorption (linear part of adsorption curve (Fig. 16)). According to the above-mentioned working hypothesis, the rate of desorption can be constant only if the diffusion

412

L. TER M1NASSIA2~-SARAGA

proceeds through a calm liquid layer, called "diffusion layer" of mean thickness e, in which a constant concentration gradient exists. We assume therefore t h a t (Fig. 18) a convection current could exist beyond the depth e; these currents are thought to carry a w a y the desorbed molecules and to maintain in this p a r t of the substrate an infinitely small solute concentration, since the substrate volume is very large. This hypothesis has led (47) to a law for the variation of the film area as a function of time: lnS=

caD -~-e t = c t e -

cte -

D K~--~ t,

[8]

where D = diffusion constant of the molecules constituting the desorbing monolayer. The hypothesis was verified in a simple direct way. The trough containing the experimental substrate was heated b y a weak lamp. The convection current intensity was thus increased without producing any substantial change in the temperature of the substrate.

x :o

×__.~

..........

.......,y. o0o 0~ooo : °°B ~° o Q

~C o

FIG. 18. Representation of diffusion layer and convection eurrents. log ,S

2.3 2,2 2.1 2.0 1.9

0

I'

1200

2400

sec f L

3600 4BOO 6000

FIG. 19. Influence of liquid movement on desorption of laurie acid; p = 6 dynes/cm.; 19.5 ± 0.5°C.

SPREAD MONOLAYERS, ADSORPTION, AND DESORPTION

2,4'

413

log 3

2.3~ 2,2

P{'dynes~m )

2.1.

~ ' ~ ~ _

" "

1

9

•0 20 40 FIG. 20. Desorption at constant pressure for laurie acid. Variable desorption. Substrate: 10-~N HC1; 19.5 =t=0.5°C. Since e decreases with the increase in the intensity of the convection currents, the desorption rate should increase according to relation [8]; this increase was in fact observed (Fig. 19).

Variable rate of desorption (parabolic portion of the desorption curve (Fig. 16)). During the time interval corresponding to the variable rate of desorption, the molecules which have desorbed immediately after spreading of the film will not yet have reached t h e depth e; the concentration gradient at the surface will vary as a function of time during the same interval. After these first desorbed molecules have traveled more than the distance the concentration gradient will become constant and the rate of desorption will also become constant. The theoretical law (47) giving the variation of the film area as a function of time will, under these conditions, take the following form:

cod

In S = in So - 2 y

~

= In S o - 2K~

d

~¢~.

[9]

According to this last relation In S is a linear function of ~v/t- (see Fig. 20). Influence of the film pressure. The rate of desorption increases with the film pressure during the interval studied (2-20 dynes/cm.) (Fig. 17). The desorption intensity is defined either by k~ (slope of the tangent at the origin of the curves represented in Fig. 20) for the variable rate of desorption or by kp (slope of the linear portion, Fig. 16) for the constant rate of desorption. According to relations [8] and [9]:

[10]

k~

K~ D

414

L. TER MINASSIAN-SARAGA

where Kd is a function of the film pressure only (D and e being constant). The relations [10] could be experimentally verified, since the rates k~ and k~ are proportional to Kd (Figs. 21 and 22). The slope of the straight line shown on Fig. 21 is proportional to D. The value of D = 7.4 × 10-G cm?/ sec. was obtained at 19.50C. This result is satisfactory in view of the results obtained by other methods (see Table II). The value of e can now be calculated from the slope of the line representing k~ as a function of Kd. It is found to be 1.2 ram. Several authors have assumed the existence of a diffusion layer in the case of heterogeneous physical processes (27, 29, 35, 41). The values calculated for e by these authors are generally lower, since xv x10"3,sec I ~

20

I0

1

o

!

5

K~I ¢m'1

!0

FIo. 21. Variation of the desorption Fate as a function of desorption ratio for laurie acid at 19.5°C. Variable state of desorption.

K;1Q'4~-I &O

/

'

25 ~

Kdcm-1

0

;

I'0

"

FIG. 22. Variation of the desorption rate as a function of the desorption ratio for laurie acid at 19.5°C. Stationary rate of desorption.

SPREAD MONOLAYERS~ ADSORPTION, AND DESORPTIO~

415

the experiments were carried out in such a manner that the liquid phase was strongly stirred. When the stirring of the liquid was caused by convection currents (29) only, the calculated e value (0.5 mm.) is of the same order of magnitude as the one found by the present method.

Experimental evidence for the existence of a diffusion layer and of convection currents. According to Benard (26) the convection currents are made visible by the motion of aluminum particles in a water suspension. A suspension of this type was placed in an optical cell, which was illuminated along a vertical direction and examined along a horizontal plane. When a monolayer of myristic acid is spread at the surface of the aqueous suspension, the aluminum particles leave the surface region and migrate beyond a depth e = 0.5 ram. (Fig. 23). According to Benard, the interpretation of this experiment is the following: the convection currents of the substrate stop at a depth of 0.5 ram., which corresponds approximately to the thickness of the diffusion layer determined by means of desorption measurements (e = 1.2 ram.). Moroever, this experiment confirms the views of Landt and Volmer (33) and of Merigoux (38), who assumed that owing to the friction which may exist between a film and the liquid substrate, there will be no movement near the surface of the liquid. The layer e disappears when the suspension is stirred but reappears again after a few minutes and will then persist for several hours. Desorption of myristic acid (47). It was found that myristic acid monolayers, which are considered to be stable, desorb very slowly; the desorption curves (Fig. 24) for these films are of the same form as those found for laurie acid. It was therefore concluded that: TABLE II

Values of Diffusion Constants by Various M~thods Authors D X 10s cm2/sec, t(°C.) Substances M e B a i n et al. (37) 8.1 25 K Laurate Lamm et al. (32) 7.5 20 Na Laurate T h i s work Cale. (Einstein relation) (30)

7.4 5.8

19 19

Laurie acid Laurie acid

surface oF theliquid calm layer. (F..= O,S ram) suspension FIG. 23. Apparatus for examining diffusion layer.

416

L. TER MINASSIAN-SARAGA 0

2.1

2

.

20

0

5

~

40

60

4 800

7200

_

2.0 0

2400

FzG. 24. Desorption at constant pressure for myristic acid. Substrate : 10-2N HC1 ~, 20°C. P dynes/c m 50

0

0

,

,

,

250

500

7,50

i' s e ¢

i 000

FIG. 25. Desorption at constant area for lauric acid; • ealeulated; - - experimental. The-desorption of myristic acid m a y also be diffusion-controlled, the diffusion proceeding inside a diffusion layer. The concentration of the region of substrate immediately below the film m a y then have the equilibrium value ca. (B) Desorption at Constant Area (44, 47). This study was carried out b y means of the surface manometer, for lauric acid spread on 10-2N tiC1. The film is spread and then compressed to a given area. The decrease of pressure as a function of time (Fig. 25) is recorded. I n this case ca, the concentration in the liquid layer immediately below the film, is also variable. I t was shown that, provided these variations are very small during an experiment, the r a t e of diffusion at the surface can be written:

d~ dt

ca(t)%/D %,/~

1 dca Kd dt

SPREAD MONOLAYERS, ADSORPTION, AND DESORPTION

417

This equation, when integrated and transformed, leads to the relation - l n ca = C%ft- or Co

in ~ p0

-C%¢/t -,

[11]

which forms the basis of the extrapolation method shown in Fig. 9. CONCLUSION

The study of the behavior of spread oxyhemoglobin has shown that in definite substrate conditions the molecular weight of this spread substance (as for other proteins) may be close to that of the native protein in solution. The measuring of surface potential for spread heterogeneous films of myristic acid has shown that the electrode interferes with the film below. It was found that the surface potentials of very dilute films of fatty acids spread on distilled water are negative and tend to a negligible value when the substrate pH decreases. The study of the desorption of fihns maintained under constant pressure has shown that the desorption of the fatty acid molecules (lauric and inyristic acids) may be diffusion-controlled; the diffusion proceeds through a layer immediately below the film, this layer being of macroscopic thickness. During desorption, an extremely thin layer of substrate adjacent to the film has the concentration e~, which is that of a solution in equilibrium with an adsorbed film which is identical with the spread film. Theoretical laws were proposed in agreement with the above-mentioned mechanism. The experimental verifications of these laws were found to be satisfactory: the variation of the film area at constant pressure as a function of time could be predicted and the correct value of the diffusion constant was calculated. It appears therefore that in this case the possibly existing potential barrier to desorption would have to be very low. 4 It was therefore assumed that the stability of the spread monolayers of certain fatty acids may be due to a very low rate of desorption. The author wishes to express her thanks to Dr. H. A. Zutrauen for his assistance in translating this paper.

REFERENCES 1. 2. 3. 4. 5. 6.

ADAM, !~. I{., AND JESSOP, G., Proc. Roy. Soc. (London) All0, 423 (1926). GVASTALLA, J., Compt. rend. 206, 993 (1938). GUASTALLA, J., Compt. rend. 208, 973 (1939). GVASTALLA, J., Compt. rend. 208, 1078 (1939). GUASTALLA, J., Cahiers phys. 13, 5 (1943). GVASTALLA, J., Cahiers phys: 10, 30 (1942).

4 Ward and Brooks (48) were led to an analogous conclusion when studying the transfer of f a t t y acids of low molecular weight across an oil-water interface.

418 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

33. 34. 35. 36.

37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

L.

'TER MINASSIAN-SARAGA

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