Penetration of octadecanol monolayers by sodium dodecyl sulfate

Colloids and Surfaces, 47 (1990) 233-244 Elsevier Science Publishers B.V., Amsterdam -

233 Printed in The Netherlands

Penetration of Octadecanol Monolayers by Sodium Dodecyl Sulfate D. VOLLHARDT and M. WITTIG Central Institute of Organic Chemistry, Academy of Sciences of G.D.R., Rudower Chaussee 5, Berlin 1199 (G.D.R.) (Received 16 February 1989; accepted 7 November 1989)

ABSTRACT To obtain quantitative information on penetrated monolayers a reliable experiment with high accuracy has been designed where the insoluble monolayer enclosedbetween two barriers is shifted over different subsolutions of a multicompartment trough. An equilibrium surface pressure-area isotherm of the octadecanol monolayer has been measured on substrates containing various concentrations of sodium dodecyl sulfate (SDS). A theoretical approximation derived by Alexander and Barnes is the starting point for a theoretical model to quantitatively describe the penetration behaviour of the surfactant and to obtain information on the equilibrium molecular composition of the penetrated monolayer. The penetration isotherm is calculated by fitting an exponential equation to the experimental data. The quantitative analysis of the penetration system involves the characterization of the singlecomponent properties.

INTRODUCTION

The understanding of the interaction between monolayers and other materials dissolved in the aqueous subphase is of considerable practical and theoretical interest [ 11. A multitude of published data about the phenomenon of penetration of an insoluble monolayer by dissolved surface-active materials, is available from a biological viewpoint. Nevertheless, the situation is very unsatisfactory because the design and interpretation of penetration experiments were often incorrect. The conventional techniques of penetration experiments, such as kinetic measurements, are subjected to serious restrictions. The disadvantages of these techniques have to be taken into account when investigating the equilibrium state of a penetrated monolayer. For example, the injection method [ 2,3] has been mostly used to mechanically distribute the soluble component after its injection beneath the insoluble monolayer. However, the monolayer stability is affected by the mechanical stirring. In case the monolayer is spread immediately after sweeping the surface [ 4,5] perfect conditions

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234

for spreading cannot be ensured as the soluble surfactant molecules are already adsorbed during the spreading process. Despite the great interest, only a small number of papers has been concerned with the theory of this topic. There is an obvious need to explore fundamental aspects of the equilibrium situation as prerequisite to an understanding of all aspects of penetration reactions. The thermodynamic analysis of applying the Gibbs’ equation involves a term for the insoluble monolayer. In 1955 Pethica [6] derived a relationship between the surface excess of a penetrant and the measured increase in surface pressure at constant area, implicitly assuming negligible specific interactions between both species. Assuming a close-packed cluster model for the monolayer a simple theory of the penetration process based on the concept of “accessible areas’ was developed [ 81. Here, a close-packed cluster model is assumed for the monolayer. In case a linear relationship exists between the adsorption onto a monolayercovered surface and the reciprocal value of the area per monolayer molecule, the parameters of the proposed equation yield the apparent cross-sectional area of a monolayer molecule and the adsorption of the surfactant in the “accessible area”. In contrast, Fowkes’ analysis is based on the consideration of an osmotic equilibrium between the surfactant dissolved in the bulk and that penetrated into the monolayer [ 71. The treatment is based on the assumption that the monolayer mixtures show ideal behaviour up to high surface concentrations of the surfactant. Progress has been made by Alexander and Barnes [91 deriving a more general expression starting from Gibbs’ equation. Approximations were made also involving Pethica’s equation which is suitable for experimental conditions. The subject of the present paper is to design a reliable penetration experiment with high accuracy and to evaluate the partial molar area (A,) as well as the relative adsorption (r,) of the dissolved surfactant. EXPERIMENTAL

To avoid the disadvantages of the conventional penetration technique discussed above a new experimental procedure for penetration measurements has been developed [lo]. A similar procedure was proposed by Fromherz [ 111 for the spectroscopic investigation of protein adsorption at lipid monolayers. It is based on the idea that a monolayer kept under a defined surface pressure is swept over subphases of different composition. In Fig. 1 the procedure of different steps of the monolayer transfer is shown. Enclosed between two barriers the spread monolayer can be moved over different subphases of a partitioned trough. This technique offers the possibility to create better defined initial conditions for penetration experiments. Thus,

235

I

aqueous

solution

1

barrier

1

aqueous

solution

2

insohble monoloyer

IU

surfactant

solution

Fig. 1. Procedure of the penetration measurements using the sweeping technique.

the two components

of the penetration system to be investigated can be characterized independently from each other. If the monolayer is spread on an aqueous subphase in partition I and the aqueous solution of the soluble component is contained in partition III (line 1) the surface pressure of both species can be measured, e.g., the surface pressure (x)-area (A) characteristics of the insoluble monolayer as well as the time dependence of the surface tension and the equilibrium surface tension-concentration curve for the soluble surfactant. The monolayer is then transported to the surfactant-containing subphase over partition 2 acting as a sluice (line 2) in order to remove adhering bulk liquid. Penetration experiments carried out in partition III according to the experimental procedure allow the study of the equilibrium conditions of the penetrated monolayer, the kinetics of penetration, as well as the effect of compression and expansion on the penetrated monolayer. Additional information on the stability of the penetrated monolayer can be obtained when the penetrated monolayer, being in equilibrium with the subsolution, is swept back to partition I containing pure water without surfactant. Some details of the trough used in this sweeping technique are shown in Fig. 2. To optimize the accuracy of the measurements, the practical design of the apparatus is aimed at a large area of the trough combined with favorable material properties. A circular teflon trough is used, 8 mm deep, with a total area of 158 000 mm2. The trough can be partitioned by a maximum of 4 walls. A special barrier coupling allows the movement of both barriers separately as well as simultaneously by an infinitely variable thyristor feed back circuit in the range between 3 and 40 angular degrees per minute. The small distance between the walls of the parts and the borders of the circular trough is important for the functionality of the sweeping mechanism.

236

8

Fig. 2. Scheme of the penetration trough: (1) trough; (2,4) barriers; (3) barrier drive; (5) wall of partition; (6) data receiver; (7) diving equipment; (8) cam gear; (9) driving motor.

\ 5..

a -5 2

4..

bi 3--

2..

6

6

8

10 12 15 20 30 compression ro+e

Fig. 3. Surface pressure of a compressed aqueous surface cleaned by repeated sweeping and sucking-off: (1) directly after cleaning the surface and rinsing with pure nitrogen; (2) 180 min after cleaning the surface and rinsing with pure nitrogen; (3) 180 min after cleaning the surface without rinsing with nitrogen; (4) 180 min after cleaning the surface and normal laboratory atmosphere.

A moving coil instrument with span-wire suspension has been used as the data receiver. For maintaining constant surface pressure servo electronics has been used. The barrier drive has been controlled by the data receiver. Deviations of the measured surface pressure from the desired one have been corrected by the movable barrier until both values were identical. The motor control works as a phase-shifting control. The constant regime

237

is guaranteed for a regulation ratio greater than 1: 10. Deviations from the set value are smaller than 2%, independent of load and speed. The surface tension has been measured by the Wilhelmy method using polished slides of 0.1 mm thickness and 18 mm edge length. The trough is protected by a flat revolving piacryl cover which can be fixed to the trough as well as simultaneously moved along with the barriers and the Wilhelmy balance. Contamination of the aqueous surface from the adjacent gaseous phase can be effectively prevented by rinsing with pure nitrogen. For cleaning the water surface is repeatedly swept by the compression barrier leaving a sector smaller than two angular degrees followed by suction with a clean capillary. In Fig. 3 it is clearly demonstrated that the purity of the adjacent gaseous phase is important to avoid experimental artefacts. Further experimental details are given elsewhere [ 12 1.

THEORY

The thermodynamic equilibrium of the penetration system can be considered by the application of Gibbs adsorption equation including a term for the insoluble monolayer dx=RTr,dlna,+RTr,dlna,

(1)

Here n is the surface pressure, ri = ni/A the surface excess concentration, Z-‘i the relative adsorption of component i related to water, ai is the absolute activity of component i, and the subscript m is the monolayer and s is the solute penetrant. Alexander and Barnes proposed an approximation to calculate the surface densities of the penetrant from surface tension data for the restricted condition n, << n, most frequently encountered when A, is small 2 RT AM d&/da M-&)&(AM-&+&)

(2)

where c, is the molality of the bulk phase, A the surface area Ai =A/ni, the area per surface excess mole of component i, & the partial molar area of the solute penetrant, & the partial molar area of insoluble monolayer without solute penetrant, and ni the surface excess amount or Gibbs adsorption of component i. According to Ref. [9] it is defined: A =nM&+n,&. We have adopted this approach to calculate &,. Thus, solving the quadratic equation for & we obtain

238

2RT th/dln

2 RTA,d&/dn C, -

&(A,

-AM)

1

2 RT AM d&,&h AM (A,

-AM)

(3) A, and r, are calculated by the relationship 44 AM-&,

r;

=L A,

(4)

Direct measurements of & and d&/&r are impossible in penetrated monolayers, but their values can be evaluated as proposed by Pethica. According to him, & of the penetrated monolayer is assumed to be equal to the area in the pure monolayer at the same surface pressure. Thus, Eqns (2) and (3) can be the starting point to calculate & and r, from the penetration pressures (AK) measured. Once & and aAM/& are known, the right hand side of Eqn (3) can be calculated in dependence of ax/& c,. Contrary to surfactant adsorption at a clean surface there are no penetration isotherms (II, In c,) with physical parameters for the penetration of soluble species at a monolayer-covered surface. It seemed inevitable, therefore, to determine the expression da/aln c, necessary for calculating r, graphically. However, in case of saturation adsorption this is identical with the slope of experimental penetration isotherms at constant monolayer area, AM. An essential progress in analysing penetration data is based on the idea to tit the measured penetration pressures, da to the exponential equation of the molality of soluble surfactants, c,, of the form dx=B, exp(& 102 c, +& log c,)

(5)

as proposed by Vilallonga et al. for alcohol adsorption [ 131. The parameters Bi (i= l-3) were obtained by transforming Eqn (5) in a quadratic equation In da=ln

B, +Bz log’ c, +& log c,

On differentiating

ak

-= aln

C,

(6)

Eqn (6) we have

2 Bz log c, +BQ B, exp(B, log2 c, +& log c,) 2.303

(7)

Inserting the parameters B, obtained by fitting to Eqn (6)) the derivative ax/ dln c, of the fitted penetration isotherm can be determined. Substitution into Eqn (3 ) leads to calculated values of & and, thus, of r,.

239

The theoretical consideration of Alexander and Barnes [ 91 led to a general equation for the change in equilibrium surface pressure when the concentration of a soluble surfactant is varied in a subphase covered by a constant amount of insoluble monolayer on a fixed surface area. The boundary conditions of the approximation, adopted above as the starting point for calculating theoretical values of A, and r,, agree with the experimental conditions applied. On the other hand, for large values of AM and not too low bulk surfactant concentrations the general equation of Alexander and Barnes [9] is reduced to Pethica’s equation.

However, Pethica applied this equation for experimental conditions consistent with those proposed by Alexander and Barnes for the boundary conditions of the approximation [ Eqn (2) 1. Therefore, it was desirable to compare r, values calculated by both equations. The derivative dx/aln c, of Eqn (8) was calculated using Eqn (7) in the same way as described above. RESULTS

The analysis of the penetration of octadecanol monolayers by sodium dodecyl sulfate (SDS) involves the characterization of the single-component properties. The surface pressure (x)-area (A) isotherm and the area (A)-time (t) curve at 30 mN m-’ of octadecanol are shown in Fig. 4. The measured n-A isotherm coincides with literature data [ 141. The relatively high stability of the octadecanol monolayers, even at high surface pressures, is indicated by the A - t curve at 30 mN m-l. Owing to these properties we selected octadecanol as the insoluble component for penetration experiments. The equilibrium surface tension properties of the soluble components were investigated. Figure 5 shows the surface tension (a) of aqueous SDS solutions dependence on the solute concentration (c,) . The continuous curve was obtained by fitting Frumkin’s universal equations [ 151 to the experimental r~log c, data. The adsorption properties can be characterized by three adjustable parameters, r”, u” and a’, applying Frumkin’s surface equation of state a= -RTP

ln(l-I-‘/r=)-a’

(I-‘/r”)’

(9)

where r” is the saturation adsorption, u” the bulk-surface distribution coefficient characterizing the surface activity, a’ a surface nonideality parameter,

240

:

(b)

(a)

E

s

&I

,-.

3G

20

10

L u.2

cm

0.20

4,

30

0.19

A/nn?/mo/ecu/e

Fig. 4. Octadecanol monolayer: (t ) curve at 30 mN m- ‘.

;

65

S 9 b

6o

60

to 790min

90

720 tlmin

(z) -area (A ) isotherm;

(a) surface pressure

(b ) area (A ) -time

55

50

L5 \,.

LO 703

I 10-2

cs0.s jmol drnm3

Fig. 5. Equilibrium surface tension solutions at 295 K.

(a)-concentration

isotherm of aqueous sodium dodecyl sulfate

241

n the surface pressure, r the surfactant adsorption, and c the solution concentration. For the purified SDS sample used the following parametervalues were calculated [ 16,171: u” = 14.40+ 0.83 pm01cmm3 P=

7.81+ 0.48 pmol rnb2

a’ = 0.94 + 0.06 The fitting to the experimentalcurve is characterizedby the medium standarddeviationof the singlemeasurementsfor the surfacepressure:m(x) = 0.46 mN m-l. In Fig. 6 equilibriumpenetrationisothermof both components is shown for the conditions: A MzO.192 nm2/molecule, 10m6mol dmm3I csns I50 10m3mol dmb3 The experimentalvalues for equilibriumpenetration are marked by crosses.

E f 2 9

LO

35

30

25

20

15

10

5

I

10‘6

:

..~.

b

10-s

:

:..:...!

:

lo-4

.

.

. . .

..I

10-3

Csoslnwldm‘3

Fig. 6. Equilibrium penetration isotherm of the system octadecanol monolayer/SDS ditions AM = 0.192 nm2/molecule, lo-’ mol dme3 li csns 5 5. 10m3 mol dmm3.

for the con-

242

The smooth curve presents the penetration isotherm calculated by fitting the exponential Eqn (5) to the experimental data. It can be easily seen that a good fit is possible for the measured concentration range. DISCUSSION

The calculation of r, is based on the fitted penetration isotherm for the investigated octadecanol monolayer with a particular A = 0.192 nm’/molecule in dependence on the bulk concentration of sodium dodecyl sulfate, csns (Fig. 7). The experimental conditions are consistent with the boundary conditions of Eqns (3) and (4) for the calculation of r, and As. However, it seemed useful to analyse the available penetration isotherm with Pethica’s Eqn (8) for comparison. In Fig. 7 r,, calculated by both methods, is plotted against csns. It follows that the r, data calculated by Eqns (3) and (4) are lower than those applying Pethica’s Eqn (8). This difference amounts to 21.8-26.8% for SDS concentrations used. If r, is substituted by A, according to Eqn (4) Pethica’s Eqn (8) takes the form dn RT =aln C, A,

(11)

Compared with Eqn (8) it is apparent that both equations differ on the righthand side of Eqn (2). The expression within the bracket is equivalent to the difference between the I?, values calculated by both methods. Thus it is evident that these data express the error range in the case where the theoretical boundary conditions deviate from the experimental ones. Information on the molecular composition of the penetrated monolayer in equilibrium can be completed by A, data calculated by means of Eqn (3) and

05

7.0

1.5

Fig. 7. Surface excess concentration of SDS in a penetrated octadecanol monolayer (A =0.192 nm’/molecule) calculated by Eqns (3) and (4) [ X-X-X 1, and Eqn (8) [*--. I.

243 TABLE 1 ASvalues calculated by Eqn (3) for AM=0.192 nm*/molecule c, (mol dmU3)

& (nm*/molecule)

&

(nm*/molecule)

1.10-6 2.10-6 6.10-’ 1*1o-5 2.10-5 6.10-5 1*10-4 2.10-4 6.10-4 1*1o-3 2.10-3 6.10-3

15.42 12.80 a.37 7.02 5.72 4.07 3.50 2.88 2.14 1.88 1.59 1.24

0.1913 0.1911 0.1907 0.1904 0.1901 0.1892 0.1887 0.1879 0.1862 0.1853 0.1839 0.1811

is given in Table 1 for A, = 0.192 nm2 of the octadecanol monolayer in contact with SDS solutions within the concentration range 10e6 mol dmV3 I~sns15-10-~ mol dmm3. As expected the data of A8 calculated between 15.42 nm2/SDS molecule and 1.24 nm2/SDS molecule are much higher than those of the partial area of insoluble molecules, AM. Further information can be obtained comparing SDS penetration into the octadecanol monolayer under the above experimental conditions with the SDS adsorption at a clean surface without monolayer. For both cases r, versus csns is plotted on a logarithmic scale (Fig. 8). It can be seen that the slope of the penetration curve is smaller than that of the adsorption curve. On the other hand, at smaller concentrations of the soluble surfactant, csDs, more SDS penetrates into the monolayer-covered surface than is adsorbed at the clean surface. For this system the results suggest two competing factors in the penetration process: (i) the interaction between soluble and insoluble components of the system prevailing at very low concentrations of the soluble species; and (ii) the small accessible area reducing the penetration of the soluble component in the main concentration range of the soluble species. To obtain more information further experimental results of monolayer penetration, including a wide range of soluble surfactant concentration and monolayer coverage, should be compared. Then, specific interactions can be substantiated by the Goodrich criterion proposed in Ref. [ 181.

//

244

Y

E 0 p

c

I

lo-‘0

10“'

lo-”

/

d

/

“’

76’3

/8 t’

B ,J

/“’

AH’

2’

AS” x

L

0

penetration monolayer adswption

octadecanol at clean surface

/

,o-‘4 &’

M6

,

10”

,

I 70-L csDstmot2

-3

Fig. 8. Penetration of an octadecanol monolayer (AM=0.192 nm’/molecule) by SDS compared with SDS adsorption at a clean aqueous surface: surface excess concentration of SDS versus solute concentration of SDS. REFERENCES

8 9 10 11 12 13 14 15 16 17 18

G.L. Gaines Jr, Insoluble Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. F.A. Vilallonga, E.R. Garrett and M. Cereijido, J. Pharm. Sci., 61 (1972) 1720. W. Rettig, D. Stibenz, G. Geyer and H.-D. Diirfler, Colloid Polym. Sci., 259 (1981) 568. J.M. Schulman and A.H. Hughes, Biochem. J., 29 (1935) 1242. Y. Hendrikx and L. Ter-Minassian Saraga, Adv. Chem. Ser., 144 (1975) 177. B.A. Pethica, Trans. Faraday Sot., 51 (1955) 1402. F.M. Fowkes, J. Phys. Chem., 65 (1961) 355. M.A. McGregor and G.T. Barnes, J. Colloid Interface Sci., 49 (1974) 362. D.M. Alexander and G.T. Barnes, J. Chem. Sot. Faraday Trans. 1,76 (1980) 118. D. Volihardt, Mater. Sci. Forum, 25-26 (1988) 541. P. Fromherz, Biochim. Biophys. Acta, 255 (1971) 382. D. Vollhardt, D.Sc. Thesis, Academy of Sciences of G.D.R., Berlin, 1982. F.A. Vilallonga, E.R. Garrett and J.S. Hunt, J. Pharm. Sci., 66 (1977) 1229. J. Ralston and T.W. Healy, J. Colioid Interface Sci., 42 (1973) 629. E.W. Lucassen-Reynders, Prog. Surf. Membr. Sci., 10 (1976) 253. G. Czichocki, D. Voiihardt and H. Seibt, Tenside, 18 (1981) 320. D. Vollhardt and G. Czichocki, Coiioids Surfaces, 11 (1984) 209. E.H. Lucassen-Reynders, J. Colloid Interface Sci., 85 (1982) 178.