Energy Relations in Monolayer Formation: The Spreading of Long

536

G. E. BOYD

Vol. 62

ENERGY RELATIONS I N MONOLAYER FORMATfON : THE SPREADING OF LONG-CHAIN FATTY ACIDS ON AQUEOUS SURFACES' BY G. E . B O Y D ~ Contribution from Kent Chemical Laboratory, University of Chicago, Chicago, Illinois Received November 9,196Y

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 7, 2015 | http://pubs.acs.org Publication Date: May 1, 1958 | doi: 10.1021/j150563a006

The temperature variations of the equilibrium spreading pressure re of tridecylic, myristic, pentadecylic and palmitic acids on clean aqueous surfaces were measured and the molar latent heats, entropy, free energy and enthalpy changes accompanying monolayer formation were estimated using experimental values for the area occupied per molecule ue in the equilibrium film. The entropy and enthalpy changes for spreading from the crystalline acids were found dependent on the chain length and on r e . The absolute entropy of the equilibrium palmitic acid monolayer a t 298.16"K.and u. = 22.5 sq. A. molecule-' was estimated as 126.1 e.u. The observed entropy increases on spreading from the crystals were found to be appreciably smaller than that required for a mobile two-dimensional gaseous monolayer, indicating that the molecules in the equilibrium film do not possess translational freedom of movement. The entropy changes are sufficiently large, however, to allow for rotation about the long axes of the molecules.

Measurements of the energy changes accompanying the formation of insoluble surface films by spreading from bulk phases a t various temperatures promise to be of assistance in laying a foundation for an exact thermodynamic analysis of the energy relations in monolayer formation. I n earlier worka with palmitic acid and cetyl alcohol it was concluded that the van der Waals attractions between the n-paraffin chains of long chain fatty acids and alcohols must be of primary importance in determining the energy differences between the bulk crystalline and liquid states of these compounds and their equilibrium films. Accordingly, in this work the effects of chain length on the entropy and enthalpy changes in monolayer formation were investigated by conduc+tingthe necessary measurements on the films of several long-chain fatty acids spread on slightly acid aqueous surfaces. The equations4 employed to evaluate the molar latent heat of spreading (Qm)s, the changes in the molar free energy (AFm)sand the molar enthalpy (AH,), of spreading are (Qrn)s

(AFm)s =

= T(dre/bT)ufN(ue

-N

Sue

(AHrn),

Qs

a

-

~

redcf = - r e N ( u e

(AFrnh

f (&rn)s

8

)

(1)

- us)

(2)

(3)

where ire is the equilibrium spreading pressure of the monolayer, ue is the molecular area in the equilibrium film and us the molecular area occupied by the bulk phase, which can be fixed negligibly small relative to g e . Two types of experimental data are required for the use of eq. 1-3: measurements of the variation of rewith temperature T and measurements which will give the variation of ue with T . The aqueous sub-phase upon which the equilibrium monolayer is formed by spreading must not interact with the bulk phase so as t o change the composition of the latter during the measurements of r e . If the identity of the bulk phase remains unaltered the monolayer in equilibrium with it may be employed as a reference state for the calculation of film activities (1) Presented before the Symposium on Properties of Monolayers, Division of Colloid Chemistry, 132nd National Meeting, American Chemical Society, September 8-13, 1957, New York, N. Y . (2) Oak Ridge National Laboratory, Oak Ridge, Tennessee. (3) G. E. Boyd and J. Schubert, THISJOURNAL, 61, 1271 (1957). (4) W. D. Harkins, T,F. YQung and G. E. Boyd, J . Chem. Phys., 8, 954 (1940)v

in studies of the interaction of the film with the subphase.s Experimental Part The CI&S n-fatty acids used were taken from the preparations donated to this Laboratory by Professor E. E. Reid.6 These compounds were additionally purified by sublimation in a high vacuum a t temperatures slightly above their melting points. The water used in the preparation of the 0.01N acid solutions upon which the fatty acids were spread was doubly distilled from alkaline permanganate and condensed in block tin. The sulfuric and hydrochloric acids em loyed were of reagent grade. furface pressure measurements were performed from 0 to 45' using a thermostated surface trough equipped with the horizontal-type surface balance described previpsly.7 Temperatures were maintained constant to &0.1 , and were measured using two single-junction copper-constantan thermocouples, one placed in the air approximately one mm. above the film and the other one mm. below the film in the aqueous sub-phase. It was assumed when the temperature indicated by the two thermocouples differed by no more than 0.1' that the temperature of the film was known. Surface pressures as low as 0.03 dyne cm.-l could be detected. After temperature equilibrium, several hundred small crystals of fatty acid were deposited onto the clean aqueous surface before the surface balance by remote manipulation. The area included between the float and barrier of the film balance, which was available for film spreading, was about 500 sq. cm. The criterion for spreading equilibrium was the constancy of the surface pressure to within 0.1 dyne cm.-l for 15 minutes: A number of rate of spreading determinations were performed which showed that constant values were attained within a few minutes a t temperatures above 20'. At the lowest temperature periods as long as three hours were necessary. Spreading from the melted fatty acids was extremely rapid and the reversibility of the final pressure could be demonstrated in contrast to spreading from their crystals. Measurements of reabove 45' using the ring method have been reported on three of the compounds employed in this work.* A special study using both the surface balance and the ring method simultaneously to measure surface pressures on the same film showed that identical values were given by both procedures. The concordance of measurements by both techniques may be seen from the curve for palmitic acid in Fig. 1. The curves shown for temperatures above the melting points of the fatty acids were computed from empirical equations given by Harkins.8 The intersections of the ( r , , T ) curves for spreading from the crystalline solids and from the melts define the melting point under the conditions of measurement. Owing to the slight solubility of water in the fatty acids at these temperatures, it is to be expected that the melting point temperature on the surface (5) A. Frumkin and A. Pankratov, Acta Physicoehirn., U R S S , 10, 55 (1939). (6) E. E. Reid and J. D. Meyer, J. A m . Chem. Soc., 66, 1574 (1933). (7) G. C. Nutting and W. D. Harkins, i b i d . , 61, 1180 (1939). (8) W. D. Harkins, "Physioal Chemistry of Surface Films," Reinhold Publ. Corp., New York, N. Y . , 1952, p. 182.


May, 1958

ENERGY RELATIONS IN MONOLAYER FORMATION

537

EQUILIBRIUM SPREADING PRESSURES OF LONG CHAIN FATTY ACIDS ON AN AQUEOUS SUBSOLUTION OF pH = 2.0

I

I

Fig. 1.-Equilibrium spreading pressures for long-chain fatty acids on aqueous sub-solutions of pH 2.0. (Solid points on curves for C1, and Cls acids from ref. 16; solid points on curve for CMacid from ref. 8.) will be somewhat lowerg than that for the pure compounds. Such an effect seems to appear in Fig. 1 where the “surface melting points” of myristic, pentadecylic and palmitic acids are: 53.9,51.8 and 62.3‘ compared with 54.4,52.3 and 62.9’ for the anhydrous compounds. The areas per molecule in the films in equilibrium with the excess bulk fatty acids were read from the pressure-area isotherms of their respective monolayers measured a t various temperatures. These monolayers were spread from solutions of the fatty acid in carefully purified ligroin of concentrations varying between 0.5 and 1.0 mg./ml. An accurately known volume (0.1074 ml.) of solution contained in a constant volume pipet was placed dropwise onto about 1300 sq. cm. of clean surface. The first compression of the film was begun after allowing 15 minutes for the volatilization of the ligroin and for complete spreading. Isotherms measured on the monolayers of tridecylic acid a t various temperatures are shown in Fig. 2. Molecular areas of the equilibrium tridecylic acid films at the desired temperatures were determined using the data in Fig. 1 to find r etogether with the isotherm in Fig. 2 to obtain ue. An analogous procedure was employed using published isotherms for myristic,lOJl pentadecylic‘ and palmitic‘ acids to find ue for these acids and their variations with temperature. It was not possible to obtain reliable ( r , a ) isotherms for the evaluation of ue at the highest temperatures because of the solubility of the monolayers in the aqueous sub-solution. Repeated experiments were performed a t 52” on myristic acid films without success. Even with extremely rapid film compressions it was not possible to achieve a film pressure of re without solubility losses so extensive as to vitiate the ue value observed. Molecular areas a t these higher temperatures accordingly were estimated using the Langmuir equation for liquid-expanded monolayers.12 The temperature a t which the equilibrium film becomes expanded can be estimated accurately a8 the locus of the intersection of the (?re,T)and ( n , T ) variations, where T k is the “kink-point” or transition pressure of the liquid expanded to the “intermediate” monolayer state. These temperatures were 13.6, 18.1, 46.7 and 49.1’ for tridecylic, myristic, pentadecylic and palmitic acids, respectively. (9) C. W. Hoerr, W. 0. Pool and A. W. Ralston, Oil and Soap, 19, 126 (1942). (10) G. C. Nutting and W. D. Harkins, J. Ant. Chem. Soc., 61, 2040 (1939). (11) N. IC. Adam and G. Jeasop, Proc. Roy. SOC.,(London), 8112, 362 (1926). (12) I. Langmuir. J . Chem. Phys., 1, 756 (1933).

Molecular areas (in sq. A.) in the equilibrium liquid expanded film are given by12 ue = u0 kT/.(r, TO)where the limiting (empirical) molecular area uo is given by uo = 12.0 0.1281, where t is the temperature in “C., and T Ois the spreading coefficient for the hydrocarbon portion of the molecule which is related to the chain length by T O = 4.8 - 1.3n, where n is the number of carbon atoms. A check on the moiecular areas estimated using the Langmuir relation may be obtained by computing values of ue at the melting points from the equation b e = (&m)dNTf[(dne/dT)a - (dre/dT)11 (4) where (Qm)fis the molar heat of fusion and where (dreldT). and (dn,/dT)I are the temperature variations of the spreading pressures for the solid and liquid fatty acid, respectively, a t the fusion temperature Tf. The values of (&m)r and Tfemployed were those for the anhydrous acids.15 However, as noted earlier, the melting temperature on the surface is approximately 0.5” lower than Tr owing to the solubility of water in the fatty acids. Possibly, also, the magnitude of (&m)fis slightly lowered. Experimental ue values together with the theoretically estimated curves are shown in Fig. 3. A fair concordance between the values derived from two independent series of isotherm measurements‘opll on myristic acid may be seen, although the values of ue below 15: appear to be high uniformly by approximately two sq. A. per molecule. There is a rough agreement of both sets of data with the curve (dashed line) computed from the Langmuir equation. Unfortunately, inaccuracies as great as 15.0q10 appear to occur in the experimental data, mainly because of film solubility but also possibly because of other interferences which may arise during the time required to determine a complete isotherm. Since a value of ue a t but one, usually high, film pressure is required a t a given temperature, a modification of this procedure should give much higher accuracy. Difficulties because of rapid solubility losses might be overcome using an extrapolation technique based on an assumed rate mechanism for the film diss~lution.~‘

+

-

+

Experimental Results The spreading pressure-temperature variations shown in Fig. 1 appear to be linear within the limits of experimental error. Accordingly these data were fitted by least squares methods to the empiri(13) A. M. King and M. E. Garner, J. Chem. Soc., 1849 (1929). (14) L. Ter Minassian-Saraga, J. CoZZ. Sei., 11, 398 (1956).

538

G. E. BOYD

Vol. 62

1

0


SURFACE P R E S S U R E A R E A ISOTHERMS FOR T R I D E C Y L I C A C I D ON 0.04N HZSO,

"

I

A

ooooo

0 A

0

I

I

I

20.0 Fig. 2.-Surface

I

I

I

25.0 30.0 35.0 40.0 45.0 A R E A P E R M O L E C U L E ( s q . L.),

pressure-area isotherms for tridecylic acid monolayers on 0.01 N H2SOa. by * on each isotherm.)

cal equation r e

=

a (T

I

- To)

(5)

with the constants and their standard deviations (S.D.) being given in Table I. The wide range of linearity of the spreading pressure with temperature observed is perhaps unexpected. Such a behavior, however, is consistent

( r evalues

1

I

50.0 from Fig. 1 indicated

with the Gibbs adsorption relation if the molecular area in the equilibrium film is approximately constant, as has been Pointed out by Langmuir. l6 Inspection of Fig. 1 and Table I permits several general observations: (a) the magnitudes of the spreading Pressures show an alternation with the (15)

I. Langmuir, J . ~rankzinInstitute, ais, 143 (1934).

ENERGY RELATIONS IN MONOLAYER FORMATION

May, 1958


TABLE I SUMMARY OF CONSTANTS IN EQ.5 FOR THE SPREADINQ OF MONOLAYERS FROM CRYSTALLINE LONC;-CHAIN FATTY ACIDS a,

Acid

(dyne c m . - ’ deg.-I)

S.D.

Ta (OK.)

Tridecylic Myristic Pentadecylic Palmitic

0.643 .659 .543 .556

0.0151 .0105 ,0062 .0066

253.2 271.4 257.7 275.2

number of carbon atoms in the long chains. The odd-numbered carbon atom acid shows a higher r e than the adjacent even-numbered acid with one less carbon atom. For example, at 25” the spreading pressures for the C12,C13,C14,CI6,C16 and C17 longchain fatty acids are 27.3, 28.9, 17.7, 22.0, 12.8 and 16.4 dyne respectively. (b) The temperature variation of redepends on the number of carbon atoms. Thus, the even-numbered carbon atom acid has the larger (dne/dT),. (c) Above the melting point there appear to be no “odd-even” effects either in r e or in (dre/dT). The temperature variations, (dne/dT), observed in this work agree with those for myristic and pentadecylic acids reported by Nutting and Harkinsl8 but not with an earlier value of 0.553 dyne cm.-l deg.-l for myristic acid reported by Myers and Harkins.l’ Values of (dae/dT) for pentadecylic and palmitic acids which have been reported in the comprehensive studies of Cary and Rideall* are in agreement with our values (Table I). However, the spreading pressure values reported by these latter workers using the ring method appear to be from 1.5 to 5.ci dyne cm.-l too low. The evaluationlg of the variable correction factor, which is essential if the pull on the ring is to be converted into a surface tension, had not been accomplished at the time of Cary and Rideal’s work. Measurements of the temperature variations of the spreading pressure of palmitic acid have been reported using the Wilhelmy slide method.20 These results agree fairly closely with those in Fig. 1. However, discontinuities in the slope of the ( r e , T) relation were found at 40 t o 42.5’ and a t 47.5 to 50” which were not observed in this work.

Discussion The results from a numerical treatment of the experimental data according to eq. 1-3 are presented in Table I1 from which it may be seen that: (1) monolayer formation by spreading from the crystalline fatty acids is accompanied by the absorption of a latent heat and by entropy and enthalpy increases. Thus, a monolayer can be formed with the absorption of heat if adsorption occurs from a crystalline phase. This fact appears to be an exception t o the rule for adsorption from gases and solutions where “all adsorption processes are exothermic.”21 (16) G. C. Nutting and W. D. Harkins, J . Am. Chem. Soc., 61, 1702 (1939). (17) R. J. Myers and W. D. Harkins, J . Chem. Phys., 4, 716 (1936). (18) A. Cary and E. K. Rideal, Proc. R O V .SOC.(London), A109,328 (1925). (19) W. D. Harkins and H. F. Jordan, J . Am. Chem. Soc., 62, 1751 (1930). (20) A. A. Trapeznikov, Compt. rend. aead. Sei., URSS, 41, 275 (1945).

539

TABLE I1 ENERQY CHANQES I N THE FORMATION O F %-FATTY EQUILIBRIUM MONOLAYERS

2, Acid Tridecylic

Myristic

Pentadecylic

Palmitic

t, “C.

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 41.5 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 54.4 54.4 10.0 15.0 20.0 25.0 ‘30.0 35.0 40.0 45.0 50.0 52.3 52.3 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 62.9 62.9

T ,

dyne om.-’ 12.9 16.1 19.3 22.5 25.8 29.0 32.2 35.4 39.6 4.5 7.8 11.1 14.4 17.7 21.0 24.3 27.6 30.9 34.2 36.1 36.1 13.8 16.5 19.3 22.0 24.7 27.4 30.1 32.8 35.5 36.8 36.8 7.3 10.0 12.8 15.6 18.4 21.2 23.9 26.7 29.5 33.3 33.9 33.9

A.

malec.-L 20.6 21.4 22.8 26.2 26.3 26.5 26.8 27.0 27.2 24.2 24.7 25.6 30.1 29.7 29.5 29.5 29.7 29.9 30.3 30.6 30.6 20.5 20.8 21.1 21.4 21.6 21.9 22.5 26.9 29.8 30.0 30.0 21.7 21.8 22.5 23.1 23.8 24.5 26.6 31.3 31.7 32.0 32.5 32.5

(&m)s,

kcal. mole-’ 5.20 5.50 5.98 6.99 7.13 7.31 7.52 7.69 7.92 6.39 6.64 7.00 8.38 8.43 8.49 8.63 8.82 9.04 9.29 9.51 -2.47 4.69 4.84 4.98 5.13 5.28 5.51 6.59 7.52 7.64 -2.93 5.01 5.12 5.37 5.61 5.90 6.15 6.78 8.10 8.33 8.54 8.75 -2.97

ACID

(ASm)., -(A(Acal. F&., Hrn)., mole-’ kcal. kcal. deg - 1 mole -1 mole -1 4.82 19.0 0.38 19.8 .50 5.01 .63 5.34 21.1 .85 6.14 24.3 .97 6.15 24.3 6.20 24.5 1.11 24.8 1.25 6.28 25.0 1.38 6.32 25.2 1.55 6.37 23.0 0.16 6.23 23.4 .28 6.36 24.3 .41 6.59 7.76 28.6 .62 28.3 .76 7.67 28.0 .89 7.60 28.0 1.03 7.60 28.2 1.18 7.64 28.4 1.33 7.71 28.7 1.49 7.80 29.0 1.59 7.92 7.53 1.59 -4.06

.

-

16.3 16.5 16.7 16.9 17.1 17.6 21.0 23.3 23.5 8.99 17.4 17.5 18.0 18.5 19.1 19.6 21.3 25.1 25.4 25.6 26.0 8.85

-

-

0.50 4.19 .59 4.25 .68 4.31, .77 4.36 .86 4.41 .98 4.53 1.27 5.42 1.52 6.00 1.59 6.05 1.59 -4.52 0.23 4.78 .32 4.80 .42 4.96 .52 5.09 .63 5.27 .75 5.40 .92 5.86 1.20 6.90 1.35 6.98 1.53 7.00 1.59 7.16 1.59 -4.56

(2) Spreading from the melted compounds is accompanied by the evolution of heat and by entropy and enthalpy decreases. Such behavior suggests that the molecules in the monolayer may be more (‘ordered” than in the melt. However, the hydrocarbon chains in the latter appear to be more closely packed than in the film. X-Ray diffraction measurements on the liquid fatty acidsz2have shown two spacings, one (4.55 A.) which is independent of the number, N , of carbon atoms in the chain, and one which increases linearly with N . In the melts the molecules are arranged longitudinally in parallel collinear chains, although, in passing from the crystalline to the liquid state, there is an increase in the distance associated with the crosssection of the molecule and a decrease in the long spacings. The average molecular cross-section in the melt is 20.6 sq. A. while in the film it is approximately 30 sq. B.;therefore, an entropy increase might have been expected if the packing of the hy(21) S. Brunauer, “Physical Adsorption,” Princeton Univ. Press, Princeton, N. J., 1943, p. 5. (22) R. W. Morrow, P h y s . Reu., 31, 10 (1928).

540

G. E. BOYD

Vol. 62

carboxyl groups in the monolayer are constrained to a single plane, whereas in the melt they may be randomly distributed in three dimensions. Accordingly, the film will possess a degree of longrange order not found in the liquid which has only 3 25.0short-range order. An additional entropy decrease LL from this source may be expected, therefore. I (3) The latent heats of spreading are 30.0.L smaller for the odd-numbered than for the evenm d numbered carbon atom acids. A similar behavior $ 25.0is observed with the latent heats of fusion, and both phenomena are related to the fact that the binding z w in the crystal of the odd carbon atom acid is less -1 3 than in the "even" acid with one less carbon atom. 30.0(4) All of the thermodynamic quantities for 0 I spreading from crystals increase with the area per molecule occupied in the equilibrium film. The 25.0 enthalpy increases, in fact, appear to vary linearly with g g . The enthalpies for the different acids, however, may not be compared because of the differences in the temperatures for each a t the same molecular area. The ( A H m ) s values for palmitic I ; $0 215 310 :5 d5 20 515 610 615 70 acid, however, may be corrected to 25" using reTEMPERATURE ('CJ cently published heat capacity datazafor the acid, Fig. 3.-Variation of the molecular area ue in the equi- assuming Cp for the monolayer is negligibly smalLZ4 librium film with temperature. (Solid points on curve for Clr acid from ref. 11; points indicated by 8 estimated from Heat capacity measurements for the other crystalline fatty acids employed in this study have not eq. 4.) been reported. The desired heat capacities needed 1o.c for correcting (AH,), to 25" can be obtained, howI I I I I I I I I I I I ever, if it is assumed that the speciJic heats of all the long, straight-chain compounds containing the same number of carbon atoms are the same a t the where Tm,p.is same reduced temperature, TITmep., .. 9.c the melting point temperature. Fortunately ac-a E curat,e data have been reported26 for tridecane, tetradecane, pentadecane and hexadecane which may 8.0 be used for constructing the necessary (Cp, T/Tm.p.) interpolation plot. Figure 4 shows a plot of the 2 molar enthalpies of spreading corrected to 25" Q, against ae. It may be seen that the corrected 7.0 molar enthalpy does not increase linearly with the za area per molecule in the equilibrium monolayer. J Also, the longer the hydrocarbon chain of the fatty .Lca However, because of the acid the greater (AH,),. & 6.C strong dependence of (AH,), on ae the uncertaina > ties in ge are such as to preclude estimates of the 1 a variation of the spreading enthalpy with the numkr ber of carbon atoms in the chain. 6 5.0 A value of 126.1 e.u. may be obtained for the absolute entropy of the equilibrium palmitic acid monolayer a t 298.16"K. using the measured valuez3 of 108.1 e.u. for the crystalline compound together 4.c with the change in entropy (AS,), of 18.0 e.u. to form the monolayer with ue = 22.5 sq. A. per molecule. This film entropy, when corrected to a suitI I I I I I I I I I I I able standard state, may be compared with the eii21 2 2 2 3 24 25 2 6 27 2 8 - 29 30 31 3: of gaseous palmitic acid in its standard tropy, SO(g), MOLECULAR AREA ( s q A ) state (ideal gas at unit fugacity, 298.16'K.). Using Fig. 4.-Variation of ( A H , , ) , at 298.16"K. with the area per the empirical equationz6 molecule in the equilibrium film. VARIATION OF MOLECULAR AR'EA

U


2

E

4b

0

(3

v)

drocarbon chains alone were important. Hydration of the polar carboxyl groups in the monolayer will give a negative entropy contribution, but this does not appear to be sufficient to overcome the positive contribution from the chains (ca. 10 e.u.) so as-to give a net entropy decrease of 8-9 e.u. The

(23) H. E. Wirth, J. W. Droege and J. H. Wood, THIS JOURNAL, 60, 917 (1956). (24) This is a good approximation for the surfaces of pure liquids a t temperatures well removed from their critical points. Cf.,ref. 8, p. 80-83. (25) H. L. Binke, M. E. Gross, G. Waddington and H. M. Huffman, J. Am. Chem. SOC.,76, 333 (1954). (26) J. W. Cobble, J. Chem. Phys., 21, 1451 (1953).

ADSORPTION OF POLYVINYL ACETATE

May, 1958

+

So(g) = S’(trans1.) 9.2N - So(.) = 3/2 R In ilf 26.00 9.2N

+

+

- So(a)

(6)

where M is the molecular mass, N the number of “skeletal” bonded atoms in the compound and is the entropy loss because of the presence of doubly-bonded oxygen in the molecule. Thus, = 205.8 e.u. while SO(f) 126.1 R In ( a o / a e ) = 134.1 e.u. so that an entropy loss of 71.7 e.u. occurs in transferring palmitic acid from the gaseous to the film standard states. A decrease of 42.7 e.u. would have occurred if only the translational entropy of the gas were lost. The larger value found accordingly suggests that appreciable losses of internal entropy must also occur in going from the Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 7, 2015 | http://pubs.acs.org Publication Date: May 1, 1958 | doi: 10.1021/j150563a006

’=

+

541

gas to the film, and that the molecules in the fiIm do not possess translational freedom of movement. On the other hand, the entropy increase on spreading from the crystal is sufficientIy large to allow for the rotation of the molecules about their long axes in the film. Measurements of the heat of rotational pre-melting in hexadecano12’ give an entropy increase of about 13 e.u. A similar increase might be expected in the spreading of palmitic acid if axial rotation of the film molecules occurs. The (Ah’,), of spreading for the acid is 18.0 B.U. so that other internal degrees of freedom in addition may also be released. (27) G. 5. Parks and R. D. Rowe, ibid., 14, 507 (1846).

THE ADSORPTION OF POLYVINYL ACETATE’ BY J. KORAL,~ ROBERT ULLMAN AND F. R. EIRICH institute for Polymer Research, Polytechnic Institute of Brooklyn, Brooklyn, N . Y . Received M a y 16, IS67

The purpose of this work was to investigate the adsorption of polyvinyl acetate on to solid surfaces under a variety of conditions. Among the variables which were considered were the adsorbent, the moIecular weight of the polymer, the distribution of molecular weights in the polymer, the presence of small numbers of active groups on the polymer, the solvent from which adsorption takes place, the temperature and the regularity of the adsorbent surface. The rates of adsorption and desorption were also determined. The results of these experiments were analyzed in terms of a simple model of polymer molecules in solution and of present theories of polymer adsorption. A reasonable picture of the polymer molecule adsorbed on a solid surface was created which explains the known facts for the case where the adsorbent is completely or nearly completely covered by the polymer. The results of this icture are in good agreement with estimates of the thickness of the adsorbed layer which were determined from the flow ofdilute polymer solution through a fine capillary.

A. Introduction The adsorption of polymers from solution on to surfaces differs from the adsorption of small molecules in several respects. Firstly the polymer consists of repeated identical chemical groups along the molecule, any one or all of which may be bound to the adsorbent surface, in contrast to smaller molecules which usually adsorb at a single adsorbent site. Secondly, by virtue of its great mass, a polymer molecule diffuses very slowly and further, because of its size and low mobility, it does not penetrate into pores on a solid surface which are easily accessible t o smaller molecules. As a result, it is to be expected that polymer adsorption would be very sensitive to geometric irregularities 011 an adsorbent surface. Thirdly, most polymer molecules are flexible to some degree and possess many internal degrees of freedom some of which are lost or restricted when adsorption takes place. The extent to which the intraniolecular configurations of the polymer molecule are limited is an important factor in the analysis of polymer adsorption. The purpose of the work reported here was to investigate systematically a single polymer, polyvinyl acetate (and to a lesser extent, a simple derivative, a partially hydrolyzed polyvinyl acetate) under a great variety of experimental conditions, and to analyze the results of the experiments in terms of current models of the polymer adsorption process. It was intended that the study be (1) This work was supported in part by the Office of Naval Reaearoh. (2) Submitted in partial fulfillment of the requirements for the Doctor of Philosophy degree a t the Polytechnic Institute of Brooklyn.

sufficiently complete so that a fairly clear picture of the important factors in this particular case of polymer adsorption would emerge. Of the many variables in polymer adsorption which influence or may influence the quantity of adsorbed polymer and the structure of the adsorbed layer, the following were considered: (1) the chemical structure of the polymer molecule, especially the presence of chemical groups (perhaps only a very few) on the polymer molecule which are particularly effective in promoting adsorption: (2) the molecular weight of the polymer; (3) the distribution of molecular weights in the polymer; (4) the solvent from which adsorption takes place; (5) the temperature; (6) the adsorbent; (7) the presence of other chemical species in the solvent or on the adsorbent surface; (8) the geometric smoothness or roughness of the adsorbed surface. I n addition, it is important to know the degree of reversibility of the adsorption process, the rate at which adsorption takes place, and the dependence of these quantities on other parameters such as solvent, temperature and chemical structure. B. Experimental 1. Materials. a. Adsorbents. (1) Iron Powder.Iron powder prepared from iron carbonyl by Antara Chemicals Division of the General Dyestuff Corporation was used. The sample was an “SF” type, has a chemical analysis of 98.2-98.8% iron, 0.50-0.70% carbon, 0.10-0.13% oxygen and 0.55-0.75% nitrogen. The company states that the average particle diameter is 3 f i . The powder was kept in a vacuum desiccator which was opened only to weigh out samples. (2) Tin Powder.-Finely divided, highly purified tin powder was purchased from the Fisher Scientific Company.

Energy Relations in Monolayer Formation: The Spreading of Long

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