Monolayer studies

Monolayer Studies IV. Surface Films of Emulsion Latex Particles E R W I N SH E PPA R D AND NOUBAR T C H E U R E K D J I A N Physical Research, S. C. Johnson and Son, Inc., Racine, Wisconsin 53403 Received April 23, 1968 An i n v e s t i g a t i o n of the spreading of emulsion latex particles on water is described. When t h e particles are spread with the aid of organic liquids, one particle thick surface films are formed. These films are best characterized b y t h e i r greater thickness in comparison with conventional monolayer films. E q u a t i o n s are developed to calculate all average d i a m e t e r of the spread particles from the projected area of an equal mass of monodisperse hard spheres of the same d e n s i t y and the experimental limiting area o b t a i n e d from the pressure-area isotherm. T h e limiting areas of four polys t y r e n e latexes studied are 114, 147, 292, and 365 cm~/mg; t h e calculated average d i a m e t e r s are 138, 107, 54, and 43 mu, respectively. The average diameters as obtained b y electron microscopic techniques are: 154, 110, 66, and 46 mt~, respectively. INTRODUCTION

Two general classes of materials, amphipathie molecules and natural and synthetic polymers, are known to form stable monolayers at the air/water interface. This paper reports a third class of substances, emulsion latexes, which also form stable monolayers on water. The technique of spreading and the development of a method to calculate an average diameter of the spread latex particles from pressure-area isotherms are also given. EXPERIMENTAL Four samples of polystyrene latex and one poIymethylmethacrylate latex were prepared using sodium lauryl sulfate as emulsifier and ammonium persulfate as initiator. Different particle size distributions were achieved by varying the polymerization conditions. Sedimentation velocity, electron microscopy, and film balance techniques were used to characterize the five systems. Sedimentation velocity measurements were made in 1:1 ethanol-water suspensions and electron microscopic particle size analyses were made on

the original aqueous dispersions and on the systems containing the organic spreading aids. The polystyrene latex films were formed from a dispersion, 0.4 gm of solids in 100 ml of dispersant, which contained 65 ml of i-propanol and 18 ml of n-hexane, with the remainder being water. Similarly, the polymethylmethaerylate system contained 0.4 gm of solids in 90 ml of ethanol and 1 ml of benzene, the remainder being water. In both eases, the original dispersion was diluted with deionized water, followed by the alcohol and either the hexane or the benzene. A Hamilton mierosyringe was used to introduce the samples on deionized, distilled water contained in a Teflon-coated aluminum trough. Film pressure changes were automatically reeorded with a Wilhelmy film balance which was modified in a similar way as described for the analytical balance by Mauer (1). A constant film compression rate of 10 era2/ mg/min was used throughout to obtain the pressure-area isotherms. The electron microscope was used to examine the spread polystyrene latex films

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SHEPPARD AND TCHEUREKDJIAN

TABLE I The latex surface films are thick enough to ]~ESULTS OF SPREADING POLYSTYRENELATEXES show interference colors when spread. The ON W~TER progress of compression a n d / o r expansion of Collapse pressure, ~rc Limiting area, Ao the film may be observed, therefore, with the Sample number (dynes/cm) (cm2/mg) naked eye. The existence of pressure gradi1 20 :h 1.2 147 ::l: 7 ents throughout the film is a distinct possi2 17 :t: 1 114 ::h 6 bility since boundaries due to incomplete and 3 21 :t: 1.2 365 ~ 10 defective packing of the film are noted. This 4 18 ::t: 1 292 ± 9 is due to the presence of one particle thick clusters or patches of latex particles several when transferred after collapse. The shadow- millimeters in length near ~ = 0. When the lsransfer technique was applied to the speci- film is compressed, the patches do not remens. Additional relevant experimental de- arrange easily to form a homogeneous and uniform film. The quasi-equilibrium contails have been reported elsewhere (2, 3). ditions prevailing during the compression RESULTS AND DISCUSSION a n d / o r expansion and the strong particleparticle interactions apparently do not lend Table I contains the limiting areas and collapse pressures for the polystyrene latex themselves to a uniform packing and fusion samples spread at the air/water interface. of the edges. Long-chain fatty acids, at low pressures, The pressure-area (Tr-A) plot for Sample 2 is shown in Fig. 1 and is a typical isotherm for probably cluster together. Because of the the latexes which were studied. The experimental conditions were optimized to achieve I I maximum film spreading. Figure 2A-C represents electron micrographs of polystyrene latex Samples 2 and 4, and of a polymethylmethacrylate sample, respectively. The photographs represent X l l , 3 5 0 magnification and were obtained from the aqueous dilutions of the original dispersions. No differences were noted in o particle morphology and size distributions when the organic spreading aids were incorporated prior to electron microscopy. The organic additives used here function as do o) the conventional spreading solvents, that is, E they facilitate the spreading of the film-forming substance. In addition, the presence of residual polar groups and adsorbed emulsitier molecules on the latexes enhances spreading and the formation of one particle thick films on the water surface. Electron micrographs of transferred latex films after collapse showed occasional unO( IO0 200 covered regions (2). Examination of the Film area, cm~/rng film at the edge of these voids indicated that the film was one particle thick except where F I G . 1. A t y p i c a l p r e s s u r e - a r e a isotherm for during collapse folding had occurred to form p o l y s t y r e n e l a t e x f i l m s o n w a t e r a t 2 0 ° C ( S a m p l e multilayers. 2). Journal of Colloid and Interface Science, Vel. 28, No. 3/4, November-December 1968

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483

Fro. 2. Electron micrographs of latexes--magnification N 11,350. (A) Polystyrene--Sample 2. (B) Polystyrene--Sample 4. (C) Polymethylmethacrylate. small size of the molecules, however, if boundaries (intersections of patches) existed in a compressed film, it would be difficult to discern their presence b y electron microscopy. So, at higher pressures, the eompressed;~films appear smooth and homogeneous (3). The general reproducibility of the 7r-A results (Table I) is in itself an indication of the stability and the spreading of the latex films. The spread films apparently remain on the water surface in a nondispersible array even though they were formed from stable dispersions. Small particles with finite contact angles are known to float. Young's equation COS 0

--

(rL/X

--

crL/w

GWIA

may be used to calculate the extent of wetting of the particles by water. In the equation 0 is the angle of contact and zL/x, o-L/w, and zwA are the surface tensions of latex particle/

air, latex particle/water, and water/air interfaces, respectively. A contact angle of 70 ° is estimated if the following approximate surface tension values are used in the equation: ¢L/A = 35 dynes/cm, ~L/~ = 10 dynes/cm,

and

Zw/x = 72 dynes/em. The adsorbed emulsifier molecules are expected to anchor the polystyrene particles to the water after the spreading is completed. Because of the film thicknesses involved, the polystyrene films studied here may be visualized as being the oil phase of an oil-water system as shown in Fig. 3. The structure of this interface may be inferred, then, from previous O/W interfaeial studies. Emulsifier molecules orient such that the aliphatie chain lies in or on the latex and the sulfate group lies in the water instead of desorbing from the latex particle surface and adsorbing at the air/water interface. About a 20 % eor-

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SHEPPARD

AND TCHEUREKDJIAN

0lL

+

÷

-I-

"t~

t-

WATER

emulsifier

(A)"

water c l u s t e r

2o Ho

If the contribution of the sulfate group toward the energy of adsorption is assumed to be the same in both instances, the latex particle/water interface should be favored over the air/water interface. With regard to the stability and the collapse of the spread latex films a collapse mechanism similar to the one proposed b y Schuller (5) is also postulated to hold even though the particles are not monodispersed and are not hard spheres. Figure 4 shows an electron micrograph of Sample 3 film transferred after collapse. The micrograph was taken at X 8,000 magnification a n d t h e photographic print was enlarged to X 56,000 magnification. Other collapse structures were given earlier (2); the indications are that collapse sets in the film before all the holes are completely covered with the latex particles. The possibility of the emulsifier having completely covered the holes is excluded on the basis of the evidence given above. A pressure of 1 d y n e / e m distributed over one-fifth of a latex film 1000 A thick is equivalent to a three-dimensional pressure of about 0.5 atm. For a uniform 20 A film such

emulsifier (~,

water cluster

FIG. 3. Schematic diagram of latex particles floating on water. (A) Side view; (B) top view. reetion of the reported area would be necessary if all the emulsifier used in the polymerization were adsorbed at the air/waterinterface. Mixed film studies which are under way are expected to yield important information regarding this point. The free energy change per methylene group on transfer from an aqueous solution to hydrocarbon/water interface is about --815 cal/mole and about --625 eal/mole for transfer to the air/water interface (4).

FIG. 4. Electron micrograph of transferred polystyrene latex film (Sample 3) after collapse-magnification X 56,000. Direction of shadowing indicated by the arrow.

Journal of Colloid and Interface Ncience, VoI. 28, N o . 3/4, N o v e m b e r - D e c e m b e r

1968

MONOLAYER

as a stearic acid monolayer, a 1 d y n e / c m pressure would correspond to a pressure of 5 arm. At their collapse pressures, the corresponding values are 10 and 200 arm, respectively. Permanent deformation of the particles occurs apparently at pressures less than 7r~ since the transferred latex films appear to contain deformed particles. Some particle deformation is due to plasticization, of course, since they are in contact with the water and were exposed to the organic liquids prior to the compression of the film. The pressure-area isotherms are used to calculate the average particle size of spread latexes according to the following reasoning. Since the systems studied are polydisperse, an infinite variety of structures are possible in the surface layer. Any proposed packing arrangement will, in general, not resemble the true interfacial structure. Furthermore, the packing of the particles of Fig. 2A, Sample 2, will be quite different from that of Fig. 2B, Sample 4. Sample 4 is considerably more polydisperse. The standard deviations of the samples are 3 and 6 mm respectively. The packing of a given latex system will differ from one determination to another, but the reproducibility of ~r-A plots was better than 5 % in many eases. The average particle size of the spread latex particles is calculated as follows. The experimental limiting areas are equated to calculated film areas of equivalent monodisperse spherical particles packed in an hexagonal or cubic arrangement (2). One milligram of polystyrene of density p 1.05 g m / e m ~, subdivided into spherical partieles of diameter d in m#, occupies an area A in square centimeters, as given in Eq. [1], where V is the total volume. A -

3V 2d-

1.428 X 104 d

[1]

When the particles are packed in an hexagonal arrangement the area becomes Ah -

1.574 X 104 d,,.

[2]

STUDIES.

IV

485

For cubic packing, the area becomes Ao -

1.819 X 104 dc

[3]

The average diameter of the spread particles is calculated from either Eq. [2] or Eq. [3] or from a combination of the two depending upon the type of packing assumed. For systems where the sticking coettieient is expected to be high, for example, Eq. [3] should be weighted more. Table II summarizes the estimates of average particle size of the samples studied by electron microscopic, ultraeentrifugal, and film balance techniques. Equations [1], [2], and [3] were used to obtain the film balance diameters. The polymethylmethacrylate latex sample was expected to perform in an analogous manner. The pressure-area plots were not reproducible, however, when the amount of the material introduced, the concentration, and the equilibration time were altered. Figure 2C indicates part of the reason for this anomalous behavior. The estimated average particle size after correcting for deformation is about 100 m~. The solution polymer has a relatively high glass transition temperature and so the latex was expected to be hard and not easily deformable. Water is known to be a good plasticizer in many systems but the observed electron microscopic and film balance results were totally unexpected. Nonetheless, the results for the polystyrene TABLE

II

PARTICLE SIZES OF POLYSTYRENE LATEXES Sample number

a

1 2 3 4

97 125 39 49

Particle diameters (m#) b c d 107 138 43 54

124 160 50 62

110 154 46 66

112 131 54 57

(a f r o m E q . [1], b f r o m E q . [2], c f r o m E q . [3], d from electron microscopic number average, and e from sedimentation velocity.)

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S H E P P A R D AND T C H E U R E K D J I A N

systems indicate that monolayer techniques can be useful in studying the properties of emulsion polymers. ACKNOWLED GMENTS The authors thank Dr. Don A. Albright and R. P. Bronson for the electron microscope work and Dr. J. W. Berge for the ultracentrifugal

analyses.

REFERENCES 1. MAUm~, F. A., Rev. Sci. Instr. 9.5, 598 (1954). 2. SHEPPARD, E. AND TCHEUREKDffIAN, N., Kolloid-Z, u. Z. Polymere, In press. 3. SHEPPARD, E., BRONSON, R. P., AND TCHEURmKI)JIAN, N., J. Colloid Sci. 9.0, 755 (1965). 4. MUKERJEE, P., Advan. Colloid and Inlerface Sci. 1, 241 (1967). 5. ScHvLImn, H., KolIoid-Z. u. Z. Polymere 216-7, 380 (1967).

Journal of Colloid and Interface Science, VoL 28, No. 3/4, ]Vovember-December1968