Ind. Eng. Chem. Res. 1993,32,373-385
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Sensitivities of Droplet Size and Stability in Monomeric Emulsions Kevin Fontenott and F. Joseph Schork* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
The stability of a monomeric emulsion is directly dependent on the size of the monomer droplets. The droplet diameter is in turn significantly influenced by a variety of parameters. Both size and stability are important when emulsions consisting of small droplets are polymerized. These parameters were studied for the monomer methyl methacrylate, although the monomers styrene and vinyl acetate were also considered. Conductance was developed as a predictive tool for providing a measurement of emulsion stability. These indications were verified by shelf life stabilities and droplet size measurements. The key parameters which affect size and stability were found to be cosurfactant concentration and monomer water solubility. Coalescence was found to play an important role in stability as well.
Introduction The size of the monomer droplets can play a key role in determining the locus of particle nucleation in emulsion polymerizations. As the droplet diameter decreases, the total droplet surface area increases and additional surfactant molecules become adsorbed at the water-droplet interface to maintain stability. The competitive position of monomer droplets for capture of free radicals during polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. For a typical recipe, micelles are not likely to be present when the droplet diameter is lege than 0.5 pm. The emulsion is then referred to as a miniemulsion. For clarity, emulsions which have droplets larger than this are hereby referred to as macroemulsions. Since miniemulsions are relatively new as compared to macroemulsions, much remains to be learned about preparation of miniemulsions and the measurement of submicrometer droplet size distributions. Thermodynamically, emulsions are unstable due to the energy required to maintain the large surface areas in droplet form. They have an inherent tendency to break down. Stability of droplets plays an important role in the preparation and polymerization of monomeric miniemulsions. All things being equal, the smaller the droplets, the more stable the emulsion. Normally, submicrometer droplets are not sufficiently stable to remain in existence for a significant amount of time due to Ostwald ripening. Stability agents are necessary. Both long chain alkanes and long chain alcohols have been used as stability agents in miniemulsions. These have traditionally been referred to as cosurfactants, although they may not actually play a surfactant role. The most important parameter affecting the stability of an emulsion is the size of the droplets. This in turn is affected by the emulsification procedure and the type and amount of surfactant and cosurfactant. Methods for preparation of miniemulsions have been studied and outlined elsewhere (Barnette and Schork, 1987;Daniels et al., 1988,Durbin et al., 1979;Grimm et al., 1983). When a fatty alcohol is used as the cosurfactant, the optimum cosurfactant/surfactant ratio has been determined to be 1:2 to 1:3 (Azad et al., 1976;Choi et al., 1985;El-Aasser et al., 1984,Lack et al., 1987). The optimum length of the hydrocarbon chain for fatty alcohols is 16 (El-Aasseret al.,
* To whom correspondence should be addressed. 'Current address: Eastman Chemical Co.,P.O. Box 1972, Kingsport, TN 37662.
1984;Lack et al., 1987). For long chain alkanes there is not a specific chain length which provides optimal stability. Instead, longer chains provide greater stability until a plateau is reached (Ugelstad et al., 1980). The stability is also known to plateau at increasing cosurfactant/surfactant ratio once a 3 to 1 molar ratio is obtained (Delgado, 1987). The stability of emulsions has been shown to decrease with increasing water solubility (Brouwer et al., 1986),and oils with higher water solubilities are easier to emulsify. Emulsion stability is also a function of the organic-phase density. Much work has been performed in determining optimal conditions for emulsion stability. However, very little experimental work has been reported in the literature in the area of monomer droplet size and how it correlates to stability. Ugelstad et aL (1974,1980)and Azad et ai. (1976) showed qualitative comparisons of droplet size to emulsion recipe and final latex particle size. They used osmium tetroxide (OsOJ staining to get cold stage electron microscope pictures. Choi et d. (1980)gave average droplet sizes from electron microscope pictures for the styrene titration of mixed emulsifier systems. Average diameters range from 128 to 540 nm. Choi et aL (1985)criticized both the OsOl staining technique and the dynamic light scattering technique due to systematic errore in the procedures. Instead, a freeze-fracturing technique was used to obtain size information for styrene miniemulsions. These measurements provided only ranges and not a distribution or mean. Rodriguez (1988)measured the droplet diameter for methyl methacrylate (MMA) and styrene (STY) miniemulsions with a Coulter Electronics submicron particle analyzer. The aqueous phase of the emulsion was used as diluent for the measurement. Droplet size decreased with increasing hexadecane concentration, and MMA miniemulsions consisted of smaller droplets than corresponding STY miniemulsions. The results given were in the range of 91-124 nm for MMA and 175-226 nm for styrene. These numbers appear to be intensity weighted averages. Several researchers have used techniques which utilize a camera and light microscope. Barnette and Schork (1987)obtained data for MMA macro- and miniemulsions. Their miniemulsion data were in error due to the inability to discern droplets below 0.5 pm in diameter. The data provided for macroemulsions are believed to have been accurate and revealed an average droplet diameter of about 10 pm. Davis and Smith (1976)obtained size data for various organics in aqueous solutions. They considered the effect of adding of small amounts of long chain alkanes and alcohols on the stability of emulsions. A small amount of HD was found to provide great enhancement to the
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374 Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993
stability of droplets in the 1-5-pm range. They suggested the mechanism for instability was most likely diffusion and not coalescence. Submicrometer droplets were not obtained since they did not use high intensity shear. In this paper, the factors which influence droplet diameters and stability in monomeric emulsions are provided. The work focuses on MMA, although measurements were also made with STY and vinyl acetate (VAC). A model was developed which predicts size and distributions in polydisperse emulsions over time. An empirical equation was derived to predict droplet size as a function of shear, surfactant, and cosurfactant in MMA miniemulsions.
Theory Emulsion stability has been studied for quite a long time (Lifshitz and Slyozov, 1961; S c h h a n and Cockbain, 1940, and a variety of factors are known to influence it. These include, but are not limited to, droplet size distribution, surfactant type and concentration, aqueous solubility of the dispersed phase, temperature, surface tension, ionic strength, and percent disperse phase. Surface science plays a major role in stability as does thermodynamics. There are several mechanisms by which emulsions can break down, with each affecting the number of droplets. The exact pathway followed depends on many factors. The mechanisms are: flocculation, creaming, coalescence, and diffusion. Flocculation occurs when two monomer droplets collide and stick together but have insufficient energy to overcome the interfacial energy barrier to form one droplet of larger size. It is a reversible phenomenon. Flocculation in a stationary emulsion occurs via Brownian motion and/or sedimentation. Very small droplets have relatively high levels of random thermal movement. Brownian motion results in random collisions of droplets with varying amounts of kinetic energy. Sedimentation flocculation resulta from the different settling velocities of droplets of varying size. Large droplets will rise (or settle) faster than small ones and therefore collide with smaller ones. Reddy and Fogler (1981b) and Melik and Fogler (1988) have delineated the regions of the various types of flocculation as determined by droplet size, differential density, temperature, and continuous-phase viscosity. In emulsions under shear,flocculation can also occur due to eddy forces. Coalescence occurs when two or more droplets collide and form one larger droplet. It is a nonreversible process since the interparticle potential energy achieves the primary minimum energy well. This may occur some time after flocculation or immediately if the colliding droplets have sufficient kinetic energy to overcome electrostatic repulsion forces. Creaming is most important in stationary emulsions. Without shear, the difference in density between the aqueous and disperse phase results in the rising (or settling) of the droplets, This can be followed by flocculation and coalescence. Emulsions are also unstable through diffusion. If the disperse phase is even slightly soluble in the aqueous phase, maw transfer will occur from the smaller droplets to the larger on-. This phenomenon, Ostwald ripening, was first theorized by Ostwald in 1901. The driving force is the free energy due to interfacial tension. Two types of diffusional stability are currently debated in the literature: thermodynamics and diffusional resistance. Monomer diffusion can play an important role in polymerization of emulsions when mass transport becomes limiting. Thermodynamic stability can be gained through the use of relatively water insoluble cosurfactants. Quantitative Analyses. (a) Creaming and Coalescence. A stationary emulsion will cream whenever there
exists a difference in the density between the aqueous phase and the disperse phase. Creaming velocities can be calculated from the Stokes terminal velocity equation (Davies, 1972): Ustokes
=
d,2lAPk
mc
(1)
where d, is the particle diameter, Ap is the difference in densities between the two phases, and p c is the viscosity of the continuous phase. This equation is strictly valid for dilute suspensions. Interparticle forces affect the terminal velocity in concentrated suspensions. If the particles in question are actually liquid, internal circulation may increase this velocity. This contribution depends on the droplet diameter. In stationary emulsions, droplet or particle coalescence results from Brownian motion and creaming. Reddy and Fogler (Reddy and Fogler, 1981a-c; Reddy et al., 1981) in a series of papers developed equations which determine the size distribution in stationary emulsions as a function of location and time. Both flocculation and creaming are considered. The authors also show the regions of importance for each of the mechanisms. Brownian motion is seen to be important when the droplet diameter is below 4 pm in diameter. Creaming becomes negligible when the droplet diameter falls below 0.4 pm. The authors also provide experimental data to verify the equations. Diameters were measured by transmission electron microscopy (TEM) and varied between 0.2 and 10 pm. (b) Ostwald Ripening and Monomer Transport. Ostwald ripening was theorized in 1901 by Ostwald and quantified some 60 years later by Lifshitz and Slyozov (1961). It was recently reviewed in the literature (Voorhees, 1985). Ripening occurs in an emulsion due to the lack of monodispersity of the droplets. If all droplets are the same size, ripening does not occur. If each droplet contains some water insoluble component, ripening will occur only to a limited extent since shrinkage reaults in concentration of the insoluble component, a thermodynamically unfavorable process. However, if all components are somewhat soluble, Ostwald ripening occurs at a rate given by d(r3) (2) -=w= (41/w1 + 42/w2)-l dt where 4; is the volume fraction of component i and oiis the Ostwald ripening rate for component i. It is predicted by (3) where yi is the interfacial tension, D , is the diffusivity in the aqueous phase, Viis the molar volume, and 4ia"L is the volume fraction in the continuous phase at saturation, all of component i. This equation results in a size distribution that is independent of time and is given as (Kabalnov et al., 1987) 34eu2exp(-l/(l - 2y/3)) P(u) = P(r/i) = 2 5 / 3 (+ ~ 3)'13(3/2- u ) ' / ~ (4) for 0 < u < 312, and P(u)= 0 for u > 312. This equation has been experimentally verified for emulsions in the 110-pm range. It suggests that although the average radius, F, may increase over time, the relative distribution about the mean does not change. The Ostwald ripening theory is based on the assumption that the diffusion through the continuous phase is the
Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993 375 limiting mass-transfer resistance. It is more or less a bulk theory. Since there is potential for several resistances to exist, it is important to consider events on the microscopic scale, that being mass-transfer theory as applied to a monomeric emulsion. The equation for monomer transport at a individual particle (or droplet) can be given as
where Mp is the moles of monomer in the particle, Kojs the overall mass-transfer coefficient at the interface, Np is the number of particles, up is the average surface area of a particle, and [MI: is the concentration of monomer in the particle that would be in equilibrium with the current concentration of monomer in the aqueous phase. KOis given by 1/K, = l / k d
+ h/Di + m/k,
(6)
where kd and k , are the disperse-phase and continuousphase mass-transfer coefficients, respectively, Ar is the width of the particle-water interface, m is the slope of the equilibrium curve between the two phases, and Di is the diffusivity of monomer in the interface. The value for [MI: can be determined by solving the appropriate thermodynamic equations. These are available in the literature (Ugelstad et al., 1982). Addition of a swelling agent can increase the equilibrium concentration of monomer within the particle. This would reduce the driving force for shrinkage of a particle and enhance emulsion stability. The other theory postulated as the mechanism of enhanced droplet stability resulting from cosurfactants involves the formation of a liquid crystal. Here, the surfactant and cosurfactant form a liquid crystal at the monomer-water interface which acta as a barrier to coalescence and mass transfer. This results in a decrease in the mass-transfer coefficient which decreases the swelling rate. This theory is applied to cosurfactants with polar end groups such as long chain fatty alcohols. The support for this theory lies in the method of preparation of the emulsion as well as in experimental interfacial tension measurements. It is well-known that preparation of a stable monomeric emulsion with fatty alcohol cosurfactants requires preemulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol cosurfactant in the water prior to monomer addition, it is believed that a crystallized structure forms from the two surfactants. The monomer diffuses through the aqueous phase to swell the crystalline structures. For long chain alkanes that are strictly oil soluble, homogenization of the premixed oil phase is required to produce a stable emulsion. Although both cosurfactants produce relatively stable emulsions, Azad et al. (1976) have shown that the alkanes will produce emulsions of higher stability. A 1:3 molecular ratio of surfactant to cosurfactant has been shown to provide optimal stability in emulsion systems where the cosurfactant is a fatty alcohol. Shah (1971) postulated that this ratio is due to an optimum alignment of surfactant and cosurfactant molecules at the interface in microemulsions.
Experimental Methods Materials and Equipment. Methyl methacrylate (MMA, CH2=C(CH3)C02CH3; MW = 100.13), inhibited, was supplied by Rohm and Haas. The inhibitor (meth-
ylethyl hydroquinone, 10 ppm) was not removed to avoid polymerization induced from exposure to ultraviolet light. Styrene (STY, C6H5-CH=CH2;MW = 104.15), inhib ited, supplied by Fisher Scientific, was used as received. Vinyl acetate (VAC, H3CCOOCH=CH2;MW = 86.19), inhibited, supplied by Aldrich, was used as received. Sodium dodecyl sulfate (SLS, C12H250S03"a; MW = 288.38), 99% specially pure, supplied by BDH Limited, Poole, England (available in U.S. through G d a r d Schlesinger Chemicals Manufacturing Co., Carle Place, New York), was used as received. Surface tension measurements revealed no dip at the critical micelle concentration (cmc) which would indicate impurities. The value of the cmc at room temperature (20 "C) in pure water was found through conductance measurements to be 7.4 f .15 mmol/(L of H20). Hexadecane (HD, CH3(CH2)&H3; MW = 226.431, certified grade, supplied by Fisher Scientific, 99.3% pure, was used as received. Water was deionized (DI) prior to use (conductance was measured to be
