Particle Size Distribution II - American Chemical Society

Chapter 18

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 16, 2015 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch018

Eluant Composition Effects on the Separation Factor in Capillary Hydrodynamic Fractionation (CHDF) 1

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J. Venkatesan, J. G. DosRamos, and C. A. Silebi 1

Department of Chemical Engineering and Emulsion Polymers Institute, Lehigh University, Bethlehem, PA 18015-3590 Matec Applied Sciences, Hopkinton, MA 01748 2

The effects of the molecular weight, concentration and eluant velocity on the separation factor of monodisperse polystyrene latexes in CHDF are reported for several surfactants. The surfactants used include ionic and nonionic surfactants with different hydrophobic groups and hydrophilic-lypophilic balance (HLB). The results show that the molecular weight and concentration of the nonionic surfactants have a significant effect on the separation factor. When the high molecular weight surfactant contains considerable amounts of ionic impurities, their separation factor is smaller than either the cleaned surfactant or a lower molecular weight surfactant relatively free of ionic impurities. With the lower molecular weight ionic surfactant, sodium lauryl sulfate (SLS), a decrease in its concentration results in increases in the separation factor. In general, at equivalent ionic strengths the values of the separation factor were greater with nonionic surfactants than with the low molecular weigth ionic surfactant. The separation factors determined with the lower molecular weight anionic surfactant were used to determine the hydrodynamic thickness of the layer of non-ionic surfactants adsorbed on the latex particles. An estimation of the thickness of the adsorbed nonionic surfactant led us to conclude that the osmotic repulsion is effective at distances greater than the thickness of the adsorbed surfactant. The application of flow through packed column methods to characterize the size of colloidal particles was first described in 1971 by Krebs and Wunderlich (1), who observed a difference in elution times when polymer latex particles were suspended in a fluid pumped through columns packed with porous silica. Five years later H . Small(2), using columns packed with nonporous beads, pioneered the development of column particle chromatography; this technique has come to be known as hydrodynamic chromatography (HDC). One important characteristic of H D C is that it allows the size analysis of colloidal particles in different environments, such as 0097-6156/91/0472-0279$06.00/0 © 1991 American Chemical Society

In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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P A R T I C L E S I Z E D I S T R I B U T I O N II

those used in the final formulation of polymer latexes in which different pH, surfactants, thickeners and electrolytes are used at various concentrations. The separation mechanism in H D C has been described in terms of the convected Brownian motion of the particles through the interstitial region, modeled as an array of interconnected capillary tubes, and subject to colloidal forces and steric exclusion effects(3-5). According to the theoretical analysis, the mechanism of size separation by flow through conduits is due to two effects: (i) the laminar velocity profile of the fluid inside the conduit and (ii) the steric exclusion of the particles from the slower velocity streamlines next to the wall of the capillary. Because the smallest particles can approach the wall most closely, where the velocity approaches zero, they will on the average be the slowest to move down the capillary. Thus, because of these two effects, die average velocity of the particles will be greater than that of the eluant, with the average velocity of the particle increasing with the size of the particle. In addition to these two effects, the average velocity of the particle is also affected by the interaction between the particles and the wall of the capillary arising from the double layer electrostatic repulsion and the van der Waals attraction. Increasing the electrostatic repulsion between the particles and the surface of the packing will force the particles to radial positions further away from the wall of the capillary. Not surprisingly, a few years later, narrow bore capillary tubes were used to separate by size micrometer sized particles(6-8). Although these studies reported differences in elution times of different size submicron particles, the resolution obtained was not good enough to obtain analytical separations of bimodal mixtures of submicrometer sized particles, primarily due to the large diameters of the capillaries used. In all these earlier investigations, the diameter of the capillaries used were greater than 100 micrometers. Recently, de Jaeger et al. (9) improved the resolution of particle separation in capillary tubes by using gelatines of high molecular weights and several synthetic water soluble polymers which, when dissolved in the eluant stream, adsorb on both the capillary wall (reducing the effective capillary diameter) and the particle surface (increasing the effective particle size). These investigators were able to obtain partial separations of submicrometer particles from samples containing mixtures of different monodisperse standards. Although the values of the separation factor, Rf (dimensionless rate of transport, experimentally obtained from the ratio of peak elution time of a molecular species to the peak elution time of the colloidal particle), reported by de Jaeger et al. were greater than those obtained by Small (2) in separations by flow through non-porous packed columns (HDC), the fractionation obtained was not better than that in HDC, primarily due to excessive axial dispersion caused by the increase in eluant viscosity and the large diameter of the capillary tube. Although the best gelatine based eluant system used by De Jaeger et al. (9) in their capillary hydrodynamic separations was a high molecular weight gelatine, these investigators found that the differences observed were not due solely to variations in the molecular weight of the gelatine in the eluant, which creates a steric barrier both on the capillary wall and the particles, but also to the fact that the gelatine is an amphoteric polyelectrolyte, providing an electrostatic repulsion between the particles and the capillary wall which forces the particles to travel in higher velocity streamlines away from the capillary wall. In their experiments with polyvinylalcohol and carboxymethylcellulose in the eluant, the separation obtained resulted in smaller values of R than those obtained for the gelatines. Furthermore, the use of polystyrene sulfonic acid in the eluant made it impossible to obtain a stable baseline. Nonionic surfactants have also been used by de Jaeger et al. in order to increase the separation factor in capillary tubes. In particular, De Jaeger et al. also studied the effect of concentration of Pluronic F109, a nonionic water-soluble block copolymer f



18. V E N K A T E S A N E T A L .

Eluant Composition Effects on Separation

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surfactant (molecular weight 14,600 Daltons and H L B >24) produced by B A S F , and found that a 1.5% by weight concentration of Pluronic gave the highest Rf value and the narrowest fractogram. Results with other nonionic surfactants in flow through packed columns H D C have also shown a very small increase in the separation factor (10). Apart from these studies, no systematic studies have been carried out on the effect of well-defined long chain nonionic surfactants on the separation factor. In particular, the influence of the molecular weight of a specific polymer and the length of either the hydrophobic or hydrophilic groups in the molecule of different homologues on the same latex can be expected to affect the separation behavior. We have recently reported the analytical separation of submicrometer particles by capillary hydrodynamic fractionation using smaller diameter capillaries(l 1-14). Our theoretical results show that the repulsive interaction between the surface of the capillary and the colloidal particles play a significant role on the fractionation. Knowledge of the interaction potential between the particles and the wall of the capillary in the presence of adsorbed layer of polymer thus provide important information for the prediction of the separation factor of the collodal particles. Thus, a more detailed understanding of the adsorption characteristics on colloidal particles would also make it possible to predict the effect of adsorbed polymers on the elution behavior of the colloidal particles in capillary hydrodynamic fractionation (CHDF). Interfacial properties of polymer solutions are of interest in many fields. One aspect which has attracted great interest is the interaction between solid surfaces separated by a polymer solution. The presence of polymer chains may influence the forces between the surfaces in two different ways, depending on the nature of the interactions between the macromolecules and the surfaces: (i) if this interaction is attractive to both surfaces, the polymer chains are adsorbed. In such a case, the effective interaction between the surfaces is repulsive (15-17). (ii) When the surface-polymer interaction is repulsive or vanishes, the situation is quite different. The concentration of polymer vanishes at the surface and increases with the distance from the surface, reaching the bulk concentration at a distance of the order of the correlation length of the polymer in solution. This leads to a positive interfacial energy and therefore to an attraction between the surfaces since the system tends to minimize its totalfreeenergy(18). Since both the ionic strength and the presence of nonionic surfactants in the eluant can affect the separation factor, in this study we used eluant solutions containing different concentrations of an anionic surfactant: sodium lauryl sulfate (SLS); as well as solutions of two nonionic water-soluble surfactants of different molecular weights: Pluronic (BASF) and Triton (Rohm and Haas). These two nonionic surfactants were chosen for our study since nonionic surfactants of the polyethylene oxide)/ alkyl ether type are extensively used as emulsifiers and dispersants of polymer latexes and are known to be adsorbed onto most of them including polystyrene latexes( 19,20) as well as silica surfaces(20). Another important factor in the selection of the nonionic surfactants is based on the interaction between the particle surface and the surfactant molecules. It has been reported that, in the adsorption of anionic and nonionic emulsifier on polystyrene latex particles, the adsorption decreases with decrease in hydrophobicity of the surfactant i.e. increasing H L B . A n optimum H L B range was found to be 14-15 for maximum adsorption of nonionic surfactants at a polystyrene latex particle. The effects of the surfactant's molecular weight, concentration and type on the separation factor at different eluant average velocities are reported. Several studies in our labs and elsewhere on the adsorption of these anionic and nonionic surfactants on latex particles of different chemical composition have been published (19-21), allowing us to choose the appropriate concentration of the surfactants in the eluant.


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Experimental Instrument. The C H D F flow system used has been described previously (6,7); a schematic diagram of the instrument is shown in Figure 1. It can be described briefly as follows: from a reservoir, a dilute solution of surfactant is pumped by a Laboratory Data Control Mini-Pump (Model 2396) equipped with a pulse dampener. The surfactant solution flows through the injection valve (Rheodyne 7125), where 20 |xl of the sample is loaded and injected into the continuously flowing surfactant solution. The sample is diluted before injection to a weight fraction of approximately 1% solids with a solution having the same composition as the eluant pumped through the microcapillary. In this study, the eluant used is an aqueous solution of either a nonionic or an anionic surfactant. Prior to use, the eluting solution is filtered through a 1.0 micrometer Nucleopore filter membrane to prevent extraneous material from fouling the system. The eluting solution is delivered to the microcapillary at a constant pressure using a Milton Roy Mini-Pump. Since the flow in the microcapillary is laminar (the calculated Reynolds numbers were always less than 1), and the eluant is a newtonian fluid, the inside diameter of the fused silica capillary tube used in this study was determined hydrodynamically from pressure drop-average fluid velocity measurements, using the Hagen-Poiseuille equation (22). The inside diameter obtained from the Hagen-Poiseuille equation was 6.5 um. The length of the microcapillary used was 2 m. The detector used is a Laboratory Data Control Model SM4000 variable wavelength spectrophotometer. Materials. A series of very uniform polystyrene latices with diameters ranging from 0.088-0.357 \im were used in this study. Typical sample preparation consisted of diluting a few drops of the concentrated latex dispersion in about 2 ml of the corresponding eluant used in the C H D F unit. Following sonication for about 1 minute, the sample was injected into the CHDF eluting stream. Sodium lauryl sulfate (SLS), 98% pure (Stephan Chemical Company), was purified by recrystallization from boiling ethanol, followed by extraction with anhydrous ethyl ether (Fisher, certified grade) and then dried in a vacuum oven. In this study we used two kinds of commercially available nonionic surfactants: Pluronic (BASF Corp.) surfactant and Triton (Rohm and Haas) surfactant. Pluronics are water soluble polyethylene oxide-polypropylene oxidepolyethylene oxide block copolymers where the propylene oxide is the hydrophobic group which will be preferentially adsorbed onto the surface of the more

SURFACTANT RESERVOIR

MAKEUP FLUID

COMPUTER

PUMP CHDF TUBE

UV DETECTOR

PUMP WASTE

Figure 1: Schematic diagram of C H D F apparatus.


18. V E N K A T E S A N E T A L .

Eluant Composition Effects on Separation

hydrophobic polymer surface of the latex particles. The molecular structures of these surfactants are: Pluronic H O - ( C H C H 0 ) - C H 2 C H ( C H 3 ) 0 ) Y - ( C H 2 C H 2 0 ) X H, polyethylene oxide-polypropylene oxide-polyethylene oxide block copolymer. Pluronics of three molecular weights were used in this work: Pluronic (BASF) L35, F68 and F127 with molecular weights of 1900 (hydrophobe: 950), 8400 (hydrophobe: 1680) and 12600 (hydrophobe: 3780) respectively, their corresponding H L B values are: 18-23, >24 and 18-23 in order of increasing molecular weight. The Triton surfactant used are octyl phenol polyether alcohols. The molecular structure of the Triton used in this work is: C s H n C e H ^ C ^ R i J x O H with values of x = 10, 40 and 70 (with commercial names Triton X100, X408 and X705); their molecular weights are: 628, 1966 and 3286 respectively, with corresponding H L B values of 13.5, 17.9 and 18.7. They will be denoted OP-E10, OP-E40 and OP-E70, respectively. The Pluronic surfactants were used in two forms: first, as received from the manufacturer and second, after removing the ionic species present in the commercial surfactant. The presence of ionic species, as impurities or byproducts from the manufacturing process of the surfactant, in some of these nonionic surfactants was made evident by comparing the conductance measurements (shown in Table I for the three molecular weights used in this study) for aqueous solutions using the cleaned and the commercial Pluronic and Triton surfactants at a concentration of 0.5% by weight. Also included in Table I are the conductivities of several concentrations of the anionic surfactant SLS. The electrolytes in the surfactant solution were removed by stirring an ion exchange resin in the solution until the electrolyte level was brought down to the desired concentration. The electrolyte level was monitored by conductance measurements. The ion exchange resin comprised of a mixture of a cationic exchange resin (20-50 mesh hydrogen form) and an anionic exchange resin (20-50 mesh chlorine form), manufactured by Bio-Rad laboratories. The total amount of ion exchange resin used was approximately 10% of the weight of surfactant in solution. The cationic and anionic resins were mixed in 50:50 weight ratio. The water was distilled and deionized, with a conductivity less than 1 \iS. 2


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Table I. Conductance of 0.5% by Weight Aqueous Solutions of Nonionic Surfactants and several Molar Concentrations of SLS Surfactant

Molecular weight (Daltons)

Conductivity (\iS) Commercial

Pluronic L35 Pluronic F68 Pluronic F127 Triton X100 Triton X405 Triton X705 SLS l m M SLS 0.5 m M SLS 0.1 m M

1900 8400 12600 628 1966 3286 288 288 288

0.9 7.3 100.0 7.5 11.0 8.7

Cleaned 0.9 1.5 1.3

— — —

57.5 33.0 7.8


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The Separation Factor: Particle separation has been quantified experimentally in terms of the separation factor, Rf, determined experimentally by the ratio of elution times associated with the marker peak and the particle peak. Thus (1) where t and tp denote the elution time of the marker and particle peaks respectively. In order to prevent flocculation of the latexes, induced by the ionic marker species, both the marker species and the monodisperse standards are injected separately and their elution times measured.


m

Results and Discussion The long chain nonionic surfactant should be used at a concentration at which the latex particles will be completely covered with the adsorbed long chain nonionic surfactant, since it is well known that partial coverage may result in flocculation of the colloidal dispersion by particle bridging. Adsorption studies have shown that a concentration range of 1.5-3 times the critical micellar concentration of the nonionic surfactant will be sufficient to completely cover the particle surface with the adsorbed surfactant (19). Since full coverage is not reached at the critical micelle concentration but at slightly higher concentrations, the affinity of the hydrocarbon tail to itself must be greater than its affinity to the slightly more polar surface of the polymer particle. The effect of the ionic strength on the separation factor in C H D F has been previously reported (11) and is further illustrated in Figure 2 for the microcapillary used in this study. As shown in this figure, the separation factor increases with both increasing particle size and decreasing eluant ionic strength.

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Particle Size Distribution II - American Chemical Society

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