Journal of Colloid and Interface Science 256, 3–15 (2002)doi:10.1006/jcis.2002.8597

FEATURE ARTICLE

Simple Colloids in Simple Environments Explored in the Past, ComplexNanoids in Dynamic Systems to be Conquered Next: Some Enigmas,

Challenges, and Strategies

P. Somasundaran

NSF/IUC Research Center for Advanced Studies in Surfactants, Langmuir Center for Colloids and Interfaces, Columbia University, New York, New York 10027

Received March 6, 2002; accepted July 12, 2002

The behavior of colloids is governed to a great extent by theirinteractions with various chemicals which in turn are influencedby their surface properties including electrostatic charge. In thepast, mechanisms of charge generation on simple solids, such asoxides, and salt types, such as calcite, and adsorption of simplespecies, such as chlorides, alkyl sulfonates, and water-soluble poly-electrolytes, on them have been explored and well understood.Microstructures of adsorbed layers have been probed using spec-troscopic, electrokinetic, and scanning microprobe techniques andthe proposed aggregation of surfactants and polymers at interfacesto form solloids1 (“Surface cOLLOIDS” such as hemimicelles) hasbeen verified. Interactions in simple systems have also received con-siderable attention. The challenge now lies in understanding com-plex real-life systems consisting of mixed multicomponent solidsin media containing a variety of organic and inorganic species.The most exciting results will possibly come from studies of thecolloidal behavior of dynamic natural colloids such as microbes.In this treatise some of the enigmas in real life systems and thechallenges involved in understanding and utilizing them are ex-plored. Future colloids and nanoids2 will be designed so that theadsorbed layers, like the living entities, will respond to exter-nal perturbations and reorient/reconform themselves for optimumbehavior. Based on the dynamics, one can envisage novel materi-als and processes that will perform a specific function in responseto externally controlled perturbations. These novel systems willfind applications for functional coatings and films, signal transmis-sion devices, nanoelectronic circuits, and drug delivery and toxinextraction. C© 2002 Elsevier Science (USA)

1 Solloids refer to surface-mediated colloids or colloids made up of any chem-ical moieties on surfaces. While they include such entities as hemimicelles, ad-micelles, etc., it is noted that these latter items cannot truly represent assembliesmade up of differently shaped and structured surfactants, polymers, proteins, ortheir mixtures.

2 In contrast to colloids which cover a wide size range, nano particles in the1- to 100-nm range are referred to here as nanoids to emphasize the significanceof their supramolecular nano size range.

3

INTRODUCTION

Colloids respond to various forces in nature in remarkableways and in turn enhance numerous facets of our own lives.Our best discoveries have been made by simulating nature andthus the artificial colloids in commercial or household systemsexhibit behavior similar to that of those that occur naturally. Allthese systems, natural or man-made, are dynamic in nature andentertain many important interactions between their variouscomponents. Basic colloidal systems can be represented by thetetrahedron shown in Fig. 1 with surfactants, polymers/proteins,solids, solvents, and gases/oils occupying different criticalpositions, the behavior of the systems being determined by thedynamics of the interactions between the moieties in cruciallocations. Future scientific and technological advances dependon fully understanding such interactions and utilizing themefficiently.

While the properties of single components in water underequilibrium conditions are well understood today, those of realdynamic multicomponent systems have yet to be explored tothe extent that their behavior could be predicted. Interactionsin single and, to some extent, binary systems have been stud-ied in the past, for example, to determine the adsorption prop-erties of surfactants and polymers on solids. Complex naturalor industrial systems can, however, be understood completelyonly if the nature of the dynamics of interactions among sev-eral components is known. A knowledge of the colloidal be-havior of mixed solids in mixed surfactant/polymer solutionsand their synergetic/antagonistic responses, both under staticand dynamic conditions, has to be generated to develop re-liable predictive methodologies and therefrom to design newtechnologies or to modify the current ones. For example, thedevelopment of new drugs or new safe delivery and removalsystems depends on building a knowledge base of interactionsamong lipids, polymers and proteins as well as such minerals asapatite.

0021-9797/02 $35.00C© 2002 Elsevier Science (USA)

All rights reserved.

4 P. SOMASU

Gas/Oil

Surfactants

Polymers/Proteins

Solids

Solvents

FIG. 1. Diagram illustrating possible interactions among usual componentsin colloidal systems.

Anyone working on multicomponent colloidal systems, nor-mally encountered in industries such as minerals, specialtychemicals, personal care and cosmetic products, agricultureproducts, paints and inks, textiles, or microelectronics, wouldhave encountered many complex and sometimes puzzling prob-lems that remain unresolved today. Components relevant to mostof these colloidal systems are minerals, surfactants, polymers,proteins, bubbles, and droplets. This treatise will start with someof the simpler single component systems of minerals, surfac-tants, and polymers; move toward increasingly complex sys-tems; and end with selected real-life examples made up of thecomponents that are of significant relevance to us. Challengesin understanding the interfacial responses of colloidal playersto perturbations in system parameters such as pH and ionicstrength, or in forces such as that due to electrostatic fields,are explored. The development of smart micro- or macrosys-tems will depend to a large extent upon our ability to monitorand control the concentration as well as the conformation andeven the orientation of the species and structures at interfacesusing some of these parameters and forces.

MINERALS

While properties of minerals are determined primarily by theirconstituent species and the surrounding medium (1), they arealso influenced markedly by the conditions under which theyare prepared, stored, and introduced into the system (2a, 2b).For example, in the case of apatite, the point of zero charge of asynthetic sample is about 7, while that of a natural one is about5.5 and, as expected, is shifted markedly by the addition of itsconstituent species such as phosphate, calcium, and even fluoridewhich can substitute into the mineral lattice (3). Furthermore,its zeta potential is dependent on the method of preparation andcleaning. The point of zero charge (pzc) of even a simple solidsuch as alumina has been shown to vary from 3 to 10, dependingon the source, the method of preparation, and the technique usedfor monitoring the electrokinetic properties (4a). In the case ofquartz, variations in the cleaning and aging conditions alone can

alter its pzc from below 2 to as high as 6 (2b). Much of the con-

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troversy in literature on the behavior of mineral solids can in factbe attributed to these variations, particularly from laboratory tolaboratory. It is important to recognize that these properties canundergo further critical changes if other solids are also presentin the solution. This is especially true in the case of semisolubleminerals such as calcite and apatite which can undergo dissolu-tion and then precipitation of the dissolved species on the surfaceof each other. This effect has been clearly illustrated in the caseof the apatite/calcite system. Thus, apatite surface is convertedto that of calcite if left in calcite supernatant, but surprisingly thereverse conversion where calcite is converted to apatite is alsopossible if the former is left in a supernatant of the latter (Fig. 2)(5–7). It should be noted that such surface alterations can pro-duce drastic effects on processes such as flotation; the effectof apatite supernatant on calcite flotation using oleate has beenshown to be to depress it completely. Interestingly, the effectof its own supernatant on the flotation of calcite is significant,suggesting the importance of the kinetics of bulk interactionsbetween the surfactant and the dissolved species in comparisonto that of its adsorption on the mineral. Similar effects have beenobserved for a number of other systems as well (8–12).

Challenges. In the case of minerals, in view of the widevariations reported in surface properties, it is important to firstestablish the optimum conditions for the preparation and treat-ment of particles to generate “ideal” surfaces lest discrepanciescontinue in the literature on the behavior of colloidal particles.The natural question then is what constitutes an ideal or repre-sentative solid and what values should be used for surface prop-erties for developing theories regarding the behavior of particles.

FIG. 2. Zeta potential results illustrating surface conversion effects of cal-cite supernatant on apatite and vice versa (5–7).

COMPLEX NANOIDS IN

Moreover, how should such surfaces be obtained and conserved?One proposed way of obtaining a nearly ideal surface is to ageit under isoionic/isoelectric conditions that might yield surfacecomposition and structure close to that of the bulk (4a). Asindicated previously, marked alterations in the interfacial andcolloidal behavior of individual components are observed whenmany solids are present in the system. It would be useful to iden-tify the species and processes responsible for such conversionsand correlate with the thermodynamical demands of the system,as has been done for the conversion of calcite to apatite and viceversa on the basis of the solution chemistry of the apatite/calcitesystem (5, 6). Identification of the surface reactions responsi-ble for conversions in mixed solid systems is important as itcan help in designing reagents that can complex with interferingspecies and thus prevent the detrimental effects observed in theprocessing of solids (8, 13–15). Similarly, a thorough knowl-edge of the reasons behind the source-, preparation-, and aging-dependent variations in surface properties is critical for makingfurther progress in the increasingly difficult task of separatingsolids from each other in the ultrafine and nano ranges. Themajor challenges in this area were explored during two NationalScience Foundation workshops on the beneficiation of mineralsthat were conducted almost two decades ago (16, 17). Unfor-tunately, almost all the challenges raised at that time for theproduction, separation, and processing of solid fines still remainto be met (18).

SURFACTANTS

Modification of surfaces by the adsorption of surfactants orthe prevention of such adsorption is crucial for the successful op-eration of many commercial processes. Thus, while separationof valuable materials from wastes or ores by flotation dependson the selective adsorption of surfactants on specific solids in amixture, micellar flooding for enhanced oil recovery depends onthe minimum adsorption of surfactants on reservoir rocks. How-ever, wettability does not appear to be controlled by adsorptionalone (Fig. 3). It can be seen that in these cases not only is theamount of adsorbed surfactant important but also the manner inwhich it adsorbs. For example, orientation of hydrophobic tailstoward the bulk solution and away from the mineral surface isimportant for flotation, while the reverse orientation is requiredfor keeping the rock oil repellant for enhanced oil recovery.

In the case of surfactants also, the behavior of single systemshas been well studied (19–24). An example is the adsorptionisotherm of dodecyl sulfonate on alumina which is character-ized by four distinct regions. Using a multipronged approachinvolving advanced spectroscopic techniques based on fluores-cence, electron spin resonance, NMR, and resonance Raman,along with the conventional techniques of electrokinetics, flota-tion, and aggregation, the formation of solloids (surface colloidsor assemblies on the solid) with orientation that has marked ef-

fects on such interfacial processes as flotation and flocculationhas been demonstrated (Fig. 3) (25). This approach identified

DYNAMIC SYSTEMS 5

FIG. 3. Correlation of adsorption of doecyl sulfonate on alumina particleswith their hydrophobicity (25a, 25b).

the isotherm to be the result of different combinations of elec-trostatic and hydrophobic mechanisms dominating in each re-gion. While initially only electrostatic forces are in effect, abovea certain adsorption density, association between the adsorbedhydrocarbon chains begins to dominate the adsorption. Once thesurface charge is nearly neutralized by the surfactant counteri-ons, only the associative interactions are operative. The plateaucorresponding to the maximum adsorption is dictated by micelleformation in the bulk or monolayer coverage. The initial partsof the isotherm are adequately represented by the equation

�δ = 2rC exp

(− zeψδ + nφ

kT

),

where �δ is the adsorption density, C is the bulk concentration,r the radius of the adsorbate, z is its charge, ψδ is the potential atthe Stern plane, n is the number of carbon atoms in the chain, andφ is the association energy per CH2 group. There are models thattake into account important parameters such as the heterogeneityof the samples (26, 27). Nevertheless, no universal model thatcan adequately represent the adsorption of surfactants in gen-eral on solids in different media exists. Adsorption will dependon the nature of the functional group as well as on the prop-erties of the solid and the solvent. While an increase in chainlength does increase the adsorption from aqueous solutions asexpected, even an apparently minor change in the position of thefunctional groups or branching can alter adsorption by orders ofmagnitude, with corresponding changes in the hydrophobicityof the substrate (Fig. 4) (28–30); the reasons for these effectsare not yet completely clear. Another interesting effect is thatof ethoxyl groups used in controlling the wettability of solids.Surfactants containing 40 ethoxyl groups have been reported toimpart hydrophobicity in a narrow concentration window, but

the ones containing 10 ethoxyl groups yielded a half-window atmuch higher concentrations (31). A puzzling observation in this

6 P. SOMASU

FIG. 4. Effect of the position of the ionic sulfonate groups with respectto the methyl groups in the ring (para vs meta) on the hydrophobicity of theparticles (28).

regard was the lack of adsorption of ethoxylated compounds onalumina in comparison with their marked adsorption on silica,which has been only partially accounted for by considering acid–base properties of the solid (32a). This behavior is quite oppositeto that of the sugar-based surfactants which adsorb on aluminaand titania with solloidal aggregation but not on silica (32b, 33).The implication of the nature of the surfactant structure formedon solids in practical processes such as flotation, detergency,and micellar flooding should be noted. While we have some un-derstanding of the general effect of the structure, it is not clearhow exactly the molecules are oriented in such a structure orhow the structures are affected by the type of surfactant, solid,and medium or by the history of the systems (mixing, purity,oxidation, etc.). Most importantly, very little is known about thedynamics of the system, i.e., the manner in which the moleculesapproach or leave the surface or reconform while in the adsorbedstate itself. Similarly, there is no information available on howthe molecules respond when the system is perturbed: do theyrearrange individually, collectively, or in patches, and are theintermediate conformations the ones that rearrange to the finalstructure?

Nonaqueous systems. In contrast to adsorption in aque-ous systems, adsorption from nonaqueous systems has receivedscant attention. In a study of the adsorption of surfactants on alu-mina from cyclohexane, the cationic surfactant/dimethyl dode-cylamine was found to adsorb more on acidic surfaces, whereasthe anionic Aerosol OT adsorbed on basic surfaces, suggestingthat adsorption probably occurs through interactions betweenthe polar groups on the surfactant and the solid (Fig. 5) (34).Increase in the solvent polarity caused a decrease in adsorption.Correlation of the properties of the solid, the surfactant, and themedium showed the maximum adsorption to occur when therewas maximum difference between the solubility parameters of

the surfactant and the solvent and between those of the solidand the solvent and minimum difference between those of the

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FIG. 5. Adsorption of anionic Aerosol OT and cationic dodecyl amineon alumina and silica in cyclohexane solution (34), Aerosol OT/silica (�),Aerosol OT/alumina (�), dimethyldodecyl amine/silica (�), dimethyldodecylamine/alumina (�).

surfactant and the solid (35). Floculation of polar materials innonpolar medium is prevented by the addition of a surfactantsuch as Aerosol OT. However, this is effective only within aspecific concentration window of water (Fig. 6) (36). Electronspin resonance experiments suggest that above monolayer ad-sorption, subsequent water uptake leads to the formation of afloating water film on the solid which in turn can induce flocu-lation by capillary bridging (37). Surprisingly, flocculation wasalso observed when the system was dry! This is proposed to bedue to the need to have some water on the surface of alumina toallow the ionization of the adsorbed surfactant molecules.

Mixed surfactants. While most of the fundamental work onsurfactants has been conducted till now with single surfactants, itis recognized that commercial and natural systems are invariablymixtures. The components in a mixture could enhance the effi-ciency of processes due to synergy or even antagonism amongthem under different conditions. While there has been some sig-nificant development in the treatment of solutions the effects ofsuch surfactants (50) have not yet been adequately explained byany theory. In most cases, adsorption seems to be determined by

FIG. 6. Effect of water on the stability of alumina suspension in cyclohexane(36).

COMPLEX NANOIDS IN

FIG. 7. Concentration of monomers of the nonionic ethoxylated nonyl phe-nol and cationic tetradecyl trimethyl ammonium chloride from their mixtures(39).

the concentration of the monomers rather than by the concen-tration of the micelles, yet quantitative treatment is hamperedby the difficulty in describing the monomer concentration itself.For example, for a mixture of cationic tetradecyl trimethyl am-monium chloride and nonionic ethoxylated nonyl phenol, theconcentration of the cationic monomer could be reasonably de-scribed by the regular solution theory, but the concentration ofthe nonionic surfactant was very different from the theoreticalvalues (Fig. 7). This has been proposed to be due to the formationof more than one type of micelle and their coexistence. Whileintuitively unacceptable, coexistence of numerous phases or, ingeneral, diversity is hardly uncommon in nature.

Challenges. Earlier works with surfactants have probablyresulted in as many questions as answers. The important andchallenging ones include how exactly do the surfactants orientindividually and collectively and why is the hydrophobicity sodrastically affected by the position of the functional groups as inthe case of para- and meta-xylene alkyl sulfonates, what is therole of groups such as the methyls in shielding the main chainfrom the ionic group and thus possibly improving the hydropho-bicity of the surfactant and in the case of hydrophilically mod-ified surfactants such as the ethoxylated alkyl sulfonates, howare the ethyl groups orienting that affects the wettability of thesolid surface? Equally probing are questions such as why do the

ethoxylated surfactants adsorb more on silica than on alumina,while the reverse is true for sugar-based surfactants, and what

DYNAMIC SYSTEMS 7

determines adsorption from nonaqueous media, and what arethe properties of the adsorbed layer that are responsible for theobserved behavior of particles in such suspensions? Some of theother questions that warrant attention include what leads to syn-ergy in some cases and antagonism in others in the case of mixedsurfactants, why do some components from a mixture preferen-tially adsorb at interfaces, do different types of micelles coexist,what leads to their stability and can the reasons be quantitativelystated and utilized to create new nanoids, and, finally, can suchnanoids be assembled systematically for creating new super-structures and micromachines with unique capabilities. Ideally,a universal relationship or a set of relationships needs to be de-veloped between the structure of surfactants and their behaviorat interfaces. The most intriguing and challenging question is re-garding the dynamics of the interfacially active molecules whentheir adsorbed layers are perturbed. Indeed, there is a need inthis regard for developing new techniques to explore such dy-namics on a molecular scale. This is particularly true for systemscontaining more than one surfactant. Information on exchangesbetween various species and on how each rearranges as anotherapproaches would be useful for controlling the behavior of thesesystems and for developing smart nanosystems. Clearly, what isneeded is a systematic multipronged attack to study the adsorp-tion, wettability, charge, and conformational characteristics of aseries of surfactants with functional groups of various lengths,positions, and branches.

POLYMERS

Polymers are widely used currently for a variety of purposessuch as dispersion/flocculation, deposition/coating, adhesion,rheology, and insulation. Among the properties that affect theirperformance are adsorption and conformation, which in turnare determined by molecular weight, polydispersity, and func-tionalization, particularly with ionic and hydrophobic groups.Although synergetic combinations have attracted the most at-tention, their effects still need to be quantified in terms of thefundamental properties of polymers, solids, and solvents. Somekey behaviors and anomalies are outlined below.

Molecular weight effects. Polymers with high molecularweight are usually employed as flocculants while those with lowweight serve as dispersants. Recently, however, it was discoveredthat low molecular weight “dispersants” can act as flocculants ifused in ultra low dosages (40a). The zeta potential of particlesdid not show any change due to the addition of such polymersunder ultra low dosage conditions. Interestingly, as few as 10of the polymer molecules per particle were sufficient for mea-surable flocculation (40b), suggesting that patchwise bridgingis the reason for this type of behavior.

Molecular weight distribution. Polymers such as polyacry-lates adsorb on solids due to, among others, electrostatic, hy-

drophobic, and covalent forces, with marked effects on sus-pension stability. While normally adsorption increases with

8 P. SOMASU

FIG. 8. Competitive adsorption between low-molecular-weight (4600) andhigh-molecular-weight (million) polystyrene sulfonates n-hematite (43a–43c).

molecular weight (41, 42), the opposite was the case with theadsorption of polystyrene sulfonate on hematite under low-ionic-strength conditions (Fig. 8) (43). It has been proposed thatthis preferential adsorption of low-molecular-weight species onhematite is due to the ability of the smaller anionic molecules todiffuse to the surface faster than the larger ones and then keepsimilarly charged larger ones away by means of electrostatic re-pulsion by the adsorbed anionic species. When the ionic strengthis higher, the electrostatic repulsion is minimized, allowing thelarger species to get to the surface and even displace the smallerones. Higher adsorption of the low-molecular-weight specieswas observed also in the case of the adsorption of polyacrylateson porous alumina when pores were relatively inaccessible tothe larger molecules (44). Preferential adsorption has also beenfound to take place in certain systems when the solid density ofthe suspensions is increased. Surprisingly, both the adsorptionand conformation of the polymer seem to change as the solidloading is increased (45). Control of such behavior might providea means of countering severe dispersion problems encounteredduring the processing of concentrated slurries.

Challenges. Past work with polymers has generated innu-merable questions on their interactions with solids. For example,in the case of floculation by the so called “dispersants,” is theflocculation a dynamic effect where, even though no measur-able change may be detected on the “net” zeta potential, enoughpolymers may dangle out at a given moment to tether otherparticles? Or, is it instead a cooperative effect, where polymerassemblies form on such particles which in turn can bridge par-ticles with each other to form flocs? A complex task in thisregard is modeling the adsorption and the resultant flocculation,while taking into account all the forces, especially the dynamiceffects. In the case of preferential adsorption of smaller poly-mers under particular conditions and their displacement by largerones under certain other conditions, it would be useful to under-stand the dynamics of such displacements and correlate themwith changes in particle properties such as zeta potential during

adsorption. Some of the questions that remain to be answered

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include whether molecular weight gradient or heterogeneity ofthe potential on the surface lead to regions of lower and highermolecular weight polymers on the surface; whether a surfacepattern could be generated in a single step or programmed stepsusing this approach; in the case of porous particles whether thelarger molecules slowly snake into the pores and displace thesmaller molecules, and whether novel controlled surface reac-tions could be facilitated depending on the size and shape of thepores and the chirality of the species. Indeed, it will be usefulto have a clear understanding of the following: what are the ef-fects of such preferential adsorption on flocculation/dispersion,deposition/coating, rheology, and wettability; if the loss of sta-bility of dispersions on long-term storage will be eliminated bymonitoring and controlling the migration of polymers on thesurface; and whether concentrated suspensions might be bet-ter stabilized by controlling the chromatographic separation ofmolecules.

POLYMERS/SURFACTANTS/HYDROPHOBICALLYMODIFIED POLYMER COMBINATIONS

As mentioned earlier, in most natural and commercial sys-tems, more than one reagent is present and they often performdrastically differently from single ones (46a, 46b). Thus, a com-bination of anionic polystyrene sulfonate and cationic polyacry-lamide was able to flocculate alumina completely, while neitherone was able to cause any significant effect when used alone(46c). Another example of such unexpected dynamic effects isthe forced adsorption of anionic dodecyl sulfonate on anionicsilica by polyethylene oxide, with the subsequent eviction ofpolyethylene oxide off the surface by the sulfonate (47)! Fur-thermore, the sequence of addition of polymers and polymer/surfactant combinations has a measurable influence on thecoiling of the polymer even in the adsorbed state and onthe flocculation/dispersion behavior of the suspension (Figs. 9and 10) (48, 49).

Some polymers that contain both hydrophilic and hydropho-bic functional groups can flocculate or disperse hydrophobic orhydrophilic particles in aqueous or nonaqueous media depend-ing on how much or how it is added (Fig. 11) (50). These poly-mers can also form hydrophobic or hydrophilic microdomainsand act as carriers of reagents (42, 51a, 51b). Furthermore,the rheological behavior of these solutions is very sensitive tothe presence of such domains even when present in very smallamounts.

Challenges. The major questions in reference to the dual- ormultipolymers or polymer/surfactant systems include whetherthe observed codispersion effects are due to synergy or antago-nism in adsorption only or are also due to changes in the config-uration or orientation of the molecules, and how a change in ei-ther would affect the other. In the dodecyl sulfonate/polyethyleneoxide case, one might ask why the guest sulfonate species evictedthe host polyethylene oxide, whether the complexation betweenthe two in solution was more favored than that on the inter-

face, and, finally, what the dynamics of these complexations

N

COMPLEX NANOIDS I

0

200

400

600

800

1000

1200a

5×10-4 1×10-3 1.5×10-3

SDS AloneSDS/PAAPAA/SDS

SDS Concentration, M

Tur

bidi

ty, N

TU

0

1

10

100

1000

10000

0.1 1 10 100

0

0.1

0.2

0.3

Tur

bidi

ty, N

TU

Coi

ling

inde

x (I

e/Im

)

Concentration of Percol 757, ppm

0.1 M NaCl, pH=810 ppm py-PAA

TurbidityCoiling index

b

FIG. 9. (a) Flocculation of an alumina by polymer–surfactant (polyacrylicacid + sodium dodecyl sulfate) combination with different addition sequences(48). (b) Dual-polymer effects on conformation and flocculation of alumina withcationic Percol as a function of anionic polyacrylic acid (49).

are: did the complex change after adsorption, causing its des-orption? Clearly, it would be fascinating to follow the historyof molecules in systems composed of several such players.Some of the critical questions in the case of hydrophobically

Dual Polymer Flocculation Dispersion of Micro-flocs

FIG. 10. Effect of addition mode for dual-polymer flocculation.

DYNAMIC SYSTEMS 9

0.0

0.1

0.2

0.3

0 200 400 600 800 1000

Concentration of DAPRAL, ppm

Set

tlin

g R

ate,

mm

/s

0

20

40

60

80

100

Sol

id S

ettl

ed, %

Alumina/TolueneAlumina/waterGraphite/water

Hydrophobic chains

Hydrophilic chains

DAPRAL

FIG. 11. Dispersion of alumina and graphite with a hydrophobically mod-ified polymer, DAPRAL, in water and toluene solutions (50a, 50b).

modified polymers include what is the role of adsorption den-sity and the conformation of hydrophobically modified poly-mers on solids in the same molecule producing the flocculatedand dispersed states in aqueous and nonaqueous media using thesame molecule; how is the state of the suspension determinedby the way the functional groups on the polymer orient and howis the orientation governed by the polarity of the solid and themedium; is the adsorption/orientation reversible and if so whatare the dynamics involved; what is the size/shape and polarity ofthe microdomains formed, and how do these properties respondto perturbations in solution properties such as pH, ionic strength,and temperature.

LIPOSOME/SURFACTANT INTERACTIONS

Most biosystems are composed of proteins, carbohydrates,cholesterols, lipids, minerals, and many other rarer components.Interactions among these components determine the stability ofthe systems. Even simple combinations of the relevant speciesstudied show new and interesting effects. Thus, when a sur-factant such as dodecyl sulfonate is added to a liposome madeup of phosphatidyl choline and phosphatidic acid, initially thesize of the liposome increases and subsequently it is solubilized(52). Furthermore, while the addition of cholesterol stabilizesliposomes, proteins destabilize them; the reasons for this effectare still far from evident (53). Electron spin resonance studieshave shown the polarity and viscosity of liposomes to change tothat of micelles of dodecyl sulfonate at sufficiently high concen-trations of the surfactant (Fig. 12a) (53).These results suggestthat liposome stabilization by dodecyl sulfonate involves the ad-sorption of sulfonate on them, leading to an increase in size, andtheir subsequent disintegration into mixed micelles composed ofliposome components and dodecyl sulfonate, resulting in a dras-tic decrease in size (Fig. 12b). The actual processes by whichsuch disintegration takes place are not known. Some preliminaryresults suggest that phosphatidic acid would exit first, leadingto the weakening of the liposome structure and its dissolution.

Similarly, the mechanisms by which species such as cholesterol

10 P. SOMASU

Low S

DS

Liposome

High SDS

Mixed micelleSDS

PC

PA

b

FIG. 12. (a) Change in rotational correlation time of 5-doxyl stearic acidin liposome and buffer due to interactions with sodium dodecyl sulfate (53).(b) Depiction of solubilization of liposomes under low and high sodium do-decyl sulfate (SDS) concentrations (60). PC-Phosphatidyl Choline and PA-Phosphatidic Acid.

stabilize liposomes and proteins destabilize them are also notknown.

Challenges. In the case of the interactions of surfactantswith liposomes, the key question is regarding the mechanism thatdetermines the formation of mixed micelles from liposomes andthe surfactant versus the adsorption of the latter on the liposomes.In this area some of the avenues that warrant investigation arehow does the disintegration of liposomes take place; does somespecies exit first or do the liposomes simply fragment in onestep, and, if so, what critical events determine such a process;why is phosphatidic acid forced out by the added surfactant; andwhere are cholesterol and protein located in liposomes, why dothey affect the stability of the liposomes, and, finally, how doesit relate to the stability of biomembranes such as those that makeup skin or blood vessels?

APPLICATIONS

(a) Flocculation

As indicated earlier, under conditions of similar adsorptiondensity, flocculation or dispersion of a suspension can be ob-

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tained depending on the experimental path (Fig. 13) (54). Alsoa dispersant can give rise to flocculation and vice versa under dif-ferent dosage or usage conditions (40a, 40b). When more thanone polymer is used, the sequence of addition can also deter-mine the state of dispersion (46c–49). There are also biosystemswhere aggregation can take place, usually due to perturbationsin their surrounding chemical or physical environment. Thus,blood will clot when exposed to air and microbes will clumpwhen they encounter an appropriate substrate.

Challenges. The fact that flocculation or dispersion can beobtained at the same adsorption density raises questions aboutthe roles of other properties like configuration or orientationsuch as what is the optimum configuration of the polymer forflocculation/dispersion; can the floc structure be controlled bytuning in a particular configuration; why “dispersants” act asflocculants under certain conditions and vice versa; how doessequential introduction of reagents affect the configuration ofthe adsorbed species and the subsequent colloidal state of thesuspension; what is the dynamics (step by step processes) of anychanges in the configuration and dispersion and how can it bemonitored; and can the processes be described by a thermody-namic or kinetic model which can predict the effects of systemparameters, i.e., properties of the solvent, polymer/surfactant,solid, or electrolyte?

(b) Deposition/Coating/Nanocomposites

It has been observed that one-dimensional and two-dimensional polymer adsorption as well as flocculation can beobtained under low-polymer dosages (Fig. 14) (55). In one ofour studies, the adsorbed cationic polymer was spotted by deco-rating it with negatively charged silica spheres. In fact, differentpatterns (tiles, ellipsoids, rings, and discs) could be obtained bypretreating the substrate with various chemicals. Adsorption ofthe cationic polymer itself on glass was catastrophic and com-plete in less than a minute under relatively low dosage condi-tions (56).

8

Tra

nsm

itta

nce,

%

Set

tlin

g R

ate,

cm

/min

0

20

40

60

80

100

3 4 5 6 7 8 9 10 110

1

2

3

4

5

6

7

pH

Fixed pH

TransmittanceSettling

pH Upshifted

FIG. 13. Effect of mode of pH change (fixed versus shifted) on the floccu-

lation of alumina suspensions as measured by turbidity of the supernatant andsettling rate (54).

11

adsto

COMPLEX NANOIDS IN DYNAMIC SYSTEMS

FIG. 14. (a) One- and (b) two-dimensional and (c) binary deposition of

Most interestingly, zwitterionic particles were found to becapable of depositing on a substrate with similar negative chargewhile anionic particles of the same “net” charge were not! It wasproposed that this preferential deposition was due to the selectivestretching of the tethers on the particles: those tethers that arecomplimentary to the substrate reaching out and those that areantagonistic retracting. AFM experiments in fact showed thatonly the zwitterionic particles experienced an attractive force,while the anionic ones felt a repulsive force even though theycarried the same net charge (Fig. 15) (7). The similarity betweenthe behavior of latex particles and microbes with fimbriae is tobe noted.

Challenges. Some of the important issues here are determin-ing what controls the oriented deposition of polymers and if it isdue to surface heterogeneity, whether it is the result of chemicalor morphological variations, and what causes the catastrophic

orption which begins only after several seconds but proceedscompletion in less than a minute? In the case of zwitteri-

silica particles on silica mediated by controlled polymer adsorption (55).

onic particles, it would be instructive to learn if the stretchingof the complimentary hairs and the contraction of the antago-nist ones can be monitored in real time and whether they can becontrolled/perturbed by external fields? An intriguing thoughtthat follows is whether the forces in nature that stimulate/retardthe behavior of microbes with such tethers can be simulated tocreate smart particles?

(c) Nanocomposites and Nanofilms

Nanocomposites have been successfully synthesized usingcore–shell type particles produced by coating core particles withnano shell particles, layer by layer (8, 59). These particles havemechanical properties superior to those of ordinary composites.The production of such nanocomposites was facilitated by thecontrolled adsorption of polymers with appropriate functional

groups. Uniform distribution of particles with desired chemicalhomogeneity could be attained.

12

processes fail whas to be curr

P. SOMASUNDARAN

C

FIG. 14—

Challenges. In the synthesis of new types of nanocompos-ites, it would be useful to generate polymers with tethers thatcan deform as desired under different force fields, so that thenanocomposites made with them could be used for intelligentoperations. The challenge here would be to generate such com-posites with different exterior or interior patterns by the selectiveremoval or rearrangement of particles.

(d) Flotation

Minerals can be separated from each other using surfac-tants that adsorb selectively only on some of them. Adsorp-tion of such surfactants and other additives on single mineralsystems is governed by electrostatic, covalent, hydrogen bod-ing, and, in some cases, hydrophobic forces. When more thanone mineral is present, however, the adsorption phenomenonis very different and not predictable. Most importantly, these

hen applied to ultrafine minerals, the type thatently exploited due to the depletion of higher

ontinued

quality ore bodies. It has been noted that fine bubbles gener-ated by vacuum or electrolysis are capable of floating ultrafineparticles.

Challenges. There is a dire need at present for surfactantsand polymers that can selectively adsorb on desired mineralswhen present together. This is especially true when interferenceby dissolved species needs to be controlled. Although the effec-tiveness of microbubbles in flotation suggests new opportunities,why the fine bubbles are more effective, particularly with fineand ultrafine particles, is not known. Equally unclear is whetherthe observed phenomenon is due to increased capillary pres-sure inside the finer bubbles or some other dynamic process thatoperates to attach and detach particles during collision and thesubsequent transport of the pulp.

(e) Biosurfaces

The most fascinating challenges of the future probably lie inthe biointerfacial systems. The interactions of cells and microbes

13

same net negative zeta pticles (57).

COMPLEX NANOIDS IN DYNAMIC SYSTEMS

FIG. 14—C

FIG. 15. Variation of forces felt by anionic and zwitterionic particles of

otential as they approach negatively charged silica par-

ontinued

with each other and external agents are responsible for mostbeneficial and adverse physiological events. Growth as well asdestruction of cells begin and progress with interfacial and col-loidal interactions such as adsorption, permeation, and aggrega-tion. As described earlier, the disintegration of membranes andliposomes by dodecyl sulfonate was found to be caused by theadsorption of the latter, followed by mixed micellization. Theliposomes thus affected first grow in size and then fragment intosmaller units (60). Solubilization of phospholipids by surfac-tants has been studied by Lichtenberg et al. (61) and de la Mataet al. (62). Microbes themselves are found to multiply muchmore rapidly when contacted with certain mineral substrates.Interestingly, their structure is markedly altered by the typesof minerals and dissolved species that they encounter. Forexample, Bacillus polymixa grown in the presence of calciteis coated more with carbohydrate than proteins, while thosegrown in the presence of quartz are coated more with proteins(63). We also find the same material, pretreated chemically oreven thermally, to adsorb proteins very differently and possi-

bly control cell attachment (64). Thus, titania, treated by heat-ing, oxidation, or surfactant coating, adsorbed fibronectin very

14 P. SOMASU

differently. Moreover, titania that was oxidized and then treatedwith butanol adsorbed more fibronectin than others. Treatmentof minerals for extraction of values as well as removal of pollu-tants has also been achieved successfully in recent times by thecontrolled selection and mediation of microbes (65, 66). Clearlythis opens up new avenues for treating pollutants and recoveringvalues from wastes.

Challenges. A myriad of questions arise while examiningthe results obtained thus far in biosystems. For example, there islittle information available at present on the nature of forces thatdetermines the attachment of microbes to surfaces and chemi-cals such as surfactants and polymers to microbes. The currentmodels based on DLVO theory or their modified versions can-not describe adequately the marked differences in the behaviorof microbes or even simple liposomes. What causes, for exam-ple, dodecyl sulfonate to adsorb on the liposomes first and whattriggers it to solubilize them? When surfactants or antimicrobialagents interact with microbes, where do they act: on the exteriorsurface, inside the membranes, or on the interior surfaces, and,once there, what is the dynamics involved in terms of both chem-ical and physical changes? It would indeed be interesting if theactual molecular movements could be monitored in real time. Inour efforts with ESR to understand such phenomena, we foundthe ESR signal to disappear totally upon contact with microbes.How the ESR radical was converted and whether this techniquecould be used to monitor the presence of bacteria instantly areyet to be determined. In a similar vein, what causes a microbeto tolerate having its exterior envelope in contact with one sub-strate but not other and whether such a phenomenon could beutilized to protect living cells from carcinogens or toxics arealso unknown. Some of the other questions that need answersinclude the following: in the case of titania treated with butanol,was there cooperative adsorption involved, and if so what con-figuration and orientation of the activator would be best for theprotein uptake and can configuration be controlled so that onlyhost tissue cells will attach to an implant but not the pathogenicbacteria? Also of interest are questions as to how proteins un-fold when contacted with certain surfaces and polymers, andwhy they crystallize under certain conditions; what role suchchanges in conformation play in biological phenomena involvedin dementia and Alzheimer’s disease; and, finally, how confor-mation affects the growth or destruction of cells and whetherconformational control can be exercised to promote repair oforgans. There are many such vital questions and opportunitiesin the biointerfacial area, a field that could be considered to beonly in its infancy in many ways.

CONCLUDING REMARKS

Clearly, the work done thus far using simple systems has givenimportant insights into many colloidal phenomena that are ba-sic to processes in complex systems. Colloidal processes in most

natural and commercial systems, which are invariably dynamic,are not, however, understood or modeled with any reliable pre-

NDARAN

dictive power. Thus, in multicomponent mineral systems surfaceconversions take place that can totally destroy the selectivityrequired in beneficiation operations. Reagent schemes are re-quired that can control such interferences. Opportunities existto explore and utilize the dynamics of adsorption processes, par-ticularly of tethers and polymers that can conform or reconformto cause dispersion, adhesion, or coating as required. In biosys-tems also, we have very little information on the dynamics of thecolloidal processes involved as in the case of the interaction ofmicrobes while they disintegrate or deposit on surfaces. Oppor-tunities are abundant, particularly to examine and probe inter-facial and colloidal phenomena on the atomic scale. With newprobes for scanning at nanolevels and possibilities for probesthat can follow the dynamics, a new horizon is appearing. Thechallenges have never been more fascinating.

ACKNOWLEDGMENTS

The work discussed above has been made possible by the support over theyears of the Department of Energy, the Environmental Protection Agency, theNational Science Foundation, and a number of companies (Amoco ProductionCo., Akzo-Nobel, ARCO, Cabot, Chevron Oil Field Research Co., Cognis,Colgate–Palmolive, Dispersion Tech., Exxon, FIPR, Gulf Res. & Dev. Co.,Halliburton, Hercules, IBM, INCO, International Specialty Products, KoreaNickel, Lipo Chemicals, Marathon Oil Co., Mobil, Mitsubishi, Nalco, PQ Corp.,Rhodia, Seppic, Shell Dev. Co., Shipley, Sun Chemical, Texaco, Union Oil Co.,and Unilever Research). Indeed, the credit for many of the findings discussedgoes to my students and postdoctoral associates and collaborators who did nothesitate to venture into unknown territories.

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