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Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 1–6

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

ermeation of a cationic polyelectrolyte into meso-porous silicaart 1. Factors affecting changes in streaming potential

ing Wua, Martin A. Hubbea,∗, Orlando J. Rojasa,b, Sunkyu Parka

North Carolina State University, Department of Forest Biomaterials, Campus Box 8005, Raleigh, NC 27695-8005, USADepartment of Forest Products Technology, Faculty of Chemistry and Materials Sciences, Helsinki University of Technology, P.O. Box 3320, FIN-0215 TKK, Espoo, Finland

r t i c l e i n f o

rticle history:eceived 11 September 2009eceived in revised form9 November 2009ccepted 25 November 2009vailable online 2 December 2009

a b s t r a c t

A recently developed streaming potential (SP) strategy was used for the first time to investigate fac-tors affecting permeation of the cationic polyelectrolyte poly-(diallyldimethylammonium chloride) fromaqueous solution into silica gel particles. Factors affecting cationic polyelectrolyte permeation wereconsidered, including polyelectrolyte dosage, molecular mass, solution pH, and electrical conductivity.Samples were equilibrated for approximately 20 h before testing. The magnitude of change in streamingpotential, which was taken as evidence of permeation, increased with increasing polyelectrolyte dosage,

eywords:olyelectrolytesilica gelermeationtreaming potentialiffusion

with decreasing molecular mass, and with decreasing pH in the range 11 to 3. The pH effect supportsa mechanism in which excessively strong electrostatic attraction between the polyelectrolyte and thesubstrate immobilizes macromolecules at or near the entrances to the pore network, thus inhibitingpermeation of like-charged macromolecules. The same mechanism is consistent with observations thatpermeation increased with increasing electrical conductivity, though the latter observation also could beexplained in terms of conformational changes.

orosity

. Introduction

Our interest in the permeation of polyelectrolytes into verymall spaces has been motivated by both practical and theoreti-al concerns. An understanding and ability to predict permeationehavior is important when applying polyelectrolyte treatmentsuring enhanced oil recovery [1–3], papermaking [4–8], andiomass hydrolysis [9–11]. Theoretically it has been proposed thatinetic and thermodynamic factors dominate the permeation ofacromolecules in confined spaces [8,12–15]. From a colloidal and

nterfacial perspective, permeation may depend on the factors con-idered in our research, such as electrostatic forces and the effectiveize of macromolecules in solution, among others.

Experimentally, it has been relatively difficult to obtain directvidence of polyelectrolyte adsorption within mesoporous sub-trates (pore size 2–50 nm according to IUPAC). Definitive workelated to permeation into pores within this size range has beenased on adsorption isotherms [16,17]. For instance, Alince and

an de Ven [16] presented some work obtained by J. Day, whoeasured adsorption isotherms for cationic polyelectrolytes of dif-

erent molecular mass interacting with silica gel in suspension.iscontinuities in the functions of adsorbed amount vs. the poly-

∗ Corresponding author. Tel.: +1 919 513 3022.E-mail address: hubbe@ncsu.edu (M.A. Hubbe).

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.11.042

© 2009 Elsevier B.V. All rights reserved.

mer radius (based on solution viscosity) were explained based ongeometrical considerations, comparing the sizes of the pores ver-sus the effective size of the polyelectrolytes. Recently, Horvath etal. [7] used a more direct approach based on fluorescently labeledpolyelectrolytes and confocal microscopy to study factors affectingthe permeation of cationic polyelectrolytes into cellulosic fibers.By using batches of polyelectrolytes labeled with contrasting colortags, they demonstrated that the initial layer of polyelectrolytesadsorbing onto the cellulosic fibers tended to remain near to theirouter surfaces, rather than diffusing further into the pore struc-ture. Later-arriving polyelectrolyte molecules had to squeeze pastalready-adsorbed molecules during specified long-term exposureperiods to diffuse further into the fibers. It was proposed that theinitially adsorbed polyelectrolyte layers tended to inhibit furtherpermeation due to charge-charge repulsion.

Research by the present authors has focused on effects of ionicstrength and dosage of commercial high mass cationic polyelec-trolytes on the streaming potential of either silica gel or cellulosicfibers in aqueous suspensions [18–21]. It was found that the signof streaming potential could be changed in a reversible mannerbetween net positive and net negative, just by changing the ionic

strength and electrical conductivity of the solution. The effectswere explainable in terms of the electrostatic double layer, sincetheir full development at charged surfaces is predicted to bestrongly suppressed within pore spaces that are small relative tothe Debye–Hückel reciprocal length parameter [22–24].

2 N. Wu et al. / Colloids and Surfaces A: Physic

Table 1Levels of independent variables and default conditions.

Factors Level*

Poly-DADMAC dosage, % on drymass of silica gel

0.1, 1, 3, 10, 30

Molecular mass ofpoly-DADMAC

High mass as received

Dialyzed high massVery low mass as received

Solution pH 3, 7, 10

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Electrical conductivity (�S/cm) 10, 100, 1000, 10000

quilibraton time: 18–24 h.* Bold font denotes the default condition.

Some of the streaming potential tests results in the cited workrovided evidence of substantial permeation of the polyelectrolyte

nto the silica gel permeation, particularly at a very high poly-er concentration [18,21]. It was unclear, however, whether the

ffect was due to the high concentration per se, or mainly dueo a sufficient concentration of very-low-mass oligomers (impu-ities), which were capable of permeating into small spaces at higholyelectrolyte concentrations. These results led us to choose theresent experimental conditions, which were designed to answerhe above questions, and to look at the factors affecting poly-lectrolyte penetration into mesoporous silica gel particles moreomprehensively, as shown in Table 1. When considering each ofhe factors, default variable levels (marked in bold font) of theemaining factors were used.

A packed bed cell was used in which the silica gel sample wasontained between a pair of 200-mesh stainless steel screens (seeig. 1). This was a modification to the streaming potential equip-ent used in the group’s past research [25,26], as described earlier

21]. In the original one-screen system, silica gel particles (or, alter-atively, fibers of various types) were periodically collected on acreen through which the suspending medium (aqueous solution)assed when high pressure was applied, and a magnetic stirrer wassed to keep the silica gel particles suspended when the pressureas released. However, it was found that the magnetic stirring

ended to grind the silica gel particles, and this effect eventuallyould result in a markedly decreased flow rate through a mat ofarticles forming on the screen. The grinding effect resulted in a

oss in accuracy of the method. In addition, the particles’ tendencyo fall away from the screen surface after cessation of the pres-ure application could be expected to contribute to a sedimentationotential effect, whose magnitude would be difficult to determine.

ig. 1. Sample cell for the streaming potential jar (SPJ) apparatus to allow for apacked bed” format during semi-automated streaming potential tests.

ochem. Eng. Aspects 364 (2010) 1–6

Those problems were overcome by the modified streaming poten-tial setup used in this investigation.

2. Experimental

2.1. Materials

The water used in the experiments was deionized witha Pureflow system. Inorganic chemicals were of reagentgrade. The cationic polyelectrolytes were linear poly-(diallyldimethylammonium chloride) (poly-DADMAC) samplesfrom the Aldrich, catalogue numbers 52,237-6 (very low mass)and 40,903-0 (high mass). The nominal molecular masses of theproducts are given as 5k–20k and 400k–500k ranges, respectively.

The mesoporous silica gel used in the experiments was obtainedas catalogue S745-1 from Fisher Scientific (also known as DavisilSilica Gel 150), which was 60–100 mesh (except where noted) witha nominal pore size of 15 nm. Certain control experiments werealso carried out with a non-porous silica powder, having a meshsize of “200 and finer” (Fisher Scientific product cat. no. S153-3).See Table 2 for properties of these silica gel particles.

2.2. Dialysis and gel permeation chromatography (GPC)

A Spectra/Por® Biotech cellulose ester (CE) dialysis membranewas used for separating the poly-DADMAC solutions with nom-inal 400k–500kDa molecular weight into fractions, of which theretained, high-mass fraction was saved. The molecular mass cut-offvalue was 100,000 Da, and the tube diameter was 10 mm. Dial-ysis was carried out with initial poly-DADMAC concentrations inthe range 40–85 g/L, and the external solution consisted of 0.158%sodium chloride (Fisher). The external solution was replaced atapproximately 3, 8.5, and 13 h after the start, and the retainedsolution was collected after about 24 h.

A Waters 2695 GPC system with a Waters 2996 PhotodiodeArray refractive index detector was equipped with 300 nm-porosityand 30 nm-porosity (particle size 10 �m) columns connectedin series. The packing material consisted of OH-functionalizedmethacrylate-copolymer-network. The flow rate was 1.0 ml/min,and the column temperature was maintained at 35 ◦C. The sys-tem was calibrated with four broad molecular mass poly-DADMACstandards obtained from the supplier of the columns. Results ofGPC showed that most of the lower-mass component of the poly-DADMAC was removed by the dialysis.

2.3. Equilibrium of adsorbate and adsorbent

Aqueous solutions were prepared with solutions containing10−4 M sodium bicarbonate (for purposes of buffering the pH nearneutral), plus sufficient sodium sulfate to reach the electrical con-ductivity values (at about 24 ◦C) as specified later in this report.Silica gel was added to each of six beakers, which were stirred(approx. 100 rpm) by impeller to keep the particles suspended.Poly-DADMAC treatment was based on the dry mass percentagerelative to silica gel. After approximately 18–24 h, the streamingpotential was measured.

2.4. Packed bed and steaming potential tests

Electrokinetic tests were carried out with “Streaming PotentialJar” (SPJ) apparatus [25] fitted with the packed bed described above

[21], as illustrated in Fig. 1. It consisted of two 200-mesh screens(316L stainless steel) to hold the solid material (silica particles).The metal probes, for detection of the electrical potential differ-ences, were composed of silver alloy wires (45% Ag, 30% Cu, 25% Zn)[see 25]. Before typical measurement of streaming potential, one

N. Wu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 1–6 3

Table 2Characteristics of silica gel particles.

Silica type Pore size (nm) Pore volume (cm3/g) Particle size (�m and mesh sizes) Surface area (m2/g)

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Fig. 2 shows streaming potential as a function of high masspoly-DADMAC treatment in salt buffer with 1000 �s/cm con-ductivity. The streaming potential vs. treatment level datawere fitted to a fourth-order regression model of the type

Fisher chemical silica Non-porous 0Davisil® 645 15 1.15Ground up Davisil® 645 15 1.15

creen at one end of the packed bed fitting was unscrewed, leav-ng the part with the other screen held in a clamp with a beakernderneath. After the specified equilibration with poly-DADMAColutions (see Section 2.3), the particles in the beaker were allowedo sediment for at least 5 min. Most of the supernatant solution waset aside for use in rinsing, and the respective silica gel particles,aving a mesh size of 60–100, were agitated so that the concen-rated mixture could be poured through the opened packed bedtting and retained on a 200 mesh screen. Filtrate was collected inclean beaker. A syringe was filled with some of the supernatant

olution and used to ensure nearly quantitative transfer of the fil-erable solids into the packed bed. Finally, the unscrewed part wasssembled again.

Immediately before the streaming potential tests, the silica geln the packed bed was rinsed by passing fresh buffer solutionhrough it.

The streaming potential test was performed by carrying out sev-ral repeated measurement cycles involving successive applicationf high pressure, zero (ambient) pressure, and then sufficient vac-um to return the filtrate to the jar at the end of the measurementycle (see also Ref. [25]). The difference of potential detected by thewo electrodes on either side of the screen between high pressurend zero applied pressure was defined as the streaming potential.“high pressure” of 207 kPa was applied for at least 8 s (default),

xcept that a 16 s period was used to obtain more stable results inhe absence of sodium sulfate. These tests were carried out with thereated silica gel particles freshly resuspended in the default bufferolution (pH 7, 1000 �S/cm).

. Results and discussion

.1. Repeatability of streaming potential measurements

A typical streaming potential test involved five or more cyclesf measurement. In each cycle, the SPJ’s software automati-ally recorded one streaming potential value (potential differenceensed by the two electrode on either side of the screen betweenigh pressure and low pressure), as described in more detail else-here [21,25]. Results from the first cycle were routinely discarded,

ince the electrical signals were generally not as stable as for theubsequent tests. The relative difference among the remaining fourr more cyclical measurements was typically less than 10% of theean recorded potential. It is possible that this relative signal dif-

erence was related to how well the electrical potential signal wastabilized near the end of the high pressure application and duringhe period between 6 and 8 s after the pressure had been releasedo ambient pressure [25].

Five independent replicate experiments (each based on fiver more cycles of pressure and vacuum) were carried out at theenter point of the experimental conditions (see Table 2), i.e. test-ng with an electrical conductivity of 1000 �S/cm, pH of 7, andn addition level of 1% of very-low-mass poly-DADMAC basedn silica gel solids. The mean streaming potential obtained under

hose conditions was −1.30 mV, which was markedly less nega-ive than −13.4 mV, the result obtained under the same conditionsn the absence of polyelectrolyte. The standard deviation amonghe independent replications of the default treated condition was.29 mV, which implies a 95% confidence interval for the mean

Floated (200 mesh and finer) 0.737150–250 �m (60–100 mesh) 282Not measured 292

value between −1.66 and −0.94 mV. Occasional larger differencesin streaming potential results were tentatively attributed to subtledifferences in the pore size of other properties of silica gel samplesobtained from different reagent jars.

3.2. Tests with dialyzed, high-mass poly-DADMAC

Our GPC results showed that the dialysis process removedthe smaller component of commercial poly-DADMAC, making itspolydispersity to decrease considerably. Fig. 2 shows results oftests to compare streaming potential values at various levels oftreatment of the silica gel with dialyzed vs. as-received high-mass poly-DADMAC. Addition of relatively small amounts of thepolyelectrolyte (either undialyzed or dialyzed) was sufficient toincrease the measured streaming potential from the initial valueof −8 mV up to the range −4 to −2 mV, which is in agreement withearlier test results of this type [18]. This change has been attributedto progressive build-up of an adsorbed layer of macromolecules onthe outer surfaces of the silica gel particles. It was proposed that rel-atively small amounts of the polyelectrolyte would permeate intothe silica gel particles’ nanopore network (nominal pore size 15 nm)due to the relatively high molecular mass of the poly-DADMACsamples (nominally 400–500 kDa). Thus, it is possible to explaina net negative streaming potential in the system after adsorptionresulting from the balance of two opposing effects: on one handflow past the outer surfaces of the gel particles (giving a positivestreaming potential contribution), and on the other hand the effectof liquid passing through the particles (giving a negative stream-ing potential contribution). In agreement with this interpretation,follow-up tests with non-porous silica under the same aqueousconditions yielded strongly positive streaming potential values of+31.3 mV, compared to −70.1 mV for the non-porous silica in theabsence of the polyelectrolyte.

Fig. 2. Results of streaming potential tests at increasing levels of either undyal-ized (top curves) or dialyzed (bottom curves) high-mass poly-DADMAC to silica gelsuspensions under the default conditions.

4 Physicochem. Eng. Aspects 364 (2010) 1–6

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As shown in Fig. 5, the pH at which the system was equili-brated had a large effect on the streaming potential measured.In these tests, again the streaming potential was measured withpoly-DADMAC-free buffer with 1000 �S/cm conductivity, in order

N. Wu et al. / Colloids and Surfaces A:

= ˇ0 + ˇ1x + ˇ2x2 + ˇ3x3 + ˇ4x4 + ε to give suitable fits withdjusted R2 values of approximately 0.93 for each of the plottedines.

When comparing the two streaming potential profiles in Fig. 2,he most distinguishing differences corresponding to undialyzeds. dialyzed samples were at the highest levels of poly-DADMACreatment. In the case of the systems treated with (high molecular

ass) dialyzed poly-DADMAC, the measured streaming potentialemained negative throughout the range of dosages considered. Byontrast, positive values of streaming potential were obtained atighest levels of as-received, high-mass poly-DADMAC. For exam-le, at an addition level of 10% polymer to SiO2 solids (log(%oly-DADMAC) = 1), the regression lines gave streaming potentialalues of 2.9 and −1.4 mV, respectively for the undialyzed vs. dia-yzed poly-DADMAC. This difference was significant at the 95%onfidence level. These results support a hypothesis proposed ear-ier [18–20], namely that low-mass material in the as-receivedundialyzed) poly-DADMAC [27] could permeate into the silica gel,nd would have a significant effect on steaming potential testsarried out at very high concentrations in the bulk solution.

Alternative explanations for the observations described aboveere also considered. For instance, it can be argued that the high-ass poly-DADMAC molecules in solution adopt coiled shapes that

revent them from efficiently covering 100% of the outer surface ofsilica particle. In such a situation, the presence of a lower-massopulation of oligomers in the sample might be expected to “fill in”he uncovered areas, possibly leading to a more positive stream-ng potential. In contrast, in the case of adsorption onto a silicael particle, the low-mass oligomers presumably would be able toigrate into the pore structure of the silica gel particles, render-

ng the outer surfaces of the particles less positive. The suggestedffects, however, are unlikely to take place, because it is knownhat high-mass polymers of moderately high affinity compete fordsorption sites, even to the extent that they tend to displace lower-ass polymers of the same type [28,29]. Also, while the average

onformation of dissolved polymers could be described as sphericaln shape, especially under conditions of low ionic strength, they arexpected to adopt flatter conformations in the adsorbed state, espe-ially when interacting with a substrate of opposite charge [14].n additional reason to reject the explanation give above is that itoes not account for the effect of dialysis on results of experimentsarried out with just silica gel as the substrate, as in Fig. 2.

Another issue that may affect the streaming potential resultsrom systems of mesoporous particles is that the volumetric flowround such particles is typically many factors of ten greater thanhe flow through the particles. However, it should be noted thattreaming potential effects arise due to double-layers that are lim-ted to the region within a few nm of the surfaces. Because thepplied pressure can be expected to govern the velocity gradientdjacent to the wetted surfaces, one can expect the contribution tolectrokinetic effects to be approximately proportional to the wet-ed surface area, as long as a continuous network of pores exists. As

entioned earlier, deviations from this rule can be expected whenhe electrical conductivity is very low [18,22–24], but not in theresence of the default of buffer solution used during the presentork.

.3. Effects of polyelectrolyte dosage and molecular mass

Fig. 3 shows the results of streaming potential tests carried out

ithin a broader range of poly-DADMAC treatment levels during

ver-night equilibration. Streaming potential was evaluated withresh buffer solution (poly-DADMAC free buffer with 1000 �S/cmonductivity), instead of the original liquid with which the silica gelad been equilibrated. By this approach it was possible to observe

Fig. 3. Effect of dosage of high-mass cationic polymer in the bulk solution duringequilibration (relative to the dry mass of silica gel) on the streaming potential mea-sured in the presence of fresh default buffer. Note that the error limits are smallrelative to the size of the plotted symbols.

changes associated just with poly-DADMAC adsorption and per-meation during a defined period of equilibration. As shown, thedegree of permeation tended to increase with increasing cationicp-DADMAC treatment during the equilibration period.

Fig. 4 shows the results of similar tests with both very-low-mass and high-mass poly-DADMAC (as received). In the case ofthe very-low-mass cationic polymer (upper curve), the observedstreaming potential rose almost in direct proportion with the loga-rithm of polymer dosage. Interestingly, the results for high-masspoly-DADMAC showed a somewhat different trend, with a lessapparent effect of dosage. These results are tentatively attributedto (a) a relatively strong adsorption of high-mass polyelectrolyteonto the outer surfaces of the gel particles, reaching near-saturationat relatively low treatment levels, and (b) a substantially less ten-dency to permeate into the mesoporous structure, compared withthe very-low-mass poly-DADMAC.

3.4. Effects of pH

Fig. 4. Effect of cationic polymer dosage (very low vs. high mass) on the streamingpotential of silica gel particles, with tests carried out in the presence of fresh defaultbuffer.

N. Wu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 1–6 5

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tion for an effect of electrical conductivity can be based on changesin the radii of gyration of the polyelectrolytes in solution; indeedthe tests related to molecular mass (see Figs. 2 and 4) already hadestablished that “smaller” molecules are more able to permeateinto the silica gel. Issues related to geometrical constraints, effects

ig. 5. Effect of pH on the streaming potential of silica gel particles treated withery-low-mass poly-DADMAC and evaluated with fresh default buffer solution. Theimit bars shown 95% confidence intervals for the observations.

o observe changes associated with poly-DADMAC adsorption andermeation, not those due to the direct effect of pH on thetreaming potential of the substrate itself. As shown, the leasthange in streaming potential, relative to the −13 mV measuredor untreated silica gel under the same electrolyte conditions,as observed at the highest pH of equilibration. The greatest

hange in streaming potential, even achieving positive SP values,as observed when the equilibration was at pH values below

.The trends shown in Fig. 5 directly contradict a possible hypoth-

sis that permeation might be favored by increasing strength oflectrostatic attraction between the polyelectrolyte and the sub-trate. It is well known that SiO2 surfaces become more negativeith increasing pH [30–32] and that the charge characteristics ofositively charged poly-DADMAC are essentially pH-independent33]. Thus, a stronger electrostatic interaction is expected at highH, presumably providing a greater driving force favoring eventualermeation into the negatively charged pores.

A more successful way to explain the effect of pH is based onhe principle of trapped, non-equilibrium states [14,34]. Accord-ngly, it is reasonable to expect that a relatively strong electrostaticnteraction might result in polyelectrolyte molecules becomingssentially immobilized at their positions of first contact with thetrongly negative substrate, i.e. on the outer surfaces and at orear the entrances into the porous network within the silica gel.ubsequent movement of the adsorbed polyelectrolytes, either byransient desorption or by reptation [14], would be greatly sup-ressed due to multiple simultaneous points of attachment. Byontrast, at relatively low pH, where the negative charge densityf the silica surface is expected to be much weaker [30–32], oneould expect polyelectrolytes to be more able to continue theirermeation into the pore network, thus allowing additional macro-olecules to occupy their recently vacated positions, and so on. This

ame mechanism also was found to explain many of the effectsecently observed by Horvath et al. [7] in their study of cationicolyelectrolyte adsorption into cellulosic fibers.

Fig. 6 shows corresponding results obtained for systems treatedith high-mass undialized poly-DADMAC at the 0.2% level and

ompared with a control experiment (in the absence of poly-ADMAC). Again, the testing was done with fresh buffer solutionaving a conductivity of 1000 �S/cm. It is clear from these resultshat although the cationic polymer had a large effect on the stream-

ng potential at each of the pH values considered, the sign ofotential was not reversed. These results, which agree with previ-us findings [18], are attributed to the fact that the interior surfacesithin mesopores were not covered by cationic polymer, and such

Fig. 6. Effect of pH on the streaming potential of silica gel particles treated withhigh-mass poly-DADMAC (0.2% level) and evaluated with fresh default buffer. Thecontrol tests were without the presence of poly-DADMAC.

surfaces can be expected to contribute a negative component to theoverall electrokinetic effect.

3.5. Effects of salt concentration

Reasons to expect that the concentration of a monomericelectrolyte in the solution might affect permeation of a cationicpolyelectrolyte into silica gel include (a) the more condensed con-formation of polyelectrolytes [35], and (b) a weakening of theelectrostatic interactions between the polyelectrolyte and sub-strate surfaces with increasing salt levels [14,36]. As shown inFig. 7, there was only a very modest increase in the positive valueof streaming potential with increasing conductivity during theover-night equilibration in the presence of very-low-mass poly-DADMAC. Though this finding is consistent with the mechanismdescribed in the previous subsection, i.e. an expected weakeningof the electrostatic attraction between the macromolecules andthe substrate with increasing salt, the results suggest that sucha mechanism did not have a large effect. An alternative explana-

Fig. 7. Effect of electrical conductivity during equilibration (adjusted by sodiumsulfate addition) on the streaming potential of silica gel treated with very-low-mass versus high-mass poly-DADMAC at the 1% level on SiO2 solids with evaluationcarried out with default buffer.

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N. Wu et al. / Colloids and Surfaces A:

f salt ions, and desorption will be further considered in subsequentrticles.

One of the unresolved questions from the present work is whyhe effects of changed conductivity were not more pronounced,specially in light of the more marked changes that resulted fromiffering pH values during equilibration of the silica gel witholy-DADMAC solutions. One might argue, for instance, that aigher electrical conductivity should have had a similar effect toreduction in pH, since both conditions would tend to decrease

lectrostatic attraction forces between the polyelectrolyte and theubstrate. It is notable, however, that the high end of the saltoncentration range in the present work was not as high as thatonsidered in relevant previous studies [36]. Tests at higher saltevels would be helpful in the future to shed more light on thesessues.

Two further sets of issues that lie beyond the scope of the presentrticle are time-related effects on streaming potential and mea-urements of adsorbed amounts. These and other effects will bexplored more fully in future publications.

. Conclusions

A streaming potential protocol was used for the first time tobtain evidence of different factors affecting cationic polyelec-rolyte penetration into silica gel particles from aqueous solutionsuring a fixed period of equilibration.

The tendency for poly-(diallyldimethylammonium chloride) toermeate into silica gel particles increased with increasing bulkoncentration and with decreasing molecular mass of the poly-lectrolyte. Salt concentration did not have a marked effect onermeation within the range considered.

The tendency of the cationic polyelectrolyte to permeate intohe silica gel also increased markedly with decreasing pH. Theseesults were attributed to strong immobilization of the polyelec-rolyte under conditions favoring strong electrostatic attractionetween the polyelectrolyte and the substrate. By contrast, con-itions of low pH are expected to weaken the interaction forces,hus favoring easier migration adjacent to oppositely chargedurfaces.

cknowledgments

The authors wish to acknowledge the support of the graduatetudent Ning Wu by the Petroleum Research Fund, and the admin-stration of the fund by the National Science Foundation (grant no.6310-AC5).

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