Adsorption of surfactants and polymers at the solid-liquid interface

COLLOIDS AND ELS E V I E R

Colloids and Surfaces A: Physicochemicaland Engineering Aspects 123-124 (1997) 491-513

A

SURFACES

Adsorption of surfactants and polymers at the solid-liquid interface P. Somasundaran *, S. Krishnakumar Langmuir Center for Colloids and Interfaces, Columbia University, New York, N Y 10027, USA Received 30 July 1996; accepted 6 August 1996

Abstract

Surfactants, polymers and their mixtures are widely used in several important industrial processes such as dispersion-flocculation, flotation, dewatering, paints and pigments and oil recovery. Many types of surfactants (anionic, cationic or non-ionic) and polymers (charged and otherwise) are used individually or in combination to achieve desired surface modifications. The adsorption of these molecules at the solid-liquid interface depends on several factors such as the nature of the substrate, solvent, adsorbate species, the presence of secondary competing-cooperative species, temperature and even mode of mixing. The nature of the adsorbed layer determines the surface modification achieved and this in turn depends on the adsorption mechanisms and the conditions prevailing during and after adsorption. Several spectroscopic techniques such as fluorescence, electron spin resonance and Raman spectroscopy have been employed to investigate the structure of various adsorbed layers and to obtain correlations with observed changes in the dispersion properties. In this paper, we review our previous work on adsorption of various ionic and non-ionic surfactants, polymers and their mixtures and mechanisms involved, in aqueous and non-aqueous media. © 1997 Published by Elsevier Science B.V. Keywords: Adsorption; Polymers; Solid-liquid interface; Surfactants

1. Introduction

Adsorption of molecules on solids from solution is important in controlling a variety of interfacial processes such as mineral flotation, flocculationdispersion, blood clotting, self-assembly and enhanced oil recovery. Adsorption results from energetically favorable interactions between the solid adsorbate and the solute species and is often a complex process since it can be influenced by all solid, solvent and solute components of the system. Several interactions such as electrostatic attraction, covalent bonding, hydrogen bonding or non-polar

* Corresponding author.

interactions between the adsorbate and the adsorbate species, and lateral interaction between the adsorbed species as well as their desolvation can contribute to the adsorption and desorption processes. Adsorption can be considered to be a process of selective partitioning of the adsorbate species to the interface in preference to the bulk and is the result of interactions of such species with the surface species on the solid. The interactions responsible for adsorption can be either physical or chemical in nature. Adsorption can be broadly classified into two categories, physical adsorption and chemical adsorption, depending on the nature of the forces involved [1 ]. Physical adsorption is usually weak and reversible and involves small

0927-7757/97/$17.00 © 1997 Published by ElsevierScience B.V. All rights reserved. PH S0927-7757(96) 03829-0

492

P. Somasundaran, S. Krishnakumar / ColloMs Smfaces A: Physicochem. Eng. Aspects 123 124 (1997) 491-513

energy changes, van der Waals forces and electrostatic forces are primarily responsible for physical adsorption which is also characterized by a high rate of adsorption and formation of multilayers [2]. Chemical adsorption occurs through covalent bonding between the adsorbate and the surface species on the solid. Chemical adsorption normally involves an activation stage and is characterized by relatively high energy changes and a low rate of adsorption. Such adsorption is usually strong and irreversible and is limited to a monolayer. A distinction between physical and chemical adsorption can usually be made from the temperature dependence of the adsorption process. In the case of physical adsorption the adsorption generally decreases with temperature while in the case of chemisorption it increases with temperature. However, it must be noted that the distinction between physical and chemical adsorption is arbitrary and in many cases an intermediate character of adsorption is encountered. In some cases such as adsorption of gases on metal surfaces, physisorption may take place initially and may be followed by adsorbent adsorbate reactions, resulting in chemisorption [3]. Adsorption isotherms are commonly used to describe adsorption processes and these represent a functional relationship between the amount adsorbed and the activity of the adsorbate at a constant temperature. The adsorption density, which is the amount of adsorbate removed from solution to the interface, can be expressed as [4] Fi = 2rC exp

- A Gaa~° ) \

(1)

RT

where Fi is the adsorption density in the plane 6, which is at the distance of closest approach of counterions to the surface, r is the effective radius of the adsorbed ions, C (tool ml-1) is the bulk concentration of the adsorbate, R is the gas constant, T is the absolute temperature and AGaa~° is the standard free energy of adsorption. In practice, however, the adsorption density is measured as depletion of the adsorbate from the solution: V F = (Cf - Ci) - W

(2)

where F (molg -1) is the adsorption density, Cr (moll -1) and Ca (moll -1) are the initial and final concentrations of the adsorbate, V (1) is the volume of solution and W (g) is the mass of the adsorbent. The net driving force for adsorption AG ° can be considered to be the sum of a number of contributing forces: AG,as ° = AGelec° + AGchem° + AGc_c ° + AGo s °AGH ~:+ AG.2o ° + . . .

(3)

where AGelec° is the electrostatic interaction term, AGchern ° is the chemical term due to covalent bonding, AGc_~° is the lateral interaction term due to the cohesive chain chain interaction among adsorbed long chain surfactant species, AGc_s° is due to interaction between hydrocarbon chains and hydrophobic sites on the solid, AGH° is the hydrogen bonding term, and AGn2o ° is the solvation or desolvation term resulting from hydration of the adsorbate or any other species from the interface during adsorption. For each surfactant-solid-solvent system, several of the above terms can be significant depending on the mineral and the surfactant type, surfactant concentration, background electrolyte and solvent temperature. For non-metallic minerals, electrostatic attraction and lateral interaction effects are considered to be the major factors determining adsorption while, for salt-type minerals such as calcite and sulfides such as galena, the chemical term often becomes significant. In organic liquids the electrostatic forces are minimal and adsorption depends on the hydrophobic and hydrophilic interactions among the constituents.

2. Surfactant adsorption in aqueous media 2.1. Ionic smfactants

A typical Somasundaran-Feurstenau type adsorption isotherm of charged surfactants on oppositely charged surfaces is shown in Fig. 1 [5] where the adsorption of negatively charged sodium dodecylsulfate (SDS) on positively charged alumina is shown. This isotherm is characterized by

P. Somasundaran, S. Krishnakumar / Colloids' Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 491-513 I0-9

o .E

adsorption the following equation can be used to evaluate the change in adsorption density:

i

SOS/ALUMINA 0.1 M NaCI, gH 6.5

"rt,"

10-~o

d In F~ -

-

dlnC g i0-~I u~

~.I

i0_12

_./

i~

10"13

i0-14

HMC

I

I

I I1~11~

i0-5

i

1

i tll/ll

I

10-4

I

I IIIIll

I

i0-~

,

i lit

10-2

RESIDUAL DODECYLSULFATE, moles/ liler

Fig. 1. Adsorptionisothermof SDS on aluminaat pH 6.5 showing a four-regionisotherm. four regions, attributed to four different dominant mechanisms being operative in each region. The mechanisms involved in these regions may be viewed as follows. Region I, which has a slope of unity under constant ionic strength conditions, is characterized by the existence of electrostatic interactions between the ionic surfactant and the oppositely charged solid surface. The adsorption in region I can be examined by an exact form of the Gouy-Chapman equation for the diffuse layer by considering an ion exchange process between the dodecylsulfate ions and the chloride ions. The total adsorption density of the negative ions is given by 0"d

r_

= --

zF

493

(4)

o-d hardly changes in this region 1 and hence F_ remains constant and the increase in sulfonate adsorption is due to exchange between chloride ions and this part of the isotherm has a slope of 1. Region II is marked by a conspicuous increase in adsorption which is attributed to the onset of surfactant aggregation at the surface through lateral interaction between hydrocarbon chains [6, 7]. Such colloidal aggregates are generically referred to as "solloids" (surface colloids) [8] and include aggregates such as hemimicelles, admicelles and self-assemblies [9]. In the presence of specific

d(zF~9~/RT) - l - -

dlnC

~b

dn

RTdlnC

(5)

In region II all three terms in Eq. (5) are effective since both electrostatic and lateral interaction forces are contributing to the adsorption. Region III exhibits a marked decrease in the slope of the isotherm and this is ascribed to the increasing electrostatic hindrance to surfactant adsorption following interfacial charge reversal caused by the adsorption of the charged species in region III and beyond. In this region the second and third terms on the right-hand side of Eq. (5) will be very small and of opposite signs. The equation predicts a slope of unity for this region which is in agreement with the experimentally obtained value. Region IV and the plateau in it correspond to the maximum surface coverage as determined by micelle formation in the bulk or monolayer coverage, whichever is attained at the lowest surfactant concentration; further increase in surfactant concentration does not alter the adsorption density. A schematic representation of adsorption by lateral interactions is given in Fig. 2.

2.1.1. Characterization of the adsorbed layer by various techniques The adsorbed layer in the above case undergoes systematic structural changes along the isotherm. A number of spectroscopic methods have been used to characterize the microproperties of the adsorbed layer. Fluorescence spectroscopy, with pyrene as the photosensitive probe, was used to determine the micropolarity and size of these aggregates. Steady state fluorescence reveals a sharp increase in micropolarity, as shown in Fig. 3 (measured as the ratio 13/11 of the vibrational peaks of the pyrene spectrum), at concentrations corresponding to that of the onset of solloid formation at the alumina-water interface [ 10]. The measured polarity is higher than that of SDS micelles in solution, suggesting a more compact structure with less solvent penetration at the solid-liquid interface. Dynamic or lifetime fluorescence meas-

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 491 513

494

2;ili

;.

Fig. 2. Schematic representation of the correlation of surface charge and the growth of aggregates: (a) reverse orientation model; (b) bilayer model.

1.5

=.

l

PYRENE 1N ADSORBED LAYER SOS/ALUMINA .1M NoCI, pH 6.5

SDSM,CELLE~.IMNoC,~ ~ &

1

/,

............................~(.............w.."5R~.~2:!.M.NQ.c!?...........

o

14 r o

~

,I,,,ri

I

IO"5

I 11~1111 I 1 llllIII i IO "4

1o-3

I

t ) l l [ l

1o -2

RESIDUAL OOOECYLSU1-FATE , mole,,/I]ler

Fig. 3. I3/I1 fluorescence parameter of pyrene in SDS in alumina slurries.

urements were used [ 11 ] to estimate the size of the adsorbed aggregates. The aggregates in region II appear to be of relatively uniform size (120-160) while in region III there is a marked growth in the aggregate size (above 250). In region II, the surface is not fully covered and enough positive sites remain as adsorption sites. The transition from region II to III corresponds to the isoelectric point (IEP) of the mineral, and adsorption in region III is proposed to occur through the growth of existing

aggregates rather than the formation of new aggregates due to lack of positive adsorption sites. Such adsorption is possible because of the hydrophobic interaction between the hydrocarbon tails of the surfactant molecules already adsorbed and the adsorbing molecules. The new molecules adsorbing at the solid-liquid interface can be expected to orient with the ionic head towards the water since the solid particles possess a net negative charge similar to that of the surfactant under these conditions. Additional information was obtained by conducting electron spin resonance (ESR) studies by co-adsorbing a paramagnetic probe, doxyl stearic acid, together with the surfactant [12,13]. Information on micropolarity and microviscosity can be obtained by measuring the hyperfine splitting constant AN and the rotational correlation time To. The latter is the time required for a complete rotation of the nitroxide radical about its axis [14]. The surfactant adsorption itself was found to be not significantly affected by the presence of small quantities of the probe. The changes in the ESR spectrum on surfactant adsorption have been used to quantify the microviscosity of

P. Somasundaran, S. Krishnakumar/ ColloidsSuJfacesA: Physicochem.Eng. Aspects 123-124 (1997) 491-513 the adsorbed layer. The hyperfine splitting constants of 16-doxyl stearic acid measured in dodecyl sulfonate solloids (hemimicelles) ( 1 5 . 0 G ) are indicative of a less polar environment in comparison with its value for water ( 1 6 . 0 G ) and SDS micelles (15.6 G), corroborating the results obtained from the fluorescence studies. Similarly, microviscosities estimated from ~c measurements and calibrated against r~ measured in ethanol-glycerol mixtures give reasonably high microviscosity values for the solloids (Fig. 4). When the position of the nitroxide group was varied from 5 to 12 to 16 along the stearic acid chain, it experienced a different viscosity within the solloid. These observations may be explained by assuming a model for the adsorption of the probe in which the carboxylate group is bound to the alumina surface (Fig. 5). Such a model would require us to attribute greater mobility for the nitroxide moiety near the SDS H20 interface (as in the 16doxyl stearic acid case) and less mobility for the 5-doxyl case. This work is the first reported indication of variations in microviscosity within a surfactant solloid as estimated by any known technique.

495

In addition to these two techniques, excited state resonance Raman spectroscopy, using Tris(2,2'bipyridyl) ruthenium (II) chloride, Ru (bpy)~ +, as a reporter molecule [15], was employed to study the nature of the solloids. Significant perturbations in the vibrational spectrum of this molecule were observed and correlated with the formation and microstructure of the solloids in this case.

2.1.2. Effect of functional group of surfactants on adsorption The structure of the adsorbed layer depends on the packing of the molecules which in turn depends on the mutual repulsion and steric constraints among adsorbate species [16]. Adsorption isotherms o f 4 C l l 3,5-paraxylene sulfonate (Para-1), 4 C l l 2,5-paraxylene sulfonate (Para-2) and 4C11 2,4-metaxylenesulfonate (Meta) on alumina from water are shown in Fig. 6. Average aggregation numbers of the solloids increase from 17 to 76 with increase in adsorption in all three cases. The aggregation numbers of the two paraxylene sulfonates are similar throughout the range studied. However, at higher adsorption densities, the aggre-

80% GLYCEROL

75% GLYCEROL

L

g

20G

! 'U

F t

0

__j

SOSALOM,NA l/if

/

I

Jr,.--

;l

till

23

40

~

60

J

~

/___....__

sos M,CELLE (0.1M NaCI)

80 100 120 VISCOSITY, CP

140

M,CELLE

(0.1M NaCI, ph 6.5)

"

160

180

2C0

Fig. 4. Comparison of ESR spectra of 16-doxylstearic acid in solloids, micelles and ethanol glycerolmixtures and corresponding rotational correlation times.

496

P. Somasundaran, S. Krishnakumar / Colloids Surjaces A: Physicochem. Eng. Aspects 123-124 (1997) 491-513

/

/

"7 7

\

>!>

/

/ I

+ 4- ÷

+

+

I

4-

+

+

4-

+

4-

(b)

(a)

Fig. 5. Schematic model of nitroxide probe incorporation into SDS colloids: (a) 16-doxyl stearic acid; (b) 5-doxyl stearic acid.

800.00 O X

ICO.00 O

O o

,, P a r a l I0,00

Z Ill

r-~

c He t a

,/

$,.-

1.00

i

a Pora2

Z 0 rl O~

0.10

.

0.50

.

.

.

.

.

.

1.00

RESIDUAL

l

loc.oo

lo.a3 £ULFONATE

CONe..

kmal/m3x

~cc"d.oo 10 5

Fig. 6. Adsorption of alkylxylene sulfonates on alumina with the average aggregation numbers as determined by dynamic fluorescence.

gation number of the metaxylenesulfonate is lower than that of the paraxylenesulfonates. This suggests higher steric hindrance to the packing of the surfactant molecules in the solloids of the metaxylenesulfonate. Microcalorimetric studies have

shown that, at low adsorption densities, the adsorption is enthalpy driven while at higher adsorption densities the entropic term becomes dominant [17]. At higher adsorption densities the adsorption entropy is higher for the paraxylene

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 491-513

sulfonates, indicating tighter packing of molecules in the solloids. On the basis of this, the effect of the change in position of functional groups on the aromatic ring of the alkylxylene suifonates on adsorption can be attributed to the steric constraints on the packing of surfactant molecules in their aggregates.

2.2. Adsorption of non-ionic surfactants The adsorption of non-ionic ethoxylated surfacrants has become of much interest recently owing to their potential applications in processes such as detergency, cosmetics and enhanced oil recovery. The adsorption of non-ionic surfactants differs from that of ionic surfactants largely because of the absence of electrostatic interactions. Non-ionic ethoxylated alcohols exhibit strong adsorption on silica but not on some other minerals such as alumina. Since hydrogen bonding is relatively weak in comparison with electrostatic and chemical bonding, the nature of the water structure at the solid-liquid interface will be of particular importance for the adsorption of non-ionics. The lack of adsorption, for example, on certain minerals such as alumina is speculated to be due to the fact that the surfactant molecules are unable to disrupt the rigid water layer surrounding the substrate. The adsorption of dodecyloxyheptaethoxyethyl alcohol (C12EO8) alcohol on silica [18] displays an isotherm similar in shape to the isotherm of dodecylsulfate on alumina (Fig. 7). The absence of electrostatic repulsion between the adsorbed nonionic species results in this case in a higher slope for the hemimicellar region and absence of region III. Adsorption of this type of surfactant depends on the degree of ethoxylation as well as the length of the alkyl chain [19] (Figs. 8 and 9). At constant chain length, the extent of adsorption at low concentrations is greater for surfactants with higher degree of ethoxylation. However, the plateau adsorption is higher for the surfactants with lower degree of ethoxylation. A linear relationship is obtained when the parking area at plateau adsorption is plotted as a function of the ethoxylation number. This yields a parking area per-OCH2CH2 segment of 9.2 A2, suggesting direct adsorption of the ethylene oxide chains on the

497

silica surface. On the contrary, the alkyl chain length affects only the onset of plateau adsorption. This is in line with the decrease in critical micelle concentration (CMC) with increase in hydrocarbon chain length. On the basis of these observations hydrogen bonding is proposed to be the driving force for adsorption at low concentrations while the higher uptake of longer EO surfactants at such concentrations is due to the cumulative hydrogen bonding interactions of the EO chains with the hydroxylated silica surface. In contrast, at higher concentrations hydrophobic chain-chain interactions become more significant as evidenced by the progressive increase in slope of the adsorption isotherm with a decrease in EO number (the smaller the EO number the lesser is the steric hindrance to chain-chain interactions by the ethoxyl groups). A new class of sugar-based non-ionic surfactants has attracted more attention recently owing to the fact that they are easily biodegradable. Information on adsorption of such surfactants on solid substrates is however very limited in literature. Smith et al. [23] measured adsorption of three alkyl polyglucosides on titanium dioxide. They postulated that the hydroxyl groups on the surfactants are slightly acidic in nature and can hydrogen bond with the basic OH groups on the titania particles. Results obtained in our laboratories [20] show that 13-D-dodecyl maltoside adsorbs strongly on alumina and hematite but not on silica (Fig. 10). Also, molecular parking area calculations based on the adsorption plateau suggest the existence of surfactant bilayers on the solid.

2.3. Adsorption of surfactant mixtures Surfactants are generally used as mixtures to serve different purposes. A typical feature of the adsorption of ionic non-ionic surfactant mixtures and oppositely charged ionic surfactant mixtures is the synergistic interaction at the interface as well as in solution. Adsorption of non-ionic ethoxylated alcohols on alumina is negligible, but it can be enhanced by several orders of magnitude by coadsorption of an anionic surfactant. Similarly, anionic surfactants do not adsorb on negatively charged silica, but substantial adsorption can be

498

P. Somasundaran, S. Krishnakumar /' Colloids Surfaces A: Physicochem. Eng. Aspects 123 124 (1997) 491 513

10-9 C12H25 - (OC2H4)8-OH/SILICA t'2:

T

el

--~- 10-10

pH=

I

F

= 50 ° C 6.9

= 0.03M NaCI

I0-11

O

c

10-12

A

f

10"13 10-6

10-5

10-4

10-3

I0-2

RESIDUAl, ALCOHOL CONCENTRATION, kmol/m 3 Fig. 7. Adsorption of alkylaryl ethoxylated alcohol (ClzEOs) on silica.

10

-9

i

~

i

l l l , l

I

,

,

i

i i , , ,

i

,

,

i

i , ; ~ l

I

i

,

~

~ i h l l

I

i

C9HIg- C6H 4 -(OC2H41 m - O H / S I L I C A "E .~ z

T = 23-27"C ~H = 6.8 - 7 . 5

~

(:3,

/f

1 0 "10

u t~

~

£3

~,,

o

o

o

0

0 e~ <

= ] 0 -~I o ,j

IO'lZ

l

10-~

~ m= 20 o m= 40 t

t

t Ilil[[

I

I

I IlllI!

i0-~ RESIDUAL

I

10-4 ALCOHOL

[

I I Ir~l

~

I

t

t lttfll

CONCENTRATION,

|

I

lO-Z

I0-3 kmol/m3

Fig. 8. Adsorption density of nonylphenoxyethoxylated alcohols on silica as a function of EO chain length.

induced by co-adsorption of a non-ionic ethoxylated alcohol which adsorbs strongly by itself on silica. Enhanced surface activity of mixtures of nonionic and anionic surfactants has been demonstrated for many systems. Sodium p-octylbenzene-

sulfonate adsorbs three orders of magnitude more than C12EO 8 on alumina (Fig. 11) [21]. However, when adsorption is conducted from a 1:1 mixture of these surfactants the ethoxylated alcohol adsorbs to a greater extent than the sulfonate (Fig. 12). It is proposed that the initial electrostatic

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A. Physicochem. Eng. Aspects 123-124 (1997) 491-513 76 9 I

i

i

i

I

1

i

ii

I

,

1

i , i ,ii

I

,

i

i J iiil

i

1

i

499

I

Cnl'12 n,,.~ C 6H 4 - ( 0 C ~ I t 4 1 1 0 - O H / S I L I C A T = 23-27"C

eJ f~

pH = 7.0-7.6 S / L = 0.2 r= O

B

c ,=

Z O

-I0

I0

e, rr O t,o o .J 0

B u <

E

13~zl i

i

i

t

1 ttll

i

I

t

i

[ i l1

i

i

Illl

I

I

I

10-4 10- 3 10-~ R E S I D U A L ALCOHOL CONCENTRATION, k m o l / r n 3

10 6

I

i

1 I1|

1o-Z

Fig. 9. A d s o r p t i o n d e n s i t y o f E O l o a l c o h o l s o f different h y d r o c a r b o n c h a i n l e n g t h s o n silica.

l x l 0 "5

• ' •lxlO-6lxlO "7 Si02 o


lxlO "$

& Fe203 l x l 0 -9 ........

10~

........ 100' ...... 10"-'00 . . . . . DM Residual Concen~-ation (uM)

10000

Fig. 10. A d s o r p t i o n o f [3-dodecyl-maltoside o n different solids.

adsorption of the sulfonate provides a sufficient number of hydrophobic sites for solloid-type adsorption of the ethoxylated alcohol. As a result of the synergism the overall adsorption of the sulfonate is also enhanced and the concentration of solloid formation is lowered by more than an order of magnitude. The presence of the non-ionic

surfactant between the sulfonate ions in the solloids enhances the sulfonate adsorption by reducing the electrostatic repulsion between the ionic sulfonate head groups. Similar results have been obtained with SDS and C l z E O 8. Fluorescence probing of the adsorbed layer conducted in this case [22] revealed that the micropolarity of the

500

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123 124 (1997) 491-513

l x l 0 -5-

l x l 0 -6-

5

~

/

T = 50 ° C 0.03 M N a C I p H 7.8-8.2

.J

-

l x l O "7-

~c80s

•~ . l x l O "8-

C 12E08~A:]~

>

< l x l O -9-

lxl0-Io

i

lx10"6

i

Ix10"5

lx10"4

l x l 0 -3

I x i 0 -2

lxI0-:

R e s i d u a l Surfactant Conc., k m o l / m 3

Fig. 11. Adsorption of C8-benzene sulfonate and alcohol on alumina.

10 9

,

, st-

10

]'

C12EO 8

~

CeCS

....

/

aggregates. Another interesting observation is the constancy of the micropolarity suggesting that the aggregates do not change much with concentration. Dynamic fluorescence estimation of the relative aggregation numbers in these systems reveals a higher aggregation number than for pure SDS. Clearly the reduction in electrostatic repulsion is an important reason for the observed increase in aggregation number (Fig. 13). Table 1 shows the aggregation number of each surfactant in the mixed layer. At low adsorption densities the number of C l z E O 8 molecules is lower than that of SDS suggesting that the C12EO8 aggregation depends on SDS adsorption. As the adsorption increases the lateral chain interactions become the dominant mechanism and the numbers of SDS and C l z E O 8 in the aggregates become more or less equal. Similar synergistic effects have also been observed in the case of adsorption of cationic-nonionic surfactant mixtures at the alumina-water interface [23]. The adsorption of mixtures of the cationic tetradecyl trimethyl ammonium chloride (TTAC) and non-ionic pentadecylethoxylated nonyl phenol (NP-15) was studied at several ratios.

'

IV

~ 0 - 5 IF

Z

:

10

~:

< 10t~

//'

t-

,

]

, s-: "

i

:

10 la 10 4

10 s

10-:

Fig. 12. Adsorption of C8-benzene sulfonate and C12EOs alcohol from their mixture at an initial composition of 50 mol.% sulfonate-50 mol.% alcohol.

lI

j~:18

133~

10

R E S H ) U A I . Sl ] R F A C T A N T C O N C k m o l m 3

141 -

7-

^ -c < 10 5

"

50% Ct,£0 ~

/

/~b)

10 6

~:'-a6

a-

f I

10 --~ L

m~'

0

10

0

[] O 5

C:2EOa OO O5

54

L /

SDS-C12EO8 adsorbed layer is higher than that of the adsorbed layer of pure SDS, but similar to that of SDS-C12EO 8 micelles in solution. This suggests that the ethoxyl chain is the key factor determining the polarity of the adsorbed layer

10 -5

10 -4

10-3

Total Residual C o n c e n t r a t i o n

10-2

kmol/m 3

Fig. 13. Total adsorption of SDS and h I SDS-CI2EO s mixtures on alumina with aggregation numbers of surfactant in the adsorbed layer.

P. Somasundaran, S. Krishnakumar / Colloids Surfiwes A: Physicochem. Eng. Aspects 123 124 (1997) 491 513

501

Table 1 Variation in surfactant aggregation number in the mixed adsorbed layer of sodium dodecylsulfate and dodecyloxyheptaethoxyethyl alcohol Total adsorption (mol m -2)

Surfactant:pyrene (adsorbed/'layer)

ko

ke (ns ~)

n

(ns 1)

1.1 x 10 ? 2.3 x 10 7

157 173

0.015 0.013

0.18 0.055

0.68 0.77

61 72

46 61

107 133

3.2 x 10 -7 6.2 x 10 - 7

179 187

0.010 0.0079

0.015 0.011

0.79 1.60

74 82

67 77

141 128

1.9 x 10 6

194

0.0071

0.0055

0.86

85

83

168

The NP-15 does not adsorb on alumina by itself possibly because the ethylene oxide groups are strongly hydrated in solution and the adsorption force is not strong enough to cause displacement of these water molecules. As expected, the cationic TTAC does adsorb strongly on alumina at pH 10 and exhibits an isotherm typical of that of ionic surfactant adsorption on an oppositely charged surface. This adsorption changes significantly in the presence of the non-ionic surfactant NP-15. As in the previous case, solloid aggregation occurs at lower TTAC concentrations as the NP-15 concentration is increased (Fig. 14). Interestingly, at relatively high NP-15 amounts (ratio 1:4) no evidence of solloid formation is observed. The increase in TTAC adsorption at low concentrations

Nagg SDS

Total

C,zEO,

is attributed to charge screening by the non-ionic surfactant which reduces the electrostatic repulsion between the cationic head groups. Plateau adsorption of TTAC decreases owing to the competition of the bulky nonyl phenol with TTAC for adsorption sites (Fig. 15). The adsorption density of NP15 from its mixtures with TTAC reveals enhancement of adsorption of NP-15 by TTAC due to solubilization in the TTAC surface aggregates. Calculation of the monomer concentrations of the individual surfactants performed using the regular solution approximation revealed the adsorption of TTAC from the mixtures to correspond to its monomer activity. However, a similar correlation does not exist for the non-ionic surfactant and one can conclude that NP-15 adsorption depends not only on its monomer activity but also on the

4x10 -6-

l x l 0 "6-

t"l

TTAC only

O

TTAC:N'P- 15 4:1

A

TTAC:NP- 15 1:1

,:,

E

1:0

TTAC:N'P- 15 1:4

ixlO -6

O

Q

/

- - . _ &

]

/

lxlO-7

/TAu

lxlO "7-

.~ lxl0 -s

o

<

0

0

N lx10.8 -

Jd ~,J

lxl0 9 lxl0 -6

• J

"A

~

~ -6010

lxl0 -5

"

.0" ~" J

l x l 0 -4

Z

lxl0 -3

l x i 0 -2 5x10 -2

TTAC Residual Concentration. kmoI/m 3 Fig. 14. Adsorption of TTAC in the presence and absence of N P - 1 5 at p H 10; IS, 0.03 M N a C 1 .

lxl0 -9

~

2x10-10 2x10 -6

lxl0 -5

I Ixl0 -4

O

TTAC:NP 1~

11

A

TTAC :."¢P-15

1:4

lxi0 -3

ixl0 -2

5x10 -2

Residual Concentration o f NP- 15, kmob'm 3 Fig. 15. Adsorption of NP-15 on alumina in the presence of varying amounts of TTAC at p H 10; IS, 0.03 M N a C I .

502

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 491-513

adsorption of TTAC and the adsorbed layer structure. It is to be noted that the regular solution theory failed to predict the change in even the bulk monomer concentration of NP-15. Fig. 16 shows the adsorption of NP-15 when TTAC is pre-adsorbed on alumina. In this case also NP-15 adsorbs synergistically because of the adsorbed TTAC and the NP-15 adsorption increases with TTAC adsorption density. It is also observed that once the TTAC forms hemimicelles at the alumin~water interface there is no further effect of increasing TTAC adsorption density on the NP-15 adsorption. It should also be noted that NP-15 adsorption density in this case is lower than that from mixtures, leading to the inference that NP-15 adsorption depends to a large extent on the structure of the adsorbed layer. When it is present in mixtures, NP-15 can adsorb as the TTAC layer evolves but, when the TTAC is pre-adsorbed, adsorption is more difficult as it should change the TTAC adsorbed layer first and this may involve a large activation energy. Both SDS and C12EO 8 adsorb on kaolinite by themselves. Adsorption of C12EO8 on kaolinite is influenced by the presence of anionic SDS. The plateau C12EO 8 adsorption in all cases is higher from its mixtures with SDS than in its absence [24]. These higher plateau adsorption values are 7 x 1 0 -7-

?-

"/

/<#/ ~'5 o,6/

.

~

1x10 -l° l x l 0 "6

Surfactant adsorption in non-aqueous media is important in various technological processes such as ceramic and magnetic tape, ink, paint and pigment manufacture. Adsorption in this case is usually governed by the polarity differential between the solvent, solute and substrate and this can be related to the acid-base characteristics of these components. 3.1. Ionic surfactants on oxide surfaces

_ z""," .~...........

/

, //

l x l 0 -9

-"

/7' y

3. Surfactants in non-aqueous media

The density of the surface hydroxyls determines the relative basicities of many minerals and this can be inferred from the IEPs of the minerals in aqueous solutions (the lower the IEP the more acidic is the mineral). Alumina has a high surface hydroxyl density (10-15 OH nm-2), a high IEP

~-C

1x,07

proposed to be due to additional adsorption of C12EO 8 in reverse orientation. At low concentrations, the chain-chain interactions between the adjacently adsorbed SDS and C12EO8 molecules provide additional energy gain for the adsorption process. Such hydrophobic interactions at the interface will result in the formation of the hydrophobic microdomains into which the hydrocarbon chains of additional C12EO8 molecules can become incorporated as a second layer. This is schematically shown in Fig. 17. The evolution of such an adsorbed layer clearly changes the wetting behavior of the particle surface.

'

-

l x l 0 -5

T T A C : 0,97 m M

O

T T A C : 5.25 m M

s

T T A C : 0.2 m M

,t,

A d d e d together: T T A C 2 m M l x l 0 -4

l x l 0 -3

l x l 0 -2

JL /F//////////7////

//7////

NP-15 Residual conc., kmol/m 3

(a)

Fig. 16. Effectof TTAC order of addition on NP-15 adsorption on alumina; varying amounts of pre-adsorbed TTAC and NP15 added together with TTAC.

(b)

(c)

Fig. 17. Schematicrepresentationof the adsorption mechanisms for SDS-C12EO8mixtures on kaolinite.

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 491-513

(8.5) and is a basic oxide. On the contrary, silica has a lower hydroxyl density (3-4 OH nm-Z), a low IEP (less than 2.5) and is an acidic oxide. The density of the hydroxyl groups and hence the basicity of the surface can be controlled also by dehydroxylation of the surface by extended heating at elevated temperatures. Fig. 18 shows the adsorption isotherms of Aerosol-OT ( A O T ) on alumina and silica from cyclohexane [25]. It can be seen that the anionic surfactant has a greater affinity for the basic oxide than for the acidic oxide. The situation is reversed in the case of the adsorption of the cationic surfactant dimethyl dodecylamine (DDA). D D A adsorbs more on acidic silica than on alumina (Fig. 19). Calculations based on a plateau adsorption value of 3 x 1 0 - 6 mol m 2 on alumina give a parking area of about 0.55 nm 2 for the AOT molecule. This is in good agreement with the published values for AOT parking areas at the water-xylene and water-isooctane [26] interfaces suggesting that it adsorbs at the alumina-cyclohexane interface as a monolayer in an orientation perpendicular to the adsorbent with the hydrocarbon chain extending into the solution. Fig. 18 also shows the adsorption of AOT on dehydroxylated alumina. Dehydroxylation, in this case, is 6

~

e,l

E ~-6

5

L

0

/

Alumina

('4

E

i

503

.~

.~---''~

Silica

~"

5

J~" //

-6

E 4

/ /

/

/

/

x

3

/ / ~x

(n -0 dO

2

tl i A /

O_

/

o el

O

<

0

0

I

I

5

10

15

1

I

'

20

25

50

Residual C o n c e n f r a f i o n ,

35

6

x 10 , m o l / I

Fig. 19. A d s o r p t i o n of cationic D D A on different solids.

achieved by heating the alumina at 900°C for 48 h and verified by the disappearance of the OH vibration bands at 3800cm -1 from the IR spectrum of the alumina sample. Dehydroxylation increases the acidity of the surface and hence reduces the adsorption of the anionic AOT. It is clear from these results that AOT adsorbs through interactions with the hydroxyl groups on the oxide surface. These results also suggest that for a given surfactant the relative acidity of the oxide surface is one of the major factors that determine the extent of interaction.

E o

3.2. Adsorption on graphite

4

x

>-

0

,,. , . , , ' "

°=

2

~.

• •

,/_,;' ...

I'l:i ,'~'.,"

0

Alumina Silica

0

7 i

Dehydroxylafed, I r Aluminal P 0

D

Residual

10

15

20

Concenfrafionx

50

25

35

6

10 , m o l / I

Fig. 18. A d s o r p t i o n i s o t h e r m o f O T on different solids from cyclohexane.

The adsorption isotherm of AOT on graphite from cyclohexane follows a very interesting pattern as shown in Fig. 20 [27]. The adsorption increases sharply in the initial part suggesting high affinity of the surfactant for the solid at low concentrations. The adsorption then appears to reach a plateau and calculations based on apparent monolayer coverage at this level yields a parking area of approximately 1.03 nm 2 (AOT molecule) -1. This molecular parking area corresponds to an AOT molecule adsorbing flatly on the solid-liquid interface. At higher AOT concentrations, above 10-2mol 1-1, a further sharp increase in the

504

P, Somasundaran, S. Krishnakumar / Colloids Surfaces A." Physicochem. Eng. Aspects 123-124 (1997) 491 513 5 ,

to form aggregates at the interface as shown in Fig. 21(b). This leads to a sharp increase in adsorption density as well as a decrease in settling rate due to the disappearance of the interparticle aggregation observed at low surface coverages. Formation of such reverse hemimicelle-like aggregates at the interface has been reported previously for the adsorption of 1-decanol on graphitized carbon black from non-polar solvents [28].

E -6

E

t~ O

×

6~ ¢"D

2

-

3.3. Surfactant desorption and effect of soh,ent © "C

:

5 Residuat

'2:

"5

~_~"~

Cencenfrofion

__':~"

50

55

5

x 10 , m o l / I

Fig. 20. Adsorption of AOT on graphite from cyclohexane.

absorbed amount occurs and this reaches to about 5 times the initial plateau value at about 3 x 1 0 2 mol 1-x At low surface coverages the polar head group of the flatly adsorbing AOT is exposed to the solvent with which it is not compatible. In such a case the AOT molecules, as shown in Fig. 21(a), can form interparticle aggregates that would effectively create a polar microdomain to shield the head groups from the solvent. Such an interparticle aggregation can account for the increased settling rate at low coverages. As the AOT concentration is increased the adsorbed molecules are proposed

While adsorption studies yield information on the interactions responsible for adsorption, desorption studies give information regarding the reversibility of the adsorption and hence a qualitative handle on the strength of the solute-solid interactions. Fig. 22 shows data for the desorption of AOT, pre-adsorbed on alumina from cyclohexane, into solvents of different polarities. It can be seen that as the solvent polarity is increased the amount of surfactant desorbed rises sharply above a certain value of the dielectric constant. As the solvent polarity increases, the surfactant interacts more with the solvent with a concomitant reduction in the surfactant-solid interaction. This increase in solvent-surfactant interaction with increasing polarity is also evident from the data for aggregation of AOT in different solvents [29]. Aggregation was observed in solvents of low polarities and the extent of aggregation decreased as the polarity of the solvent was increased as a result of increased interaction of the surfactant with the solvent. Most

2::o

A A

A A

A Fig. 21. Schematic representation of adsorption of AOT on graphite (a) at low concentrations by interparticle aggregation and (b) at high concentrations through intraparticle aggregation.

P. Somasundaran, S. Krishnakumar / Colloids Surfitces A: Physicochem. Eng. Aspects 123 124 (1997) 491 513 100

80 -

60 i

ca

40

i

20 k

/0

0

10

been estimated in some cases by considering them as supercooled liquids. Alternatively, these can be theoretically calculated from their structural formulae using the group theory approach [32]. For solid surfaces the interaction parameter can be estimated using a procedure similar to that used for polymers wherein the polymer is characterized by the center of its volume of solubility in a threedimensional plot of the interaction parameters of various solvents [33]. The net adsorption energy E, is the affinity of the solute j for the solid surface s less the affinity of the solvent i for the surface and the affinity of the solute for the solvent [34]:

Et =JsE - i s E-iJ E 20

50

aO

Dielectri.c

50

60

72,

(6)

93

Constant

Fig. 22. Desorption of A O T pre-adsorbed solvents of different polarities.

505

on alumina in

interestingly, beyond a certain solvent polarity the affinity of the surfactant for the oxide surface starts to increase again as indicated by the decrease in the desorbed amount. At higher solvent polarities, the hydrocarbon chain of the surfactant is less compatible with the solvent and tends to form aggregate structures essential to remove the hydrocarbon parts from the bulk solvent. This can be accomplished via micelle formation in solution or adsorption at a less polar interface. Evidently the latter mechanism is favored in this case. Studies with different oxides (Fig. 23) revealed similar trends with the curves shifting to the right in the low dielectric constant regime and moving to lower desorption values in the higher dielectric constant region as the acidity of the oxide is increased. The acidity of the oxides [30] increases in the order alumina
A similar approach is adopted to develop a generalized model for surfactant adsorption in this case. Simple algebraic differences between the interaction parameters are taken as being proportional to the strength of their mutual interaction. The larger the absolute value of 6 i - 6j, the weaker is the interaction between the two species, i and j: net adsorption =f(l,Ssolid -- 6solven t l, 16solute -- 6solvent I,IC~solid -- ~solute I)

(7)

These three binary interactions can be combined in a number of different ways. The simplest case is to take a linear combination of these terms as shown below 6ef f = abs(A [£]solid -- 6solvent [ "~ Bl6solute -- ~soivent [ -- C[ ~solid -- ~solute [)

(8)

where 6eft is the effective interaction parameter and A, B, C are numerical constants and will depend on specific system interactions such as electrostatic forces or lateral interaction among adsorbed molecules. It can be seen from the earlier discussion that for good adsorption this sum should be large or in other words the larger the value of 6eff the greater is the adsorption tendency. Conversely, one can argue that a low value of 6~ff will favor surfactant partitioning into the bulk solvent. Eq. (8) has been tested for a number of different surfactant solvent-solid systems [35]. Thus adsorption in non-polar media is governed by the relative interactions of the system components as described by their interaction

506

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123 124 (1997) 491 513 100

i

~

i

I

---

80

= o

i

-q.

0 Alumina • Smea D Hematite • Rutile

60

r', o

4-0

/ •i /'

:."

i

I

".

I

i/? l/. 2

"

I : :'

'/'Yo

'.:

|i., ,:

2,0

:'' _,

'/ -:

1:/ 0 0

10

20

30

aO

Dielectric

F i g . 23. D e s o r p t i o n

of AOT

50

60

70

80

Constant

from different solids into solvents of different polarities.

parameters. The adsorption of ionic surfactants on hydrophilic substrates proceeds to a monolayer and there is no further evidence of lateral interactions or aggregation among the adsorbed molecules. However, when the substrate is hydrophilic reverse solloids can form on the surface owing to the interactions among the polar head groups of the adsorptive molecules.

4. Polymers in aqueous media Polymers can exist in different conformations both in the solution and in the adsorbed state. Adsorption of polymeric materials onto solid surfaces can be quite different from that of small molecules in that the polymer adsorption is greatly influenced by the multifunctional groups that it possesses [36]. This stems from the widely varying sizes and configurations available for the polymer. In addition macromolecules usually possess many functional groups each having a potential to adsorb on one or more given surfaces.

4.1. Adsorption of polyelectrolytes Adsorption isotherms of polymers are determined similarly to those for surfactants, i.e. by

measuring the depletion of the polymer concentration in the solution after contact with the solid. Linear flexible macromolecules can assume different conformations at the interface and also exhibit varying degrees of attachment to the surface [37]. At the solid-liquid interface the macromolecules usually prefer a conformation that allows for maximum segment-surface contact. Since the attachment of one segment will increase the probability of neighboring segments to be adsorbed and the number of functional groups per molecule is large, multiple bonding between the polymer and the surface is favored. The result is normally an interfacial conformation consisting of sequences of adsorbed segments ("trains") alternating with free three-dimensional "loops" extending away from the surface and with the chain terminating at either end in two free dangling "tails" [38] (Fig. 24). Adsorption causes a loss of entropy of the polymer. Therefore, for adsorption to be favorable, the interaction energy per segment-surface attachment must be large. For polyelectrolytes, the major driving force for adsorption is the electrostatic attraction. To investigate this the adsorption of polyacrylic acid (PAA) on Sumitomo AKP alumina was studied under various pH conditions. Fig. 25 shows the effect of pH on the maximum adsorption

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 491-513

~

LOOPS

507

80

t ~

60

A . ~ Q. , x ' ~

TAIL

40

D

AKPS0

I

O

PAA 50ppm

A

PAA 500ppm

I

.

P~]000ppm

& " ~ "', ~ ~

\

~ 20

'9

\

6

7"\?

TRAINS

3 Fig. 24. Schematic representation of an adsorbed polymer at a plane interface showing the presence of trains, loops and tails.

4

5~

~ -20

~

10

11

NO

'X .A

-40 •

C~- ..0

-60 0.9 -80 0.8 Fig. 26. Variation in ~ potential of alumina suspensions with adsorbed PAA.

0.7 ~-~ 0.6 E =.~ 0.5 E. ~

0.4

.<

0.3 0.2 0.1 2

r

i

3

4

5

i

E

I

q

I

[

6

7 pH

8

9

10

11

12

Another important factor affecting polymer adsorption is the polymer molecular weight [35]. For non-porous substrates where adsorption is restricted only by the extent of accessible surface, the general observation is that adsorption increases with increase in chain length. This is observed in the case of PAA adsorption on non-porous AKP50 alumina as shown in Fig. 27 and is expected because the number of polymer-segment bonds increases with chain length of the polymer mole-

Fig. 25. Effect of pH on the adsorption of PAA on alumina.

density of PAA on alumina [39]. As can be seen, there is a decrease in the adsorption density with an increase in pH until the IEP of alumina is reached. Fig. 26 shows the variation in ~ potential of alumina suspensions in the presence and absence of adsorbed PAA. As can be seen clearly, the adsorption of the negatively charged polymer decreases the ~ potential significantly and also lowers the IEP of the alumina. These results suggest that electrostatic forces are predominantly responsible for the adsorption of PAA on alumina. However, even under conditions where the surface is highly negatively charged (pH 10) and the polymer highly ionized there is some adsorption observed. This may be attributed to the fact that at pH 10 there are still some positive sites on the alumina surface which can anchor some polymer segments.

0.9 O 0.8

I"

/

0.7

f

13 "~0.6 E

~o.5 o 0.4 .~

<

[] 5000 O 150000 Z~ 800000

0.3 0.2 0.1 0--

I 0

I

500

1000 Residual

Concentration,

I 1500

2000

ppm

Fig. 27. Effect of PAA molecular weight on its adsorption on non-porous AKP50 alumina.

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A." Physicochem. Eng. Aspects 123-124 (1997) 491 513

508

cule. However, the adsorption density of PAA on the porous Linde alumina decreases with increase in molecular weight (Fig. 28). This is attributed to the exclusion of the higher molecular weight polymers from a large fraction of the pores which are accessible only to the smaller molecules.

4.1.1. Polymer conformation by spectroscopic techniques PAA can exist in different conformations depending on the solvent, pH, and ionic strength conditions (Fig. 29) [40]. Such a flexibility also influences its adsorption characteristics on solids and in turn affects subsequent suspension behavior. Using a fluorescent labelled polymer and by monitoring the extent of excimer formation it was shown that the polymer at the interface could have a stretched or coiled conformation at the interface 0.8 0.7 0.6 ~.~ 0.5 E ~" 0.4

t

_aO _

--

depending on the pH. The rationale behind the use of this technique is the observation that the extent of excimer formation which depends on the interaction of an excited state pyrene of the polymer pendant group with another pyrene group in the ground state has a direct bearing on the polymer conformation. This may be understood with reference to Fig. 30 which shows that, at low pH, there is a better probability for intramolecular excimer formation between pyrene groups resulting from a favorable coiled conformation. Similarly, a low probability for the excimer formation at high pH may be understood as a consequence of the repulsion between the highly ionized carboxylate groups in the polymer and the subsequent stretching of the polymer chain. This difference is reflected in the nature of their fluorescence spectra as seen in Fig. 31. Also, these studies have demonstrated that the PAA adsorbed in the stretched form on alumina at high pH could not become coiled because the conformation could not be altered by lowering the pH subsequently (Fig. 32) [41]. In contrast, the polymer adsorbed in the

-O-

_

.......

A

0,3

.<

[] 5000

_;o -? ~o,

,_,5. 800000

a...

0.2 O 150000

0.1

LOw ~H

1 500

1000

Re,dual

1500

Coacen~a6on,

2000

ppm

Fig. 30. Schematic representation of the correlation of the extent of excimer formation and intrastrand coiling of pyrenelabeled PAA.

Fig. 28. Effect of PAA molecular weight on its adsorption on p o r o u s Linde alumina.

Monomer

,I l t'

.

.

.

-+

.

.

.

.

.

.

.

+ + +--+

(a) pH 4.4

.

High ~

Eximer ph 49

.

+ . . . .

+--

(b) pH 10.5

Fig. 29. Schematic representation of PAA conformation under different p H conditions.

350

400

450 5()0 Wavelength (nm)

550

600

Fig. 31. Fluorescence emission spectra of adsorbed polymer at two p H values.

P. Somasundaran, S. Krishnakumar / Colloid~ Surfitces A." Physicochem. Eng. Aspects 123-124 (1997) 491 513 a)

low ph

b)

4-'4-4"4, expanded/solution

4. ae t" 4" coiled/solution

)

high ph

P

L

T

V

low ph

high ph

E

B

expanded,'adsorbed strong binding

coiled/adsorbed loose binding

high ph

low ph F

partiall~ expanded/adsorbed

expanded/adsorbed

Fig. 32. Schematic representation of the adsorption process of pyrene-labeled PAA on alumina. A) At low pH polymer is coiled in solution which leads to B) adsorption in coiled form C) Subsequent raising of the pH causes some expansion of the polymer D) Polymer at high pH in solution is extended and binds E) strongly to the surface in this conformation F) Subsequent lowering of pH does not allow for sufficient intrastrand interactions for coiling to occur. coiled form at low p H did stretch out when the p H was increased.

4.1.2. Determination of bound fraction of the adsorbed polymer using electron spin resonance spectroscopy ESR spectroscopy is a powerful analytical tool, which can provide information on the mobility of specific molecules and thus fluidity of the microdomain environment. In our study, PAA with attached nitroxyl groups (molar ratio 1:100) was used to investigate polymer conformation on solid surfaces [42]. The number of polymer segments bound to the surface ("trains") can be estimated from the complex ESR spectra obtained from labeled polymer adsorbed on alumina surface

509

(Fig. 33). These spectra are composed of two distinct components: non-restricted ("free rotational"), corresponding to the parts of polymer molecules in solution ("loops" and "tails"), and restricted ("frozen"), corresponding to the parts of polymer molecules directly bound or in very closed contact with solid surfaces. The relative increase in the peak at 3235 G in the series of spectra collected at increasing polymer concentration reveals an increase in non-restricted spectral component. This suggests that with an increase in the number of polymer molecules at the solid-liquid interface, a larger and larger fraction of the polymer molecules are pushed away from the solid surface. Using computer simulation, we decomposed the observed spectra and calculated the ratio of spectral components. The fraction of segment bound (relative number of "trains") together with the adsorption isotherm of polymer are given in Fig. 34. It can be seen that the number of free polymer segments (loops and tails) is at a maximum at the polymer adsorption plateau, indicating intermolecular repulsion of the polymer molecules at high packing density.

5. Adsorption of polymer-surfactant mixtures Mixtures of polymers and surfactants are widely used in various commercial formulations since these mixtures possess special interfacial and colloidal properties. Adsorption of polymer surfactant complexes has been studied and they are also known to have very interesting surface-modifying properties. For example, it has been shown that interactions between a cationic polymer and an anionic surfactant promote the flotation of anionic quartz. In one study using polyethylene oxide (PEO) and SDS, the former of which normally does not adsorb on alumina, was induced to adsorb by preadsorbed SDS [43]. There was near complete adsorption of the polymer once the solloid aggregates of the surfactant formed at the interface. Interestingly, the conformation (as observed by fluorescence emission) of the co-adsorbed PEO was markedly influenced as a result of its interaction with SDS with the polymer being completely

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A." Physicochem. Eng. Aspects 123-124 (1997) 491-513

510 2800

FREE ROTATIONAL COMPONENT

2.00

2000

1600

~1200

800

400

0 52120

3200

3 2 4i 0

52 ~ 0

Field,

A 3280

3500

lauss

Fig. 33. E S R spectra o f n i t r o x i d e - l a b e l e d P A A s h o w i n g t w o c o m p o n e n t s c o r r e s p o n d i n g t o f a s t m o t i o n ( t a i l s )

1

o.soj

1

~o o

0,8

~

0.9 ~=

{0.6

. o . g !_

°251 / _E o.o i j° _o

_

. . . . . . . . .

I

\

/

c

z

~ 0.4

andtrains(slowmotion).

o.15i{

i

O,

0 7 "d

0.10 0.2

-0.6

L 0.05 i

0

I

r 10

i , ,) 100 1000 Residual eoneen~'ation, ppm

,,,

0.5 I0000

Fig. 34. F r a c t i o n of p o l y m e r segment b o u n d on a l u m i n a surface as a function o f P A A concentration.

stretched at high surfactant concentrations (Fig. 35). In contrast to the above, dodecylsulfate, which does not adsorb on silica (because of electrostatic repulsion), was forced to adsorb by preadsorbed PEO (Fig. 36). The mechanism of inter-

i/' . . . . . . . ....... ""

0,00 i 10 -6

10 -5

~ / 10 -4

rpypEOpy: 15 ppm] 05 M NoZS04 ] 10 -5

SDS RESIDUAL CONCENTRATION, kmol/'rn 3 Fig. 35. C o n f o r m a t i o n o f pyrene-labeled polyethylene glycol at the a l u m i n a - w a t e r interface in the presence o f p r e - a d s o r b e d SDS.

action between the surfactant and the polymer at the silica interface could be similar to that in bulk solution: association between the ether oxygen of

P. Somasundaran, S. Krishnakumar / Colloids Surfaces A. Physicochem. Eng. Aspects 123-124 (1997) 491-513 0.00007 ! • ©

0.00006

£~

O.00005

o

0.00004

o

0.00003

~,Z ~ F<~''~" '~ r

PEO]

23000 M.W. PEO

~/~/f'~'~

SILICA: 25 mZ/g PREADSORBED RED 4 m g / 9 p H ~ 6.5

E

z"

NO P R E A D S O R B E D 5000 M W. PEO

:!

~a..

/!

< 0.00,:,02

/

]

I

S

0.00001 V

~ ""

O.O0O0O i~ i0 -5

,( m ~ 2 " l ~ 10 -4

'! J.i 1 :,[.I ,I 10-2

10-3

,

, ~J

SDS RESIDUAL CONCENTRATION, k m o l / m 3

511

the binding of SDS to pre-adsorbed PEO, suggesting that the adsorbing tendency is reduced once the complex forms in solution. There was, however, no effect of the pre-adsorbed PEO amount on the SDS uptake at the silica interface. In fact at higher pre-adsorbed PEO levels there was some desorption of PEO as the SDS adsorbs. These results suggest that there is a minimum amount of PEO required to activate the uptake of SDS and this amount is lower than that required for interactions in solution. Thus, the surface is seen in some way to activate the polymer-surfactant interaction. Clearly various types of polymer-surfactant interactions can be designed to achieve desirable interfacial properties.

Fig. 36. Adsorption of SDS on silica with pre-adsorbed PEO.

6. Summary and conclusions PEO with the surfactant head group. Such binding occurs in a narrow concentration range and saturation adsorption is reached near the CMC of the surfactant. The sharp increase in adsorption is attributed to the strong interaction between the hydrocarbon chains of the surfactant molecules that are anchored on the polymer chain rather than on the silica surface. It is to be noted that there was a measurable difference in the adsorption of dodecylsulfate depending on whether adsorption took place from a bulk solution of PEO and SDS or from SDS solution onto pre-adsorbed PEO (Fig. 37). Adsorption of SDS from P E O - S D S mixtures was consistently lower than 0.00007 O ADSORPTION ONTO PREADSORBED PEO . z~. ADSORPTION FROM PED-SDS MIXTURES

0.00006

< "5 E

z" o E o

0 0000S

0.00004

~ , ,

//

PEO: 5000 M.W. 2 SILICA: 25 m /g

S

pH ~ 6.5

II /

]l

0,00003 ,

0,00002

z}/

0.00001 [ O.O000O L lO-S

i0 -4

10 .3

SDS RESIDUAL CONCENTRATION.

iO -2 kmol/m 3

Fig. 37. Adsorption of SDS onto silica from PEO-SDS mixtures showing effect of mode of addition.

Adsorption of surfactants and polymers at the solid liquid interface depends on the nature of the surfactant or polymer, the solvent and the substrate. In addition, system properties such as temperature and even system history can be expected to affect adsorption significantly. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. At low concentrations the adsorption is mainly by ion exchange (electrostatic interaction). At higher concentrations lateral interactions among the adsorbed molecules causes a sharp rise in adsorption which plateaus out once the CMC of the surfactant is reached. In some cases, at high concentrations a second layer of surfactant can adsorb with a reverse orientation. Non-ionic surfactants adsorb primarily through hydration or hydrogen bond interactions and this depends on the hydration characteristics of the substrate. In this case the lateral interactions are much stronger owing to the absence of electrostatic repulsion between the surfactant head groups and this leads to a sharp increase in the adsorption corresponding to solloid formation. The adsorption of non-ionic surfactants can be greatly enhanced by the presence of ionic surfactants and vice versa. Experiments have shown that non-ionic and ionic surfactants when used in mixtures can be forced to adsorb on substrates on which they

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do not exhibit much adsorption by themselves. The synergistic effect of these mixtures on adsorption is hypothesized to be due to reduction in charge repulsion leading to better packing of the ionic surfactants while for the non-ionic surfacrants the increase in adsorption is due to their solubilization in the hydrophobic microdomains formed by the ionic surfactants. While adsorption of simple components can be expressed in terms of the properties of the components and the solid, the adsorption of mixtures cannot yet be described theoretically. In fact interactions even in bulk solution do not follow, for example, the regular solution theory in many cases. In organic media, adsorption depends on the relative acid-base characteristics of the solute, solvent and substrate. Adsorption of ionic surfactants on oxide surfaces proceeds to a monolayer and there is no evidence of lateral interaction between the adsorbed molecules. However, on hydrophobic surfaces, interactions are possible among the polar groups of the adsorbed molecules leading to the formation of reverse solloids and a concomitant increase in adsorption density somewhat resembling solloid formation in aqueous media. Polymer adsorption in aqueous media is controlled by the polymer charge, molecular weight, solvent, solution conditions (pH, ionic strength) and porosity of the substrate. Polyelectrolyte adsorption proceeds mainly by electrostatic interactions while non-ionic PEO adsorbs by hydrogen bonding interactions similarly to the non-ionic surfactants. However, the adsorbed layer structure is very different in the case of polymers from that of the surfactants. A polymer can have various configurations at the interface and this has been studied for the case of PAA on alumina by fluorescence and ESR spectroscopy, The adsorbed layer in this case can be thought of as a fuzzy polymer layer with trains, loops and tails. Also, it is clear that the reorientation of the polymer molecules at the interface takes place in response to changes in the surrounding environment such as changing pH in the case of PAA on alumina. Polymersurfactant mixtures exhibit synergism in adsorption akin to ionic~on-ionic surfactant mixtures. The origin of this synergism lies in the ten-

dency of polymers and surfactants to interact in solution at low concentrations via interactions among the hydrophobic moieties. In the case of mixed systems, the order of addition is found to be an important parameter determining the equilibrium adsorption.

Acknowledgment The authors wish to acknowledge National Science Foundation (NSF-CTS-9311940), Engineering Research Center, University of Florida, Environmental Protection Agency (RR823301-01-0) for financial support.

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