water interface

COLLOIDS AND SURFACES

CollOids and Surfaces A: Physicochemical and Engineering Aspects 94 (1995) 125-130

ELSEVIER

A

Adsorption of SDS and PVP at the air/water interface * J.P. Purcell a, R.K. Thomas a, J. Penfold b, A.M. Howe a b

C

Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK Rutherford Appleton Laboratory, Chilton, Didcot, axon, OX11 OQX. UK C Kodak Ltd., Headstone Drive. Harrow, Middlesex HAl 4TY, UK

Received 26 September 1993; accepted 6 July 1994

Abstract The technique of specular reflection of neutrons has been used to investigate the adsorption from polyvinylpyrroli.done/sodium dodecyl sulphate (PVP/SDSI) solutions at the air/water interface. Isotopic substitution was used to highlight features of the adsorbed layer and to distinguish between solvent and solute in this layer. At low SDS concentrations the presence of the polymer causes an increase in the adsorption of surfactant (at 1 x 10- 4 M SDS, the area per molecule of SDS is 2271>.2 in the absence of polymer, 100,.\2 in 0.5% PVP and 85,.\2 in 2.0% PVP). On increasing the surfactant concentration this difference is reduced until, at high SDS concentrations, the surface concentration of surfactant is higher in the absence of PVP (at 0.01 M SDS, the area per SDS molecule is 42,.\2,44,.\2 and 46,.\2 at 0%, 0.5% and 2.0% PVP respectively). Keywords: Adsorption; Polyvinylpyrrolidone; Sodium dodecyl sulphate

1. Introduction

Specular reflection of neutrons is a technique that has recently been applied to the study of the structures of interfaces. It has been used successfully to elucidate structural information on adsorbed surfactant layers at both the liquid/air [1,2] and solid/liquid interfaces [3]. This technique has recently been extended to the study of polymer conformation in solution, on such systems as poly(dimethylsiloxane) in toluene [4] and polyethyleneoxide in water [5]. Here, it is used to investigate a more complex situation, the adsorbed layer in an aqueous solution of both surfactant,

* Correspondmg author. Presented at the Polymers at Interfaces conference, held at Bnstol Umversity, 8-10 September. 1993.

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0927-7757/95/$09.50 © 1995 Elsevier SCience B.Y. All rights reserved SSDI 0927-7757(94)02980-6

sodium dodecyl sulphate (SDS), and polymer, polyvinylpyrrolidone (PVP). Polymer/surfactant solutions in general, and PVP/SDS in particular, have been analysed using a variety of experimental techniques. Some of the more noteworthy methods employed are surface tension [6], solubilization [7], dialysis [8,9], conductivity measurements [10] and radio tracing [11]. These methods have demonstrated that the behaviour of these solutions differs substantially from that of the pure polymer or pure surfactapt, with three different regions being observed. Fig. 1 shows a schematic representation of a surface tension vs. log of surfactant concentration plot, illustrating the position of the transition points. The labelling of these two transition points, T 1 and T z , was introduced by Jones [12] in his studies of SDS-poly(ethylene oxide) solutions. This is in contrast with aqueous surfactant solutions where only

J.P. Purcell et at leallaids Surfaces A' Physicachem Eng. Aspects 94 ( 1995) 125-130

126

' ....

, \ \ \

c

o

"wc

\ \ \ \

\ \

Q)

\

+-'

\

Q)

o

{g

\

T1·'-----;.\--~

:::J (f)

C.M.C.

Log concentration Fig. 1. Schematic representatIOn of surface tension YS. concentration plot of a surfactant in the presence of a co.mplexing polymer.

one break point at the critical micellization concentration (CMC) is observed (see Fig. 1). The three different regions are generally explained in terms of a polymer-surfactant interaction. Initially, at low surfactant concentrations it is thought (on the basis of electrical conductance measurements) that single SDS ions may be binding to the polymer [9]. On increasing the surfactant concentration the first transition point, T 1 is reached. In PVP/SDS solutions T 1 occurs at about 2 x 10- 3 M SDS. This point represents the onset of formation of polymer/surfactant complexes. These complexes consist of SDS molecules clustered in subunits [13] which are themselves adsorbed on the polymer resembling a "string of beads" [14]. On further addition of SDS to the solution more of these polymer-bound micelles are formed until the polymer is saturated at T z. At this point the formation of regular surfactant micelles commences and the behaviour exhibited is then the same as that of pure SDS solutions.

2. Experimental details Chain-deuterated SDS was obtained from Promochern Ltd. and recrystallised from ethanol. The water used was of ultrahigh purity, from an

Elgastat water purifier (Elgastat UHQ, Elga, UK), and the DzO was supplied by MSD Isotopes. The Teflon sample troughs were soaked overnight in Decon 90 and rinsed in ultrapure water. The PVP had a molecular weight of 700000 and was obtained from BDH. The neutron reflection experiments were carried out on the CRISP reflectometer at the Rutherford Appleton Laboratory in Didcot, UK. The reflectometer is described in detail elsewhere [15]. The instrument was calibrated using DzO, and a fixed grazing angle of incidence of 1.5 a was employed throughout. All measurements were carried out at 25 a C using either a multidetector or a single detector, i.e. either a well-shielded single 3He gas detector or a one-dimensional multidetector with a positional resolution in either the vertical or horizontal plane of less than 1 mm [16]. The multidetector data established the absence of small-angle scattering, and a flat background was subtracted before the analysis was preformed.

3. Results and discussion Neutron reflectivity profiles were obtained for SDS solutions with different concentrations of PVP (0%,0.5% and 2.0%). For all of these experiments deuterated SDS in a null-reflecting solution, either with or without PVP was used. The scattering length density of PVP at 0.206 x 10- 6 is very close of that of air. Thus, a HzO : DzO ratio of 1 : 0.0597 is needed to give a null-reflecting PVP solution. Assuming a single layer of scattering length density p and thickness r, the area per molecule, A, at the surface can be calculated using the relationship [17] A

= In,b.jpr

(1)

i

where n is the number of atoms of i and b is the scattering length of atom i. The surface excess can then be evaluated from [17]

10 16

A=NT

(2)

127

I.P Purcell et at /Colloids Surfaces A: Physlcochem. Eng. Aspects 94 (1995) 125-130

where N is Avogadro's number and r is the surface excess expressed in mol cm - 2 and the area per molecule is in A. 2 • The surface excesses of SDS, derived from the reflectivity measurements, are shown in Fig. 2 for 0%, 0.5% and 2.0% PVP. At low SDS concentrations the surface excesses of SDS in all cases increase rapidly with increasing SDS concentration. The 2.0% PVP solution exhibits consistently higher surface excesses than the 0.5% PVP, which in turn is consistently higher than the pure SDS solutions, but it is only at the very low SDS concentrations that these differences are significant. At 0.0001 M SDS the differences in surface excesses are large and correspond to areas per molecule of SDS of 85 A. 2 in 2.0% PVP, 100 A. 2

4

N---

S

3

<.>

;:::, 0

5 S

2

S >< !-.

in 0.5% PVP, and 227 A. 2 in 0% PVP. These changes in the amount of SDS adsorbed at the surface are clearly demonstrated by the reflectivity profiles shown in Fig. 3. The three reflectivity profiles obtained with 0.001 M SDS differ dramatically. The 2.0% PVP solution reflects most strongly, followed by the 0.5% PVP solution, with the 0% PVP solution having a weak signal. A further profile, obtained from 0.00001 M SDS in 2% PVP is also included in Fig. 3. This demonstrates that at this very low SDS concentration adsorption occurs. In contrast, there is no measureable signal from 0.00001 M SDS in 0% or 0.5% PVP. Above 0.0001 M SDS the differences are within the error range ± 5 A. 2 • This is compatible with the surface tension curves [6J where, at low SDS concentrations, the pure SDS has the highest surface tension, and an increase in the PVP concentration results in a decrease in surface tension. This would suggest that at low SDS concentrations the presence of PVP results in an increase in the amount of SDS adsorbed at the surface. PVP is itself surface active, and the presence of PVP could lower the energy for SDS adsorption in the same way as it lowers the energy for aggrega-

o-t--..---r----,.--,---,----,--........--l 0000

0.001

0.002

0003

10. 4. - - - - - - - - - - - - - - - - - - - , 00.0001 M SOS <>0.0001 M SOS .5% PVP +0.0001 M SOS 2 % PVP XO.OOOOl M SOS 2% PVP

0.004

SDS Concentration (M)

t4

itf

Hn

4.0-,------------------. 0%

N---

3.8

>

S

, !

IlfHI< <,

f

~f

05%

;g o

5

f

3.6

2.0%

S

.....

~

3.4

~

3.2 +----,--,---.-,---.--.,.---..-.,.---.----1 0.002 0.004 0.006 0.008 0.010 0.012

5DS Concentration (M) Fig. 2. Surface excesses of SDS as a function of total SDS concentratIon. The low SDS concentration region is shown above, with the higher SDS concentration region shown below.

0.100

0.150

0.200

0.250

Momentum transfer K(.~-11 Fig. 3. Neutron reflectivity profiles of 0.0001 M SDS III various PVP concentrations and 0.00001 M SDS/2.0% PVP.

128

I P Purcell et al./Colloids Surfaces A: Physicochem. Eng. Aspects 94 (1995) 125-130

tion in solution (T 1 < CMC), via reduced repulsion of headgroups and removal of water from the chain region. This could then account for the increased concentration of PVP at the interface. Alternatively, it is possible that the aggregates formed below T 1, (where single SDS ions are thought to be binding to the polymer) are also surface active, and that it is the adsorption of these that results in increased amounts of SDS at the surface. However, as the behaviour of the PVP was not investigated directly in these experiments it is not possible to confirm which of the above is taking place. At 0.003 M SDS all three curves intersect and above this concentration the order is reversed. The pure SDS solutions now have the highest surface excesses followed by the 0.5% PVP, with the 2.0% PVP now having the lowest values, although the differences here are less than at the low SDS concentrations. It can be concluded from the above that the presence of PVP does influence the amount of SDS adsorbed at the surface. At low SDS concentrations at the point of greatest difference, 0.0001 M SDS, there appears to be a 60% increase in the amount of SDS present in going from a pure SDS solution to one containing 2.0% PVP. At the higher concentration of 0.01 M SDS, the situation is reversed and the amount of SDS at the surface is decreased on addition of PVP. Two further sets of measurements were carried out to examine the effects of addition of PVP on the adsorption of SDS at the interface over a wider range of conditions. In the first of these, reflectivity profiles of 0.04 M SDS in 10- 3 M NaCl with 0.5%, 1.0% and 5.0% PVP solutions were obtained. These

concentrations were chosen as a similar range had been investigated using radiotracing methods [11]. The profiles are shown in Fig. 4 and the results of the single-layer analysis are shown in Table 1. From Fig. 4 it is immediately obvious that there is no systematic change in reflectivity with amount of PVP present. The highest reflectivity is obtained from the 5.0% PVP solution, the lowest from the 1.0%, with the 0.5% falling in between. The 5.0% and 1.0% PVP solutions, with a difference of ± 4 A2 , have the greatest difference in the areas per molecule. Given that there is an error of ± 5 A2 in arriving at the areas per molecule, these differences are not significant. This is in contrast with

10"3.------------------.

•

•

!

10-6

0.05

0.10

0.15

0.25

•

,

0.30

Fig. 4. Neutron reflectIvity profiles of 0.5% PVP (e), 1.0% PVP (1":.), and 5.0% PVP (+), III 0.04 M SDS/1O- 3 M NaC!.

PVP concentration

Layer thickness

(w/w%)

(A)'

Area per molecule (A2)b

Surface excess (molcm- 2 )

0.5 1.0 5.0

18 18 18

38

40 36

4.4 4.2 4.6

± 2.0 A. b±5.oA2.

0.20

+

•

K/A-1

Table I Effect of an increase in PVP concentration in constant concentratIOn 0.04 M SDS/1O- 3 M NaCI solution

a

t

r x 10- 10

I P. Purcell et uf./Colloids Surfaces A. Physicochem. Eng. Aspects 94 ( 1995) 125-130

the findings of Chari and Hossain [11]. From their radiotracing experiments they inferred that increasing the PVP concentration from 0.2% to 3.0% resulted in a decrease of 1/3 in the concentration of SDS at the surface. A similar set of experiments was carried out below T I (at 0.0008 M SDS), this time no NaCI was used, but a wider range of PVP concentrations was investigated. The reflectivity profiles are shown in Fig. 5 and the results summarised in Table 2. In this case the changes in area per molecule of SDS are very slight. The most substantial difference of 4.6 A2 occurs between the 0.5% and 8.0% solutions. This is still within the error margin, and so it 10-3, - - - - - - - - - - - - - - - - - ,

0.05

0.10

015 KIA"

020

025

030

Fig. 5. Neutron reflectivity profiles of 0.5% PVP (D). 1.0% PVP (0 ). 20% PVP ( x ). 5.0% PVP (/::,) in 0.0008 M SDS.

appears again that at this concentration the addition of PVP does not result in a significant change in the concentration of SDS at the interface.

4. Summary

The effect of addition of PVP to an SDS solution depends on the concentration of SDS. At low SDS concentrations (0.0001 M) dramatically different reflectivity profiles are obtained on addition of PVP. These are compatible with an increase of up to 60% in the amount of SDS adsorbed at the interface. This cannot be ·explained fully by these experiments, and could be due to a lowering in the energy for SDS adsorption by the PVP. Another possibility is that the aggregates suggested by Fishman and Eirich are more surface active than SDS monomers [9]. Experiments were also carried out on two constant SDS concentrations with varying amounts of added PVP. Neither of these showed a significant change in the adsorption of SDS on changing the PVP concentration. One of these, 0.04 M SDS in 10- 3 M NaCl had previously been studied [11], where a decrease of 1/3 in the amount of SDS adsorbed at the interface on addition of PVP was reported.

Acknowledgements

We thank the Science and Engineering Research Council for their support of this project. LP.P. also

Table 2 Effect of mcreasing PVP concentration in 0.0008 M SDS solutions PVP concentration

Layer thickness

Area per molecule

(w/w%)

(A)a

(A')b

0.5 1.0 2.0 5.0 8.0

19.9 17.0 17.0 19.0 19.0

60

a

± 2.0 A.

b

±5.oA2.

57

56 59

55

129

Surface excess (molcm- 1 ) 2.8 2.9 3.0 2.8 30

r x 10 -10

130

J.P. Purcell et al IColloids Surfaces A' Physlcochem. Eng. Aspects 94 ( 1995) 125-130

thanks Kodak Limited for funding in the form of a studentship.

References [1] E.M. Lee, RK. Thomas, 1. Penfold and RC. Ward, 1. Phys. Chern., 93 (1989) 381. [2] 1. Penfold, E.M. Lee and R.K. Thomas, Mol. Phys., 93 (1989) 381. [3] E.M. Lee, RK. Thomas, P.G. Cummins, EJ. Staples, 1. Penfold and A.R Rennie, Chern. Phys. Lett., 162(3) (1989) 196. [4] O. Guiselin, L.T. Lee, B. Farnoux and A. Lapp, 1. Chern. Phys. 96( 6) (1991) 4632. [5] A.R Renme, R.J. Crawford, E.M. Lee, RK. Thomas, T.L. Crowley, S. Roberts, M.S. Qureshi and R.W. Richards, Macromolecules, 22 (1989) 3466.

[6] H. Lange, Kolloid Z. Z. Polym., 243 (1971) 101. [7] M. Murata and H. Ari, 1. CollOId Interface SCI., 37(1) (1971) 475. [8] H. Ari, M. Murata and K. Shinoda, 1. Colloid Interface Sci., 37(1) (1971) 223. [9] M.L. Fishman and F.R Eirich, 1. Phys. Chern., 75(20) (1971) 3135. [10] M.L. Fishman and F.R Einch, J. Phys. Chern., 79(25) (1975) 2470. [11] K. Chari and T. Hossain, J. Phys. Chern., 95 (1991) 3302. [12] M.N. Jones, J. Colloid Interface Sci., 23 (1967) 36. [13] B. Cabane and R. Duplessix, J. Phys., (Paris), 43 (1982) 1529. [14] E.D. Goddard, 1. Soc. Cosmet. Chern., 41 (1990) 23. [15] 1. Penfold, RC. Ward and W.G. WillIams, 1. Phys. E., 20 (1987) 1411. [16] 1. Penfold and RK. Thomas, 1. Phys: Condens. Matter, 2 (1990) 1369. [17] 1.R Lu, A. Marrocco, T.J. Su, R.K. Thomas and 1. Penfold, 1. Colloid Interface Sci., 158 (1993) 303.