- Email: [email protected]
The effect of solvent quality on the interaction between two layers of adsorbed Polyvinyl Alcohol (PVA)
Colloids and Surfaces,
25 (1987) 77-90
Elsevier Science Publishers
B.V.. Amsterdam
77
-
Printed
in The Netherlands
The Effect of Solvent Quality on the Interaction between Two Layers of Adsorbed Polyvinyl Alcohol (PVA) Th. GOTZE and H. SONNTAG Zentralinstitut (G.D.R.)
(Received
fiir Physikalische
11 November
Chemie, Akademie
der Wissenschaften
1985; accepted in final form 14 January
der DDR, 1199 Berlin
1987)
ABSTRACT The energy-distance profiles of homopolymers with a narrow molecular weight distribution in different electrolyte solutions were measured. The results presented are for polymer solutions of constant concentration. We found that the interaction energy between the surfaces always takes the form of repulsion. This is in agreement with the theoretical prediction of a restricted equilibrium [ 261 according to which the amount of polymer between the surfaces does not change during compression. The thickness of adsorbed polymer at the interface is equivalent to a few radii of gyration of a free coil in solution. In the lyotropic sequence we found the thickness increased with increasing radii of cations. We noticed irreversible behaviour (dependent on time) due to the first compression and found different compression/decompression cycles.
INTRODUCTION
The application of macromolecules as stabilizers or destabilizers of colloidal dispersions has become more and more important in many industrial processes. The conditions for stabilization are that polymer molecules must be adsorbed onto the particles and the adsorbed layers must prevent the particles from coming close enough for van der Waals forces to become effective. Many theories have been put forward but fundamental studies and experimental measurements of charged and uncharged macromolecules adsorbing on the particle surface have lagged behind theoretical developments. There have been many studies on the adsorption of macromolecules onto polystyrene or aerosil particles [ l-31 ; rheological measurements have been carried out on polymer stabilized particles [ 41 and the thickness of adsorbed polymer layers has been measured by ellipsometry [ 51. Other authors have considered the bound fraction p as a measure of the conformation of adsorbed polymer molecules [ 6-81, but this parameter gives an incomplete picture and therefore is not suitable as a criterion for the stablization of particles. Until now there has only been one
0166-6622/87/$03.50
0 1987 Elsevier Science Publishers
B.V.
78 TABLE 1 Molecular weights and intrinsic viscosities of PVA samples No 1
2 3 4
n;i,(gmol-‘)
qHjO(0.1 dm,‘g-‘)
41700 48900 97100 120300
0.74 0.81 0.98 1.12
method of estimating the conformation of adsorbed molecules, namely, neutron scattering [ 91. We are most interested in cases where two adsorbed polymer layers, covering solid surfaces, interact. In this paper we deal with the interaction forces between polyvinyl alcohol (PVA) layers adsorbed onto quartz glass filaments immersed in aqueous media. The influence of different electrolytes in the polymer solution on the interaction forces between the filaments has been investigated; the addition of electrolytes was found to alter the quality of the solvent. Several attempts have been made to make direct measurements of forces between polymer surfaces, but they have tended to suffer from the unreliability of the technique used or from poorly characterized and very polydisperse polymers. Only a few works have produced quantitatively reliable results [ 10-131. The aim of the present work is to explore the influence of the solvent parameter on the interaction between two solid surfaces covered with an adsorbed polymer layer and therefore to throw some light on the conformation of adsorbed macromolecules. EXPERIMENTAL
Material
Water was double distilled and filtered through a 50-nm filter. All chemicals were of analytical grade. Polyvinyl alcohol samples were fractionated, and fully hydrolyzed samples prepared from the commercial ones (M,/M,1.2; see Table 1) . The aqueous polymer solutions were filtered through a 0.45-pm filter. The filaments were prepared from quartz glass (Herasil III) with a surface roughness of a few nanometers. The polymer concentration was in all solutions lop3 g cmp3 (q& = 10”). Apparatus
and procedure
The apparatus used in these experiments was adapted from that described previously [ 141. In this experiment, we did not apply a compensation system
79
to fix the vertical filament in the zero position and only the deformation of the taut band was measured. Also, we used a small glass cell (volume - 25 cm3) within a steel box [ 151. The two quartz glass filaments (diameter - 1 mm) were immersed in the liquid. The separation between the two quartz surfaces was additionally controlled by a differential spring mechanism: the first double cantilever spring is about a thousand times stiffer than the second cantilever steel spring so that a one micrometer movement is reduced to a one nanometer displacement between the two quartz surfaces. In principle, the same system is reported in Ref. [ 161. The calibration of the differential spring mechanism and of the piezoelectric column (6.5 nm V-’ ) with respect to the distance is done with a device for capacitive length-measurement before and after an experiment, with a resolution of a few nanometers. The apparatus was placed on a special optical table to reduce vibration due to external noise within a thermally isolated room (AT-t 1 ‘C ) which reduces thermal drift during the course of the experiment. Before the measurement was taken, the force-distance profile in the electrolyte solution was determined. The surfaces were then separated to about 4 mm and the electrolyte solution replaced by filtered PVA solution. The surfaces were allowed to incubate in the polymer solution for at least 8 h to reach an equilibrium. Following this period of incubation, the force-distance profile was determined again. The measurements were repeated in short time intervals (1.5 min) to prevent any thermal or mechanical disturbances. The forces were measured both in compression and decompression in two cases. The velocity of approach and withdrawal was in all cases constant (14 nm s-l ) . The results reported are based on at least two independent measuring intervals and always on measurements at different contact positions for the same pair of filaments. The difference between the time intervals was 1 h. RESULTS
The energy-distance profile between bare quartz glass surfaces immersed in lo-” and lo-’ mol dmp3 KC1 solution at pH -5.6 is shown in Fig. 1. The energy axis gives the interaction energy per unit area calculated with the Derjaguin approximation [ 171. The data are shown in a semi-log plot to allow the large variation in V(d) to be included. At lower salt concentration, an exponentially increasing repulsion with decreasing d was detected at d I60 nm, and at higher salt concentration similar behaviour was observed but with deviations from theoretical predictions. For all measurements in polymer solution a separation was achieved at which the energy between the quartz glass samples increased steeply with no measurable decrease in separation. Compression measurements by ultracentrifugation have indicated that for polystyrene particles covered with PVA samples at comparable values of interaction energy, the adsorbed layer is about 9-13
80 Log v/ lm‘*
1 -3
-4
-5
‘t
-6 ,L
0
Fig. 1. Energy-distance cases).
IO-:’ mol drn- ” KC1 (pH -5.6
in both
an error of about ? 2 nm ) and cannot be compressed further [ 32 ] . that twice the compressed adsorbed layer would be comparable with the closest approach of the filaments. This assumption was necessary because we cannot measure directly the distance between the quartz glass samples. Compression profiles were measured up to a surface separation of 20 nm. In all figures the onset of interaction was estimated for a given energy of interaction of 8.10M6J m-‘. Figure 2 shows the energy-distance profile between quartz glass surfaces in lo-* mol dm-” LiCl/PVA 4 solution and in lop2 mol dm-” KCl/PVA 4 solution. The energy commences at separations of about 110 nm and 140 nm. Down to these separations a monotonically increasing repulsion was observed. The energy varied in both cases approximately linearly with d, but with different slopes, At a separation of about 70 nm, the energy-distance profile of PVA in LiCl solution “leveled out” but in KC1 solution a steep rise was observed. Down to separations of d = 50 nm, a monotonic rise in energy was again evident. The energy-distance profile measured on the first approach was reversible. Figure 3 shows the energy-distance profile of PVA 1 in 10W2 mol dm-” KC1 and in lo-’ mol dm-” NaCl solution. Measurements commenced at a separanm
( within
profile in (@) IO-’ and (m)
It was assumed
-6i--L---‘-L
0
20
-A
40
60
80
100
120 d/m
Fig. 2. Energy-distance profile in PVA 4 solution and KC1 (broken line).
_,L_,L,,L 0
20
40
60
80
100
120 d/m
Fig. 3. Energy-distance profile in PVA 1 solution and KC1 (broken line).
140 -
(@,,= 10 “) in 10 ’ mol drn-” LiCl (solid line)
140 -
(r& = 10 “) in 10 a mol drn- .i NaCl (solid line )
Fig. 4. Energy-distance profile in PVA 1 solution ($,= 10 “) in lo-’ mol drn-” NaCl on first compression (solid line) and decompression (broken line) and compressions following initial approach ( 00 x ) .
‘\
\ \
I
I
I I I \
\,
‘1\
_-_I~-
20
-I
40
60
80
~
__~.I_
100
_
L
120 140 d/nm --
Fig. 5. Energy-distance profile in PVA 1 solution (@,= 10 ‘) in 10 ” mol dm bation for 8 h (broken line) and 23 h (solid line).
.’ NaCl after incu-
83
-5 t
LI_-b
-6
20
0
/ 40
60
80
100
120
140 d/nm-
Fig. 6. Energy-distance profile in PVA 3 solution and KC1 (broken line).
-3
($,= lo-“)
in 10m2 moldm-”
RbCl (solidline)
I
t : E n
‘5 0
-4 .
-5
-6
I-I_--LA_-.-A
0
20
40
60
80
100
120
140
d/ nrn-
Fig. 7. Energy-distance profile in PVA 2 solution and KC1 (broken line).
(&= lo-“)
in lo-’
mol drn-” CsCl (solid line)
84
Fig. 8. Energy-distance profiles in PVA 2 solution (@,= lo-“) in 10 ’ mol drn-I’ BaCl, on first compression (solid line, 0 )/decompression (broken line, x ) and 1 h later (9) first compression/( 0 ) decompression and in 10 -’ mol dm ’ KC1 (dotted line).
4
-3
^: E n > g -4
-5
-6
I
-I
1--L
20
40
Fig. 9. Energy-distance shown in Fig. 8.
60
profile
80
100
J
140 d/mT~-c
120
on compression
and decompression
following
initial
approach
as
85
tion of about 75 and 50 nm, respectively. Down to these separations a steep rise in the energy was observed but with different slopes for each electrolyte concentration. In Fig. 4, the energy-distance profile of PVA 1 in lop2 mol dme3 NaCl solution is shown in detail. The profile measured on the first approach was reversible, with a margin of error of about t5 nm. Upon separation, the surfaces’ V(d) fell to the initial position but the decompression led to a change in the V(d) profile. The repulsive interaction was dependent on time when the same PVA 1 sample was studied; in particular, when the surface was left to stand overnight (23 h) following compression the change in the energy-distance profile was different from that shown in Fig. 5. The incubation of surfaces for longer times resulted in a monotonically increasing repulsion commencing at a value of d E 80 nm. Figure 6 shows the energy-distance profile of PVA 3 in lop2 mol drn-” KC1 and in lo-’ mol dmd3 RbCl solution. The molecular weight of PVA 3 is somewhat lower than in Fig. 2. The same trend was observed for PVA 3 in LiCl solution but the interaction energy was lower at distances from 50
Let us first consider the interaction between the quartz glass filaments in pure electrolyte solution. In general the energy-distance profiles correspond to the DLVO theory, although the measurement in lo-* mol dmp3 KC1 solution indicates a deviation. Using the Derjaguin approximation for curved surfaces one can calculate the repulsive electrostatic double-layer interaction energy per unit area (valid for distances greater than the Debye length) [ 30,311.
-6 :
-1_
c
0
20
GO-
60
80
100
120
140 d/W-
Fig. 10. Energy-distance profile in PVA 2 solution (qi,= lo-“) in IO-’ mol drn-.” BaCl, on first compression ( l )/decompression (9) cycle after incubation for 25 h.
energy
F
area --2nR-
64nkT
ptanh’ K
(ey//4kT) exp ( - Ed)
where R is the mean radius of the filaments, n the number of ions per unit volume, k is Boltzmann’s constant, T is the absolute temperature, e is electronic charge, K is the Debye-Hiickel constant and I,Yis the surface potential. The experimental values in the lop3 mol dmP3 KC1 solution fit the linear part of the above equation. The effective surface potential is about 90 mV. This is comparable with the values of surface potential measured for aerosil particles dispersed in electrolyte solution [ 291. The slope of the energy-distance function in lo-* mol dmP3 KC1 deviates strongly from the theoretical values. The surface potential has higher values. The reason for this is the higher error of the measurements at smaller distances, as we cannot measure the separation of the surfaces. The mean value obtained for the ratio (measured exponential decay length/theoretical Debye length) in the two KC1 solutions, together with the range of distance over which slopes were measured, was:
lo-”
mol dm-“:
1.12
(range lo-60 nm)
87 TABLE 2 Molecular weight, unperturbed
radius of gyration
(Ri ) and onset of interaction
No
n;i, (gmol-‘)
Ri (nm)
d (nm) [in lo-’ mol dm-3 KC1 solution]
3 2 1
97100 48900 41700
14.6 10.6 10.0
5.5 R; 6.0 R; 5.5 R;
(cl)
106’ mol dme3: 1.33 (range 5-30 nm) Let us now consider the energy-distance profile between the quartz glass filaments covered with PVA in different electrolyte solutions. The results of measurements in different 1,l electrolyte solutions (lyotropic sequence) in Figs 2-7 confirm the appreciable differences in the adsorption behaviour of polymers. Before we discuss dependence on the addition of electrolyte, we must notice that the different range of interaction is dependent on the molecular weight of the PVA samples. The unperturbed radii of gyration, Ri, of the actual PVA samples we used are not known, but it is possible to generalise using values from other papers [ 23 ] (Table 2). The onset of interaction between adsorbed layers in lo-’ mol dmp3 KC1 solution commences at surface separations d= (5.5-6.0) Ri where Ri is the unperturbed radius of gyration of the respective polymers. The effective thickness of polymer on the quartz glass surfaces in thus some 3 Ri. Our experimental values are comparable with those for polyethylenoxide (PEO) with a hydrodynamic thickness of about 2 Rt in moderately good solvents [ 31, ellipsometrical measurements for various polymers adsorbed on planar surfaces from 0-solvents [ 181 and with measurements of PEO onto mica surfaces in a moderately good solvent [ 12,131. According to the theory of polymer adsorption of Scheutjens and Fleer [ 191, the theoretically predicted values must be higher than 3 Ri because for adsorbed polymers in an athermal solvent and in dilute polymer solution ( qbx= 10p6) the root-mean-square ( rms) layer thickness is about 3 Rz for an average number of segments r= 1000. With increasing @,, the rms layer thickness increases. A better agreement between theory and experiment was obtained in lithium chloride, rubidium chloride and caesium chloride electrolyte solution, the thickness of adsorbed layers being about (4-6) Ri. For a semi-dilute solution (& = 10p3) and an athermal solvent, the theory predicted values of about (4-7) Ri (r=500-1000). The differences in rms layer thickness due to solvent quality are small, the value for a f3-solvent being somewhat higher than for an athermal solvent [ 191. Figures 2-7 indicate a change of interaction energy as a result of the influence of alkaline ions. The alkaline ions are adsorbed in the sequence
Li > Na > K > Rb > Cs [ 201, that is, in the order of coagulation concentration decrease [ 21,221. In the absence of polymer the Stern potential reflects the electrostatic interaction with respect to the different volume of alkaline counterions. Tadros and Lyklema [ 271 found that at a fixed ionic strength the surface charge density for precipitated silica increased in the order Li < Na < K < Cs due to geometric effects dependent on ion volumes at pH > 7 in 10-l mol dmp3 electrolyte solution. However, their results with different types of glass powders indicated no relation between the affinity of a given cation and the glass surface and the tendency of the glass to respond to that cation [ 281. Therefore the occurrence of specific effects at quartz glass in pure electrolyte solution is unlikely although at full coverage by PVA about 30% of the Stern layer is still void of segments [ 231. There are two possible reasons for this change in adsorption behaviour. Firstly, the hydration number of alkaline ions decreases from Li to Cs, which means that the interaction between polymer segments and solvent molecules increases and the solvent quality becomes higher. Secondly, since the change in solvent quality alone cannot explain the dramatic increase in the interaction range, as in the case of PVA 2 in CsCl solution, we could add the decrease in the amount adsorbed due to the electrolyte solution as an additional explanation. This would be in line with the theory of Scheutjens and Fleer since a change to a better quality of solvent and a change in adsorption energy lead to an increase in the rms layer thickness at a bulk volume fraction of 10p3. Calculations of the free interaction enthalpy according to the HVO theory [ 24,251 indicate a similar dependence on solvent quality and on the coverage by tails. According to the HVO theory, the average number of statistical segments is about 20-25% per tail of the total number of segments with respect to our measurements of PVA in KC1 solutions. With increasing solvent quality and decreasing amount adsorbed per unit area, the range of interaction between the polymer adsorbed layers commences at higher separations and the number of segments in tails increases too. An exception in the lyotropic sequence is the behaviour of PVA 4 in LiCl solution. But from electrochemistry, for example, a deviation in comparison with other alkaline ions is well know. Some properties are similar to those of alkaline earth ions. The possibility of irreversible behaviour after the first compression was investigated for two polymer/electrolyte systems. The present findings indicate that the interaction between adsorbed layers may be represented by an irreversible type of behaviour for the first energy-distance profile. The thickness of the adsorbed layer in 109’ mol dmp3 BaCl, solution is somewhat smaller after 1 h (Fig. 8). We find that in the case of the first approach there is a compression/decompression cycle. The cycles of compression and decompression after the first approach and withdrawal are reproducible and of the same order in the energy-distance profile (Fig. 9). The effective thickness of PVA
89
2 from the surface in lop2 mol dmp3 BaCl, solution is on the first approach z 4 Ri and on the following approach z 3 Rz. Owing to deterioration of the solvent quality, the adsorbed amount increases (increasing hydration of barium ions). The flocculation concentration decreases strongly [ 231. The results in BaCl, solution suggest there is an irreversible compression dependent on time. The energy-distance profile in lo-’ mol dmp3 NaCl solution indicates a reversible behaviour of the energy-distance profile of PVA 1 at the quartz glass surfaces (Fig. 4)) although there is always a compression and decompression cycle. Figures 8 and 9 indicate no change in distance at the beginning of the decompression cycle, due to the adhesion between the compressed adsorbed layers. The measurements of time dependence of polymer adsorption indicate the continuous buildup with time of thicker layers of adsorbed material (Figs 5 and 10). The adsorbance increases with time. These results are consistent with other findings [ 13 ] .
REFERENCES
2 3
9 10 11
12 13 14 15 16 17 18 19 20 21 22 23
M.C. Barker and M.J. Garvey, J. Colloid Interface Sci., 74 (1979) 2. D. Heath and Th.F. Tadros, Faraday Discuss. Chem. Sot., 76 (1983) 1. M.A. Cohen Stuart, F.H.W.H. Waajen, T. Cosgrove, B. Vincent and T.L. Cowley, Macromolecules, 17 (1984) 1825. Th.F. Tadros, J. Colloid Interface Sci., 93 (1983) 2. M. Kawaguchi and A. Takahasi, Macromolecules, 16 (1983) 1465. M.A. Cohen Stuart, Thesis, University of Wageningen, 1980. A.T. Clark, J.D. Robb and R. Smith, J. Chem. Sot. Faraday Trans. 1,72 (1976) 1489. K.G. Barnett, T. Cosgrove, B. Vincent, A.N. Burgess, T.L. Cowley, T.A. King, Th.F. Tadros and J.D. Turner, Polymer 22 (1981) 283. K.G. Barnett, T. Cosgrove, T.L. Crowley, Th.F. Tadros and B. Vincent, in Th.F. Tadros (Ed.), The Effect of Polymers on Dispersion Properties, Academic Press, London, 1981. J.N. Israelachvili, M. Tirrell, J. Klein and Y. Almong, Macromolecules, 17 (1984) 204. P.F. Luckham and J. Klein, J. Chem. Sot. Faraday Trans. 1,80 (1984) 865. J. Klein and P.F. Luckham, Nature 300 (1982) 5891. J. Klein and P.F. Luckham, Nature 308 (1984) 836. L. Knapschinski, W. Katz, B. Ehmke and H. Sonntag, Colloid Polym. Sci., 260 (1982) 1153. Th. Gotze, H. Sonntag and Ya. Rabinovitch, J. Colloid Polym. Sci., 265 (1987) 5. J.N. Israelachvili and G.E. Adams, J. Chem. Sot. Faraday Trans 1,74 (1978) 975. B.V. Derjaguin, Kolloidn. Zh., 69 (1934) 155. R.R. Stromberg, D.J. Tutas and E, Passaglia, J. Phys. Chem., 69 (1965) 3955. J.M.H.M. Scheutjens and G.F. Fleer, J. Phys. Chem., 84 (1980) 178. A. Kuhn, Kolloidchemisches Taschenbuch, Akademische Verlagsanstalt Geest&Partig K.G., Leipzig, 1948. J. Lyklema, Chem. Techn., 46 (1974) 873. J. Lyklema, III. Intern. Vortragung tiber grenzflachenaktive Stoffe, Berlin, Abhandlungen, Akademie Verlag, 1967, p. 542. G.F. Fleer, Thesis, University of Wageningen, 1971.
90
24 25 26 27 28 29 30 31 32
Fph. Hesselink, A. Vrij and J.Th. Overbeek, J. Phys. Chem., 75 (1971) 2094. F.Th. Hesselink, J. Polym. Sci. Polym. Symp., 61 (1977) 439. J.M.H.M. Scheutjens, Thesis, University of Wageningen, 1985. Th.F. Tadros and J. Lyklema, J. Electroanal. Chem., 17 (1968) 267. Th.F. Tadros and J. Lyklema, J. Elektroanal. Chem., 22 (1969) 1. H. Sonntag and H. Pilgrimm, Prog. Colloid Polym. Sci., 61 (1976) 87. B.V. Derjaguin and J. Landau, Acta Physicochim. URSS, 14 (1941) 633. E.J.W. Verwey and J.Th. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. K. Strenge, personal communication.
25 (1987) 77-90
Elsevier Science Publishers
B.V.. Amsterdam
77
-
Printed
in The Netherlands
The Effect of Solvent Quality on the Interaction between Two Layers of Adsorbed Polyvinyl Alcohol (PVA) Th. GOTZE and H. SONNTAG Zentralinstitut (G.D.R.)
(Received
fiir Physikalische
11 November
Chemie, Akademie
der Wissenschaften
1985; accepted in final form 14 January
der DDR, 1199 Berlin
1987)
ABSTRACT The energy-distance profiles of homopolymers with a narrow molecular weight distribution in different electrolyte solutions were measured. The results presented are for polymer solutions of constant concentration. We found that the interaction energy between the surfaces always takes the form of repulsion. This is in agreement with the theoretical prediction of a restricted equilibrium [ 261 according to which the amount of polymer between the surfaces does not change during compression. The thickness of adsorbed polymer at the interface is equivalent to a few radii of gyration of a free coil in solution. In the lyotropic sequence we found the thickness increased with increasing radii of cations. We noticed irreversible behaviour (dependent on time) due to the first compression and found different compression/decompression cycles.
INTRODUCTION
The application of macromolecules as stabilizers or destabilizers of colloidal dispersions has become more and more important in many industrial processes. The conditions for stabilization are that polymer molecules must be adsorbed onto the particles and the adsorbed layers must prevent the particles from coming close enough for van der Waals forces to become effective. Many theories have been put forward but fundamental studies and experimental measurements of charged and uncharged macromolecules adsorbing on the particle surface have lagged behind theoretical developments. There have been many studies on the adsorption of macromolecules onto polystyrene or aerosil particles [ l-31 ; rheological measurements have been carried out on polymer stabilized particles [ 41 and the thickness of adsorbed polymer layers has been measured by ellipsometry [ 51. Other authors have considered the bound fraction p as a measure of the conformation of adsorbed polymer molecules [ 6-81, but this parameter gives an incomplete picture and therefore is not suitable as a criterion for the stablization of particles. Until now there has only been one
0166-6622/87/$03.50
0 1987 Elsevier Science Publishers
B.V.
78 TABLE 1 Molecular weights and intrinsic viscosities of PVA samples No 1
2 3 4
n;i,(gmol-‘)
qHjO(0.1 dm,‘g-‘)
41700 48900 97100 120300
0.74 0.81 0.98 1.12
method of estimating the conformation of adsorbed molecules, namely, neutron scattering [ 91. We are most interested in cases where two adsorbed polymer layers, covering solid surfaces, interact. In this paper we deal with the interaction forces between polyvinyl alcohol (PVA) layers adsorbed onto quartz glass filaments immersed in aqueous media. The influence of different electrolytes in the polymer solution on the interaction forces between the filaments has been investigated; the addition of electrolytes was found to alter the quality of the solvent. Several attempts have been made to make direct measurements of forces between polymer surfaces, but they have tended to suffer from the unreliability of the technique used or from poorly characterized and very polydisperse polymers. Only a few works have produced quantitatively reliable results [ 10-131. The aim of the present work is to explore the influence of the solvent parameter on the interaction between two solid surfaces covered with an adsorbed polymer layer and therefore to throw some light on the conformation of adsorbed macromolecules. EXPERIMENTAL
Material
Water was double distilled and filtered through a 50-nm filter. All chemicals were of analytical grade. Polyvinyl alcohol samples were fractionated, and fully hydrolyzed samples prepared from the commercial ones (M,/M,1.2; see Table 1) . The aqueous polymer solutions were filtered through a 0.45-pm filter. The filaments were prepared from quartz glass (Herasil III) with a surface roughness of a few nanometers. The polymer concentration was in all solutions lop3 g cmp3 (q& = 10”). Apparatus
and procedure
The apparatus used in these experiments was adapted from that described previously [ 141. In this experiment, we did not apply a compensation system
79
to fix the vertical filament in the zero position and only the deformation of the taut band was measured. Also, we used a small glass cell (volume - 25 cm3) within a steel box [ 151. The two quartz glass filaments (diameter - 1 mm) were immersed in the liquid. The separation between the two quartz surfaces was additionally controlled by a differential spring mechanism: the first double cantilever spring is about a thousand times stiffer than the second cantilever steel spring so that a one micrometer movement is reduced to a one nanometer displacement between the two quartz surfaces. In principle, the same system is reported in Ref. [ 161. The calibration of the differential spring mechanism and of the piezoelectric column (6.5 nm V-’ ) with respect to the distance is done with a device for capacitive length-measurement before and after an experiment, with a resolution of a few nanometers. The apparatus was placed on a special optical table to reduce vibration due to external noise within a thermally isolated room (AT-t 1 ‘C ) which reduces thermal drift during the course of the experiment. Before the measurement was taken, the force-distance profile in the electrolyte solution was determined. The surfaces were then separated to about 4 mm and the electrolyte solution replaced by filtered PVA solution. The surfaces were allowed to incubate in the polymer solution for at least 8 h to reach an equilibrium. Following this period of incubation, the force-distance profile was determined again. The measurements were repeated in short time intervals (1.5 min) to prevent any thermal or mechanical disturbances. The forces were measured both in compression and decompression in two cases. The velocity of approach and withdrawal was in all cases constant (14 nm s-l ) . The results reported are based on at least two independent measuring intervals and always on measurements at different contact positions for the same pair of filaments. The difference between the time intervals was 1 h. RESULTS
The energy-distance profile between bare quartz glass surfaces immersed in lo-” and lo-’ mol dmp3 KC1 solution at pH -5.6 is shown in Fig. 1. The energy axis gives the interaction energy per unit area calculated with the Derjaguin approximation [ 171. The data are shown in a semi-log plot to allow the large variation in V(d) to be included. At lower salt concentration, an exponentially increasing repulsion with decreasing d was detected at d I60 nm, and at higher salt concentration similar behaviour was observed but with deviations from theoretical predictions. For all measurements in polymer solution a separation was achieved at which the energy between the quartz glass samples increased steeply with no measurable decrease in separation. Compression measurements by ultracentrifugation have indicated that for polystyrene particles covered with PVA samples at comparable values of interaction energy, the adsorbed layer is about 9-13
80 Log v/ lm‘*
1 -3
-4
-5
‘t
-6 ,L
0
Fig. 1. Energy-distance cases).
IO-:’ mol drn- ” KC1 (pH -5.6
in both
an error of about ? 2 nm ) and cannot be compressed further [ 32 ] . that twice the compressed adsorbed layer would be comparable with the closest approach of the filaments. This assumption was necessary because we cannot measure directly the distance between the quartz glass samples. Compression profiles were measured up to a surface separation of 20 nm. In all figures the onset of interaction was estimated for a given energy of interaction of 8.10M6J m-‘. Figure 2 shows the energy-distance profile between quartz glass surfaces in lo-* mol dm-” LiCl/PVA 4 solution and in lop2 mol dm-” KCl/PVA 4 solution. The energy commences at separations of about 110 nm and 140 nm. Down to these separations a monotonically increasing repulsion was observed. The energy varied in both cases approximately linearly with d, but with different slopes, At a separation of about 70 nm, the energy-distance profile of PVA in LiCl solution “leveled out” but in KC1 solution a steep rise was observed. Down to separations of d = 50 nm, a monotonic rise in energy was again evident. The energy-distance profile measured on the first approach was reversible. Figure 3 shows the energy-distance profile of PVA 1 in 10W2 mol dm-” KC1 and in lo-’ mol dm-” NaCl solution. Measurements commenced at a separanm
( within
profile in (@) IO-’ and (m)
It was assumed
-6i--L---‘-L
0
20
-A
40
60
80
100
120 d/m
Fig. 2. Energy-distance profile in PVA 4 solution and KC1 (broken line).
_,L_,L,,L 0
20
40
60
80
100
120 d/m
Fig. 3. Energy-distance profile in PVA 1 solution and KC1 (broken line).
140 -
(@,,= 10 “) in 10 ’ mol drn-” LiCl (solid line)
140 -
(r& = 10 “) in 10 a mol drn- .i NaCl (solid line )
Fig. 4. Energy-distance profile in PVA 1 solution ($,= 10 “) in lo-’ mol drn-” NaCl on first compression (solid line) and decompression (broken line) and compressions following initial approach ( 00 x ) .
‘\
\ \
I
I
I I I \
\,
‘1\
_-_I~-
20
-I
40
60
80
~
__~.I_
100
_
L
120 140 d/nm --
Fig. 5. Energy-distance profile in PVA 1 solution (@,= 10 ‘) in 10 ” mol dm bation for 8 h (broken line) and 23 h (solid line).
.’ NaCl after incu-
83
-5 t
LI_-b
-6
20
0
/ 40
60
80
100
120
140 d/nm-
Fig. 6. Energy-distance profile in PVA 3 solution and KC1 (broken line).
-3
($,= lo-“)
in 10m2 moldm-”
RbCl (solidline)
I
t : E n
‘5 0
-4 .
-5
-6
I-I_--LA_-.-A
0
20
40
60
80
100
120
140
d/ nrn-
Fig. 7. Energy-distance profile in PVA 2 solution and KC1 (broken line).
(&= lo-“)
in lo-’
mol drn-” CsCl (solid line)
84
Fig. 8. Energy-distance profiles in PVA 2 solution (@,= lo-“) in 10 ’ mol drn-I’ BaCl, on first compression (solid line, 0 )/decompression (broken line, x ) and 1 h later (9) first compression/( 0 ) decompression and in 10 -’ mol dm ’ KC1 (dotted line).
4
-3
^: E n > g -4
-5
-6
I
-I
1--L
20
40
Fig. 9. Energy-distance shown in Fig. 8.
60
profile
80
100
J
140 d/mT~-c
120
on compression
and decompression
following
initial
approach
as
85
tion of about 75 and 50 nm, respectively. Down to these separations a steep rise in the energy was observed but with different slopes for each electrolyte concentration. In Fig. 4, the energy-distance profile of PVA 1 in lop2 mol dme3 NaCl solution is shown in detail. The profile measured on the first approach was reversible, with a margin of error of about t5 nm. Upon separation, the surfaces’ V(d) fell to the initial position but the decompression led to a change in the V(d) profile. The repulsive interaction was dependent on time when the same PVA 1 sample was studied; in particular, when the surface was left to stand overnight (23 h) following compression the change in the energy-distance profile was different from that shown in Fig. 5. The incubation of surfaces for longer times resulted in a monotonically increasing repulsion commencing at a value of d E 80 nm. Figure 6 shows the energy-distance profile of PVA 3 in lop2 mol drn-” KC1 and in lo-’ mol dmd3 RbCl solution. The molecular weight of PVA 3 is somewhat lower than in Fig. 2. The same trend was observed for PVA 3 in LiCl solution but the interaction energy was lower at distances from 50
Let us first consider the interaction between the quartz glass filaments in pure electrolyte solution. In general the energy-distance profiles correspond to the DLVO theory, although the measurement in lo-* mol dmp3 KC1 solution indicates a deviation. Using the Derjaguin approximation for curved surfaces one can calculate the repulsive electrostatic double-layer interaction energy per unit area (valid for distances greater than the Debye length) [ 30,311.
-6 :
-1_
c
0
20
GO-
60
80
100
120
140 d/W-
Fig. 10. Energy-distance profile in PVA 2 solution (qi,= lo-“) in IO-’ mol drn-.” BaCl, on first compression ( l )/decompression (9) cycle after incubation for 25 h.
energy
F
area --2nR-
64nkT
ptanh’ K
(ey//4kT) exp ( - Ed)
where R is the mean radius of the filaments, n the number of ions per unit volume, k is Boltzmann’s constant, T is the absolute temperature, e is electronic charge, K is the Debye-Hiickel constant and I,Yis the surface potential. The experimental values in the lop3 mol dmP3 KC1 solution fit the linear part of the above equation. The effective surface potential is about 90 mV. This is comparable with the values of surface potential measured for aerosil particles dispersed in electrolyte solution [ 291. The slope of the energy-distance function in lo-* mol dmP3 KC1 deviates strongly from the theoretical values. The surface potential has higher values. The reason for this is the higher error of the measurements at smaller distances, as we cannot measure the separation of the surfaces. The mean value obtained for the ratio (measured exponential decay length/theoretical Debye length) in the two KC1 solutions, together with the range of distance over which slopes were measured, was:
lo-”
mol dm-“:
1.12
(range lo-60 nm)
87 TABLE 2 Molecular weight, unperturbed
radius of gyration
(Ri ) and onset of interaction
No
n;i, (gmol-‘)
Ri (nm)
d (nm) [in lo-’ mol dm-3 KC1 solution]
3 2 1
97100 48900 41700
14.6 10.6 10.0
5.5 R; 6.0 R; 5.5 R;
(cl)
106’ mol dme3: 1.33 (range 5-30 nm) Let us now consider the energy-distance profile between the quartz glass filaments covered with PVA in different electrolyte solutions. The results of measurements in different 1,l electrolyte solutions (lyotropic sequence) in Figs 2-7 confirm the appreciable differences in the adsorption behaviour of polymers. Before we discuss dependence on the addition of electrolyte, we must notice that the different range of interaction is dependent on the molecular weight of the PVA samples. The unperturbed radii of gyration, Ri, of the actual PVA samples we used are not known, but it is possible to generalise using values from other papers [ 23 ] (Table 2). The onset of interaction between adsorbed layers in lo-’ mol dmp3 KC1 solution commences at surface separations d= (5.5-6.0) Ri where Ri is the unperturbed radius of gyration of the respective polymers. The effective thickness of polymer on the quartz glass surfaces in thus some 3 Ri. Our experimental values are comparable with those for polyethylenoxide (PEO) with a hydrodynamic thickness of about 2 Rt in moderately good solvents [ 31, ellipsometrical measurements for various polymers adsorbed on planar surfaces from 0-solvents [ 181 and with measurements of PEO onto mica surfaces in a moderately good solvent [ 12,131. According to the theory of polymer adsorption of Scheutjens and Fleer [ 191, the theoretically predicted values must be higher than 3 Ri because for adsorbed polymers in an athermal solvent and in dilute polymer solution ( qbx= 10p6) the root-mean-square ( rms) layer thickness is about 3 Rz for an average number of segments r= 1000. With increasing @,, the rms layer thickness increases. A better agreement between theory and experiment was obtained in lithium chloride, rubidium chloride and caesium chloride electrolyte solution, the thickness of adsorbed layers being about (4-6) Ri. For a semi-dilute solution (& = 10p3) and an athermal solvent, the theory predicted values of about (4-7) Ri (r=500-1000). The differences in rms layer thickness due to solvent quality are small, the value for a f3-solvent being somewhat higher than for an athermal solvent [ 191. Figures 2-7 indicate a change of interaction energy as a result of the influence of alkaline ions. The alkaline ions are adsorbed in the sequence
Li > Na > K > Rb > Cs [ 201, that is, in the order of coagulation concentration decrease [ 21,221. In the absence of polymer the Stern potential reflects the electrostatic interaction with respect to the different volume of alkaline counterions. Tadros and Lyklema [ 271 found that at a fixed ionic strength the surface charge density for precipitated silica increased in the order Li < Na < K < Cs due to geometric effects dependent on ion volumes at pH > 7 in 10-l mol dmp3 electrolyte solution. However, their results with different types of glass powders indicated no relation between the affinity of a given cation and the glass surface and the tendency of the glass to respond to that cation [ 281. Therefore the occurrence of specific effects at quartz glass in pure electrolyte solution is unlikely although at full coverage by PVA about 30% of the Stern layer is still void of segments [ 231. There are two possible reasons for this change in adsorption behaviour. Firstly, the hydration number of alkaline ions decreases from Li to Cs, which means that the interaction between polymer segments and solvent molecules increases and the solvent quality becomes higher. Secondly, since the change in solvent quality alone cannot explain the dramatic increase in the interaction range, as in the case of PVA 2 in CsCl solution, we could add the decrease in the amount adsorbed due to the electrolyte solution as an additional explanation. This would be in line with the theory of Scheutjens and Fleer since a change to a better quality of solvent and a change in adsorption energy lead to an increase in the rms layer thickness at a bulk volume fraction of 10p3. Calculations of the free interaction enthalpy according to the HVO theory [ 24,251 indicate a similar dependence on solvent quality and on the coverage by tails. According to the HVO theory, the average number of statistical segments is about 20-25% per tail of the total number of segments with respect to our measurements of PVA in KC1 solutions. With increasing solvent quality and decreasing amount adsorbed per unit area, the range of interaction between the polymer adsorbed layers commences at higher separations and the number of segments in tails increases too. An exception in the lyotropic sequence is the behaviour of PVA 4 in LiCl solution. But from electrochemistry, for example, a deviation in comparison with other alkaline ions is well know. Some properties are similar to those of alkaline earth ions. The possibility of irreversible behaviour after the first compression was investigated for two polymer/electrolyte systems. The present findings indicate that the interaction between adsorbed layers may be represented by an irreversible type of behaviour for the first energy-distance profile. The thickness of the adsorbed layer in 109’ mol dmp3 BaCl, solution is somewhat smaller after 1 h (Fig. 8). We find that in the case of the first approach there is a compression/decompression cycle. The cycles of compression and decompression after the first approach and withdrawal are reproducible and of the same order in the energy-distance profile (Fig. 9). The effective thickness of PVA
89
2 from the surface in lop2 mol dmp3 BaCl, solution is on the first approach z 4 Ri and on the following approach z 3 Rz. Owing to deterioration of the solvent quality, the adsorbed amount increases (increasing hydration of barium ions). The flocculation concentration decreases strongly [ 231. The results in BaCl, solution suggest there is an irreversible compression dependent on time. The energy-distance profile in lo-’ mol dmp3 NaCl solution indicates a reversible behaviour of the energy-distance profile of PVA 1 at the quartz glass surfaces (Fig. 4)) although there is always a compression and decompression cycle. Figures 8 and 9 indicate no change in distance at the beginning of the decompression cycle, due to the adhesion between the compressed adsorbed layers. The measurements of time dependence of polymer adsorption indicate the continuous buildup with time of thicker layers of adsorbed material (Figs 5 and 10). The adsorbance increases with time. These results are consistent with other findings [ 13 ] .
REFERENCES
2 3
9 10 11
12 13 14 15 16 17 18 19 20 21 22 23
M.C. Barker and M.J. Garvey, J. Colloid Interface Sci., 74 (1979) 2. D. Heath and Th.F. Tadros, Faraday Discuss. Chem. Sot., 76 (1983) 1. M.A. Cohen Stuart, F.H.W.H. Waajen, T. Cosgrove, B. Vincent and T.L. Cowley, Macromolecules, 17 (1984) 1825. Th.F. Tadros, J. Colloid Interface Sci., 93 (1983) 2. M. Kawaguchi and A. Takahasi, Macromolecules, 16 (1983) 1465. M.A. Cohen Stuart, Thesis, University of Wageningen, 1980. A.T. Clark, J.D. Robb and R. Smith, J. Chem. Sot. Faraday Trans. 1,72 (1976) 1489. K.G. Barnett, T. Cosgrove, B. Vincent, A.N. Burgess, T.L. Cowley, T.A. King, Th.F. Tadros and J.D. Turner, Polymer 22 (1981) 283. K.G. Barnett, T. Cosgrove, T.L. Crowley, Th.F. Tadros and B. Vincent, in Th.F. Tadros (Ed.), The Effect of Polymers on Dispersion Properties, Academic Press, London, 1981. J.N. Israelachvili, M. Tirrell, J. Klein and Y. Almong, Macromolecules, 17 (1984) 204. P.F. Luckham and J. Klein, J. Chem. Sot. Faraday Trans. 1,80 (1984) 865. J. Klein and P.F. Luckham, Nature 300 (1982) 5891. J. Klein and P.F. Luckham, Nature 308 (1984) 836. L. Knapschinski, W. Katz, B. Ehmke and H. Sonntag, Colloid Polym. Sci., 260 (1982) 1153. Th. Gotze, H. Sonntag and Ya. Rabinovitch, J. Colloid Polym. Sci., 265 (1987) 5. J.N. Israelachvili and G.E. Adams, J. Chem. Sot. Faraday Trans 1,74 (1978) 975. B.V. Derjaguin, Kolloidn. Zh., 69 (1934) 155. R.R. Stromberg, D.J. Tutas and E, Passaglia, J. Phys. Chem., 69 (1965) 3955. J.M.H.M. Scheutjens and G.F. Fleer, J. Phys. Chem., 84 (1980) 178. A. Kuhn, Kolloidchemisches Taschenbuch, Akademische Verlagsanstalt Geest&Partig K.G., Leipzig, 1948. J. Lyklema, Chem. Techn., 46 (1974) 873. J. Lyklema, III. Intern. Vortragung tiber grenzflachenaktive Stoffe, Berlin, Abhandlungen, Akademie Verlag, 1967, p. 542. G.F. Fleer, Thesis, University of Wageningen, 1971.
90
24 25 26 27 28 29 30 31 32
Fph. Hesselink, A. Vrij and J.Th. Overbeek, J. Phys. Chem., 75 (1971) 2094. F.Th. Hesselink, J. Polym. Sci. Polym. Symp., 61 (1977) 439. J.M.H.M. Scheutjens, Thesis, University of Wageningen, 1985. Th.F. Tadros and J. Lyklema, J. Electroanal. Chem., 17 (1968) 267. Th.F. Tadros and J. Lyklema, J. Elektroanal. Chem., 22 (1969) 1. H. Sonntag and H. Pilgrimm, Prog. Colloid Polym. Sci., 61 (1976) 87. B.V. Derjaguin and J. Landau, Acta Physicochim. URSS, 14 (1941) 633. E.J.W. Verwey and J.Th. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. K. Strenge, personal communication.