Conformational transition of polyvinylpyrrolidone in aqueous solutions of sodium dodecyl sulphate

Eur. Polym. J. Vol. 24, No. 5, pp. 493--496, 1988 Printed in Great Britain. All rights reserved

0014-3057/88 $3.00+0.00 Copyright © 1988 Pergamon Press plc

CONFORMATIONAL TRANSITION OF POLYVINYLPYRROLIDONE IN AQUEOUS SOLUTIONS OF SODIUM DODECYL SULPHATE TERESA GARCiA L6PEZ DE SA, Jos~ L. ALLENDE RIA~O a n d LUIS M. GARRIDO Departmento de Farmacologia, Facultad de Veterinaria, Universidad Complutense de Madrid, Av. Puerta de Hierro s/n, 28040 Madrid, Espafia (Received 14 April 1987; in revised f o r m 28 July 1987)

Abstract--The intrinsic viscosity of polyvinylpyrrolidone (PVP) was measured at 20-35 ° in water containing sodium dodecyl sulphate (SDS) and NaC1 (0.05N). Four fractions of mol. wt 2.75 × 104-3.5 x 105 were used. The SDS concentration was varied but always remained below the critical micellar concentration (cmc). The partial specific volumes of the solutions were measured for the fraction of mol. wt 3.5 x 105. The values of the constants a and K of the Mark-Houwink-Sakurada equation were determined. The a values were in general high, as they should be for good solvents. By means of the Stockmayer-Fixman method, K o (the short-range interaction parameter) and B (the long-range interaction parameter) were calculated, from which a possible conformational change was detected for the 0.08% SDS concentration at all temperatures. This conformational transition was verified by the data obtained from partial volumes. INTRODUCTION

where the relative viscosity r/r = r//r/0, q0 is the viscosity of the pure solvent and the specific viscosity ~]sp = ?]r - - 1. The concentration of PVP is c, and the intrinsic viscosity is defined as

Study of the ternary system, w a t e r / s o d i u m dodecylsulphate (SDS)/polyvinylpyrrolidone (PVP) is interesting because o f the interactions between the macromolecule a n d the surfactants which lead to a d s o r p t i o n a n d complex f o r m a t i o n [1, 2]. F o r the same reason, o t h e r similar polymers, such as poly (vinyl alcohol) a n d copolymers o f vinyl alcohol with vinyl acetate, were studied [3]. Each p y r r o l i d o n e ring o f P V P has a high dipolar m o m e n t [4], which m a k e s it interesting to analyse the n a t u r e of the u n i o n between P V P a n d SDS, a surfactant with amphilic c h a r a c t e r for which the critical micellar c o n c e n t r a t i o n (cmc) is minimal at 25 °. N a C l was a d d e d as a strong electrolyte to be present as a screen for the charges o f the anionic heads o f the s u r f a c t a n t a d s o r b e d o n the PVP, thus preventing the solution f r o m b e h a v i n g like a polyelectrolyte. The SDS c o n c e n t r a t i o n was n o t increased further because cmc decreases w h e n N a C l is added.

[r/] = lira rt~p. c~0

C

Densities were measured with a densitometer. Precision for the measured specific volumes was estimated to be around 0.0001 cmSg - ~. The temperature was controlled by a digital thermostat with a maximum deviation of 0.02 ° In a solution with a volume V containing x,g solvent and x2g solute, and having a specific volume v, the partial volume of the solute v2 is obtained as follows [7]: V = v(x, + x2)

and Ov v2=( O~v ) x,=~(x,+x2)+v c3v

x~ ----kv

~X 2 X 1 + X 2

EXPERIMENTAL The fractions of PVP (Merck) had mol. wt 27,500, 40,000, 160,000 and 350,000. The dispersion index I = ~tw/k3n < 1.2. The number-average molecular weight was verified by means of a Hewlett-Packard 502 osmometer. The viscometric determinations were made using a Ubbelhode capillary viscometer, modified to allow dilutions. The error in the intrinsic viscosity was not greater than 1%. Measurements were taken at six concentrations of the polymer, in order to extrapolate to infinite dilution; in general, concentrations were < 1%. Fluctuations of the temperature were <0.02 °. Intrinsic viscosities were obtained from Huggins' [5] and Kraemer's [6] equations,

~v =v +(l--w2)

w2 is the weight fraction of the solute. The specific volume of dilute solutions depends linearly on the concentration of PVP, v = vo + A w z

v2=vo+A,

c

C

(1)

v0 is the specific volume of the pure solvent. For each solvent, the specific volumes of the components were measured in various PVP concentrations which were never above 1.3% Plotting v vs w2 and fitting it to equation (1) by the method of least squares, v0 and A were obtained. For dilute solutions,

r&~ = [r/] + k t [r/]2c, In r/~ = [r/] -- k 2[r/]2c,

ow /

which makes it possible to obtain partial specific volumes (v2) for PVP in the various solvent systems. 493

TERESAGARCiA I.,6PEZ DE SA et al.

494

[n] dig -1

In] dL g-t 1.5

1.5

m

Mw 350.000

q

-~'~"~'¢'~.o--.o

1.0

0.5 (

0

0

Mw 350.000

1.0

Mw16o.ooo

0.5 0

B

.

2

0.10 r

0 i

~ ~

Mw 40.000 Mw 27.500

0.04 0.08 0.12 SDS concentration (wt %) Fig. 1. Intrinsic viscosity vs surfactant (SDS) concentration in 0.05 N NaC] for several fractions at 20°.

~ 0.10

Fig. 3. Intrinsic viscosity vs suffactant (SDS) concentration in 0.05 N NaCI for several fractions at 30°.

1.5

1.5

( 1.0

Mw 350.000

1.0

0.5 ~~ ' ~ ~ - o

Mw 160.000

0.5 0

~

"~--~'"~"~'~

Mw 350.000 Mw 160.000

D

w,O _

0.20

.,0000

Mw 27.500

0.10

Mw 2'/.500

I I I 0.04 0.08 0.12 SDS concentration (wt %) Fig. 2. Intrinsic viscosity vs surfactant (SDS) concentration in 0.05 N NaCI for several fractions at 25 °.

RESULTS AND DISCUSSION Figures 1-4 show the variation of intrinsic viscosity with SDS concentration for the fractions 27,500, 40,000, 160,000 and 350,000. In all the solutions, the concentration o f NaCI was fixed at 0.05 N. A small minimum can be observed at the 0.08% concentration. It is more marked in the fractions with low molecular weight. By using the techniques of surface tension, dialysis and dye solubility, Arai [8-11] studied aqueous solutions of PVP with SDS at a fixed concentration of 0.1 N NaC1, and obtained two transition points: one coincides with the beginning of SDS adsorption on PVP below cmc, and the other above cmc, which indicates the end of adsorption. In the present studies, the minimum (0.08% point) coincides with the first transition point. Generally, the viscosity minim u m and its later increase are not very noticeable due to the screening effect of NaC1. The a and K parameters of the M a r k - H o u w i n k - S a k u r a d a equation were determined, [r/] = K M a,

Mw40.000

Mw 27.500 I I I 0.04 0.08 0.12 SDS concentrotion (wt %)

In] d[ g-1

0.10 r I

m

0.2(5

[n] dig -1

o2o

Mw 160.000

I I I 0.04 0.08 0.12 SDS concentration (wt %) Fig. 4. Intrinsic viscosity vs surfactant (SDS) concentration in 0.05 N NaCl for several fractions at 35°.

and rather high a values, as expected for good solvent, were produced (Table 1). Gargallo and Radic [12, 13] determined the value of a by using water at 25 ° as the solvent and found a slightly higher value for the water/NaCl (0.05 N) solvent than in the present study. It was also observed that there was a linear relation between [?']]M -1/2 and M 1/2 at each dodecyl concentration; for this reason, the S t o c k m a y e r - F i x m a n equation [14] was chosen to fit the data. This equation was also used by other authors [13] in the study of PVP viscosity in various solvents. Table 1. Mark-Houwink-Sakurada constant a for water/SDS/NaCI (0.05 N) a

c, 20° 25° 30° 0.12 0.80 0.84 0.85 0.10 0.83 0.88 0.86 0.08 0.90 0.93 0.89 0.06 0.85 0.88 0.86 0.04 0.83 0.84 0.85 0.02 0.81 0.82 0.80 0.00 0.79 0.78 0.77 c~= SDS concentration (% by wt).

35° 0.81 0.82 0.87 0.85 0.84 0.83 0.81

Conformational transition of PVP

495

(

60,

I

40

co

A

6O

-

50

-- 50

60

O~ x

- 40 ~.~

'o~ 40

to

B

,< 30

--30 o

I

30

30

to

tO

L - -

10

L

20

20 ~ . . _ . , ~ . . ~ . _ , ~

- 20

10

101

-- 10

I

I

0.04 0.08 0.12 SDS concentration (wt %)

I

I

0.04 0.08 0.12 SDS concentration (wt %)

Fig. 5. Ko and B vs surfactant (SDS) concentration in 0.05 N NaC1 at 20 °.

Fig. 7. K o and B vs surfactant (SDS) concentration in 0.05 N NaCI and 30 °.

When Ko and B (Figs 5-8) were determined, there was a sudden change of K o at the 0.08% concentration at all temperatures, but especially at 25 ° . This points to a possible conformational change for this SDS concentration. It is further observed that the long-range B parameter varies inversely with Ko; however, this may not be taken as a general rule. There is also some correlation between the constants Ko and B and exponent a of the Mark-Houwink-Sakurada equation. The higher the value of a of the Mark-Houwink-Sakurada equation. The higher the value of a, the lower the Ko valu~ and the higher the B value. This is due to the fact that, in good solvents, the solute-solvent interaction gives more freedom of rotation to the adjoining segments; for this reason, the unperturbed size of the

molecule decreases and greater long-range interaction takes place. The behaviour in relation to temperature is very similar in all four cases studied. At 25 °, the possible conformational changes appears to be more marked. With increasing temperature, this change becomes

60 ¸

60

50

5O

less noticeable. Intrinsic viscosity decreases with temperature, l o a n et al., [15-17] have studied aqueous solutions o f poly(methylene N , N - d i m e t h y l p i p e r i d i n i u m chloride) with variable ionic forces to prevent polyelectrolyte behaviour, a n d viscosity variations vs t e m p e r a t u r e were o b t a i n e d with different ionic strengths. W i t h the lowest ionic strength (i.e. 0.1 N) viscosity shows little c h a n g e with temperature, but a sudden decrease is observed a b o v e 50 ° .

6°t 5O

-

6O

-

50

g3 x

(Xl x

5

'~ 4o

40

40

-40 Bt ~

% 3o

30

o

30

B --30 o

- - 20

- - 10

,en

.L

L 20

20

20

10

10

1(]

I

I

I

0.04 0.08 0.12 SOS concentration ( w t %)

Fig. 6. K o and B vs surfactant (SDS) concentration in 0.05 N NaCI at 25 °.

I

I

I

0.04 0.08 032 SDS concentration (wt %)

Fig. 8. Ko and B vs surfactant (SDS) concentration in 0.05 N NaCI at 35 °.

496

TERESAGARCIA I.,~PEZ DE SA et al.

Table 2. Partial specificvolume (cm3/g)of PVP in water/SDS/NaCl (0.05 N) /)2

cs 20° 25° 30° 0.12 0.8125 0.8043 0.7907 0.08 0.8141 0.8058 0.7943 0.04 0.8005 0.8017 0.8043 0.00 0.7443 0.7532 0.7687 c~= SDS concentration (% by wt).

35° 0.7862 0.7946 0.8015 0.7737

40° 0.7810 0.7823 0.7962 0.7651

In the present study, viscosity generally tends to decrease with temperature [18-20], but the behaviour is not entirely regular, as in the case described above. It is known that intrinsic viscosity for a polymer may increase or decrease with temperature. Experimentally, intrinsic viscosity increases with temperature in bad solvents. Consequently, in the present case the tendency indicates good solvents. Although in theory all systems have a temperature, with maximum viscosity, within the range studied, this maximum could not be found. Turro [2 l] studied the interaction o f PVP with SDS by means of photoluminescence. The results confirmed those obtained by the classical methods of surface tension, conductivity and viscosity. It may therefore be concluded that the viscometric results obtained in the present study are valid and comparable to those found by using other techniques. At temperatures between 30 and 40 ° the partial specific volume of PVP (Table 2) increases with surfactant concentration to a m a x i m u m at 0.04% concentration; from this point it slightly decreased. Between 20 and 25 ° , this decrease does no take place while the v2 value stabilizes. This behaviour agrees with that for viscosity. With a fixed SDS concentration, v2 generally decreases slightly with temperature. This may be caused by the fact that the dielectric constant of the medium decreases with temperature. Carlfors et aL [22] studied the volumes of PVP in water; the results are in general as little higher than those of this study. This effect is due to the presence o f N a C l - - b y which the volume is reduced because of the screening of the charges. The ;( parameter o f the polymer-solvent interaction was also calculated: X

1

BN g Vo

2

2v 2

with B having been obtained using the S t o c k m a y e r Fixman equation. V0 is the solvent molar volume and v2 the partial specific volume of the solute. The X parameter consists o f two terms, one enthalpic (K) and the other entropic (~- ~,):

x=~+½-~,. The former refers to the heat of mixing and the latter to the number of available configurations for the macromolecule in the dilute solution.

Table 3. ;~ Parameter for several temperatures x cS 20° 25° 30° 35° 0.12 0.486 0.486 0.484 0.487 0.08 0.484 0.480 0.484 0.485 0.04 0.483 0.482 0.483 0.483 0.00 0.482 0.483 0.485 0.484 cs = SDS concentration (% by wt).

The results of the present study (Table 3) have values <0.5 as expected for good solvents. There are no great temperature variations. It is known from the work of Kresheck [3] that SDS adsorption is almost athermic and therefore the contribution of Z is basically entropic. It is also observed that there are no great variations of X with the sufactant concentration at each temperature. When adsorption occurs, the macromolecule does not expand very much as a result of the electric double charge effect. The number of configurations also hardly varies.

REFERENCES

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