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Effect of solvent composition on the association behavior of pectin in methanol–water mixtures
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 1164–1172
www.elsevier.com/locate/europolj
Effect of solvent composition on the association behavior of pectin in methanol–water mixtures Ingunn Tho a, Anna-Lena Kjøniksen a
b,*
, Kenneth D. Knudsen c, Bo Nystro¨m
b
Department of Pharmaceutics, Institute of Pharmacy and Molecular Biotechnology, Im Neuenheimer Feld 366, University of Heidelberg, D-69120 Heidelberg, Germany b Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway c Department of Physics, Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway Received 10 May 2005; accepted 27 October 2005 Available online 9 December 2005
Abstract Turbidity, small-angle neutron scattering (SANS), and dynamic light scattering measurements have been carried out on semidilute systems of pectin in methanol–water media of various composition ratios. Structural and dynamical properties of pectin dissolved in water–methanol mixtures (case I) are compared with the corresponding conditions when pectin was dissolved in water before the prescribed amounts of methanol were added (case II). The turbidity rises as the percentage of methanol in the mixture increases and this trend is much stronger when the first procedure is used to dissolve pectin. The wavelength dependence of the turbidity indicates that larger association complexes are formed for the samples prepared according to case I. These findings indicate that at a given methanol–water composition, the preparation procedure in case I gives rise to poorer thermodynamic conditions of the system. Local structures probed by SANS experiments do not reveal any significant differences between the systems prepared by the two different procedures. The dynamic light scattering (DLS) results show that increasing methanol concentration in the mixture promotes the formation of association complexes and the disengagement relaxation time of individual chains or clusters is longer at higher percentage of methanol for case II. This can be attributed to stronger entanglement effects in case II. The features from DLS can be rationalized in the framework of the coupling model for constrained and interconnecting systems. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pectin; Methanol–water mixtures; Associations; Thermodynamic properties; Structures; Dynamics
1. Introduction Pectin is a non-toxic, heterogeneous polysaccharide present in the cell wall of most plants, and has a wide range of applications in pharmaceutical *
Corresponding author. Tel.: +47 22 85 55 08. E-mail address: [email protected] (A.-L. Kjøniksen).
formulations as well as in cosmetics and food industry. Pectin consists mainly of linearly connected a(1 ! 4)-D-galacturonic acid residues occasionally interrupted by (1 ! 2) linked a-L-rhamnose, which introduces a kink in the backbone. Natively, a considerable proportion of the galacturonic acid residues of the backbone are methyl-esterified. Pectins in which the degree of esterification (DE) of the galacturonic acid residues is >50% are known as
0014-3057/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.10.015
I. Tho et al. / European Polymer Journal 42 (2006) 1164–1172
high methoxyl (HM) and <50% are regarded as low methoxyl (LM). It is well established that the degree of methoxylation is important for the type of interactions operating in semidilute and gelling solutions of these systems [1]. In solutions of HM pectin, intermolecular chain associations and gelation are governed by a combination of hydrogen bonds and hydrophobic interactions [1]. This type of pectin form gels at acidic pH in presence of large amounts of sugar to reduce the water activity [2]. In aqueous solutions of LM pectins hydrogen bonded intermolecular complexes are expected to play a predominant role in the chain association process. For this type of pectin gelation occurs in the presence of divalent cations such as calcium, which acts as a bridge between pairs of carboxyl groups. Furthermore, gelation of aqueous solutions of LM pectin upon cooling under acidic conditions in the absence of divalent cations has also been reported [3]. Recently, shear-induced gelling was reported for aqueous solution of a LM pectin sample after exposure to oscillatory shear over a certain time [4,5]. Pectin has increasingly gained acceptance as a carrier polymer for sustained-drug release [6,7] and site-specific drug delivery to the colon [8–11]. Different dosage forms have been investigated, such as tablets [8–10], pellets [12–14], films [11,15,16], microcapsules and hydrogel beads [6,17–19]. Pectins muco-adhesive properties can be utilized for drug delivery over the sublingual mucosa [20] as well as the intestinal wall [21]. The drug release property of pectin based dosage forms can be modified for instance by cross-linking the pectin chains with calcium ions to form the insoluble calcium pectinate [8,14] as well as by forming complexes with positively charged polymers [15,16]. An interesting approach is to formulate drug delivery systems where gelation or complexation will occur in situ [7] i.e. when the dosage form has reached the place of action, in contrast to systems where the polymer is in its wanted state (gel or cross-linked) prior to administration. In a recent study [22], the thermodynamic and rheological properties of pectin in methanol–water mixtures were investigated. By increasing the percentage of methanol in the mixture, the ternary pectin–methanol–water system becomes poorer and the viscosity increases because of enhanced associations, stabilized through hydrogen bonds. In the cited paper, pectin was dissolved in mixtures of methanol–water of various compositions, and most of
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the attention was focused on the rheological features in connection with the shear-induced gelation [22] of pectin. In the present work, we will show that if pectin is dissolved in water before methanol is added to reach the prescribed methanol–water composition, the thermodynamics and dynamical features of the system are significantly different from those in the corresponding methanol–water mixtures. This study demonstrates the importance of the dissolution procedure of a polymer when a binary solvent is utilized. For many practical applications with polymers, a complex solvent is frequently employed and the aim of this article is to elucidate the complication that may arise when the solutions are prepared under different conditions. This work will show that the order of adding the solvent components in dissolving the polymer will significantly affect the physical properties of the system. 2. Experimental section 2.1. Materials and solution preparation A low-methoxyl pectin sample designated classic CU701 and lot. no. 0903185, was supplied from Herbstreith & Fox KG, Germany. According to the specifications from the manufacturer, this sample has a degree of methylation (DM) of 35% and the galacturonic acid content is 88%. To remove impurities, the sample was centrifuged for 12 h at 3800 rpm and dialyzed against water for 7 days and freeze-dried prior to use. From capillary viscometry [22] on dilute aqueous solutions of pectin in the presence of sodium hexametaphosphate (suppressing the tendency of forming aggregates) the molecular weight was determined from intrinsic viscosity data to be about 50 000. The freeze-dried polymer was dissolved in methanol–water mixtures (case I) or in water and then methanol was added to the desired methanol–water ratio (case II), and semidilute solutions with a total polymer concentration of 1 wt% were prepared. The pH of these solutions is around 3, and prior to measurement, the solutions were allowed to equilibrate for 12 h. All the measurements were carried out on solutions at 25 °C. 2.2. Turbidity experiments The transmittances of 1 wt% solutions of pectin were measured with a temperature controlled Helios Gamma (Thermo Spectronic, Cambridge, UK)
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spectrophotometer over a wide wavelength range. The apparatus is equipped with a temperature unit (Peltier plate) that gives a good temperature control over an extended time. The turbidity s of the samples can be determined from the following relationship: s = (1/L) ln(It/I0) where L is the light path length of the cell (1 cm), It is the transmitted light intensity, and I0 is the incident light intensity. The results from the spectrophotometer will be presented in terms of turbidity. 2.3. Small-angle neutron scattering (SANS) The small-angle neutron scattering measurements were performed at the SANS installation (beam port 2) at the Institute for Energy Technology (IFE) reactor at Kjeller, Norway. The wavelength was set by means of a selector (Dornier), using a high FWHM for the transmitted beam (Dk/k = 20%), and maximized flux on the sample. The distance was varied from 1.0 m to 3.4 m and ˚ and 10.2 A ˚ , giving a the wavelength between 5.1 A ˚ 1. The wavevector (q) range from 0.008 to 0.25 A wavevector is given by q = (4pn/k)sin(h/2), where h is the scattering angle and n is the refractive index of the medium. The high viscous pectin–methanol–water mixtures were filled in 1 mm quartz cuvettes, which were equipped with a detachable sidewall to facilitate the introduction of the high-viscous samples. To avoid any leakage from the cuvette, a gasket of viton was placed between the sidewalls and this resulted in a pathlength of the cell of 1.5 mm. The measuring cells were placed onto a copper-base for good thermal contact and mounted in the sample chamber. The path between the sample and the detector was evacuated to reduce air scattering. Standard reductions of the scattering data, including transmission corrections, were conducted by incorporating data collected from empty cuvette, beam without cuvette, and blocked-beam background. The data were transformed to an absolute scale (coherent differential cross-section (dR/dX)) by calculating the normalized scattered intensity from direct beam measurements [23]. 2.4. Dynamic light scattering Dynamic light scattering experiments were conducted by means of a standard laboratory-built light scattering spectrometer with vertically polarized incident light of wavelength k = 514.5 nm sup-
plied by an argon ion laser (Lexel laser, model 95). The beam was focused onto the sample cell through a temperature-controlled chamber (the temperature constancy being controlled within ±0.05 °C) filled with refractive index matching silicone oil. The sample solutions were filtered in an atmosphere of filtered air through 5 lm filters directly into precleaned 10 mm NMR tubes (Wilmad Glass Company) of highest quality. In the present work the full homodyne intensity autocorrelation function g2(q, t) was measured with an ALV-5000 multiple-s digital correlator. The correlation functions were recorded in the real time ‘‘multiple-s’’ mode of the correlator, in which 256 time channels are logarithmically spaced over an interval ranging from 0.2 ls to almost an hour. Depending on polymer concentration, the experiment duration was in the range 5 min–1.5 h and each measurement was repeated two or more times. In ergodic systems (like the pectin solutions considered in this study), the scattered field obeys Gaussian statistics and the measured correlation function g2(q, t) can be related to the theoretically amenable first-order electric field correlation function g1(q, t) by the Siegert relationship [24] g2(q, t) = 1 + Bjg1(q, t)j2, where B is usually treated as an empirical factor. Amphiphilic polymers dissolved in aqueous media usually form chain associations, and it has been found that the decay of the correlation function of these systems [25–27] can initially be described by a single exponential, followed at longer times by a stretched exponential h i b g1 ðtÞ ¼ Af expðt=sf Þ þ As exp ðt=sse Þ ð1Þ with Af + As = 1. The quantities Af and As are the amplitudes for the fast and the slow relaxation mode, respectively. Analyses of time correlation functions of concentration fluctuations in the domain qnh < 1, where nh is the hydrodynamic screening length in the semidilute regime, have shown that the first term (short-time behavior) on the righthand side of Eq. (1) is related to the cooperative dif2 fusion coefficient Dc ðs1 f ¼ Dc q Þ. The second term (long-time behavior) is expected to be associated with disengagement relaxation of individual chains [28,29] or cluster relaxation [30]. The variable sse is some effective relaxation time, and b (0 < b 6 1) is a measure of the width of the distribution of relaxation times. The mean value of the slow relaxation time is given by
I. Tho et al. / European Polymer Journal 42 (2006) 1164–1172
ss
Z
1
h i b exp ðt=sse Þ dt ¼ ðsse =bÞCð1=bÞ
ð2Þ
0
where C(b1) is the gamma function of b1. In the analysis of correlation function data, a nonlinear fitting algorithm (a modified Levenberg– Marquardt method) was employed to obtain bestfit values of the parameters Af, sf, sse, and b appearing on the right-hand side of Eq. (1). 3. Results and discussion 3.1. Turbidimetry and SANS Turbidimetric measurements have been used to reveal the formation of large-scale associations in the solutions, and these experiments were specifically undertaken to ascertain if the turbidity is dependent upon the order of addition of the solvent components. Furthermore, it is of interest to assess if the average size of the aggregates changes significantly as the composition of the solvent changes. Fig. 1 shows the effect on the turbidity of the methanol–water ratio and of the order of adding the solvent components for a semidilute 1 wt% pectin sample. When pectin is dissolved in a methanol– water mixture (case I), the turbidity increases strongly as the methanol contents in the mixture increases, which suggest the evolution of large aggregates because the thermodynamic conditions of the system become gradually poorer as the meth-
3 Open symbols are measured after one week
Case I Case II
Turbidity (cm-1)
1
0.1 λ = 800 nm
0.06 0
5
10
15
20
25
Concentration of methanol
Fig. 1. Effect of methanol on the turbidity of 1 wt% pectin solution, where pectin has been dissolved either in a methanol– water mixture (case I) or in water before the prescribed amounts of methanol were added (case II). The open symbols represent data that were measured after one week. No time effect was observed.
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anol percentage increases. In the situation when pectin is dissolved in water before methanol is added (case II), the rise of the turbidity is weaker and much higher values of the methanol–water ratio can be reached before macroscopic phase separation occurs. This shows that when pectin is dissolved in methanol–water mixtures, large association structures are formed with increasing percentage of methanol; while for case II the overall thermodynamic properties of the system are deteriorated to a less extent as the percentage of methanol in the mixture increases. Thus if the sample is dissolved in water (good solvent for pectin) prior to the addition of methanol, the alteration of the thermodynamics of the system is less marked, and the growth of aggregates is less pronounced. In other words, pectin has lower tendencies to associate when methanol is added to the aqueous pectin solution. To reveal possible long-time effects in the association process, turbidity measurements on some samples (open symbols in Fig. 1) were repeated after 1 week, but no change in the value of the turbidity was observed. These results clearly demonstrate that the dissolution process of a polymer in a binary solvent can be significantly affected by the sequence by which the solvents are added. For spherical objects, the following generalized expression has been given [31,32] for the specific turbidity of a system as a function of the wavelength (k) of the light used s ¼ kkn ð3Þ c c!0 where s is the turbidity at concentration c, k is a constant and the value of n is a function of particle size and relative refractive index (particle/medium). For objects of diameter much smaller than the wavelength used n = 4 (Rayleigh scattering). For larger aggregates, n decreases gradually and the value of n may be helpful for a qualitative characterization of the growth process of the aggregates at different conditions. In Fig. 2, the influence of wavelength on the turbidity, on the form of log–log representations, is depicted for 1 wt% pectin dissolved in methanol– water mixtures prepared according to the different procedures (cases I and II) described above. The divergence between the results obtained from cases I and II becomes stronger at higher contents of methanol in the mixture. The wavelength dependence of s can be described by a power law, and the value of the scaling exponent n decreases as
-1
Turbidity (cm )
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10
Case I Case II
0
10
-1
10
-2
10
-1
10
-1
10
-2
2 wt % methanol 400
Turbidity (cm )
10
0
600
4 wt % methanol
800
0
10
-1
10
-2
400
10
400
600
800
0
10
-1
10
-2
6 wt % methanol
600
800
8 wt % methanol 400
600
800
-1
Turbidity (cm )
Wavelength (nm) 10
0
10
-1
10
-2
of n falls off when the methanol contents increase and this tendency is stronger for the samples prepared according to case I. This indicates growth of aggregates as the methanol percentage in the mixture increases, and the association complexes become larger under the conditions of case I. This is again an indication of the better solvent power in case II for pectin, with less inclination to form large interchain aggregates. Fig. 4 shows the SANS results for the two preparation methods at the methanol–water compositions indicated. The general picture that emerges is that for the small distances (local dimensional scale) probed in these experiments (up to d = 2p/ ˚ ), the difference between cases I and qmin 600 A II is small at both 4 and 10 wt% methanol. This is in contrast to the turbidity results, which displayed a marked distinction between the preparation methods at moderate and high methanol percentages in the mixture. This result signalizes that there exists no prominent structural differences on local scales
10 wt % methanol 3
400
600
800
Case I Case II
2
-1
Fig. 2. Wavelength dependence of the turbidity for the case I and case II systems with 1 wt% pectin at the methanol percentages indicated.
I (cm )
Wavelength (nm)
the percentage of methanol in the mixture increases. Fig. 3 illustrates the effects of methanol addition (cases I and II) on the power law exponent n for 1 wt% pectin solutions. For both cases, the value
1
4 wt % methanol 0.3 6x10
-3
10
-2
10
-1
3x10
-1
10
-1
3x10
-1
3.0
3 Case I Case II
2.5
2
I (cm )
1.5
-1
n
2.0
1
1.0
0.5 -n
10 wt % methanol
τ∼ λ
0.3
0.0 0
5
10
15
20
25
Concentration of methanol (wt %)
Fig. 3. Effect of methanol concentration on the power law exponent n (see Eq. (3)), expressing the wavelength dependence of the turbidity, for the case I and case II systems with 1 wt% pectin.
6x10
-3
10
-2
-1
q (Å )
Fig. 4. SANS scattered intensity I(q), plotted versus the scattering vector q for the case I and case II systems with 1 wt% pectin at the methanol percentages indicated.
I. Tho et al. / European Polymer Journal 42 (2006) 1164–1172
g1 (t)
between the samples from the different preparation methods.
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4 2 wt % methanol
0.2
4 wt % methanol
0.0 10
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10
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0.4 6 wt % methanol
0.2
0.0 -7 -5 -3 -1 1 3 10 10 10 10 10 10
8 wt % methanol
0.0 -7 -5 -3 -1 1 3 10 10 10 10 10 10
t (s) 1.0 0.8 Case I Case II
0.6 0.4 10 wt % methanol
0.2
1.0
0.2
0.0
g1 (t)
Normalized time correlation data for 1 wt% solutions of pectin in methanol–water mixtures (cases I and II) of various compositions are depicted in Fig. 5 in the form of semilogarithmic plots. The general trend for both cases is that as the methanol percentage in the mixture increases, the long-time tail of the correlation function is shifted toward longer times. At all conditions, the correlation functions can be well described by Eq. (1). Fig. 6 shows a comparison of the correlation functions for the cases I and II at the methanol percentages indicated. For methanol concentrations above 4 wt%, the decay of the slow relaxation time for case II is shifted toward longer times as compared with the corresponding conditions of case I.
g1 (t)
3.2. Dynamic light scattering
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0.0
Case I
10
-7
10
-5
10
-3
10
-1
10
1
10
3
t (s)
0.8
Fig. 6. A comparison of correlation functions for the cases I and II at a scattering angle of 90° and at the methanol percentages indicated. Every third data point is shown.
0.6 1
g (t)
0 wt % Methanol 2 wt % Methanol 4 wt % Methanol 6 wt % Methanol 8 wt % Methanol 10 wt % Methanol
0.4 0.2 0.0 10
-7
10
-6
10
-5
10
-4
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-3
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-1
10
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Case II
1.0 0.8
1
g (t)
0.6 0.4 0.2 0.0 10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
t (s)
Fig. 5. First-order electric field correlation function versus time at a scattering angle of 90° for the case I and case II systems with 1 wt% pectin at the methanol percentages indicated. Every third data point is shown.
Effects of methanol concentration on the fast (sf) and slow (ss) relaxation times and the stretched exponent b, all parameters determined with the aid of Eq. (1), are illustrated in Fig. 7 for 1 wt% pectin solutions prepared according to the two procedures (cases I and II). The fast relaxation time is virtually the same for both cases at all methanol–water ratios. This observation correlates with the SANS data that showed little difference on the local length scale. In the semidilute polymer concentration regime, the results of the fast relaxation time can be analyzed in the framework of the ‘‘blob’’ model, where a cooperative diffusion coefficient Dc associated with network deformations can be defined [33] by Dc kBT/6pg0nh. Here kB is the BoltzmannÕs constant, g0 is the viscosity of the solvent at temperature T, and the hydrodynamic correlation length nh can be viewed as a characteristic mesh size of the transient network. The results presented in Fig. 7a, showing an increase in sf, and therefore a reduction in Dc, suggest that nh increases as the value of the methanol–water ratio increases. It can be argued that the methanol-induced association
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τ f (s)
10
0
10
-1
10
-2
10
-3
10
-4
10
-5
(a)
Case I Case II
τs (s)
0 10
3
10
2
10
1
10
0
10
-1
10
-2
2
4
6
8
10
0
2
4
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8
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0
2
4
6
8
10
(b)
1.0
(c) 0.8
β
0.6 0.4 0.2 0.0
Methanol concentration (wt %)
Fig. 7. Effect of methanol concentration on the fast relaxation time (a), the slow relaxation time (b), and the stretched exponent (c) for the case I and case II systems with 1 wt% pectin. The values of sf, ss, and b have all been obtained from fits with the aid of Eq. (1).
of chains gives rise to a reorganization of the network to a more heterogeneous one (consisting of bundles of close packed chains) with a larger average mesh size. This is a typical behavior [34–36] when a more heterogeneous network evolves. The similar features of sf or nh at different methanol concentrations for cases I and II indicates formation of networks which are characterized by average mesh sizes of the same magnitude for the two cases. The slow relaxation time (Fig. 7b) rises as the methanol percentage in the mixture increases and higher values of ss are observed for case II than for case I at the highest methanol concentrations. The rise suggests that the disengagement relaxation of individual chains is slowed down as the percentage of methanol in the mixture increases, probably due to progressively enhanced interchain associations. The longer relaxation times for the case II sys-
tems as compared to the corresponding case I systems, reflects that the intensity of entanglement couplings is stronger for the case II samples. The case II systems are, as discussed above, at better thermodynamic conditions than the corresponding ones of case I, and therefore the polymer chains under case II conditions are more extended and more inclined to form entanglements. The results of the slow relaxation mode can be interpreted in the framework of the coupling approach elaborated by Ngai [30,37,38]. This model provides a general description of dynamics and relaxation processes in constrained and interacting systems [38], and this approach has been resorted to in the analysis of dynamical features in associating and gelling polymer systems of various natures [25,27,34–36]. The results of the slow relaxation time and the stretched exponent b in Fig. 7 can be interrelated and rationalized by the coupling model, which is semiempirical in the sense that it does not identify the particular mechanism or the exact nature of the interaction responsible for the coupling. In this approach, the coupling parameter a(b = 1 a), the effective relaxation time ss , the characteristic time for unconstrained relaxation s0, and the crossover time tc are linked to each other through the expression 1=b ss ¼ tcb1 s0
ð4Þ
The value of b is a direct measure of the coupling strength of the relaxation mode to its complex surroundings. A decreasing value of b (see Fig. 7c) indicates enhanced coupling effects and Eq. (4) predicts longer relaxation times of the slow mode. The lower values of b for case II are consistent with the observed higher values of the slow relaxation time for this case. This scenario supports stronger coupling effects for the case II systems. 4. Conclusions In this work, we have provided some novel information about the dissolution of pectin in a binary solvent consisting of methanol and water. By using different experimental techniques, it is demonstrated that the physical properties of the polymer are different depending on how the polymer is dissolved. A comparison of structural and dynamical properties of pectin dissolved in methanol–water mixtures (case I) with those where pectin was dissolved in water before the prescribed amounts of methanol were added (case II) revealed significant differences.
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At corresponding conditions, turbidity experiments unveiled stronger turbidity and larger association complexes for the case I systems. The SANS experiments, operating on a more local dimension scale, disclosed no significant difference between the systems, probably because the large-scale structures consist of bundles of closed-packed chains, with little local rearrangement. The DLS experiments exposed longer slow relaxation times for case II systems. This was attributed to stronger coupling effects for case II systems due to higher intensity of entanglements. The reported findings should be kept in mind when working with pectin and other swelling polysaccharides. The approach can be taken advantage of in formulating systems for e.g. drug delivery purposes. Acknowledgements B.N. and K.D.K. gratefully acknowledge support from the Norwegian Research Council through a NANOMAT Project (158550/431). K.D.K. also thanks the Marie Curie Industry Host Project (Contract no. G5TR-CT-2002-00089) for support. References [1] Thakur BR, Singh RK, Handa AK. Chemistry and uses of pectin—a review. Crit Rev Food Sci Nutri 1997;37:47–73. [2] Oakenfull D, Scott A. Hydrophobic interaction in the gelation of high methoxyl pectins. J Food Sci 1984;49(4):1093–8. [3] Gilsenan PM, Richardson RK, Morris ER. Thermally reversible acid-induced gelation of low-methoxy pectin. Carbohydr Polym 2000;41(4):339–49. [4] Kjøniksen A-L, Hiorth M, Roots J, Nystro¨m B. Shearinduced association and gelation of aqueous solutions of pectin. J Phys Chem B 2003;107:6324. [5] Kjøniksen A-L, Hiorth M, Nystro¨m B. Association under shear flow in aqueous solutions of pectin. Eur Polym J 2005;41:761–70. [6] Sungthongjeen S, Pitaksuteepong T, Somsiri A, Sriamornsak P. Studies on pectins as potential hydrogel matrices for controlled-release drug delivery. Drug Dev Ind Pharm 1999;25:1271–6. [7] Kubo W, Miyazaki S, Dairaku M, Togashi M, Mikami R, Attwood D. Oral sustained delivery of ambroxol from in situ-gelling pectin formulations. Int J Pharm 2004;271: 233–40. [8] Rubinstein A, Radai R, Ezra M, Pathak S, Rokem JS. In vitro evaluation of calcium pectinate: a potential colonspecific drug delivery carrier. Pharm Res 1993;10:258–63. [9] Ashford M, Fell J, Attwood D, Sharma H, Woodhead P. Studies on pectin formulations for colonic drug delivery. J Controlled Release 1994;30:225–32.
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[10] Ahrabi SF, Madsen G, Dyrstad K, Sande SA, Graffner C. Development of pectin matrix tablets for colonic delivery of model drug ropivacaine. Eur J Pharm Sci 2000;10:43–52. [11] Macleod GS, Fell JT, Collett JH. Studies on the physical properties of mixed pectin/ethyl cellulose films intended for colonic drug delivery. Int J Pharm 1997;157(1):53–60. [12] Tho I, Anderssen E, Dyrstad K, Kleinebudde P, Sande SA. Quantum chemical descriptors in the formulation of pectin pellet produced by extrusion/spheronisation. Eur J Pharm Sci 2002;16:143–9. [13] Tho I, Sande SA, Kleinebudde P. Disintegrating pellets from a water-insoluble pectin derivative produced by extrusion/ spheronisation. Eur J Pharm Biopharm 2003;56(3):371–80. [14] Tho I, Sande SA, Kleinebudde P. Cross-linking of amidated low-methoxylated pectin with calcium during extrusion/ spheronisation: effects on pellet size and shape. Chem Eng Sci 2005;60:3899–907. [15] Hiorth M, Tho I, Sande SA. The formation and permeability of drugs across free pectin and chitosan films prepared by a spraying method. Eur J Pharm Biopharm 2003;56(2):175–81. [16] Ofori-Kwakye K, Fell JT, Sharma HL, Smith A-M. Gamma scintigraphic evaluation of film-coated tablets intended for colonic or biphasic release. Int J Pharm 2004;270:307–13. [17] Muhiddinov Z, Khalikov D, Speaker T, Fassihi R. Development and characterization of different low methoxy pectin microcapsules by an emulsion–interface-reaction technique. J Microencapsulation 2004;21(7):729–41. [18] Liu L, Cooke PH, Coffin DR, Fishman ML, Hicks KB. Pectin and polyacrylamide composite hydrogels: effect of pectin on structural and dynamic mechanical properties. J Appl Polym Sci 2004;92:1893–901. [19] Munjeri O, Collett JH, Fell JT, Sharma HL, Smith AM. In vivo behavior of hydrogel beads based on amidated pectins. Drug Deliv 1998;5:239–41. [20] Miyazaki S, Kawasaki N, Nakamura T, Iwatsu M, Hayashi T, Hou W-M, et al. Oral mucosal bioadhesive tablets of pectin and HPMC: in vitro and in vivo evaluation. Int J Pharm 2000;204:127–32. [21] Zand S, Mitragotri S. Intestinal patches for oral drug delivery. Pharm Res 2002;19(4):391–5. [22] Tho I, Kjøniksen A-L, Nystro¨m B, Roots J. Characterization of association and gelation of pectin in methanol–water mixtures. Biomacromolecules 2003;4:1623. [23] Wignall GD, Bates FS. Absolute calibration of small-angle neutron scattering data. J Appl Crystallogr 1987;20(1): 28–40. [24] Siegert AJF. Massachusetts Institute of Technology. Radiation Laboratory Report No. 465, 1943. [25] Nystro¨m B, Walderhaug H, Hansen FK. Dynamic crossover effects observed in solutions of a hydrophobically associating water-soluble polymer. J Phys Chem 1993;97(29):7743–52. [26] Mortensen K, Brown W, Jørgensen E. Phase behavior of poly(propylene oxide)–poly(ethylene oxide)–poly(propylene oxide) triblock copolymer melt and aqueous solutions. Macromolecules 1994;27(20):5654–66. [27] Nystro¨m B, Lindman B. Dynamic and viscoelastic properties during the thermal gelation process of a nonionic cellulose ether dissolved in water in the presence of ionic surfactants. Macromolecules 1995;28(4):967–74. [28] Douglas JF, Hubbard JB. Semiempirical theory of relaxation: concentrated polymer solution dynamics. Macromolecules 1991;24(11):3163–77.
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[29] Wang CH, Zhang XQ. Quasielastic light scattering and viscoelasticity of polystyrene in diethyl phthalate. Macromolecules 1993;26(4):707–14. [30] Ngai KL. Dynamics of semidilute solutions of polymers and associating polymers. Adv Colloid Interface Sci 1996;64:1–43. [31] Heller W, Vassay E. Tyndall spectra, their significance and application. J Chem Phys 1946;14:565–6. [32] Heller W, Bhatnagar HL, Nakagaki M. Theoretical investigations on the light scattering of spheres. XIII. The ‘‘wavelength exponent’’ of differential turbidity spectra. J Chem Phys 1962;36:1163–70. [33] De Gennes PG. Scaling concepts in polymer physics. Ithaca, NY: Cornell University Press; 1979. [34] Tsianou M, Kjøniksen AL, Thuresson K, Nystro¨m B. Light scattering and viscoelasticity in aqueous mixtures of oppo-
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European Polymer Journal 42 (2006) 1164–1172
www.elsevier.com/locate/europolj
Effect of solvent composition on the association behavior of pectin in methanol–water mixtures Ingunn Tho a, Anna-Lena Kjøniksen a
b,*
, Kenneth D. Knudsen c, Bo Nystro¨m
b
Department of Pharmaceutics, Institute of Pharmacy and Molecular Biotechnology, Im Neuenheimer Feld 366, University of Heidelberg, D-69120 Heidelberg, Germany b Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway c Department of Physics, Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway Received 10 May 2005; accepted 27 October 2005 Available online 9 December 2005
Abstract Turbidity, small-angle neutron scattering (SANS), and dynamic light scattering measurements have been carried out on semidilute systems of pectin in methanol–water media of various composition ratios. Structural and dynamical properties of pectin dissolved in water–methanol mixtures (case I) are compared with the corresponding conditions when pectin was dissolved in water before the prescribed amounts of methanol were added (case II). The turbidity rises as the percentage of methanol in the mixture increases and this trend is much stronger when the first procedure is used to dissolve pectin. The wavelength dependence of the turbidity indicates that larger association complexes are formed for the samples prepared according to case I. These findings indicate that at a given methanol–water composition, the preparation procedure in case I gives rise to poorer thermodynamic conditions of the system. Local structures probed by SANS experiments do not reveal any significant differences between the systems prepared by the two different procedures. The dynamic light scattering (DLS) results show that increasing methanol concentration in the mixture promotes the formation of association complexes and the disengagement relaxation time of individual chains or clusters is longer at higher percentage of methanol for case II. This can be attributed to stronger entanglement effects in case II. The features from DLS can be rationalized in the framework of the coupling model for constrained and interconnecting systems. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pectin; Methanol–water mixtures; Associations; Thermodynamic properties; Structures; Dynamics
1. Introduction Pectin is a non-toxic, heterogeneous polysaccharide present in the cell wall of most plants, and has a wide range of applications in pharmaceutical *
Corresponding author. Tel.: +47 22 85 55 08. E-mail address: [email protected] (A.-L. Kjøniksen).
formulations as well as in cosmetics and food industry. Pectin consists mainly of linearly connected a(1 ! 4)-D-galacturonic acid residues occasionally interrupted by (1 ! 2) linked a-L-rhamnose, which introduces a kink in the backbone. Natively, a considerable proportion of the galacturonic acid residues of the backbone are methyl-esterified. Pectins in which the degree of esterification (DE) of the galacturonic acid residues is >50% are known as
0014-3057/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.10.015
I. Tho et al. / European Polymer Journal 42 (2006) 1164–1172
high methoxyl (HM) and <50% are regarded as low methoxyl (LM). It is well established that the degree of methoxylation is important for the type of interactions operating in semidilute and gelling solutions of these systems [1]. In solutions of HM pectin, intermolecular chain associations and gelation are governed by a combination of hydrogen bonds and hydrophobic interactions [1]. This type of pectin form gels at acidic pH in presence of large amounts of sugar to reduce the water activity [2]. In aqueous solutions of LM pectins hydrogen bonded intermolecular complexes are expected to play a predominant role in the chain association process. For this type of pectin gelation occurs in the presence of divalent cations such as calcium, which acts as a bridge between pairs of carboxyl groups. Furthermore, gelation of aqueous solutions of LM pectin upon cooling under acidic conditions in the absence of divalent cations has also been reported [3]. Recently, shear-induced gelling was reported for aqueous solution of a LM pectin sample after exposure to oscillatory shear over a certain time [4,5]. Pectin has increasingly gained acceptance as a carrier polymer for sustained-drug release [6,7] and site-specific drug delivery to the colon [8–11]. Different dosage forms have been investigated, such as tablets [8–10], pellets [12–14], films [11,15,16], microcapsules and hydrogel beads [6,17–19]. Pectins muco-adhesive properties can be utilized for drug delivery over the sublingual mucosa [20] as well as the intestinal wall [21]. The drug release property of pectin based dosage forms can be modified for instance by cross-linking the pectin chains with calcium ions to form the insoluble calcium pectinate [8,14] as well as by forming complexes with positively charged polymers [15,16]. An interesting approach is to formulate drug delivery systems where gelation or complexation will occur in situ [7] i.e. when the dosage form has reached the place of action, in contrast to systems where the polymer is in its wanted state (gel or cross-linked) prior to administration. In a recent study [22], the thermodynamic and rheological properties of pectin in methanol–water mixtures were investigated. By increasing the percentage of methanol in the mixture, the ternary pectin–methanol–water system becomes poorer and the viscosity increases because of enhanced associations, stabilized through hydrogen bonds. In the cited paper, pectin was dissolved in mixtures of methanol–water of various compositions, and most of
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the attention was focused on the rheological features in connection with the shear-induced gelation [22] of pectin. In the present work, we will show that if pectin is dissolved in water before methanol is added to reach the prescribed methanol–water composition, the thermodynamics and dynamical features of the system are significantly different from those in the corresponding methanol–water mixtures. This study demonstrates the importance of the dissolution procedure of a polymer when a binary solvent is utilized. For many practical applications with polymers, a complex solvent is frequently employed and the aim of this article is to elucidate the complication that may arise when the solutions are prepared under different conditions. This work will show that the order of adding the solvent components in dissolving the polymer will significantly affect the physical properties of the system. 2. Experimental section 2.1. Materials and solution preparation A low-methoxyl pectin sample designated classic CU701 and lot. no. 0903185, was supplied from Herbstreith & Fox KG, Germany. According to the specifications from the manufacturer, this sample has a degree of methylation (DM) of 35% and the galacturonic acid content is 88%. To remove impurities, the sample was centrifuged for 12 h at 3800 rpm and dialyzed against water for 7 days and freeze-dried prior to use. From capillary viscometry [22] on dilute aqueous solutions of pectin in the presence of sodium hexametaphosphate (suppressing the tendency of forming aggregates) the molecular weight was determined from intrinsic viscosity data to be about 50 000. The freeze-dried polymer was dissolved in methanol–water mixtures (case I) or in water and then methanol was added to the desired methanol–water ratio (case II), and semidilute solutions with a total polymer concentration of 1 wt% were prepared. The pH of these solutions is around 3, and prior to measurement, the solutions were allowed to equilibrate for 12 h. All the measurements were carried out on solutions at 25 °C. 2.2. Turbidity experiments The transmittances of 1 wt% solutions of pectin were measured with a temperature controlled Helios Gamma (Thermo Spectronic, Cambridge, UK)
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spectrophotometer over a wide wavelength range. The apparatus is equipped with a temperature unit (Peltier plate) that gives a good temperature control over an extended time. The turbidity s of the samples can be determined from the following relationship: s = (1/L) ln(It/I0) where L is the light path length of the cell (1 cm), It is the transmitted light intensity, and I0 is the incident light intensity. The results from the spectrophotometer will be presented in terms of turbidity. 2.3. Small-angle neutron scattering (SANS) The small-angle neutron scattering measurements were performed at the SANS installation (beam port 2) at the Institute for Energy Technology (IFE) reactor at Kjeller, Norway. The wavelength was set by means of a selector (Dornier), using a high FWHM for the transmitted beam (Dk/k = 20%), and maximized flux on the sample. The distance was varied from 1.0 m to 3.4 m and ˚ and 10.2 A ˚ , giving a the wavelength between 5.1 A ˚ 1. The wavevector (q) range from 0.008 to 0.25 A wavevector is given by q = (4pn/k)sin(h/2), where h is the scattering angle and n is the refractive index of the medium. The high viscous pectin–methanol–water mixtures were filled in 1 mm quartz cuvettes, which were equipped with a detachable sidewall to facilitate the introduction of the high-viscous samples. To avoid any leakage from the cuvette, a gasket of viton was placed between the sidewalls and this resulted in a pathlength of the cell of 1.5 mm. The measuring cells were placed onto a copper-base for good thermal contact and mounted in the sample chamber. The path between the sample and the detector was evacuated to reduce air scattering. Standard reductions of the scattering data, including transmission corrections, were conducted by incorporating data collected from empty cuvette, beam without cuvette, and blocked-beam background. The data were transformed to an absolute scale (coherent differential cross-section (dR/dX)) by calculating the normalized scattered intensity from direct beam measurements [23]. 2.4. Dynamic light scattering Dynamic light scattering experiments were conducted by means of a standard laboratory-built light scattering spectrometer with vertically polarized incident light of wavelength k = 514.5 nm sup-
plied by an argon ion laser (Lexel laser, model 95). The beam was focused onto the sample cell through a temperature-controlled chamber (the temperature constancy being controlled within ±0.05 °C) filled with refractive index matching silicone oil. The sample solutions were filtered in an atmosphere of filtered air through 5 lm filters directly into precleaned 10 mm NMR tubes (Wilmad Glass Company) of highest quality. In the present work the full homodyne intensity autocorrelation function g2(q, t) was measured with an ALV-5000 multiple-s digital correlator. The correlation functions were recorded in the real time ‘‘multiple-s’’ mode of the correlator, in which 256 time channels are logarithmically spaced over an interval ranging from 0.2 ls to almost an hour. Depending on polymer concentration, the experiment duration was in the range 5 min–1.5 h and each measurement was repeated two or more times. In ergodic systems (like the pectin solutions considered in this study), the scattered field obeys Gaussian statistics and the measured correlation function g2(q, t) can be related to the theoretically amenable first-order electric field correlation function g1(q, t) by the Siegert relationship [24] g2(q, t) = 1 + Bjg1(q, t)j2, where B is usually treated as an empirical factor. Amphiphilic polymers dissolved in aqueous media usually form chain associations, and it has been found that the decay of the correlation function of these systems [25–27] can initially be described by a single exponential, followed at longer times by a stretched exponential h i b g1 ðtÞ ¼ Af expðt=sf Þ þ As exp ðt=sse Þ ð1Þ with Af + As = 1. The quantities Af and As are the amplitudes for the fast and the slow relaxation mode, respectively. Analyses of time correlation functions of concentration fluctuations in the domain qnh < 1, where nh is the hydrodynamic screening length in the semidilute regime, have shown that the first term (short-time behavior) on the righthand side of Eq. (1) is related to the cooperative dif2 fusion coefficient Dc ðs1 f ¼ Dc q Þ. The second term (long-time behavior) is expected to be associated with disengagement relaxation of individual chains [28,29] or cluster relaxation [30]. The variable sse is some effective relaxation time, and b (0 < b 6 1) is a measure of the width of the distribution of relaxation times. The mean value of the slow relaxation time is given by
I. Tho et al. / European Polymer Journal 42 (2006) 1164–1172
ss
Z
1
h i b exp ðt=sse Þ dt ¼ ðsse =bÞCð1=bÞ
ð2Þ
0
where C(b1) is the gamma function of b1. In the analysis of correlation function data, a nonlinear fitting algorithm (a modified Levenberg– Marquardt method) was employed to obtain bestfit values of the parameters Af, sf, sse, and b appearing on the right-hand side of Eq. (1). 3. Results and discussion 3.1. Turbidimetry and SANS Turbidimetric measurements have been used to reveal the formation of large-scale associations in the solutions, and these experiments were specifically undertaken to ascertain if the turbidity is dependent upon the order of addition of the solvent components. Furthermore, it is of interest to assess if the average size of the aggregates changes significantly as the composition of the solvent changes. Fig. 1 shows the effect on the turbidity of the methanol–water ratio and of the order of adding the solvent components for a semidilute 1 wt% pectin sample. When pectin is dissolved in a methanol– water mixture (case I), the turbidity increases strongly as the methanol contents in the mixture increases, which suggest the evolution of large aggregates because the thermodynamic conditions of the system become gradually poorer as the meth-
3 Open symbols are measured after one week
Case I Case II
Turbidity (cm-1)
1
0.1 λ = 800 nm
0.06 0
5
10
15
20
25
Concentration of methanol
Fig. 1. Effect of methanol on the turbidity of 1 wt% pectin solution, where pectin has been dissolved either in a methanol– water mixture (case I) or in water before the prescribed amounts of methanol were added (case II). The open symbols represent data that were measured after one week. No time effect was observed.
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anol percentage increases. In the situation when pectin is dissolved in water before methanol is added (case II), the rise of the turbidity is weaker and much higher values of the methanol–water ratio can be reached before macroscopic phase separation occurs. This shows that when pectin is dissolved in methanol–water mixtures, large association structures are formed with increasing percentage of methanol; while for case II the overall thermodynamic properties of the system are deteriorated to a less extent as the percentage of methanol in the mixture increases. Thus if the sample is dissolved in water (good solvent for pectin) prior to the addition of methanol, the alteration of the thermodynamics of the system is less marked, and the growth of aggregates is less pronounced. In other words, pectin has lower tendencies to associate when methanol is added to the aqueous pectin solution. To reveal possible long-time effects in the association process, turbidity measurements on some samples (open symbols in Fig. 1) were repeated after 1 week, but no change in the value of the turbidity was observed. These results clearly demonstrate that the dissolution process of a polymer in a binary solvent can be significantly affected by the sequence by which the solvents are added. For spherical objects, the following generalized expression has been given [31,32] for the specific turbidity of a system as a function of the wavelength (k) of the light used s ¼ kkn ð3Þ c c!0 where s is the turbidity at concentration c, k is a constant and the value of n is a function of particle size and relative refractive index (particle/medium). For objects of diameter much smaller than the wavelength used n = 4 (Rayleigh scattering). For larger aggregates, n decreases gradually and the value of n may be helpful for a qualitative characterization of the growth process of the aggregates at different conditions. In Fig. 2, the influence of wavelength on the turbidity, on the form of log–log representations, is depicted for 1 wt% pectin dissolved in methanol– water mixtures prepared according to the different procedures (cases I and II) described above. The divergence between the results obtained from cases I and II becomes stronger at higher contents of methanol in the mixture. The wavelength dependence of s can be described by a power law, and the value of the scaling exponent n decreases as
-1
Turbidity (cm )
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10
Case I Case II
0
10
-1
10
-2
10
-1
10
-1
10
-2
2 wt % methanol 400
Turbidity (cm )
10
0
600
4 wt % methanol
800
0
10
-1
10
-2
400
10
400
600
800
0
10
-1
10
-2
6 wt % methanol
600
800
8 wt % methanol 400
600
800
-1
Turbidity (cm )
Wavelength (nm) 10
0
10
-1
10
-2
of n falls off when the methanol contents increase and this tendency is stronger for the samples prepared according to case I. This indicates growth of aggregates as the methanol percentage in the mixture increases, and the association complexes become larger under the conditions of case I. This is again an indication of the better solvent power in case II for pectin, with less inclination to form large interchain aggregates. Fig. 4 shows the SANS results for the two preparation methods at the methanol–water compositions indicated. The general picture that emerges is that for the small distances (local dimensional scale) probed in these experiments (up to d = 2p/ ˚ ), the difference between cases I and qmin 600 A II is small at both 4 and 10 wt% methanol. This is in contrast to the turbidity results, which displayed a marked distinction between the preparation methods at moderate and high methanol percentages in the mixture. This result signalizes that there exists no prominent structural differences on local scales
10 wt % methanol 3
400
600
800
Case I Case II
2
-1
Fig. 2. Wavelength dependence of the turbidity for the case I and case II systems with 1 wt% pectin at the methanol percentages indicated.
I (cm )
Wavelength (nm)
the percentage of methanol in the mixture increases. Fig. 3 illustrates the effects of methanol addition (cases I and II) on the power law exponent n for 1 wt% pectin solutions. For both cases, the value
1
4 wt % methanol 0.3 6x10
-3
10
-2
10
-1
3x10
-1
10
-1
3x10
-1
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3 Case I Case II
2.5
2
I (cm )
1.5
-1
n
2.0
1
1.0
0.5 -n
10 wt % methanol
τ∼ λ
0.3
0.0 0
5
10
15
20
25
Concentration of methanol (wt %)
Fig. 3. Effect of methanol concentration on the power law exponent n (see Eq. (3)), expressing the wavelength dependence of the turbidity, for the case I and case II systems with 1 wt% pectin.
6x10
-3
10
-2
-1
q (Å )
Fig. 4. SANS scattered intensity I(q), plotted versus the scattering vector q for the case I and case II systems with 1 wt% pectin at the methanol percentages indicated.
I. Tho et al. / European Polymer Journal 42 (2006) 1164–1172
g1 (t)
between the samples from the different preparation methods.
1.0
1.0
0.8
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0.4 2 wt % methanol
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8 wt % methanol
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t (s) 1.0 0.8 Case I Case II
0.6 0.4 10 wt % methanol
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g1 (t)
Normalized time correlation data for 1 wt% solutions of pectin in methanol–water mixtures (cases I and II) of various compositions are depicted in Fig. 5 in the form of semilogarithmic plots. The general trend for both cases is that as the methanol percentage in the mixture increases, the long-time tail of the correlation function is shifted toward longer times. At all conditions, the correlation functions can be well described by Eq. (1). Fig. 6 shows a comparison of the correlation functions for the cases I and II at the methanol percentages indicated. For methanol concentrations above 4 wt%, the decay of the slow relaxation time for case II is shifted toward longer times as compared with the corresponding conditions of case I.
g1 (t)
3.2. Dynamic light scattering
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0.0
Case I
10
-7
10
-5
10
-3
10
-1
10
1
10
3
t (s)
0.8
Fig. 6. A comparison of correlation functions for the cases I and II at a scattering angle of 90° and at the methanol percentages indicated. Every third data point is shown.
0.6 1
g (t)
0 wt % Methanol 2 wt % Methanol 4 wt % Methanol 6 wt % Methanol 8 wt % Methanol 10 wt % Methanol
0.4 0.2 0.0 10
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g (t)
0.6 0.4 0.2 0.0 10
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10
-6
10
-5
10
-4
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-3
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-2
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-1
10
0
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1
10
2
t (s)
Fig. 5. First-order electric field correlation function versus time at a scattering angle of 90° for the case I and case II systems with 1 wt% pectin at the methanol percentages indicated. Every third data point is shown.
Effects of methanol concentration on the fast (sf) and slow (ss) relaxation times and the stretched exponent b, all parameters determined with the aid of Eq. (1), are illustrated in Fig. 7 for 1 wt% pectin solutions prepared according to the two procedures (cases I and II). The fast relaxation time is virtually the same for both cases at all methanol–water ratios. This observation correlates with the SANS data that showed little difference on the local length scale. In the semidilute polymer concentration regime, the results of the fast relaxation time can be analyzed in the framework of the ‘‘blob’’ model, where a cooperative diffusion coefficient Dc associated with network deformations can be defined [33] by Dc kBT/6pg0nh. Here kB is the BoltzmannÕs constant, g0 is the viscosity of the solvent at temperature T, and the hydrodynamic correlation length nh can be viewed as a characteristic mesh size of the transient network. The results presented in Fig. 7a, showing an increase in sf, and therefore a reduction in Dc, suggest that nh increases as the value of the methanol–water ratio increases. It can be argued that the methanol-induced association
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τ f (s)
10
0
10
-1
10
-2
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-3
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-4
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-5
(a)
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0 10
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0
2
4
6
8
10
(b)
1.0
(c) 0.8
β
0.6 0.4 0.2 0.0
Methanol concentration (wt %)
Fig. 7. Effect of methanol concentration on the fast relaxation time (a), the slow relaxation time (b), and the stretched exponent (c) for the case I and case II systems with 1 wt% pectin. The values of sf, ss, and b have all been obtained from fits with the aid of Eq. (1).
of chains gives rise to a reorganization of the network to a more heterogeneous one (consisting of bundles of close packed chains) with a larger average mesh size. This is a typical behavior [34–36] when a more heterogeneous network evolves. The similar features of sf or nh at different methanol concentrations for cases I and II indicates formation of networks which are characterized by average mesh sizes of the same magnitude for the two cases. The slow relaxation time (Fig. 7b) rises as the methanol percentage in the mixture increases and higher values of ss are observed for case II than for case I at the highest methanol concentrations. The rise suggests that the disengagement relaxation of individual chains is slowed down as the percentage of methanol in the mixture increases, probably due to progressively enhanced interchain associations. The longer relaxation times for the case II sys-
tems as compared to the corresponding case I systems, reflects that the intensity of entanglement couplings is stronger for the case II samples. The case II systems are, as discussed above, at better thermodynamic conditions than the corresponding ones of case I, and therefore the polymer chains under case II conditions are more extended and more inclined to form entanglements. The results of the slow relaxation mode can be interpreted in the framework of the coupling approach elaborated by Ngai [30,37,38]. This model provides a general description of dynamics and relaxation processes in constrained and interacting systems [38], and this approach has been resorted to in the analysis of dynamical features in associating and gelling polymer systems of various natures [25,27,34–36]. The results of the slow relaxation time and the stretched exponent b in Fig. 7 can be interrelated and rationalized by the coupling model, which is semiempirical in the sense that it does not identify the particular mechanism or the exact nature of the interaction responsible for the coupling. In this approach, the coupling parameter a(b = 1 a), the effective relaxation time ss , the characteristic time for unconstrained relaxation s0, and the crossover time tc are linked to each other through the expression 1=b ss ¼ tcb1 s0
ð4Þ
The value of b is a direct measure of the coupling strength of the relaxation mode to its complex surroundings. A decreasing value of b (see Fig. 7c) indicates enhanced coupling effects and Eq. (4) predicts longer relaxation times of the slow mode. The lower values of b for case II are consistent with the observed higher values of the slow relaxation time for this case. This scenario supports stronger coupling effects for the case II systems. 4. Conclusions In this work, we have provided some novel information about the dissolution of pectin in a binary solvent consisting of methanol and water. By using different experimental techniques, it is demonstrated that the physical properties of the polymer are different depending on how the polymer is dissolved. A comparison of structural and dynamical properties of pectin dissolved in methanol–water mixtures (case I) with those where pectin was dissolved in water before the prescribed amounts of methanol were added (case II) revealed significant differences.
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At corresponding conditions, turbidity experiments unveiled stronger turbidity and larger association complexes for the case I systems. The SANS experiments, operating on a more local dimension scale, disclosed no significant difference between the systems, probably because the large-scale structures consist of bundles of closed-packed chains, with little local rearrangement. The DLS experiments exposed longer slow relaxation times for case II systems. This was attributed to stronger coupling effects for case II systems due to higher intensity of entanglements. The reported findings should be kept in mind when working with pectin and other swelling polysaccharides. The approach can be taken advantage of in formulating systems for e.g. drug delivery purposes. Acknowledgements B.N. and K.D.K. gratefully acknowledge support from the Norwegian Research Council through a NANOMAT Project (158550/431). K.D.K. also thanks the Marie Curie Industry Host Project (Contract no. G5TR-CT-2002-00089) for support. References [1] Thakur BR, Singh RK, Handa AK. Chemistry and uses of pectin—a review. Crit Rev Food Sci Nutri 1997;37:47–73. [2] Oakenfull D, Scott A. Hydrophobic interaction in the gelation of high methoxyl pectins. J Food Sci 1984;49(4):1093–8. [3] Gilsenan PM, Richardson RK, Morris ER. Thermally reversible acid-induced gelation of low-methoxy pectin. Carbohydr Polym 2000;41(4):339–49. [4] Kjøniksen A-L, Hiorth M, Roots J, Nystro¨m B. Shearinduced association and gelation of aqueous solutions of pectin. J Phys Chem B 2003;107:6324. [5] Kjøniksen A-L, Hiorth M, Nystro¨m B. Association under shear flow in aqueous solutions of pectin. Eur Polym J 2005;41:761–70. [6] Sungthongjeen S, Pitaksuteepong T, Somsiri A, Sriamornsak P. Studies on pectins as potential hydrogel matrices for controlled-release drug delivery. Drug Dev Ind Pharm 1999;25:1271–6. [7] Kubo W, Miyazaki S, Dairaku M, Togashi M, Mikami R, Attwood D. Oral sustained delivery of ambroxol from in situ-gelling pectin formulations. Int J Pharm 2004;271: 233–40. [8] Rubinstein A, Radai R, Ezra M, Pathak S, Rokem JS. In vitro evaluation of calcium pectinate: a potential colonspecific drug delivery carrier. Pharm Res 1993;10:258–63. [9] Ashford M, Fell J, Attwood D, Sharma H, Woodhead P. Studies on pectin formulations for colonic drug delivery. J Controlled Release 1994;30:225–32.
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