J. Cent. South Univ. (2012) 19: 1817-1822
DOI: 10.1007/s11771-012-1214-z
Complexation of starch with dodecylamine
LI Hai-pu(李海普)1, 2, LI Bin(李彬)1, 2, ZHANG Sha-sha(张莎莎)1, 2, ZOU Jie-hui(邹洁辉)1, 2
1. Key Laboratory of Resources Chemistry of Nonferrous Metals of Ministry of Education
(Central South University), Changsha 410083, China;
2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract: The chemical nature of the interaction of starch and dodecylamine (DDA), which generally act as depressant and collector, respectively, in the reverse flotation of bauxite, was investigated using starch-iodine tests. The results obtained from the blue-value measurements for starch+DDA+iodine system indicate the formation of the inclusion complex for amylose-DDA system at low DDA concentration (<2 mmol/L). However, it is less likely for amylopectin-DDA system with short helix. UV-Vis spectra of starch-iodine complexes show that each helix of amylose can accommodate two DDA molecules locating separately at its two ends, and in the helical cavity there is room available for the upcoming iodine. When concentrated DDA is tested, amylose-DDA system exhibits no characteristic starch-iodine color, owing to the presence of a compact coating of DDA molecules on starch via hydroxyl/amine hydrogen bonding. 1H NMR spectroscopy and surface tension determination help to clarify the interaction mechanism of amylose with DDA.
Key words: inclusion complex; amylose; starch; dodecylamine; reverse flotation
1 Introduction
Over decades, starch has been practically used as depressant and flocculant in the minerals processing industry for froth flotation [1], benefiting mainly from its abundant hydrophilic hydroxyls along with high availability and easy modification. The extensive application of starch in the minerals field encourages in turn the intense mechanism studies concerning its interaction with various mineral particles and co-modifiers. Factors like the relative molecular mass, functional groups, and structures of native or chemically modified starches have been correlated to their performance [2]. By now, the majority of research has agreed that these starch additives implement their role by firstly adsorbing on the surface of targeted mineral particles, and then modifying the hydro-philicity/hydro- phobicity of the solid surface and shifting the surface electrokinetic potential [3]. When starch works together with collector, their interaction behavior is generally interpreted as competitive or co-adsorption onto mineral surface [4]. However, no detailed information was yet available indicating the chemical interaction but for hydrogen bonds between starch and collector [5].
The two major components of natural starch are linear amylose and highly branched amylopectin. The α-1,4 linked glucose component of amylose promotes the formation of a helix structure of 8 ? pitch with 6 glucose units per turn. The ~5 ? wide central cavity of this kind of helix has a hydrocarbon-like interior surface formed by the hydrogen atoms attached to carbon atoms [6], and it shows the ability to form inclusion complexes with iodine [7] or lipids [8-9]. The starch-guest complexation also has long been recognized in food industry [10-11].
The well-known starch-iodine complex is believed to consist of polyiodides in a supramolecular host-guest compound that gives rise to a dark-blue color [6]. This color producing reaction has been developed successfully as a sensitive method of detecting starch, and also helpful in determining the complexing abilities of guest molecules with starch. Among them, aliphatic amines such as diethylamine or diphenylamine have been previously communicated to fail to give the inclusion complexes with amylose in aqueous solution due to the great affinity of the amines with water [12]. However, from our recent practices of depressing diaspore with starch in reverse flotation, it was noticed that the presence of collector dodecylamine (DDA) at low concentration did cause a perceptible drop of blue values of starch samples [13], providing a clue for the possible inclusion complexation of starch with the aliphatic amine that bears 12 carbons. Accordingly, it is hypothesized that the DDA molecule could penetrate the hydrophobic cavity of the starch helix, especially when the high similarity of iodide mandrel to DDA molecule in terms of the hydrophobic long chain and the suitable cross-sectional area (3 ? for the latter) was taken into account.
The present work has been carried out with the intention to find the evidence and behavior of the complexation of starch and DDA collector by means of starch-iodine reaction. As far as we know, there has been no such research in the field of froth flotation. The influence of the formation of the starch-DDA complex on the depressant performance was also studied from the surface-tension standpoint.
2 Material and methods
2.1 Materials and reagents
Amylose and amylopectin were separated from potato starch in our lab [14]. The starch solutions were freshly prepared by dispersing starch particles into a small amount of absolute ethanol and then dissolving them in hot distilled water. The mean DP (degree of polymerization) of the obtained amylose samples was determined to be about 70 [15]. Dodecylamine (DDA) in analytical grade was used as the collector. The ionic strength of solutions was maintained at unity by suitable additions of a stock KCl solution.
2.2 Determination of blue value
The blue value of each starch sample was determined by the Gilbert procedure with slight modification [16]. DDA solution (5 mL) or water (5 mL) was poured in a beaker charged with 10 mL starch solution (0.1 g/L); 5 min later, iodine solution (5 mL) was added. The sample was scanned from 400 to 800 nm on a UV-visible spectrophotometer (UV-2100, Unico Instrument Co., Ltd). Blue value was calculated as the optical density to light of wavelength (λmax) corresponding to the maximum absorbance of a mixture of 1 mg of starch per 100 mL of solution contained in a 4 cm cell.
2.3 Nuclear magnetic resonance spectroscopy (NMR)
1H NMR spectra were recorded at 500 MHz with a Bruker AV 500 spectrometer. Amylose and DDA were dissolved in DMSO-d6 and the chemical shifts were calibrated using the residual signals of deuterated solvent (2.50×10-6).
2.4 Surface tension
The surface tension of starch aqueous solution was measured by Du Nouy ring tensiometer (DT-102, Zibo Huakun Electrical Equipment Co., Ltd, China). The procedure was similar to that reported in Ref. [17].
3 Results and discussion
3.1 Starch-DDA interaction by starch-iodine reaction
The iodine-blue results of the respective amylose and amylopectin samples are depicted in Fig. 1 as a function of DDA concentration.
Fig. 1 Iodine-blue values of starches as function of DDA concentration ([I]=7.88?10-2 mmol/L, [Starch]=0.05 mg/mL)
In the absence of DDA, the blue values of the amylose (1.02) and amylopectin (0.19) derived from potato starch are found to be comparable to the results in early reports [18]. When firstly being pre-treated with 0.2 mmol/L DDA, the amylose system exhibits a blue value (0.47) nearly half of its initial value, seemingly implying that some helix cavities are pre-occupied by DDA molecules and are, at least partly, unavailable for iodine reaction afterwards.
With further increase of DDA concentration up to 2.0 mmol/L, the blue value of amylose decreases steadily but marginally to 0.39. It is accordingly suggested that at such a concentration, DDA molecules still aren’t able to occupy all the reserved sites for iodine since there are always almost a fixed quantity of iodines that can sustain the DDA pre-treatment. The amylose-iodine complex has been well known for its linear polyiodide chain responsible for the deep color. In our case, the DDA-pretreated amylose should still enable to accommodate the polyiodide chain within, while DDA molecules could presumably settle merely in the specific location in the cavity. In sharp contrast, the blue value of amylopectin varies only slightly at a low level (0.17-0.19) with the increase of DDA concentration (0.2-2.0 mmol/L), indicative of a negligible interaction of DDA molecules with the highly branched amylopectin. Based on these observations, it is presumed that the behavior of DDA molecule to interact with starch is very similar to that of iodine, with respect to the stronger binding strength with amylose than amylopectin as well as their localized binding mode.
It is attractive to investigate if the DDA penetration into amylose could be dispelled by further increasing the iodine concentration. As shown in Fig. 2, the blue value of the untreated amylose sample ascends almost linearly with the increase of iodine concentration up to 0.5?10-2 mmol/L, followed by a gentle upslope by further multiplying the addition of iodine. The iodine amount involved in this turning point (0.5?10-2 mmol/L) could give hints for the maximum available hydrophobic cavities contained in the current amylose. Once the cavities are saturated, the additional iodine will have no chance to participate in the color reaction to uplift the blue value further.
Fig. 2 Iodine-blue values of amylose-iodine complexes in absence or presence of DDA as function of iodine concentration ([Starch]=0.05 mg/mL, [DDA]=0.8 mmol/L)
When pre-treated with 0.8 mmol/L DDA, the amylose starts with an abrupt rise in its blue value and ceases its rapidly increasing trend at 0.37 when the iodine concentration reaches 0.25?10-2 mmol/L (Fig. 2). From this, it is inferred that the maximum cavities available for iodine greatly shrink due to the pre-occupation by DDA molecules, which is consistent with the finding in Fig. 1. In addition, an extra supply of iodine into this system causes only minor changes in blue value (0.36-0.41), suggesting a strong amylose-DDA binding which is hard to be broken by any extra iodine. This observation as such directly leads to another question: is it possible for DDA molecules to cut down the polyiodide chain in already formed starch-iodine complex and replace some iodine atoms with themselves?
Additional tests were conducted to investigate the effect of adding DDA to a starch-iodine system. The UV-Vis spectra were therefore stacked for a comparison about the interaction among starch, DDA, and iodine at respective specified concentration (Fig. 3).
It is seen from Fig. 3 that the maximum absorbance (1.28) of untreated amylose-iodine system appears at a wavelength of 640 nm (λmax) as the control (labeled as amylose+I2). An interested issue here is the order of reagent addition. On one hand, when DDA is introduced afterwards into the above system, the amylose+I2+DDA system experiences an obvious drop in the maximum absorbance to 1.05 with the corresponding λmax slightly blue shifting to 630 nm. It is evident that DDA does essentially dispel more or less iodine from the formed amylose-iodine complex and facilitate a takeover, which is in agreement with the previous finding that DDA exhibits relatively stronger binding ability to starch than iodine does. However, the concentration dependence of substitution of iodine with DDA couldn’t be carried out smoothly, due to the severe macro-phase separation brought by the higher concentration of DDA. On the other hand, as exactly expected for the DDA pre-treated system (amylose+DDA+I2), the maximum absorbance (0.50) decreases greatly and the corresponding λmax (560 nm) blue shifts by a wide margin, relative to those of the control.
Fig. 3 UV-Vis spectra of starch-iodine complexes in absence or presence of DDA ([I]=7.88?10-2 mmol/L, [Starch]=0.05 mg/mL, [DDA]=0.8 mmol/L)
Previous works have concluded that the range of λmax of the starch-iodine complex depends on the helical turns of starch [19], as listed in Table 1. This method can be applied to differentiate amylose and amylopectin, and also is useful in estimating the occupancy of iodine in starch-iodine complex by counting the change of the corresponding helix turns.
Table 1 Color producing reaction of starch-iodine complexes and its corresponding wavelength of maximum absorbance [19]
When comparing the λmax of above two systems, amylose+I2 (640 nm) and amylose+DDA+I2 (560 nm), a blue shift of 80 nm indicates the fall of the iodine occupancy. Alternatively, this change could be described as a shrinking from >10 to 7 as measured by helix turns. It is concluded that the introduction of DDA apparently reduces more than three turns of each helix when compared with the initially formed amylose-iodine complex. Considering the length of the carbon chain of DDA, which is about 1.52 nm and nearly equal to the length of two helical turns of amylose, it is plausibly proposed that each amylose molecule could accommodate two DDA molecules, one at either end of the amylose. When considering the hydrophilic nature of the polar head (—NH2) of DDA and the existence of linear polyiodide chain for giving rise to the intense optical absorption at 560 nm, it is speculated that the polar end of DDA does not enter the helical cavity. The proposed binding mode of DDA with amylose competing against iodine is illustrated in Scheme 1.
Scheme 1 Proposed complexation of DDA with amylose (hydroxyl groups omitted for clarity): (a) Amylose+DDA; (b) Amylose+DDA+I2
Amylopectin system has the similar situation (Fig. 3), but for the less lose of apparent helix turns as measured by the blue shift of λmax from 560 nm (amylopectin+I2) to 540 nm (amylopectin+DDA+I2), which possibly refers to the average attendance of no more than one DDA molecule in each branch of amylopectin+I2 adduct.
In the extended blue-value tests concerning amylose+DDA+I2 system (data is not shown), it is found that when highly concentrated DDA was used the only definite color that could be recognized was due to I2 instead of the starch-iodine system. At such a high concentration, DDA molecules could possibly first plug the starch by inclusion interaction and then wrap it entirely via intermolecular hydrogen bonds between hydroxyl groups of starch and amine groups of DDA. As such, the coat around the starch might be too compact to allow for the inclusion of iodine into the helix.
The 1H NMR spectroscopy was used to fundamentally investigate the amylose-DDA interaction in a comparative perspective. In Fig. 4, the 1H NMR spectra of amylose+DDA system show the signals both due to amylose and DDA at low and high DDA concentrations, respectively. The chemical shifts for amylose are well consistent with those of previously reported study [20]. At low concentration of DDA (Fig. 4(a)), the single peak due to the nine end methylene groups (Hb) attached to the methyl group of DDA becomes slightly broadened and is indeed expected to be upfield shifted at least by 0.01×10-6 from 1.25×10-6 to 1.24×10-6, while the amylose signals are almost intact. This observation strongly suggests that the hydrophobic interaction contributes to the amylose-DDA inclusion complexation. In sharp contrast, at a high concentration of DDA (Fig. 4(b)), all the peaks due to the hydroxyl functionality of amylose become weak and very broad while the signals for the protons attaching the carbon atoms of amylose are essentially the same, indicating the occurrence of hydrogen bonding between hydroxyl groups of starch and amine groups of DDA.
Fig. 4 1H NMR spectra of amylose-DDA samples ([Starch]=0.05 mg/mL): (a) [DDA]=0.8 mmol/L; (b) [DDA]= 1.6 mmol/L
In a broad view, the chemical interaction of aliphatic amine with amylose could be described as competition among water/amine hydrogen bonding, amine/amylose hydrophobic inclusion complexation, and hydroxyl/amine hydrogen bonding. If the carbon chain of amine is not too long, the water/amine interaction would dominate the situation and could not form the inclusion complex, just as early reported for aliphatic amines with less than ten carbon atoms [12, 21]. As the number of carbon atoms of alkyl chain of amine is increased up to 12, the hydrophobic interaction would play a leading role to generate the amylose-amine inclusion complex at a low concentration of guest molecules. When the starch helixes have been all plugged and the amine concentration is still continuously increased, the hydroxyl/amine interaction would be the principal force to bind amine and starch.
3.2 Effect of starch-DDA interaction on surface tension
Surface tension measurements were carried out to study the surface activity of DDA influenced by its interaction with amylose.
Figure 5 shows the surface tension as a function of the DDA concentration in the absence and presence of amylose. It can be clearly seen that the presence of amylose is helpful in increasing the surface tension, especially in the moderate concentration range. Combined with the previous observation in starch-iodine test, it could be deduced that under such a condition amylose could have included some of the DDA molecules into its helical cavity so as to ‘hide them’ and weaken their hydrophobic effect. Being trapped in the helix of starch, the collector would not be expected to exhibit a high surface activity during co-adsorption.
Fig. 5 Surface tension of amylose affected by DDA concentration ([Starch]=0.05 mg/mL)
This phenomenon is very similar to that for waxy maize starch (largely of amylopectin) and carboxymethyl starch with low degree of substitution, which, as we have addressed before [13, 22], can enhance the DDA adsorption onto diaspore surface while depress its flotation by trapping but not dispelling the collector. However, the trend for increasing the surface tension of DDA by the formation of amylose-DDA inclusion complex appears unremarkable when the DDA concentration increases further, which should be due to the increasing interaction between the polar groups of amylose and DDA.
4 Conclusions
1) Starch-iodine reaction shows that the pre-treatment of amylose with DDA at low concentration causes a sharp drop in blue-value, indicating the partial occupation of helical cavities by DDA molecules by forming inclusion amylose-DDA complex.
2) As revealed by the starch-iodine tests for amylose+DDA+iodine and amylose+iodine+DDA systems with different orders of reagent addition, DDA is inferred to exhibit a stronger affinity to the interior hydrophobic cavity of amylose than iodine does. Based on the analysis of the numbers of apparent helical turns, it is deduced that each amylose helix can be plugged by two DDA molecules, one at each end of the amylose molecule.
3) When treated with concentrated DDA, amylose would be compactly coated with DDA by predominantly hydroxyl/amine hydrogen bonding, as verified by starch-iodine reaction, surface tension measurement, and 1H NMR spectroscopy.
4) The inclusion complexation of amylose with DDA can help to weaken the hydrophobic effect of DDA collector to some extent without dispelling its attachment. In contrast to amylose, amylopectin shows poor complexing ability to DDA due to the short helix.
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(Edited by YANG Bing)
Foundation item: Project(50804055) supported by the National Natural Science Foundation of China
Received date: 2011-08-12; Accepted date: 2012-01-13
Corresponding author: LI Hai-pu, Associate Professor, PhD; Tel: +86-731-88876961; E-mail: lihaipu@csu.edu.cn
