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On the new possibility of applying oscillating liquidmembrane systems for molecular recognition substances responsible for taste
DESALINATION Desalination 173 (2005) 61~57
ELSEVIER
www.elsevier.com/locate/desaI
On the new possibility of applying oscillating liquid membrane systems for molecular recognition substances responsible for taste Maria Szpakowska a*, E12bieta Ptocharska-Jankowska a, Otto B. Nagy b "Commodity Science Laboratory, Faculty of Economics and Management, Gdatbk University of Technology, ul. Narutowieza 11/12, 80-952 Gdazisk, Poland Tel. +48 (58) 347-1107; Fax: +48 (58) 348-6009; email: [email protected], [email protected] bLaboratoire de Chimie Organique Physique, Univers#e Catholique de Louvain, Batiment Lavoisier, PI. Pasteur 1, 1348 Louvain la Neuve, Belgium Received 23 February 2004; accepted 29 June 2004
Abstract
It was previously suggested that liquid membrane oscillator systems might be used for the manufacture of taste sensors. The influence of substances responsible for taste belonging to bitterness, sweetness, sourness and saltiness on oscillation patterns of a liquid membrane oscillator with cationic surfactant benzyldimethyltetradecylammonium chloride was examined. It was concluded that independently of the nature of the membrane solvent present in the liquid membrane phase, the oscillation characteristics are sensitive to the investigated taste substances in a specific way. Keywords: Liquid membrane oscillator; Liquid membrane; Cationic surfactant; Oscillations
1. I n t r o d u c t i o n
Liquid membranes are important tools in separation science, e.g., for water purification o f toxic metals [1] and other substances [2]. Oscillatory liquid membrane systems allow also the identification and quantitative determination o f certain water pollutants [3,4]. Artificial systems can be used for modeling the oscillatory behaviour o f biological mem-
branes [5]. Very interesting is the behaviour o f cells in the human body responsible for taste. In the estimation o f food quality, the organoleptic methods are mainly used. However, these methods are not objective enough. In recent years much effort has been made for designing a taste sensor based on lipid membranes [6,7]. On the other hand, it has been shown recently that a bulk liquid membrane oscillator can be sensitive to a
Presented at PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland and Slovakia), September 7-11, 2003, Tatranskd Matliare, Slovakia. 0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.06.209
62
M. Szpakowska et al. / Desalination 173 (2005) 61-67
certain group of substances [8,9]. On account of their unique feature of oscillation (amplitude, frequency, phase) [10], it is suggested that they would be more sensitive to different molecules than lipid membranes. The influence o f substances responsible for taste belonging to four classes (bitterness, sweetness, sourness and saltiness) on oscillation characteristics of an oscillator with benzyldimethyltetradecylammonium chloride (BDMTAC) is presented. The bulk liquid membrane oscillator consists of three phases: donor aqueous, containing BDMTAC and ethanol; membrane (m), containing picric acid in organic solvent (nitrobenzene or nitromethane); and acceptor aqueous, containing various substances responsible for taste.
2. Experimental
At the bottom of the thermostated U-shape glass tube (Fig. I), the liquid membrane solution (m) was introduced. In the two branches of the glass tube aqueous phases were introduced simultaneously. The composition of three phases was the following: • aqueous donor phase; 4 ml of BDMTAC (5x 10 -3 M) in ethanol (1.5 M), water mixture; • liquid membrane: 5 ml of picric acid (HPi, 1.5 × 10 -3 M) in nitromethane or nitrobenzene; • aqueous acceptor phase: 4 ml of taste substances in water [sucrose, (0.1 M), NaC1 (0.1 M), CH3COOH (0.1 M), quinine hydrochloride (0.05 M)]. The electrochemical potential difference between the two aqueous phases, Ea/d,was measured by means of two Ag/AgCI/CI reference electrodes using a voltmeter controlled by a PC (sampling speed 5 s-l). Each experiment was repeated at least four times. The oscillation curves were self-similar due to a certain chaotic behaviour of the system.
2
d
a
Fig. 1. Scheme of the liquid membrane oscillator. d, aqueous donor phase; m, liquid membrane; a, aqueous acceptor phase; 1, Ag/AgC1/CI- electrodes; 2, millivoltmeter controlled by a PC.
3. Results and discussion
Practical applications of liquid membrane oscillators require detailed knowledge of their functioning. For this purpose we built different oscillation systems and investigated the influence on oscillations of various physicochemical factors such as liquid membrane type and its composition, surfactant and ethanol concentration changes and taste substances in aqueous phases. The first set of results conceming the system is with a nitromethane membrane containing picric acid. Fig. 2 shows the influence of surfactant concentration in the donor aqueous phase. It can be seen that in the absence of surfactant no oscillation is observed at all (Fig. 2a). When the added surfactant concentration is of the order of critical micelle concentration (cmc) (~1 × 10-3M), the system becomes unstable and shows a slight oscillating behaviour at the beginning of the
63
M. Szpakowska et al. / Desalination 173 (2005) 61-67
200 .,.,"
0
~' 200 o
200 if,,r~
I
I
I
600
1200
1800
I
2400
I
3000
3600
t(s) Fig. 2. Surfactant (BDMTAC) concentration effect on oscillation characteristics. Liquid membrane: HPi (1.5 ×10-3 M) in nitromethane;aqueous acceptor phase: sucrose (0.1 M); aqueous donor phase. (a) 0 M BDMTAC, (b) 1× 10 3MBDMTAC, (c) 5× 10-3M BDMTAC in ethanol (1.5 M), water mixture. experiment till about 300 s (Fig. 2b). After this period no oscillation is observed. A dramatic change occurs when the surfactant concentration is much above the cmc (Fig. 2c). Oscillations start from almost the very beginning of the experiment. These initial small amplitude oscillations disappear after about 800 s and are replaced by a different kind of oscillation pattern. The peaks are large, having a great amplitude, and they change phase with time. Furthermore, higher frequency bursting oscillations can be observed in the time interval 800-1800 s. These results clearly show that the presence o f surfactant is indispensable for the appearance o f oscillations. Furthermore, the characteristics o f these latter strongly depend on surfactant concentration. The presence of alcohol in the donor phase is also necessary for the oscillation [10]. For the oscillator containing surfactant
BDMTAC, oscillations are observed above 0.5 M ethanol concentration. Fig. 3 presents the influence of substances responsible for taste on the oscillation pattern. The various taste substances were added to the aqueous acceptor phase. Donor and membrane phases were kept constant. In the case of an oscillator containing sucrose in the acceptor phase (Fig. 3a), four types o f peaks were observed: narrow peaks o f small amplitude (no. 1) followed by wider peaks o f small amplitude (no. 5). Around 1000 s wide peaks of the opposite phase arise (no. 2). These types of peaks observed at the end o f the experiment are characterized by phase change. Between these wide peaks, small narrow peaks are observed (no. 4). Compared to the effect o f sucrose on the oscillation curve, the presence o f sodium chloride fundamentally modifies the oscillation pattern
64
M. Szpakowska et al. / Desalination 173 (2005) 61-67 |
i
|
i
i
1 a
200 0
5
b
200 ~iLi'l~lI~. 2 ~,,~.5.... ~ ,--~
4
2
2
4
4
0
200
513
2
~i~k~,, c-~
4
2
2
4
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d 200 0
4
4
4
4
4
I
I
I
600
1200
1800
4
4 I
2400
4
4 I
3000
3600
t(s) Fig. 3. Oscillation pattems of liquid membrane oscillator with nitromethane. Donor phase: BDMTAC (5 x 10-3M) in ethanol (1.5 M), water mixture, m: HPi (1.5x 10-3M) in nitrobenzene; aqueous acceptor phase composition: (a) sucrose (0.1 M),
(b) NaC1 (0.1 M), (c) citric acid (0.1 M), (d) quinine hydrochloride (0.05 M).
(Fig. 3b). The initial oscillations (no. 5) have a higher frequency. They are followed by symmetrical oscillations creating a group of oscillations (no. 3). Interestingly enough, the wide, small amplitude peaks (no. 2) are again visible, but they have opposite phases to those observed for sucrose. However, this phase remains constant throughout the experiment. It should also be noted that the presence of NaCI suppresses the bursting oscillations.
Citric acid brings about a similar effect (Fig. 3c). However, the higher frequency oscillations (nos. 1 and 5) have smaller amplitudes, and they are followed by a group of peaks (nos. 3, 2 and 4). At the beginning the same phase is observed as for sucrose together with bursting oscillations (around 600 s). In the following interval (from 1200 to 3500 s), the phase is completely opposite, but constant without bursting oscillation. The most spectacular effect is brought
65
M. Szpakowska et al. / Desalination 173 (2005) 61-67 -\
400 200 0
200 0 C
400: 200 0
d
400
t
L
600
t
!
I
1200
I
I
I
1800
2400
3000
3600
t(s) Fig. 4. Oscillation patterns of liquid membrane oscillator with nitrobenzene, d: BDMTAC (5 x 10-3 M) in ethanol (1.5 M), water mixture, m: HPi (1.5 x 10- 3M) in nitrobenzene; aqueous aceeptor phase composition. (a) sucrose (0.1 M), (b) NaC1
(0.1 M), (c) citric acid (0.1 M), (d) quininehydrochloride(0.05 M).
about by quinine hydrochloride (Fig. 3d). The oscillation pattem is completely changed and only small amplitude oscillations (no. 4) are observed with the constant phase. However, a region without oscillation (700-1300 s) appears. The influence of taste substances on oscillation patterns is quite different when nitrobenzene was used as the membrane solvent. The presence of sucrose in the aqueous acceptor phase
produces a very regular oscillation pattern (Fig. 4a). After an induction period (-760 s) where only two peaks at -300s are observed, large amplitude oscillations appear. The frequency changes, but the amplitudes remain reasonably constant (330-350 mV) till the end of the experiment. Adding sodium chloride to the acceptor phase provokes relatively few changes in the oscillation pattern (Fig. 4b). The induction
M. Szpakowska et al. / Desalination 173 (2005) 61-67
66
35
a
b
30 25
20 n
n 15
I5
5
5
10 5
2
10
4 0
0 .3,50
-2,50
-1,50
-0,50
-3,50
0,50
ig F
35
-2,50
3O
25
25
0,50
-0,50
Ig F
35
30
-1,50
d
20
2O n
n 15
15 r
10
I0f
0
-3,50
0
-2,50
-1,50
-0,50
0,50
L
--------------------~
-3,50
-2,50
-1,50
-0,50
0,50
lg F lg F Fig. 5. Histograms for the systems with (a) sucrose, (b) NaCI, (c) citric acid, (d) quinine hydrochloride in the acceptor phase. period (till 850 s) is without oscillations. The constant amplitude of peaks remain almost the same as in the sucrose case, but the frequency of peaks is modified. Furthermore, windows without oscillations appear during the intervals 16001900 s and 2100-2500 s. Again, as for systems with nitromethane, the effect of citric acid is similar to that of sodium chloride (Fig. 4c). However, the initial time is shorter (760 s), the frequencies are longer and the intervals without oscillations appear earlier.
The effect of quinine hydrochloride (Fig 4 d), is very different from the previous cases. The oscillation pattern shows at first an induction period up to -330 s, followed by regular small amplitude oscillations in the constant phase. These oscillations disappear abruptly at 1140 s. After that interval no oscillations are observed at all. In order to interpret the above results obtained for the oscillator containing nitromethane, histograms were established. They present the rela-
67
M. Szpakowska et al. / Desalination 173 (2005) 61-67
tionship between frequency (F) and number of peaks (n) for systems containing various substances responsible for taste (Fig. 5). Due to the fact that oscillations are rather irregular the range of frequencies is represented by a rectangular. As it can be seen the histograms are different for each system. All observed types o f peaks were noted only for the system containing sour substance (Fig. 5c). It is very interesting that in the case of bitter substance (Fig. 5d) represented by quinine hydrochloride only one type o f peaks (no. 4) is observed. It can be concluded that histograms might be used for molecular recognition of substances responsible for taste belonging to four classes. To distinguish taste substances present in an oscillator with nitrobenzene, the range of oscillations from 750 to 1200 s was analyzed (Fig. 4). The number of peaks with their amplitude and frequency in this region is summarized in Table 1. In the case o f a sucrose system (Fig. 4a), oscillations are rather irregular in the examined region but have almost the same amplitude. For the system with NaC1 (Fig. 4b), the amplitude of peaks is smaller and frequency fluctuates with time. For the system with an acid substance (Fig. 4c), amplitude and frequency o f peaks is diminishing significantly. Only five peaks are observed in the examined region (Table 1). The presence of a bitter substance in the oscillator changes the oscillation pattern very dramatically (Fig. 4d). In this case fourteen peaks of higher frequency and small amplitude in the analyzed region are observed. It can be concluded that, independently of the nature of the membrane solvent, oscillation characteristics are different for the taste substances belonging to four different classes. In particular, the presence of a bitter substance changes the oscillation pattern significantly. However, more studies are necessary to generalize this conclusion. Furthermore, these systems might be characterized more quantitatively by constructing
Table 1 Characteristicsof peaks in the range of 750-1200 s for an oscillator with nitrobenzene containing different taste substances Acceptor phase
No. of peaks
Range of amplitude, mV
Frequency x 102, 1/s
Sucrose (0.1 M) Sodium chloride (0.1 M) Citric acid (0.1 M) Quinine hydrochloride (0.0 5M)
6 5°
330-350 270-300
1.0-1.8 0.9-2.0
5
98-130
0.8-1.0
14
20-30
2.9-4.2
"After first peak at 850 s, the regular oscillation started from 950 s. their attractors in phase space. The appropriate calculations are in progress.
References [1] R.D. Noble and J.D. Way, Liquid membranes: Theory and Applications, A.C.S. Symp. Series, No. 374, Washington, DC, 1987. [2] A.M. Eyal and E. Bressler, Biotechnol.Bioeng.,41 (1993) 287-295. [3] T. Yoshidome, T. Higashi, M. Mitsushio and S. Kamata, Chem. Lett., (1998) 855-856. [4] T. Yoshidome, T. Higashi and S. Kamata, Chem. Lett., (2000) 550-551. [5] R. Larter, Chem. Rev., 90 (1990) 355-381. [6] K. Toko, Meas. Sci. Technol., 9 (1998) 1919-1936. [7] K. Toko, Mat. Sci. Eng. C, 4 (1996) 69-92. [8] K. Yoshikawa, T. Omochi and Y. Matsubara, Biophys. Chem., 23 (1986) 211-214. [9] M. Szpakowska,I. Czaplicka, A. Magnuszewskaand O.B. Nagy, Proc. 13th IGWT Symp. Commodity Science in Global Quality Perspective, Maribor, 1 (2001) 757-761. [10] M. Szpakowska, I. Czaplicka, J. Szwacki and O.B. Nagy, Chem. Pap., 56 (2002) 20-23.
ELSEVIER
www.elsevier.com/locate/desaI
On the new possibility of applying oscillating liquid membrane systems for molecular recognition substances responsible for taste Maria Szpakowska a*, E12bieta Ptocharska-Jankowska a, Otto B. Nagy b "Commodity Science Laboratory, Faculty of Economics and Management, Gdatbk University of Technology, ul. Narutowieza 11/12, 80-952 Gdazisk, Poland Tel. +48 (58) 347-1107; Fax: +48 (58) 348-6009; email: [email protected], [email protected] bLaboratoire de Chimie Organique Physique, Univers#e Catholique de Louvain, Batiment Lavoisier, PI. Pasteur 1, 1348 Louvain la Neuve, Belgium Received 23 February 2004; accepted 29 June 2004
Abstract
It was previously suggested that liquid membrane oscillator systems might be used for the manufacture of taste sensors. The influence of substances responsible for taste belonging to bitterness, sweetness, sourness and saltiness on oscillation patterns of a liquid membrane oscillator with cationic surfactant benzyldimethyltetradecylammonium chloride was examined. It was concluded that independently of the nature of the membrane solvent present in the liquid membrane phase, the oscillation characteristics are sensitive to the investigated taste substances in a specific way. Keywords: Liquid membrane oscillator; Liquid membrane; Cationic surfactant; Oscillations
1. I n t r o d u c t i o n
Liquid membranes are important tools in separation science, e.g., for water purification o f toxic metals [1] and other substances [2]. Oscillatory liquid membrane systems allow also the identification and quantitative determination o f certain water pollutants [3,4]. Artificial systems can be used for modeling the oscillatory behaviour o f biological mem-
branes [5]. Very interesting is the behaviour o f cells in the human body responsible for taste. In the estimation o f food quality, the organoleptic methods are mainly used. However, these methods are not objective enough. In recent years much effort has been made for designing a taste sensor based on lipid membranes [6,7]. On the other hand, it has been shown recently that a bulk liquid membrane oscillator can be sensitive to a
Presented at PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland and Slovakia), September 7-11, 2003, Tatranskd Matliare, Slovakia. 0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.06.209
62
M. Szpakowska et al. / Desalination 173 (2005) 61-67
certain group of substances [8,9]. On account of their unique feature of oscillation (amplitude, frequency, phase) [10], it is suggested that they would be more sensitive to different molecules than lipid membranes. The influence o f substances responsible for taste belonging to four classes (bitterness, sweetness, sourness and saltiness) on oscillation characteristics of an oscillator with benzyldimethyltetradecylammonium chloride (BDMTAC) is presented. The bulk liquid membrane oscillator consists of three phases: donor aqueous, containing BDMTAC and ethanol; membrane (m), containing picric acid in organic solvent (nitrobenzene or nitromethane); and acceptor aqueous, containing various substances responsible for taste.
2. Experimental
At the bottom of the thermostated U-shape glass tube (Fig. I), the liquid membrane solution (m) was introduced. In the two branches of the glass tube aqueous phases were introduced simultaneously. The composition of three phases was the following: • aqueous donor phase; 4 ml of BDMTAC (5x 10 -3 M) in ethanol (1.5 M), water mixture; • liquid membrane: 5 ml of picric acid (HPi, 1.5 × 10 -3 M) in nitromethane or nitrobenzene; • aqueous acceptor phase: 4 ml of taste substances in water [sucrose, (0.1 M), NaC1 (0.1 M), CH3COOH (0.1 M), quinine hydrochloride (0.05 M)]. The electrochemical potential difference between the two aqueous phases, Ea/d,was measured by means of two Ag/AgCI/CI reference electrodes using a voltmeter controlled by a PC (sampling speed 5 s-l). Each experiment was repeated at least four times. The oscillation curves were self-similar due to a certain chaotic behaviour of the system.
2
d
a
Fig. 1. Scheme of the liquid membrane oscillator. d, aqueous donor phase; m, liquid membrane; a, aqueous acceptor phase; 1, Ag/AgC1/CI- electrodes; 2, millivoltmeter controlled by a PC.
3. Results and discussion
Practical applications of liquid membrane oscillators require detailed knowledge of their functioning. For this purpose we built different oscillation systems and investigated the influence on oscillations of various physicochemical factors such as liquid membrane type and its composition, surfactant and ethanol concentration changes and taste substances in aqueous phases. The first set of results conceming the system is with a nitromethane membrane containing picric acid. Fig. 2 shows the influence of surfactant concentration in the donor aqueous phase. It can be seen that in the absence of surfactant no oscillation is observed at all (Fig. 2a). When the added surfactant concentration is of the order of critical micelle concentration (cmc) (~1 × 10-3M), the system becomes unstable and shows a slight oscillating behaviour at the beginning of the
63
M. Szpakowska et al. / Desalination 173 (2005) 61-67
200 .,.,"
0
~' 200 o
200 if,,r~
I
I
I
600
1200
1800
I
2400
I
3000
3600
t(s) Fig. 2. Surfactant (BDMTAC) concentration effect on oscillation characteristics. Liquid membrane: HPi (1.5 ×10-3 M) in nitromethane;aqueous acceptor phase: sucrose (0.1 M); aqueous donor phase. (a) 0 M BDMTAC, (b) 1× 10 3MBDMTAC, (c) 5× 10-3M BDMTAC in ethanol (1.5 M), water mixture. experiment till about 300 s (Fig. 2b). After this period no oscillation is observed. A dramatic change occurs when the surfactant concentration is much above the cmc (Fig. 2c). Oscillations start from almost the very beginning of the experiment. These initial small amplitude oscillations disappear after about 800 s and are replaced by a different kind of oscillation pattern. The peaks are large, having a great amplitude, and they change phase with time. Furthermore, higher frequency bursting oscillations can be observed in the time interval 800-1800 s. These results clearly show that the presence o f surfactant is indispensable for the appearance o f oscillations. Furthermore, the characteristics o f these latter strongly depend on surfactant concentration. The presence of alcohol in the donor phase is also necessary for the oscillation [10]. For the oscillator containing surfactant
BDMTAC, oscillations are observed above 0.5 M ethanol concentration. Fig. 3 presents the influence of substances responsible for taste on the oscillation pattern. The various taste substances were added to the aqueous acceptor phase. Donor and membrane phases were kept constant. In the case of an oscillator containing sucrose in the acceptor phase (Fig. 3a), four types o f peaks were observed: narrow peaks o f small amplitude (no. 1) followed by wider peaks o f small amplitude (no. 5). Around 1000 s wide peaks of the opposite phase arise (no. 2). These types of peaks observed at the end o f the experiment are characterized by phase change. Between these wide peaks, small narrow peaks are observed (no. 4). Compared to the effect o f sucrose on the oscillation curve, the presence o f sodium chloride fundamentally modifies the oscillation pattern
64
M. Szpakowska et al. / Desalination 173 (2005) 61-67 |
i
|
i
i
1 a
200 0
5
b
200 ~iLi'l~lI~. 2 ~,,~.5.... ~ ,--~
4
2
2
4
4
0
200
513
2
~i~k~,, c-~
4
2
2
4
2
d 200 0
4
4
4
4
4
I
I
I
600
1200
1800
4
4 I
2400
4
4 I
3000
3600
t(s) Fig. 3. Oscillation pattems of liquid membrane oscillator with nitromethane. Donor phase: BDMTAC (5 x 10-3M) in ethanol (1.5 M), water mixture, m: HPi (1.5x 10-3M) in nitrobenzene; aqueous acceptor phase composition: (a) sucrose (0.1 M),
(b) NaC1 (0.1 M), (c) citric acid (0.1 M), (d) quinine hydrochloride (0.05 M).
(Fig. 3b). The initial oscillations (no. 5) have a higher frequency. They are followed by symmetrical oscillations creating a group of oscillations (no. 3). Interestingly enough, the wide, small amplitude peaks (no. 2) are again visible, but they have opposite phases to those observed for sucrose. However, this phase remains constant throughout the experiment. It should also be noted that the presence of NaCI suppresses the bursting oscillations.
Citric acid brings about a similar effect (Fig. 3c). However, the higher frequency oscillations (nos. 1 and 5) have smaller amplitudes, and they are followed by a group of peaks (nos. 3, 2 and 4). At the beginning the same phase is observed as for sucrose together with bursting oscillations (around 600 s). In the following interval (from 1200 to 3500 s), the phase is completely opposite, but constant without bursting oscillation. The most spectacular effect is brought
65
M. Szpakowska et al. / Desalination 173 (2005) 61-67 -\
400 200 0
200 0 C
400: 200 0
d
400
t
L
600
t
!
I
1200
I
I
I
1800
2400
3000
3600
t(s) Fig. 4. Oscillation patterns of liquid membrane oscillator with nitrobenzene, d: BDMTAC (5 x 10-3 M) in ethanol (1.5 M), water mixture, m: HPi (1.5 x 10- 3M) in nitrobenzene; aqueous aceeptor phase composition. (a) sucrose (0.1 M), (b) NaC1
(0.1 M), (c) citric acid (0.1 M), (d) quininehydrochloride(0.05 M).
about by quinine hydrochloride (Fig. 3d). The oscillation pattem is completely changed and only small amplitude oscillations (no. 4) are observed with the constant phase. However, a region without oscillation (700-1300 s) appears. The influence of taste substances on oscillation patterns is quite different when nitrobenzene was used as the membrane solvent. The presence of sucrose in the aqueous acceptor phase
produces a very regular oscillation pattern (Fig. 4a). After an induction period (-760 s) where only two peaks at -300s are observed, large amplitude oscillations appear. The frequency changes, but the amplitudes remain reasonably constant (330-350 mV) till the end of the experiment. Adding sodium chloride to the acceptor phase provokes relatively few changes in the oscillation pattern (Fig. 4b). The induction
M. Szpakowska et al. / Desalination 173 (2005) 61-67
66
35
a
b
30 25
20 n
n 15
I5
5
5
10 5
2
10
4 0
0 .3,50
-2,50
-1,50
-0,50
-3,50
0,50
ig F
35
-2,50
3O
25
25
0,50
-0,50
Ig F
35
30
-1,50
d
20
2O n
n 15
15 r
10
I0f
0
-3,50
0
-2,50
-1,50
-0,50
0,50
L
--------------------~
-3,50
-2,50
-1,50
-0,50
0,50
lg F lg F Fig. 5. Histograms for the systems with (a) sucrose, (b) NaCI, (c) citric acid, (d) quinine hydrochloride in the acceptor phase. period (till 850 s) is without oscillations. The constant amplitude of peaks remain almost the same as in the sucrose case, but the frequency of peaks is modified. Furthermore, windows without oscillations appear during the intervals 16001900 s and 2100-2500 s. Again, as for systems with nitromethane, the effect of citric acid is similar to that of sodium chloride (Fig. 4c). However, the initial time is shorter (760 s), the frequencies are longer and the intervals without oscillations appear earlier.
The effect of quinine hydrochloride (Fig 4 d), is very different from the previous cases. The oscillation pattern shows at first an induction period up to -330 s, followed by regular small amplitude oscillations in the constant phase. These oscillations disappear abruptly at 1140 s. After that interval no oscillations are observed at all. In order to interpret the above results obtained for the oscillator containing nitromethane, histograms were established. They present the rela-
67
M. Szpakowska et al. / Desalination 173 (2005) 61-67
tionship between frequency (F) and number of peaks (n) for systems containing various substances responsible for taste (Fig. 5). Due to the fact that oscillations are rather irregular the range of frequencies is represented by a rectangular. As it can be seen the histograms are different for each system. All observed types o f peaks were noted only for the system containing sour substance (Fig. 5c). It is very interesting that in the case of bitter substance (Fig. 5d) represented by quinine hydrochloride only one type o f peaks (no. 4) is observed. It can be concluded that histograms might be used for molecular recognition of substances responsible for taste belonging to four classes. To distinguish taste substances present in an oscillator with nitrobenzene, the range of oscillations from 750 to 1200 s was analyzed (Fig. 4). The number of peaks with their amplitude and frequency in this region is summarized in Table 1. In the case o f a sucrose system (Fig. 4a), oscillations are rather irregular in the examined region but have almost the same amplitude. For the system with NaC1 (Fig. 4b), the amplitude of peaks is smaller and frequency fluctuates with time. For the system with an acid substance (Fig. 4c), amplitude and frequency o f peaks is diminishing significantly. Only five peaks are observed in the examined region (Table 1). The presence of a bitter substance in the oscillator changes the oscillation pattern very dramatically (Fig. 4d). In this case fourteen peaks of higher frequency and small amplitude in the analyzed region are observed. It can be concluded that, independently of the nature of the membrane solvent, oscillation characteristics are different for the taste substances belonging to four different classes. In particular, the presence of a bitter substance changes the oscillation pattern significantly. However, more studies are necessary to generalize this conclusion. Furthermore, these systems might be characterized more quantitatively by constructing
Table 1 Characteristicsof peaks in the range of 750-1200 s for an oscillator with nitrobenzene containing different taste substances Acceptor phase
No. of peaks
Range of amplitude, mV
Frequency x 102, 1/s
Sucrose (0.1 M) Sodium chloride (0.1 M) Citric acid (0.1 M) Quinine hydrochloride (0.0 5M)
6 5°
330-350 270-300
1.0-1.8 0.9-2.0
5
98-130
0.8-1.0
14
20-30
2.9-4.2
"After first peak at 850 s, the regular oscillation started from 950 s. their attractors in phase space. The appropriate calculations are in progress.
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