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Effects of magnetic field on water investigated with fluorescent probes
A: Physicochemical
Colloids and Surfaces and Engineering Aspects
COLLOIDS AND SURFACES
A
109 (1996) 167-173
Effects of magnetic field on water investigated with fluorescent probes Ko Higashitani *, Jun Oshitani, Norio Ohmura Department ofchemical
Engineering, Kyoto University, Yoshida, Sakyo, Kyoto 606-01, Japan
Received 1 September 1995; accepted 29 September 1995
Abstract Aqueous solutions with dissolved fluorescent probes were exposed to a magnetic field, and the effects of the magnetic exposure were investigated by measuring the fluorescence emission intensity with a spectrofluorophotometer. The data were compared with those of magnetic exposure effects on colloidal solutions reported previously [K. Higashitani, K. Okuhara and S. Hatade, J. Colloid Interface Sci., 152 (1992) 125; K. Higashitani, H. Isari, K. Okuhara, A. Kage and S. Hatade, J. Colloid Interface Sci., 172 (1995) 3831. It is found that (1) the degree of magnetic effects increases with the magnetic exposure and becomes constant at a certain exposure time in the case of solutions of fluorescent probes with an alkyl chain, but no effect was observed for solutions of probes without an alkyl chain; (2) the magnetic effects decay with the concentration of alcohol in the solution, the solution temperature, and the standing time of the solution after the magnetic exposure; and (3) if it is presumed that an ordered structure of water molecules around the hydrophobic chain of probes is formed by the magnetic exposure, not only are the results in the present study but also are our previous results for colloidal particles explained consistently. Keywords:
Aqueous solutions; Fluorescence; Fluorescent probes; Magnetic field effects; Memory effect; Water structure
1. Introduction
All the materials on earth, including living organisms, have been exposed to the geomagnetic field and the occasions on which we are exposed to an anthropogenic magnetic field, such as magnetic resonance imaging, are increasing. Hence it is fundamentally important to investigate the effects of magnetic exposure on materials. The effects of a magnetic field on aqueous solutions [l-11] and biomaterials [12,13] have been investigated in various fields, but these effects have been mysterious in the sense that the effects arise from the exposure of materials to a magnetic field * Corresponding 0927-7757/96/$15.00
author.
0 1996 Elsevier Science B.V. All rights reserved
SSDZ 0927-7757(95)03483-S
of low flux density and they remain even after the magnetic exposure is completed. These phenomena are classified into the following categories: (1) phenomena for magnetic materials which are understandable using current electromagnetic theory; and (2) phenomena for non-magnetic materials which are not understandable by current electromagnetics. We are interested only in the latter. Recently Higashitani et al. reported quantitative data concerning the effects of magnetic exposure on the rapid coagulation rate of polystyrene latices (PSL) in solutions [9], data relating to magnetic exposure effects on the formation of CaCO, crystals from CaCl, and Na,CO, solutions [lo], and data concerning magnetic exposure effects on the zeta potential and diffusivity of PSL in electrolyte solutions as well as a possible mechanism to
168
K. Higashitani
et al. JCoNoids Surfaces A: Physicochem.
explain the data [ll]. It is now certain that magnetic exposure effects on colloidal solutions do exist even if the colloids are non-magnetic and the magnetic field is of low flux density, and the effects have been considered to be related to a conformational change of water molecules, ions or hydrated ions adsorbed onto the particle surface. The data reported so far are rather macroscopic in character. In the present study, we employed fluorescent probes to obtain information on the molecular level. The fluorescent probes were dissolved in water and the change of fluorescence emission intensity caused by the magnetic exposure was evaluated under various experimental conditions. The results were compared with those relating to magnetic exposure effects on the zeta potential and coagulation rate constant obtained in preceding experiments [ 9,111.
2. Experimental The data on the fluorescent probes employed in this study are summarized in Table 1. Two kinds of probes were used. The probes with fluorescent benzene nuclei and a non-fluorescent alkyl chain are illustrated in the left-hand column, and the probes with the same fluorescent benzene nuclei only are in the right-hand column, although a short alkyl chain exists in the case of 1-pyrenebutanoic acid (PB). Pure water with a relative resistance of 17.3 MW R cm, which was purified by distillation and reverse osmosis, was used. Aqueous solutions were prepared by adding water to 1.0 mg of the fluorescent probes in large-size glassware and sonieating the solution with an ultrasonic oscillator until the probes appeared to have dissolved completely. Since some probes were not very soluble in water, the probe concentration, C,, was determined as follows. After evaporating water from the sample solution, the residue on the glassware was dissolved using a good solvent (ethanol) and the fluorescence emission intensity of the solution was compared with that of reference solution. When the effect of alcohol on the magnetic exposure effects was examined, water-ethanol mixtures of various concentrations were employed. The solutions prepared were kept in a temperature-
Eng. Aspects 109 (1996) 167-I 73
controlled bath at 25 f 0.1 “C and all the measurements were carried out at this temperature unless otherwise specified. The wavelengths for the maximum excitation and emission intensities were determined by use of a spectrophotofluorometer (Shimadzu RF-540), and all the measurements were then carried out using these wavelengths. A uniform magnetic field was generated by a pair of permanent magnets installed in a rectangular iron yoke. The distance between the magnets was 2.6 cm and the magnetic flux density was 0.42 T. The experimental procedure is shown schematically in Fig. 1. Glass tubes containing 4.0 cm3 samples of the fluorescent solution were placed between a pair of magnets and exposed to the magnetic field for a given period. These samples are termed “magnetized” henceforth. The fluorescence emission intensity was measured using the spectrofluorophotometer immediately after the magnetic exposure had been completed. However, when the duration of the magnetic effect after exposure was examined, the samples were left standing for a given period in a temperaturecontrolled bath before measuring the fluorescence intensity. The same experiments, but without the magnetic exposure, were also carried out as a reference. The magnetic effect was evaluated through the ratio of the fluorescence emission intensity for a magnetized solution against that of the non-magnetized solution, Z,/I. Measurements of I, and I were repeated about 20 times for each sample, and the magnitude of I,/1 was evaluated using their averaged values. The deviation from unity indicates the degree of the magnetic effect. Fluorescent Solutions
I
I
Magnetic Exposure Flux Density : 0.42T ExposureTie : O-45min.
Standing Period Period : 0-lOhours I
I
J( L Fig. 1. Schematic procedure.
JI
Measurement of Fluorescent Emission Intensity drawing
of
the
experimental
exposure
K. Higashitani et al./CoNoids
Surfaces A: Physicochem.
Eng. Aspects 109 (1996) 167-I 73
Table 1 Florescent probes with and without an alkyl chain 2-9anduoyloxy
stearic acid (AS) HO
CH3(CH2)15-&&OH
3.6 bis dimetbylamino-lo-dodecyl acridinium bromide (DADAB)
4-heptadecyl umbelliferone (HUF)
3,6 bis dimethylsminoaaidine (DAA)
Umbelliferone (UF)
Homo ClMU
S-N-octadecanoylsmino fluorescein (OAFL)
Fluomscein (FL)
, T cOoH
HO /
NHCO(CH+CH3
12-1-pyrene dodezanoie acid (PD)
I”/‘,‘” ’:
1-pyrene butsnoic acid (PB)
169
K Higashitani et ai./CoNoids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 167-l 73
170
All the experiments room.
were carried out in a dark
3. Results and discussion 3.1. Magnetic eflects on solutions of 2,9-anthroyloxystearic conditions
acid under various
Fig. 2 shows the dependence of I,,$ on the exposure time t, for 2,9-anthroyloxystearic acid (AS). It is clear that the magnitude of I,/1 increases with increasing t, after an induction time and it becomes about 1.08 at t, > 30 min. It was found in the preceding experiments on colloidal solutions that the reduced zeta potential and the reduced coagulation rate decrease with increasing t, and become constant at t, > 10 min, and that the maximum reduction of the zeta potential and coagulation rate was about 8% and 15%, respectively [ 111. These results are very similar to those given in Fig. 2, except that a longer exposure time is needed for ZJI to be constant. It is important to know whether the change in the fluorescence emission intensity was caused either by the direct reaction of the fluorescent probes to the magnetic wave or by the change in the surrounding environment, to which the probes
0.95’ 0
’ 10
’
’
20
I
30
’
’
40
are usually very sensitive. If the magnetic effect is attributable to the former mechanism, the magnitude of I,/1 is expected to be finite even if the properties of the medium are changed. Fig. 3 shows the dependence of the I,/I value at t, = 30 min on the volume fraction C, of ethanol in water. It is clear that the value of Z,/I decreases with increasing C, and becomes zero at C, > 0.1. Since the fluorescence intensity increases with the ethanol concentration, this reduction of I,/I with t, does not necessarily indicate that the probes do not react directly to the magnetic wave. However, it is found that this dependence of Z,/I on the alcohol concentration is very similar to that of the zeta potential of colloidal particles [ 141. Hence it is plausible to assume that the magnetic effect is attributable to the change of the configuration of water molecules surrounding the fluorescent probes. Fig. 4 shows the dependence of the I,/I value on the solution temperature T. It is clear that the magnetic effect disappears at T> 30°C. A similar temperature dependence of the magnetic effect was found also for the zeta potential [ 141. This coincidence between the different experiments implies that the magnetic effects are closely related to the thermal properties of the water molecules, and the effects are destroyed by the motion of the water
’
50
CaL-1
te [min] Fig. 2. Dependence 10e7 kmol mm3.)
of
I,/I
on
t. for
AS.
(C, =2.83
x
Fig. 3. Dependence of I,/1 lo-’ kmol rnm3; t,=30 min.)
on
C,
for
AS.
(C,=2.83
x
K Higashitani et al./Colloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 167-l 73
171
memory lasted for about 5 h in this case, but for more than 6 days in the case of the zeta potential and coagulation rate constant of colloidal particles [ 111. The reasons for its existence are mysterious. However, supposing that the water molecules are in a quasistable state, as mentioned above, the memory may exist until it is destroyed by an external disturbance. 1,oo
. . .._..____......___.................................
.
t
0
Fig. 4. Dependence me3; t,= 30 min.)
10
of 1,/I
20
30
T
[“Cl
40
1 50
on T for AS. (C, = 2.83 x lo-’ kmol
molecules. Hence it seems that water molecules in the magnetized solution are in a quasistable state. In the above experiments, the fluorescence emission intensity was measured immediately after the magnetic exposure was completed. Here, the solutions were left standing for a given period t, after the exposure. Fig. 5 shows the dependence of I,,lI on t,. The value of 1,/r gradually decreases with time. Since the magnetic effect remains after the magnetic exposure, we may say that there exists a “memory” with respect to the magnetic effects. The
3.2. EfSects of the alkyl chain of thefluorescent probe
In this section, the role of the alkyl chain of the fluorescent probe was investigated using the four pairs of probes illustrated in Table 1. Each pair has the same or similar fluorescent benzene nuclei, but a different side-chain. Fig. 6 shows the dependence of 1,/Z on t, for the DADAB and DAA probes. It is clear that no magnetic effect is observed for DAA, while the value of I,/I for DADAB increases with t, and becomes 1.15 at t, 825 min. Similar correlations of 1,/Z vs. t, were found between another two pairs of probes, as shown in Figs. 7 and 8. These results can be summarized as follows. (1) Magnetic effects were observable only for probes with a long alkyl chain. (2) The I,,,/1 value increases with the magnetic
1.3
,95 0.95’ 0
’ 2
I 4
. ts
Fig. 5. Dependence mm3; t,=30min.)
of I,/!
on
’ 6
’ 8
I . 10
’ 12
0
10
20
30
te [min]
[hour]
t, for AS. (C, =2.83 x lo-’ kmol
Fig. 6. Dependence (DADAB: C,=4.80 lo-* kmol mm3.)
of I,/1 on t, for DADAB x 10e9 kmol me3; DAA:
and DAA. C, =2.28 x
K. Higashitani et al./Colloids Surfaces A: Physicochem. Eng. Aspects 109 ( 19961 167-I 73 1.10
i_I.0 PD PB
0.95
’ 0
a
I
10
20
’
’
’
’
30
40
50
0.95 0
10
te [min]
20 te
Fig. 7. Dependence of I,/1 on t, for HUF and UF. (HUF: C,=9.55 x 10m9kmol rne3; UF: C,= 1.02 x lo-* kmol mm3.)
30
40
50
[min]
Fig. 9. Dependence of I,/1 on t, for PD and PB. (PD: C,= 1.08 x 10e7 kmol m-3; PB: C,=4.65 x lo-* kmol me3.)
be more or less proportional to the length of the alkyl chain of the probes. The reason why the I,/I value for the PD probe becomes constant at the shorter exposure time, compared with the other probes, is not known at present. 3.3. Possible mechanism
0.95 -
0
10
20
30
40
50
te [min] Fig. 8. Dependence of 1,/I on t, for OAFL and FL. (OAFL: C,=7.50 x lo-@ kmol mm3; FL: C,=1.88 x lOma kmol mm3.)
exposure and becomes constant after a sufficiently long exposure time. (3) A longer exposure time is needed for the magnetic effect to become constant probe solutions than colloidal solutions [ 111. Fig. 9 shows a comparison between the relationship 1,/I vs. t, for the PD probe and that for the PB probe. Both probes have an alkyl chain but the length is different. It is worth noting that the Z,/Z value for the PB probe is slightly larger than zero, and so the degree of the magnetic effect may
Although the above results give us information at the molecular level, they are still not enough to clarify the detailed mechanism of the magnetic effects. Nevertheless, it is important to discuss the possible mechanisms. A comparison of the present results with those relating to the magnetic effects on the colloidal solutions [9,11] is also important in order to confirm their reliability and to estimate the mechanism. It is clear that a long alkyl chain of the probes plays a vital role for the magnetic exposure effects. Since the magnetic field does not act directly on the fluorescent probes, and probes without a long alkyl chain exhibit no magnetic effect, it is plausible to assume that the magnetic effects are caused by the change of the orientation of the water molecules around the alkyl chain. It is known that water molecules around hydrophobic molecules are ordered so that they can participate in hydrogen bond formation, as in bulk water. Suppose that the ordered structure of the water molecules sur-
K. Higashitani et al.lColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 167-173
rounding the hydrophobic chain is altered by the magnetic exposure such that the alkyl chain is distorted, then the fluorescence emission intensity may vary because the distortion influences the fluorescent benzene nuclei. In the preceding paper, the following mechanism was proposed. The magnetic exposure alters the conformation of the water molecules, ions and hydrated ions adsorbed on the particle surface in such a way that the effective adsorbed layer becomes thicker and ordered. This mechanism is consistent with the hypothesis concerning probe solutions described above. As for probes with a long alkyl chain, the value of I,/I increases with t, and becomes constant. This behaviour is very similar to that of the zeta potential and coagulation rate, but the exposure time to obtain the maximum magnetic effect is much longer and the memory of the magnetic effect decays much faster compared with those for colloidal solutions. If the above hypothetical mechanism is accepted, the reason for these differences in the time dependence of the magnetic effects may be explained as follows. Since the alkyl chain is flexible, ordering of the structure of water molecules around the chain is prevented and the destruction of the ordered structure is accelerated by the thermal motion of the chain. However, this is not the case for colloidal particles. Our hypothesis that the structure of the water molecules around the hydrophobic chain of the probes and the surface of the colloids is more or less ordered by the magnetic exposure is consistent with most of the results we have reported so far.
4. Conclusions (1) The emission intensity of fluorescent probes with an alkyl chain increases with magnetic exposure, and the maximum constant value is obtained after some exposure time. However, no magnetic effect was observed for probes without an alkyl chain. (2) Magnetic effects on fluorescent probe solutions decrease with the alcohol concentration of the aqueous solution, the solution temperature and
173
the standing time of the solution after the magnetic exposure. These results are considered to be closely related to the motion of the alkyl chain of the probe and the water molecules surrounding it. (3) If we assume that the magnetic exposure makes the structure of water molecules around the hydrophobic chain of probes more or less ordered, not only the results of the present study but also those of the preceding experiments concerning magnetic effects on colloidal solutions are explained consistently.
Acknowledgment
The authors are grateful for financial support by the Magnetic Health Science Foundation.
References [II
S.S. Dushkin and V.N. Ievstratov, in Magnetic Water Treatment in Chemical Undertaking, Khmiya, Moscow, 1986. [II K. Yamamoto, S. Sugimoto, T. Kimura, R. Akiyama and R. Kobayashi, J. Jpn. Goetherm. Energy Assoc., 25 (1988) 31. and H. Yamamoto, J. Toyota Coil. c31 K. Nakashima Technol.. 20 (1987) 67. Nippon Kagaku Kaishi, c41 A. Chiba and T. Ogawa, 357 (1988). and M. Takatsuji, Solid State Phys., 17 CSI T. Kaneo (1982) 530. IIs1 I.J. Lin and J. Yotvat, J. Magn. Magn. Mater., 83 (1990) 525. R. Colale and G. Paiaro, Chim. Ind., 69 171 L. Pandolof, (1987) 88. Vatten, 35 (1979) 309. [81 F.T. Ellingsen and H. Kristiansen, K. Okuhara and S. Hatade, J. Colloid [91 K. Higashitani, Interface Sci., 152 (1992) 125. A. Kage, S. Katamura, K. Imai and [lOI K. Higashitani, S. Hatade, J. Colloid Interface Sci., 156 (1993) 90. H. Iseri, K. Okuhara, A. Kage and [Ill K. Higashitani, S. Hatade, J. Colloid Interface Sci., 172 (1995) 383. Cl21 J.M. Barnothy, M.F. Barnothy and I. Boszormeny-Nagy, Nature, 177 (1956) 577. and S. Ueno, in Ziba no Seitai [I31 T. Shiga, M. Miyamoto eno Eikyo, Teraspaia, Tokyo, 1991. [I41 H. Iseri, M.Sc. Thesis, Kyushu Institute of Technology, 1994.
Colloids and Surfaces and Engineering Aspects
COLLOIDS AND SURFACES
A
109 (1996) 167-173
Effects of magnetic field on water investigated with fluorescent probes Ko Higashitani *, Jun Oshitani, Norio Ohmura Department ofchemical
Engineering, Kyoto University, Yoshida, Sakyo, Kyoto 606-01, Japan
Received 1 September 1995; accepted 29 September 1995
Abstract Aqueous solutions with dissolved fluorescent probes were exposed to a magnetic field, and the effects of the magnetic exposure were investigated by measuring the fluorescence emission intensity with a spectrofluorophotometer. The data were compared with those of magnetic exposure effects on colloidal solutions reported previously [K. Higashitani, K. Okuhara and S. Hatade, J. Colloid Interface Sci., 152 (1992) 125; K. Higashitani, H. Isari, K. Okuhara, A. Kage and S. Hatade, J. Colloid Interface Sci., 172 (1995) 3831. It is found that (1) the degree of magnetic effects increases with the magnetic exposure and becomes constant at a certain exposure time in the case of solutions of fluorescent probes with an alkyl chain, but no effect was observed for solutions of probes without an alkyl chain; (2) the magnetic effects decay with the concentration of alcohol in the solution, the solution temperature, and the standing time of the solution after the magnetic exposure; and (3) if it is presumed that an ordered structure of water molecules around the hydrophobic chain of probes is formed by the magnetic exposure, not only are the results in the present study but also are our previous results for colloidal particles explained consistently. Keywords:
Aqueous solutions; Fluorescence; Fluorescent probes; Magnetic field effects; Memory effect; Water structure
1. Introduction
All the materials on earth, including living organisms, have been exposed to the geomagnetic field and the occasions on which we are exposed to an anthropogenic magnetic field, such as magnetic resonance imaging, are increasing. Hence it is fundamentally important to investigate the effects of magnetic exposure on materials. The effects of a magnetic field on aqueous solutions [l-11] and biomaterials [12,13] have been investigated in various fields, but these effects have been mysterious in the sense that the effects arise from the exposure of materials to a magnetic field * Corresponding 0927-7757/96/$15.00
author.
0 1996 Elsevier Science B.V. All rights reserved
SSDZ 0927-7757(95)03483-S
of low flux density and they remain even after the magnetic exposure is completed. These phenomena are classified into the following categories: (1) phenomena for magnetic materials which are understandable using current electromagnetic theory; and (2) phenomena for non-magnetic materials which are not understandable by current electromagnetics. We are interested only in the latter. Recently Higashitani et al. reported quantitative data concerning the effects of magnetic exposure on the rapid coagulation rate of polystyrene latices (PSL) in solutions [9], data relating to magnetic exposure effects on the formation of CaCO, crystals from CaCl, and Na,CO, solutions [lo], and data concerning magnetic exposure effects on the zeta potential and diffusivity of PSL in electrolyte solutions as well as a possible mechanism to
168
K. Higashitani
et al. JCoNoids Surfaces A: Physicochem.
explain the data [ll]. It is now certain that magnetic exposure effects on colloidal solutions do exist even if the colloids are non-magnetic and the magnetic field is of low flux density, and the effects have been considered to be related to a conformational change of water molecules, ions or hydrated ions adsorbed onto the particle surface. The data reported so far are rather macroscopic in character. In the present study, we employed fluorescent probes to obtain information on the molecular level. The fluorescent probes were dissolved in water and the change of fluorescence emission intensity caused by the magnetic exposure was evaluated under various experimental conditions. The results were compared with those relating to magnetic exposure effects on the zeta potential and coagulation rate constant obtained in preceding experiments [ 9,111.
2. Experimental The data on the fluorescent probes employed in this study are summarized in Table 1. Two kinds of probes were used. The probes with fluorescent benzene nuclei and a non-fluorescent alkyl chain are illustrated in the left-hand column, and the probes with the same fluorescent benzene nuclei only are in the right-hand column, although a short alkyl chain exists in the case of 1-pyrenebutanoic acid (PB). Pure water with a relative resistance of 17.3 MW R cm, which was purified by distillation and reverse osmosis, was used. Aqueous solutions were prepared by adding water to 1.0 mg of the fluorescent probes in large-size glassware and sonieating the solution with an ultrasonic oscillator until the probes appeared to have dissolved completely. Since some probes were not very soluble in water, the probe concentration, C,, was determined as follows. After evaporating water from the sample solution, the residue on the glassware was dissolved using a good solvent (ethanol) and the fluorescence emission intensity of the solution was compared with that of reference solution. When the effect of alcohol on the magnetic exposure effects was examined, water-ethanol mixtures of various concentrations were employed. The solutions prepared were kept in a temperature-
Eng. Aspects 109 (1996) 167-I 73
controlled bath at 25 f 0.1 “C and all the measurements were carried out at this temperature unless otherwise specified. The wavelengths for the maximum excitation and emission intensities were determined by use of a spectrophotofluorometer (Shimadzu RF-540), and all the measurements were then carried out using these wavelengths. A uniform magnetic field was generated by a pair of permanent magnets installed in a rectangular iron yoke. The distance between the magnets was 2.6 cm and the magnetic flux density was 0.42 T. The experimental procedure is shown schematically in Fig. 1. Glass tubes containing 4.0 cm3 samples of the fluorescent solution were placed between a pair of magnets and exposed to the magnetic field for a given period. These samples are termed “magnetized” henceforth. The fluorescence emission intensity was measured using the spectrofluorophotometer immediately after the magnetic exposure had been completed. However, when the duration of the magnetic effect after exposure was examined, the samples were left standing for a given period in a temperaturecontrolled bath before measuring the fluorescence intensity. The same experiments, but without the magnetic exposure, were also carried out as a reference. The magnetic effect was evaluated through the ratio of the fluorescence emission intensity for a magnetized solution against that of the non-magnetized solution, Z,/I. Measurements of I, and I were repeated about 20 times for each sample, and the magnitude of I,/1 was evaluated using their averaged values. The deviation from unity indicates the degree of the magnetic effect. Fluorescent Solutions
I
I
Magnetic Exposure Flux Density : 0.42T ExposureTie : O-45min.
Standing Period Period : 0-lOhours I
I
J( L Fig. 1. Schematic procedure.
JI
Measurement of Fluorescent Emission Intensity drawing
of
the
experimental
exposure
K. Higashitani et al./CoNoids
Surfaces A: Physicochem.
Eng. Aspects 109 (1996) 167-I 73
Table 1 Florescent probes with and without an alkyl chain 2-9anduoyloxy
stearic acid (AS) HO
CH3(CH2)15-&&OH
3.6 bis dimetbylamino-lo-dodecyl acridinium bromide (DADAB)
4-heptadecyl umbelliferone (HUF)
3,6 bis dimethylsminoaaidine (DAA)
Umbelliferone (UF)
Homo ClMU
S-N-octadecanoylsmino fluorescein (OAFL)
Fluomscein (FL)
, T cOoH
HO /
NHCO(CH+CH3
12-1-pyrene dodezanoie acid (PD)
I”/‘,‘” ’:
1-pyrene butsnoic acid (PB)
169
K Higashitani et ai./CoNoids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 167-l 73
170
All the experiments room.
were carried out in a dark
3. Results and discussion 3.1. Magnetic eflects on solutions of 2,9-anthroyloxystearic conditions
acid under various
Fig. 2 shows the dependence of I,,$ on the exposure time t, for 2,9-anthroyloxystearic acid (AS). It is clear that the magnitude of I,/1 increases with increasing t, after an induction time and it becomes about 1.08 at t, > 30 min. It was found in the preceding experiments on colloidal solutions that the reduced zeta potential and the reduced coagulation rate decrease with increasing t, and become constant at t, > 10 min, and that the maximum reduction of the zeta potential and coagulation rate was about 8% and 15%, respectively [ 111. These results are very similar to those given in Fig. 2, except that a longer exposure time is needed for ZJI to be constant. It is important to know whether the change in the fluorescence emission intensity was caused either by the direct reaction of the fluorescent probes to the magnetic wave or by the change in the surrounding environment, to which the probes
0.95’ 0
’ 10
’
’
20
I
30
’
’
40
are usually very sensitive. If the magnetic effect is attributable to the former mechanism, the magnitude of I,/1 is expected to be finite even if the properties of the medium are changed. Fig. 3 shows the dependence of the I,/I value at t, = 30 min on the volume fraction C, of ethanol in water. It is clear that the value of Z,/I decreases with increasing C, and becomes zero at C, > 0.1. Since the fluorescence intensity increases with the ethanol concentration, this reduction of I,/I with t, does not necessarily indicate that the probes do not react directly to the magnetic wave. However, it is found that this dependence of Z,/I on the alcohol concentration is very similar to that of the zeta potential of colloidal particles [ 141. Hence it is plausible to assume that the magnetic effect is attributable to the change of the configuration of water molecules surrounding the fluorescent probes. Fig. 4 shows the dependence of the I,/I value on the solution temperature T. It is clear that the magnetic effect disappears at T> 30°C. A similar temperature dependence of the magnetic effect was found also for the zeta potential [ 141. This coincidence between the different experiments implies that the magnetic effects are closely related to the thermal properties of the water molecules, and the effects are destroyed by the motion of the water
’
50
CaL-1
te [min] Fig. 2. Dependence 10e7 kmol mm3.)
of
I,/I
on
t. for
AS.
(C, =2.83
x
Fig. 3. Dependence of I,/1 lo-’ kmol rnm3; t,=30 min.)
on
C,
for
AS.
(C,=2.83
x
K Higashitani et al./Colloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 167-l 73
171
memory lasted for about 5 h in this case, but for more than 6 days in the case of the zeta potential and coagulation rate constant of colloidal particles [ 111. The reasons for its existence are mysterious. However, supposing that the water molecules are in a quasistable state, as mentioned above, the memory may exist until it is destroyed by an external disturbance. 1,oo
. . .._..____......___.................................
.
t
0
Fig. 4. Dependence me3; t,= 30 min.)
10
of 1,/I
20
30
T
[“Cl
40
1 50
on T for AS. (C, = 2.83 x lo-’ kmol
molecules. Hence it seems that water molecules in the magnetized solution are in a quasistable state. In the above experiments, the fluorescence emission intensity was measured immediately after the magnetic exposure was completed. Here, the solutions were left standing for a given period t, after the exposure. Fig. 5 shows the dependence of I,,lI on t,. The value of 1,/r gradually decreases with time. Since the magnetic effect remains after the magnetic exposure, we may say that there exists a “memory” with respect to the magnetic effects. The
3.2. EfSects of the alkyl chain of thefluorescent probe
In this section, the role of the alkyl chain of the fluorescent probe was investigated using the four pairs of probes illustrated in Table 1. Each pair has the same or similar fluorescent benzene nuclei, but a different side-chain. Fig. 6 shows the dependence of 1,/Z on t, for the DADAB and DAA probes. It is clear that no magnetic effect is observed for DAA, while the value of I,/I for DADAB increases with t, and becomes 1.15 at t, 825 min. Similar correlations of 1,/Z vs. t, were found between another two pairs of probes, as shown in Figs. 7 and 8. These results can be summarized as follows. (1) Magnetic effects were observable only for probes with a long alkyl chain. (2) The I,,,/1 value increases with the magnetic
1.3
,95 0.95’ 0
’ 2
I 4
. ts
Fig. 5. Dependence mm3; t,=30min.)
of I,/!
on
’ 6
’ 8
I . 10
’ 12
0
10
20
30
te [min]
[hour]
t, for AS. (C, =2.83 x lo-’ kmol
Fig. 6. Dependence (DADAB: C,=4.80 lo-* kmol mm3.)
of I,/1 on t, for DADAB x 10e9 kmol me3; DAA:
and DAA. C, =2.28 x
K. Higashitani et al./Colloids Surfaces A: Physicochem. Eng. Aspects 109 ( 19961 167-I 73 1.10
i_I.0 PD PB
0.95
’ 0
a
I
10
20
’
’
’
’
30
40
50
0.95 0
10
te [min]
20 te
Fig. 7. Dependence of I,/1 on t, for HUF and UF. (HUF: C,=9.55 x 10m9kmol rne3; UF: C,= 1.02 x lo-* kmol mm3.)
30
40
50
[min]
Fig. 9. Dependence of I,/1 on t, for PD and PB. (PD: C,= 1.08 x 10e7 kmol m-3; PB: C,=4.65 x lo-* kmol me3.)
be more or less proportional to the length of the alkyl chain of the probes. The reason why the I,/I value for the PD probe becomes constant at the shorter exposure time, compared with the other probes, is not known at present. 3.3. Possible mechanism
0.95 -
0
10
20
30
40
50
te [min] Fig. 8. Dependence of 1,/I on t, for OAFL and FL. (OAFL: C,=7.50 x lo-@ kmol mm3; FL: C,=1.88 x lOma kmol mm3.)
exposure and becomes constant after a sufficiently long exposure time. (3) A longer exposure time is needed for the magnetic effect to become constant probe solutions than colloidal solutions [ 111. Fig. 9 shows a comparison between the relationship 1,/I vs. t, for the PD probe and that for the PB probe. Both probes have an alkyl chain but the length is different. It is worth noting that the Z,/Z value for the PB probe is slightly larger than zero, and so the degree of the magnetic effect may
Although the above results give us information at the molecular level, they are still not enough to clarify the detailed mechanism of the magnetic effects. Nevertheless, it is important to discuss the possible mechanisms. A comparison of the present results with those relating to the magnetic effects on the colloidal solutions [9,11] is also important in order to confirm their reliability and to estimate the mechanism. It is clear that a long alkyl chain of the probes plays a vital role for the magnetic exposure effects. Since the magnetic field does not act directly on the fluorescent probes, and probes without a long alkyl chain exhibit no magnetic effect, it is plausible to assume that the magnetic effects are caused by the change of the orientation of the water molecules around the alkyl chain. It is known that water molecules around hydrophobic molecules are ordered so that they can participate in hydrogen bond formation, as in bulk water. Suppose that the ordered structure of the water molecules sur-
K. Higashitani et al.lColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 167-173
rounding the hydrophobic chain is altered by the magnetic exposure such that the alkyl chain is distorted, then the fluorescence emission intensity may vary because the distortion influences the fluorescent benzene nuclei. In the preceding paper, the following mechanism was proposed. The magnetic exposure alters the conformation of the water molecules, ions and hydrated ions adsorbed on the particle surface in such a way that the effective adsorbed layer becomes thicker and ordered. This mechanism is consistent with the hypothesis concerning probe solutions described above. As for probes with a long alkyl chain, the value of I,/I increases with t, and becomes constant. This behaviour is very similar to that of the zeta potential and coagulation rate, but the exposure time to obtain the maximum magnetic effect is much longer and the memory of the magnetic effect decays much faster compared with those for colloidal solutions. If the above hypothetical mechanism is accepted, the reason for these differences in the time dependence of the magnetic effects may be explained as follows. Since the alkyl chain is flexible, ordering of the structure of water molecules around the chain is prevented and the destruction of the ordered structure is accelerated by the thermal motion of the chain. However, this is not the case for colloidal particles. Our hypothesis that the structure of the water molecules around the hydrophobic chain of the probes and the surface of the colloids is more or less ordered by the magnetic exposure is consistent with most of the results we have reported so far.
4. Conclusions (1) The emission intensity of fluorescent probes with an alkyl chain increases with magnetic exposure, and the maximum constant value is obtained after some exposure time. However, no magnetic effect was observed for probes without an alkyl chain. (2) Magnetic effects on fluorescent probe solutions decrease with the alcohol concentration of the aqueous solution, the solution temperature and
173
the standing time of the solution after the magnetic exposure. These results are considered to be closely related to the motion of the alkyl chain of the probe and the water molecules surrounding it. (3) If we assume that the magnetic exposure makes the structure of water molecules around the hydrophobic chain of probes more or less ordered, not only the results of the present study but also those of the preceding experiments concerning magnetic effects on colloidal solutions are explained consistently.
Acknowledgment
The authors are grateful for financial support by the Magnetic Health Science Foundation.
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