Received on  Dec. 29, 2003; Support by the National Natural Science Foundation of China (No.20276002)

Abstract Fluorescence whitening agents (FWAs) tender yellowing or gloomy cellulose whiter image, because FWAs can absorb UV and emit blue light, therefore can compensate the defection of blue light of fiber from elderly soil or cellulose's oxidation, and make fiber reflect the complex visible light, even make fiber reflect more light than the original.  Distyrylbiphenyl and ditriazinestyryl are the widely used kinds of FWAs in detergent and paper industry. It was surprisingly found that when into the aqueous solution of the products were added polypyrrolidone(PVP), diethyltriaminopentoacetic acid(DTPA), or phthalocyanine sulfonic acid (PCs), an enhancement of 0.2-12 times of fluorescence produced. To VBL there is a remarkable red shift in excitation and emission; for emission spectrum there were double peaks at the same level. Also CBS-PVP system and VBL-PVP system gained strong time-enduring after preparation of the solution. It is deduced that it was intra-molecular force, namely molecular absorption between FWAs and the additives so that produced electron flow from PVP, DTPA, and PCs to the FWAs molecules, that caused the change of performance and red shift in spectrum of the FWAs. According to calculation of the inorganic(I) and organic property(O), it was deduced that on the base of similarity of the ratio of I/O, linear polymer, or the compounds with large molecular weight containing nitrogen atoms were favorable to the enhancement of fluorescence and red shift.
Keywords fluorescence whitening agents, detergents, the ratio of inorganic to organic property, N－containing compounds， intramolecule force

Fluorescence whitening agents (FWAs) are compounds, which is a kind of widely used class of colorless dyes. They absorb solar radiation in the near ultraviolet region and fluorescence in the blue, which compensates deficiency of blue light for yellowing or gloomy fiber. For many materials manufactured from natural textile fibers, any undesirable yellow coloring came from oxidation of cellulose of fiber or old soil, can be counteracted effectively by these fluorescent dyes to enhance their white appearance [1-2].
    In detergents and pulp industry the FWAs often used are stilbene-based compounds ^[3,4,5]: 1. FWA CBS, 4，4'-bis（2-disulfostyryl）biphenyl.

2. VBL, or CXT-100, CXT-200, their chemical name are the derivatives of ditriazine styryl. CXT has a chlorine atom on the group of aniline of the triazine.

    Many researches on enhancing performance of FWAs have been reported these years ^[6,7,8]. Generally, the work is mainly to change the structure of FWAs. In fact, distyryl-biphenyl can give much more fluorescence strength than the type of styryl triazine. The latter still has its great market because of the cheap cost. Much more modification has been done for the two kinds, such as adding halogen to phenyl group, or changing the substitute to electron donating or electron-withdrawing groups to keep the molecule suitable to be used in some field. Tendering FWA other function is another trend ^[9,10].
    Apart from the structure of molecules, there are some other factors to influence fluorescence, such as the polarity of solvents, pH value, and etc ^[11]. Solvent effect gave a great influence to the performance of fluorescence ^[12]. In this paper it is described the effectiveness of molecular absorption to the performance of FWAs. The results of experiments tell that in aqueous solution there was some intramolecular force to influence fluorescence performance.
1 APPARATUS AND MATERIALS
All the FWAs were got from Beijing Shunyi Fine Chemical factory; the other materials were got from the local markets with chemical grade. Fluorescence determination was done with F-4010 Fluorescence Spectrophotometer (HITACHI).

2 EXPERIMENTAL
2.1 Preparation of FWA-additives system
The FWA solutions were made by dissolving them into distilled water and adjusted to pH9 with KOH. All the additives were dissolved in distilled water to get certain solution prior to be adjusted to pH 9 with KOH.
2.2 Determination of fluorescence
A scanning slit width of 4 mm and scanning speed of 20 mm/min were used.

3 RESULTS AND DISCUSSION
3.1 CBS-PVP system
3.1.1 Influence of concentration of PVP to CBS-PVP system
Figure 1 was from different samples 1# to 10# with the same concentration of CBS but with random beginning time. It was found from Figure 1 that PVP had remarkable effect on CBS. Whatever the beginning value of samples were tested, when the samples were added same amount of PVP there were always a number of points on the lines after which the line became smooth.

Fig.1 Effect of concentration of PVP to CBS-PVP system

3.1.2 Fluorescence duration of CBS-PVP system
After the solutions of CBS-PVP were prepared they were stored in the dark place before determination. The amount of CBS were 2mL in all samples: from 1# to 10#, PVP was 0, 0.5. 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5mL, respectively)
    For CBS solution, with time increasing after preparation, the fluorescence declined greatly, even to 77% of the original value. While with the presence of PVP together the decline of fluorescence of solution can be avoided greatly. When PVP reached a certain concentration, the fluorescence remained at more than 90%. It was obvious that PVP had an effect of stabilizing FWA CBS solution time duration. Because after PVP reached a certain amount the relative fluorescence strength did not increase any more there were several lines that were remained at the similar levels.

Fig.2 Declining of fluorescence of CBS-PVP with time

3.1.3 Influence of temperature to CBS-PVP system
FWA CBS solutions were divided into 2 parts, one of which was placed at ambient temperature under dark, the other part was placed at low temperature. Table 1 listed the results.

Table 1 Effect of temperature to CBS-PVP system

    The fluorescence strength in 1^st column in Table 1 was determined immediately. The fluorescence strength increased with the addition of PVP. When the samples were placed under room temperature in the dark, the relative one without PVP declined to 68%, but the one with PVP and placed at low temperature had the relative fluorescence strengh increased to 103%.
3.1.4 Fluorescence spectrum of CBS-PVP system
The fluorescence spectrum of CBS-PVP did not get big change under the existence of PVP.
3.2 VBL-PVP system
3.2.1 Concentration of PVP to VBL-PVP system

It was obvious from Table2 that PVP tendered a great enhancement to the performance of VBL solution.

3.2.2 Time effect to VBL-PVP system
0.04% of VBL-system solution were prepared and put under dark. The fluorescence with increasing of PVP showed a bigger stability with time. After 48hrs the solution without PVP kept as less as 55% of original value, but for that with more than 2mL of PVP, the value kept to over 95%.

Table 3. Change of fluorescence of VBL-PVP system (28ºC)

3.2.3 The influence of temperature to VBL-PVP system
VBL-PVP solutions were divided into 2 parts. One part was placed under dark, the other in refrigerator. After a period their fluorescence values were compared with each other.

Table 4 Temperature to VBL-PVP system

    From Table 4 it was found that under lower temperature the fluorescence of FWAs could be kept. Without PVP at 28ºC after 77hrs of preparation of solution the value declined to 56.5%; at 8ºC the value kept to 69.7%. The presence of PVP will prevent the fluorescence of the VBL solution from declining with the storage time. Under low temperature it appeared a greater ability to keep the fluorescence. With 2mL of PVP the fluorescence of VBL-PVP at 28ºC remained in 97.9%, at 8ºC with 2.0mL of PVP the value remained in 101.7%,and with 2.5mL of PVP the value at 8ºC remained in 111.5% respectively.
3.2.4 The fluorescence spectrum of VBL-PVP system
There is great change of VBL-PVP system in spectrum. The maximum of excitation from 330nm increase to 349nm; the emission spectrum appeared double peaks, the one was at 415nm, the second was at 435nm, and the two peaks kept at the same level.
3.3 The effects of other materials
3.3.1. CBS-urea and VBL-urea system
Table 5 is the data of CBS-urea and VBL-urea system. It was saying that urea had no effect on their fluorescence. CBS and VBL were at 0.01% and 0.04%, respectively. Urea was at 0.2%. The pH value kept at 9.

Table 5 the effect of urea to CBS(C) and VBL(V)

3.3.2 CBS-DTPA and VBL-DTPA systems
From Table 6 it seemed that DTPA to CBS had an enhancement effect of fluorescence, but less effect to VBL.

Table 6 Data of CBS(C)-DTPA and VBL(V)-DTPA

3.3.3 CBS-PCs and VBL-PCs systems
It was obvious that PCs to CBS and VBL had great effect on the enhancement of relative fluorescence and the effect to the latter was much greater.

Table 7 Data of CBS(C)-PCs and VBL(V)-PCs systems

3.4 Deductions of the mechanism of enhancement of fluorescence and change of spectrum of FWAs
The experiments showed that there were 3 factors to affect the characteristics of FWAs: the ratio of inorganic to organic property; the situation of with nitrogen atoms; and the size of molecules, linear polymers or large molecule.
    The ratios of inorganic to organic property for the additives were calculated according to the theory of solvents in Table 8.
    The theory of solvents is based on the melting points of compounds. It is not like thermodynamics that is the theory for macroscopical world; neither like quantum theory that is for microcosmic world. It comes from the statistics for the melting points of compounds. The typical values are for carbon atom and hydroxide. The organic value of every carbon atom equals to 20; the inorganic value of every hydroxyl group equals to 100. Every compound has its position on the ordinate, which has horizontal axis for organic value, and has ordinate for inorganic value. From the original point of the coordinate a line can be run to a certain point of some compound. That line is named the same-ratio line (the ratio of inorganic to organic value) on which the compounds or solvents generally have good solubility with each other. Surrounding the same-ratio line there are continuously soluble area, sub-soluble area, insoluble area.

Table 8 the position of FWAs and the additives on coordinate of solvents

    The experiments showed that PVP had a remarkable enhancement effect to the fluorescence of CBS and VBL, CXT. Next were PCs and DTPA.
    On the Figure of solvents , PVP occupies a long section of same-ratio line, because it is a polymer, and its ratio is near CBS and VBL. DTPA and PCs are all near CBS or VBL in different degree. VBL, PVP and PCs have more Nitrogen atoms in their molecules PVP is a kind of water-soluble macro polymer with weak property of cationic surfactants. Its molecules comprise a number of pyrrolidone units ^[1,13]. On the other hand the whole molecule of PVP has a great elasticity, and remarkable freedom of rotation within the chain. The pyrrolidone units are like leaves, which are even free to rotate to paste on the surface of FWAs in aqueous solution. PVP molecules are full of electrons on Nitrogen atoms. Therefore, it can be expected that the free electrons could flow to the molecule of FWA. At the same time the electron on nitrogen atoms in VBL molecules can flow to PVP molecules. This may lead to form new kinds of complexes of CBS-PVP and VBL-PVP came from charge-transfer. This could make VBL molecule's conjugation field change to a great extent, and is the cause to produce double peaks in emission spectrum and a big shift in maximum emission. This led up to produce very great enhancement of performance.
   The experiments showed that the largest equilibrium constants are 18.6:1 for the single unit of PVP to CBS and 24.3:1 for the single unit of PVP to VBL, 24.9:1 to CXT. PVP was like a rope or a net that tie up the molecules of FWAs, which make the complexes are stable to produce the positive fluorescence phenomena.
    Some N-containing compounds and the others were tested. With imidazole, p-methylbenzen sulfonate, caprolactam, dimethylformamide, hydroquinone, phthalic anhydride, salicylic acid, diethyl oxalate, diethyltriaminopentacetic acid (DTPA), urea, phthalocyanine sulfonic acid(PCs), only PVP, DTPA, and PCS gave the positive effect. PVP was the best. Others gave different effects; even some of them gave negative results.
    Urea and some compounds,such as caprolactam containing nitrogen provided negative results. Maybe they could form complex with FWA at the beginning. but unfortunately the complexes were not stable enough like that of PVP-FWAs to inhibit rotation of molecules to produce stable new performance and spectrum. pyrrole is a very weak base(Kb=2.5X10^-14); caprolactam, N,N-dimethylformamide are all amide, also do not have enough electron to flow to others. On the graph of solvents, some compounds have similar ratio of I/O to that of FWAs, but they are set far from the position of FWAs. PVP is a long chain molecule with a member of pyrrolidone units which could rotate freely to be comfortable, so the pyrollidone units could put themselves on the surface of FWA molecules plainly and extended. PCs are the large molecules containing 8 nitrogen atoms and with a large conjugated plain ^[14], that made the molecules absorb FWA molecules strongly. DTPA is a compound with not long a chain and with three groups of amino and five acetic groups in its molecules, which also had an advantage to absorb FWAs molecules. Among the additives tested only PVP, PCs, DTPA are near the FWAs in the ration of organic to inorganic number. The phenomena could be explained by forming high grade structural-ordered aggregate and self-organization in solution^[15,16].
    Molecules, which are able to rotate, bend or twists have a tendency to lose excited state energy through molecular collision and other vibration processes. Any increase in limiting movement will impose a barrier to rotation about the ethylenic double bound. So the factor that limits the movement of FWAs could have positive effect for fluorescence, and leads to an increasing fluorescence yield and lifetime. Decreasing solvent polarity or increasing viscosity leads to an increasing in fluorescence yield and lifetime. Forming of non-radioactivity, cis-isomer, or twisted configuration is all-unfavorable to fluorescence.
    In the experiments the solvent polarity and viscosity were not responsible for the spectrum change and fluorescence enhancing, because the additives added were so less, as not enough to influence polarity and viscosity of aqueous solution.
    Water sometimes has a quenching effect on fluorescence through hydrogen bonding. Maybe these additives surrounded the FWAs decreased the quenching of water to some extent.

REFERENCES
[1] Liu Yun. Detergent--principle, materials, processing, composition. Beijing Chemical Industry Press, China, 1998, 50-126.
[2] Zhang Xiaowen, Liu Yun. J. Beijing Institute of Light Industry, 1998, 16 (2): 46-54.
[3] Axel Bader, Leverkusen; Dieter Arilt,Cologne; Florin Seng, US. 5177255 (1993)
[4] Kurt Weber, Basel, Claude Eckhardt. US 5145991 (1992)
[5] Bu Ping. Detergent & cosmetics, 1996, 2: 43-462.
[6] Dong Zhongsheng. Dye Industry, 2000,2: 27-30.
[7] Xu Hua. Fine and Special Chemists, 1999, 18: 16-17.
[8] Wu Tianyuan. Builder for Pint and Dye，1997, 14 (2): 35-37.
[9] Liu Yun, Guo Guangxun, Zhang Jun. J. Surfactants & Detergents, 2001, 4: 151-154.
[10] Liu Yun.1996 international conference on surctants/detergents,Sep.1996, Nanjing, China
[11] Chen Guozhen. Fluorescence Analysis. Science press, China，1990: 75-83.
[12] Liu Yun, Zhang Weicheng. Chromatographia, 1998, 48(7/8): 548-554.
[13] Zhen Huozhu, M J Rosen. Surfactant Industry, 1990, 2: 24-27.
[14] Liu Yun, Zhangjun, Sun YuE. 91 AOCS annual conference, San Diego, CA, U S A, 2000.
[15] Hirotaka Hirata，Nahoko Iimura. Journal of Colloid and Interface Science，2003，268 (1): 215-220.
[16] Asim Kumar Ray, Sumanta Bhattacharya, Manas Banerjee et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2002) :1375-1378.

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