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Spreading and adhesion of ASA on hydrophilic and hydrophobic SiO2
Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 217–224
Spreading and adhesion of ASA on hydrophilic and hydrophobic SiO2 J. Lindfors ∗ , S. Ylisuvanto, T. Kallio, J. Laine, P. Stenius Helsinki University of Technology, Laboratory of Forest Products Chemistry, P.O. Box 6300, FIN-02015 TKK, Finland Received 12 October 2004; accepted 22 January 2005
Abstract The spreading kinetics and adhesion of liquid alkenyl succinic anhydride (ASA) on hydrophilic and hydrophobic SiO2 surfaces was studied. Contact angles were measured both in air and in aqueous environment. ASA spread fairly well on both surfaces in air, but when immersed in aqueous solution it spread only on the hydrophobic surface. The contact angle in water on the hydrophilic surface was ≈170◦ . The work of adhesion of ASA to both surfaces in air was about 60 mJ/m2 . When immersed in water the work of adhesion of ASA on the hydrophobic surface was 22 mJ/m2 , and less than 1 mJ/m2 on the hydrophilic surface. In the pH range used in papermaking there was no significance of pH on spreading of ASA, but at high pH effects on the surface tensions due to hydrolysis of ASA were observed. Ca2+ -ions had no effect on the spreading of ASA on the hydrophilic surface but strongly influenced spreading on the hydrophobic surface. ASA is extensively used as a hydrophobing agent and tends to form detrimental deposits on equipment used for manufacture of paper. From the results it can be concluded that best way to prevent fouling on paper machines by hydrophobic substances is to maintain the process surfaces wetted by water and to choose surface materials that stay hydrophilic as consistently as possible. © 2005 Elsevier B.V. All rights reserved. Keywords: Alkenyl succinic anhydride; Paper machine; Adhesion; Contamination; Hydrophobing agent
1. Introduction Alkenyl succinic anhydrides (ASA) emulsions are widely used as hydrophobing agents in alkaline papermaking [1]. The basic structure of ASA is shown in Fig. 1(A). The composition of the emulsion and the detailed structure of the ASA vary (e.g. emulsifiers used, olefin chain length and branching), but the main function of the emulsions, i.e., conferring hydrophobicity to papers by deposition of ASA on the fibres, always remains [2]. A persistent problem associated with the use of ASA is the contamination of process equipment by formation of hydrophobic deposits [3,4]. A reasonable assumption is that this contamination for the most part occurs because of hydrophobic interactions, such as recently discussed by, for example, Yamaguchi et al. [5]. The objective of this study was to obtain further insight into this contamination problem by investigation of the spreading of ASA on different surfaces. Dynamic contact ∗
Corresponding author. Tel.: +358 9 451 5657; fax: +358 9 451 4259. E-mail address: [email protected] (J. Lindfors).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.01.028
angle measurements were used for the determination of the spreading kinetics and adhesion of ASA on selected model surfaces in air and in water. The type of measurements in a liquid cell used by us, have been described earlier by Svitova et al. [6]. The wetting kinetics in solid–liquid–liquid system was studied, for example, by Bartell and Zuidema [7] and by Schultz et al. [8]. Determination of the work of adhesion to silica using contact angle measurements was recently discussed by Della Bona et al. [9]. The effect of pH and Ca2+ -ion content was studied by measurements in aqueous environment. pH is a major factor influencing the hydrolysis of ASA and the dissociation of hydrolysed ASA (see Fig. 1(B) and (C)), and it can thus have a strong influence on the contamination behaviour of ASA in paper machines [10,11]. The salt formed by the ASA anion with Ca2+ is insoluble (see Fig. 1(D)) and it has been reported that Ca2+ -ion for this reason contributes significantly to fouling [11]. In this paper we describe an evaluation of the effects of the hydrophobicity/hydrophilicity of the surface material in different mediums on the spreading and adhesion of ASA.
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Fig. 1. The structure of (A) ASA, (B) hydrolysed ASA, (C) dissociated form of (B) and (D) the binding of Ca2+ to C.
The results make it possible to draw conclusions about the contamination behaviour of ASA on different surfaces and its dependence on process conditions.
2. Materials and methods 2.1. Contact angles and surface tensions A CAM 200 contact angle goniometer (KSV Instruments Ltd, Helsinki, Finland) was used for determination of contact angles and surface tensions. The arrangement of the instrumentation when investigating liquid–liquid–solid systems is shown in Fig. 2. The size of the ASA drops was approximately 10 l in air and 30 l in water. Surface tensions and contact angles were calculated by axisymmetric drop shape analysis (ADSA) using software delivered with the instrument. This analysis is based on fitting the full equations for drop profiles for pendant drops (surface tension) and sessile drops (contact angle) derived from the Young–Laplace equation. The fitting method is based on the principle described by Jennings and Pallas [12].
2.2. Atomic force microscopy, AFM AFM measurements were performed using a NanoScope IIIa Multimode scanning probe microscope (Digital Instruments, Inc., Santa Barbara). The images were scanned in tapping mode in air using commercial Si cantilevers (Digital Instruments) with a resonance frequency of about 300 kHz. The only image processing used was flattening. Four images were recorded for each surface and representative micrographs were chosen for presentation. The roughness of the surfaces was evaluated by determination of the roughness factor, r, from the 3D-AFM-images: r =1+
Sdr 100
(1)
where Sdr is the ratio between the real interfacial area and the projected area. 2.3. Time-of-flight secondary ion mass spectrometry, ToF-SIMS The ToF-SIMS spectrometer PHI TRIFT II at Top Analytica Ltd., Turku, Finland was used. The instrument and experimental procedure have been described by Schueler [13]. High-mass-resolution spectra in positive secondary ion mode over the mass range 2–2000 Da were acquired using a Ga ion gun. The primary ion current was 600 pA. The acquisition time was 2 min and the time per channel 138 ps. Two areas of 100 m × 100 m on each sample were analysed. ToF-SIMS ion images were recorded from an area of 50 m × 50 m. The acquisition time of the mapping was 10–20 min. The analysis depth was <2 nm. The primary data were evaluated using the WinCadence Data Reduction software in off-line mode. 2.4. Alkenyl succinic anhydride, ASA
Fig. 2. Scheme of the measurement set-up. Note that the density of ASA is lower than the density of water, so that the substrate is located above the ASA droplet.
Pure ASA was supplied by Raisio Chemicals Oy, Raisio, Finland. According to the suppliers, the purity of the ASA was ≈99%. Its melting point was ≈5 ◦ C. The density of ASA
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surface was 100 ± 2◦ . Standard deviations are based on five measurements. The morphology of the surfaces was characterised by AFM and their chemical composition was verified by ToFSIMS analysis.
3. Results Fig. 3. Partial positive TOF-SIMS spectrum of ASA. Molecular peaks corresponding to chain lengths C16 (363) and C18 (391) are indicated.
was determined to be 0.956 kg/dm3 . The ASA was not further purified. The composition of the hydrocarbon chains was approximately 50% C16-olefin and 50% C18-olefin. No dispersants were used in the preparation of the ASA. ToF-SIMS spectra of ASA were recorded in order to verify its composition. Fig. 3 shows part of the positive ToF-SIMS spectrum of ASA, which has molecular peaks corresponding to chain lengths C16 (363) and C18 (391). The liquid ASA was used as such. 2.5. Water The water was purified UHQ water in equilibrium with air (prepared with ELGA PURELAB UHQ). Its conductivity was ≈5.6 S. 2.6. Substrates The silica surfaces were those of smooth silicon wafers (from Okmetic Oy, Helsinki, Finland) [14]. They were cleaned in the following way: The wafers were first immersed in alkaline and acidic hydrogen peroxide solutions (first 10 min at 75–85 ◦ C in alkaline, then 10 min at 75–85 ◦ C in acidic solution) [15]. Then they were rinsed with water and ethanol. The advancing contact angle of water on the SiO2 surface was 20 ± 2◦ . Hydrophobic silica surfaces were prepared by methylating the SiO2 surface; the cleaned and dried surface was immersed in 0.05% dichlorodimethylsilane in xylene for 2 h. The method is described in greater detail in [16]. The advancing contact angle of water on the methylated
3.1. Surface topography Topographical AFM images of the surfaces are presented in Fig. 4. From the images it is clear that the native silicon wafer was very smooth. The roughness of the methylated surface, as determined from the AFM images using Eq. (1), was also low. The effect of roughness on contact angles was estimated using the Wenzel equation [17]: cosθW = r cos θY
(2)
where θ W and θ Y are the contact angles measured on the rough and smooth surface, respectively. The effect of roughness was found to be negligible (maximum value obtained for r was 1.001) and was, therefore, ignored in the contact angle analyses. 3.2. ToF-SIMS of surfaces Some ToF-SIMS ion images, of both methylated and untreated silicon surface, are shown in Fig. 5. The images show that the methylation was successful and resulted in a reasonably homogenous surface layer on the silicon wafer. Organic ions containing silicon such as 43 (CH3 Si+ ), 45 (SiOH+ ), 59 (CH3 OSi+ ), 73 ((CH3 )3 Si+ ), 117 (C3 H9 OSi2+ ), 133 (C4 H13 OSi2+ ), 147 (C5 H15 OSi2+ ), 191 (C5 H15 O2 Si3+ ), 207 (C5 H15 O3 Si3+ ), 221 (C7 H21 O2 Si3+ ) and 281 (C7 H21 O4 Si4+ ) were emitted from the hydrophobic surface. Parts of the ToFSIMS spectrum corresponding to some of these peaks (43, 45, 59 and 73) are presented in Fig. 6. The spectrum showed that the intensity of these ions was much lower on the hydrophilic surface. In particular, the intensity of ((CH3 )3 Si+ ) was high on the treated surface, which proves that the surface methylation was successful.
Fig. 4. Tapping mode AFM images of hydrophilic (on the left) and hydrophobic (on the right) surfaces.
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Fig. 5. TOF-SIMS ion images of hydrophilic (A) and hydrophobic (B) surfaces. The analysis depth was <2 nm. In the images a brighter colour indicates higher ion content. The bars in the images are 10 m.
3.3. Surface and interfacial tensions The surface tension of ASA and the interfacial tensions of ASA in water and water solutions at different pH and Ca2+ ion concentrations were measured. The results are shown in Fig. 7. 3.4. Wetting dynamics The kinetics of spreading of ASA on hydrophilic and hydrophobic surfaces in water is illustrated by the results in
Fig. 8(A) and (B). Five measurements were performed on each surface. The figures show that the reproducibility of the measurements was rather good; in particular on the hydrophobic surface. The spread in the measurements on the hydrophilic surfaces was about ±5◦ . In the following the discussion will be restricted to representative curves for each case. The most critical stage in the measurements with respect the reproducibility was the attachment of the droplet to the surface. Recording of drop images was started manually, which made it difficult to exactly identify the time (t = 0 s) at which the drop attached to the surface. Therefore,
Fig. 6. Positive TOF-SIMS spectrum peaks corresponding to ions: 43, 45, 59 and 73 (A, hydrophilic surface; B, hydrophobic surface).
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Fig. 7. ASA surface tensions in different chemical environments (pH was adjusted with HCl and NaOH solutions). Standard deviation: 0.2 mN/m (five measurements).
Fig. 9. Contact angles of ASA on the surfaces in water.
there was some scattering of results at the beginning of measurements, where also the change in contact angle was the fastest. Fig. 9 shows the spreading of ASA on hydrophilic and hydrophobic surfaces immersed in water. On both surfaces, contact angle equilibrium was reached within a 1-min-period, as in all other measurements. Equilibrium angles used for calculations of work of adhesion were taken at t = 1 min. In water attainment of equilibrium was slower on the hydrophobic surface than on the hydrophilic surface, probably just because the final contact angle was much lower. As expected, ASA did not wet the hydrophilic surface (final contact angle 170 ± 4◦ ) while it did spread on but did not completely wet the hydrophobic surface (final contact angle 20 ± 2◦ ). In air (Fig. 10) spreading on the hydrophobic surface was faster and resulted in a higher final contact angle than on the hydrophilic surface. Both surfaces were wetted by the ASA (final contact angles 48 ± 2◦ and 23 ± 2◦ , respectively), but the difference between the two surfaces was not as significant as it was in water. The effect of the concentration of Ca2+ -ions on the spreading of ASA on the hydrophobic surface is shown in Fig. 11. Concentrations below 10 mM had no significant influence on
the results. Only 100 mM CaCl2 solution, where the effect was very significant, was further investigated. Spreading curves of ASA on hydrophilic and hydrophobic surfaces under different conditions (pH 3, pure water, pH 10 and 100 mM CaCl2 ) are presented in Fig. 12(A) and (B). Changes in the chemical environment did not affect the spreading on the hydrophilic surface, but on the hydrophobic surface the contact angles increased in the presence of Ca2+ -ions.
Fig. 10. Contact angles of ASA on the surfaces in air.
Fig. 8. (A) Contact angle of ASA on hydrophilic SiO2 in water. (B) Contact angle of ASA on hydrophobic SiO2 in water.
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Fig. 11. The effect of CaCl2 concentration on behaviour of spreading of ASA on hydrophobic surface.
4. Discussion 4.1. Surface and interfacial tensions The interfacial tensions are considerably lower than the surface tension of ASA. When the ASA was contacted with aqueous solutions at pH 3 and 10 only minor changes in the interfacial tension were observed after 1 min, compared to the value determined in pure water. On the other hand, when the ASA drop was kept in contact with these solutions for 5 min, the interfacial tension at pH 3 decreased by 2 mN/m and at pH 10 it decreased to approximately half of the initial value in water. The interfacial tension value in water also decreased slightly when the contact time was prolonged to 5 min. These changes in interfacial tensions very likely are due to hydrolysis reactions and dissociation of reaction products of ASA, especially at high pH. These reactions are not very fast, but a clear decrease in surface tension is observed after a couple of minutes. To get an idea about the extent of the hydrolysis reactions, the surface tension of the water in contact with an ASA drop in the measurement cell was also determined as a function of time. There were no changes in surface tension, which implies that the amount of hydrolysis products of ASA dissolved into the water was at least so low that no measurable diffusion of such molecules to the surface of the water takes place. As seen from the molecular struc-
tures of the molecules (Fig. 1(C)) they should be strongly surface active. From these results, and from the excellent reproducibility of the spreading experiments, we conclude that dissolved hydrolysis products did not significantly affect the results recorded after 1 min contact time. This, of course, does not exclude that ASA molecules located at the l/l interface are rapidly hydrolysed when contacted with the aqueous solution. The interfacial tensions measured in CaCl2 -solutions were considerably smaller than those in pure water. It was also obvious that the effect of Ca2+ on the interfacial tension was greater and much faster than the effect of pH changes. Presumably, Ca2+ -ion reacts with ASA at the l/l interface, forming an adsorbed compound that results in a significantly lower interfacial tension than that of pure ASA. 4.2. Equilibrium contact angles Young’s equation [18] for a drop of ASA (b) on a solid surface (s) immersed in water (w) is γb/w cosθL = γs/w − γs/b
(3)
where the contact angle θ L is the interfacial angle measured through ASA and γ i/j denotes the interfacial tension between phases i and j. Young’s equation in air for the solid (s)–ASA (b)–vapour (v) system is γb/v cosθb = γs/v − γs/b
(4)
and for the solid (s)–water (w)–vapour (v) system γw/v cosθw = γs/v − γs/w
(5)
From Eqs. (2)–(4) follows the Bartell–Osterhof equation [19], which can be used to calculate θ L from measurements of contact angles in air and surface tensions: γb/w cosθL = γb/v cosθb − γw/v cosθw
(6)
Measured interfacial contact angles of ASA on hydrophilic and hydrophobic SiO2 surfaces are compared with angles calculated from Eq. (6) in Table 1. The calculation results in values of cos θ L smaller than −1 for the hydrophilic
Fig. 12. (A) Spreading of ASA on hydrophilic surface in different circumstances. (B) Spreading of ASA on hydrophobic surface in different circumstances.
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Table 1 Interfacial contact angles of ASA in water on hydrophilic and hydrophobic SiO2 surfaces and angles calculated from the Bartell–Osterhof equation (standard deviations based on five measurements) Sample surface
Liquid
Surface tension (mJ/m2 )
Interfacial tension (mJ/m2 )
Hydrophilic SiO2
Water ASA
72.8 ± 0.5 33.2 ± 0.2
11.7 ± 0.2
Hydrophobic SiO2
Water ASA
72.8 ± 0.5 33.2 ± 0.2
11.7 ± 0.2
surface and larger than 1 for the hydrophobic surface. The same phenomenon was observed also by Johnson and Dettre [18] for cases where the surface tensions of liquids were below 28 mN/m. The surface tension of the ASA was rather close to this value (33.2 mN/m). The measured interfacial angle on the hydrophilic surface was very high while the interfacial angle on the hydrophobic surface was relatively small. However, these facts do not explain the discrepancy. According to Eq. (6). γb/v cosθb − γw/v cosθw cosθL = γb/w
(7)
The values in Table 1 for hydrophilic surfaces yield cos θ L = −3.3, which is clearly absurd. Assuming that the measured surface and interfacial tensions of ASA are correct, to get a value close to −1, the contact angle of water should be much higher. Such a situation might occur if the surface were not homogeneous, i.e., there would be hydrophobic patches on it. In view of the ToF-SIMS and AFM analysis, which proved the surface to be rather homogenous and smooth, this does not seem likely. Moreover, if the other values in Table 1 are correct, θw would have to be ≈120◦ . Other options would be that the true ASA contact angle is much lower (not likely), or that the true interfacial tension of ASA/water is much higher, which is not impossible. It should be of the same order as the surface tension of ASA. Eq. (5) assumes no adsorption/desorption; if the ASA surface is hydrolysed by water this is of course an adsorption (chemisorption) process that will change the interfacial tension. The values for the hydrophobic surface in Table 1 yield cos θ L = 2.8. To get a value close to 1 (or slightly less), either the contact angle of water should be higher (not totally impossible, but unlikely), the true contact angle of ASA should be lower (not impossible, if there are hydrophilic patches on the surface), or the true interfacial tension should be much higher (not unlikely, as discussed above). Thus, the possible reasons for the unrealistic values of cos θ L seem to be contact angle hysteresis or the assumption that the measured interfacial tensions are really those of pure ASA. Contact angle hysteresis can evidently explain only a small part of the inconsistency since roughness or heterogeneity were found to be insignificant. Thus, the main factor seems to be the interfacial tension. As already stated, a possible reason for the very low measured interfacial tensions could be the hydrolysis of ASA molecules at the interface
Angle in air (◦ )
Calculated interfacial angle (◦ )
Measured interfacial angle (◦ )
20 ± 2 25 ± 2
180
170 ± 4
100 ± 2 47 ± 2
0
25 ± 2
Table 2 Adhesion of water and ASA to hydrophilic and hydrophobic surfaces Hydrophilic surface In air Water ASA
(mJ/m2 )
141 63
Hydrophobic surface
In liquid (mJ/m2 )
In air (mJ/m2 )
In liquid (mJ/m2 )
23 0.18
60 56
1 22
in contact with the aqueous solutions. If this reaction is very fast, the true values of ASA interfacial tensions were never recorded, but only the interfacial tension in the presence of the reaction products. 4.3. Adhesion of ASA The work of adhesion (WAa ) of ASA to the solid in air is given by: WAa = γs/v + γb/v − γs/b
(8)
Adhesion was calculated here using the Dupr´e equation: WAa = γl/v (1 + cosθb )
(9)
Correspondingly, the work of adhesion of ASA to the solid in water and aqueous solutions is WAw = γb/w (1 + cosθL )
(10)
The calculated values of the work of adhesion of water and ASA (the contact angle of water in liquid is, of course, 180 − θ b ) on the surfaces, in air and water, are given in Table 2. The adhesion of ASA is also shown in Fig. 13.
Fig. 13. Work of adhesion of ASA on the surfaces, in air and in water.
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The work of adhesion of ASA on the substrates is considerably larger in air than it is in water. This is expected because of the clear difference between the ASA surface tensions measured in air and in water. In aqueous environment hydrolysis of ASA further decreases the adhesion due to the lowering of the interfacial tension. The difference between the adhesion to hydrophilic and hydrophobic surfaces in air is rather small (≈13% larger on the hydrophilic surface). In water the difference is larger and the order of the hydrophilic and hydrophobic surface is opposite to that measured in air. Adhesion of water to the hydrophilic surface is as low as 23 mJ/m2 , and adhesion of ASA to the hydrophobic surface is only 22 mJ/m2 . This, as already discussed above, shows that the value of γb/w measured after 1 min cannot be the value of pure ASA before hydrolysis. Actually, adhesion of unhydrolysed ASA to the hydrophobic surface should be about the same as the cohesion of ASA, i.e., ≈2 × γ b = 66, which is what one would obtain if γb/w were ≈33. This would also give more reasonable values of the contact angles using Eq. (6).
5. Conclusions A reasonable explanation of the results presented in this investigation is that the hydrolysis of ASA molecules at the ASA/aqueous solution interface is very fast. For this reason, true values of ASA interfacial tension were never recorded, but only the interfacial tension in the presence of the reaction products (adsorbed on the l/l interface). On the other hand the aqueous solubility of these products was so low that during the measurement times used they had no influence on the surface tension of the aqueous solutions in contact with the ASA. This is why calculation of interfacial contact angles according to Bartell–Osterhof equation, which assumes that all surfaces are in equilibrium, was impossible. It is obvious that, in a case of a reacting system like ASA in aqueous solution, no such equilibrium is reached. However, the results can be used for evaluation of surface hydrophilicity/hydrophobicity effects on the contamination behaviour of ASA. When endeavouring to avoid contamination of the surfaces of process equipment by hydrophobic compounds such as ASA, the work of adhesion of fouling compounds to the surfaces should to be minimised. There was virtually no spreading of ASA on the hydrophilic surface immersed in water and the work of adhesion to this surface was the smallest. ASA spread fairly well on the hydrophobic surface in water and the adhesion was markedly larger than to the hydrophilic surface. ASA spread rather well in air on both surfaces and the differences between the works of adhesion to the surfaces was not large. In both cases the adhesion in air was larger than in aqueous environment.
The spreading of ASA did not depend significantly on pH at low pH values. At high pH effects on the surface tensions due to reactions of ASA (hydrolysis) were observed. Ca2+ -ions had a marked effect on the spreading of ASA on the hydrophobic surface. There was no effect of pH or Ca2+ concentration on the spreading on the hydrophilic surface. From these results we conclude that the best way to prevent fouling on paper machines by ASA (and presumably by other hydrophobic substances) is to maintain the process surfaces wet and to choose surface materials that stay hydrophilic as consistently as possible. In the pH range used in papermaking neither pH nor calcium ion concentration will have any significant effect on the spreading of ASA as long as the surface is strongly hydrophilic. At low pH, the solubility of hydrolysis products of ASA is apparently too low to have any effect on the aqueous solution properties, at least within the time ranges investigated (several minutes).
Acknowledgement The work was funded by National Technology Agency of Finland (TEKES). It has been performed as a part of “Shine Pro”—project in “Clean Surfaces”—technology program.
References [1] J.M. Gess, Tappi J. 75 (4) (1992) 79. [2] T. Gliese, Ipw (9) (2003) 42–46. [3] J.P. Koskela, O.E.O. Hormi, J.C. Roberts, G. Peng, Appita J. 56 (2003) 213. [4] L. Neimo (Ed.), Papermaking Science and Technology, Book 4: Papermaking Chemistry, FAPET OY, Helsinki, Finland, 1999, p. 151 (Chapter 7). http://www.fapet.fi/. [5] K. Yamaguchi, M. Ito, T. Taniguchi, S. Kawaguchi, K. Nagai, Colloid Polym. Sci. 284 (2004) 366–372. [6] T. Svitova, O. Theodoly, S. Christiano, R.M. Hill, C.J. Radke, Langmuir 18 (2002) 6821. [7] F.E. Bartell, H.H. Zuidema, J. Am. Chem. Soc. 58 (1936) 1449. [8] J. Schultz, K. Tsutsumi, J.B. Donnet, J. Colloids Interf. Sci. 58 (1977). [9] A. Della Bona, C. Shen, K.J. Anusavice, Dental Mater. 20 (2004) 338. [10] R.B. Wasser, J. Pulp Paper Sci. 13 (1) (1987) J29. [11] R.M. S Savolainen, Proceedings TAPPI Papermakers Conference, Philadelphia, March 24–27, 1996, p. 289. [12] J.W. Jennings Jr., N.R. Pallas, Langmuir 4 (1988) 959. [13] B. Schueler, Microsc. Microanal. Microstruct. 3 (1992) 119. [14] J. Kekkonen, P. Stenius, Colloids Surf. A: Physicochem. Eng. Aspects 156 (1999) 357–372. [15] W. Kern, D.A. Puotinen, RCA Rev. 31 (2) (1970) 187–206. [16] H. Elwing, S. Welin, A. Askendal, U. Nilsson, I. Lundstr¨om, J. Colloids Interf. Sci. 119 (1987) 203. [17] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, sixth ed., Wiley, New York, 1997. [18] J.C. Berg, Wettability, Marcel Dekker, New York, 1993. [19] F.E. Bartell, H.J. Osterhof, Ind. Eng. Chem. 19 (1927) 1277.
Spreading and adhesion of ASA on hydrophilic and hydrophobic SiO2 J. Lindfors ∗ , S. Ylisuvanto, T. Kallio, J. Laine, P. Stenius Helsinki University of Technology, Laboratory of Forest Products Chemistry, P.O. Box 6300, FIN-02015 TKK, Finland Received 12 October 2004; accepted 22 January 2005
Abstract The spreading kinetics and adhesion of liquid alkenyl succinic anhydride (ASA) on hydrophilic and hydrophobic SiO2 surfaces was studied. Contact angles were measured both in air and in aqueous environment. ASA spread fairly well on both surfaces in air, but when immersed in aqueous solution it spread only on the hydrophobic surface. The contact angle in water on the hydrophilic surface was ≈170◦ . The work of adhesion of ASA to both surfaces in air was about 60 mJ/m2 . When immersed in water the work of adhesion of ASA on the hydrophobic surface was 22 mJ/m2 , and less than 1 mJ/m2 on the hydrophilic surface. In the pH range used in papermaking there was no significance of pH on spreading of ASA, but at high pH effects on the surface tensions due to hydrolysis of ASA were observed. Ca2+ -ions had no effect on the spreading of ASA on the hydrophilic surface but strongly influenced spreading on the hydrophobic surface. ASA is extensively used as a hydrophobing agent and tends to form detrimental deposits on equipment used for manufacture of paper. From the results it can be concluded that best way to prevent fouling on paper machines by hydrophobic substances is to maintain the process surfaces wetted by water and to choose surface materials that stay hydrophilic as consistently as possible. © 2005 Elsevier B.V. All rights reserved. Keywords: Alkenyl succinic anhydride; Paper machine; Adhesion; Contamination; Hydrophobing agent
1. Introduction Alkenyl succinic anhydrides (ASA) emulsions are widely used as hydrophobing agents in alkaline papermaking [1]. The basic structure of ASA is shown in Fig. 1(A). The composition of the emulsion and the detailed structure of the ASA vary (e.g. emulsifiers used, olefin chain length and branching), but the main function of the emulsions, i.e., conferring hydrophobicity to papers by deposition of ASA on the fibres, always remains [2]. A persistent problem associated with the use of ASA is the contamination of process equipment by formation of hydrophobic deposits [3,4]. A reasonable assumption is that this contamination for the most part occurs because of hydrophobic interactions, such as recently discussed by, for example, Yamaguchi et al. [5]. The objective of this study was to obtain further insight into this contamination problem by investigation of the spreading of ASA on different surfaces. Dynamic contact ∗
Corresponding author. Tel.: +358 9 451 5657; fax: +358 9 451 4259. E-mail address: [email protected] (J. Lindfors).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.01.028
angle measurements were used for the determination of the spreading kinetics and adhesion of ASA on selected model surfaces in air and in water. The type of measurements in a liquid cell used by us, have been described earlier by Svitova et al. [6]. The wetting kinetics in solid–liquid–liquid system was studied, for example, by Bartell and Zuidema [7] and by Schultz et al. [8]. Determination of the work of adhesion to silica using contact angle measurements was recently discussed by Della Bona et al. [9]. The effect of pH and Ca2+ -ion content was studied by measurements in aqueous environment. pH is a major factor influencing the hydrolysis of ASA and the dissociation of hydrolysed ASA (see Fig. 1(B) and (C)), and it can thus have a strong influence on the contamination behaviour of ASA in paper machines [10,11]. The salt formed by the ASA anion with Ca2+ is insoluble (see Fig. 1(D)) and it has been reported that Ca2+ -ion for this reason contributes significantly to fouling [11]. In this paper we describe an evaluation of the effects of the hydrophobicity/hydrophilicity of the surface material in different mediums on the spreading and adhesion of ASA.
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Fig. 1. The structure of (A) ASA, (B) hydrolysed ASA, (C) dissociated form of (B) and (D) the binding of Ca2+ to C.
The results make it possible to draw conclusions about the contamination behaviour of ASA on different surfaces and its dependence on process conditions.
2. Materials and methods 2.1. Contact angles and surface tensions A CAM 200 contact angle goniometer (KSV Instruments Ltd, Helsinki, Finland) was used for determination of contact angles and surface tensions. The arrangement of the instrumentation when investigating liquid–liquid–solid systems is shown in Fig. 2. The size of the ASA drops was approximately 10 l in air and 30 l in water. Surface tensions and contact angles were calculated by axisymmetric drop shape analysis (ADSA) using software delivered with the instrument. This analysis is based on fitting the full equations for drop profiles for pendant drops (surface tension) and sessile drops (contact angle) derived from the Young–Laplace equation. The fitting method is based on the principle described by Jennings and Pallas [12].
2.2. Atomic force microscopy, AFM AFM measurements were performed using a NanoScope IIIa Multimode scanning probe microscope (Digital Instruments, Inc., Santa Barbara). The images were scanned in tapping mode in air using commercial Si cantilevers (Digital Instruments) with a resonance frequency of about 300 kHz. The only image processing used was flattening. Four images were recorded for each surface and representative micrographs were chosen for presentation. The roughness of the surfaces was evaluated by determination of the roughness factor, r, from the 3D-AFM-images: r =1+
Sdr 100
(1)
where Sdr is the ratio between the real interfacial area and the projected area. 2.3. Time-of-flight secondary ion mass spectrometry, ToF-SIMS The ToF-SIMS spectrometer PHI TRIFT II at Top Analytica Ltd., Turku, Finland was used. The instrument and experimental procedure have been described by Schueler [13]. High-mass-resolution spectra in positive secondary ion mode over the mass range 2–2000 Da were acquired using a Ga ion gun. The primary ion current was 600 pA. The acquisition time was 2 min and the time per channel 138 ps. Two areas of 100 m × 100 m on each sample were analysed. ToF-SIMS ion images were recorded from an area of 50 m × 50 m. The acquisition time of the mapping was 10–20 min. The analysis depth was <2 nm. The primary data were evaluated using the WinCadence Data Reduction software in off-line mode. 2.4. Alkenyl succinic anhydride, ASA
Fig. 2. Scheme of the measurement set-up. Note that the density of ASA is lower than the density of water, so that the substrate is located above the ASA droplet.
Pure ASA was supplied by Raisio Chemicals Oy, Raisio, Finland. According to the suppliers, the purity of the ASA was ≈99%. Its melting point was ≈5 ◦ C. The density of ASA
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surface was 100 ± 2◦ . Standard deviations are based on five measurements. The morphology of the surfaces was characterised by AFM and their chemical composition was verified by ToFSIMS analysis.
3. Results Fig. 3. Partial positive TOF-SIMS spectrum of ASA. Molecular peaks corresponding to chain lengths C16 (363) and C18 (391) are indicated.
was determined to be 0.956 kg/dm3 . The ASA was not further purified. The composition of the hydrocarbon chains was approximately 50% C16-olefin and 50% C18-olefin. No dispersants were used in the preparation of the ASA. ToF-SIMS spectra of ASA were recorded in order to verify its composition. Fig. 3 shows part of the positive ToF-SIMS spectrum of ASA, which has molecular peaks corresponding to chain lengths C16 (363) and C18 (391). The liquid ASA was used as such. 2.5. Water The water was purified UHQ water in equilibrium with air (prepared with ELGA PURELAB UHQ). Its conductivity was ≈5.6 S. 2.6. Substrates The silica surfaces were those of smooth silicon wafers (from Okmetic Oy, Helsinki, Finland) [14]. They were cleaned in the following way: The wafers were first immersed in alkaline and acidic hydrogen peroxide solutions (first 10 min at 75–85 ◦ C in alkaline, then 10 min at 75–85 ◦ C in acidic solution) [15]. Then they were rinsed with water and ethanol. The advancing contact angle of water on the SiO2 surface was 20 ± 2◦ . Hydrophobic silica surfaces were prepared by methylating the SiO2 surface; the cleaned and dried surface was immersed in 0.05% dichlorodimethylsilane in xylene for 2 h. The method is described in greater detail in [16]. The advancing contact angle of water on the methylated
3.1. Surface topography Topographical AFM images of the surfaces are presented in Fig. 4. From the images it is clear that the native silicon wafer was very smooth. The roughness of the methylated surface, as determined from the AFM images using Eq. (1), was also low. The effect of roughness on contact angles was estimated using the Wenzel equation [17]: cosθW = r cos θY
(2)
where θ W and θ Y are the contact angles measured on the rough and smooth surface, respectively. The effect of roughness was found to be negligible (maximum value obtained for r was 1.001) and was, therefore, ignored in the contact angle analyses. 3.2. ToF-SIMS of surfaces Some ToF-SIMS ion images, of both methylated and untreated silicon surface, are shown in Fig. 5. The images show that the methylation was successful and resulted in a reasonably homogenous surface layer on the silicon wafer. Organic ions containing silicon such as 43 (CH3 Si+ ), 45 (SiOH+ ), 59 (CH3 OSi+ ), 73 ((CH3 )3 Si+ ), 117 (C3 H9 OSi2+ ), 133 (C4 H13 OSi2+ ), 147 (C5 H15 OSi2+ ), 191 (C5 H15 O2 Si3+ ), 207 (C5 H15 O3 Si3+ ), 221 (C7 H21 O2 Si3+ ) and 281 (C7 H21 O4 Si4+ ) were emitted from the hydrophobic surface. Parts of the ToFSIMS spectrum corresponding to some of these peaks (43, 45, 59 and 73) are presented in Fig. 6. The spectrum showed that the intensity of these ions was much lower on the hydrophilic surface. In particular, the intensity of ((CH3 )3 Si+ ) was high on the treated surface, which proves that the surface methylation was successful.
Fig. 4. Tapping mode AFM images of hydrophilic (on the left) and hydrophobic (on the right) surfaces.
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Fig. 5. TOF-SIMS ion images of hydrophilic (A) and hydrophobic (B) surfaces. The analysis depth was <2 nm. In the images a brighter colour indicates higher ion content. The bars in the images are 10 m.
3.3. Surface and interfacial tensions The surface tension of ASA and the interfacial tensions of ASA in water and water solutions at different pH and Ca2+ ion concentrations were measured. The results are shown in Fig. 7. 3.4. Wetting dynamics The kinetics of spreading of ASA on hydrophilic and hydrophobic surfaces in water is illustrated by the results in
Fig. 8(A) and (B). Five measurements were performed on each surface. The figures show that the reproducibility of the measurements was rather good; in particular on the hydrophobic surface. The spread in the measurements on the hydrophilic surfaces was about ±5◦ . In the following the discussion will be restricted to representative curves for each case. The most critical stage in the measurements with respect the reproducibility was the attachment of the droplet to the surface. Recording of drop images was started manually, which made it difficult to exactly identify the time (t = 0 s) at which the drop attached to the surface. Therefore,
Fig. 6. Positive TOF-SIMS spectrum peaks corresponding to ions: 43, 45, 59 and 73 (A, hydrophilic surface; B, hydrophobic surface).
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Fig. 7. ASA surface tensions in different chemical environments (pH was adjusted with HCl and NaOH solutions). Standard deviation: 0.2 mN/m (five measurements).
Fig. 9. Contact angles of ASA on the surfaces in water.
there was some scattering of results at the beginning of measurements, where also the change in contact angle was the fastest. Fig. 9 shows the spreading of ASA on hydrophilic and hydrophobic surfaces immersed in water. On both surfaces, contact angle equilibrium was reached within a 1-min-period, as in all other measurements. Equilibrium angles used for calculations of work of adhesion were taken at t = 1 min. In water attainment of equilibrium was slower on the hydrophobic surface than on the hydrophilic surface, probably just because the final contact angle was much lower. As expected, ASA did not wet the hydrophilic surface (final contact angle 170 ± 4◦ ) while it did spread on but did not completely wet the hydrophobic surface (final contact angle 20 ± 2◦ ). In air (Fig. 10) spreading on the hydrophobic surface was faster and resulted in a higher final contact angle than on the hydrophilic surface. Both surfaces were wetted by the ASA (final contact angles 48 ± 2◦ and 23 ± 2◦ , respectively), but the difference between the two surfaces was not as significant as it was in water. The effect of the concentration of Ca2+ -ions on the spreading of ASA on the hydrophobic surface is shown in Fig. 11. Concentrations below 10 mM had no significant influence on
the results. Only 100 mM CaCl2 solution, where the effect was very significant, was further investigated. Spreading curves of ASA on hydrophilic and hydrophobic surfaces under different conditions (pH 3, pure water, pH 10 and 100 mM CaCl2 ) are presented in Fig. 12(A) and (B). Changes in the chemical environment did not affect the spreading on the hydrophilic surface, but on the hydrophobic surface the contact angles increased in the presence of Ca2+ -ions.
Fig. 10. Contact angles of ASA on the surfaces in air.
Fig. 8. (A) Contact angle of ASA on hydrophilic SiO2 in water. (B) Contact angle of ASA on hydrophobic SiO2 in water.
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Fig. 11. The effect of CaCl2 concentration on behaviour of spreading of ASA on hydrophobic surface.
4. Discussion 4.1. Surface and interfacial tensions The interfacial tensions are considerably lower than the surface tension of ASA. When the ASA was contacted with aqueous solutions at pH 3 and 10 only minor changes in the interfacial tension were observed after 1 min, compared to the value determined in pure water. On the other hand, when the ASA drop was kept in contact with these solutions for 5 min, the interfacial tension at pH 3 decreased by 2 mN/m and at pH 10 it decreased to approximately half of the initial value in water. The interfacial tension value in water also decreased slightly when the contact time was prolonged to 5 min. These changes in interfacial tensions very likely are due to hydrolysis reactions and dissociation of reaction products of ASA, especially at high pH. These reactions are not very fast, but a clear decrease in surface tension is observed after a couple of minutes. To get an idea about the extent of the hydrolysis reactions, the surface tension of the water in contact with an ASA drop in the measurement cell was also determined as a function of time. There were no changes in surface tension, which implies that the amount of hydrolysis products of ASA dissolved into the water was at least so low that no measurable diffusion of such molecules to the surface of the water takes place. As seen from the molecular struc-
tures of the molecules (Fig. 1(C)) they should be strongly surface active. From these results, and from the excellent reproducibility of the spreading experiments, we conclude that dissolved hydrolysis products did not significantly affect the results recorded after 1 min contact time. This, of course, does not exclude that ASA molecules located at the l/l interface are rapidly hydrolysed when contacted with the aqueous solution. The interfacial tensions measured in CaCl2 -solutions were considerably smaller than those in pure water. It was also obvious that the effect of Ca2+ on the interfacial tension was greater and much faster than the effect of pH changes. Presumably, Ca2+ -ion reacts with ASA at the l/l interface, forming an adsorbed compound that results in a significantly lower interfacial tension than that of pure ASA. 4.2. Equilibrium contact angles Young’s equation [18] for a drop of ASA (b) on a solid surface (s) immersed in water (w) is γb/w cosθL = γs/w − γs/b
(3)
where the contact angle θ L is the interfacial angle measured through ASA and γ i/j denotes the interfacial tension between phases i and j. Young’s equation in air for the solid (s)–ASA (b)–vapour (v) system is γb/v cosθb = γs/v − γs/b
(4)
and for the solid (s)–water (w)–vapour (v) system γw/v cosθw = γs/v − γs/w
(5)
From Eqs. (2)–(4) follows the Bartell–Osterhof equation [19], which can be used to calculate θ L from measurements of contact angles in air and surface tensions: γb/w cosθL = γb/v cosθb − γw/v cosθw
(6)
Measured interfacial contact angles of ASA on hydrophilic and hydrophobic SiO2 surfaces are compared with angles calculated from Eq. (6) in Table 1. The calculation results in values of cos θ L smaller than −1 for the hydrophilic
Fig. 12. (A) Spreading of ASA on hydrophilic surface in different circumstances. (B) Spreading of ASA on hydrophobic surface in different circumstances.
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Table 1 Interfacial contact angles of ASA in water on hydrophilic and hydrophobic SiO2 surfaces and angles calculated from the Bartell–Osterhof equation (standard deviations based on five measurements) Sample surface
Liquid
Surface tension (mJ/m2 )
Interfacial tension (mJ/m2 )
Hydrophilic SiO2
Water ASA
72.8 ± 0.5 33.2 ± 0.2
11.7 ± 0.2
Hydrophobic SiO2
Water ASA
72.8 ± 0.5 33.2 ± 0.2
11.7 ± 0.2
surface and larger than 1 for the hydrophobic surface. The same phenomenon was observed also by Johnson and Dettre [18] for cases where the surface tensions of liquids were below 28 mN/m. The surface tension of the ASA was rather close to this value (33.2 mN/m). The measured interfacial angle on the hydrophilic surface was very high while the interfacial angle on the hydrophobic surface was relatively small. However, these facts do not explain the discrepancy. According to Eq. (6). γb/v cosθb − γw/v cosθw cosθL = γb/w
(7)
The values in Table 1 for hydrophilic surfaces yield cos θ L = −3.3, which is clearly absurd. Assuming that the measured surface and interfacial tensions of ASA are correct, to get a value close to −1, the contact angle of water should be much higher. Such a situation might occur if the surface were not homogeneous, i.e., there would be hydrophobic patches on it. In view of the ToF-SIMS and AFM analysis, which proved the surface to be rather homogenous and smooth, this does not seem likely. Moreover, if the other values in Table 1 are correct, θw would have to be ≈120◦ . Other options would be that the true ASA contact angle is much lower (not likely), or that the true interfacial tension of ASA/water is much higher, which is not impossible. It should be of the same order as the surface tension of ASA. Eq. (5) assumes no adsorption/desorption; if the ASA surface is hydrolysed by water this is of course an adsorption (chemisorption) process that will change the interfacial tension. The values for the hydrophobic surface in Table 1 yield cos θ L = 2.8. To get a value close to 1 (or slightly less), either the contact angle of water should be higher (not totally impossible, but unlikely), the true contact angle of ASA should be lower (not impossible, if there are hydrophilic patches on the surface), or the true interfacial tension should be much higher (not unlikely, as discussed above). Thus, the possible reasons for the unrealistic values of cos θ L seem to be contact angle hysteresis or the assumption that the measured interfacial tensions are really those of pure ASA. Contact angle hysteresis can evidently explain only a small part of the inconsistency since roughness or heterogeneity were found to be insignificant. Thus, the main factor seems to be the interfacial tension. As already stated, a possible reason for the very low measured interfacial tensions could be the hydrolysis of ASA molecules at the interface
Angle in air (◦ )
Calculated interfacial angle (◦ )
Measured interfacial angle (◦ )
20 ± 2 25 ± 2
180
170 ± 4
100 ± 2 47 ± 2
0
25 ± 2
Table 2 Adhesion of water and ASA to hydrophilic and hydrophobic surfaces Hydrophilic surface In air Water ASA
(mJ/m2 )
141 63
Hydrophobic surface
In liquid (mJ/m2 )
In air (mJ/m2 )
In liquid (mJ/m2 )
23 0.18
60 56
1 22
in contact with the aqueous solutions. If this reaction is very fast, the true values of ASA interfacial tensions were never recorded, but only the interfacial tension in the presence of the reaction products. 4.3. Adhesion of ASA The work of adhesion (WAa ) of ASA to the solid in air is given by: WAa = γs/v + γb/v − γs/b
(8)
Adhesion was calculated here using the Dupr´e equation: WAa = γl/v (1 + cosθb )
(9)
Correspondingly, the work of adhesion of ASA to the solid in water and aqueous solutions is WAw = γb/w (1 + cosθL )
(10)
The calculated values of the work of adhesion of water and ASA (the contact angle of water in liquid is, of course, 180 − θ b ) on the surfaces, in air and water, are given in Table 2. The adhesion of ASA is also shown in Fig. 13.
Fig. 13. Work of adhesion of ASA on the surfaces, in air and in water.
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The work of adhesion of ASA on the substrates is considerably larger in air than it is in water. This is expected because of the clear difference between the ASA surface tensions measured in air and in water. In aqueous environment hydrolysis of ASA further decreases the adhesion due to the lowering of the interfacial tension. The difference between the adhesion to hydrophilic and hydrophobic surfaces in air is rather small (≈13% larger on the hydrophilic surface). In water the difference is larger and the order of the hydrophilic and hydrophobic surface is opposite to that measured in air. Adhesion of water to the hydrophilic surface is as low as 23 mJ/m2 , and adhesion of ASA to the hydrophobic surface is only 22 mJ/m2 . This, as already discussed above, shows that the value of γb/w measured after 1 min cannot be the value of pure ASA before hydrolysis. Actually, adhesion of unhydrolysed ASA to the hydrophobic surface should be about the same as the cohesion of ASA, i.e., ≈2 × γ b = 66, which is what one would obtain if γb/w were ≈33. This would also give more reasonable values of the contact angles using Eq. (6).
5. Conclusions A reasonable explanation of the results presented in this investigation is that the hydrolysis of ASA molecules at the ASA/aqueous solution interface is very fast. For this reason, true values of ASA interfacial tension were never recorded, but only the interfacial tension in the presence of the reaction products (adsorbed on the l/l interface). On the other hand the aqueous solubility of these products was so low that during the measurement times used they had no influence on the surface tension of the aqueous solutions in contact with the ASA. This is why calculation of interfacial contact angles according to Bartell–Osterhof equation, which assumes that all surfaces are in equilibrium, was impossible. It is obvious that, in a case of a reacting system like ASA in aqueous solution, no such equilibrium is reached. However, the results can be used for evaluation of surface hydrophilicity/hydrophobicity effects on the contamination behaviour of ASA. When endeavouring to avoid contamination of the surfaces of process equipment by hydrophobic compounds such as ASA, the work of adhesion of fouling compounds to the surfaces should to be minimised. There was virtually no spreading of ASA on the hydrophilic surface immersed in water and the work of adhesion to this surface was the smallest. ASA spread fairly well on the hydrophobic surface in water and the adhesion was markedly larger than to the hydrophilic surface. ASA spread rather well in air on both surfaces and the differences between the works of adhesion to the surfaces was not large. In both cases the adhesion in air was larger than in aqueous environment.
The spreading of ASA did not depend significantly on pH at low pH values. At high pH effects on the surface tensions due to reactions of ASA (hydrolysis) were observed. Ca2+ -ions had a marked effect on the spreading of ASA on the hydrophobic surface. There was no effect of pH or Ca2+ concentration on the spreading on the hydrophilic surface. From these results we conclude that the best way to prevent fouling on paper machines by ASA (and presumably by other hydrophobic substances) is to maintain the process surfaces wet and to choose surface materials that stay hydrophilic as consistently as possible. In the pH range used in papermaking neither pH nor calcium ion concentration will have any significant effect on the spreading of ASA as long as the surface is strongly hydrophilic. At low pH, the solubility of hydrolysis products of ASA is apparently too low to have any effect on the aqueous solution properties, at least within the time ranges investigated (several minutes).
Acknowledgement The work was funded by National Technology Agency of Finland (TEKES). It has been performed as a part of “Shine Pro”—project in “Clean Surfaces”—technology program.
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