The colloidal pK, of lipophilic extractives commonly found in Pinus radiata DOUGLAS S. MGLEANT, DAVID VERCOE*, KAREN R. STACK* AND DES RICHARDSONS

SUMMARY The negative logarithm of each side of the and the nature of the species that are Equation 3 yields Equation 4 present. Effluent pH in relation to the pK, This paper uses established experimental methods to determine the colloidal pK, of the acidic lipophilic extractives of values for a variety of weak organic acids softwoods has an influence on their found in the lipophilic extractives of Pinus biodegradation (2) and the subsequent radiata at 20°C and 50°C. effluent toxicity (3,4). The pH of pulp and The 50°C colloidal pK, values, to the The tenns: and are given special names. papennaking process watcrs has an knowledge of the authors, are published These are pK, and pH respectively and influence on the deposition of these acidic for the first time in this paper. This data will are expressed as: lipophilic extractives onto pulp and hopefully lead to a better understanding of papennaking surfaces in the form of pitch how these weak organic acids interact in (5,6). The interaction of these extractives the 50°C temperature range of modern varies with pH (7). pulp and papermaking. Kanicky and Shah (8) found that the The colloidal pK, values for hehenic, By substituting these definitions into pK, values for fatty acids, as determined lignoceric and cerotic acids are also Equation 4 we derive the Henderson- by acid-base titration, is dependent on the believed to be published for the first time Hasselhach equation: chain length of the and also the in this paper. concentration. They attributed the effects to molecular association of the chemical pK, =pH - log [RCOOH] [71 species, even below critical micelle Keywords concentration (CMC). Thev were able to Colloidal pK,, resin acids, fatty acids, When 50% oi the acid is ionised (or dis- demonstrate that extrapolation of the pK, wood pitch, Pinus radiata, behenic sociated): to very dilute concentrations of -5x10-11 acid, , cerotic acid M yielded a pK, close to 5.0 for the fatty acids they investigated (CG to C12 fatty When a carhoxylic acid (RCOOH) is As a result of the ahove case (Equation 8) acid chain length). In addition Kanicky placed in water it undergoes dissociation the second half of Equation 7 is eliminated, and Shah (8) showed that once the fatty to form the hydronium ion (H30+)and its as the negative logarithm of 1 equals 0. acid concentration was above its CMC the conjugate base (RCOO-1. This reaction pK, of the fatty acid became constant. can be written as: As a result the term 'colloidal pK,' is introduced in this paper to describe the pK, of solutions of fatty and resin acids Generally chemists like to represent this Equation 7 thus becomes: ahove their CMC. equation by an equilibrium constant (K), This paper focuses on determining the PK, = pH [lo1 which is defined as: colloidal pK, of the various acidic In order to determine the pK, of a given lipophilic extractives in P. mdiata. organic acid one must determine the pH at P. radiata was selected due to its impor- which 50% of the acid is ionised, or dis- tance to the pulp and paper Industries of sociated. This can he determined hy titrat- Australia and New Zealand (9). The weak In most bench scale laboratory experiments ing the organic acid with a base. The pH organic acids examined in this paper were the concentration of water, [H,O], is at which half of the volume (V) of base selected from the lipophilic extractive relatively constant. Thus the equilibrium used to reach the equilibrium point (ep) is analytical works conducted on P. radiata constant can be redefined as an acidity the value of the pK, for that acid (Fig. 1). by others (10-12). These compounds constant (KJ. This is written as follows: This procedure does not work for acids consisted primarily of saturated and that are insoluble in water. Many of the unsaturated fatty acids with chain lengths organic acids that papennakers are inter- of 12 to 26 carbon atoms and the eight ested in have a limited . For resin acids. The pK, for some of these acids that are not soluble in water, the compounds have been determined general procedure is to form the salt, by previously by others (1.13-21) at 20% PhD Student, t Corresponding author, adding a base. The salt is then hack titrat- though some discrepancy does exist * Re~earchFellow, Pulp and Paper Research Group, School of Chemistly, ed with an acid (1). between some of the values in the literature. University of Tasmania, Private Bag 75, Hobart, Tasmania 7001. The relationship between pK, and pH Measurements of 9 Senior Specialist Pmcess Chemistry, is impoaant for papermakers in order to (C22:0), lignoceric acid (C24:O) and Research &Development Australasia. Norrke Skag Paper Mills (Australia) Ltd, Boyer, Tasmania 7140. understand the chemistry of the system cerotic acid (C26:O) will he undeaaken . . ~-~- ~. ~ ~~~ I j 362 Appita Journal Vol 58 ,No 5 12 j I i j 6% Peer Reviewed App~ta

mL of NaOH solution added Volume of Acid added

Fig. 1 Titration of a weak acid with a base. Fig. 2 pK, for an insoluble weak organic acid.

and reported for the fust time. This paper The potassium salts were then titrated with acid yields the same pK, when the also aims to determine the colloidal pK, using a weak solution (0.005 to 0.0005N) acid is titrated with base as shown in of these substances at 50%, which is a of hydrochloric acid (HCI) in order to Figure 1, measurements were made using representative temperature of modern determine the colloidal pK, value. An . Treating the acetic atid as an papermaking. average of three colloidal pK, values and insoluble acid, which it is not, the salt was their standard deviations are reported. first formed by adding 10% molar excess EXPERIMENTAL These methods were based on the work of of KOH. This salt was then titrated using The fatty acids were obtained from others (I ,8). a weak solution of HCI. A graphic repre- Sigma-Aldrich and had a purity of 99% or sentation of this titration is shown in greater, with the exception of RESULTS AND DISCUSSION Figure 3. which was 95+% purity. In order to fur- The pK, of some ofthe weak insoluble The pK, determined by this method ther improve its purity the stearic acid organic acids investigated in this paper was found to be 4.77 +I- 0.09 (point A) was recrystallised in twice. The have been reported in the literature which is in close agreement with the well resin acids were obtained from Helix- (1,8,13,19).These researchers have deter- known pK,, at 20°C, of 4.74 (23). This Biotech and had a purity of 99% or mined the pK, as the pH at which half the confirms that the method is acceptable. greater, with the exception of abietic acid volume of acid required to titrate the salt There were two difficulties in repro- which had a purity of 90 to 95%. formed from the addition of excess base. ducibility of these acid-base titrations of The majority of the substances exam- Figure 2 shows the typical s-shaped curve the insoluble weak organic acids. The fust ined are insoluble in water. As a result, the that is obtained. The first part of the curve difficulty was ensuring that the potassium potassium salts of the acids were first can be attributed to neutralisation of the salt of the acids had been formed. If too prepared using 10% ~nolarexcess of excess base present when forming the little KOH were added, not all of the acid potassium hydroxide (KOH), the ninimum soluble salt. In order to ensure only one would have been converted to the potassi- amount of distilled water and heated at inflection point, some of the researchers um salt. If too much KOH is added the 90°C for 20 minutes. At all times the prepared solutions such that there was no time required for the titration is increased, amount of acid used was above the CMC excess potassium hydroxide. due to the volume of HCI added to of the acid (22). The salt solution was In order to ensure that the pK, deter- neutralise the'excess KOH concentration then filtered through a 0.2 pm filter. mined by this method of titrating the salt [OH-]. This increased titration time

I Volume of Acid added I Fig. 3 pK, of acetic acid, treated as an insoluble acid. Fig. 4 Titration curves with increasing molar ratio of KOH to that of the analyte acid. September 2005 363 1 Peer Reviewed I

elongates the curves to the point where determining the two inflection points is ~h~ co~~oida~pKa.values of lipophilic extractives commonly found in Pinus radiata. / auite difficult. This ~roblemis shown in (STD denotes two siama- standard deviation). I Figwe 4. P& STD PK, STD In order to correct the problem found @2WC 020°C @5WC 050°C with high KOH concentrations one would increase the titrating acid's (HCI) Saturated acids I Lauric (C12:O) 7.07 0.04 6.48 0.34 concentration [H30i]. The second 5.3 problem in regards to reproducibility -7.9 occurs when the titrating acid concentration is too strong. With a very high acid Myristic (C14:O) 7.88 0.11 7.25 0.35 -fi Qa concentration only one inflection point is observed, as one decreases the titrating acid concentration both inflection points Palmitic (C16:O) 8.34 0.44 8.63 0.32 become distinguishable. This issue is 5.066 highlighted in Figure 5. 8.6-8.8b During the titrations we saw a change 9.7c in the physical nature of the solutions. Stearic (C18:O) 9.89 0.43 The photographs of what we saw are 10.15e presented in Figure 6. Kanicky and Shah (13)reported similar observations. Arachidic (C20:O) 9.82 0.27 9.28 0.17 In Figure 6 the solution became more and more cloudy from image I towards 111 Behenic (C22:O) 9.89 0.63 9.53 0.19 whereas the solution gained in particulate Lignoceric (C24:O) 10.21 0.1 9.81 0.15 matter from images 111 through V. Kanicky and Shah (13) explained image Cerot~c(C26:O) 10.21 0.09 9.83 0.12 IV in terms of the formation of crystals Unsaturated fatty acids and precipitate. Back and Steenberg (1) Elaidic (lB:l;(trans)g) 8.31 0.03 7.65 0 24 referred to image N as the flocculation 9.95e point. Langmuir made very similar obser- 8.3 vations on many of the same fatty acids, Oleic (18:l;(cis)9) 8.22 0.35 8.29 0.07 though he was looking at surface tension 5.026 instead of pH (24). Rayleigh (25) and 9.W Duprk (26) noted difficulty in understand- Linoleic (18:2;(cis)9.12) 7.43 0.25 ing the phenomena associated with Figure 9.24e 6. These solutions contain a mixture of free molecules, associated molecules, Linolenic 6.83 0.03 6.26 0.06 micelles and colloids. The colloidal (18:3;(cis)9,12,15) 8.28e nature of the solutions is obvious from the Resin acids visual observations and based on the work Abietic 7.26 0.11 6.18 0.04 by Kanicky (ZI), the pK, that is being measured is a pK, of the colloids and not the true molecular pK,. Colloidal stability is of particular importance to the pulp and Dehydroabietic 6.77 0.15 6.18 0.07 paper industry, as it relates to pitch depo- 5.84 5.71 h 0.09 sition (27,28). Further work is needed in order to understand the colloidal forma- lsopimaric tion and colloid stability of the solutions Neoabietic 7.07 0.01 6.23 0.14 and the effect pH is having, particularly why the colloidal pK, is so much higher than the molecular pK,. pK, rather than molecular pK, is work by Figure 7 shows that as fatty acid chain A summary of the colloidal pK, values Suckling et al. (29) that sliowed the pH at length increases so does colloidal pK, (8). determined is presented in Table 1. The which resin components were soluble in The shorter chain length (C1:O to C9:O) reason that colloidal pK, was examined fatty acids have a pK, values between 4 to rather than 'molecular pK,' is that the water correspond with colloidal pK, values authors believe tliat the colloidal pK, is reported in Table 1. Additionally work by 5. There is a critical chain length some- more relevant to papermakers as the acid Sundberg et al. (30) showed the pH at where between 9 and 12 carbons where concentrations usually found, in the paper- which colloidal resin droplets were &s- the colloidal pK, starts to trend upwards making waters, are above -5x10 11 M @'). solved into the water phase correspond to on a steeper slope with increasing chain Supporting this decision to use colloidal the colloidal pK, values repo~tedin Table 1. length. The increase in colloidal pK, 364 Appita Journal Vol 58 No 5 Decreasing [H,Ot] in the titrating acid

pH

Volume of Acid added

Fig. 5 Titration curves with decreasing molarity of the Fig. 6 Photographs from the titration of . titrating acid (HCI). (Similar observations were made for each acid studied). values starts to taper off to about 10 when acids are further away from the acidic This result of the colloidal pK, values the fatty acids have 18 or more carbon proton than the double bonds of abietic being lower at 50°C than at 20°C is impor- atoms in their straight chain. This is an acid. Due to this greater withdrawal dis- tant for papermakers in predicting the interesting result as it indicates that the tance the colloidal pK, values for neoabi- type of species present at aparticular pH higher chain length-fatty acids have a col- e6c and isopimaric acids are lower than and hence the colloidal behaviour and loidal pK, that is basic (i.e. above 7). the colloidal PK, value for abietic acid at deposition tendency of the extractives. Further work is needed to explain this. 200~.lt is unclear why abietic acid does The effect of degree of saturation of not follow the electron-withdrawing CONCLUSlONS the fatty acid in relation to colloidal pK, group interpretation at 500C, Measurements of the colloidal pK, values shows that, as degree of unsaturation Taking standard expen- for insoluble fatty acids and resin acids increases, the colloidal pK, decreases at mental error into account the colloidal 20%. A similar result was observed by found in p, ra,jiata have been determined PK, values are sligbtf~lower at 5O0C than Kanicky and Shah (13).The effect is less at 2o0cand 50°C using a method that con- clearly demonstrated at 50°C due to at 20°C. These temperature biased col- .,fiS the insoluble acids to their experimental errors. loid* PKa values are expected given the sponding soluble salts by adding a slight. The colloidal pK, of resin acids was inverse relationship between pK, and excess of base and then titrating with i i found to be between 6 and 7. The trends in the defini- acid. The values at 20°C are higher than 1 j would appear to be driven by the electron- tion of PK,. shown in Equation 11. those values expected for pK, of short Withdrawing groups present in the various chain we& acids. Values are reported for 1 / resin acid structures. Of the four resin behenic acid and lignoceric acid and 1 acid strnctures shown in Figure 8, dehy- pKa = InlO.RT [Ill cerotic acid for the first time. droabietic acid's aromatic ring is the The values that have been determined I strongest electron-withdrawing group. Where: R is the gas constant in this paper are reported as a colloidal 1 Consequently dehydroabietic acid has the T is temperature in Kelvin pK, as the solutions investigated are in a lowest colloidal pK, value at 20°C. The AG, is the Gihbs energy of colloidal state being above their CMC. double bonds in neoabietic and isopimaric the acid in an aqueous solution. The colloidal nature of the species present

I

Fatiy acid chain length (C.:O) I Fig. 7 Fatty acid chain length versus colloidal pK,. (Circles (@) denote literature pK, values (311, diamonds (+) 20% and squares (m) 50°C denote experimental colloidal pK, values.) Fig. 8 Resin acid structures. September 2005 365 '5% / Applta peer .Reviewed

appears to be affecting the values deter- (5) Dreisbach, D.D. and Michalopoulos, D.L. - cules at interfaces in relatian to various tech- mined. Further work is planned in order Understanding the behavior of pitch in pulp nological processes: Effect of chain length on and paper mills, Toppi J. 72(6):129 (1989). pK, of fany acid salt solutions, Langmuir to understand how the colloidal properties (6) Allen, L.H. - The importance of pH in control- affect the measured pK,. ling metal-soap deposition, Tappi J. 71(1):61 16(1):172 (2000). The colloidal pK, values determined (1988). Christodoulou, A.P. and Rosano, HZ. -Effect for the fatty acids show that the values are (7) McLean, D.S. - The effect of wood extractive of pH and nature of monovalent cations on sur- composition on pitch deposition, MSe Thesis face isotherms of saturated C16 to CZ2soap significantly higher than the typical mole- School of Chemistry, University of Tasmania, manolayers, Adv. Chem. Ser. 84210 (1968). cular pK, values for weak acids @K, = 4- Hobart, p.106 (2003). 5) and higher still as the chain length (8) Kanicky, J.R. and Shah, D.O. - Effect of Heikkila, P.E., Dcamer, D.W. and Cornell, increases and the degree of saturation premicellar aggregation on pKa of fatty acid D.G. - Solution of fatty acids from monolayers soap solutions, Lnngntuir 19(6): 2034 (2003). increases. Above C18 the colloidal pK, spread at the air-water interface: Identification (9) Uprichard, J.M. - Introduction to chapter 1, of phase transfomatians and the estimation of values of higher chain length fany acids Pulp and Paper from Radiatu Pine, Appita surface charge, J. LipidRes. 11(3):195 (1970). reach a constant value of about 10. The and Forest Research, Carltan, p.2 (2002). Vercoe, D., Stack, K., Blackman, A., Yates. B. resin acids have colloidal pK, values (10) McDonald, I.R.C. and Porter, LJ. - Resin and fatly acid composition of Pinus rodiota whole and Richardson. D. -An innovative aoproach between 6 and 7 and the trends can be wood, and its relation to the yield and compo- to characlerising the interactions leading to explained by the differences in electron- sition of New Zealand tall oil, N. Z. Jl Sci. wood pitch deposition., J. Wood Chem. 12(2):352 (1969). withdrawing groups present. Technol. 24(2): In press (2004). The colloidal pK, values were found to (11) Suckling,ID.,Gallagher, S.S. andEde, RM.- A new methad for softwood exmctives analysis Wmdholz, M. - The Merck Index, Merck & be lower at 50%. This can be explained using high performance liquid chromatography, Co.,Inc.,Rahway,N.J., U.S.A. (1983). through thermodynamics. Holzforschung 44(5):339 (1990). Lan-muir, I. - The constitution and fundamen- (12)Wallis. A.F.A. and Wearne, R.H. - Characterisation of resin in radiata pine woods, tal properties of solids and liquids. Part 11. ACKNOWLEDGEMENTS bisulfite pulps and mill pitch samples,Appitn J. Liquid8,J.h. Chem. Sac. 39(9): 1848 (1917). This work was funded with industry sup- 50:409 (1997). Rayleigh, L. - On the tension of recently (13) Kanicky, JR. and Shah, D.0 - Effect of famed liquid surfaces, Pmc. R. Soc. London A port from Norske Skog Paper Mills degree, type, and position of nnsatnration on 47: 281 (1890). (Australia) Pty. Ltd. Artaches 'Tom' the pK(a) of long-chain fatty acids, J. Colloid Kazarian of the University of Tasmania's Interface Sci. 256(1):201 (2002). Dupk, A. - Theorie nlgchanique de la chuleur, Gnuthier-Villars, Paris, p.484 (1869). School of Chemistry for the Russian (14) Bruun, H. -An interpretation of the effect of ionization upon monolayer properties of rosin Sihvonen, A.L., Sundberg, K., Sundberg, A. translation of reference 17. acids,Acta Chem. Scand. 6955 (1952). and Holmbom, B. - Stability and deposition (15) Nykn, V. and Back, E. -The ionization con- tendency of colloidal wood resin, Nord. Pulp REFERENCES stant, solubility product and solubility of lauric and , Acta Chem. Scand. Pnp. Res. J. 13(1):64 (1998). (1) Back, E. and Steenberg, B. - Simultaneous 12(6):1305 (1958). Garver, T.M. and Yuan,'H. - Measuring the determination of ionization consant, solubility (16) Nyrbn, V. and Back, E. - The ionization response of pitch control strategies, Pulp Pap. pmduct and solubility for slightly soluble acids constant, solubilihi ~roductand solubility of . . Can. 103(9):24 (2002). and bases. Electrolytic constants for abietic ahietic and dehydroabietic acid, Acto Chem. acid, Act= Chem. Scond. 4810 (1950). Scand. 12(7):1516 (1958). Suckling, ID., Hua, HL. and Uprichard, JM. (2) Werker, A.G. and Hall, ER. - The influence of (17) Tolstikov, G.A., hismetov, MR. Andrusenko, -Factors affecting resin removal from radiata pH on the growth-linked biodegradation kinet- A.A. and Goryaev, M.I. - Synthetic transfor- pine mechanical pulps, Appita J. 43(3):217 ics of selected resin acids found in pulp mill mation of nsin acids m. The effect of the con- (1990). effluent, TAPPI J. 82(7):169 (1999). formation tmnsfer and dissociation constants Sundberg, K., Pettersson, C., Eckennan,C. and (3) McLeay, D.J., Walden. C.C. and Munm, JR - of diterpene acids, lzv. Nats. Akad. Nauk Resp. Iniluence of dilution water on the toxicity of Kaz. Ser. Khim. 18:71 (1968). Holmbom, B. - Preparation and properties of a haft pulp and paper mill effluent. Including (18) White, J.R. - Dissociation constants of higher model dispersion of colloidal wood resin from mechanisms of effect, Waer Res. 13:151(1979). alkyl phosphate esters, phosphonic acids, Nonvay spruce, J. Pulp Pap. Sci. 22(7):1248 (4) Leach, JM. and Thakore,A.N.- Identification phosphonous acids, phosphinic acids and (1996). of the constituents of kraft pulping effluent that carbonylic acids, J. Am. Chem. Soc. 72:1859 are toxic to iuvenile Coho salmon (1950). Lide, D.R. - CRC handbook of chemistry (Oncorhynchus kisutch), I. Fish. Res. Board (19) Kanicky, I.R., Poniatowski, AP., Mehta, NR. and physics, CRC Press, New York (1999). Can. 30(4):479 (1973). and Shah, D.O. - Caoperativity among mole- Rcvird msnussnpi recived forpublicalan 169.M.

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