Silica에 의한 Asphalt 모델 성분의  흡착특성

박 상권, 박 상준*, 윤 현희*

동국대학교 화학공학과, 경원대학교 화학공학과*

Adsorption Behavior of Asphalt Model Compounds on Silica

Sangkwon Park, Sang Joon Park*, Hyon Hee Yoon*

Department of Chemical Engineering, Dongguk University,

Department of Chemical Engineering, Kyungwon University

Introduction

During the last several decades, asphalt has served as a binder for pavement due to its low cost, workability, strength, durability and imperviousness.  However, with increasing traffic loads and weathering stress, numerous pavement failures continue to occur.  These prematurely spoiled roads cost a fortune to reconstruct every year.

Asphalt is a colloidal system of organic molecules which consist primarily of hydrocarbons and their derivatives.  Elements of asphalt contain nitrogen, sulfur, oxygen and small amount of metals as well as carbon and hydrogen.  Up to date, several attempts to investigate asphalt-aggregate bonding in terms of adsorption of asphalt molecules onto substrate surface have been made [1-4]. Although such studies have contributed to the understanding of asphalt-aggregate bonding, many questions still remain unanswered.  For example, such studies have not adequately addressed important adsorption parameters such as isotherms, monolayer amounts, free energy changes for adsorption, etc., which are critical variables to understand asphalt-aggregate interactions.  In this study, adsorption behavior of seven model compounds containing asphalt functionalities which are known to interact strongly with aggregates were investigated in order to provide information on better understanding of asphalt-aggregate bonding.  Adsorption isotherms of model compounds were experimentally determined.  Two adsorption models, Langmuir and Freundlich models, were applied to the experimental isotherms to estimate relavant adsorption parameters.

Experimental

Dimethylsulfoxide (99.9%), 1-butanol (99.9%), 2-butanone (99.9%), triethylamine (>99%), naphthalene (99%), propionic acid (>99%), and ethylacetate (>99.9%) (all from Aldrich) were used.  Toluene (>99.9%, HPLC grade, Aldrich) was used as a solvent.  The minute amount of moisture contained in each liquid was removed before use by type 4A molecular sieves.  Silica gel with surface area of about 300 m2/g (Davison Chemical Division, W. R., Grace & Co., U. S. A.) was used as an adsorbent.  The silica was heated to 140℃ for 24 hours before use to remove adsorbed moisture.  Benzene (>99.9%, HPLC grade, Aldrich) was used as an internal standard for the concentration analysis of model compound solutions by gas chromatography (GC).

First, adsorption equilibrium time of model compounds was experimetally determined.  Nine adsorption solutions were prepared by dissolving a specific model compound in 50 ml toluene at initial concentration of 50 mmol.  After oven-dried silica samples were added to Erlenmeyer flasks containing solutions, the flasks were shaked in a orbit shaker (Lab-line Instruments Inc., U.S.A.) at 250 r.p.m. and constant temperature of 25±0.1℃.  Eight samples of the same amount were taken from each flask at time intervals of 0, 1, 2, 5, 10, 20, 40, 60, and 80 minutes.  Each sample was filtered using a filter unit comprising a syringe, filter holder, and teflon membrane filter with the pore size of 0.22 □m-diameter to purge suspended silica particles in the sample solution.  Then, known amount of bezene was added to the samples.  The samples were quantatitively analyzed by the GC (Varian model 3700 with capillary columns of DB-Wax and DB-1).

Second, adsorption isotherms of model compounds were examined.  The same procedures as above were repeated except that eight adsorption solutions of 50 ml with different concentrations were prepared and upon completion of filtration the silica sample was dried in a vacuum desiccator overnight and kept in a dry place for further analysis for the future.

Results and Discussion

Figure 1 shows the results of adsorbed amount of dimethylsulfoxide and 1-butanol as a function of time.  As shown in Figure 1, the adsorption of  two model compounds was completed within few minutes.  Adsorptions of other model compounds were completed within at least 40 minutes (not shown here) and, thus, a common adsorption equilibrium time was taken as 40 minutes.  Figures 2 presents the adsorption isotherms of dimethylsulfoxide and 1-butanol.  As shown in Figure 2, the isotherm of dimethylsulfoxide has a stiff increase at low concentration, which implies that dimethylsulfoxide molecules has such a high affinity toward the silica surface.  On the other hand, the isotherm of 1-butanol more gradually increases than that of dimethylsulfoxide does, which means that 1-butanol has lower affinity than that of dimethylsulfoxide.  Similar analysis and interpretation were made for other model compounds(isotherms are not shown here.).

The adsorption data were plotted according to the following Langmuir model:

C/q = C/qm + 1/(bqm)	(1)

where C, q, qm, b denote the equilibrium concentration, the adsorbed amount, the monolayer amount, the constant, respectively.  In Figure 3, the Langmuir plots of dimethylsulfoxide and 1-butanol are given.  From the slope and the intercept of the C/q versus C plot, qm and b were calculated.  The resulting qm values were in the order: 1-butanol > dimethylsulfoxide > triethylamine > 2-butanone > propionic acid > ethylacetate > naphthalene.  The adsorption equilibrium constant, K, was obtained by plotting the adsorption data according to another form of the Langmuir model, C'/X = C'/Xm + 1/(KXm) where C', X and Xm are the mole fraction of model compound, the adsorbed amount (□mol/g silica) and the monolayer amount (□mol/g silica) [5].  The standard free energy change for adsorption (□G°) was calculated from the equation of □G°= -RT lnK where R is gas constant and T is temperature.  As a result, dimethylsulfoxide yielded the largest □G°value.

The following Freundlich model were applied to the adsorption data:

q = kC1/n	(2)

where k and n are the constants.  Figure 4 shows the Freundlich plots of dimethylsulfoxide and 1-butanol.  The slope and the intercept of this plot are 1/n and k, respectively.  As a result, dimethylsulfoxide showed the largest n value.  Since the heat of adsorption, □H, is related to the constant n by the equation of □H = nRT ln□ where □ is the surface coverage [6], dimethylsulfoxide appears to yield the largest heat of adsorption and, thus, the strongest adsorption under the assumption of same surface coverage.

References

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Association of Asphalt Paving Technologists, 46, 151(1977).

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