Solubility of 1-Adamantanamine hydrochloride in Six Pure Solvents between 283.15 K and 333.15 K

Abstract

The solubility of 1-adamantanamine hydrochloride (1-AH) in ethanol, acetic acid, distilled water, N -methylpyrrolidone (NMP), N,N -dimethylformamide (DMF) and dimethylacetamide (DMAC) between 283.15 K and 333.15 K was measured using a laser monitoring observation technique. Results of these measurements were correlated with the NRTL equation and a semi-empirical equation. For six solvents studied, the data are well fitted with the two equations, which can be used as a useful model in the production process of 1-AH.

Keywords:
Solubility; 1-Adamantanamine hydrochloride; Modified Apelblat equation

INTRODUCTION

1-Adamantanamine hydrochloride (abbreviated 1-AH CAS Registry No. 665-66-7, structural formula listed as Figure 1) (Schild and Sutton, 1965; Oxford and Schild, 1965), is a synthetic organic compound clinically used as an antiparkinsonism agent, as well as an antiviral drug (Davies et al., 1964; Van Voris et al., 1981; Bryson, 1982; Paci et al., 2001). Since 1-AH is highly soluble, relatively non-toxic and bio logically stable (Schild and Sutton, 1965) it merited further extensive investigation.

Figure 1
Structure of 1-Adamantanamine hydrochloride.

As we know, the solubility of a drug is not only essential information in the drug discovery process, but also an important property in the recrystallization stage of solid drugs (Shayanfar et al., 2008; Nti-Gyabaah et al., 2008).

Crystallization processes are key steps that de termine the quality of the final product. Crystal habit plays an important role in affecting the crystal prod uct physicochemical properties, such as solubility, dissolution rate, compressibility, and bulk density, that have an effect on the product biological activity and production cost. The solubility of solid com pounds in different solvents played a crucial role in the determination of proper solvents and the devel opment and operation of the crystallization process (Wu et al., 2010; Pankaj and Murthy, 2010).

Therefore, the solubility of 1-AH in different sol vents directly affects the size of crystal formation, crystal habit, yield, and cost of production. Hence, it is necessary to know the solubility of 1-AH in pure and mixed solvents. However, it was found that there were few reported experimental solubility data of 1-AH.

In recent research, the dynamic method is used as a common approach in solubility measurement (Liu et al., 2011), which incorporates laser techniques to monitor the dissolution process of the solid solute. Given the overwhelming advantages of the dynamic method (Jouyban-Guaramaleki et al., 2014; Qiao et al., 2014), it is also used in this research to measure the solubility of 1-AH in six pure organic solvents, including ethanol, acetic acid, distilled water, NMP, DMF and DMAC between 283.15 K and 333.15 K at atmospheric pressure. In addition, the experimental solubility results in pure solvents were correlated with the modified Apelblat equation and the NRTL equation, which proved good agreement with experi mental data.

EXPERIMENTAL SECTION

Materials

ADA-NH₃Cl used during the solubility measure ments had a mass purity of 0.998 and was purchased from China Langchem Inc. Its mass fraction purity was determined by HPLC and was purified through crystallization twice in distilled water before utiliza tion. The X-ray diffraction (XRD) spectra of samples are shown in Figure 2. Other reagents were analyti cal research grade reagents from Shanghai Chemical Reagent Co.

Figure 2
XRD pattern of 1-Adamantanamine hydrochloride: a, samples; b, standard XRD pattern.

Apparatus and Procedure.

The solubility of 1-AH was determined by a laser monitoring dynamic method. The experimental in strument and procedure were similar to those de scribed in the previous literature ^14-18 . A predeter mined excess mass of solvent (about 30 g) and a known mass of solute were added to a jacketed glass vessel (about 200 mL) with the laser light adjusted accordingly. The solution in the vessel was main tained at a constant temperature by water circulating through the outer jacket from a thermostatic water bath (type MPG-10C, China). The temperature of the solution was determined by a mercury glass ther mometer with an uncertainty of ± 0.05 K. The pro cess of dissolution was facilitated by continuously stirring at a desired temperature. At the beginning of the experiment, the intensity of the laser beam dropped due to a large number of undissolved 1-AH particles suspended in the solution. As the solid par ticles dissolved, the intensity of the laser beam in creased. When the solid particles dissolved com pletely, the laser beam intensity reached a maximum level, and the solution in the vessel was clear. Then additional solid solute of known mass (about 1 mg to 3 mg) was added to the solution in the vessel.

This process was repeated several times until the maximum intensity of the laser beam started to de cline after the last addition of solute. The time inter val depended on the dissolution speed of 1-AH, usu ally more than 60 min. When the intensity of the laser beam could no longer reach 90% of the maxi mum, the mixture was considered to be in phase equilibrium. The total amount of the solute added to the vessel was recorded. Then the undissolved solute was separated and identified to be 1-AH by X-ray diffraction (XRD). Through all of the experiments in this work, polymorphic transformation was not found.

The weight of all the chemicals was measured by an electronic analytical balance (Sartorius CP124S, Germany) with the precision of ± 0.0001 g. In order to ensure the accuracy of the experimental data, all of the above processes were repeated more than three times, and the average value was taken as the final experimental value. The standard uncertainty of the measured solubility values was estimated to be less than 2%. The uncertainty in the solubility values can be due to uncertainties in the weighing procedure, temperature measurements, excess addition of solute, and instabilities of the water bath.

THERMODYNAMIC MODELS

Modified Apelblat Equation

The Apelblat equation is the commonly used (Hefter and Tomkins, 2003; Wang et al., 2005; Li et al., 2010; Apelblat and Manzurola, 1997; Kondepudi and Prigogine, 2002) semi-empirical expression which is used to correlate experimental solubilities with calculated ones and to evaluate the influence of tem perature on the mole fraction solubility of the solute.

According to the solid-liquid phase equilibrium theory, the relationship between solubility and tem perature is generally modeled by (Apelblat and Man zurola, 1997):

(1) $\ln {\mathit{x}}_1=-\frac{\Delta {\mathit{H}}_{\mathit{f},1}}{{\mathit{RT}}_{\mathit{f},1}}\left(\frac{{\mathit{T}}_{\mathit{f},1}}{\mathit{T}}-1\right)-\frac{\Delta {\mathit{C}}_{\mathit{pf},1}}{\mathit{R}}\left(\frac{{\mathit{T}}_{\mathit{f},1}}{\mathit{T}}-1\right)+\frac{\Delta {\mathit{C}}_{\mathit{pf},1}}{\mathit{R}}\ln \frac{{\mathit{T}}_{\mathit{f},1}}{\mathit{T}}-\ln {\mathit{\gamma }}_1$

where x₁, y₁, ∆Hf,1, ∆C_pf,1, T_f,1, R and T stand for the mole fraction of the solute, activity coefficient, en thalpy of fusion, difference in the solute heat capaci ty between the solid and liquid at the melting tem perature, melting temperature of the solute, gas con stant, and equilibrium temperature in the saturated solution, respectively. The values of ∆H_f, ∆C _pf, T_f were estimated by ASPEN PLUS software version 8.4 (∆H_f =-1.3673×10⁸ J/kmol, ∆C _pf=23.0446 kJ/(kmol∙K), T _f = 300 ℃).

Equation (2) can be written as

2 $\ln {\mathrm{\chi }}_1=A+\frac{B}{T}+\ln T$

where x₁ is the mole fraction solubility of 1-AH, T stands for the absolute temperature (K), A, B and C are the dimensionless parameters.

NRTL Model

In the binary system, the activity coefficient can be calculated by the following formula (Domanska and Marciniak, 2003):

3 $\ln {\mathrm{\gamma }}_1={\mathrm{\chi }}_2^2\left[\frac{{\mathrm{\tau }}_{21}{G}_{21}^2}{{({\mathrm{\chi }}_1+G_{21}{\mathrm{\chi }}_2)}^2}+\frac{{\mathrm{\tau }}_{12}{G}_{12}^2}{{({\mathrm{\chi }}_2+G_{12}{\mathrm{\chi }}_1)}^2}\right]$

where,

4 $\begin{array}{cc}{\mathit{G}}_{12}=\exp (-{\mathit{\alpha }}_{12}{\mathit{\tau }}_{12})& {\mathit{G}}_{21}=\exp (-{\mathit{\alpha }}_{21}{\mathit{\tau }}_{21})\end{array}$

5 $\begin{array}{cc}\begin{array}{cc}{\mathit{\tau }}_{12}=\frac{{\mathit{g}}_{12}{\mathit{g}}_{12}}{\mathit{RT}}\hfill & \hfill \end{array}& {\mathit{\tau }}_{21}=\frac{{\mathit{g}}_{21}{\mathit{g}}_{21}}{\mathit{RT}}\end{array}$

where ∆g₁₂ (= g₁₂ - g₂₂) and ∆g₂₁ (=g₂₁ - g₁₁) are cross in teraction energy parameters, independent of tempera ture and composition. In addition, α₁₂ is a constant that reflects the non-randomness of the mixture and its value generally varies between 0.20 and 0.47 (Wei and Pei, 2008). Different values of α₁₂ were chosen to correlate the solubility data of 1-AH. It turns out that α₁₂ = 0.30 is the most suitable value because of the smallest relative deviation for the measurement system.

RESULTS AND DISCUSSION

The mean values were used to calculate the mole fraction solubility x₁ based on

(6) ${\mathit{x}}_1=\frac{{\mathit{m}}_1/{\mathit{M}}_1}{{\mathit{m}}_1/{\mathit{M}}_1+{\mathit{m}}_2/{\mathit{M}}_2}$

where m₁ and m₂ are the mass of the solute and sol vent respectively, M₁ and M₂ the molecular weight of the solute and solvent respectively. The solubility data of 1-AH in distilled water, acetic acid, ethanol, DMF, NMP and DMAC between 283.15 K and 333.15 K are listed in Table 1.

Table 1
Mole fraction solubility (x₁) of 1-adamantanamine hydrochloride in selected solvents with the temperature range from 278.15 to 333.15 K and pressure p = 0.1 MPa^a.

T (K)	100x1exp	100x1calc		T (K)	100x1exp	100x1calc
		Apelblat	NRTL			Apelblat	NRTL
DMAC
278.35	1.863	1.873	1.815	307.95	2.700	2.730	2.678
282.75	2.007	2.029	2.033	313.15	2.803	2.818	2.805
288.05	2.213	2.207	2.292	318.15	2.857	2.881	2.888
292.45	2.405	2.345	2.406	322.85	2.898	2.922	2.934
298.65	2.536	2.519	2.499	328.95	2.961	2.949	2.985
302.65	2.620	2.618	2.575	333.15	2.979	2.950	2.932
DMF
278.35	1.414	1.436	1.430	307.95	1.829	1.835	1.859
282.75	1.503	1.502	1.501	313.15	1.873	1.892	1.887
288.25	1.602	1.582	1.577	318.15	1.905	1.943	1.859
292.45	1.662	1.641	1.641	322.85	1.975	1.986	1.973
298.65	1.731	1.723	1.737	328.95	2.047	2.037	2.050
302.65	1.784	1.773	1.806	333.15	2.094	2.068	2.099
NMP
283.45	1.849	1.854	1.928	312.85	4.178	4.118	4.250
288.15	2.226	2.225	2.310	318.15	4.399	4.403	4.467
293.25	2.643	2.644	2.728	323.15	4.525	4.606	4.590
298.45	3.075	3.073	3.158	328.15	4.690	4.738	4.752
303.25	3.467	3.453	3.546	333.35	4.874	4.800	4.932
308.25	3.809	3.820	3.885
Acetic acid
293.35	1.132	1.125	1.138	318.15	2.999	2.943	3.003
298.05	1.377	1.353	1.386	323.25	3.601	3.573	3.596
303.35	1.606	1.664	1.617	328.15	4.339	4.299	4.327
308.15	1.978	2.005	1.990	333.15	5.097	5.184	5.081
313.25	2.456	2.440	2.465
Water
278.05	2.660	2.660	2.771	308.15	6.399	6.331	6.414
282.95	3.193	3.193	3.284	313.25	6.928	6.953	6.937
288.45	3.833	3.838	3.902	318.65	7.481	7.572	7.485
293.05	4.356	4.407	4.410	323.35	7.986	8.064	7.987
298.35	5.128	5.082	5.165	328.05	8.485	8.504	8.484
303.35	5.777	5.723	5.801	333.12	9.013	8.914	9.014
Ethanol
283.55	4.126	4.201	4.040	313.25	6.452	6.461	6.390
287.85	4.651	4.590	4.759	318.15	6.700	6.692	6.661
293.35	5.162	5.069	5.219	323.25	6.884	6.878	6.889
298.05	5.466	5.453	5.478	328.15	7.012	7.004	7.047
303.35	5.822	5.850	5.792	333.15	7.094	7.079	7.126
308.15	6.081	6.169	6.047

• ^a
  The standard uncertainties u are u(T) = 0.1 K, ur(p) = 0.05, ur(x) =0.03

To evaluate the correlation results and select the most suitable model for 1-AH solubility in pure sol vents, the relative deviation (RD %) and the average relative deviation (ARD %) were calculated. The relative deviation and the average relative deviation are defined as

(7) $\mathit{RD}\%=\frac{{\mathit{x}}_{1,\mathit{i}}^{\exp }-{\mathit{x}}_{1,\mathit{i}}^{\mathit{cal}}}{{\mathit{x}}_{1,\mathit{i}}^{\exp }}$

(8) $\mathit{ARD}\%=\frac{100}{\mathit{N}}\sum _{\mathit{i}=1}^{\mathit{N}}|\frac{{\mathit{x}}_{1,\mathit{i}}^{\exp }-{\mathit{x}}_{1,\mathit{i}}^{\mathit{cal}}}{{\mathit{x}}_{1,\mathit{i}}^{\exp }}|$

The parameters of the mentioned equations and the root mean square deviations ( RMSD) are listed in Table 2. The RMSD is defined as (Douglas, 1997):

(9) $\mathit{RMSD}={{\displaystyle \left[\frac{\sum _{\mathit{i}=1}^{\mathit{N}}({\mathit{x}}_{1,\mathit{i}}^{\exp }-{\mathit{x}}_{1,\mathit{i}}^{\mathit{cal}}{)}^2}{\mathit{N}1}\right]}}^{1/2}$

Table 2
Parameters of Equation (2) for 1-adamantanamine hydrochloride in Different Solvents.

Solvent	Apelblat				NRTL
	A	B	C	RMSD%	Δg12 (J·mol−1)	Δg21 (J·mol−1)	RMSD%
DMAC	187.88	-9324.5	-28.133	0.027	2725.3	14731.3	0.039
DMF	52.886	-3102.2	-8.1694	0.02	-3524.8	22243.5	0.02
NMP	437.00	-21630	-64.578	0.043	1626.3	11973.3	0.20
acetic acid	-118.93	2139.7	18.859	0.048	-1061.8	3247.6	0.47
water	261.07	-13673	-38.297	0.058	1377.1	7453.6	1.61
ethanol	217.28	-10890	-32.235	0.05	9692.3	9193.3	0.05

Table 3 lists the ARD % of different correlation models. The average relative deviations of the two models are 0.86% (Apelblat) and 1.13% (NRTL). Therefore, the Apelblat model fits well with the experimental solubility data of 1-AH in pure solvents.

Table 3
ARD% of different models in pure solvents.

Solvent	Water	DMF	DMAC	NMP	Acetic acid	Ethanol
Apelblat	0.6750	0.8992	0.7742	0.6316	1.4729	0.7050
NRTL	0.9688	0.9081	1.3524	2.3161	0.4149	0.8532

We can conclude from Figure 3 that: (1) Solubili ty of 1-AH is the lowest in DMF and the highest in water when the temperature is higher than 310 K; this may be because of the intermolecular interaction between solvent and solute molecules. 1-AH is a salt and most salts can be dissolvent in water, especially in hot water. In addition, the structure of 1-AH is much more complicated and is harder to disperse in organic solvents. Hence, solvents which have com plicated structures such as DMF show a lower solu bility value than in water. (2) Solubility increases with temperature in all the selected solvents; (3) the solubility curves of 1-AH in NMP and ethanol show almost the same curvature, which may mean that both solubilities have the same sensitivity to temper ature though their solubility values are different. The reason for this phenomenon needs to be studied further.

Figure 3
Mole fraction solubility of 1-Adamantanamine hydrochloride (x₁) in different solvents between 278 K and 333 K: ◄, NMP; ▷ acetic acid; ▼, DMAC; ▲, water; △, DMF; ▽, ethanol.

The solid line solubility curve was calculated by the modified Apelblat equation.

CONCLUSIONS

The solubility of 1-adamantanamine hydrochlo ride (1-AH) in ethanol, acetic acid, distilled water, N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and dimethylacetamide (DMAC) between 283.15 K and 333.15 K were measured using a laser monitoring observation technique. The solubilities in all selected solvents are functions of temperature and increase with the rise of temperature.

The modified Apelblat equation based on solid-liquid phase equilibrium principles and the NRTL equation were used to correlate the solubility data of 1-AH in these solvent systems. The RDs of the mod ified Apelblat Equation among all of these values does not exceed 1.82%. The average relative devia tion of the two models are 0.86% (Apelblat) and 1.13% (NRTL). Therefore, the modified Apelblat equation fits well with the experimental solubility data of 1-AH in pure solvents.

The solubility values calculated by the modified Apelblat equation and NRTL equation show good agreement with experimental values. Both the exper imental solubility and correlation equation can be used as essential data in the purification process of 1-AH, as well as good support for further development of solubility models for 1-AH.

Acknowledgement

We are grateful for the financial support of the National Natural Science Foundation of China (No. NNSFC 21306158), and General project of Hunan Provincial Education Department (grant no. 13C911).

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Publication Dates

• Publication in this collection
  Jan-Mar 2017

History

• Received
  21 Dec 2014
• Reviewed
  02 Nov 2015
• Accepted
  08 Dec 2015