A new laboratory procedure for the evaluation of the mean molecular weight (mean relative molecular mass) of petroleum oils with high accuracy is described. The density and dynamic viscosity of three commercial petroleum oils are measured at different temperatures. These experimental data are used to calculate the kinematic viscosity as a function of the temperature, a typical task in an introductory-level laboratory course in physical chemistry. This new procedure offers an alternative to the classic use of a capillary viscometer. The kinematic viscosity data are used to evaluate the mean molecular weight of the petroleum oils. A set of equations is used that fits the data provided by the ASTM standard D 2502-04. (This standard is commonly used in the petroleum industry to calculate the mean molecular weight of petroleum oils.) The results confirm that viscosity measurements obtained with the new procedure can be used to evaluate the mean molecular weight of petroleum oils with high accuracy.

We describe experiments on Bénard–Marangoni convection which permit a useful understanding of the main concepts involved in this phenomenon such as, for example, Bénard cells, aspect ratio, Rayleigh and Marangoni numbers, Crispation number and critical conditions. In spite of the complexity of convection theory, we carry out a simple and introductory analysis which has the additional advantage of providing very suggestive experiments. As a consequence, we recommend our device for use as a laboratory experiment for undergraduate students of the thermodynamics of nonlinear and fluid physics.

The ASTM standard D 2502-92 (reapproved 2004) provides a means of calculating the mean molecular weight (relative molecular mass) of petroleum oils from kinematic viscosity measurements. It is applicable to samples with molecular weights in the range from 250 to 700 g/mol and is intended for use with average petroleum fractions. Nevertheless, this estimation was carried out by using a viscosity-molecular weight chart that involves evident interpolation errors. In this work, we have used both of Hirschler's equations as numerical methods in order to carry out an analytical estimation of the mean molecular weight of petroleum oils from kinematic viscosity measurements. The data provided by the named Hirschler-Maroto equation are in excellent agreement with the ASTM chart for a viscosity index range that covers the majority of commercial oils. Finally, by using the Hirschler-Maroto equation we have designed a PC program in order to make the evaluation of molecular weight of petroleum oils easier for engineers and professionals. The use of this program, which can be downloaded free from the Internet, only requires the insertion of kinematic viscosity data, which remove any graphical or interpolation errors. Nevertheless, the Hirschler-Maroto equation and the PC program must be cautiously used in order to avoid important errors when applied to the higher molecular weight constituents of petroleum.

Mines of Linares, Jaen, Spain, have been exploited from the age of the Phoenicians, Carthaginians and Romans. These silver and lead mines reached their maximum splendour at the end of the 19th century and at the beginning of the 20th century. Nevertheless, all the mining works finished in the 1970s and the diverse machinery was sold. Only the shafts remain since then which has now permitted carrying out interesting free fall experiments using smooth spheres of both cork and cast iron. The experiments were facilitated by the fact that the tubular shape of the shafts provides excellent transmission of sound, which made feasible the recording of the impact sound of the spheres with water at the bottom of the shafts at distances of up to 200 m. By using these experimental data, we have carried out an evaluation of the drag coefficient for the movement of smooth spheres through the air in the laminar regime with Reynolds number in the interval 10(3) to 2 x 10(5). This evaluation was in excellent agreement with the literature data. From the theoretical point of view, the analysis of the free fall movement includes a variety of concepts such as Newton's second law, the drag force, Archimedes principle and the velocity of sound, which makes these experiments very attractive for both physics teachers and physics students at university level. Finally, an easy experiment is proposed in this paper which has permitted an approximate evaluation of the drag coefficient for smooth spheres to be carried out in a laboratory environment.

A classical experience in a physics student laboratory is to determine the surface tension of a liquid versus the temperature and to check the linear appearance of the obtained graph. In this work we show a simple method to estimate the critical temperature of three liquids by using experimental data of surface tension at different temperatures. By a logarithm fitting between surface tension and temperature, the critical temperature can be determined and compared with data from the literature. For two liquids (butanol and nitrobenzene) the comparison is acceptable but the differences are too high for the third liquid (water). By discussing the results it seems to be clear that the difference between the critical temperature of the liquid and the maximum temperature of the surface tension measurements is the determining factor in obtaining acceptable results. From this study it is possible to obtain more information on the liquid characteristics from surface tension measurements that are currently carried out in a student laboratory. Besides, in this paper it is shown how to select the most suitable liquids which provide both acceptable values for the critical temperature and measurements of the surface tension at moderate temperatures. The complementary use of numerical methods permits us to offer a complete experience for the students with a simple laboratory experiment which we recommend for physics students in advanced university courses.

A Mariotte bottle is a device that provides a constant effusion velocity for liquids. We discuss a Mariotte bottle that has been designed to study the flow regime and the transition from laminar to turbulent flow. Several straight, smooth, circular glass tubes with different lengths and sections were inserted into the lower part of the bottle so that the rate of flow could be measured in very different experimental conditions by using a precision balance. Reynolds numbers in the interval 1014⩽R⩽6098 were obtained, showing that the flow regime is laminar for R⩽3000. There is a transition in the flow regime for R in the interval 3000–4000 and a turbulent flow regime for higher R. Because the device is very simple and the results obtained are very clear and exemplary, we recommend using this device as a laboratory experiment for physics or engineering students who require a knowledge of fluid mechanics.

In this work, an evaluation of the Lorentz law has been carried out by a Barlow wheel. Basically, a Barlow wheel is a primitive dynamo-electric machine (invented by P. Barlow in 1828) which consists of a metallic cogwheel, which moves around a horizontal axis. The theoretical analysis includes the Lorentz law and the Petroff law, which characterizes the friction between the axis and the bearing. In the stationary state (when the angular velocity is constant), a simple formula can be obtained which relates the current intensity and the angular velocity. Experimental data approximately confirms this formula, and this device can be used as a simple and classical laboratory experiment in basic electromagnetism courses. Some modern complements were used for the acquisition of data, as a light barrier connected to a counter, a voltammeter and a direct-current power supply Therefore, the Barlow wheel, originally designed for qualitative demonstrations, can be used as a quantitative and useful assembly in a laboratory of General Physics.

A novel and useful process of heterocoagulation between bare and surfactant-coated latexes is studied. Basically, this process consists of the heterocoagulation of two identical latexes, i.e. of the same size and with charges of the same sign, but distinguished by the degree of coverage by a nonionic surfactant (Triton X-100). The different critical coagulation concentrations (ccc) of this type of sample permitted us to analyze the influence of the ionic strength in the heterocoagulation process between both colloidal samples. Different ratios (2:1, 1:1 and 1:2) of the bare and surfactant-coated latexes were used during the experiments. In all cases, the heterocoagulation rate constants were lower than the homocoagulation rate constant in diffusion conditions; however, for an ionic strength higher than the ccc of both systems, similar values were found for the rate constants.

The heterocoagulation kinetics of two latexes bearing surface charges of opposite sign was studied. The doublet formation kinetic constant was used to determine the evolution of the kinetics. It has been adsorbed a non-ionic surfactant onto the anionic latex in order to increase its stability. Aggregation experiments were performed for both latexes bared. Also, measurements were carried out with the anionic latex partially covered. Comparison between these two results showed small differences in the kinetic constant. The study of the heteroaggregation kinetic constant versus ionic concentration showed a trend opposite to that of the homocoagulation case. The experimental results were compared with simulations. The model used for simulations takes into account the interaction forces between particles to perform the random paths for clusters. Comparison showed good agreement. The inclusion of attractive forces was found to be essential for describing processes faster than diffusion limited aggregations.

We perform a Brownian dynamics simulation for describing the first stages of cluster formation. Special attention has been paid to systems composed of particles of opposite charges. Results are compared with Light scattering experiments, obtaining a very good agreement, which validates the simulation model.

Turbidimetric and nephelometric techniques have been used to study the homocoagulation and heterocoagulation of aqueous dispersions of monodisperse latices. Cationic and anionic latices of similar particle size (361 and 370 nm) and different surface charge density (+16.4 and −3.6 μC cm−2) were used. The colloidal stability of both latices was modified by adsorption of a non-ionic surfactant (Triton X-100). The stability factors and the rate constants (KD,F) for doublet formation were determined for the homocoagulation process when the latices were partially or totally covered by the surfactant. Similar KD,F values were obtained for both latices, even when they were totally or partially covered by the surfactant. These results were confirmed by spectrophotometric and nephelometric measurements at different wavelengths and particle concentrations. The heterocoagulation rate constant (KD12) was calculated from turbidity and nephelometry data as a function of the wavelength and total particle concentration. The heterocoagulation process was studied for cationic and anionic latices: bare cationic latex–bare anionic latex; bare anionic latex–totally covered cationic latex; and totally covered anionic and cationic latices. Even in the last case heterocoagulation occurs, but the kinetic constant (KD12) was lower in comparison with the process when both bare latices were used or when one of them was saturated by the surfactant. The KD12 values were calculated versus the wavelength and particle concentration of the suspension, and these results were confirmed by nephelometry measurements. Thus, a sterically stabilized latex could be coagulated by colloids of opposite sign of surface charge.

In this work, the homocoagulation and heterocoagulation rates of aqueous dispersions of uniform spherical particles of surfactant-free cationic and anionic latexes were measured with a spectrophotometer. Two polymer colloids with similar particle sizes and different surface charge densities were used. The homocoagulation and heterocoagulation rate constants (Kd and Kd12 respectively) were determined as a unction of the total particle number N and the wavelength λ of the spectrophotometer. Constant values of Kd and Kd12 were found when N and λ had specific values. The decreasing of the rate constants for lower λ or higher N values was explained as a consequence of the appearance of multiple light scattering at some experimental conditions. It was possible to detect critical N and λ values which prevent this phenomenon. By calculating the effective volume of solid in the cell at the critical wavelength Vef(λc), it is possible to define an experimental parameter NVef which provides a critical value above which the multiple light scattering takes place.

The purpose of this paper is to apply the classical DLVO theory to explain the colloid stability of two model colloids with similar size and different sign and value of the surface charge. For this comparison the hydrodynamic interaction and the presence of hydration forces (extended DLVO theory) have been taken into account. The experimental stability factor and the experimental doublet rate constant in diffusion conditions were compared with those evaluated theoretically. The mathematical treatment permits an easy evaluation and interpretation of the different adjustable parameters such as the Hamaker constant, diffuse layer potential and the hydration layer thickness. The theoretical and experimental comparison shows that the “extended DLVO theory” only permits to explain the stability curves Log[W]/Log[KBr] in a semiquantitative way by using, for the evaluation of the total interaction potential VT, a value of the Hamaker constant (A) similar to the classical theoretical one for polystyrene particles dispersed in water. In the case of the anionic latex, it was necessary to admit the presence of a hydration layer of a thickness similar to the radius of the hydrated/dehydrated counterion. On the other hand, by using the experimental doublet rate constant in diffusion conditions, we obtain a lower value of the Hamaker constant (A), but within the range of the A values usually found in previous studies.

We have developed a simple theory within the Rayleigh–Gans–Debye (RGD) approximation to follow the homocoagulation process by a turbidity measurement. We have considered all the contributions in the aggregate from particle–particle pairs, with none, one and two particles between them and a von Smoluchowski process. It has been possible to obtain a theoretical description of the experimental absorbance vs. time curves for the first minute in most general cases. Due to the dependence on the RGD approximation, the smaller the particle is, the better agreement between experimental data and theoretical calculations is obtained. We have used this model to develop a technique to calculate kinetic constants from experimental turbidity measurements from homocoagulation of polystyrene latexes. Great agreement has been found between kinetic constants obtained by this method and others by fitting the initial slope. In this new method we can adjust up to 60s of experimental data (for the particle concentrations we have used), to be compared with 4 or 5s that can be used in adjusting the initial slope in the classical turbidimetric technique.