Comparison of thickness values obtained with different analytical techniques

membranes studied, measured with the seven analytical techniques. The seven techniques operate on the basis of a broad range of physical principles. The SEM and TEM measurements are based on image analyses of electron micrographs of the cross sections of membrane samples; AFM and profilometry measurements are based on profiling the active layer height with respect to a solid substrate on which the active layer has been isolated; RBS and QCM estimates are based on the measurement of their areal mass (more specifically, RBS measures atomic areal density and QCM measures mass areal density); and ellipsometry measurements are based on fitting an electromagnetic zero approximation model to light reflection data. Therefore, consistent thickness results among several techniques serve as a strong indicator that the

thickness values are accurate estimates of the true value of the active layer thickness. Similarly, substantial discrepancies between the values measured with one technique and consistent values

measured by several techniques likely indicate that the former technique does not provide

accurate results. Accordingly, analysis of variance (ANOVA) was performed at 95% confidence level of the thickness values in Figure 2.7 to compare the results obtained with the different techniques for each of the membranes.

The ANOVA results indicate that for all six membranes studied, except for the SW30HR membrane, thickness values measured with SEM and TEM were significantly higher than thickness values measured with the non-electron microscopy techniques (AFM, RBS, QCM, profilometry and ellipsometry). For five membranes (NF270, NF90, XLE, SWC4+ and

SW30HR) out of the six studied, the thickness values obtained for each membrane with the non- electron microscopy techniques were not statistically different from each other. While for the ESPA3 membrane, QCM gave an average thickness value that was statistically different from the ones obtained using the other non-electron microscopy techniques, the difference was of only 21% (compared to a difference of 44% for the value obtained with TEM) and could be the combined result of sample variability or level of error in the assumption of a polyamide density value of 1.24 g·cm-3.

Given that the six membranes studied cover a broad range of performance levels (including NF, brackish water RO and seawater RO) and three commonly used active layer materials (fully aromatic polyamide, semi-aromatic poly(piperazinamide) and poly(vinyl alcohol), it is concluded that: (1) AFM, RBS, QCM, profilometry and ellipsometry provide consistent thickness values for any given membrane studied, and thus they can be regarded as appropriate analytical techniques for the determination of active layer thickness of TFC membranes; and (2) SEM and TEM, in general, produce higher thickness values compared to

rough estimates of active layer thickness. A possible reason for the overestimations of active layer thickness using SEM and TEM is the subjective nature of the judgment of the location of the polyamide-polysulfone interface.

Based on the previous discussion, AFM, RBS, QCM, profilometry and ellipsometry all can be regarded as appropriate techniques to measure the thickness of the active layers of RO/NF membranes; however, each of these technique has several advantages and disadvantages as discussed in Sections 2.3.2-2.3.4 and shown in Table 2.1, and therefore, the preferred technique to use will depend on circumstances such as equipment availability, time available for sample preparation and analysis, etc. Among the various advantages and disadvantages listed in Table 2.1, the size of the sample, the need for major assumptions, and level of subjectivity needed to estimate the active layer thickness are three key features to evaluate the appropriateness of a technique, as these three aspects all directly affect the accuracy of the active layer thickness results. Based on the discussions in previous sections regarding advantages and disadvantages of each technique and corresponding summary in Table 2.1, ellipsometry is the only technique that has relatively large analysis scale, and does not require major assumptions and or subjective judgment to estimate active layer thickness. Therefore, ellipsometry is considered as the

preferred technique to use to measure active layer thickness in RO/NF membranes. However, if an ellipsometer is not available, AFM and profilometry are the next preferred alternatives, since the ANOVA tests performed in this study showed that thickness results obtained by AFM and profilometry analyses were not significantly different from those obtained by ellipsometry analyses, and AFM and profilometry do not require major assumptions or subjective judgment to estimate active layer thickness.

Figure 2.7. Measured thicknesses of the active layers of the NF270, NF90, XLE, ESPA3, SWC4+ and SW30HR membranes obtained by SEM, TEM, AFM, RBS, QCM, profilometry and ellipsometry analyses. For each membrane, the reported thickness value and corresponding error represent the average and standard deviation, respectively, for multiple samples. The thickness values obtained with the five techniques that produced consistent results among each other (i.e., AFM, RBS, QCM, profilometry and ellipsometry) ranged from 14.44.7 nm (NF270) to

176.324.5 nm (SW30HR).

In the case that the active layer density of the membrane of interest were known, RBS or QCM would be ideal techniques to measure active layer thickness, since they both perform analyses on samples with large areas, do not require subjective judgment to estimate active layer thickness, and ANOVA tests indicate that the thickness results obtained by RBS and QCM analyses are not significantly different from those obtained by ellipsometry analyses for five out of six membranes studied. To choose between RBS and QCM, one should evaluate the

availability of RBS equipment and the time needed to prepare samples for QCM, since

instruments needed to perform RBS tests are not widely available and are very expensive, and sample preparation for QCM tests (isolating active layers onto QCM sensors) is time consuming. Given that SEM and TEM in general overestimate the thickness of RO/NF membrane active layers, one should only use SEM or TEM for the characterization of active layer thickness when accurate thickness values are not required, but rather rough estimates of active layer thickness are acceptable.